Observation of Growth/Damping Rate of Kinetic Waves in Magnetosheath Turbulence

Abstract:

The energy conversion between waves and particles is a fundamental physics in collisionless plasmas. In previous studies, the appearance of spectral bump is usually speculated to be a growing result of waves due to a certain instability. As a further step, particle velocity distribution with a field-aligned beam or thermal anisotropy is investigated and adopted to indirectly estimate the possible frequency-dependent growth-rate of kinetic waves based on the linear Vlasov-Maxwell theory. However, the direct evidence of real-time growth/damping rate of kinetic waves has yet to be reported. The term J·E is a good indicator of the ongoing energy conversion between electromagnetic energy and particle energy: J·E>0 and J·E<0 represent the “motor/dissipation-effect” and “dynamo-effect”, respectively. Prior works often calculate the integral over the time sequence of J·E, and give the amount without distinguishing the scale-dependence. In this study, we expand it to calculate the energy conversion in the time and scale dimensions, , where is the wavelet coefficient vector of current fluctuations, and is the conjugate wavelet coefficient vector of electric field fluctuations. The growth or damping rate is then approximated by dividing the energy conversion rate by the sum of energy for magnetic fluctuations and electric fluctuations. As an example, we apply this technique to the cases of ion-cyclotron waves in the magnetosheath turbulence. Thermal anisotropy with T_perp/T_para>1 of protons is in association with the power spectral bump of circularly-polarized magnetic fluctuations. Moreover, the scale-dependence of energy conversion is explored with this new method, with some direct evidence of growth being presented in the results.

Acknowledgement: This is a preliminary work result. We would like to thank the MMS science team for providing the calibrated high-quality data online at https://lasp.colorado.edu/mms/sdc/public/.