Statistics of whistler mode waves in the outer radiation belt: Cluster STAFF‐SA measurements

Abstract

[1] ELF/VLF waves play a crucial role in the dynamics of the radiation belts and are partly responsible for the main losses and the acceleration of energetic electrons. Modeling wave-particle interactions requires detailed information of wave amplitudes and wave normal distribution over L-shells and over magnetic latitudes for different geomagnetic activity conditions. We performed a statistical study of ELF/VLF emissions using wave measurements in the whistler frequency range for 10 years (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) aboard Cluster spacecraft. We utilized data from the STAFF-SA experiment, which spans the frequency range from 8 Hz to 4 kHz. We present distributions of wave magnetic and electric field amplitudes and wave normal directions as functions of magnetic latitude, magnetic local time, L-shell, and geomagnetic activity. We show that wave normals are directed approximately along the background magnetic field (with the mean value of  -the angle between the wave normal and the background magnetic field, about 10 ı -15 ı ) in the vicinity of the geomagnetic equator. The distribution changes with magnetic latitude: Plasmaspheric hiss normal angles increase with latitude to quasi-perpendicular direction at 35 ı -40 ı where hiss can be reflected; lower band chorus are observed as two wave populations: One population of wave normals tends toward the resonance cone and at latitudes of around 35 ı -45 ı wave normals become nearly perpendicular to the magnetic field; the other part remains quasi-parallel at latitudes up to 30 ı . The observed angular distribution is significantly different from Gaussian, and the width of the distribution increases with latitude. Due to the rapid increase of  , the wave mode becomes quasi-electrostatic, and the corresponding electric field increases with latitude and has a maximum near 30 ı . The magnetic field amplitude of the chorus in the day sector has a minimum at the magnetic equator but increases rapidly with latitude with a local maximum near 12 ı -15 ı . The wave magnetic field maximum is observed in the night sector at L > 7 during low geomagnetic activity (at L 5 for K p > 3). Our results confirm the strong dependence of wave amplitude on geomagnetic activity found in earlier studies.

Intense duskside lower band chorus waves observed by Van Allen Probes: Generation and potential acceleration effect on radiation belt electrons

“…The adopted spectral parameters for the nightside chorus waves are simply extracted from the previous statistical model of Horne et al [2005] (rather than the real-time observations). Recently, some new chorus wave models depending on the geomagnetic activities have been developed [e.g., Li et al, 2011;Meredith et al, 2012;Agapitov et al, 2013;Spasojevic and Shprits, 2013]. These models could give the nightside chorus waves with larger amplitude than that of Horne et al [2005], and consequently the corresponding diffusion coefficients would exceed those of the duskside chorus observed by RBSP-B in this event.…”

“…The global distribution of chorus waves has been statistically analyzed based on the observations of Combined Release and Radiation Effects Satellite (CRRES) [Meredith et al, 2001;Horne et al, 2005], Time History of Events and Macroscale Interactions during Substorms (THEMIS) [Li et al, 2009a], and other satellites [e.g., Meredith et al, 2012;Agapitov et al, 2013]. Their analyses clearly demonstrated the significant dependence of chorus wave intensity on substorm activity AE, magnetic shell L, magnetic latitude , and magnetic local time MLT.…”

Electron pitch-angle diffusion: resonant scattering by waves vs. nonadiabatic effects

“…We consider that electrons are scattered by lower band chorus waves, which are the most intense whistlermode emissions in the inner magnetosphere (see Meredith et al, 2012;Agapitov et al, 2013, and references therein). D α 0 α 0 for the resonant wave-particle interactions is obtained by using the quasi-linear theory (Glauert and Horne, 2005, see also references therein) with a wave-frequency distributionB 2 (ω) = K exp(−(ω m − ω) 2 /δω 2 ), where K is a normalization coefficient and parameters ω m and δω are the frequency of the maximum wave power and the bandwidth, respectively, taken from Orlova et al (2012).…”

“…The averaging procedure over electron bounce oscillations is described in Orlova and Shprits (2011). We consider chorus wave amplitudes of 100 pT, in agreement with Cluster and CRRES measurements at L ∼ 7 during disturbed periods (Shprits et al, 2007;Agapitov et al, 2013). For D α 0 α 0 calculation, we use the T01s magnetic field model presented in Fig.…”

Probability of relativistic electron trapping by parallel and oblique whistler-mode waves in Earth's radiation belts

“…37,40,42,58,64,67 Multiple spacecraft observations of intense emissions of parallel (relative to the background magnetic field) whistler-mode waves during geomagnetic storms and substorms 1,33,38 have led to the suggestion that these parallel waves could be responsible for almost all the recorded variations of electron fluxes. In addition, oblique wave damping due to Landau resonance with a dense population of suprathermal electrons 20,36 often sensibly decreases the amplitudes of oblique whistler-mode waves along their propagation.…”