The In Situ Signature of Cyclotron Resonant Heating

Abstract

The dissipation of magnetized turbulence is an important paradigm for describing heating and energy transfer in astrophysical environments such as the solar corona and wind; however, the specific collisionless processes behind dissipation and heating remain relatively unconstrained by measurements. Remote sensing observations have suggested the presence of strong temperature anisotropy in the solar corona consistent with cyclotron resonant heating. In the solar wind, in situ magnetic field measurements reveal the presence of cyclotron waves, while measured ion velocity distribution functions have hinted at the active presence of cyclotron resonance. Here, we present Parker Solar Probe observations that connect the presence of ion-cyclotron waves directly to signatures of resonant damping in observed proton-velocity distributions. We show that the observed cyclotron wave population coincides with both flattening in the phase space distribution predicted by resonant quasilinear diffusion and steepening in the turbulent spectra at the ion-cyclotron resonant scale. In measured velocity distribution functions where cyclotron resonant flattening is weaker, the distributions are nearly uniformly subject to ion-cyclotron wave damping rather than emission, indicating that the distributions can damp the observed wave population. These results are consistent with active cyclotron heating in the solar wind.

Observations of Cross Scale Energy Transfer in the Inner Heliosphere by Parker Solar Probe

“…It was also shown by [228] that these features are consistent with the expectations of quasilinear diffusion in velocity space in the presence of waves. A recent paper studies proton VDFs in the presence of cyclotron waves [229]. The VDFs show signatures consistent with qualisilinear diffusion in the velocity space, indicating a possibility of cyclotron heating.…”

“…At the kinetic scale, the cascade is modified by kinetic physics as well as dissipation [229]. The spectral slopes in the imbalanced inner heliospheric turbulence routinely show a steep transition range with powerlaw ∼ f −4 just below proton kinetic scales [199,201,202].…”

“…The transfer of energy into internal degrees of freedom has been shown to happen via wave-particle interactions, stochastic heating as well as in intermittent locations [163,185,225,233]. Distribution functions in the presence of waves show signatures expected of quasilinear diffusion in the velocity space [228,229] . Larger PVI values have been shown to associate favourably with hotter plasma, consistent with simulations as well as 1au observations [231].…”

“…The differences in the inferences of [234] and [165] could potentially stem from the different nature of these measures, or from other considerations such as enhanced possibility of wave damping [116,229], or from yet largely unexplored clustering properties of the PVI events [232] or the transition of turbulence to monofractal behaviour at sub-proton scales potentially owing to scale invariance of current sheets in the kinetic range [236]. Moreover, LET and PVI are scale dependent quantities.…”