Nonadiabatic heating in magnetic reconnection

Abstract

Plasma transport process as a fundamental problem in magnetospheric physics is often associated with strong nonadiabatic heating. At the magnetopause, observations show an increase of specific entropy (i.e., S = p∕ ) by 2 orders of magnitude from the magnetosheath into the magnetosphere. In the near-Earth magnetotail, particle injection requires strongly entropy depleted plasma bubbles, and their evolution can be strongly modified in the presence of nonadiabatic heating. In this study, one of the critical plasma transport mechanisms, magnetic reconnection, is investigated as a nonadiabatic process in the framework of MHD. It is important to examine whether magnetic reconnection can provide sufficient nonadiabatic heating to explain the observed plasma properties and to identify plasma conditions that allow such strong nonadiabatic heating. We demonstrate that the entropy can indeed strongly increase associated with magnetic reconnection provided that the plasma beta (i.e., the ratio of thermal to magnetic energy density) is low in the inflow region of reconnection.

On the Dawn‐Dusk Asymmetry of the Kelvin‐Helmholtz Instability Between 2007 and 2013

“…Both reconnection in KH vortices (Nishino et al, ; Nykyri et al, ) and ion‐scale wave generation by the KHI (Johnson & Cheng, ; Johnson et al, ; Moore & Nykyri, ; Moore et al, ) has been linked to ion heating. Furthermore, while Ma and Otto () showed that significant heating due magnetic reconnection is possible only for very low magnetosheath plasma beta, it may be possible that compression of the magnetic field by the KHI can generate low beta regions, so that shocks associated with 3‐D reconnection in KH vortices may provide effective heating. This effect may be further enhanced for low solar wind Alfvén Mach numbers.…”

“…This suggests that asymmetric development of physical mechanisms at the magnetopause is responsible for this asymmetric heating and transport of cold component ions. Ma and Otto (2014) showed that specific entropy increase (measure of nonadiabatic heating), during magnetic reconnection at the Earth's magnetopause is possible only if magnetosheath plasma beta is very low ( ≪1). Because magnetosheath beta typically is of the order of unity, magnetic reconnection is likely not the sole process responsible for this heating.…”

Thin current sheet formation in response to the loading and the depletion of magnetic flux during the substorm growth phase

“…Referring to our dual‐current‐sheet formation, we argue that the near‐Earth reconnection is more likely to cause plasma injection into the inner magneotosphere because the midtail reconnection is excepted to create larger flux tube volume and flux tube entropy than the near‐Earth reconnection. In addition Ma and Otto [] have shown that nonadiabatic heating can strongly increase the flux tube entropy for an extended reconnection outflow region and low plasma beta. This implies an additional increase of an already larger flux tube entropy with increasing distance of the tail reconnection site.…”

“…For the MFD case, the magnetic field is significantly reduced below values of B z ∼1 nT in a large portion of the near-Earth plasma sheet, consistent with the divergent slow convection at 10 R E radial distance which forms in response to the sunward flow boundary condition. The reduction of magnetic field near-geosynchronous distances is a typical property of the substorm growth phase [McPherron, 1972;Kokubun and McPherron, 1981]; however, this reduction is not present for lobe compression only. This strong reduction of the near-Earth magnetic field requires significant transport of magnetic flux out of the near-Earth region, which is provided by the divergent convection through the sunward outflow boundary condition.…”

Comparison Between Fluid Simulation With Test Particles and Hybrid Simulation for the Kelvin‐Helmholtz Instability

“…This implies nonadiabatic heating processes are triggered in the KH instability, which includes but is not limited to magnetic reconnection. Nevertheless, the specific entropy enhancement is still less than the observation (Ma & Otto, 2014). The parallel term has relatively smaller enhancement, and it can also decrease in the edge of the vortex region, suggesting the first adiabatic invariant is no longer conserved in this condition.…”