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Solar observation is the scientific endeavor of studying the Sun and its behavior and relation to the Earth and the remainder of the Solar System. Deliberate solar observation began thousands of years ago. That initial era of direct observation gave way to telescopes in the 1600s followed by satellites in the twentieth century.

Prehistory

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Stratigraphic data suggest that solar cycles have occurred for hundreds of millions of years, if not longer; measuring varves in precambrian sedimentary rock has revealed repeating peaks in layer thickness corresponding to the cycle. It is possible that the early atmosphere on Earth was more sensitive to solar irradiation than today, so that greater glacial melting (and thicker sediment deposits) could have occurred during years with greater sunspot activity.[1][2] This would presume annual layering; however, alternative explanations (diurnal) have also been proposed.[3]

Analysis of tree rings revealed a detailed picture of past solar cycles: Dendrochronologically dated radiocarbon concentrations have allowed for a reconstruction of sunspot activity covering 11,400 years.[4]

Early observations

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Solar activity and related events have been regularly recorded since the time of the Babylonians. In the 8th century BC,[5] they described solar eclipses and possibly predicted them from numerological rules. The earliest extant report of sunspots dates back to the Chinese Book of Changes, c. 800 BC. The phrases used in the book translate to "A dou is seen in the Sun" and "A mei is seen in the Sun", where dou and mei would be darkening or obscuration (based on the context). Observations were regularly noted by Chinese and Korean astronomers at the behest of the emperors, rather than independently.[5]

The first clear mention of a sunspot in Western literature, around 300 BC, was by the ancient Greek scholar Theophrastus, student of Plato and Aristotle and successor to the latter.[6] The Royal Frankish Annals record that beginning on 17 March AD 807 a large sunspot that was visible for eight days and incorrectly conclude that it was a transit of Mercury.[7]

The earliest surviving record of deliberate sunspot observation dates from 364 BC, based on comments by Chinese astronomer Gan De in a star catalogue.[8] By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.[9]

A large sunspot was observed at the time of Charlemagne's death in AD 813.[10] Sunspot activity in 1129 was described by John of Worcester and Averroes provided a description of sunspots later in the 12th century;[11] however, these observations were also misinterpreted as planetary transits.[12]

The first unambiguous mention of the solar corona was by Leo Diaconus, a Byzantine historian. He wrote of the 22 December 968 total eclipse, which he experienced in Constantinople (modern-day Istanbul, Turkey):[13]

at the fourth hour of the day ... darkness covered the earth and all the brightest stars shone forth. And it was possible to see the disk of the Sun, dull and unlit, and a dim and feeble glow like a narrow band shining in a circle around the edge of the disk.

— Leo Diaconus[13]

Black and white drawing showing Latin script surrounding two concentric circles with two black dots inside the inner circle
A drawing of a sunspot in the Chronicles of John of Worcester[14]

The earliest known record of a sunspot drawing was in 1128, by John of Worcester.[14]

In the third year of Lothar, emperor of the Romans, in the twenty-eighth year of King Henry of the English...on Saturday, 8 December, there appeared from the morning right up to the evening two black spheres against the sun.

— John of Worcester, The Chronicle of John of Worcester, cited in Albert Van Helden, 1996.[15]

Another early observation was of solar prominences, described in 1185 in the Russian Chronicle of Novgorod.[13]

In the evening there as an eclipse of the sun. It was getting very gloomy and stars were seen ... The sun became similar in appearance to the moon and from its horns came out somewhat like live embers.

— Russian Chronicle of Novgorod[13]

17th and 18th centuries

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Sunspots in 1794 Samuel Dunn Map

Giordano Bruno and Johannes Kepler suggested the idea that the sun rotated on its axis.[16] Sunspots were first observed telescopically on 18 December 1610 (Gregorian calendar, not yet adopted in England) by English astronomer Thomas Harriot, as recorded in his notebooks.[17] On 9 March 1611 (Gregorian calendar, also not yet adopted in East Frisia) they were observed by Frisian medical student Johann Goldsmid (latinised name Johannes Fabricius) who subsequently teamed up with his father David Fabricius, a pastor and astronomer, to make further observations and to publish a description in a pamphlet in June 1611.[18] The Fabricius' used camera obscura telescopy to get a better view of the solar disk, and like Harriot made observations shortly after sunrise and shortly before sunset. Johann was the first to realize that sunspots revealed solar rotation, but he died on 19 March 1616, aged 26 and his father a year later. Several scientists such as Johannes Kepler, Simon Marius, and Michael Maestlin were aware of the Fabricius' early sunspot work, and indeed Kepler repeatedly referred to it his writings. However, like that of Harriot, their work was otherwise not well known. Galileo Galilei almost certainly began telescopic sunspot observations around the same time as Harriot, given he made his first telescope in 1609 on hearing of the Dutch patent of the device, and that he had managed previously to make naked-eye observations of sunspots. He is also reported to have shown sunspots to astronomers in Rome, but we do not have records of the dates. The records of telescopic observations of sunspots that we do have from Galileo do not start until 1612, for when they are of unprecedented quality and detail as by then he had developed the telescope design and greatly increased its magnification.[19] Likewise Christoph Scheiner had probably been observing the spots using an improved helioscope of his own design. Galileo and Scheiner, neither of whom knew of the work of Harriot or Fabricius vied for the credit for the discovery. In 1613, in Letters on Sunspots, Galileo refuted Scheiner's 1612 claim that sunspots were planets inside Mercury's orbit, showing that sunspots were surface features.[18][20]

Although the physical aspects of sunspots were not identified until the 20th century, observations continued.[21] Study was hampered during the 17th century due to the low number of sunspots during what is now recognized as an extended period of low solar activity, known as the Maunder Minimum. By the 19th century, then-sufficient sunspot records allowed researchers to infer periodic cycles in sunspot activity. In 1845, Henry and Alexander observed the Sun with a thermopile and determined that sunspots emitted less radiation than surrounding areas. The emission of higher than average amounts of radiation later were observed from the solar faculae.[22] Sunspots had some importance in the debate over the nature of the Solar System. They showed that the Sun rotated, and their comings and goings showed that the Sun changed, contrary to Aristotle, who had taught that all celestial bodies were perfect, unchanging spheres.

