15 research outputs found
LargeâAmplitude Oscillatory Motion of Mercuryâs CrossâTail Current Sheet
We surveyed 4Â years of MESSENGER magnetic field data and analyzed intervals with observations of largeâamplitude oscillatory motions of Mercuryâs crossâtail 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 â 25Â 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 yâz plane, which suggests that these flapping waves are kinkâtype waves propagating in the crossâtail direction of Mercuryâs magnetotail. The occurrence of flapping waves shows a strong preference in Mercuryâs duskside plasma sheet. We compared our results with the magnetic doubleâgradient 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 KâH waves) sources.Key PointsLargeâamplitude oscillations of Mercuryâs crossâtail current sheet (or flapping waves) with period of ~4 â 25Â s were observedFlapping motion of Mercuryâs crossâtail current sheet warped and tilted the current sheet in the yâz planeFlapping waves preferentially occur in Mercuryâs 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
FOREWORD
We analyzed MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during 319 crossings of Mercuryâs crossâ tail current sheet. We found that the measured BZ in the current sheet is higher on the dawnside than the duskside by a factor of â 3 and the asymmetry decreases with downtail distance. This result is consistent with expectations based upon MHD stress balance. The magnetic fields threading the more stretched current sheet in the duskside have a higher plasma beta than those on the dawnside, where they are less stretched. This asymmetric behavior is confirmed by mean current sheet thickness being greatest on the dawnside. We propose that heavy planetary ion (e.g., Na+) enhancements in the duskside current sheet provides the most likely explanation for the dawnâ dusk current sheet asymmetries. We also report the direct measurement of Mercuryâs substorm current wedge (SCW) formation and estimate the total current due to pileup of magnetic flux to be â 11Ă kA. The conductance at the foot of the field lines required to close the SCW current is found to be â 1.2Ă S, which is similar to earlier results derived from modeling of Mercuryâs Region 1 fieldâ aligned currents. Hence, Mercuryâs regolith is sufficiently conductive for the current to flow radially then across the surface of Mercuryâs highly conductive iron core. Mercury appears to be closely coupled to its nightside magnetosphere by mass loading of upward flowing heavy planetary ions and electrodynamically by fieldâ aligned currents that transfer momentum and energy to the nightside auroral oval crust and interior. Heavy planetary ion enhancements in Mercuryâs duskside current sheet provide explanation for crossâ tail asymmetries found in this study. The total current due to the pileup of magnetic flux and conductance required to close the SCW current is found to be â 11Ă kA and 1.2Ă S. Mercury is coupled to magnetotail by mass loading of heavy ions and fieldâ aligned currents driven by reconnectionâ related fast plasma flow.Key PointsHeavy planetary ion enhancements in Mercuryâs duskside current sheet provide explanation for crossâ tail asymmetries found in this studyThe total current due to the pileup of magnetic flux and conductance required to close the SCW current is found to be almost equal to 11Ă kA and 1.2Ă SMercury is coupled to magnetotail by mass loading of heavy ions and fieldâ aligned currents driven by reconnectionâ related fast plasma flowPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138879/1/jgra53698.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138879/2/jgra53698_am.pd
Ionâscale structure in Mercuryâs magnetopause reconnection diffusion region
The strength and time dependence of the electric field in a magnetopause diffusion region relate to the rate of magnetic reconnection between the solar wind and a planetary magnetic field. Here we use ~150âms measurements of energetic electrons from the Mercury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft observed over Mercuryâs dayside polar cap boundary (PCB) to infer such smallâscale changes in magnetic topology and reconnection rates. We provide the first direct measurement of open magnetic topology in flux transfer events at Mercury, structures thought to account for a significant portion of the open magnetic flux transport throughout the magnetosphere. In addition, variations in PCB latitude likely correspond to intermittent bursts of ~0.3â3âmV/m reconnection electric fields separated by ~5â10âs, resulting in average and peak normalized dayside reconnection rates of ~0.02 and ~0.2, respectively. These data demonstrate that structure in the magnetopause diffusion region at Mercury occurs at the smallest ion scales relevant to reconnection physics.Key PointsEnergetic electrons at Mercury map magnetic topology at ~150âmsFirst direct observation of flux transfer event openâfield topology at MercuryModulations of the reconnection rate at Mercury occur at ion kinetic scalesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/133575/1/grl54476_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133575/2/grl54476.pd
Solar wind forcing at Mercury: WSA-ENLIL model results
Analysis and interpretation of observations from the MESSENGER spacecraft in orbit about Mercury require knowledge of solar wind âforcingâ parameters. We have utilized the Wang-Sheeley-Arge (WSA)-ENLIL solar wind modeling tool in order to calculate the values of interplanetary magnetic field (IMF) strength (B), solar wind velocity (V) and density (n), ram pressure (~nV2), cross-magnetosphere electric field (VâĂâB), AlfvĂŠn Mach number (MA), and other derived quantities of relevance for solar wind-magnetosphere interactions. We have compared upstream MESSENGER IMF and solar wind measurements to see how well the ENLIL model results compare. Such parameters as solar wind dynamic pressure are key for determining the Mercury magnetopause standoff distance, for example. We also use the relatively high-time-resolution B-field data from MESSENGER to estimate the strength of the product of the solar wind speed and southward IMF strength (Bs) at Mercury. This product VBs is the electric field that drives many magnetospheric dynamical processes and can be compared with the occurrence of energetic particle bursts within the Mercury magnetosphere. This quantity also serves as input to the global magnetohydrodynamic and kinetic magnetosphere models that are being used to explore magnetospheric and exospheric processes at Mercury. Moreover, this modeling can help assess near-real-time magnetospheric behavior for MESSENGER or other mission analysis and/or ground-based observational campaigns. We demonstrate that this solar wind forcing tool is a crucial step toward bringing heliospheric science expertise to bear on planetary exploration programs
