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Observation of high-shear bifurcated electron-scale current sheet at the magnetopause
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The internal structure of thin current sheets is important for magnetic reconnection and other energy transfer mechanisms, but is not well understood. In this paper we report observations of a high-shear current sheet of width comparable to electron spatial scales, with bifurcated structure embedded within a magnetopause with thickness of several ion-scales. The current sheet has features consistent with guide-field magnetic reconnection close to the electron diffusion region, such as strong out-of-plane currents, Hall electric and magnetic fields, and agyrotropic electron distributions. These distributions have crescent-shaped components due to finite gyroradius effects at the boundary of the current sheet. The crescent distributions carry perpendicular currents with magnitudes comparable to the parallel currents and are thus an integral part of the current system supporting the magnetic field structure. Electrons behave adiabatically in the vicinity of the current sheet where  they convect with the magnetic field into the magnetic reconnection inflow region. These results provide an important step forward in understanding magnetic reconnection and the dynamics of electrons within thin electron-scale current sheets.

National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-307954OAI: oai:DiVA.org:uu-307954DiVA: diva2:1049009
Available from: 2016-11-23 Created: 2016-11-23 Last updated: 2016-12-01
In thesis
1. Electron-scale physics in space plasma: Thin boundaries and magnetic reconnection
Open this publication in new window or tab >>Electron-scale physics in space plasma: Thin boundaries and magnetic reconnection
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Most of the observable Universe consists of plasma, a kind of ionized gas that interacts with electric and magnetic fields. Large volumes of space are filled with relatively uniform plasmas that convect with the magnetic field. This is the case for the solar wind, and large parts of planetary magnetospheres, the volumes around the magnetized planets that are dominated by the planet's internal magnetic field. Large plasma volumes in space are often separated by thin extended boundaries. Many small-scale processes in these boundaries mediate large volumes of plasma and energy between the adjacent regions, and can lead to global changes in the magnetic field topology. To understand how large-scale plasma regions are created, maintained, and how they can mix, it is important understand how the processes in the thin boundaries separating them work.

A process in these thin boundaries that may result in large scale changes in magnetic field topology is magnetic reconnection. Magnetic reconnection is a fundamental process that transfers energy from the magnetic field to particles, and occurs both in laboratory and astrophysical plasmas. It is a multi-scale process involving both ions and electrons, but is only partly understood

Space above the Earth's ionosphere is essentially collisionless, meaning that information, energy, and mass transfer have to be mediated through means other than collisions. In a plasma, this can happen through interactions between particles and electrostatic and electromagnetic waves. Instabilities that excites waves can therefore play a crucial role in the energy transfer between fields and particles, and different particle populations, for example between ions and electrons.

In this thesis we have used data from ESA's four Cluster and NASA's four Magnetospheric Multiscale (MMS) satellites to study small-scale – the scale where details of the electron motion becomes important – processes in thin boundaries around Earth. With Cluster, we have made detailed measurements of lower-hybrid waves and electrostatic solitary waves to better understand what role these waves can play in collisionless energy transfer. Here, the use of at least two satellites was crucial to estimate the phase speed of the waves, and associated wavelength, as well as electrostatic potential of the waves. With MMS, we have studied the electron dynamics within thin boundaries undergoing magnetic reconnection, and found that the current is often carried by non-gyrotropic parts of the electron distribution. The non-gyrotropy was caused by finite gyroradius effects due to sharp gradients in the magnetic field and plasma density and temperature. Here, the use of four satellites was crucial to deduce the spatial structure and thickness of the boundaries. Before the MMS mission, these observations of electron dynamics have never been possible in space, due to instrumental limitations of previous missions. All these findings have led to better understanding of both our near-space environment and plasma physics in general.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 68 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1453
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-307955 (URN)978-91-554-9755-2 (ISBN)
Public defence
2017-01-20, Polhemsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2016-12-21 Created: 2016-11-23 Last updated: 2016-12-28

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