Sunspots were rarely recorded between 1650 and 1699. Later analysis revealed the problem to be a reduced number of sunspots, rather than observational lapses. Building upon Gustav Spörer's work, the wife-and-husband team of Annie Maunder and Edward Maunder suggested that the Sun had changed from a period in which sunspots all but disappeared to a renewal of sunspot cycles starting in about 1700. Adding to this understanding of the absence of solar cycles were observations of aurorae, which were absent at the same time, except at the very highest magnetic latitudes [23]

The lack of a solar corona during solar eclipses was also noted prior to 1715.[24]

The period of low sunspot activity from 1645 to 1717 later became known as the "Maunder Minimum".[25] Observers such as Johannes Hevelius, Jean Picard and Jean Dominique Cassini confirmed this change.[20]

19th century

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Solar spectroscopy

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After the detection of infra-red radiation by William Herschel in 1800 and of Ultraviolet radiation by Johann Wilhelm Ritter, solar spectrometry began in 1817 when William Hyde Wollaston noticed that dark lines appeared in the solar spectrum when viewed through a glass prism. Joseph von Fraunhofer later independently discovered the lines and they were named Fraunhofer lines after him. Other physicists discerned that properties of the solar atmosphere could be determined from them. Notable scientists to advance spectroscopy were David Brewster, Gustav Kirchhoff, Robert Wilhelm Bunsen and Anders Jonas Ångström.[26]

Solar cycle

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Samuel Heinrich Schwabe (1789–1875). German astronomer, discovered the solar cycle through extended observations of sunspots
Rudolf Wolf (1816–1893), Swiss astronomer, carried out historical reconstruction of solar activity back to the seventeenth century
400 year history of sunspot numbers.

The cyclic variation of the number of sunspots was first observed by Samuel Heinrich Schwabe between 1826 and 1843.[27] Rudolf Wolf studied the historical record in an attempt to establish a history of solar variations. His data extended only to 1755. He also established in 1848 a relative sunspot number formulation to compare the work of different astronomers using varying equipment and methodologies, now known as the Wolf (or Zürich) sunspot number.

Gustav Spörer later suggested a 70-year period before 1716 in which sunspots were rarely observed as the reason for Wolf's inability to extend the cycles into the 17th century.

Also in 1848, Joseph Henry projected an image of the Sun onto a screen and determined that sunspots were cooler than the surrounding surface.[28]

Around 1852, Edward Sabine, Wolf, Jean-Alfred Gautier and Johann von Lamont independently found a link between the solar cycle and geomagnetic activity, sparking the first research into interactions between the Sun and the Earth.[29]

In the second half of the nineteenth century Richard Carrington and Spörer independently noted the migration of sunspot activity towards the solar equator as the cycle progresses. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed by Edward Walter Maunder and Annie Scott Dill Maunder in the early twentieth century (see graph). Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce a time-series diagram.

The sunspot butterfly diagram. This modern version is constructed (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

Half a century later, the father-and-son team of Harold and Horace Babcock showed that the solar surface is magnetized even outside of sunspots; that this weaker magnetic field is to first order a dipole; and that this dipole undergoes polarity reversals with the same period as the sunspot cycle (see graph below). These observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole.

Time vs. solar latitude diagram of the radial component of the solar magnetic field, averaged over successive solar rotation. The "butterfly" signature of sunspots is clearly visible at low latitudes. Diagram constructed (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

Photography

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The Sun was photographed for the first time, on 2 April 1845, by French physicists Louis Fizeau and Léon Foucault. Sunspots, as well as the limb darkening effect, are visible in their daguerrotypes. Photography assisted in the study of solar prominences, granulation and spectroscopy. Charles A. Young first captured a prominence in 1870. Solar eclipses were also photographed, with the most useful early images taken in 1851 by Berkowski and in 1860 by De la Rue's team in Spain.[29]

Rotation

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Early estimates of the Sun's rotation period varied between 25 and 28 days. The cause was determined independently in 1858 by Richard C. Carrington and Spörer. They discovered that the latitude with the most sunspots decreases from 40° to 5° during each cycle, and that at higher latitudes sunspots rotate more slowly. The Sun's rotation was thus shown to vary by latitude and that its outer layer must be fluid. In 1871 Hermann Vogel, and shortly thereafter by Charles Young confirmed this spectroscopically. Nils Dúner's spectroscopic observation in the 1880s showed a 30% difference between the Sun's faster equatorial regions and its slower polar regions.[29]

Space weather

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The first modern, and clearly described, accounts of a solar flare and coronal mass ejection occurred in 1859 and 1860 respectively. On 1 September 1859, Richard C. Carrington, while observing sunspots, saw patches of increasingly bright light within a group of sunspots, which then dimmed and moved across that area within a few minutes. This event, also reported by R. Hodgson, is a description of a solar flare. The widely viewed total solar eclipse on 18 July 1860 resulted in many drawings, depicting an anomalous feature that corresponds with modern CME observations.[26]

For many centuries, the earthly effects of solar variation were noticed but not understood. E.g., displays of auroral light have long been observed at high latitudes, but were not linked to the Sun.

In 1724, George Graham reported that the needle of a magnetic compass was regularly deflected from magnetic north over the course of each day. This effect was eventually attributed to overhead electric currents flowing in the ionosphere and magnetosphere by Balfour Stewart in 1882, and confirmed by Arthur Schuster in 1889 from analysis of magnetic observatory data.

In 1852, astronomer and British major general Edward Sabine showed that the probability of the occurrence of magnetic storms on Earth was correlated with the number of sunspots, thus demonstrating a novel solar-terrestrial interaction. In 1859, a great magnetic storm caused brilliant auroral displays and disrupted global telegraph operations. Richard Carrington correctly connected the storm with a solar flare that he had observed the day before in the vicinity of a large sunspot group—thus demonstrating that specific solar events could affect the Earth.

Kristian Birkeland explained the physics of aurora by creating artificial aurora in his laboratory and predicted the solar wind.

