Coherent wave activity in Mercury's magnetosheath

This study presents a statistical overview of coherent wave activity in Mercury's magnetosheath. Left‐handed electromagnetic ion cyclotron waves are commonly found behind the quasi‐perpendicular section of the bow shock, where they are present in ~50% of the spacecraft crossings of the magnetosheath. Their occurrence distribution maximizes within the magnetosheath, approximately halfway between the bow shock and the magnetopause, and the waves are generally strongly Doppler shifted up to frequencies above the local ion cyclotron frequency. Downstream of the quasi‐parallel shock, the magnetosheath often exhibits large‐amplitude pulsations with wave periods around 10 s and peak‐to‐peak amplitudes of up to 100 nT that dominate the magnetic field structure. These waves are circularly left‐hand polarized with wave vectors in the direction of the local shock normal. The data suggest that they have been generated upstream of the shock and transmitted into the downstream region. Their occurrence rates maximize at the near‐parallel shock, where they are present approximately 10% of the time, and where they also show their largest wave powers. Some evidence is also found of waves with a right‐handed polarization in the spacecraft frame. These consist of both whistler waves above the local ion cyclotron frequency and ion cyclotron waves propagating against the magnetosheath flow with Doppler shifts exceeding the intrinsic wave frequency, which results in a change in their apparent polarization. These waves are in minority compared to the left‐handed observations, which indicates a preference for ion cyclotron waves propagating in the direction of the plasma flow.Key PointsWe investigate the properties of magnetosheath waves at MercuryIon cyclotron waves are common in the magnetosheath downstream of the quasi‐perpendicular shockLarge‐amplitude waves up to 100 nT peak to peak are observed downstream of the quasi‐parallel shockPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115984/1/jgra52042_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115984/2/jgra52042.pd

Cyclic reformation of a quasi-parallel bow shock at Mercury: MESSENGER observations

[1] We here document with magnetic field observations a passage of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft through Mercury's magnetosphere under conditions of a quasi-parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25-min-long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasiperiodicity allows us to study all aspects of the shock reconfiguration, including ultra-low-frequency waves in the upstream region and large-amplitude magnetic structures observed in the vicinity of the magnetosheath-solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak-to-peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large-amplitude magnetic structures, as is generally the case at the terrestrial bow shock under quasi-parallel conditions

ION ACCELERATION AT THE QUASI-PARALLEL BOW SHOCK: DECODING THE SIGNATURE OF INJECTION

Collisionless shocks are efficient particle accelerators. At Earth, ions with energies exceeding 100 keV are seen upstream of the bow shock when the magnetic geometry is quasi-parallel, and large-scale supernova remnant shocks can accelerate ions into cosmic rays energies. This energization is attributed to diffusive shock acceleration, however, for this process to become active the ions must first be sufficiently energized. How and where this initial acceleration takes place has been one of the key unresolved issues in shock acceleration theory. Using Cluster spacecraft observations, we study the signatures of ion reflection events in the turbulent transition layer upstream of the terrestrial bow shock, and with the support of a hybrid simulation of the shock, we show that these reflection signatures are characteristic of the first step in the ion injection process. These reflection events develop in particular in the region where the trailing edge of large-amplitude upstream waves intercept the local shock ramp and the upstream magnetic field changes from quasi-perpendicular to quasi-parallel. The dispersed ion velocity signature observed can be attributed to a rapid succession of ion reflections at this wave boundary. After the ions' initial interaction with the shock, they flow upstream along the quasi-parallel magnetic field. Each subsequent wave front in the upstream region will sweep the ions back toward the shock, where they gain energy with each transition between the upstream and the shock wave frames. Within three to five gyroperiods, some ions have gained enough parallel velocity to escape upstream, thus completing the injection process.Comment: 30 pages, 10 figures. Accepted for publication in The Astrophysical Journa

MESSENGER observations of dipolarization events in Mercury's magnetotail

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95196/1/jgra22066.pd

MESSENGER orbital observations of large‐amplitude Kelvin‐Helmholtz waves at Mercury's magnetopause

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95418/1/jgra21747.pd

MESSENGER observations of multiscale Kelvin‐Helmholtz vortices at Mercury

Observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in Mercury's magnetotail demonstrate for the first time that Na+ ions exert a dynamic influence on Mercury's magnetospheric system. Na+ ions are shown to contribute up to ~30% of the ion thermal pressure required to achieve pressure balance in the premidnight plasma sheet. High concentrations of planetary ions should lead to Na+ dominance of the plasma mass density in these regions. On orbits with northward‐oriented interplanetary magnetic field and high (i.e., >1 cm−3) Na+ concentrations, MESSENGER has often recorded magnetic field fluctuations near the Na+ gyrofrequency associated with the Kelvin‐Helmholtz (K‐H) instability. These nightside K‐H vortices are characteristically different from those observed on Mercury's dayside that have a nearly constant wave frequency of ~0.025 Hz. Collectively, these observations suggest that large spatial gradients in the hot planetary ion population at Mercury may result in a transition from a fluid description to a kinetic description of vortex formation across the dusk terminator, providing the first set of truly multiscale observations of the K‐H instability at any of the diverse magnetospheric environments explored in the solar system

