MESSENGER Observations of Fast Plasma Flows in Mercury's Magnetotail

Dewey, R. M.; Raines, J. M.; Sun, W.; Slavin, J. A.; Poh, Gangkai

doi:10.1029/2018gl079056

Cited by 27 publications

(56 citation statements)

References 47 publications

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“…Indeed, for quiet solar wind conditions when MESSENGER crossed the nightside plasma sheet (in 2013–2015), Dewey et al. (2018) showed that the average solar wind density roughly halved to around 3.1 cm −3 , to which our modeled solar wind densities are in a reasonable range of values. It is therefore likely that the low solar wind ion density in the plasma sheet of our model is indeed a result of undisturbed, low ram pressure conditions.…”

Section: Simulation Resultssupporting

confidence: 54%

“…By investigating multiple dipolarization events in Mercury's wakeside, Dewey et al. (2018) found fast plasma flows in Mercury's vicinity that move duskward with an average velocity magnitude of 300 km/s. Within surface altitudes analyzed by these authors, our modeled solar wind has a bulk velocity in the range of 200–700 km/s in Figure 3f, encompassing the observed value.…”

Section: Simulation Resultsmentioning

confidence: 99%

“…(2013) showed that the velocity of sodium neutrals is about three orders of magnitude smaller than the bulk velocity of the solar wind. The tenuous sodium exosphere is partially photoionized, and sodium ions with an average density in the range of 5.1·10 −3 –2 cm −3 have been observed by the Fast Imaging Plasma Spectrometer (FIPS Andrews et al., 2007) throughout Mercury's magnetosphere (Dewey et al., 2018; DiBraccio et al., 2015; Gershman et al., 2014; Korth et al., 2014; Raines et al., 2014, 2015). These in situ observations suggest the density of the exospheric ions to be multiple orders of magnitude lower than the average upstream solar wind proton density of about 60 cm −3 (Winslow et al., 2013), thereby suggesting that the sodium exo‐ionosphere has no dynamic influence on the current systems within the magnetosphere.…”

Section: Introductionmentioning

confidence: 99%

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Influence of Mercury's Exosphere on the Structure of the Magnetosphere

et al. 2020

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Mercury is embedded in a tenuous and highly anisotropic sodium exosphere, generated mainly by plasma‐surface interactions. The absolute values of the sodium ion density are still under debate. Observations by MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) instrument suggest the density of exospheric ions to be several orders of magnitude lower than the upstream solar wind density, indicating that the sodium exosphere has no substantial influence on the magnetospheric current systems. However, MESSENGER magnetic field observations of field line resonances revealed sodium ion densities comparable to the upstream solar wind density. To investigate how a dense exosphere would affect the current systems within Mercury's magnetosphere, we apply an established hybrid (kinetic ions, fluid electrons) model and conduct multiple model runs with gradually increasing exospheric density, ranging from no sodium ions at all to comet‐like configurations. We demonstrate how a sufficiently dense exosphere leads to self‐shielding of the sodium ion population from the ambient electric field and a significant inflation and symmetrization of Mercury's magnetosphere, which is decreasingly affected by the dipole offset. Once the sodium ion density is sufficiently high, Region 2 field‐aligned currents emerge close to the planet. The modeled Region 2 currents are located below the orbit of MESSENGER, thereby providing a possible explanation for the absence of these currents in observations. The sodium exosphere also closes a significant fraction of the Region 1 currents through Pedersen and Hall currents before the “guiding” magnetic field lines even reach the planetary surface. The modeled sodium ion and solar wind densities agree well with observations.

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Section: Simulation Resultssupporting

confidence: 54%

Section: Simulation Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Influence of Mercury's Exosphere on the Structure of the Magnetosphere

et al. 2020

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“…In previous studies at Mercury, Sun et al () has shown clear dawn‐dusk asymmetry of dipolarization fronts in the near‐Mercury‐neutral‐line region with more dipolarization fronts on the dawnside plasma sheet than on the duskside plasma sheet. The following studies on the dipolarization fronts in the near‐Mercury plasma sheet, proton energization and heating, energetic electrons and proton bulk flows have shown the similar dawn‐dusk asymmetries (Dewey et al, , ; Sun et al, ).…”

Section: Messenger Observations In the Nightside Plasma Sheetmentioning

confidence: 96%

Studying Dawn‐Dusk Asymmetries of Mercury's Magnetotail Using MHD‐EPIC Simulations

et al. 2019

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MESSENGER has observed a lot of dawn-dusk asymmetries in Mercury's magnetotail, such as the asymmetries of the cross-tail current sheet thickness and the occurrence of flux ropes, dipolarization events, and energetic electron injections. In order to obtain a global pictures of Mercury's magnetotail dynamics and the relationship between these asymmetries, we perform global simulations with the magnetohydrodynamics with embedded particle-in-cell (MHD-EPIC) model, where Mercury's magnetotail region is covered by a PIC code. Our simulations show that the dawnside current sheet is thicker, the plasma density is larger, and the electron pressure is higher than the duskside. Under a strong interplanetary magnetic field driver, the simulated reconnection sites prefer the dawnside. We also found the dipolarization events and the planetward electron jets are moving dawnward while they are moving toward the planet, so that almost all dipolarization events and high-speed plasma flows concentrate in the dawn sector. The simulation results are consistent with MESSENGER observations.

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“…Here we have surveyed all four years of the MESSENGER magnetometer (MAG) (sample rate 20 s; Anderson et al, 2007) and the Fast Imaging Plasma Sensor (FIPS) (complete energy per charge and composition scan of ions up to 13 keV/e in 10 s) (Andrews et al, 2007). Due to the placement of FIPS on the MESSENGER spacecraft, full plasma moments can only be derived under certain sets of assumptions (Raines et al, 2011;Gershman et al, 2012, Gershman et al, 2013Dewey et al, 2018). Because those assumptions are not typically met in the magnetosheath plasma analyzed here, we present only energy spectra and observed ion densities without correction for any plasma outside the field of view (Raines et al, 2013).…”

Section: Disappearing Dayside Magnetosphere Eventsmentioning

confidence: 99%

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

et al. 2019

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MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements taken during passes over Mercury's dayside hemisphere indicate that on four occasions the spacecraft remained in the magnetosheath even though it reached altitudes below 300 km. During these disappearing dayside magnetosphere (DDM) events, the spacecraft did not encounter the magnetopause until it was at very high magnetic latitudes, ~66 to 80°. These DDM events stand out with respect to their extremely high solar wind dynamic pressures, Psw ~140 to 290 nPa, and intense southward magnetic fields, Bz ~ −100 to −400 nT, measured in the magnetosheath. In addition, the bow shock was observed very close to the surface during these events with a subsolar altitude of ~1,200 km. It is suggested that DDM events, which are closely associated with coronal mass ejections, are due to solar wind compression and/or reconnection‐driven erosion of the dayside magnetosphere. The very low altitude of the bow shock during these events strongly suggests that the solar wind impacts much of Mercury's sunlit hemisphere during these events. More study of these disappearing dayside events is required, but it is likely that solar wind sputtering of neutrals from the surface into the exosphere maximizes during these intervals.

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MESSENGER Observations of Fast Plasma Flows in Mercury's Magnetotail

Cited by 27 publications

References 47 publications

Influence of Mercury's Exosphere on the Structure of the Magnetosphere

Influence of Mercury's Exosphere on the Structure of the Magnetosphere

Studying Dawn‐Dusk Asymmetries of Mercury's Magnetotail Using MHD‐EPIC Simulations

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

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