20th century

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Observatories

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Early in the 20th century, interest in astrophysics grew in America, and multiple observatories were built.[30]: 320  Solar telescopes (and thus, solar observatories), were installed at Mount Wilson Observatory in California in 1904,[30]: 324  and in the 1930s at McMath–Hulbert Observatory.[31] Interest also grew in other parts of the world, with the establishment of the Kodaikanal Solar Observatory in India at the turn of the century,[32] the Einsteinturm in Germany in 1924,[33] and the Solar Tower Telescope at the National Observatory of Japan in 1930.[34]

Around 1900, researchers began to explore connections between solar variations and Earth's weather. Smithsonian Astrophysical Observatory (SAO) assigned Abbot and his team to detect changes in the radiation of the Sun. They began by inventing instruments to measure solar radiation. Later, when Abbot was SAO head, they established a solar station at Calama, Chile to complement its data from Mount Wilson Observatory. He detected 27 harmonic periods within the 273-month Hale cycles, including 7, 13, and 39-month patterns. He looked for connections to weather by means such as matching opposing solar trends during a month to opposing urban temperature and precipitation trends. With the advent of dendrochronology, scientists such as Glock attempted to connect variation in tree growth to periodic solar variations and infer long-term secular variability in the solar constant from similar variations in millennial-scale chronologies.[35]

Coronagraph

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Until the 1930s, little progress was made on understanding the Sun's corona, as it could only be viewed during infrequent total solar eclipses. Bernard Lyot's 1931 invention of the Coronagraph – a telescope with an attachment to block out the direct light of the solar disk – allowed the corona to be studied in full daylight.[26]

Spectroheliograph

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American astronomer George Ellery Hale, as an MIT undergraduate, invented the spectroheliograph, with which he made the discovery of solar vortices. In 1908, Hale used a modified spectroheliograph to show that the spectra of hydrogen exhibited the Zeeman effect whenever the area of view passed over a sunspot on the solar disc. This was the first indication that sunspots were basically magnetic phenomena, which appeared in opposite polarity pairs.[36] Hale's subsequent work demonstrated a strong tendency for east-west alignment of magnetic polarities in sunspots, with mirror symmetry across the solar equator; and that the magnetic polarity for sunspots in each hemisphere switched orientation from one solar cycle to the next.[37] This systematic property of sunspot magnetic fields is now commonly referred to as the Hale–Nicholson law,[38] or in many cases simply Hale's laws.

Solar radio bursts

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Main article: Solar radio emission

The introduction of radio revealed periods of extreme static or noise. Severe radar jamming during a large solar event in 1942 led to the discovery of solar radio bursts.

Satellites

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Many satellites in Earth orbit or in the heliosphere have deployed solar telescopes and instruments of various kinds for in situ measurements of particles and fields. Skylab, a notable large solar observational facility, grew out if the impetus of the International Geophysical Year campaign and the facilities of NASA. Other spacecraft, in an incomplete list, have included the OSO series, the Solar Maximum Mission, Yohkoh, SOHO, ACE, TRACE, and SDO among many others; still other spacecraft (such as MESSENGER, Fermi, and NuSTAR) have contributed solar measurements by individual instruments.

Modulation of solar bolometric radiation by magnetically active regions, and more subtle effects, was confirmed by satellite measurements of the total solar irradiance (TSI) by the ACRIM1 experiment on the Solar Maximum Mission (launched in 1980).[39] The modulations were later confirmed in the results of the ERB experiment launched on the Nimbus 7 satellite in 1978.[40] Satellite observation was continued by ACRIM-3 and other satellites.[41]

Measurement proxies

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Direct irradiance measurements have been available during the last three cycles and are a composite of multiple observing satellites.[41][42] However, the correlation between irradiance measurements and other proxies of solar activity make it reasonable to estimate solar activity for earlier cycles. Most important among these proxies is the record of sunspot observations that has been recorded since ~1610. Solar radio emissions at 10.7 cm wavelength provide another proxy that can be measured from the ground, since the atmosphere is transparent to such radiation.

Other proxy data – such as the abundance of cosmogenic isotopes – have been used to infer solar magnetic activity, and thus likely brightness, over several millennia.

Total solar irradiance has been claimed to vary in ways that are not predicted by sunspot changes or radio emissions. These shifts may be the result of inaccurate satellite calibration.[43][44] A long-term trend may exist in solar irradiance.[45]

Other developments

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The Sun was, until the 1990s, the only star whose surface had been resolved.[46] Other major achievements included understanding of:[47]

  • X-ray-emitting loops
  • Corona and solar wind
  • Variance of solar brightness with level of activity and verification of this effect in other solar-type stars
  • The intense Fibril state of the magnetic fields at the visible surface of a star like the sun
  • The presence of magnetic fields of 0.5×105 to 1×105 gauss at the base of the conductive zone, presumably in some fibril form, inferred from the dynamics of rising azimuthal flux bundles.
  • Low-level Electron neutrino emission from the Sun's core.[47]

21st century

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Nasa's 2006 prediction. At 2010/2011, the sunspot count was expected to be at its maximum, but in reality in 2010 it was still at its minimum.

The most powerful flare observed by satellite instrumentation began on 4 November 2003 at 19:29 UTC, and saturated instruments for 11 minutes. Region 486 has been estimated to have produced an X-ray flux of X28. Holographic and visual observations indicate significant activity continued on the far side of the Sun.

