Energy budget and mechanisms of cold ion heating in asymmetric magnetic reconnection

Abstract

Cold ions (few tens of eV) of ionospheric origin are commonly observed on the magnetospheric side of the Earth's dayside magnetopause. As a result, they can participate in magnetic reconnection, changing locally the reconnection rate and being accelerated and heated. We present four events where cold ion heating was observed by the Magnetospheric Multiscale mission, associated with the magnetospheric Hall E field region of magnetic reconnection. For two of the events the cold ion density was small compared to the magnetosheath density, and the cold ions were heated roughly to the same temperature as magnetosheath ions inside the exhaust. On the other hand, for the other two events the cold ion density was comparable to the magnetosheath density and the cold ion heating observed was significantly smaller. Magnetic reconnection converts magnetic energy into particle energy, and ion heating is known to dominate the energy partition. We find that at least 10–25% of the energy spent by reconnection into ion heating went into magnetospheric cold ion heating. The total energy budget for cold ions may be even higher when properly accounting for the heavier species, namely helium and oxygen. Large E field fluctuations are observed in this cold ion heating region, i.e., gradients and waves, that are likely the source of particle energization.

MMS Observations of Multiscale Hall Physics in the Magnetotail

“…These oscillations at the lower‐hybrid frequency (∼15 Hz) cannot be resolved by the FPI ion time resolution (150 ms). The waves propagate along the − M direction, which is similar to lower‐hybrid waves driven by interaction between cold and hot ions that have been observed at the magnetopause (Graham et al, ), suggested as a mechanism for heating cold ions (Toledo‐Redondo et al, ). Around 17:57:12.1 UT MMS observed a j M current sheet with a peak current of 180 nA/m 2 and a thickness of ∼24 km (∼1.8 ρ ce ), denoted by the first vertical line in Figure .…”

Concomitant Double Ion and Electron Populations in the Earth's Magnetopause Boundary Layers From Double Reconnection With Lobe and Closed Field Lines

“…The aim of the present section is to report on the statistical analysis of individual energy spectra in a large‐scale fashion, to determine their spatial distribution, occurrence, and how they are ordered by interplanetary conditions (IMF). It should be noted that the magnetosphere also contains even colder ions of ionospheric origin (André & Cully, ; Chappell et al, ; Sauvaud et al, ), but once inside the LLBL they are heated and mostly indistinguishable from ions of magnetosheath origin (Dargent et al, ; Toledo‐Redondo et al, , ). They are thus not addressed in the present study.…”

Signatures of Cold Ions in a Kinetic Simulation of the Reconnecting Magnetopause

“…Cold ions are heated and accelerated in the separatrix region (Divin et al, ; Toledo‐Redondo, André, Vaivads, et al,; Toledo‐Redondo et al, , ). Once in the exhaust, they mix with magnetosheath ions and, therefore, become hard to distinguish from other ions in spacecraft data (André et al, ; Li et al, ; Toledo‐Redondo et al, ).…”

“…Owing to this heating, cold ions take part in the energy budget of magnetic reconnection (Aunai et al, ) in some yet unknown proportion. A recent study based on four independent observations by MMS of the reconnecting magnetopause with cold ions showed that cold ions take at least 10–25% of the energy spent into particle heating (Toledo‐Redondo et al, ). To study cold ion heating, numerical simulations have the advantage of being able to flag ions and then discriminate cold ions from other populations, even once heated.…”