User:Mccauley.pi/sandbox/Extreme ultraviolet wave (solar physics)

An Extreme ultraviolet (EUV) wave is a large-scale wave-like disturbance observed in the solar corona at extreme ultraviolet (EUV) wavelengths. They appear as large bubbles expanding in all directions away from an eruption site, often traveling across a significant fraction of the solar disk. EUV waves were initially referred to as EIT waves in reference to the EUV Imaging Telescope (EIT) that facilitated their discovery in 1997. They originate in solar active regions, which are regions of highly-concentrated magnetic fields that often include a sunspot at their base in the photosphere. They immediately follow solar flares and precede coronal mass ejections (CMEs). However, all such eruptions do not exhibit EUV waves, which are primarily associated with impulsive (abrupt) flares of moderate-to-high intensity. The physical nature of EUV waves is debated. Some scientists argue that they are "true" waves, in this case fast-mode magnetohydrodynamic shock waves, while others argue that they are "pseudo-waves", which may refer to several effects that appear to an observer as a traveling wave but are not actually waves in the physical or mathematical sense. EUV waves sometimes exhibit two distinct brightness fronts, which has also led to a "hybrid" interpretation that includes both true and pseudo- waves.

History and observations
Extreme ultraviolet radiation is blocked by Earth's atmosphere, and therefore the observation of an EUV wave requires the use of a space telescope. The first observation of an EUV wave was recorded in 1997 by the EUV Imaging Telescope (EIT) on-board the Solar and Heliospheric Observatory (SOHO), a NASA satellite launched in 1995. In unprocessed original images, EUV waves appear as outward moving shells of increased brightness that are relatively faint compared to the flare site. Because they are not very bright compared to the surrounding features, special image processing techniques are generally required to reveal the true extent of their size and the amount of distance they can cover. A year after their discovery, a team led by Barbara Thompson of NASA's Goddard Spaceflight Center applied a "running difference" technique to show that EUV waves can travel millions of kilometers across nearly the entire solar disk in the span of around 30 minutes. This initial study estimated a speed of 245 kilometers per second (km s-1), and subsequent research has typically found speeds of between 200 and 400 km s-1. Since their discovery using SOHO's EIT, a number of additional telescopes have been used to study EUV waves. These include the Transition Region and Coronal Explorer (TRACE), the EUV Imaging (EUVI) telescopes on the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft, the Sun Watcher using Active Pixel system (SWAP) telescope on the PROBA-2 satellite, and the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO).

Nomenclature and related phenomena
The terminology used to describe EUV waves in the scientific literature is still evolving and is not always consistent. They were originally called EIT waves after the telescope used for their discovery, but they are now more often referred to as EUV waves in reference to the wavelength of light primarily used to observe them. However, there exist other wave and wave-like phenomena that are also observed at EUV wavelengths, including a number that were discovered after the EUV waves discussed here. EUV wave may therefore also be used as an umbrella term to encompass all of these features, of which the large-scale disturbances described in this article are simply the largest and most dramatic examples. This has motivated the development of more precise terminology that has not yet solidified, primarily because a consensus on the physical interpretation has not been reached. Global coronal waves, large-scale coronal waves, coronal propagating fronts, and large-scale coronal propagating fronts have all been used to describe this type of EUV wave. Other types of waves observed in EUV observations of the solar corona include waves trapped inside the magnetic loops of an active region, waves propagating along much longer magnetic loops that connect multiple active regions, and waves that propagate outward along open magnetic field lines within coronal holes. These features are fundamentally different than the large-scale EUV waves described here, and they are not generally referred to as EUV waves except when discussed in the broader context.

Physical interpretations
The physical nature of EUV waves is debated, and this debate is reflected in the varying nomenclature discussed in the previous section. EUV waves may alternatively be referred to as large-scale coronal waves and large-scale coronal propagating fronts because there is not yet consensus on whether or not they are "true" waves. While they do look very much like shock waves, it is possible to produce the appearance of a shock wave without actually being one in the physical or mathematical sense. All interpretations involve the compression and/or heating of coronal plasma, which leads to the increase in EUV radiation.