Sunspot and infrared spectral line measurements made in the latter part of the first decade of the 2000s suggested that sunspot activity may again be disappearing, possibly leading to a new minimum.[48] From 2007 to 2009, sunspot levels were far below average. In 2008, the Sun was spot-free 73 percent of the time, extreme even for a solar minimum. Only 1913 was more pronounced, with no sunspots for 85 percent of that year. The Sun continued to languish through mid-December 2009, when the largest group of sunspots to emerge for several years appeared. Even then, sunspot levels remained well below those of recent cycles.[49]

In 2006, NASA predicted that the next sunspot maximum would reach between 150 and 200 around the year 2011 (30–50% stronger than cycle 23), followed by a weak maximum at around 2022.[50][51] Instead, the sunspot cycle in 2010 was still at its minimum, when it should have been near its maximum, demonstrating its unusual weakness.[52]

Cycle 24's minimum occurred around December 2008 and the next maximum was predicted to reach a sunspot number of 90 around May 2013.[53] The monthly mean sunspot number in the northern solar hemisphere peaked in November 2011, while the southern hemisphere appears to have peaked in February 2014, reaching a peak monthly mean of 102. Subsequent months declined to around 70 (June 2014).[54] In October 2014, sunspot AR 12192 became the largest observed since 1990.[55] The flare that erupted from this sunspot was classified as an X3.1-class solar storm.[56]

Independent scientists of the National Solar Observatory (NSO) and the Air Force Research Laboratory (AFRL) predicted in 2011 that Cycle 25 would be greatly reduced or might not happen at all.[57]

References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Solar observation

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Solar observation is the scientific practice of examining the Sun's physical properties, , atmosphere, and dynamic phenomena through diverse techniques, from ancient naked-eye to advanced space-based , enabling insights into solar variability and its broader impacts. This field encompasses the detection of features such as sunspots, solar flares, and coronal mass ejections (CMEs), which are critical for modeling the Sun's 11-year activity cycle and predicting events. Historical records of solar observation date back over two millennia, with the earliest documented sunspot sightings appearing in Chinese, Korean, Japanese, , and Indian texts as early as 364 BC. These naked-eye observations provided initial evidence of solar variability, though limited by the absence of telescopes and the danger of direct viewing. The advent of telescopic observation in the early 17th century, pioneered by in 1610–1611, revolutionized the field by allowing detailed drawings of s and sparking debates on their nature as planetary companions or solar surface features. Subsequent milestones include the identification of the by Samuel Schwabe in 1843 through systematic sunspot counts and the establishment of the International Sunspot Number series from 1700 onward, which has tracked long-term patterns like the (1645–1715), a period of unusually low activity. Modern solar observation relies on sophisticated ground- and space-based methods to overcome atmospheric distortion and capture multi-wavelength data. Ground techniques include pinhole projectors for safe projection of the solar image and filtered telescopes, such as those using (H-alpha) filters to reveal chromospheric structures like prominences and filaments. Space missions, unhindered by Earth's atmosphere, employ instruments like the (SDO), launched in 2010, which uses the Atmospheric Imaging Assembly (AIA) to image the corona in (EUV) wavelengths every 10 seconds at 725 km resolution, the Extreme Ultraviolet Variability Experiment (EVE) to measure fluctuations, and the Helioseismic and Magnetic Imager (HMI) for mapping surface magnetic fields and probing the solar interior via helioseismology. These approaches have revealed the Sun's and the origins of , enhancing our understanding of phenomena invisible from the ground. The importance of solar observation lies in its role in forecasting , as solar eruptions can disrupt operations, power grids, and communications on , potentially causing economic losses comparable to major disasters. By monitoring and magnetic activity, observations contribute to studies, revealing how variations in solar output influence 's atmosphere and oceans over long timescales. Ongoing international efforts, including fleets of solar observatories, ensure continuous 24/7 monitoring to mitigate these risks and advance .

Ancient and Pre-Telescopic Observations

Prehistoric Evidence

Prehistoric evidence for solar observation relies on indirect proxies preserved in geological and paleoclimatic archives, which reveal long-term variations in solar activity through their influence on Earth's environment. These records predate written and provide insights into solar variability over millions of years, demonstrating that the Sun's output has fluctuated in ways that affected terrestrial conditions. Such evidence is derived from natural patterns, isotopic compositions in organic materials, and atmospheric deposition in polar ice, all modulated by solar-driven changes in flux and . One of the earliest indicators comes from rock formations, where exhibit cyclic layering attributable to solar influences. In the Elatina Formation of , dated to approximately 680 million years ago, varve thicknesses show rhythmic variations with periods resembling solar activity cycles, including an 11-year signal and longer harmonics, suggesting solar variability influenced seasonal sediment deposition during the late era. These cycles, preserved in glacial varves, indicate that fluctuations impacted and processes even in ancient geological times. Direct prehistoric evidence includes megalithic structures aligned with solar events, such as the solstice markers at sites like in , constructed around 3000 BCE, which demonstrate early human tracking of solar cycles for ritual and calendrical purposes. Tree-ring records from offer a more recent but still prehistoric perspective, extending back over 11,400 years and linking solar magnetic activity to variations in atmospheric (¹⁴C). Annual growth rings in ancient bristlecone pines and oaks preserve ¹⁴C levels, which increase during periods of low solar activity due to enhanced cosmic ray penetration and subsequent production of cosmogenic isotopes in the atmosphere. Reconstructions from these tree rings reveal sunspot-like activity patterns, with high solar magnetic strength suppressing ¹⁴C production and correlating with grand solar maxima over the . For instance, the past 11,400 years show only about 10% of the time at levels of solar activity as high as recent decades, highlighting episodic solar variability. Ice core data from polar regions provide complementary evidence through beryllium-10 (¹⁰Be) isotopes, which track solar minima and maxima over millennia by recording cosmic ray-induced production rates. During low solar activity, weakened heliospheric magnetic fields allow more galactic cosmic rays to reach , increasing ¹⁰Be deposition in layers; conversely, high activity reduces ¹⁰Be. and ice cores document this for the past several thousand years prior to written records. These proxies thus capture changes indirectly, as variations in solar magnetic activity and total modulate cosmic ray flux, influencing isotope production without direct solar viewing.

