Abstract
Most theoretical wave models require the power in the wave magnetic field in order to determine the effect of chorus waves on radiation belt electrons. However, researchers typically use the cold plasma dispersion relation to approximate the magnetic wave power when only electric field data are available. In this study, the validity of using the cold plasma dispersion relation in this context is tested using Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) observations of both the electric and magnetic spectral intensities in the chorus wave band (0.1–0.9 fce). Results from this study indicate that the calculated wave intensity is least accurate during periods of enhanced wave activity. For observed wave intensities >10−3 nT2, using the cold plasma dispersion relation results in an underestimate of the wave intensity by a factor of 2 or greater 56% of the time over the full chorus wave band, 60% of the time for lower band chorus, and 59% of the time for upper band chorus. Hence, during active periods, empirical chorus wave models that are reliant on the cold plasma dispersion relation will underestimate chorus wave intensities to a significant degree, thus causing questionable calculation of wave-particle resonance effects on MeV electrons.
Highlights
Relativistic electron fluxes in the outer radiation belt (3 < L < 7) are highly dynamic during periods with enhanced geomagnetic activity [e.g., Onsager et al, 2002; Green et al, 2004; Borovsky and Denton, 2009; Hartley et al, 2014]
The measured spectral intensity of the electric and magnetic wave field is observed for a sample case where whistler mode chorus waves are present
This study addresses the applicability of using the cold plasma dispersion relation to calculate magnetic field spectral intensities, SB, from electric field spectral intensities, SE, in a plasma that may contain a significant warm/hot component [cf
Summary
Relativistic electron fluxes in the outer radiation belt (3 < L < 7) are highly dynamic during periods with enhanced geomagnetic activity [e.g., Onsager et al, 2002; Green et al, 2004; Borovsky and Denton, 2009; Hartley et al, 2014]. It is well established that radial diffusion, which violates the third adiabatic invariant of particle motion, can transport electrons inward and increase their energy through betatron and Fermi acceleration processes [e.g., Schulz and Lanzerotti, 1974] This type of acceleration requires positive gradients in the phase space density versus L shell profile inward of the source region. Recent studies have shown that persistent local maxima in the phase space density profiles exist between L = 4 and L = 5.5 [e.g., Green and Kivelson, 2004; Chen et al, 2007b; Turner et al, 2010; Shprits et al, 2012; Reeves et al, 2013] These studies demonstrate that local acceleration mechanisms, such as wave-particle interactions, which violate the first and second adiabatic invariants of particle motion, are at work within the outer belt region.
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