A brief review of “solar flare effects” on the ionosphere

The study of solar flare effects (SFEs) on the ionosphere is having a renaissance. The development of GPS ground and satellite data for scientific use has opened up new means for high time resolution research on SFEs. At present, without continuous flare photon spectra (X rays, EUV, UV, and visible) monitoring instrumentation, the best way to model flare spectral changes within a flare is through ionospheric GPS studies. Flare EUV photons can increase the total electron content of the subsolar ionosphere by up to 30% in ∼5 min. Energetic particles (ions) of 10 keV to GeV energies are accelerated at the flare site. Electrons with energies up to several MeV are also created. A coronal mass ejection (CME) is launched from the Sun at the time of the flare. Fast interplanetary CMEs (ICMEs) have upstream shocks which accelerate ions to ∼10 keV to ∼10 MeV. Both sources of particles, when magnetically connected to the Earth's magnetosphere, enter the magnetosphere and the high-latitude and midlatitude ionosphere. Those particles that precipitate into the ionosphere cause rapid increases in the polar atmospheric ionization, disruption of transpolar communication, and cause ozone destruction. Complicating the picture, when the ICME reaches the magnetosphere ∼1 to 4 days later, shock compression of the magnetosphere energizes preexisting 10–100 keV magnetospheric electrons and ions, causing precipitation into the dayside auroral zone (∼60°–65° MLAT) ionospheres. Shock compression can also trigger supersubstorms in the magnetotail with concomitant energetic particle precipitation into the nightside auroral zones. If the interplanetary sheath or ICME magnetic fields are southwardly directed and last for several hours, a geomagnetic storm will result. A magnetic storm is characterized by the formation of an unstable ring current with energetic particles in the range ∼10 keV to ∼500 keV. The ring current decays away by precipitation into the middle latitude ionosphere over timescales of ∼10 h. A schematic of a time line for the above SFE ionospheric effects is provided. Descriptions of where in the ionosphere and in what time sequence is provided in the body of the text. Much of the terminology presently in use describing solar, interplanetary, magnetospheric, and ionospheric SFE-related phenomena are dated. We suggest physics-based terms be used in the future.