A statistical study of EMIC waves observed by Cluster: 2. Associated plasma conditions

Abstract

This is the second in a pair of papers discussing a statistical study of electromagnetic ion cyclotron (EMIC) waves detected during 10 years (2001–2010) of Cluster observations. In the first paper, an analysis of EMIC wave properties (i.e., wave power, polarization, normal angle, and wave propagation angle) is presented in both the magnetic latitude (MLAT)‐distance as well as magnetic local time (MLT)‐L frames. This paper focuses on the distribution of EMIC wave‐associated plasma conditions as well as two EMIC wave generation proxies (the electron plasma frequency to gyrofrequency ratio proxy and the linear theory proxy) in these same frames. Based on the distributions of hot H+ anisotropy, electron and hot H+ density measurements, hot H+ parallel plasma beta, and the calculated wave generation proxies, three source regions of EMIC waves appear to exist: (1) the well‐known overlap between cold plasmaspheric or plume populations with hot anisotropic ring current populations in the postnoon to dusk MLT region; (2) regions all along the dayside magnetosphere at high L shells related to dayside magnetospheric compression and drift shell splitting; and (3) off‐equator regions possibly associated with the Shabansky orbits in the dayside magnetosphere.

EMIC Waves in the Outer Magnetosphere: Observations of an Off‐Equator Source Region

“…However, statistical surveys of EMIC waves have also shown that these waves occur frequently in the outer magnetosphere across the dayside region (Allen et al, , 2016Anderson et al, 1991;Min et al, 2012;Wang et al, 2017). Theoretical/numerical analyses have predicted that there could be high-latitude EMIC wave generation regions for larger L shells due to the effects of drift-shell splitting and particles executing Shabansky orbits (Shabansky, 1971) around local minimums in the magnetic field occurring off-equator in the dayside magnetosphere (McCollough et al, 2009(McCollough et al, , 2010(McCollough et al, , 2012.…”

“…Theoretical/numerical analyses have predicted that there could be high-latitude EMIC wave generation regions for larger L shells due to the effects of drift-shell splitting and particles executing Shabansky orbits (Shabansky, 1971) around local minimums in the magnetic field occurring off-equator in the dayside magnetosphere (McCollough et al, 2009(McCollough et al, , 2010(McCollough et al, , 2012. Statistical studies of EMIC waves in the middle to outer magnetosphere (i.e., L shell ≳ 7) have suggested that these high-latitude source regions may indeed exist, given the observed wave and plasma properties along with instability threshold proxies (Allen et al, , 2016. While the dayside outer magnetosphere has been shown to be conducive for wave growth over a large latitudinal range, there have only been a couple of case studies of bidirectionally propagating EMIC waves at high latitudes (Allen et al, 2013;Liu et al, 2013).…”

Magnetospheric Multiscale Observation of Quasiperiodic EMIC Waves Associated With Enhanced Solar Wind Pressure

“…Because the magnetic energy needed for the EMIC excitation is lower in the region of the local minimum magnetic field, EMIC waves are most often observed near the magnetic equator in the inner magnetosphere due to the presence of a primarily dipolar field (e.g., Fraser et al, , ; Loto'aniu et al, ). For the outer magnetosphere in the dayside, geomagnetic field lines are distorted from dipole geometry by solar wind dynamic pressure, which forms local B minima at high latitudes and thus generating off‐equatorial EMIC waves (Allen et al, ; Liu et al, ; McCollough et al, ).…”

Test of Ion Cyclotron Resonance Instability Using Proton Distributions Obtained From Van Allen Probe‐A Observations

“…Ion cyclotron instability has also been examined based on an inverse relation of ion anisotropy with parallel plasma beta β ∥ ( ) (Allen et al, ; Anderson et al, ; Blum et al, , ; Gary et al, ; Gary et al, , ; Lin et al, ; Phan et al, ; Spasojevic et al, ). The inverse relations are given by T ⊥ / T ∥ − 1 = Q /β ∥ α , where the subscripts ∥and ⊥ refer to the directions parallel and perpendicular to the background magnetic field, respectively, and α and Q are the parameters that define specific models of inverse relationships.…”