Electron currents supporting the near‐Earth magnetotail during current sheet thinning

Abstract

Formation of intense, thin current sheets (i.e., current sheet thinning) is a critical process for magnetospheric substorms, but the kinetic physics of this process remains poorly understood. Using a triangular configuration of the three Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at the end of 2015 we investigate field‐aligned and transverse currents in the magnetotail current sheet around 12 Earth radii downtail. Combining the curlometer technique with direct measurements of ion and electron velocities, we demonstrate that intense, thin current sheets supported by strong electron currents form in this region. Electron field‐aligned currents maximize near the neutral plane Bx∼0, attaining magnitudes of ∼20 nA/m2. Carried by hot (>1 keV) electrons, they generate strong magnetic shear, which contributes up to 20% of the vertical (along the normal direction to the equatorial plane) pressure balance. Electron transverse currents, on the other hand, are carried by the curvature drift of anisotropic, colder (<1 keV) electrons and gradually increase during the current sheet thinning. In the events under consideration the thinning process was abruptly terminated by earthward reconnection fronts which have been previously associated with tail reconnection further downtail. It is likely that the thin current sheet properties described herein are similar to conditions further downtail and are linked to the loss of stability and onset of reconnection there. Our findings are likely applicable to thin current sheets in other geophysical and astrophysical settings.

Ion Anisotropy in Earth's Magnetotail Current Sheet: Multicomponent Ion Population

“…The preferentially field‐aligned anisotropic cold electrons (probably of the ionospheric origin; see, e.g., Walsh et al, ) provide p ‖, e / p ⊥, e ∼1.1 in the quiet magnetotail (Artemyev et al, ; Walsh et al, ). For some CSs, the contribution of electron anisotropy to j y can reach 100%, that is, the current density can be fully attributed to the electron anisotropy, (1/ μ 0 )( p ‖, e − p ⊥, e )( ∂B x / ∂z )/ B 2 (Artemyev et al, , , Mingalev et al, ). However, statistical estimates of the electron anisotropy are too low to explain the observed j y (Artemyev et al, ).…”

An Electron‐Scale Current Sheet Without Bursty Reconnection Signatures Observed in the Near‐Earth Tail

“…A thin current sheet of several ion inertial lengths has been frequently observed inside this magnetotail current sheet in recent years (Baumjohann et al, 2007;Zelenyi et al, 2016), thanks especially to the four-point Cluster measurements. In general, the thin current sheet is embedded in a much broader plasma sheet Petrukovich et al, 2011), and the current within such a thin current sheet is mainly carried by electrons (Artemyev et al, 2017;Baumjohann et al, 2007;Petrukovich et al, 2011;Shen et al, 2008). The closest separation of Cluster was~250 km, close to ion inertial length.…”

Force-free collisionless current sheet models with non-uniform temperature and density profiles

“…where P T is the total pressure, since B 2 ¼ jBj 2 ¼ B 2 0 for the magnetic field (8), which is independent of k. Since, in this case, P zz ¼ ðb e þ b i Þn=ðb e b i Þ, it follows that the density and temperature will also be independent of k. As can be seen from the expressions (15) and (17), Abraham-Shrauner's model has density and temperature profiles that are constant across the current sheet, in a similar way to the models discussed in Refs. 18, 33, and 38-42.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15]. Equations (1)- (3) imply that the current density is parallel to the magnetic field: j ¼ aðrÞB.…”