True wave interpretation
The true wave interpretation suggests that EUV waves are fast-mode magnetohydrodynamic (MHD) shock waves that are most-likely triggered by the onset of a coronal mass ejection (CME) immediately following a solar flare. Observational support for this interpretation includes the following points: The typical speeds of EUV waves (200—400 km s-1) are consistent with those expected of fast-mode waves given the typical densities, temperatures, and magnetic field strengths found in the corona. Fast-mode waves are compressive waves, which means that the density near the shock front is enhanced, leading to the increase in EUV radiation. Such waves can also propagate perpendicularly to the magnetic field orientation, which allows the wave to travel long distances across the solar disk. In addition to the fast-mode MHD wave scenario, some researchers have also suggested an alternative "true" wave hypothesis, slow-mode soliton waves. However, slow-mode waves cannot propagate perpendicularly to the magnetic field orientation, and therefore additional effects are required to explain that observational property of EUV waves. The distinction between fast- and slow-mode MHD shock waves relates to different properties exhibited depending on the speed of the wave compared to the sound and Alfvén speeds.

Pseudo-wave interpretation
The pseudo-wave interpretation suggests that EUV waves are not true wave phenomena, and rather the wave-like appearance is a projection effect generated by the expansion of a coronal mass ejection (CME). One possibility is that EUV waves are produced by the expanding envelope of a CME flux rope, which is a collection of twisted magnetic flux tubes. Plasma may be compressed at the CME flanks and/or heated and compressed within a shell of enhanced electric currents near the the surface of the flux rope. Another possibility is that as the CME flux rope expands laterally across the solar disk, minor magnetic reconnection events occur between the flux rope and favorably-aligned ambient coronal magnetic fields. This "reconnection front" leads to plasma heating and enhanced EUV radiation that collectively gives the appearance of a propagating wave front.

Hybrid interpretation
Observations of EUV waves sometimes exhibit two distinct brightness fronts, and this bimodality may be explained by a combination of the different true- and pseudo-wave interpretations. The hybrid interpretation suggests that that there are two EUV waves, a sharply-defined outer true wavefront and an inner more-diffuse pseudo-wave. These two components are observationally inseparable at the beginning of an eruption but become distinguishable as the true wave out-paces the pseudo-wave. The earliest observations of EUV waves did not have sufficient time and spatial resolution to reliably distinguish these two components, but more-recent studies using the Solar Dynamics Observatory have found that over half of EUV waves are bimodal. Numerical simulations have also suggested that eruptions may generate both true and pseudo- waves in the same event.

Relationship to Moreton waves
When they were discovered, EUV waves were hailed as the coronal counterparts of chromospheric Moreton waves, which were discovered in the late 1950s. Moreton waves occur in the chromosphere, the atmospheric layer beneath the corona, and they are observed primarily in H-alpha radiation, which is in visible spectrum and so can be seen by ground-based telescopes using a specific filter. The leading physical interpretation for Moreton waves is that they are byproducts, often called the "sweeping skirts", of fast-mode MHD shock waves propagating above the chromosphere in the corona.

This interpretation is based primarily on the following properties of Moreton waves: First, Moreton waves travel faster than the chromosheric sound and Alfvén speeds, and they travel farther than would be possible for a true chromospheric disturbance to travel before being dissipated given the high Mach numbers in the chromopshere. Second, Doppler observations reveal a downward motion of material of a few tens of km s-1, which suggests that Moreton waves are really a reaction to downward pressure from something propagating higher up in the corona. They are also strongly associated with Type II radio bursts, which are also associated which coronal shock waves.

All this meant that when EUV waves were discovered nearly 40 years after Moreton waves, they were quickly interpreted as the long-awaited coronal counterpart to Moreton waves. Indeed, Moreton waves are almost always accompanied by an EUV wave and early observations showed that the two wavefronts were generally aligned. However, as observations improved, inconsistencies began to emerge that called into question the straightforward interpretation that EUV waves are themselves the fast-mode MHD waves responsible for exciting Moreton waves. These include that more modern and precise observations show that EUV waves are often somewhat slower than Moreton waves. There is also not a strong correlation between EUV wave speeds and those implied by Type II radio bursts, and nor is there a strong correlation between EUV wave speeds and coronal magnetic field strength estimates. Along with several other arguments

These include:


 * 1) EUV wave and Type II radio burst speeds are not correlated
 * 2) EUV waves are often somewhat slower than Moreton waves and the speed implied by Type II radio bursts
 * 3) EUV waves sometimes appear to travel slower than the coronal sound speed
 * 4) EUV wave speeds are not

and farther than would be possible for true chromospheric disturbances, where faster means

than the sound and Alfvén speeds in the chromosphere

a a true chromospheric disturbance could because dense chromospheric the high Mach numbers in the