Global Ten-Moment Multifluid Simulations of the Solar Wind Interaction with Mercury: From the Planetary Conducting Core to the Dynamic Magnetosphere

For the first time, we explore the tightly coupled interior-magnetosphere system of Mercury by employing a three-dimensional ten-moment multifluid model. This novel fluid model incorporates the non-ideal effects including the Hall effect, inertia, and tensorial pressures that are critical for collisionless magnetic reconnection; therefore, it is particularly well suited for investigating $collisionless$ magnetic reconnection in Mercury's magnetotail and at the planet's magnetopause. The model is able to reproduce the observed magnetic field vectors, field-aligned currents, and cross-tail current sheet asymmetry (beyond the MHD approach) and the simulation results are in good agreement with spacecraft observations. We also study the magnetospheric response of Mercury to a hypothetical extreme event with an enhanced solar wind dynamic pressure, which demonstrates the significance of induction effects resulting from the electromagnetically-coupled interior. More interestingly, plasmoids (or flux ropes) are formed in Mercury's magnetotail during the event, indicating the highly dynamic nature of Mercury's magnetosphere.Comment: Geophysical Research Letters, in press, 17 pages, 4 (fancy) figure

The Structure of Martian Magnetosphere at the Dayside Terminator Region as Observed on MAVEN Spacecraft

We analyzed 44 passes of the MAVEN spacecraft through the magnetosphere, arranged by the angle between electric field vector and the projection of spacecraft position radius vector in the YZ plane in MSE coordinate system (${\theta}$ E ). All passes were divided into 3 angular sectors near 0{\deg}, 90{\deg} and 180{\deg} ${\theta}$ E angles in order to estimate the role of IMF direction in plasma and magnetic properties of dayside Martian magnetosphere. The time interval chosen was from January 17 through February 4, 2016 when MAVEN was crossing the dayside magnetosphere at SZA ~ 70{\deg}. Magnetosphere as the region with prevailing energetic planetary ions is always found between the magnetosheath and the ionosphere. 3 angular sectors of dayside interaction region in MSE coordinate system with different orientation of the solar wind electric field vector E = -1/c V x B showed that for each sector one can find specific profiles of the magnetosheath, the magnetic barrier and the magnetosphere. Plume ions originate in the northern MSE sector where motion electric field is directed from the planet. This electric field ejects magnetospheric ions leading to dilution of magnetospheric heavy ions population, and this effect is seen in some magnetospheric profiles. Magnetic barrier forms in front of the magnetosphere, and relative magnetic field magnitudes in these two domains vary. The average height of the boundary with ionosphere is ~530 km and the average height of the magnetopause is ~730 km. We discuss the implications of the observed magnetosphere structure to the planetary ions loss mechanism.Comment: 24 pages, 13 figure

Observations of Magnetic Reconnection and Plasma Dynamics in Mercury's Magnetosphere.

Mercury’s magnetosphere is formed as a result of the supersonic solar wind interacting with the planet’s intrinsic magnetic field. The combination of the weak planetary dipole moment and intense solar wind forcing of the inner heliosphere creates a unique space environment, which can teach us about planetary magnetospheres. In this work, we analyze the first in situ orbital observations at Mercury, provided by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Magnetic reconnection and the transport of plasma and magnetic flux are investigated using MESSENGER Magnetometer and Fast Imaging Plasma Spectrometer measurements. Here, we report our results on the effect of magnetic reconnection and plasma dynamics on Mercury’s space environment: (1) Mercury’s magnetosphere is driven by frequent, intense magnetic reconnection observed in the form of magnetic field components normal to the magnetopause, BN, and as helical bundles of flux, called magnetic flux ropes, in the cross-tail current sheet. The high reconnection rates are determined to be a direct consequence of the low plasma beta, the ratio of plasma to magnetic pressure, in the inner heliosphere. (2) As upstream solar wind conditions vary, we find that reconnection occurs at Mercury’s magnetopause for all orientations of the interplanetary magnetic field, independent of shear angle. During the most extreme solar wind forcing events, the influence of induction fields generated within Mercury’s highly conducting core are negated by erosion due to persistent magnetopause reconnection. (3) We present the first observations of Mercury’s plasma mantle, which forms as a result of magnetopause reconnection and allows solar wind plasma to enter into the high-latitude magnetotail through the dayside cusps. The energy dispersion observed in the plasma mantle protons is used to infer the cross-magnetosphere electric field, providing a direct measurement of solar wind momentum transferred into the system. We conclude that Mercury’s magnetosphere is a dynamic environment with constant plasma and magnetic flux circulation as a result of frequent and intense magnetic reconnection. These results are directly applicable to the understanding of geomagnetic storms at Earth, when coronal mass ejections produce solar wind parameters similar to those regularly experienced by Mercury.PhDAtmospheric, Oceanic and Space SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108893/1/gdibracc_1.pd