MESSENGER observations of Alfvénic and compressional waves during Mercury's substorms

MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) magnetic field measurements during the substorm expansion phase in Mercury's magnetotail have been examined for evidence of low‐frequency plasma waves, e.g., Pi2‐like pulsations. It has been revealed that the By fluctuations accompanying substorm dipolarizations are consistent with pulses of field‐aligned currents near the high‐latitude edge of the plasma sheet. Detailed analysis of the By fluctuations reveals that they are near circularly polarized electromagnetic waves, most likely Alfvén waves. Soon afterward the plasma sheet thickened and MESSENGER detected a series of compressional waves. These Alfvénic and compressional waves have similar durations (10–20 s), suggesting that they may arise from the same source. Drawing on Pi2 pulsation models developed for Earth, we suggest that the Alfvénic and compressional waves reported here at Mercury may be generated by the quasi‐periodic sunward flow bursts in Mercury's plasma sheet. But because they are observed during the period with rapid magnetic field reconfiguration, we cannot fully exclude the possibility of standing Alfvén wave.Key PointsThe first observation of Pi2‐like pulsations during Mercury's substormAlfvénic and compressional waves were observed in the different regions of the plasma sheetWe proposed the sources for the plasma wavesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113132/1/grl53278.pd

New Perspectives on Solar Wind-Magnetosphere Coupling

The streaming plasma in the solar wind is a never ending source of energy, plasma, and momentum for planetary magnetospheres, and it continuously drives large-scale plasma convection systems in our magnetosphere and over our polar ionosphere. This coupling between the solar wind and the magnetosphere is primarily explained by two different processes: magnetic reconnection at high latitudes, which interconnects the interplanetary magnetic field (IMF) with the planetary dipole field, and low-latitude dynamos such as viscous interaction, where the streaming plasma in the solar wind may trigger waves and instabilities at the flanks of the magnetosphere, and thereby allow solar wind plasma to enter into the system.This work aims to further determine the nature and properties of these driving dynamos, both by statistical studies of their relative importance for ionospheric convection at Earth, and by assessment and analysis of the Kelvin-Helmholtz instability at Mercury, utilizing data from the MESSENGER spacecraft's first and third flyby of the planet.It is shown that the presence of the low-latitude dynamos is primarily dependent on the IMF direction: the driving is close to non-existent when the IMF is southward, but increases to the order of a third of the total ionospheric driving when the IMF turns northward (here, the magnitude of the driving is also shown to be dependent on the viscous parameters in the solar wind). The work also discusses the saturation of the reconnection generated potential, and shows that the terrestrial response follows a non-linear behavior for strong solar wind driving both when the IMF is southward and northward.Comparative studies of different magnetospheres provide an excellent path for increasing our understanding of space-related phenomena. Here, study of the Kelvin-Helmholtz instability at Mercury allows us to investigate how the different parameters of the system affect the mass, energy, and momentum transfer at the flanks of the magnetosphere. The large ion gyro radius expected is shown to develop a dawn-dusk asymmetry in the growth rates, with the dawn side as the more unstable of the two. This effect should be particularly visible when the planet is close to perihelion. Mercury's smaller scale size combined with the relatively high spacecraft velocity is also shown to provide excellent opportunities for studying the spatial structure of the waves, and a vortex reconstruction that can explain all the large-scale variations in the Kelvin-Helmholtz waves observed during MESSENGER's third Mercury flyby is presented.QC 2011040

On the Properties of Ionospheric Convection

The solar wind interaction with the magnetosphere-ionosphere system continuously drives plasma convection in the polar regions of the ionosphere. The flow velocity and the shape of the convection pattern are closely dependent on the interplanetary conditions, in particular the direction of the interplanetary magnetic field (IMF). The main driver of the system is considered to be magnetic reconnection between the IMF and the terrestrial field, a process that is most efficient during southward IMF when the magnetic fields at the dayside magnetopause are anti-parallell, and less efficient but still present when the IMF is northward. Additional driving may be caused by waves at the magnetopause flanks, where viscous effects can lead to an energy, momentum and plasma exchange across the boundary. In this work, we make use of the characteristics of the ionospheric convection and particle precipitation to investigate the nature of the driving dynamos, and large statistical data sets for steady solar wind conditions are used to derive the general behavior of the driving processes and their dependence on interplanetary conditions. The results show that the primary dynamo responsible for the convection in the boundary layer is closely dependent on the sign of the IMF Bz component, the average potential over the boundary layer region increases from <1 kV for steady southward IMF up to the order of 10kV for strictly northward conditions with reconnection poleward of the cusps, whereas the magnitude of magnetic field only has a minor influence at most. This could for example indicate that the magnetopause is more unstable to Kelvin-Helmholtz waves for parallel rather than anti-parallel magnetic fields, or that magnetic reconnection on the dayside suppresses other processes. It is well known that the ionospheric potential drop saturates during strong driving conditions and southward IMF. The results presented here also show that the same phenomenon occurs when the IMF is northward. This gives additional information on the physics governing the solar wind-magnetosphere-ionosphere interaction, and may impose new restrictions on the theories explaining the saturation