Properties and Acceleration Mechanisms of Electrons Up To 200 keV Associated With a Flux Rope Pair and Reconnection XâLines Around It in Earth's Plasma Sheet
The properties and acceleration mechanisms of electrons (<200 keV) associated with a pair of tailward traveling flux ropes and accompanied reconnection X-lines in Earth's plasma sheet are investigated with MMS measurements. Energetic electrons are enhanced on both boundaries and core of the flux ropes. The power-law spectra of energetic electrons near the X-lines and in flux ropes are harder than those on flux rope boundaries. Theoretical calculations show that the highest energy of adiabatic electrons is a few keV around the X-lines, tens of keV immediately downstream of the X-lines, hundreds of keV on the flux rope boundaries, and a few MeV in the flux rope cores. The X-lines cause strong energy dissipation, which may generate the energetic electron beams around them. The enhanced electron parallel temperature can be caused by the curvature-driven Fermi acceleration and the parallel electric potential. Betatron acceleration due to the magnetic field compression is strong on flux rope boundaries, which enhances energetic electrons in the perpendicular direction. Electrons can be trapped between the flux rope pair due to mirror force and parallel electric potential. Electrostatic structures in the flux rope cores correspond to potential drops up to half of the electron temperature. The energetic electrons and the electron distribution functions in the flux rope cores are suggested to be transported from other dawn-dusk directions, which is a 3-dimensional effect. The acceleration and deceleration of the Betatron and Fermi processes appear alternately indicating that the magnetic field and plasma are turbulent around the flux ropes
Flux transfer event observation at Saturn's dayside magnetopause by the Cassini spacecraft
We present the first observation of a flux rope at Saturn's dayside magnetopause. This is an important result because it shows that the Saturnian magnetopause is conducive to multiple X-line reconnection and flux rope generation. Minimum variance analysis shows that the magnetic signature is consistent with a flux rope. The magnetic observations were well fitted to a constant-Îą force-free flux rope model. The radius and magnetic flux content of the rope are estimated to be 4600â8300 km and 0.2â0.8 MWb, respectively. Cassini also observed five traveling compression regions (remote signatures of flux ropes), in the adjacent magnetosphere. The magnetic flux content is compared to other estimates of flux opening via reconnection at Saturn
Seven Sisters: a mission to study fundamental plasma physical processes in the solar wind and a pathfinder to advance space weather prediction
This paper summarizes the Seven Sisters solar wind mission concept and the outstanding science questions motivating the mission science objectives. The Seven Sisters mission includes seven individual spacecraft designed to uncover fundamental physical processes in the solar wind and provides up to â 2Â days of advanced space weather warnings for 550 Earth days during the mission. The mission will collect critical measurements of the thermal and suprathermal plasma and magnetic fields, utilizing, for the first time, VenusâSun Lagrange points. The multi-spacecraft configuration makes it possible to distinguish between spatial and temporal changes, define gradients, and quantify cross-scale transport in solar wind structures. Seven Sisters will determine the 3-D structure of the solar wind and its transient phenomena and their evolution in the inner heliosphere. Data from the Seven Sisters mission will allow the identification of physical processes and the quantification of the relative contribution of different mechanisms responsible for suprathermal particle energization in the solar wind
On the Growth and Development of NonâLinear KelvinâHelmholtz Instability at Mars: MAVEN Observations
In this study, we have analyzed Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of fields and plasma signatures associated with an encounter of fully developed KelvinâHelmholtz (KâH) vortices at the northern polar terminator along Marsâ induced magnetosphere boundary. The signatures of the KâH vortices event are: (a) quasiâperiodic, âbipolarâlikeâ sawtooth magnetic field perturbations, (b) corresponding density decrease, (c) tailward enhancement of plasma velocity for both protons and heavy ions, (d) coâexistence of magnetosheath and planetary plasma in the region prior to the sawtooth magnetic field signature (i.e., mixing region of the vortex structure), and (e) pressure enhancement (minimum) at the edge (center) of the sawtooth magnetic field signature. Our results strongly support the scenario for the nonâlinear growth of KâH instability along Marsâ induced magnetosphere boundary, where a plasma flow difference between the magnetosheath and inducedâmagnetospheric plasma is expected. Our findings are also in good agreement with 3âdimensional local magnetohydrodynamics simulation results. MAVEN observations of protons with energies greater than 10Â keV and results from the WalĂŠn analyses suggests the possibility of particle energization within the mixing region of the KâH vortex structure via magnetic reconnection, secondary instabilities or other turbulent processes. We estimate the lower limit on the KâH instability linear growth rate to be âź5.84Â ĂÂ 10â3 sâ1. For these vortices, we estimate the instantaneous atmospheric ion escape flux due to the detachment of plasma clouds during the late nonâlinear stage of KâH instability to be âź5.90Â ĂÂ 1026 particles/s. Extrapolation of loss rates integrated across time and space will require further work.Key PointsMars Atmosphere and Volatile EvolutioN (MAVEN) observed magnetic field and plasma signatures consistent with the encounter of fully developed KelvinâHelmholtz (KâH) vortices along Marsâ induced magnetospheric boundary (IMB)Close agreement between 3âD magnetohydrodynamics simulation result and MAVEN observation support the scenario for KâH instability occurrence along Marsâ IMBWe estimated the instantaneous atmospheric ion escape flux due to detachment of plasma clouds from KâH instability to be âź5.9Â ĂÂ 1026Â sâ1Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/170215/1/jgra56662.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/170215/2/jgra56662_am.pd