Broadband low‐frequency electromagnetic waves in the inner magnetosphere

Abstract

A prominent yet largely unrecognized feature of the inner magnetosphere associated with particle injections, and more generally geomagnetic storms, is the occurrence of broadband electromagnetic field fluctuations over spacecraft frame frequencies (fsc) extending from effectively zero to fsc ≳ 100 Hz. Using observations from the Van Allen Probes we show that these waves most commonly occur premidnight but are observed over a range of local times extending into the dayside magnetosphere. We find that the variation of magnetic spectral energy density with fsc obeys εB=Afsc−α over several decades with a spectral breakpoint at fb ≈ 1 Hz. The values for α are lognormally distributed with α = 1.9 ± 0.6 for fsc < fb and α = 2.9 ± 0.6 for fsc > fb. A is a function of geomagnetic activity with the largest values observed over intervals of decreasing Dst index during the main phase of geomagnetic storms. At these times these waves are nearly always present in the nightside inner magnetosphere and are commonly observed from L = 3 outward. The observed variation of the electric to magnetic field amplitude with fsc is well described by a dispersive Alfvén wave model under the assumption that fsc is primarily a consequence of the Doppler shift of plasma frame structures moving over the spacecraft. The robust anticorrelation between the time rate change of the Dst index and wave spectral energy density coupled with the ability of dispersive Alfvén waves to drive transverse ion acceleration suggests that these waves may boost ion energy density in the inner magnetosphere and intensify the ring current during storm times.

Radial transport of radiation belt electrons in kinetic field‐line resonances

“…It has been shown that the observed broad spectrum of low-frequency electromagnetic field variations in the inner magnetosphere are well modeled as an ensemble of eigenmodes or KFLRS with form [Chaston et al, 2014]. It has been shown that the observed broad spectrum of low-frequency electromagnetic field variations in the inner magnetosphere are well modeled as an ensemble of eigenmodes or KFLRS with form [Chaston et al, 2014].…”

“…Figure 2c shows E ϕ in the wave frame as well as the trajectories of resonant electrons (white curves) at a number of equatorial pitch angles (α). E ϕ is given by the solution of equation (3) for k ϕ ρ i = 1 at the equator for the plasma model presented in Figures 2a and 2b [Chaston et al, 2014] above an infinitely conducting ionosphere. Alternate conductivity models or plasma profiles could also be used without changing the substance of the following discussion.…”

“…This is because electrons resonant with this harmonic can be continuously immersed in electric fields of the same orientation (as illustrated in Figure 2c). To obtain this, we employ the statistical average spectral energy densities (ε Ysc ) reported by Chaston et al [2015] represented in Figure 4. For the higher harmonics (Figures 3g and 3h) at small k ϕ ρ i the occurrence of multiple antinodes in E ϕ along the field line leads to a corresponding number of peaks in D LL as a function To relate the normalized coefficients of Figures 3f-3h to physical values requires the wave amplitude at each k ϕ ρ i .…”

“…Recent observations from the Van Allen Probes spacecraft have revealed the prevalence at L ≳ 4 during storm times of broadband electromagnetic oscillations with some of the largest amplitude electric fields observed in the magnetosphere [Chaston et al, 2015]. These oscillations have scales transverse to the magnetic field of the order of ion gyroradii and locally have the dispersive properties of kinetic Alfvén waves [Chaston et al, 2014;Malaspina et al, 2015;Moya et al, 2015]. While kinetic Alfvén waves are narrow perpendicular to the geomagnetic field they have parallel wavelengths which in the inner magnetosphere may be of the order of the field-line length.…”

“…These oscillations have scales transverse to the magnetic field of the order of ion gyroradii and locally have the dispersive properties of kinetic Alfvén waves [Chaston et al, 2014;Malaspina et al, 2015;Moya et al, 2015]. On the closed field lines of this region of space these waves may occur as eigenmodes of the geomagnetic field or kinetic field-line resonances (KFLRs) Chaston et al, 2014]. On the closed field lines of this region of space these waves may occur as eigenmodes of the geomagnetic field or kinetic field-line resonances (KFLRs) Chaston et al, 2014].…”