Origin of low proton‐to‐electron temperature ratio in the Earth's plasma sheet

Abstract

We study the proton‐to‐electron temperature ratio (Tp/Te) in the plasma sheet (PS) of the Earth's magnetotail using 5 years of Cluster observations (2001–2005). The PS intervals are searched within a region defined with −19 < X ≤ −7 RE and |Y| < 15 RE (GSM) under the condition |BX| ≤ 10 nT. One hundred sixty PS crossings are identified. We find an average value of <Tp/Te> ~ 6.0. However, in many PS intervals Tp/Te varies over a wide range from a few units to several tens of units. In 86 PS intervals the Tp/Te decreases below 3.5. Generally, the decreases of Tp/Te are due to some increase of Te while Tp either decreases or remains unchanged. In the majority of these intervals the Tp/Te drops are observed during magnetotail dipolarizations. A superposed epoch analysis applied to these events shows that the minimum value of Tp/Te is observed after the dipolarization onset during the “turbulent phase” of dipolarization, when a number of transient BZ pulses are reduced, but the value of BZ is still large and an intensification of wave activity is observed. The Tp/Te drops, and associated increases of Te often coincide either with bursts of broadband electrostatic emissions, which may include electron cyclotron harmonics, or with broadband electromagnetic emission in a frequency range from proton plasma frequency (fpp) up to the electron gyrofrequency (fce). These findings show that the wave activity developing in the current sheet after dipolarization onset may play a role in the additional electron heating and the associated Tp/Te decrease.

Ion and Electron κ Distribution Functions Along the Plasma Sheet

“…The values T i ∕T e ∼ 4 observed near Earth are in agreement with the results of Artemyev et al (2011) but are much smaller than those reported by Baumjohann et al (1989) (T i ∕T e ∼ 7 − 8). Our results, including the distribution of T i ∕T e values on the plasma sheet, are in very good agreement with those measured by Wang et al (2012) and Grigorenko et al (2016). Our results, including the distribution of T i ∕T e values on the plasma sheet, are in very good agreement with those measured by Wang et al (2012) and Grigorenko et al (2016).…”

Van Allen Probes Measurements of Energetic Particle Deep Penetration Into the Low L Region (L < 4) During the Storm on 8 April 2016

“…On the other hand, however, lower‐energy plasma sheet electrons could be convected inward to lower L than those higher‐energy electrons. Considering the ratio of temperature of plasma sheet protons to electrons ranges ~5–10 (e.g., Baumjohann, Paschmann, & Cattell, ; Grigorenko et al, ; Runov et al, ; Spence et al, ; Wang et al, ), Figure (right) shows the backward tracing results of μ = 0.25 MeV/G electrons (corresponding to ~2 keV electrons at L = 3.27), under the same condition with Figures (left) and (middle) . For 0.25 MeV/G electrons, as the test particle simulation results suggest, the plasma sheet electrons had access to the location of Van Allen Probes A but not B at the time of deep penetration, which is quite similar to the results of 1.70 MeV/G protons as shown in Figure (left).…”

Contrasting dynamics of electrons and protons in the near-Earth plasma sheet during dipolarization

“…The energy transported by BBFs dissipates in the flow braking region through various channels including the adiabatic and nonadiabatic energization of particles (e.g., Khotyaintsev et al, 2011;Fu et al, 2011;Runov et al, 2015;Grigorenko et al, 2016;, the generation of compressional oscillations (e.g., Runov et al, 2014), the propagation of Alfvén waves out of the flow braking region into the auroral region (e.g., Ergun et al, 2015;Stawarz et al, 2015) and the generation of wave activity in a wide frequency range. The last may lead to additional energization and/or particles scattering and losses via processes of wave-particle interactions (e.g., Le Contel et al, 2009;Zhou et al, 2009;Deng et al, 2010;Khotyaintsev et al, 2011;Panov et al, 2013;Fu et al, 2014;Vilberg et al, 2014;Zhang and Angelopoulos, 2014;Grigorenko et al, 2016).…”

“…Malykhin et al: Contrasting dynamics of electrons and protons 1992; Nakamura et al, 2002;Runov et al, 2009). DFs are associated with electron and ion acceleration (e.g., Apatenkov et al, 2007;Asano et al, 2010;Zhou et al, 2010;Fu et al, 2011;Birn et al, 2011;Grigorenko et al, 2017) as well as with various wave activities, e.g., whistler emissions, lower hybrid and electron cyclotron waves (e.g., Le Contel et al, 2009;Zhou et al, 2009;Deng et al, 2010;Khotyaintsev et al, 2011;Hwang et al, 2011;Vilberg et al, 2014;Grigorenko et al, 2016). BBFs with embedded DFs transport energy and mass from a remote tail source to the near-Earth plasma sheet (PS), where the high-speed flows are slowed down and diverted (e.g., Sergeev et al, 2009;Ge et al, 2011 and references therein).…”

“…The last may lead to additional energization and/or particles scattering and losses via processes of wave-particle interactions (e.g., Le Contel et al, 2009;Zhou et al, 2009;Deng et al, 2010;Khotyaintsev et al, 2011;Panov et al, 2013;Fu et al, 2014;Vilberg et al, 2014;Zhang and Angelopoulos, 2014;Grigorenko et al, 2016).…”