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Electron-scale physics in space plasma: Thin boundaries and magnetic reconnection
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.
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. , p. 68
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: urn:nbn:se:uu:diva-307955ISBN: 978-91-554-9755-2 (print)OAI: oai:DiVA.org:uu-307955DiVA, id: diva2:1049011
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
List of papers
1. Lower Hybrid Drift Waves: Space Observations
Open this publication in new window or tab >>Lower Hybrid Drift Waves: Space Observations
2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 5, p. 055001-Article in journal (Refereed) Published
Abstract [en]

Lower hybrid drift waves (LHDWs) are commonly observed at plasma boundaries in space and laboratory, often having the strongest measured electric fields within these regions. We use data from two of the Cluster satellites (C3 and C4) located in Earth's magnetotail and separated by a distance of the order of the electron gyroscale. These conditions allow us, for the first time, to make cross-spacecraft correlations of the LHDWs and to determine the phase velocity and wavelength of the LHDWs. Our results are in good agreement with the theoretical prediction. We show that the electrostatic potential of LHDWs is linearly related to fluctuations in the magnetic field magnitude, which allows us to determine the velocity vector through the relation integral delta Edt . v = phi(delta B parallel to). The electrostatic potential fluctuations correspond to similar to 10% of the electron temperature, which suggests that the waves can strongly affect the electron dynamics.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-180280 (URN)10.1103/PhysRevLett.109.055001 (DOI)000306994900014 ()
Available from: 2012-09-03 Created: 2012-09-03 Last updated: 2017-12-07Bibliographically approved
2. Finite gyroradius effects in the electron outflow of asymmetric magnetic reconnection
Open this publication in new window or tab >>Finite gyroradius effects in the electron outflow of asymmetric magnetic reconnection
Show others...
2016 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 13, p. 6724-6733Article in journal (Refereed) Published
Abstract [en]

We present observations of asymmetric magnetic reconnection showing evidence of electron demagnetization in the electron outflow. The observations were made at the magnetopause by the four Magnetospheric Multiscale (MMS) spacecraft, separated by approximate to 15km. The reconnecting current sheet has negligible guide field, and all four spacecraft likely pass close to the electron diffusion region just south of the X line. In the electron outflow near the X line, all four spacecraft observe highly structured electron distributions in a region comparable to a few electron gyroradii. The distributions consist of a core with T-vertical bar>T and a nongyrotropic crescent perpendicular to the magnetic field. The crescents are associated with finite gyroradius effects of partly demagnetized electrons. These observations clearly demonstrate the manifestation of finite gyroradius effects in an electron-scale reconnection current sheet.

Keywords
magnetic reconnection, electron demagnetization, finite gyroradius effects, electron diffusion region
National Category
Geophysics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-304453 (URN)10.1002/2016GL069205 (DOI)000380901600006 ()
Funder
Swedish National Space Board, 23/12:2 175/15
Available from: 2016-10-05 Created: 2016-10-05 Last updated: 2017-11-30Bibliographically approved
3. Slow electron phase space holes: Magnetotail observations
Open this publication in new window or tab >>Slow electron phase space holes: Magnetotail observations
2015 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 6, p. 1654-1661Article in journal (Refereed) Published
Abstract [en]

We report multispacecraft observations of slow electrostatic solitary waves in the plasma sheet boundary layer. The electrostatic solitary waves are embedded in a region with field-aligned electron flows and are interpreted as electron phase space holes. We make unambiguous velocity and length estimates of the electron holes, v(EH)approximate to 500 km/s and l(||)approximate to 2-4(De), where l(||) is the parallel half width. We do not detect any magnetic signature of the holes. The electrostatic potentials of the holes are of the order e/k(B)T(e)approximate to 10%, indicating that they can affect electron motion and further couple the electron and ion dynamics.

Keywords
slow electron holes, multispacecraft
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-253082 (URN)10.1002/2015GL063218 (DOI)000353170000006 ()
Available from: 2015-06-10 Created: 2015-05-20 Last updated: 2017-12-04Bibliographically approved
4. Slow electron holes in multicomponent plasmas
Open this publication in new window or tab >>Slow electron holes in multicomponent plasmas
Show others...
2015 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 18, p. 7264-7272Article in journal (Refereed) Published
Abstract [en]

Electrostatic solitary waves (ESWs), often interpreted as electron phase space holes, are commonly observed in plasmas and are manifestations of strongly nonlinear processes. Often slow ESWs are observed, suggesting generation by the Buneman instability. The instability criteria, however, are generally not satisfied. We show how slow electron holes can be generated by a modified Buneman instability in a plasma that includes a slow electron beam on top of a warm thermal electron background. This lowers the required current for marginal instability and allows for generation of slow electron holes for a wide range of beam parameters that covers expected plasma distributions in space, for example, in magnetic reconnection regions. At higher beam speeds, the electron-electron beam instability becomes dominant instead, producing faster electron holes. The range of phase speeds for this model is consistent with a statistical set of observations at the magnetopause made by Cluster.

Keywords
Multi-component plasma, Modified Buneman instability, Ion-electron instability, Slow electron holes
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-267330 (URN)10.1002/2015GL065390 (DOI)000363412400004 ()
Funder
Swedish Research Council, 23/12:2
Available from: 2015-11-24 Created: 2015-11-20 Last updated: 2017-12-01Bibliographically approved
5. Observation of high-shear bifurcated electron-scale current sheet at the magnetopause
Open this publication in new window or tab >>Observation of high-shear bifurcated electron-scale current sheet at the magnetopause
(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:nbn:se:uu:diva-307954 (URN)
Available from: 2016-11-23 Created: 2016-11-23 Last updated: 2016-12-01

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