Coronal hole

A coronal hole is a region of the Sun's corona that appears dark in extreme-ultraviolet (EUV) and soft-X-ray images because its plasma is cooler and more rarefied than the surrounding corona. Despite its name, a coronal hole is not an actual physical hole or void in the Sun's corona. The darkness reveals open magnetic field lines that guide plasma directly into interplanetary space, producing the fast component of the solar wind. They are composed of relatively cool and tenuous plasma permeated by magnetic fields that are open to interplanetary space. This results in decreased temperature and density of the plasma at the site of a coronal hole, as well as an increased speed in the average solar wind measured in interplanetary space.

Coronal holes were first identified unambiguously in soft-X-ray images from the 1973 Skylab mission, although eclipse photographs had hinted at polar dark regions earlier in the twentieth century. Routine mapping now combines full-disk EUV imagers with ground-based synoptic magnetographs to track hole evolution and feed space-weather forecasts.

Streams of fast solar wind originating from coronal holes can interact with slow solar wind streams to produce corotating interaction regions (CIRs). These regions can interact with Earth's magnetosphere to produce geomagnetic storms of minor to moderate intensity. During solar minima, CIRs are the main cause of geomagnetic storms.

Early observations of coronal holes date back to total solar eclipses between 1901 and 1954, when astronomers noticed polar darkenings adjacent to bright helmet streamers. These dim regions were later identified as magnetically open areas through detailed analysis. The first quantitative observations of coronal holes were made by Max Waldmeier in 1956 and 1957, who used coronagraphic images of the green emission line at 5303 Å to identify these features.

During the 1960s, coronal holes became visible in X-ray images captured by sounding rockets and in radio wavelength observations from the Sydney Chris Cross radio telescope. However, their nature remained unclear at the time. The true understanding of coronal holes emerged in the 1970s when X-ray telescopes aboard the Skylab mission operated above Earth's atmosphere, revealing detailed coronal structure.

The advent of continuous extreme ultraviolet coverage from SOHO/EIT and SDO/AIA enabled automated detection of coronal holes and systematic analysis of their area, latitude, and magnetic flux throughout Solar cycles 23–25 (1996–2019).

A coronal hole refers to regions of the corona with low emission and predominantly open magnetic flux. Polar coronal holes are large, stable features that dominate during sunspot minima and persist for months to years at the Sun's poles, serving as the primary source of ambient fast solar wind. In contrast, mid-latitude and equatorial holes emerge and decay throughout the solar cycle and are smaller, more transient features. A satellite hole is a low-latitude coronal hole that maintains a magnetic connection to a polar hole through a narrow corridor of open magnetic field lines. This distinction is important for space weather forecasting, as satellite holes can produce variable fast solar wind streams that sweep across Earth's orbital plane more frequently than the steady polar wind.

Computer models using potential-field source-surface extrapolations and global magnetohydrodynamic simulations demonstrate that magnetic fields rooted inside coronal holes remain open and extend radially outward beyond approximately 2.5 R_☉ solar radii. However, measurements of the heliospheric magnetic field at 1 AU consistently indicate more open magnetic flux than most models predict, a discrepancy known as the open-flux problem. Proposed solutions to this problem include incomplete coverage of polar magnetic fields in observations and narrow open corridors along coronal hole boundaries that remain unresolved in low-resolution magnetic field maps.