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Strong current sheet at a magnetosheath jet: Kinetic structure and electron acceleration
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0002-1046-746X
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
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2016 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 10, p. 9608-9618Article in journal (Refereed) Published
Abstract [en]

Localized kinetic-scale regions of strong current are believed to play an important role in plasma thermalization and particle acceleration in turbulent plasmas. We present a detailed study of a strong localized current, 4900 nA m(-2), located at a fast plasma jet observed in the magnetosheath downstream of a quasi-parallel shock. The thickness of the current region is similar to 3 ion inertial lengths and forms at a boundary separating magnetosheath-like and solar wind-like plasmas. On ion scales the current region has the shape of a sheet with a significant average normal magnetic field component but shows strong variations on smaller scales. The dynamic pressure within the magnetosheath jet is over 3 times the solar wind dynamic pressure. We suggest that the current sheet is forming due to high velocity shears associated with the jet. Inside the current sheet we observe local electron acceleration, producing electron beams, along the magnetic field. However, there is no clear sign of ongoing reconnection. At higher energies, above the beam energy, we observe a loss cone consistent with part of the hot magnetosheath-like electrons escaping into the colder solar wind-like plasma. This suggests that the acceleration process within the current sheet is similar to the one that occurs at shocks, where electron beams and loss cones are also observed. Therefore, electron beams observed in the magnetosheath do not have to originate from the bow shock but can also be generated locally inside the magnetosheath.

Place, publisher, year, edition, pages
2016. Vol. 121, no 10, p. 9608-9618
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-312116DOI: 10.1002/2016JA023146ISI: 000388965900020OAI: oai:DiVA.org:uu-312116DiVA, id: diva2:1062967
Available from: 2017-01-09 Created: 2017-01-04 Last updated: 2018-09-06Bibliographically approved
In thesis
1. Electron energization in near-Earth space: Studies of kinetic scales using multi-spacecraft data
Open this publication in new window or tab >>Electron energization in near-Earth space: Studies of kinetic scales using multi-spacecraft data
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Plasma, a gas of charged particles exhibiting collective behavior, is everywhere in the Universe. The heating of plasma to millions of degrees and acceleration of charged particles to very high energies has been observed in many astrophysical environments. How and where the heating and acceleration occur is in many cases unclear. In most astrophysical environments, plasma consists of negative electrons and positive ions. In this thesis we focus on understanding the heating and acceleration of electrons. Several plasma processes have been proposed to explain the observed acceleration. However, the exact heating and acceleration mechanisms involved and their importance are still unclear. This thesis contributes toward a better understanding of this topic by using observations from two multi-spacecraft missions, Cluster and the Magnetospheric MultiScale (MMS), in near-Earth space.

In Article I we look at magnetic nulls, regions of vanishing magnetic field B believed to be important in particle acceleration, in the Earth's nightside magnetosphere. We find that nulls are common at the nightside magnetosphere and that the characterization of the B geometry around a null can be affected by localized B fluctuations. We develop and present a method for determining the effect of the B fluctuation on the null's characterization.

In Article II we look at a thin (a few km) current sheet (CS) in the turbulent magnetosheath. Observations suggest local electron heating and beam formation parallel to B inside the CS. The electron observations fits well with the theory of electron acceleration across a shock due to a potential difference. However, in our case the electron beams are formed locally inside the magnetosheath that is contrary to current belief that the beam formation only occurs at the shock.

In Article III we present observations of electron energization inside a very thin (thinner than Article II) reconnecting CS located in the turbulent magnetosheath. Currently, very little is know about electron acceleration mechanisms at these small scales. MMS observe local electron heating and acceleration parallel to B when crossing the CS. We show that the energized electrons correspond to acceleration due to a quasi-static potential difference rather than electrostatic waves. This energization is similar to what has been observed inside ion diffusion regions at the magnetopause and magnetotail. Thus, despite the different plasma conditions a similar energization occurs in all these plasma regions.

In Article IV we study electron acceleration by Fermi acceleration, betatron acceleration, and acceleration due to parallel electric fields inside tailward plasma jets formed due to reconnection, the so called tailward outflow region. We show that most observations are consistent with local electron heating and acceleration from a simplified two dimensional picture of Fermi acceleration and betatron acceleration in an outflow region. We find that Fermi acceleration is the dominant electron acceleration mechanism.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 80
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1719
Keywords
magnetic reconnection, electron acceleration, electron heating, magnetosheath, magnetotail, magnetic nulls, Cluster, Magnetospheric MultiScale
National Category
Other Physics Topics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-359594 (URN)978-91-513-0437-3 (ISBN)
Public defence
2018-10-25, Polhemsalen, Ångström Laboratory 10134, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2013-4309
Available from: 2018-10-02 Created: 2018-09-06 Last updated: 2018-10-16

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Eriksson, ElinVaivads, AndrisGraham, Daniel. B.Khotyaintsev, YuriYordanova, EmiliyaAndré, Mats
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