Ancient Civilizations

Ancient civilizations across the world made pioneering observations of , laying the groundwork for understanding celestial patterns through written records and monumental alignments. In , Babylonian astronomers from the utilized clay tablets to document and predict by recognizing arithmetic progressions in solar-lunar cycles, such as the Saros cycle of approximately 18 years that allowed forecasting of occurrences. These predictions relied on meticulous tracking of lunar months and eclipse timings, enabling anticipatory calculations for events like the solar eclipse of 136 BC. In , Chinese observers recorded sunspots as early as 165 BC, with descriptions in historical texts noting dark spots on the sun's surface during periods of heightened solar activity. These accounts, preserved in later compilations such as the , also linked auroral displays—termed "red " or atmospheric lights—to solar disturbances, providing early evidence of geomagnetic effects from solar flares. Such highlighted recurring solar patterns, often interpreted through astrological lenses but grounded in direct visual observations. Egyptian and Mayan societies developed solar calendars attuned to solstices and es to regulate and rituals, with monumental structures facilitating precise tracking. The ancient Egyptian civil calendar, established around 3000 BC, divided the year into aligned with the Nile's cycle and solar events like , when the of Sirius marked the . Similarly, early Mesoamerican orientations from 1100–750 BCE represent the earliest evidence of solar and calendrical alignments, while later Mayan sites like integrated the Haab' (a 365-day solar year), where the El Castillo casts a serpent shadow during equinoxes, symbolizing seasonal transitions around 1000 CE. These civilizations employed stone megaliths and temple orientations—reminiscent of solstice-tracking structures from 2000 BC—for observing solar extremes, ensuring calendars reflected equinox balance and solstice shifts. In the , Vedic texts from around 1500 BC, particularly the , described solar eclipses as mythical events where demons like obscured the sun, while correlating these phenomena with patterns essential for . Hymns in the detailed eclipse timings and their perceived influence on seasonal rains, with ancient qualitatively noting year-to-year variations tied to solar observations. This integration of astronomical events with climatic cycles underscored early recognition of the sun's role in environmental rhythms.

Medieval Records

During the medieval period, Islamic astronomers advanced solar observation through precise measurements of eclipses and related phenomena. , working in the at in present-day , refined the length of the solar year to 365 days, 5 hours, 46 minutes, and 24 seconds by analyzing timings of solar and lunar eclipses alongside other observations, achieving an accuracy within about 2 minutes of modern values. His work, preserved in the Zij al-Sabi, built on Ptolemaic methods and influenced later European astronomy by improving predictions of solar positions. Solar eclipses were recognized as predictable events, extending ancient techniques with greater precision in timing and periodicity. In , monastic scholars contributed to solar records through eclipse documentation and early astronomical tables. The saw the development of precursor tables to later works like those of , including translations of Islamic Toledan tables into Latin around 1140 by scholars at the monastery and elsewhere, which facilitated eclipse predictions across the continent. These tables, often compiled in monastic scriptoria, correlated celestial events with earthly calamities; for instance, chroniclers like Gervase of linked the annular of 1185 to subsequent famines and plagues, interpreting it as a divine omen signaling societal distress. A notable European record is the first known illustrated depiction of sunspots by the monk in his Chronicle, dated to December 8, 1128, showing two large dark spots on the solar disk observed from , likely with the due to their exceptional size. This observation coincided with heightened solar activity, as evidenced by a red auroral display recorded five days later on December 13 in , Korea, described as a crimson vapor filling the sky, possibly resulting from a triggered by the same solar event. In , Chinese and Korean astronomers documented "dark spots" on the Sun during the 13th century, providing some of the earliest systematic naked-eye records of sunspots amid the Medieval . The dynasty's official chronicle, Songshi, includes 38 such candidates between 960 and 1279 CE, with clusters in the 13th century noting black vapors or spots visible during daylight, reflecting periods of elevated solar activity. Korean records from the dynasty similarly captured auroral phenomena linked to solar storms, enhancing the global picture of medieval solar variability.

17th to 19th Centuries

Early Telescopic Observations

The introduction of the revolutionized solar observation in the early , enabling detailed views of the Sun's surface that were previously impossible with the . English mathematician and astronomer conducted the earliest known telescopic observations of sunspots in December 1610, using a to produce sketches that captured dark patches on the solar disk. These drawings, numbering nearly 200 from 1610 to 1612, predated similar published work by several months and marked the first pictorial records of solar features through instrumentation. The first published account of telescopic sunspot observations appeared in June 1611, when German astronomer Johann Fabricius issued De Maculis in Sole observatis, describing spots as solar phenomena based on his and his father David Fabricius's sightings. Italian astronomer expanded on these initial sightings with systematic observations beginning in 1611, publishing his findings in the Letters on Sunspots in 1613. In these letters, Galileo described sunspots as transient phenomena occurring on or near the Sun's surface, using their daily motion across the disk to infer the Sun's rotation period of approximately one month—thus providing early evidence of from surface markers. This interpretation directly challenged the Aristotelian doctrine of the heavens' immutable perfection, as sunspots revealed the Sun as a changeable body akin to , sparking philosophical and scientific controversy. Jesuit astronomer Christoph Scheiner independently observed sunspots starting in 1611, publishing Tres Epistolae de Maculis Solaribus in 1612 under the pseudonym . To safely project the Sun's image without direct viewing, Scheiner employed a method using a focused onto a screen behind the , often with colored glass filters to reduce glare, allowing for precise tracing of spots over time. Initially, Scheiner argued that sunspots were not surface features but small, star-like satellites orbiting the Sun, a view intended to preserve celestial perfection; this sparked a heated debate with Galileo, who countered that the spots' irregular paths and disappearances proved they were atmospheric or surface irregularities rather than permanent bodies. Scheiner later conceded the surface-origin hypothesis in his comprehensive 1630 work Rosa Ursina sive Sol. Telescopic monitoring continued through the mid-17th century, revealing prolonged periods of anomalously low activity. The , spanning roughly 1645 to 1715, saw sunspot numbers plummet to near zero for decades, with observers like Polish astronomer recording only sporadic groups—such as 19 between 1653 and 1679—despite diligent daily projections. This era of diminished solar activity coincided with the , a time of cooler global temperatures in and , though the causal link remains a subject of ongoing research.

