Statistical Study on Electron and Ion Temperatures in the Near‐Earth Reconnection and Magnetic Pileup Regions

• Published in: Geophysical research letters 2019-12, Vol.46 (24), p.14223-14229
• Main Authors: Watanabe, K., Keika, K., Hoshino, M., Kitamura, N., Saito, Y., Giles, B. L., Paterson, W. R.
• Format: Article
• Language: English
• Subjects: Boundary layers; Earth; Earth magnetosphere; Electron energy; Electron temperatures; Entropy; Heating; Heating rate; Inflow; Ion temperature; Ions; Magnetic field; Magnetic fields; Magnetic reconnection; Magnetotails; Outflow; Plasma; Plasma heating; Profiles; Regions; Statistics; Temperature; Temperature ratio
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL084837

Plasma heating during the magnetic reconnection is not well understood, particularly in terms of adiabaticity, heating rate, and electron‐ion temperature ratio. This study focuses on macroscopic profiles of Ti, Te, and Ti/Te in the near‐Earth magnetotail, using data obtained from Magnetospheric Multiscale. Observation points are categorized into three different regions: low‐beta/high‐beta inflow region, the reconnection sites, and magnetic pileup regions. From low‐beta inflow region to high‐beta inflow region, both Ti and Te increase and electron entropy is constant. Ion entropy, on the other hand, increases across the transition. Inside the reconnection site, Ti and Te increase, and entropies remain constant. From the outflow region to the pileup region, electrons are selectively heated. Electron entropy increases in the transition. This statistical study suggests that nonadiabatic heating occurs around the plasma sheet boundary layer for ions and at the early stage of magnetic field pileup for electrons. Plain Language Summary Plasma heating processes in and around the Earth's magnetosphere are not fully understood, and it remains an open question as to why the ion‐electron temperature ratio is 5–10 in the plasma sheet in the Earth's magnetotail. As magnetic reconnection is a major process that heats plasma sheet by releasing the magnetic energy, we investigate average profiles of ion and electron temperatures in the near‐Earth reconnection region using data obtained from Magnetospheric Multiscale for a period from 1 May to 30 September 2017. We make a map of plasma parameters (temperature, entropy, etc.) to present the relative locations to the reconnection X‐point. This analysis enables us to understand where nonadiabatic heating occurs for electrons and/or ions. Key Points Reconstruction around the reconnection regions performed with MMS data is successful in separating the inflow, outflow, and pileup regions In the inflow region, electrons are adiabatically heated, while ions are not; in the reconnection region, heating is all adiabatic In the early stage of magnetic field pileup associated with fast flows, nonadiabatic heating is likely to occur particularly for electrons