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  • 1.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Liljeblad, Elisabet
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Brenning, Nils
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Nilsson, H.
    Gunell, H.
    Hamrin, M.
    On the origin of magnetosheath plasmoids and their relation to magnetosheath jets2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 9, p. 7390-7403Article in journal (Refereed)
    Abstract [en]

    We investigate localized magnetosheath and solar wind density enhancements, associated with clear magnetic field changes, and therefore referred to as magnetosheath/solar wind plasmoids, respectively. Using Cluster data, we show that there are two distinct populations of magnetosheath plasmoids, one associated with a decrease of magnetic field strength (diamagnetic plasmoids), and one with an increased magnetic field strength (paramagnetic plasmoids). The diamagnetic magnetosheath plasmoids have scale sizes of the order of 1-10 R-E, while the paramagnetic ones are an order of magnitude smaller. The diamagnetic plasmoids are not associated with any change in the magnetosheath plasma flow velocity, and they are classified as embedded plasmoids in the terminology of Karlsson et al. (2012). The paramagnetic plasmoids may either be embedded or associated with increases in flow velocity (fast plasmoids). A search for plasmoids in the pristine solar wind resulted in identification of 62 diamagnetic plasmoids with very similar properties to the magnetosheath diamagnetic plasmoids, making it probable that the solar wind is the source of these structures. No paramagnetic plasmoids are found in the pristine solar wind, indicating that these are instead created at the bow shock or in the magnetosheath. We discuss the relation of the plasmoids to the phenomenon of magnetosheath jets, with which they have many properties in common, and suggest that the paramagnetic plasmoids can be regarded as a subset of these or a closely related phenomenon. We also discuss how the results from this study relate to theories addressing the formation of magnetosheath jets.

  • 2.
    Karlsson, Tomas
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Liljeblad, Elisabet
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Raines, Jim M.
    Slavin, James A.
    Sundberg, Torbjorn
    Isolated magnetic field structures in Mercury's magnetosheath as possible analogues for terrestrial magnetosheath plasmoids and jets2016In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 129, p. 61-73Article in journal (Refereed)
    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.

  • 3.
    Liljeblad, Elisabet
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Investigation of ~mHz ULF waves in Mercury's dayside magnetosphere and their driving mechanismsManuscript (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.

  • 4.
    Liljeblad, Elisabet
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Structures and Processes at the Mercury Magnetopause2015Licentiate 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.

     

  • 5.
    Liljeblad, Elisabet
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Structures and processes in the Mercury magnetosphere2017Doctoral 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.

  • 6.
    Liljeblad, Elisabet
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Investigation of similar to 20-40mHz ULF waves and their driving mechanisms in Mercury's dayside magnetosphere2017In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 35, no 4, p. 879-884Article in journal (Refereed)
    Abstract [en]

    Ultra-low-frequency (ULF) waves in the similar to 20-40 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 waves have very similar characteristics to the waves likely driven by Kelvin-Helmholtz (KH) ULF waves (which are retained as a subset of the wave events studied in this paper) identified in a previous study. Significant ULF wave activity is observed in the dawn sector of the magnetosphere. This indicates that Mercury KH waves may be more common between 6 and 12 magnetic local time than previously predicted and that magnetospheric ULF waves in the frequency band similar to 20-40 mHz can be used as a detection tool for Hermean KH waves.

  • 7.
    Liljeblad, Elisabet
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Raines, J.
    Slavin, J.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sundberg, T.
    Zurbuchen, T. H.
    MESSENGER observations of the dayside low-latitude boundary layer in Mercury's magnetosphere2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 10Article in journal (Refereed)
    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.

  • 8.
    Liljeblad, Elisabet
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sundberg, Torbjörn
    KTH.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Observations of magnetospheric ULF waves in connection with the Kelvin-Helmholtz instability at Mercury2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 9, p. 8576-8588Article in journal (Refereed)
    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.

  • 9.
    Liljeblad, Elisabet
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Sundberg, T.
    Karlsson, Tomas
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Kullen, Anita
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Statistical investigation of Kelvin-Helmholtz waves at the magnetopause of Mercury2014In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 12, p. 9670-9683Article in journal (Refereed)
    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

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