J. P. Eastwood

Average properties of the magnetic reconnection ion diffusion region in the Earth's magnetotail: The 2001–2005 Cluster observations and comparison with simulations

[1]xa0Magnetic reconnection plays a key role in the circulation of plasma through the Earths magnetosphere. As such, the Earths magnetotail is an excellent natural laboratory for the study of reconnection and in particular the diffusion region. To address important questions concerning observational occurrence rates and average properties, the Cluster data set from 2001–2005 has been systematically examined for encounters with reconnection X lines and ion diffusion regions in the Earths magnetotail. This survey of 175 magnetotail passes resulted in a sample of 33 correlated field and flow reversals. Eighteen events exhibited electric and magnetic field perturbations qualitatively consistent with the predictions of antiparallel Hall reconnection and could be identified as diffusion region encounters. The magnitudes of both the Hall magnetic and electric field were found to vary from event to event. When normalized against the inflow magnetic field and the current sheet number density the average peak Hall magnetic field was found to be 0.39 ± 0.16, the average peak Hall electric field was found to be 0.33 ± 0.18, and the average out of plane (reconnection) electric field was found to be ∼0.04. Good quantitative agreement was found between these results and a large, appropriately renormalized particle-in-cell simulation of reconnection. In future missions, the magnitude of the total DC electric field may be a useful tool for automatically identifying ion diffusion region encounters.

Evidence for collisionless magnetic reconnection at Mars

[1]xa0Using data from Mars Global Surveyor (MGS) in combination with Particle-In-Cell (PIC) simulations of reconnection, we present the first direct evidence of collisionless magnetic reconnection at Mars. The evidence indicates that the spacecraft passed through the diffusion region where reconnection is initiated and observed the magnetic field signatures of differential electron and ion motion – the Hall magnetic field – that uniquely indicate the reconnection process. These are the first such in-situ reconnection observations at an astronomical body other than the Earth. Reconnection may be the source of Mars recently discovered auroral activity and the changing boundaries of the closed regions of crustal magnetic field.

Foreshock bubbles and their global magnetospheric impacts

[1]xa0We employ 2.5-D electromagnetic, hybrid simulations that treat ions kinetically via particle-in-cell methods and electrons as a massless fluid to study the formation and properties of a new structure named the foreshock bubble upstream from the bow shock. This structure forms due to changes in the interplanetary magnetic field (IMF) associated with solar wind discontinuities and their interaction with the backstreaming ions in the foreshock prior to these discontinuities encountering the bow shock. The leading edge of the foreshock bubble consists of a fast magnetosonic shock and the compressed and heated plasma downstream of the shock. The leading edge surrounds the core which consists of a less-dense and hotter plasma and lower magnetic field strength. Ultra low frequency turbulence is present in both the outer and core regions of the foreshock bubbles. The size of the foreshock bubble transverse to the flow direction scales with the width of the ion foreshock and at Earth corresponds to tens of RE. The size along the flow depends on the age of the bubble and grows with time. Although they expand sunward, foreshock bubbles are carried antisunward by the solar wind, and for small IMF cone angles (angle between IMF and solar wind flow) when the foreshock lies upstream of the dayside magnetosphere they collide with the bow shock. This collision is shown to have significant magnetospheric impacts. Upon encountering the bow shock, the low pressures within the core of the bubble result in the reversal of the magnetosheath flow from antisunward to sunward direction. This in turn results in the outward motion of the magnetopause and expansion of the dayside magnetosphere. The interaction is found to noticeably impact the density and energy of trapped radiation belt ions and plasma injection into the cusp. Foreshock bubbles are found to be highly effective sites for ion reflection and acceleration to high energies via first- and second-order Fermi acceleration. The interaction of the foreshock bubble with the bow shock results in the release of energetic ions into the magnetosheath. Some of these ions are subsequently injected into the cusp.

In situ observations of reconnection Hall magnetic fields at Mars: Evidence for ion diffusion region encounters

