Solar Dynamics Observatory in the context of Living With a Star

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.

Solar phenomena are natural phenomena which occur within the atmosphere of the Sun. They take many forms, including solar wind, radio wave flux, solar flares, coronal mass ejections, coronal heating and sunspots.

These phenomena are believed to be generated by a helical dynamo, located near the center of the Sun's mass, which generates strong magnetic fields, as well as a chaotic dynamo, located near the surface, which generates smaller magnetic field fluctuations. All solar fluctuations together are referred to as solar variation, producing space weather within the Sun's gravitational field.

Multispectral imaging captures image data within specific wavelength ranges across the electromagnetic spectrum. The wavelengths may be separated by filters or detected with the use of instruments that are sensitive to particular wavelengths, including light from frequencies beyond the visible light range (i.e. infrared and ultraviolet). It can allow extraction of additional information the human eye fails to capture with its visible receptors for red, green and blue. It was originally developed for military target identification and reconnaissance. Early space-based imaging platforms incorporated multispectral imaging technology to map details of the Earth related to coastal boundaries, vegetation, and landforms. Multispectral imaging has also found use in document and painting analysis.

Multispectral imaging measures light in a small number (typically 3 to 15) of spectral bands. Hyperspectral imaging is a special case of spectral imaging where often hundreds of contiguous spectral bands are available.

In solar physics, a coronal loop is a well-defined arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes. Coronal loops begin and end at two footpoints on the photosphere and project into the transition region and lower corona. They typically form and dissipate over periods of seconds to days and may span anywhere from 1 to 1,000 megametres (621 to 621,000 mi) in length.

Coronal loops are often associated with the strong magnetic fields located within active regions and sunspots. The number of coronal loops varies with the 11 year solar cycle.

Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths shorter than the hydrogen Lyman-alpha line from 121 nm down to the X-ray band of 10 nm. By the Planck–Einstein equation the EUV photons have energies from 10.26 eV up to 124.24 eV where we enter the X-ray energies. EUV is naturally generated by the solar corona and artificially by plasma, high harmonic generation sources and synchrotron light sources. Since the ultraviolet C range extends to 100 nm, there is some overlap in the terms.

The main uses of extreme ultraviolet radiation are photoelectron spectroscopy, solar imaging, and lithography. In air, EUV is the most highly absorbed component of the electromagnetic spectrum, requiring high vacuum for transmission.

The 2012 transit of Venus, when the planet Venus appeared as a small, dark spot passing across the face of the Sun, began at 22:09 UTC on 5 June 2012, and finished at 04:49 UTC on 6 June. Depending on the position of the observer, the exact times varied by up to ±7 minutes. Transits of Venus are among the rarest of predictable celestial phenomena and occur in pairs. Consecutive transits per pair are spaced 8 years apart, and consecutive pairs occur more than a century apart: The previous transit of Venus took place on 8 June 2004 (preceded by transits on 9 December 1874 and 6 December 1882); the next pair of transits will occur on 10–11 December 2117 and December 2125 within the 22nd century.