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Structures and processes in the Mercury magnetosphere
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0002-9164-0761
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 [en]
Mercury, MESSENGER, magnetosphere, processes, structures
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-207173ISBN: 978-91-7729-349-1 (print)OAI: oai:DiVA.org:kth-207173DiVA: diva2:1096534
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
List of papers
1. Statistical investigation of Kelvin-Helmholtz waves at the magnetopause of Mercury
Open this publication in new window or tab >>Statistical investigation of Kelvin-Helmholtz waves at the magnetopause of Mercury
2014 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 12, 9670-9683 p.Article in journal (Refereed) Published
Abstract [en]

A large study of Kelvin-Helmholtz (KH) waves at the magnetopause of Mercury covering 907 days of data from the MErcury Surface Space ENvironment GEochemistry Ranging spacecraft have resulted in 146 encounters of not only nonlinear KH waves but also linear surface waves, including the first observations of KH waves at the dawnside magnetopause. Most of the waves are in the nonlinear phase (90%) occur at the duskside magnetopause (93%), under northward magnetosheath magnetic field conditions (89%) and during greater magnetosheath Bz (23 nT) values than in general. The average period and amplitude is 30 ± 14 s and 14 ± 10 nT, respectively. Unlike duskside events, dawnside waves do not appear at the magnetopause flank (<6 magnetic local time). This is in agreement with previous observations and modeling results and possibly explained by finite Larmor radius effects and/or a lack of a large-scale laminar flow at the dawnside magnetopause boundary. Key Points Observing Kelvin-Helmholtz waves at the dawnside Mercury magnetopause Confirming a dawn-dusk asymmetry associated with the Kelvin-Helmholtz at Mercury Determine characteristics associated with Kelvin-Helmholtz waves

Keyword
Kelvin-Helmholtz, magnetopause, MESSENGER
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-161507 (URN)10.1002/2014JA020614 (DOI)000349161100025 ()2-s2.0-84921760749 (Scopus ID)
Funder
Swedish National Space Board
Note

QC 20150313

Available from: 2015-03-13 Created: 2015-03-12 Last updated: 2017-05-18Bibliographically approved
2. MESSENGER observations of the dayside low-latitude boundary layer in Mercury's magnetosphere
Open this publication in new window or tab >>MESSENGER observations of the dayside low-latitude boundary layer in Mercury's magnetosphere
Show others...
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
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-174177 (URN)10.1002/2015JA021662 (DOI)000366135200016 ()2-s2.0-84954385256 (Scopus ID)
Funder
Swedish National Space Board, 566176
Note

QC 20150107

Available from: 2015-10-01 Created: 2015-10-01 Last updated: 2017-05-18Bibliographically approved
3. Isolated magnetic field structures in Mercury's magnetosheath as possible analogues for terrestrial magnetosheath plasmoids and jets
Open this publication in new window or tab >>Isolated magnetic field structures in Mercury's magnetosheath as possible analogues for terrestrial magnetosheath plasmoids and jets
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2016 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 129, 61-73 p.Article in journal (Refereed) Published
Abstract [en]

We have investigated MESSENGER magnetic field data from the Mercury magnetosheath and near solar wind, to identify isolated magnetic field structures (defined as clear, isolated changes in the field magnitude). Their properties are studied in order to determine if they may be considered as analogues to plasmoids and jets known to exist in Earth's magnetosheath. Both isolated decreases of the magnetic field absolute value ('negative magnetic field structures') and increases ('positive structures') are found in the magnetosheath, whereas only negative structures are found in the solar wind. The similar properties of the solar wind and magnetosheath negative magnetic field structures suggests that they are analogous to diamagnetic plasmoids found in Earth's magnetosheath and near solar wind. The latter have earlier been identified with solar wind magnetic holes. Positive magnetic field structures are only found in the magnetosheath, concentrated to a region relatively close to the magnetopause. Their proximity to the magnetopause, their scale sizes, and the association of a majority of the structures with bipolar magnetic field signatures identify them as flux transfer events (which generally are associated with a decrease of plasma density in the magnetosheath). The positive magnetic field structures are therefore not likely to be analogous to terrestrial paramagnetic plasmoids but possibly to a sub-population of magnetosheath jets. At Earth, a majority of magnetosheath jets are associated with the quasi-parallel bow shock. We discuss some consequences of the findings of the present investigation pertaining to the different nature of the quasi-parallel bow shock at Mercury and Earth.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Mercury, Magnetosheath, Bow shock, MESSENGER, Plasmoids, Magnetosheath jets
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-192709 (URN)10.1016/j.pss.2016.06.002 (DOI)000381323800006 ()2-s2.0-85006710378 (Scopus ID)
Note

QC 20160926

Available from: 2016-09-26 Created: 2016-09-20 Last updated: 2017-05-18Bibliographically approved
4. Observations of magnetospheric ULF waves in connection with the Kelvin-Helmholtz instability at Mercury
Open this publication in new window or tab >>Observations of magnetospheric ULF waves in connection with the Kelvin-Helmholtz instability at Mercury
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
Keyword
Ultra-low frequency waves, Kelvin-Helmholtz instability, Mercury, Energy transfer magnetosphere
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-191502 (URN)10.1002/2016JA023015 (DOI)000385844000027 ()2-s2.0-84987974256 (Scopus ID)
Note

QC 20160912

Available from: 2016-08-31 Created: 2016-08-31 Last updated: 2017-05-18Bibliographically approved
5. Investigation of ~mHz ULF waves in Mercury's dayside magnetosphere and their driving mechanisms
Open this publication in new window or tab >>Investigation of ~mHz ULF waves in Mercury's dayside magnetosphere and their driving mechanisms
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Ultra-low frequency (ULF) waves in the ~mHz range are frequently observed in the Mercury magnetosphere using MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER) magnetic field data. The majority of these have very similar characteristics as the likely Kelvin-Helmholtz (KH) driven ULF waves identified in a previous study (which are retained as a subset of the wave events studied in this paper). A large quantity of the ULF waves is observed in the dawn sector of the magnetosphere. This indicates that Mercury KH waves at the dawn side may be more common than previously predicted, and that magnetospheric ULF waves in the frequency band ~20-40 mHz can be used as a detection tool for Hermean KH waves.

Keyword
ULF waves, Mercury, magnetosphere
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-207042 (URN)
Funder
Swedish National Space Board, 122/11
Note

QCR 20170517

Available from: 2017-05-13 Created: 2017-05-13 Last updated: 2017-05-18Bibliographically approved

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