[1]xa0We present Mars Global Surveyor measurements of bipolar out-of-plane magnetic fields at current sheets in Mars magnetosphere. These signatures match predictions from simulations and terrestrial observations of collisionless magnetic reconnection, and could similarly indicate differential ion and electron motion and the resulting Hall current systems near magnetic X lines. Thus, these observations may represent passages through or very near reconnection diffusion regions at Mars. Out of 28 events found at 400 km altitude with well-defined current sheet orientations, 26 have magnetic fields consistent with the expected polarities of Hall fields near diffusion regions. For these events, we find an average ratio of Hall field to main field of 0.51 ± 0.13, and an average ratio of normal to main field (reconnection rate) of 0.16 ± 0.09, consistent with terrestrial observations of reconnection. These events do not consistently correlate with the location of crustal fields or with IMF reversals, indicating that magnetic field draping alone (perhaps enhanced by high solar wind dynamic pressure) may generate current sheets capable of reconnection. For some events, we observe field-aligned electrons that may carry parallel currents that close the Hall current loop. Electron distributions around current sheets often indicate magnetic connection to the collisional exosphere. For crossings sunward of the X line, we usually observe an electron flux minimum at the current sheet, consistent with the resulting closed magnetic structure. For crossings antisunward of the X line, we do not observe flux minima, consistent with field lines open downstream. Collisionless reconnection, if common at Mars, could represent a significant atmospheric loss process.

THEMIS observations of extreme magnetopause motion caused by a hot flow anomaly

[1] On 30 October 2007, the five THEMIS spacecraft observed the cause and consequence of extreme motion of the dawn flank magnetopause, displacing the magnetopause outward by at least 4.8 RE in 59 s, with flow speeds in the direction normal to the model magnetopause reaching 800 km/s. While the THEMIS A, C, D, and E observations allowed the determination of the velocity, size, and shape of a large bulge moving tailward along the magnetopause at a speed of 355 km/s, THEMIS B observed the signatures of a hot flow anomaly (HFA) upstream of the bow shock at the same time, indicating that the pressure perturbation generated by the HFA may be the source of the fast compression and expansion of the magnetosphere. The transient deformation of the magnetopause generated field-aligned currents and created traveling convection vortices which were detected by ground magnetometers. This event demonstrates that kinetic (non-MHD) effects at the bow shock can have global consequences on the magnetosphere.

Tracing solar wind plasma entry into the magnetosphere using ion-to-electron temperature ratio

When the solar wind Mach number is low, typically such as in magnetic clouds, the physics of the bow shock leads to a downstream ion-to-electron temperature ratio that can be notably lower than usual. We utilize this property to trace solar wind plasma entry into the magnetosphere by use of Cluster measurements in the vicinity of the dusk magnetopause during the passage of a magnetic cloud at Earth on November 25, 2001. The ion-to-electron temperature ratio was indeed low in the magnetosheath (Ti/Te ∼ 3). In total, three magnetopause boundary layer intervals are encountered on that day. They all show that the low ion-to-electron temperature ratio can be preserved as the plasma enters the magnetosphere, and both with and without the observation of Kelvin-Helmholtz activity. This suggests that the ion-to-electron temperature ratio in the magnetopause boundary layer, which is usually high, is not prescribed by the heating characteristics of the plasma entry mechanism that formed these boundary layers. In the future, this property may be used to (1) further trace plasma entry into inner regions and (2) determine the preferred entry mechanisms if other theoretical, observational and simulation works can give indications on which mechanisms may alter this ratio.

Contributions to the cross shock electric field at a quasiperpendicular collisionless shock

[1]xa0The normal electric field structure of a supercritical (Mms = 5.2), quasiperpendicular (θBn = 70°) collisionless shock is examined using Cluster four-spacecraft observations of the terrestrial bow shock. Comparing the observed electric field with magnetic field and plasma observations, two different techniques find that the J × B/ne term in the generalized Ohms law accounts for a majority of the large-scale normal electric field and potential drop encountered by the ions - the solar wind ion deceleration is in good empirical agreement with the observed potential drop, confirming earlier work. Large amplitude electric field fluctuations on shorter timescales, corresponding to fine scale structure, are not observed to contribute to the ion energization.

Energetic, ∼5–90 keV neutral atom imaging of a weak substorm with STEREO/STE

[1]xa0We present imaging and high resolution energy spectra of energetic ∼5–90 keV neutral atoms (ENA) of a weak geomagnetic substorm (Dst > −8 nT and AE ≲ 200 nT), made by the Suprathermal Electron (STE) instrument on the STEREO B spacecraft. Enhanced ENA emissions were observed coming from around local midnight near the equator with different spatial distribution and/or temporal behavior at ∼5–20keV compared to ∼20–90 keV. By forward modeling using a parameterized ring-current model, we show that the ENA images imply the parent equatorial protons have pitch-angle distributions peaked at 90°, an energy spectrum consistent with in situ proton measurements at geosynchronous orbit, and a spatial asymmetry with the maximum flux at midnight for 5–20 keV and at 2240 MLT for 20–90 keV. These are the first ENA measurements at ∼5 to 26 keV from low altitude, and the first for such weak activity.