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Observations of magnetospheric ULF waves in connection with the Kelvin-Helmholtz instability at Mercury
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0002-9164-0761
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0003-1270-1616
KTH.
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0002-0349-0645
2016 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 9, 8576-8588 p.Article in journal (Refereed) Published
Abstract [en]

The magnetic field data from the MESSENGER spacecraft is investigated to establish the presence of magnetospheric ultra-low frequency (ULF) waves in connection with 131 previously observed nonlinear Kelvin-Helmholtz (KH) waves at Mercury. Distinct ULF wave signatures are detected in 44 out of the 131 magnetospheric traversals prior to or after observing the KH waves. Of these ULF events, 39 out of 44 are highly coherent at the frequency of maximum power spectral density, and occur more often on the dayside magnetosphere than away from it. The waves observed at the dayside magnetosphere, which appear mainly at the duskside and naturally following the KH wave occurrence asymmetry, are significantly different to the evening- or morningside events, and have the following distinct wave characteristics: a polarization mainly in the perpendicular (azimuthal) direction to the mean magnetic field, a wave normal angle closer to the parallel than the perpendicular direction, an absolute ellipticity increasing away from noon, almost exclusively a right-hand polarization, and frequencies in the narrow range of 0.02 − 0.04 Hz (well below the local Na +  gyrofrequency, and in the same range as the KH waves). The results strongly suggest that the large majority of the ULF waves at the dayside observed in this study are driven by KH waves at the magnetopause, and that they occur in the vicinity of a field line resonance, which in turn manifests the importance of the Kelvin-Helmholtz instability in terms of energy and momentum transport throughout Mercury's magnetosphere.

Place, publisher, year, edition, pages
John Wiley & Sons, 2016. Vol. 121, no 9, 8576-8588 p.
Keyword [en]
Ultra-low frequency waves, Kelvin-Helmholtz instability, Mercury, Energy transfer magnetosphere
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-191502DOI: 10.1002/2016JA023015ISI: 000385844000027Scopus ID: 2-s2.0-84987974256OAI: oai:DiVA.org:kth-191502DiVA: diva2:956937
Note

QC 20160912

Available from: 2016-08-31 Created: 2016-08-31 Last updated: 2017-05-18Bibliographically approved
In thesis
1. Structures and processes in the Mercury magnetosphere
Open this publication in new window or tab >>Structures and processes in the Mercury magnetosphere
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The mechanisms involved in the transfer of mass and energy from the solar wind to any planetary magnetosphere is considered an important topic in space physics. With the use of the Mercury spacecraft MESSENGER's data, it has been possible to study these processes in an environment different, yet similar, to Earth's. These data have resulted in new knowledge advancing not only the extraterrestrial space plasma research, but also the general space physics field.

 

This thesis aims to investigate mechanisms for the transfer of mass and energy into Mercury’s magnetosphere, and magnetospheric regions affected by, and processes directly driven by, these. The work includes the Kelvin-Helmholtz instability (KHI) at the magnetopause, which is one of the main drivers for mass and energy transfer on Earth, the low-latitude boundary layer (LLBL), which is in direct connection to the magnetopause and proposed to be affected by the KHI, magnetospheric ultra-low frequency (ULF) waves driven by the KHI, and isolated magnetic field structures in the magnetosheath as possible analogues to the Earth magnetosheath plasmoids and jets.

 

Kelvin-Helmholtz waves (KHW) and the LLBL are identified and characterized. The KHWs are observed almost exclusively on the duskside magnetopause, something that has not been observed on Earth. In contrast, the LLBL shows an opposite asymmetry. Results suggest that the KHI and LLBL are connected, possibly by the LLBL creating the asymmetry observed for the KHWs.

 

Isolated changes of the total magnetic field strength in the magnetosheath are identified. The similar properties of the solar wind and magnetosheath negative magnetic field structures suggest that they are analogues to diamagnetic plasmoids found on Earth. No clear analogues to paramagnetic plasmoids are found.  

 

Distinct magnetospheric ULF wave signatures are detected frequently in close connection to KHWs. Results from the polarization analysis on the dayside ULF waves indicate that the majority of these are most probably driven by the KHI. In general, likely KHI driven ULF waves are observed frequently in the Hermean magnetosphere. 

Although similar in many aspects, Mercury and Earth show fundamental differences in processes and structures, making Mercury a highly interesting planet to study to increase our knowledge of Earth-like planets.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. 53 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2017:029
Keyword
Mercury, MESSENGER, magnetosphere, processes, structures
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-207173 (URN)978-91-7729-349-1 (ISBN)
Public defence
2017-06-15, Kollegiesalen, Brinellvägen 8, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 122/11
Note

QC 20170519

Available from: 2017-05-19 Created: 2017-05-18 Last updated: 2017-05-19Bibliographically approved

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