Global model of low-frequency chorus (fLHR

Nigel P Meredith,Wen Li,Richard M Thorne,Richard B Horne,Angélica Sicard-Piet

doi:10.1002/2013gl059050

Abstract

Whistler mode chorus is an important magnetospheric emission, playing a dual role in the acceleration and loss of relativistic electrons in the Earth's outer radiation belt. Chorus is typically generated in the equatorial region in the frequency range 0.1–0.8 fce, where fce is the local electron gyrofrequency. However, as the waves propagate to higher latitudes, significant wave power can occur at frequencies below 0.1fce. Since this wave power is largely omitted in current radiation belt models, we construct a global model of low-frequency chorus, fLHR<f<0.1fce, using data from six satellites. We find that low-frequency chorus is strongest, with an average intensity of 200 pT2, in the prenoon sector during active conditions at midlatitudes (20°<|λm|<50°) from 4<L∗<8. Such midlatitude, low-frequency chorus wave power will contribute to the acceleration and loss of relativistic electrons and should be taken into account in radiation belt models.Key PointsStrong chorus waves can extend below 0.1 times local electron gyrofrequencyLow frequency chorus strongest at mid-latitudes in pre-noon sector for L*=4 to 8Low frequency chorus should be included in radiation belt models

Highlights

Chorus is a discrete, naturally occurring whistler mode emission that is generated in two bands with a gap at 0.5 fce [Tsurutani and Smith, 1974], separating the emissions into so-called lower band (0.1 fce < f < 0.5 fce) and upper band (0.5 fce < f < fce) chorus
Since this wave power is largely omitted in current radiation belt models, we construct a global model of low-frequency chorus, fLHR < f < 0.1 fce, using data from six satellites
Closer to the equator the chorus wave power is largely above 0.1 fce, with little or no low-frequency chorus, illustrating that chorus wave power tends to fall below 0.1 fce at midlatitudes

Summary

Introduction

Naturally occurring whistler mode emission that is generated in two bands with a gap at 0.5 fce [Tsurutani and Smith, 1974], separating the emissions into so-called lower band (0.1 fce < f < 0.5 fce) and upper band (0.5 fce < f < fce) chorus. The waves are generated outside the plasmapause near the magnetic equator [LeDocq et al, 1998] by cyclotron resonant interaction with suprathermal electrons [Li et al, 2008] injected into the inner magnetosphere during storms and substorms. Gyroresonant wave particle interactions with whistler mode chorus play a major role in radiation belt dynamics contributing to both the acceleration and loss of relativistic electrons [Bortnik and Thorne, 2007]. Chorus waves are thought to be largely responsible for the gradual buildup of radiation belt electrons that occurs on a time scale of 1–2 days during the recovery phase of geoeffective storms [e.g., Horne et al, 2005]. There is strong theoretical and observational evidence to suggest that chorus is the dominant source of plasmaspheric hiss [e.g., Bortnik et al, 2008; Meredith et al, 2013], which is itself responsible for the formation of the slot region between the inner and outer radiation belts [Lyons and Thorne, 1973] and the quiet time decay of energetic electrons in the outer radiation belt [Meredith et al, 2006]

Results

Discussion

Conclusion