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MESSENGER observations of the dayside low-latitude boundary layer in Mercury's magnetosphere
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
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2015 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 10Article in journal (Refereed) Published
Abstract [en]

Observations from MESSENGER's MAG and FIPS instruments during the first orbital year have resulted in the identification of 25 magnetopause crossings in Mercury's magnetosphere with significant low-latitude boundary layers (LLBLs). Of these crossings 72% are observed dawnside, and 65% for northward interplanetary magnetic field.

The estimated LLBL thickness is 450 ± 56 km, and increases with distance to noon. The Na+-group ion is sporadically present in 14 of the boundary layers, with an observed average number density of 22 ± 11% of the proton density. Furthermore, the average Na+-group gyroradii in the layers is 220 ± 34 km, the same order of magnitude as the LLBL thickness.

Magnetic shear, plasma β and reconnection rates have been estimated for the LLBL crossings, and compared to those of a control group (non-LLBL) of 61 distinct magnetopause crossings which show signs of nearly no plasma inside the magnetopause. The results indicate that reconnection is significantly slower, or even suppressed, for the LLBL crossings compared to the non-LLBL cases.

Possible processes that form or impact the LLBL are discussed. Protons injected through the cusp or flank may be important for the formation of the LLBL. Furthermore, the opposite asymmetry in the Kelvin-Helmholtz instability (KHI) as compared to the LLBL, rules out the KHI as a dominant formation mechanism. However, the KHI and LLBL could be related to each other, either by the impact of sodium ions gyrating across the magnetopause, or by the LLBL preventing the growth of KH waves on the dawnside.

Place, publisher, year, edition, pages
Blackwell Publishing, 2015. Vol. 120, no 10
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-174177DOI: 10.1002/2015JA021662ISI: 000366135200016Scopus ID: 2-s2.0-84954385256OAI: oai:DiVA.org:kth-174177DiVA: diva2:858233
Funder
Swedish National Space Board, 566176
Note

QC 20150107

Available from: 2015-10-01 Created: 2015-10-01 Last updated: 2017-12-01Bibliographically approved
In thesis
1. Structures and Processes at the Mercury Magnetopause
Open this publication in new window or tab >>Structures and Processes at the Mercury Magnetopause
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The mechanism involved in the transfer of energy, momentum and plasma from the solar wind to any planetary magnetosphere is considered one of the more important topics in space plasma physics. With the use of the Mercury spacecraft MESSENGER’s (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) data, it has been possible to study these processes in an environment different, yet similar, to Earth’s. These data have resulted in unprecedented investigations advancing not only the extraterrestrial space plasma research, but also the general space physics field.

This work aims to investigate the Kelvin-Helmholtz (KH) instability at Mercury’s magnetopause, which is believed to be one of the main drivers for the transfer of matter and energy into Earth’s magnetosphere, and the low- latitude boundary layer (LLBL) which is in direct connection to the magnetopause. The studies use data from MESSENGER’s magnetometer (MAG) and fast imaging plasma spectrometer (FIPS) instruments during the first three years in orbit. Results show that KH waves are observed almost exclusively on the duskside magnetopause, something that has not been observed at Earth. In contrast, the LLBL shows an opposite asymmetry as it occurs more often on the dawnside. Both the KH instability and the LLBL are observed mainly during northward interplanetary magnetic field. This, together with the distinct opposite asymmetry, suggests that the KH instability and LLBL are somehow connected. Previous theoretical studies, simulations and observations have shown or indicated that the sodium ions have a large impact on the Hermean magnetospheric environment, including the boundary layer where the KH instability arises. One possibility is that the sodium ions also induce the observed dawn-dusk asymmetry in the LLBL. Another explanation could be that the LLBL on its own influences the KH wave occurrence by reducing the KH wave growth rates on the dawnside where most of the LLBLs are observed. Furthermore, observations agree with some formation mechanisms that should give rise to the observed dawn-dusk LLBL asymmetry.

The processes responsible for the dawn-dusk occurrence asymmetry in both the KH instability and the LLBL are yet to be confirmed. Future work may also include determination of the contribution of KH waves to the energy and plasma transfer from the solar wind to the Hermean magnetosphere.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xi, 32 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:53
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-174306 (URN)978-91-7595-681-7 (ISBN)
Presentation
2015-10-21, Alfvén lab’s seminar room, Teknikringen 31, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 566176
Note

QC 20151005

Available from: 2015-10-05 Created: 2015-10-02 Last updated: 2015-10-05Bibliographically approved
2. 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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Liljeblad, ElisabetKarlsson, TomasKullen, Anita

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