Global morphology and spectral properties of EMIC waves derived from CRRES observations

Abstract

Gyroresonant wave-particle interactions with electromagnetic ion cyclotron (EMIC) waves are a potentially important loss process for relativistic electrons in the Earth's radiation belts. Here we perform a statistical analysis of the EMIC waves observed by the Combined Release and Radiation Effects Satellite (CRRES) to determine the global morphology and spectral properties of the waves and to help assess their role in radiation belt dynamics. Helium band EMIC waves, with intensities, B 2 w , greater than 0.1 nT 2 , are most prevalent during active conditions (AE > 300 nT), from 4 < L * < 7 in the afternoon sector, with an average percentage occurrence of 2.7%. Hydrogen band EMIC wave events, with intensities greater than 0.1 nT 2 , are also most prevalent in the afternoon sector during active conditions in the same region, but they are less frequent with an average percentage occurrence of 0.6%. The average intensity of the helium and hydrogen band EMIC waves in the region 4 < L * < 7 in the afternoon sector during active conditions is 2 nT 2 and 0.5 nT 2 , respectively, and suggests that the waves can cause strong diffusion. However, the time-averaged properties are very different, being a factor of 30-50 lower for helium and hydrogen band EMIC waves, respectively, suggesting that the overall effect will be correspondingly weaker. Nevertheless, the moderate and strong EMIC wave events with B 2 w > 0.1 nT 2 reported on here will contribute to relativistic electron loss in the Earth's radiation belts and should be included in radiation belt models.

Resonant scattering of outer zone relativistic electrons by multiband EMIC waves and resultant electron loss time scales

“…While the intensity of EMIC waves varies with the level of geomagnetic activity and the spatial location [e.g., Usanova et al, 2012;Min et al, 2012;Meredith et al, 2014], in the present study, for simplicity, we have adopted a nominal intermediate amplitude of 1 nT for all three bands of EMIC waves. This value is consistent with the AMPTE survey results that the average power spectral density typically lies between 1 and 10 nT 2 /Hz [Anderson et al, 1992a[Anderson et al, , 1992b with a peak-to-peak wave amplitude of about 1.6 nT and between the CRRES survey results and the model results used by Kersten et al [2014].…”

“…Compared to the frequently measured H + -band and He + -band EMIC waves, O + -band EMIC waves are rarely observed but were recently reported in the outer plasmasphere at L = 2-5 from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science and Electric Fields and Waves data [Yu et al, 2015]. Meredith et al, 2014]. Observed over a broad range of L shell from L = 3 to L = 10, EMIC waves have typical amplitudes in the range of~0.1-10 nT [Fraser et al, 1996;Erlandson and Ukhorskiy, 2001].…”

“…To model EMIC waves, we first assume that the three wave bands all follow a Gaussian frequency distribution, taking the form [e.g., Glauert and Horne, 2005;Meredith et al, 2014;Kersten et al, 2014] …”

“…Propagating at frequencies below the proton gyrofrequency (f cp ), electromagnetic ion cyclotron (EMIC) waves have been extensively observed in the inner magnetosphere in the frequency range 0.1-5.0 Hz, i.e., the ultralow frequency Pc1-2 band [e.g., Anderson et al, 1992aAnderson et al, , 1992bFraser et al, 1992Fraser et al, , 1996Fraser and Nguyen, 2001;Meredith et al, 2003Meredith et al, , 2014Zhang et al, 2014]. Partially controlled by the ion composition and anisotropy [e.g., Kozyra et al, 1984;Horne and Thorne, 1994] and by the location with respect to the plasmapause [Fraser and Nguyen, 2001], EMIC waves can be generated in three distinct frequency bands below the hydrogen (H + ), helium (He + ), and oxygen (O + ) ion gyrofrequencies.…”

“…Such an MLT dependence of EMIC wave occurrence is consistent with the westward drift of energetic ions, which are commonly regarded as the free-energy source population for EMIC wave excitation. More recently, Meredith et al [2014], by performing a statistical analysis of EMIC waves observed by CRRES, found that the average intensity of H + -band and He + -band EMIC waves in the region L* = 4-7 (where L* is the Roederer parameter related to the third adiabatic invariant [Roederer, 1970]) in the afternoon sector under actively disturbed conditions is 0.5 nT 2 and 2 nT 2 , respectively. While H + -band EMIC waves are most common in the outer magnetosphere (L = 7-9) on the afternoon side regardless of geomagnetic activity, He + -band EMIC waves occur most frequently in the inner magnetosphere (L = 4-7) on the prenoon to dusk side during active times [Keika et al, 2013].…”