Solar Cycle and Sunspots

In 1843, German apothecary and amateur astronomer Samuel Heinrich Schwabe announced the discovery of an approximately 11-year cycle in activity, based on his meticulous daily observations of sunspots from 1826 to 1843 using a small . Schwabe's work built upon earlier telescopic sightings of sunspots by Galileo and others in the , transforming qualitative descriptions into quantitative evidence of periodic solar variability. His findings, published in the Astronomische Nachrichten, revealed alternating periods of high and low numbers, laying the groundwork for understanding long-term solar behavior. Swiss astronomer Rudolf Wolf extended Schwabe's observations in 1852 by developing a standardized formula for the relative number, Rz=k(10g+f)R_z = k(10g + f), where gg represents the number of sunspot groups, ff the number of individual spots, and kk a correction factor accounting for observational conditions at different sites. This metric, derived from Wolf's analysis of historical records dating back to , enabled consistent tracking of solar activity across observatories and facilitated the reconstruction of past cycles. Wolf's sunspot number series, maintained at the Zurich Observatory, became the primary tool for monitoring the solar cycle's approximately 11-year periodicity. British astronomer Richard Carrington advanced the study of sunspot dynamics through his systematic observations from 1853 to 1861, culminating in the 1863 publication Observations of the Spots on the Sun. By tracking the motion of individual s over time, Carrington established the Sun's : the equatorial regions complete a rotation in about 25 days, while higher latitudes near the poles take approximately 36 days. This discovery, confirmed through precise positional measurements, indicated that the solar behaves as a fluid rather than a rigid body, influencing models of solar convection and magnetic field generation. A pivotal event during this era occurred on September 1, 1859, when Carrington and independently Richard Hodgson observed the first white-light erupting from a large group, visible against the Sun's disk for about five minutes. This intense flare, part of what is now known as the , triggered a massive that disrupted telegraph systems worldwide, causing sparks, fires, and auroras visible as far south as the . The event underscored the Sun's capacity for sudden, high-energy releases tied to activity, highlighting the need for coordinated solar monitoring.

Spectroscopy and Photography

In the early , solar spectroscopy emerged as a pivotal tool for analyzing the Sun's composition, beginning with the work of . In 1814, Fraunhofer constructed a spectroscope and meticulously mapped 574 dark absorption lines in the solar spectrum, which appeared as gaps in the otherwise continuous rainbow of colors produced by dispersing sunlight through a prism. These lines, now known as , remained unexplained for decades, as they defied contemporary understanding of light and matter. The mystery of these absorption lines was resolved in 1859 by Gustav Kirchhoff, who developed a theoretical framework linking them to gaseous absorption in the Sun's atmosphere. Kirchhoff's gas theory posited that cooler gases surrounding the hotter solar interior absorb specific wavelengths of light emitted from below, creating the dark lines observed on Earth. Collaborating with Robert Bunsen, Kirchhoff used prism spectroscopes to compare laboratory emission spectra of heated elements with the solar spectrum, successfully identifying hydrogen and sodium as key constituents in the solar atmosphere through matching absorption patterns, such as the prominent yellow D-lines for sodium. This breakthrough not only explained Fraunhofer's observations but also established spectroscopy as a method for remote chemical analysis of celestial bodies. Parallel to these spectroscopic advances, early solar photography captured the Sun's visible features, enabling permanent records beyond fleeting visual observations. In 1840, produced the first solar image using a setup with a polished metal plate sensitized to light, marking a foundational step in despite the era's technical challenges like long exposures. Building on this, designed the photoheliograph in 1857, a specialized telescope-camera hybrid equipped with wet plates that facilitated daily imaging of the solar disk and sunspots at observatories like , providing systematic documentation of solar surface dynamics. Spectroscopy and photography converged dramatically during the 1868 total solar eclipse observed by Jules Janssen in India. Using a spectroscope attached to his telescope, Janssen examined the emission lines from solar prominences—fiery gaseous extensions beyond the Sun's edge—confirming their chromospheric origin as hydrogen-dominated structures through bright-line spectra visible only during totality, while also detecting an unidentified yellow line (later known as the D3 line of ). This observation, independently corroborated by , not only validated the gaseous nature of prominences but also led to the discovery of as a new element in 1868 and paved the way for routine non-eclipse studies by demonstrating how spectral isolation could reveal solar atmospheric features.

20th Century Advances

Ground-Based Instruments

In the , ground-based solar observation advanced significantly through the development of specialized instruments that overcame atmospheric limitations and enabled detailed imaging of the Sun's dynamic layers. Building upon the spectroscopic techniques pioneered in the , astronomers created devices capable of isolating specific wavelengths and suppressing overwhelming disk brightness to study phenomena like sunspots and the corona. These innovations, often mounted at high-altitude observatories, provided foundational data on solar activity despite challenges from Earth's atmosphere. A pivotal instrument was the spectroheliograph, invented by and first deployed at in 1908 using the 60-foot solar tower telescope. This device employed a spectrograph and moving slit to capture monochromatic images of the Sun by isolating narrow wavelength bands, such as the (Hα) line at 656.3 nm, allowing visualization of the chromosphere's filaments, prominences, and spicules that are invisible in broadband light. Hale's initial Hα spectroheliogram on March 28, 1908, revealed intricate solar structures, revolutionizing the study of the Sun's outer atmosphere. Using the same spectroheliograph setup, Hale discovered solar magnetic fields in sunspots later in 1908 by observing the , where spectral lines split in the presence of a . In observations of sunspot spectra, he detected polarized line splitting proportional to field strength, quantified by the formula for longitudinal Zeeman displacement: Δλ=4.67×10−13gλ2B\Delta \lambda = 4.67 \times 10^{-13} g \lambda^2 B where Δλ\Delta \lambda is the wavelength shift in angstroms, gg is the Landé factor, λ\lambda is the central in angstroms, and BB is the strength in gauss. This breakthrough, detailed in Hale's analysis of calcium and hydrogen lines, confirmed fields up to several thousand gauss in sunspots and established as central to solar dynamics. Another landmark was the , invented by Bernard Lyot and first successfully operated at Pic du Midi Observatory in 1931. By using an occulting disk to artificially eclipse the solar disk, a Lyot stop to block diffracted light, and high-quality to minimize scattering, the instrument enabled routine observation of the faint solar corona without waiting for a total eclipse. Lyot's first photograph of the corona on July 12, 1931, captured its pearly structure and polarized light, opening the field to studies of coronal mass ejections and streamer evolution from ground sites. By the late 20th century, facilities like the Big Bear Solar Observatory, established in by the on , enhanced full-disk monitoring with vacuum towers to reduce air turbulence. Its patrol telescopes provided continuous Hα and white-light full-disk images for tracking solar flares and eruptions, while the vector magnetograph measured both magnitude and direction of photospheric fields using Zeeman splitting in , yielding insights into magnetic shear and energy buildup in active regions.

