Mercury's cross‐tail current sheet: Structure, X‐line location and stress balance

Poh, Gangkai; Slavin, J. A.; Jia, Xianzhe; Raines, J. M.; Imber, Suzanne M.; Sun, W.; Gershman, D. J.; DiBraccio, G. A.; Genestreti, K. J.; Smith, A. W.

doi:10.1002/2016gl071612

Cited by 52 publications

(101 citation statements)

References 45 publications

(56 reference statements)

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“…From the average ratio of magnetic field strength at the end of the dipolarization front to the strength at the beginning, we estimate that the typical betatron acceleration would increase a particle's perpendicular energy by a factor of ~2.3. From the ratio of dipole field line lengths above Mercury's surface from the typical nightside X‐line location at X MSM = −3 R M (Poh et al, ) to the typical dipolarization‐injection event location at L ‐shell ~1.5 R M , we estimate that typical Fermi acceleration would increase a particle's parallel energy by a factor of ~2.7. We expect electrons to behave adiabatically, whereas ions behave nonadiabatically and cannot access these rates of energization fully.…”

Section: Discussionmentioning

confidence: 88%

Energetic Electron Acceleration and Injection During Dipolarization Events in Mercury's Magnetotail

et al. 2017

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Energetic particle bursts associated with dipolarization events within Mercury's magnetosphere were first observed by Mariner 10. The events appear analogous to particle injections accompanying dipolarization events at Earth. The Energetic Particle Spectrometer (3 s resolution) aboard MESSENGER determined the particle bursts are composed entirely of electrons with energies ≳ 300 keV. Here we use the Gamma‐Ray Spectrometer high‐time‐resolution (10 ms) energetic electron measurements to examine the relationship between energetic electron injections and magnetic field dipolarization in Mercury's magnetotail. Between March 2013 and April 2015, we identify 2,976 electron burst events within Mercury's magnetotail, 538 of which are closely associated with dipolarization events. These dipolarizations are detected on the basis of their rapid (~2 s) increase in the northward component of the tail magnetic field (ΔBz ~30 nT), which typically persists for ~10 s. Similar to those at Earth, we find that these dipolarizations appear to be low‐entropy, depleted flux tubes convecting planetward following the collapse of the inner magnetotail. We find that electrons experience brief, yet intense, betatron and Fermi acceleration during these dipolarizations, reaching energies ~130 keV and contributing to nightside precipitation. Thermal protons experience only modest betatron acceleration. While only ~25% of energetic electron events in Mercury's magnetotail are directly associated with dipolarization, the remaining events are consistent with the Near‐Mercury Neutral Line model of magnetotail injection and eastward drift about Mercury, finding that electrons may participate in Shabansky‐like closed drifts about the planet. Magnetotail dipolarization may be the dominant source of energetic electron acceleration in Mercury's magnetosphere.

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Section: Discussionmentioning

confidence: 88%

Energetic Electron Acceleration and Injection During Dipolarization Events in Mercury's Magnetotail

et al. 2017

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“…However, lacking multipoint observations, the average lobe field (<| B | Lobe >) was recorded shortly before each plasma sheet passage (an average of 4 m 43 s before). A background model lobe field can then be subtracted:

| B |_{Lobe}^{Model} (| X_{MSM}^{'} |) = A | X_{MSM}^{'} |^{G} + B_{0}

where the values of the variables A , G , and B 0 were determined to be 86.4, −3.1, and 41.4, respectively, by Poh et al []. The aim of the subtraction is to provide an estimate of whether the lobe field is relatively enhanced or diminished compared to other orbits.…”

Section: Discussionmentioning

confidence: 99%

“…The fraction of crossings during which (MVA‐confirmed) flux ropes were observed plotted against the relative lobe field strength measured just prior to the plasma sheet encounter. The model lobe field strength (from equation ) [ Poh et al , ] is subtracted from the average lobe field (measured shortly before entering the plasma sheet boundary layer) to calculate the relative lobe field strength. The color bar indicates the number of crossings present in each bin.…”