Mercury’s Solar Wind Interaction as Characterized by Magnetospheric Plasma Mantle Observations With MESSENGER

We analyze 94 traversals of Mercury’s southern magnetospheric plasma mantle using data from the MESSENGER spacecraft. The mean and median proton number densities in the mantle are 1.5 and 1.3 cm−3, respectively. For sodium number density these values are 0.004 and 0.002 cm−3. Moderately higher densities are observed on the magnetospheric dusk side. The mantle supplies up to 1.5 × 108 cm−2 s−1 and 0.8 × 108 cm−2 s−1 of proton and sodium flux to the plasma sheet, respectively. We estimate the cross‐electric magnetospheric potential from each observation and find a mean of ~19 kV (standard deviation of 16 kV) and a median of ~13 kV. This is an important result as it is lower than previous estimations and shows that Mercury’s magnetosphere is at times not as highly driven by the solar wind as previously thought. Our values are comparable to the estimations for the ice giant planets, Uranus and Neptune, but lower than Earth. The estimated potentials do have a very large range of values (1–74 kV), showing that Mercury’s magnetosphere is highly dynamic. A correlation of the potential is found to the interplanetary magnetic field (IMF) magnitude, supporting evidence that dayside magnetic reconnection can occur at all shear angles at Mercury. But we also see that Mercury has an Earth‐like magnetospheric response, favoring −BZ IMF orientation. We find evidence that −BX orientations in the IMF favor the southern cusp and southern mantle. This is in agreement with telescopic observations of exospheric emission, but in disagreement with modeling.Key PointsProton and sodium ions in Mercury’s southern plasma mantle have mean number densities of ~1.5 and 0.004 cm−3, respectivelyThe highest estimates of mantle proton and sodium flux supply to the plasma sheet are 1.5 × 108 cm−2 s−1 and 0.8 × 108 cm−2 s−1, respectivelyAn average cross‐electric magnetospheric potential of ~19 kV is determined, which is enhanced for increased IMF strength and −BZPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142326/1/jgra53846.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142326/2/jgra53846_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142326/3/jgra53846-sup-0001-data_si.pd

Transport of Mass and Energy in Mercury’s Plasma Sheet

We examined the transport of mass and energy in Mercury’s plasma sheet (PS) using MESSENGER magnetic field and plasma measurements obtained during 759 PS crossings. Regression analysis of proton density and plasma pressure shows a strong linear relationship. We calculated the polytropic index γ for Mercury’s PS to be ~0.687, indicating that the plasma in the tail PS behaves nonadiabatically as it is transported sunward. Using the average magnetic field intensity of Mercury’s tail lobe as a proxy for magnetotail activity level, we demonstrated that γ is lower during active time periods. A minimum in γ was observed at R ~ 1.4 RM, which coincides with previously observed location of Mercury’s substorm current wedge. We suggest that the nonadiabatic behavior of plasma as it is transported into Mercury’s nearâ tail region is primarily driven by particle precipitation and particle scattering due to large loss cone and particle acceleration effect, respectively.Plain Language SummaryThe transport process of mass and energy within Mercury’s magnetotail remains unexplored until now. The availability of in situ magnetic field and plasma measurements from National Aeronautics and Space Administration’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft provides us with the first opportunity to study the thermodynamic properties of particles within sunward convecting closed flux tubes in the plasma sheet. In this study, we study how mass and energy are transported in Mercury’s magnetotail by investigating the relationship between the thermal pressure and number density of the plasma in Mercury’s plasma sheet given by the equation of state in magnetohydrodynamics theory. We determined, for the first time, that the plasma behaves nonadiabatically as it is transported sunward toward Mercury. We suggest that precipitation of particles due to Mercury’s large loss cone and demagnetization of particles due to finite gyroradius effect contributes to this nonadiabatic behavior of plasma in the plasma sheet. Our results have major implications in our understanding of particle sources and sinks mechanisms in Mercury’s magnetotail.Key PointsWe calculated the value of polytropic index γ for Mercury’s plasma sheet to be ~0.687, which is smaller than 5/3 (adiabatic)Nonadiabatic plasma behavior is driven by ion precipitation and ion demagnetization due to large loss cone and finite gyroradius effectWe demonstrated that γ is lower during active time and determined a relationship between γ and the location of flow breaking regionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147033/1/grl58293_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147033/2/grl58293.pd