Space-Based Observations

Space-based observations of the Sun began in the with satellites that circumvented Earth's atmospheric absorption, enabling unprecedented views in and wavelengths to study the solar corona and flares. These missions provided continuous monitoring without weather disruptions or seeing effects, revealing dynamic processes invisible from ground-based telescopes. The (OSO-1), launched in 1962, marked the first satellite dedicated to solar observations from orbit and included instruments for detection. It detected the first satellite-based emission from the Sun, demonstrating that the corona reaches temperatures exceeding 1 million K, far hotter than the . These findings confirmed theoretical models of coronal heating and highlighted the role of in maintaining such extreme conditions. In 1973, NASA's mission featured the (), a suite of solar instruments that produced (XUV) spectroheliograms of solar flares and prominences. The XUV spectroheliograph (experiment S082A) resolved fine structures in these events, such as looping prominences and flare loops, with spatial resolution of approximately 1 arcminute, offering insights into plasma dynamics during eruptions. These observations, conducted over multiple manned missions, amassed a vast dataset on coronal mass ejections and energy release mechanisms. The Solar Maximum Mission (SMM), launched in 1980 near the peak of 21, included the Active Cavity Radiometer Monitor (ACRIM) to measure total solar (TSI). It established the mean TSI value at TSI=1366±0.5TSI = 1366 \pm 0.5 W/m², with variations of about 0.1% over the 11-year sunspot cycle, linking changes directly to solar activity levels. SMM's gamma-ray and X-ray spectrometers further correlated flare emissions with these fluctuations, advancing understanding of solar output's impact on Earth's climate. Launched in 1991 by (with international collaboration), the Yohkoh satellite provided the first high-resolution imaging in both soft and hard X-rays, focusing on solar flares. Its Hard X-ray Telescope (HXT) and Soft X-ray Telescope (SXT) revealed compact sources at flare loop tops, supporting models of as the primary energy release mechanism. Observations of events like the 1992 Masuda flare showed hard X-ray emission above soft X-ray loops, indicating reconnection sites in the low corona. The (SOHO), a joint NASA-ESA mission launched in December 1995, further advanced space-based solar monitoring with instruments such as the Extreme-ultraviolet Imaging Telescope (EIT) for full-disk EUV imaging of the corona, the Large Angle and Spectrometric (LASCO) for observing CMEs from 1.1 to 30 solar radii, and the Michelson Doppler Imager (MDI) for helioseismic and measurements. SOHO provided near-continuous observations, enabling the discovery of hundreds of comets and detailed studies of origins and interior dynamics.

Helioseismology and Proxies

Helioseismology, a technique developed in the late , enables indirect probing of the Sun's interior by studying global oscillations on its surface, analogous to on . These oscillations arise from convective motions in the solar interior, generating standing that reveal properties such as , , and rotation profiles otherwise inaccessible to direct observation. The Global Oscillation Network Group (), initiated in , marked a significant advancement by deploying a worldwide network of six instruments to acquire nearly continuous Doppler velocity measurements of the solar surface, minimizing gaps due to and . Central to helioseismology are p-mode oscillations, pressure-driven with periods ranging from 3 to that propagate through the solar interior and are trapped by the surface boundary. By analyzing the frequencies and splitting of these modes, researchers map the Sun's internal rotation, revealing where the rotates faster (about 25 days) than the poles (about 35 days), with a transition to rigid rotation in the radiative interior below the . data have been instrumental in refining these maps, providing high-resolution insights into distribution and its implications for solar dynamo processes. A key analytical method in helioseismology is ray inversion, which reconstructs the radial speed profile c(r)c(r) and from observed p-mode frequencies. This involves solving an where travel times of acoustic rays between surface points are modeled using the , approximating wave propagation along curved paths in a spherically symmetric model; the resulting inversions yield localized averages of c(r)c(r), typically showing an increase from about 500 km/s in to about 7 km/s near the surface, enabling precise profiles that match standard solar models within a few percent. Such techniques, applied to observations, have constrained the depth of the to approximately 0.71 solar radii. Complementing helioseismology, cosmogenic proxies offer reconstructions of long-term solar activity and total solar irradiance (TSI) variations extending back over 10,000 years, far beyond direct measurements. (^10Be), a produced in Earth's atmosphere by and deposited in polar ice cores, serves as a primary proxy; its concentration inversely correlates with solar magnetic modulation of cosmic rays, allowing inference of past solar cycles. Analyses of (GRIP) samples spanning the reveal TSI fluctuations of up to 0.4% (about 6 W/m²), with grand minima like the showing elevated ^10Be levels indicative of reduced solar output. These reconstructions, combining ^10Be with (^14C) tree-ring data, highlight periodicities around 200 and 2,300 years in solar variability. Solar radio bursts, first systematically observed in the 1940s using early radar systems during , provided another indirect probe of solar activity through metric-wavelength emissions from the corona. These bursts were classified into Types I through V based on their dynamic spectra and durations: Type I as short, narrowband noise storms associated with active regions; Type II as drifting, harmonic emissions from shock waves; Type III as fast-drifting bursts from electron streams; Type IV as long-duration continua from flare ejecta; and Type V as post-Type III continua. Pioneering Australian observations from 1945–1947 identified these patterns, linking them to optical flares and later to coronal mass ejections (CMEs) via shock-driven Type II and particle-accelerated Type IV emissions, with about 70% of such bursts accompanying CMEs in cataloged events.

