Global And Hemispheric Distributions Of Inner Magnetospheric Lower-band Chorus Waves Observed By Van Allen Probes

As intense electromagnetic emissions observed in Earth’s magnetosphere,whistler-mode chorus waves are believed to play an important role in both creating relativistic electrons in the radiation belts and precipitating energetic electrons for atmospheric loss.Wave-particle interactions caused by chorus waves contribute remarkably to significant enhancements of radiation belt electrons and also trigger both diffuse aurorae and pulsating aurorae,which act as important processes in the magnetosphere-ionosphere coupling system.During these physical processes,lower-band chorus whose wave frequencies are characteristically below a half electron gyrofrequency,can drive the electron scattering effect that is essential to understand the processes,mechanisms and consequences of wave-particle interactions in the Earth’s inner magnetosphere.This study focuses on the spatial distribution properties of inner magnetospheric lower-band chorus waves,in terms of a detailed analysis of long-term(>100 months),high-quality EMFISIS wave instrument data from Van Allen probes.It is directed towards the distribution features of lower-band chorus wave amplitude,normalized peak wave frequency and wave normal angle that show the dependence on L-shell,magnetic local time,magnetic latitude and geomagnetic activity index,along with the occurrence pattern of the waves.Furthermore,analysis efforts are made to investigate the hemispheric distribution properties of lower-band chorus waves.The major content and conclusions of this study are summarized as follows:1.Using wave spectrum data provided by Van Allen Probes EMFISIS measurements,we have performed two methods,i.e.,the intensity of ECH waves and the gradient drop of plasma density,to identify the plasmapause location with more accuracy,and further established a more reliable database of inner magnetospheric chorus wave event database for subsequent investigation.2.Through a statistical analysis,we have obtained the variation properties of lower-band chorus wave average wave amplitude with geomagnetic latitude,and the day-night asymmetric features.It is found that lower-band chorus can extend to the latitude of 20° at the morning MLT sector,confine within ～15° at the noon sector,and become limited within ～12° at the nightside sector.Moderately strong waves are present at almost all MLT sectors at L ～ 4-6.5,with the occurrence rates typical of20%-30%.The occurrence possibility for very strong chorus waves is very weak,showing a rate < ～ 1% on the nightside at L ～ 5-6.3.Statistically,lower-band chorus emissions exhibit stronger wave amplitudes and larger normalized peak wave frequencies in the magnetically southern hemisphere,especially around 12 MLT and at 03-06 MLT sector.During periods of geomagnetic disturbances,the peak amplitude of the waves can reach ～105 p T in the southern hemisphere and ～82 p T in the northern hemisphere.The waves on both magnetic hemispheres exhibit positive correlation with AE* and apparent dawn-dusk asymmetry.Overall,the occurrence rates of lower-band chorus waves are larger in the magnetically northern hemisphere,especially at the middle latitudes within the noon and postnoon MLT sectors and at the lower portion of wave frequencies.4.The wave normal angle distributions of lower-band chorus show an overall similarity in the magnetically southern and northern hemispheres,both showing the peak values of the occurrence rates at the quasi-parallel propagation angles ～ 10° and the clear decreasing occurrence trend with increasing wave normal angle.While in the southern hemisphere the wave normal angles exhibit a weak secondary peak at the propagation angles ～70°,there does not exist such a feature in the northern hemisphere.To conclude,the detailed evaluations of the global and hemispheric distributions of lower-band chorus waves in the inner magnetosphere have provided valuable information regarding the similarities and differences of the distribution properties of lower-band chorus waves.Our results not only improve the currently existing statistical model of inner magnetospheric lower-band chorus but also support in-depth comprehension of the generation and propagation of the waves and closely associated radiation belt electron acceleration and loss dynamics via wave-particle interactions.