Upstream Ultra‐Low Frequency Waves Observed by MESSENGER’s Magnetometer: Implications for Particle Acceleration at Mercury’s Bow Shock

We perform the first statistical analysis of the main properties of waves observed in the 0.05–0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the “30 s” waves observed at the Earth’s foreshock, the normalized wave amplitude (δB/|B0|∼0.2) and occurrence rate (∼0.5%) are much smaller. This could be associated with relatively lower backstreaming proton fluxes, the smaller foreshock size and/or less stable solar wind (SW) conditions around Mercury. Furthermore, we estimate that the speed of resonant backstreaming protons in the SW reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the SW speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.Key PointsWe perform the first statistical analysis (4,536 events) of the main properties of the lowest‐frequency waves in the Hermean foreshockSmall normalized wave amplitude (0.2) and occurrence (0.5%) are likely due to low backstreaming proton flux and variable external conditionsThe normalized backstreaming protons speed (∼0.95–2.6) suggests similar acceleration processes occur at several planetary shocksPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155492/1/grl60476.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155492/2/grl60476_am.pd

MESSENGER Observations of Mercury’s Nightside Magnetosphere Under Extreme Solar Wind Conditions: Reconnectionâ Generated Structures and Steady Convection

Mercury’s nightside magnetosphere is investigated under the impact of a coronal mass ejection (CME) and a highâ speed stream (HSS) with MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) observations. The CME was shown to produce a low plasma β (ratio of thermal pressure to magnetic pressure) magnetosheath, while the HSS creates a higher β magnetosheath. Reconnection at the dayside magnetopause was found to be stronger during the CME than the HSS, but both were stronger than the average condition (Slavin et al., 2014, https://doi.org/10.1002/2014JA020319). Here we show that the CME and HSS events produced large numbers of flux ropes and dipolarization fronts in the plasma sheet. The occurrence rates for the structures were approximately 2 orders of magnitude higher than under average conditions with the rates during CME’s being twice that of HSS’s. The flux ropes appeared as quasiperiodic flux rope groups. Each group lasted approximately 1 min and had a few large flux ropes followed by several smaller flux ropes. The lobe magnetic flux accounted for around half of the Mercury’s available magnetic flux with the flux during CME’s being larger than that of HSS’s. The CME produced a more dynamic nightside magnetosphere than the HSS. Further, for the CME event, the tail magnetic reconnection produced a distorted Hall magnetic field pattern and the Xâ line had a dawnâ dusk extent of 20% of the tail width. No magnetic flux loading and unloading events were observed suggesting that, during these intense driving conditions, Mercury’s magnetosphere responded with a type of quasiâ steady convection as opposed to the tail flux loadingâ unloading events seen at Earth.Key PointsCoronal mass ejections drive more intense nightside reconnection than high speed streamsUnder extreme conditions, magnetic reconnection produces a distorted Hall magnetic field pattern in the plasma sheetContinued intense solar wind forcing does not produce substorm magnetic flux loading and unloading of tail lobe instead steady convectionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154381/1/jgra55534_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154381/2/jgra55534.pd

Large‐Amplitude Oscillatory Motion of Mercury’s Cross‐Tail Current Sheet

We surveyed 4 years of MESSENGER magnetic field data and analyzed intervals with observations of large‐amplitude oscillatory motions of Mercury’s cross‐tail current sheet, or flapping waves, characterized by a decrease in magnetic field intensity and multiple reversals of BX, oscillating with a period on the order of ~4 – 25 seconds. We performed minimum variance analysis (MVA) on each flapping wave event to determine the current sheet normal. Statistical results showed that the flapping motion of the current sheet caused it to warp and tilt in the y‐z plane, which suggests that these flapping waves are kink‐type waves propagating in the cross‐tail direction of Mercury’s magnetotail. The occurrence of flapping waves shows a strong preference in Mercury’s duskside plasma sheet. We compared our results with the magnetic double‐gradient instability model and examined possible flapping wave excitation mechanism theories from internal (e.g., finite gyroradius effects of planetary sodium ions Na+ on magnetosonic waves) and external (e.g., solar wind variations and K‐H waves) sources.Key PointsLarge‐amplitude oscillations of Mercury’s cross‐tail current sheet (or flapping waves) with period of ~4 – 25 s were observedFlapping motion of Mercury’s cross‐tail current sheet warped and tilted the current sheet in the y‐z planeFlapping waves preferentially occur in Mercury’s duskside current sheetPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156232/2/jgra55803.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156232/1/jgra55803_am.pd