21st Century Developments

Modern Ground Observatories

Modern ground observatories in the have leveraged larger apertures, advanced , and computational enhancements to achieve unprecedented resolutions in solar imaging, surpassing the limitations of earlier 20th-century instruments like spectroheliographs by providing dynamic, real-time views of solar features. These facilities focus on high-resolution studies of the solar atmosphere, particularly magnetic structures and dynamic processes, using visible and near-infrared wavelengths to probe phenomena such as sunspots and chromospheric activity. Key examples include telescopes equipped with multi-conjugate systems that correct for atmospheric , enabling diffraction-limited observations over wider fields of view. The (DKIST), located on in , represents the pinnacle of modern ground-based solar observation with its 4-meter off-axis Gregorian , which achieved first light in 2020. This design delivers a theoretical better than 0.03 arcseconds at visible wavelengths, corresponding to scales of about 20 kilometers on the solar surface, allowing detailed mapping of in the and their role in energy transfer to the upper atmosphere. Equipped with a suite of instruments including the Visible Spectro-Polarimeter () and the Visible Tunable Filter (VTF), DKIST captures spectropolarimetric data to infer vector with high sensitivity, revealing fine-scale structures in sunspots and faculae that drive solar eruptions. At Big Bear Solar Observatory in , the Goode Solar Telescope (GST), a 1.6-meter off-axis upgraded in the late , excels in near- for studying magnetism. Its system and the Near-Infrared Imaging Spectropolarimeter (NIRIS) provide diffraction-limited imaging at wavelengths around 1.56 micrometers, where the is stronger, enabling precise measurements of umbral and penumbral dynamics with resolutions approaching 0.1 arcseconds. These observations have illuminated how twisted in contribute to flux emergence and torsional motions, building on earlier capabilities but with enhanced stability and throughput. The Swedish 1-meter Solar Telescope (SST) on , , operational since 2003, specializes in chromospheric dynamics using groundbreaking . Its 85-electrode deformable mirror corrects wavefront distortions in real time, achieving resolutions of 0.1 arcseconds in the near-ultraviolet, which reveals wave propagation and in spicules and Ellerman bombs. Instruments like the CRisp Imaging SPectropolarimeter (CRISP) facilitate high-cadence imaging in H-alpha and Ca II lines, capturing transient events such as chromospheric swirls with temporal resolutions down to seconds, thus advancing understanding of energy dissipation in the solar transition region. Integration of has further revolutionized operations at these observatories, enabling real-time flare detection and automated data processing. For instance, at the National Solar Observatory's Dunn Solar Telescope in , which ceased NSO operations in the late 2010s before transfer to a consortium, AI algorithms analyzed high-cadence Ca II K-line images to identify pre-flare signatures, improving alert times for events. Similar techniques applied to ground-based networks like the Global Oscillation Network Group () now process vast datasets for predictive modeling of solar activity, enhancing the overall efficacy of modern facilities.

Recent Space Missions

The (SDO), launched in 2010 by , has provided continuous high-resolution observations of the Sun's and atmosphere throughout , capturing its maximum activity phase around 2014. Its Helioseismic and Magnetic Imager (HMI) produces magnetograms that map the Sun's photospheric magnetic fields, revealing the evolution of sunspots and active regions, while the Atmospheric Imaging Assembly (AIA) delivers (EUV) images across multiple wavelengths to track coronal dynamics and plasma heating. These instruments have been instrumental in studying the corona's response to events, contributing to our understanding of solar variability during the cycle's peak. Building on earlier space-based efforts like the Solar and Heliospheric Observatory (SOHO) from the 1990s, the Parker Solar Probe, launched in 2018, represents a leap in in-situ measurements of the solar corona and wind. By 2024, the probe achieved its closest approaches to the Sun at approximately 8.5 solar radii from the surface, enduring extreme conditions to sample the young solar wind directly. Observations have revealed switchbacks—sudden reversals in the magnetic field—within solar wind streams with velocities ranging from approximately 300 to 800 km/s, providing insights into the mechanisms accelerating the wind and heating the corona during the ascent of Solar Cycle 25. Launched in 2020 as an ESA mission with contributions, combines and in-situ instruments to probe the Sun from distances as close as 0.28 AU, enabling unprecedented views of the solar poles. For the first time, it has imaged the polar regions, revealing weaker magnetic activity and fewer sunspots compared to equatorial zones, which informs models of the Sun's global during Cycle 25. Its suite of sensors measures interplanetary magnetic fields, particles, and waves alongside coronal imagery, linking surface processes to heliospheric structures inaccessible from Earth-orbiting observatories. During , which began in 2019 and is projected to peak in 2025, X-class flares have intensified coronal studies, with an example being the X1.0 event on May 8, 2024, from NOAA 13663. These powerful eruptions, observed by missions like SDO and Parker, highlight increased solar activity, including larger coverage that causes total (TSI) dips of about 0.1% at cycle peaks due to reduced radiative output from dark umbrae. Such variations underscore the missions' role in monitoring impacts from heightened coronal mass ejections and wind turbulence.

Current Challenges and Future Prospects

One major challenge in solar observation remains the distortion caused by Earth's atmosphere, which limits the of ground-based telescopes to approximately 1 arcsecond, preventing the capture of fine-scale solar features despite advances in . Space-based missions like the (SDO) generate vast data volumes, approximately 1.5 terabytes per day, necessitating advanced techniques to process and analyze imagery for patterns such as sunspot evolution and flare precursors. Accurate space weather forecasting is hindered by uncertainties in coronal mass ejection (CME) propagation, where models like WSA-ENLIL provide 1- to 4-day advance warnings of disturbances reaching , but prediction errors can still reach several hours due to variable speeds and interactions. These limitations underscore the need for enhanced observational vantage points to improve lead times for alerts. Future prospects include missions like NASA's Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS), launched in July 2025, which study processes in Earth's to better understand solar-driven energy transfers. Similarly, the European Space Agency's mission, scheduled for launch around 2031 and positioned at the Sun-Earth L5 , will enable early detection of CMEs up to five days in advance by viewing the Sun from a 60-degree offset relative to . These efforts align with observations during the maximum phase of , predicted to peak around July 2025 with continued high activity into late 2025, including significant solar storms in November 2025, which heightened the urgency for refined predictive capabilities. Emerging multi-messenger approaches promise deeper insights by integrating detections, such as those from the Borexino experiment measuring pp-chain fluxes, with electromagnetic observations to probe the Sun's interior dynamics and activity cycles beyond surface phenomena. This synergy could refine models of solar variability, linking core fusion processes to observable flares and CMEs for more holistic understanding.

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