Abstract
Abstract. We study the kinetic structure of intense ion-scale current sheets with strong electron currents and the guide field having a bell-shape profile. We consider four crossings of the Earth magnetotail current sheet by the Cluster mission in 2003. The thickness of these current sheets is about the ion inertial length and significantly smaller than the characteristic ion gyroradius. We analyze the asymmetry of the electron velocity distribution functions and show that the electron current is provided by the small electron subpopulation interpreted as an electron beam or two counter-streaming electron beams. The beam (counter-streaming beams) has a bulk velocity of the order of the electron thermal velocity and a density (difference of beam densities) of about 1–5% of the plasma density. To describe the observed current sheets we develop a kinetic model with particle beams. The model predicts different thickness of the current sheet for different types of current carriers (one electron beam or two counter-streaming electron beams). The observed ion-scale current sheets can be explained assuming that the current is carried by one electron beam and a co-streaming ion beam. Although the ion beam does not carry a significant current, this beam is required to balance the electron current perpendicular to the current sheet neutral plane. The developed model explains the dominance of the electron current and the ion scales of the current sheets.
Highlights
The current sheet (CS) is a critical element of the Earth magnetotail (Ness, 1965, 1969), where charged particle acceleration and magnetic field energy dissipation occur (Birn et al, 2012)
The local structure of the magnetic field in the CS can be usually described as B = B0 tanh(z/L)ex + Byey + Bzez, where L is the CS thickness, B0 is the magnetic field at the CS boundary, shear By and perpendicular Bz components are generally uniform across the CS and significantly smaller than B0
(1) What is the origin of the field-aligned electron current from the kinetic point of view? (2) What determines the spatial scale of the CS? We have shown that the electron current is provided by the electron beam streaming along the magnetic field with velocities of the order of the thermal velocity and having density of about 1–5 % of the plasma density
Summary
The current sheet (CS) is a critical element of the Earth magnetotail (Ness, 1965, 1969), where charged particle acceleration and magnetic field energy dissipation occur (Birn et al, 2012). Detailed comparisons with spacecraft observations have shown that the TCS model well describes the CS structure (Artemyev and Zelenyi, 2013). For CSs analyzed by Shen et al (2008a), Nakamura et al (2008), and Artemyev et al (2013) the current is generally provided by electrons. Artemyev (2011) has generalized the TCS model to take into account the strong bell-shape By. The model assumes that ion beams are the main current carriers and predicts a CS thickness of about several ion thermal gyroradii. The thickness of the observed CSs is by an order of magnitude smaller than predicted by the TCS model These CSs differ from CSs with small By having ions for the main current carriers and a thickness of about several ion thermal gyroradii.
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