Editing Aurora (section)

== Auroral particle acceleration ==

Just as there are many types of aurora, there are many different mechanisms that accelerate auroral particles into the atmosphere. Electron aurora in Earth's auroral zone (i.e. commonly visible aurora) can be split into two main categories with different immediate causes: diffuse and discrete aurora. Diffuse aurora appear relatively structureless to an observer on the ground, with indistinct edges and amorphous forms. Discrete aurora are structured into distinct features with well-defined edges such as arcs, rays, and coronas; they also tend to be much brighter than diffuse aurora.

In both cases, the electrons that eventually cause the aurora start out as electrons trapped by the magnetic field in Earth's [[magnetosphere]]. These [[L-shell#Charged particle motions in a dipole field|trapped particles]] bounce back and forth along [[magnetic field lines]] and are prevented from hitting the atmosphere by the [[magnetic mirror]] formed by the increasing magnetic field strength closer to Earth. The magnetic mirror's ability to trap a particle depends on the particle's [[Pitch angle (particle motion)|pitch angle]]: the angle between its direction of motion and the local magnetic field. An aurora is created by processes that decrease the pitch angle of many individual electrons, freeing them from the magnetic trap and causing them to hit the atmosphere.

In the case of diffuse auroras, the electron pitch angles are altered by their interaction with various [[Waves in plasmas|plasma waves]]. Each interaction is essentially wave-particle [[scattering]]; the electron energy after interacting with the wave is similar to its energy before interaction, but the direction of motion is altered. If the final direction of motion after scattering is close to the field line (specifically, if it falls within the [[Magnetic mirror#Mirror ratios|loss cone]]) then the electron will hit the atmosphere. Diffuse auroras are caused by the collective effect of many such scattered electrons hitting the atmosphere. The process is mediated by the plasma waves, which become stronger during periods of high [[Geomagnetic storm|geomagnetic activity]], leading to increased diffuse aurora at those times.

In the case of discrete auroras, the trapped electrons are accelerated toward Earth by electric fields that form at an altitude of about 4000–12000&nbsp;km in the "auroral acceleration region". The electric fields point away from Earth (i.e. upward) along the magnetic field line.<ref>The theory of acceleration by parallel electric fields is reviewed in detail by {{cite journal|vauthors=Lysak R, Echim M, Karlsson T, Marghitu O, Rankin R, Song Y, Watanabe TH|date=2020|title=Quiet, Discrete Auroral Arcs: Acceleration Mechanisms|url=https://link.springer.com/content/pdf/10.1007/s11214-020-00715-5.pdf|journal=Space Science Reviews|volume=216|issue=92|page=92|doi=10.1007/s11214-020-00715-5|bibcode=2020SSRv..216...92L|s2cid=220509575|access-date=1 June 2021|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165316/https://link.springer.com/content/pdf/10.1007/s11214-020-00715-5.pdf|url-status=live}}</ref> Electrons moving downward through these fields gain a substantial amount of energy (on the order of a few [[electronvolt|keV]]) in the direction along the magnetic field line toward Earth. This field-aligned acceleration decreases the pitch angle for all of the electrons passing through the region, causing many of them to hit the upper atmosphere. In contrast to the scattering process leading to diffuse auroras, the electric field increases the kinetic energy of all of the electrons transiting downward through the acceleration region by the same amount. This accelerates electrons starting from the magnetosphere with initially low energies (tens of eV or less) to energies required to create an aurora (100s of eV or greater), allowing that large source of particles to contribute to creating auroral light.

The accelerated electrons carry an electric current along the magnetic field lines (a [[Birkeland current]]). Since the electric field points in the same direction as the current, there is a net conversion of electromagnetic energy into particle energy in the auroral acceleration region (an [[Electric power#Passive devices (loads)|electric load]]). The energy to power this load is eventually supplied by the magnetized solar wind flowing around the obstacle of Earth's magnetic field, although exactly how that power flows through the magnetosphere is still an active area of research.<ref>A discussion of 8 theories in use in 2020 as well as several no longer in common use can be found in: {{cite journal|vauthors=Borovsky JE, Birn J, Echim MM, Fujita S, Lysak RL, Knudsen DJ, Marghitu O, Otto A, Watanabe TH, Tanaka T|date=2020|title=Quiescent Discrete Auroral Arcs: A Review of Magnetospheric Generator Mechanisms|url=https://link.springer.com/content/pdf/10.1007%2Fs11214-019-0619-5.pdf|journal=Space Science Reviews|volume=216|issue=92|doi=10.1007/s11214-019-0619-5|s2cid=214002762|access-date=1 June 2021|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165327/https://link.springer.com/content/pdf/10.1007%2Fs11214-019-0619-5.pdf|url-status=live}}</ref> While the energy to power the aurora is ultimately derived from the solar wind, the electrons themselves do not travel directly from the solar wind into Earth's auroral zone; magnetic field lines from these regions do not connect to the solar wind, so there is no direct access for solar wind electrons.

Some auroral features are also created by electrons accelerated by dispersive [[Alfvén wave]]s. At small wavelengths, transverse to the background magnetic field (comparable to the [[Plasma parameters|electron inertial length]] or [[Plasma parameters|ion gyroradius]]), Alfvén waves develop a significant electric field parallel to the background magnetic field. This electric field can accelerate electrons to [[keV]] energies, sufficient to produce auroral arcs.<ref>{{cite thesis |type=PhD Thesis |last=Pokhotelov |first=D. |date=2002 |title=Effects of the active auroral ionosphere on magnetosphere-ionosphere coupling. |publisher=Dartmouth College |doi=10.1349/ddlp.3332}}</ref> If the electrons have a speed close to that of the wave's phase velocity, they are accelerated in a manner analogous to a surfer catching an ocean wave.<ref>{{cite web|url=https://now.uiowa.edu/2021/06/physicists-determine-how-auroras-are-created|title=Physicists determine how auroras are created|author=Richard Lewis|website=IOWA university|date=7 June 2021|access-date=8 June 2021|archive-date=8 June 2021|archive-url=https://web.archive.org/web/20210608061856/https://now.uiowa.edu/2021/06/physicists-determine-how-auroras-are-created|url-status=live}}</ref><ref>{{cite journal|vauthors=Schroeder JW, Howes GG, Kletzing CA et al|date=7 June 2021|title=Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves|journal=Nature Communications|volume=12|issue=1|page=3103|doi=10.1038/s41467-021-23377-5|pmid=34099653|pmc=8184961|bibcode=2021NatCo..12.3103S }}</ref> This constantly changing wave electric field can accelerate electrons along the field line, causing some of them to hit the atmosphere. Electrons accelerated by this mechanism tend to have a broad energy spectrum, in contrast to the sharply peaked energy spectrum typical of electrons accelerated by quasi-static electric fields.

In addition to the discrete and diffuse electron aurora, proton aurora is caused when magnetospheric protons collide with the upper atmosphere. The proton gains an electron in the interaction, and the resulting neutral hydrogen atom emits photons. The resulting light is too dim to be seen with the naked eye. Other aurora not covered by the above discussion include transpolar arcs (formed poleward of the auroral zone), cusp aurora (formed in two small high-latitude areas on the dayside), and some non-terrestrial auroras.