A statistical analysis of 351 independent bow shock crossings and 233 independent magnetopause crossings by the ISEE‐I spacecraft from 1977 to 1980 was performed to determine the average positions and shapes of the bow shock and magnetopause. The standoff distance between the magnetopause and the bow shock depends on the compressibility of the plasma which in the ‘polytropic’ approximation is related to the ratio of specific heats, γ. Standoff distances for the bow shock and magnetopause were found to be 13.7 RE (±0.2 RE) and 10.3 RE (±0.3 RE) respectively. These distances are smaller than those observed during earlier epochs. The observed thickness of the magnetosheath is that expected for the compression of a gas whose polytropic index, γ, is 1.76±0.15. This value, representative of the entire magnetosheath, is consistent with the value of 1.67 deduced from the behavior of the plasma across individual shock transitions. A value of 1.67 is expected for an adiabatic process in a collisional, monatomic gas with three degrees of freedom with lower values for non‐adiabatic processes and higher values for anisotropic heating at the shock. The observed value of 1.76 indicates that heat flux does not much affect the position of the shock while the downstream anisotropy may have a small effect.

An examination of the response of the low‐latitude H component of the Earth's magnetic field during the passage of interplanetary shocks when the interplanetary magnetic field is northward reveals that this response can be understood quantitatively in terms of the compression of a simple vacuum magnetospheric model. The compression at the surface of the Earth at 20° latitude at noon in the absence of equatorial electrojet effects is found to be 18.4 nT/(nPa)1/2. Stations below 15° latitude and above 40° appear to have additional but variable sources of current which magnify this effect. The diurnal variation of the compression is larger than expected from the simple vacuum magnetosphere, ±20% about the mean instead of ±10%. We interpret this difference to indicate that tail currents, not in the vacuum model, are as important as the magnetopause currents in determining the diurnal variation of the field at the surface of the Earth.

[1] Magnetic reconnection transfers energy from magnetic fields to particles in space, but the conditions under which reconnection is initiated are not well understood. Indirect evidence confirms the existence of reconnection where merging fields are anti-parallel, but also confirms the existence of another type of reconnection, called component reconnection. This ambiguity has resulted in considerable debate over which of these types of reconnection dominates. Here we report on a method using 3D plasma observations from the TIMAS instrument on Polar as it passes through the northern cusp to calculate the distance to the reconnection line, and subsequently trace this distance to the magnetopause. Results from 130 events reveal that in general, magnetic reconnection occurs along an extended line across the dayside magnetopause, consistent with the component reconnection scenario. These results indicate that reconnection in other regions of space (e.g., solar flares, Earth's magnetotail) should not be limited to anti-parallel magnetic field geometries. Citation: Trattner, K. J., J. S. Mulcock, S. M. Petrinec, and S. A. Fuselier (2007), Location of the reconnection line at the magnetopause during southward IMF conditions, Geophys. Res. Lett., 34, L03108,

The stability of the high-latitude reconnection site under steady northward Interplanetary Magnetic Field (IMF) conditions was investigated using observations in the Earth's magnetospheric cusp. Using proton distributions with characteristic low-energy cutoffs, the distance to the high-latitude reconnection site was monitored during four cusp crossings that had relatively steady solar wind dynamic pressure and IMF clock angle. For these four events, the reconnection site location remained approximately fixed, indicating that the high-latitude magnetic reconnection site remains stable for many minutes under steady IMF conditions. A possible explanation for this stability is that the magnetosheath flow remains sub-Alfvenic even at high latitudes because of the presence of a plasma depletion layer.

The Magnetospheric Multiscale (MMS) mission and operations are designed to provide the maximum reconnection science. The mission phases are chosen to investigate reconnection at the dayside magnetopause and in the magnetotail. At the dayside, the MMS orbits are chosen to maximize encounters with the magnetopause in regions where the probability of encountering the reconnection diffusion region is high. In the magnetotail, the orbits are chosen to maximize encounters with the neutral sheet, where reconnection is known to occur episodically. Although this targeting is limited by engineering constraints such as total available fuel, high science return orbits exist for launch dates over most of the year. The tetrahedral spacecraft formation has variable spacing to determine the optimum separations for the reconnection regions at the magnetopause and in the magnetotail. In the specific science regions of interest, the spacecraft are operated in a fast survey mode with continuous acquisition of burst mode data. Later, burst mode triggers and a ground-based scientist in the loop are used to determine the highest quality data to downlink for analysis. This operations scheme maximizes the science return for the mission.

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The shocked solar wind in the Earth's magnetosheath becomes nearly stationary at the subsolar magnetopause. At this location, solar wind protons are neutralized by charge exchange with neutral hydrogen atoms at the extreme limits of the Earth's tenuous exosphere. The resulting Energetic Neutral Atoms (ENAs) propagate away from the subsolar region in nearly all directions. Simultaneous observations of hydrogen ENAs from the Interstellar Boundary Explorer (IBEX) and proton distributions in the magnetosheath from the Cluster spacecraft are used to quantify this charge exchange process. By combining these observations with a relatively simple model, estimates are obtained for the ratio of ENA to shocked solar wind flux (about 10−4) and the exospheric density at distances greater than 10 Earth Radii (RE) upstream from the Earth (about 8 cm−3).

[1] Two types of reconnection occur at the Earth's magnetopause: component and antiparallel reconnection. Recently, an empirical model was developed that purportedly determines under what solar wind conditions one or the other type of reconnection is dominant. This empirical model is tested using observations at the magnetopause from the Cluster spacecraft. For a range of interplanetary magnetic field orientations, there are regions on the magnetopause where these two types of reconnection can be distinguished from one another by the direction of the flow of reconnection jets observed in magnetopause boundary layers. Cluster spacecraft observations at the magnetopause under these specific interplanetary magnetic field (IMF) orientations confirm model predictions of the type of reconnection. In addition, these observations show evidence of possible multiple reconnection (both component and antiparallel) occurring at the magnetopause.