Cluster statistics of thin current sheets in the Earth magnetotail: Specifics of the dawn flank, proton temperature profiles and electrostatic effects

Abstract

[1] In this paper we use the statistics of 70 crossings by the Cluster mission to study and compare properties of thin current sheets observed at the dawn and dusk flanks of the Earth magnetotail. Special attention is devoted to the current sheet embedding: we define the degree of the current sheet embedding as b e = B ext /B 0 (B 0 and B ext are magnetic field magnitudes at the thin current sheet boundary and in the lobes). We determine the current density by curlometer technique and calculate the current sheet thickness. We demonstrate that the current sheets at the dawn flank have larger b e , smaller magnitude of the current density and larger relative thicknesses in Larmor radii than the current sheets at the dusk flank. Protons in thin current sheets are divided into two populations (the current-carrying particles and the background) and the temperatures of these populations have been estimated. The distribution of the proton temperatureT p inside typical current sheet is approximated asT p ≈ T p (1 − a T (B x /B ext ) 2 ), where T p is theT p value in the central region of the current sheet. The average value ha T i ≈ 0.8. The proton current density (flow velocity in Y) is positive at the dusk flank and negative at the dawn flank, while the electron current density is positive at both flanks. This difference of the proton current density at two flanks is explained by the E × B drift due to the presence of the earthward electrostatic field E x . We develop a simple model of the earthward electrostatic field to incorporate the influence of the embedding and the dawn-dusk magnetic field component.

Proton and heavy ion acceleration by stochastic fluctuations in the Earth's magnetotail

“…For both profiles, proton energies of up to 100 keV are obtained, in agreement with observations. When a small magnetic fluctuation level is present, A 0 / 1 nT, the proton 924 F. Catapano et al: Proton and heavy ion acceleration by stochastic fluctuations in the Earth's magnetotail energy can easily reach 5-6 keV and He ++ and O + can reach 10 keV, in agreement with typical ion temperatures observed in the magnetotail (e.g, Baumjohann et al, 1989;Runov et al, 2006;Artemyev et al, 2011). For He ++ ions, notably, energies of up to 200 keV are obtained (see Fig.…”

“…Considering that the proton temperature in the magnetotail plasma sheet is observed to be of the order of 5 keV (e.g., Borovsky et al, 1997;Runov et al, 2006;Artemyev et al, 2011), a very small fluctuation level with δB ∼ 1 nT, almost always present in the magnetotail (Hoshino et al, 1994), is sufficient to heat the cold plasma coming from the solar wind to a temperature of 3-5 keV (Runov et al, 2006).…”

“…Further, relevant levels of energetic electrons and ions are also observed during periods of low geomagnetic activity (Christon et al, 1989), leaving open the search for the acceleration mechanism. In addition, the observed proton temperatures, of the order of 5-10 keV, are also much larger than the energy corresponding to their original source, but the mechanism that can generate such heated particles is not fully understood (Runov et al, 2006;Artemyev et al, 2011). Grigorenko et al (2009) pointed out that 100 keV ions are accelerated near reconnecting regions, possibly by a process related to time-dependent, patchy reconnection.…”

“…From the Cluster database we picked up four crossings with distinguished variations in the ion temperature across the sheet (see Fig. 5 in Artemyev et al, 2011). In Fig.…”

Profiles of electron temperature and <I>B</I><sub>z</sub> along Earth's magnetotail

“…Configuration of the magnetotail CS controls the rate of charged particle acceleration and following injection into the inner magnetosphere (see recent reviews by Baumjohann et al, 2007;Sergeev et al, 2012, and references therein). With the multi-spacecraft Cluster mission, transverse (along Z) structure of the CS was studied in detail (especially at the distances X ∼ −20 R E , where Cluster collected main statistics; see Runov et al, 2006;Artemyev et al, 2011b). Although the distribution of the magnetic field along the tail plays an important role in particle (mainly electron) acceleration, it is less investigated.…”