Section: Discussionmentioning

confidence: 99%

Flux ropes in the Hermean magnetotail: Distribution, properties, and formation

et al. 2017

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An automated method was applied to identify magnetotail flux rope encounters in MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) magnetometer data. The method identified significant deflections of the north‐south component of the magnetic field coincident with enhancements in the total field or dawn‐dusk component. Two hundred forty‐eight flux ropes are identified that possess well‐defined minimum variance analysis (MVA) coordinate systems, with clear rotations of the field. Approximately 30% can be well approximated by the cylindrically symmetric, linearly force‐free model. Flux ropes are most common moving planetward, in the postmidnight sector. Observations are intermittent, with the majority (61%) of plasma sheet passages yielding no flux ropes; however, the peak rate of flux ropes during a reconnection episode is ∼5 min−1. Overall, the peak postmidnight rate is ∼0.25 min−1. Only 25% of flux ropes are observed in isolation. The radius of flux ropes is comparable to the ion inertial length within Mercury's magnetotail plasma sheet. No clear statistical separation is observed between tailward and planetward moving flux ropes, suggesting the near‐Mercury neutral line (NMNL) is highly variable. Flux ropes are more likely to be observed if the preceding lobe field is enhanced over background levels. A very weak correlation is observed between the flux rope core field and the preceding lobe field orientation; a stronger relationship is found with the orientation of the field within the plasma sheet. The core field strength measured is ∼6 times stronger than the local dawn‐dusk plasma sheet magnetic field.

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“…HCS fitting only employs magnetic field measurements in the southern hemisphere to mitigate the effects from dipole magnetic field, since the MESSENGER is closer to the planet in the northern hemisphere. The measured magnetic field has been transformed into the local coordinate system in the HCS fitting (Poh et al, ; Rong et al, ; Sun et al, ). Figure e shows the fitting result.…”

Section: Magnetotail Flux Rope Embedded In Current Sheetmentioning

confidence: 99%

A Statistical Study of the Force Balance and Structure in the Flux Ropes in Mercury's Magnetotail

et al. 2019

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This study presents a statistical investigation of the force balance and structures in the flux ropes in Mercury's magnetotail plasma sheet by using the measurements of MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER). One hundred sixty‐eight flux ropes were identified from the 14 hot seasons of MESSENGER from 11 March 2011 to 30 April 2015, and 143 of them show clear magnetic field enhancements with the core field being ≥20% higher than the background magnetic field. The investigation on the force balance of these 143 flux ropes shows that magnetic pressure gradient force cannot be solely balanced by magnetic tension force, implying that thermal plasma pressure gradient force cannot be neglected in the flux ropes. We employ a non‐force‐free model considering the contribution of thermal pressure to resolve the physical properties of flux ropes in Mercury's magnetotail. Twenty‐eight flux ropes are obtained through the fitting to the non‐force‐free model. The flux ropes are found to be consistent with the flattened structures, in which the mean semimajor is ∼851 km and semiminor is ∼333 km, both are several times the local proton inertial length. The average core field is estimated to be ∼57.5 nT, and flux content is ∼0.019 MWb, much larger than the previous results obtained from force‐free flux rope model. The importance of thermal pressure gradient in the force balance of the flux ropes and the flattened structure indicates that the flux ropes in Mercury's magnetotail plasma sheet are mostly in early stage of the evolution, and still contain enough plasma to affect their magnetic structures.

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Mercury's cross‐tail current sheet: Structure, X‐line location and stress balance

Cited by 52 publications

References 45 publications

Energetic Electron Acceleration and Injection During Dipolarization Events in Mercury's Magnetotail

Energetic Electron Acceleration and Injection During Dipolarization Events in Mercury's Magnetotail

Flux ropes in the Hermean magnetotail: Distribution, properties, and formation

A Statistical Study of the Force Balance and Structure in the Flux Ropes in Mercury's Magnetotail

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