Relativistic electron pitch‐angle scattering by electromagnetic ion cyclotron waves during geomagnetic storms

Abstract

[1] During magnetic storms, relativistic electrons execute nearly circular orbits about the Earth and traverse a spatially confined zone within the duskside plasmapause where electromagnetic ion cyclotron (EMIC) waves are preferentially excited. We examine the mechanism of electron pitch-angle diffusion by gyroresonant interaction with EMIC waves as a cause of relativistic electron precipitation loss from the outer radiation belt. Detailed calculations are carried out of electron cyclotron resonant pitch-angle diffusion coefficients D aa for EMIC waves in a multi-ion (H + , He + , O + ) plasma. A simple functional form for D aa is used, based on quasi-linear theory that is valid for parallelpropagating, small-amplitude electromagnetic waves of general spectral density. For typical observed EMIC wave amplitudes (1-10nT ), the rates of resonant pitch-angle diffusion are close to the limit of ''strong'' diffusion, leading to intense electron precipitation. In order for gyroresonance to take place, electrons must possess a minimum kinetic energy E min which depends on the value of the ratio (electron plasma frequency/ electron gyrofrequency); E min also depends on the properties of the EMIC wave spectrum and the ion composition. Geophysically interesting scattering, with E min comparable to 1 MeV, can only occur in regions where (electron plasma frequency/electron gyrofrequency) !10, which typically occurs within the duskside plasmapause. Under such conditions, electrons with energy !1 MeV can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm over a time-scale of several hours to a day.INDEX TERMS: 2716 Magnetospheric Physics: Energetic particles, precipitating; 2772 Magnetospheric Physics: Plasma waves and instabilities; 2471 Ionosphere: Plasma waves and instabilities; 2730 Magnetospheric Physics: Magnetosphere-inner; KEYWORDS: relativistic electrons, magnetic storms, EMIC waves, electron precipitation, outer radiation belt, strong diffusion scattering Citation: Summers, D., and R. M. Thorne, Relativistic electron pitch-angle scattering by electromagnetic ion cyclotron waves during geomagnetic storms,

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

“…EMIC waves are enhanced during magnetic storms (Fraser et al 2010), as anisotropic energetic ring current ions are injected into the inner magnetosphere (Jordanova et al 2001a). EMIC waves can cause rapid scattering and loss for ring current ions (Jordanova et al 2001b) and relativistic electrons above 0.5 MeV (Thorne and Kennel 1971;Lyons and Thorne 1973;Albert 2003;Summers and Thorne 2003;Meredith et al 2003b). Favored regions for EMIC excitation include the overlap between the ring current and the plasmasphere (Pickett et al 2010), dayside drainage plumes (Morley et al 2009), and the outer dayside magnetosphere in association with solar wind pressure fluctuations (Arnoldy et al 2005;Usanova et al 2008;McCollough et al 2009).…”

The controlling effect of ion temperature on EMIC wave excitation and scattering

“…Figure 3 shows contours of E min − of y and f, and yf ≥ 15.4 is required for resonance with 1 MeV electrons. Previous studies [e.g., Summers and Thorne, 2003;Ukhorskiy et al, 2010] concluded that favorable conditions for lowering E min − (i.e., large wave number) can be achieved at a frequency just below W He+ , based on the cold plasma wave assumption. However, the finite He + temperature will dramatically modify the wave dispersion relation and the EMIC instability as shown in Section 2.…”

“…[4] Waves in the band just below W He+ having a large wave number, are also considered to be a potential candidate for producing resonant scattering loss of relativistic electrons (>∼MeV) [Summers and Thorne, 2003;Ukhorskiy et al, 2010]. Modeling of radiation belt dynamics typically includes EMIC scattering loss to account for the relativistic flux dropout during the storm main phase [Thorne and Kennel, 1971], but the computed scattering rates [e.g., Shprits et al 2009], except a few [e.g., Liu et al, 2010], have usually been based on cold plasma wave theory.…”

Space Weather: Physics, Effects and Predictability

“…The storm occurrence is normally indicated by a large decrease of the D st index which measures the variations in the geomagnetic NorthSouth component at equatorial stations. It is constructed using the horizontal component of the geomagnetic field observed at low to mid latitudes (Surgiara 1965;Sugiara and Kamei 1991). It was introduced as a measurement of the intensity of the ring current (Surgiara 1965).…”