Magnetic reconnection is a fundamental physical process in plasmas whereby stored 40 magnetic energy is converted into heat and kinetic energy of charged particles. 41Reconnection occurs in many astrophysical plasma environments and in laboratory 42 plasmas. Using very high time resolution measurements, NASA's Magnetospheric 43 2 Multiscale Mission (MMS) has found direct evidence for electron demagnetization and 44 acceleration at sites along the sunward boundary of Earth's magnetosphere where the 45 interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) 46 observed the conversion of magnetic energy to particle energy, (ii) measured the electric 47 field and current, which together cause the dissipation of magnetic energy, and (iii) 48identified the electron population that carries the current as a result of demagnetization 49 and acceleration within the reconnection diffusion/dissipation region. 50 51 Introduction 52

[1] One of the major outstanding questions about magnetic reconnection is where reconnection will occur at the Earth's magnetopause for specific conditions of the solar wind and the interplanetary magnetic field (IMF). There are two scenarios discussed in the literature: (1) antiparallel reconnection, which occurs where the magnetospheric magnetic field and the IMF are antiparallel (shear angle of approximately 180°) and (2) component reconnection, where shear angles between the magnetospheric field and the IMF as low as 50°have been reported. The distinction between the two reconnection scenarios is important for the energy and momentum transfer from the solar wind to the magnetosphere. Here we report on a method using three-dimensional plasma observations from the Toroidal Imaging Mass-Angle Spectrograph instrument on the Polar spacecraft as it passes through the northern magnetospheric cusp to calculate the distance to the reconnection line and subsequently trace the distance along model magnetic field lines back to the magnetopause. Results from 130 events reveal that in general, magnetic reconnection occurs along an extended line across the dayside magnetopause (i.e., consistent with the component reconnection scenario). During strong sunward or antisunward IMF conditions (B X ), however, the reconnection location resembles the antiparallel reconnection model at high latitudes and does not cross the dayside magnetopause as a single tilted reconnection line. These results show that either reconnection scenario can occur at the magnetopause, depending on the specific IMF conditions. Citation: Trattner, K. J., J. S. Mulcock, S. M. Petrinec, and S. A. Fuselier (2007), Probing the boundary between antiparallel and component reconnection during southward interplanetary magnetic field conditions,

Knowledge of the average size and shape of the near‐Earth magnetotail is an essential element for our understanding of the magnetospheric response to the influence of the solar wind. An empirical model of the near‐Earth magnetotail has been developed, which depends upon distance downtail (xGSM), the solar wind momentum flux (ρv2sw), and the zGSM component of the interplanetary magnetic field (IMF Bz). This model has been created by using the pressure balance relation to calculate a set of flare angles for the nightside magnetopause in the region −22 RE ≤ xGSM ≤ −10 RE. Observations of the magnetic field in the lobe by ISEE 2 and simultaneous observations of the magnetic field and plasma properties of the solar wind by IMP 8 were used to determine the internal and external pressure components, respectively. Examination of calculated flare angle values reveal a dependence upon downtail distance and ρv2sw. Normalized to the median downtail distance and dynamic pressure, the angle of flare of the magnetopause is found to increase linearly with decreasing Bz when the IMF is southward, but there is little variation when the IMF is northward. The empirical function derived for the flaring angle of the magnetotail is used to determine a relation for the radius of the tail. Comparisons with previous empirical models and results are also performed. In addition, values of magnetic flux within the magnetotail are calculated for times of sudden impulse events.

[1] Reconnection at the Earth's magnetopause is the mechanism by which magnetic fields in different regions change topology to create open magnetic field lines that allow energy and momentum to flow into the magnetosphere. One of the long-standing open questions about magnetic reconnection is the location of the reconnection line. There are two reconnection scenarios discussed in the literature: (1) antiparallel reconnection where shear angles between the magnetospheric field and the interplanetary magnetic field (IMF) are near 180°and (2) component reconnection where a tilted reconnection line which crosses the magnetopause in the subsolar region at shear angles not near 180°. Early satellite observations were limited to the detection of accelerated ion beams in the magnetopause boundary layer to determine the general direction of the reconnection line location with respect to the satellite. An improved view of the reconnection location at the magnetopause was determined from ionospheric emissions observed by polar-orbiting imagers which revealed that both scenarios occur. The time-of-flight effect of precipitating ions in the cusp in connection with the low-velocity cutoff method pinpointed reconnection locations and their dependency on IMF conditions. These results are summarized by the maximum magnetic shear model. This study uses confirmed magnetic reconnection locations from the THEMIS mission to test the predictions of this reconnection location model. The results reveal that the maximum magnetic shear model predicts the observed reconnection locations for dominant IMF B Y conditions very well but needs further improvement and modifications for dominant southward IMF conditions.

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[1] The interconnection of magnetic fields through magnetic reconnection at the magnetopause is the dominant process for mass, energy, and momentum transfers from the Earth's magnetosheath to the magnetosphere. Earlier studies on the location of the reconnection line during northward interplanetary magnetic field (IMF) conditions and high solar wind dynamic pressure revealed an approximately equal probability for antiparallel and component reconnection between the IMF and the geomagnetic field. Using data from the Toroidal Imaging Mass Angle Spectrograph on the Polar spacecraft, we have substantially increased our survey, selecting only events with northward IMF but no restrictions on the solar wind dynamic pressure. The distance to the reconnection line is calculated using the proven method of three-dimensional cuts of the proton distribution function in the cusp to identify the low-velocity cutoffs of precipitating and mirrored ion populations. Antiparallel and component reconnections are identified by tracing this calculated distance to the reconnection site back along the geomagnetic field to the magnetopause, where the draped IMF is used to calculate the magnetopause shear angle. We find that both reconnection scenarios, antiparallel and component reconnections, occur for the same IMF conditions. The observation of either antiparallel or component reconnection depends entirely on the location of the observing spacecraft.

Data from the Cassini Electron Spectrometer are used to investigate the location of magnetic reconnection at Saturn's magnetopause. Heated, streaming electron distributions in the boundary layer on the magnetosheath side of the magnetopause are evidence of reconnection and an open magnetopause. A model for the location of reconnection is used to compare the modeled and observed streaming direction of the heated electron distributions. Magnetic reconnection at Saturn's magnetopause is predicted and observed to occur at locations similar to those at Earth's magnetopause. Although not conclusive, the results here are consistent with the expected importance of X-line drifts in suppressing low-shear reconnection. Because of different conditions at Saturn's magnetopause, this suppression is predicted to be more severe at Saturn than at Earth.

[1] The interconnection of the interplanetary magnetic field with the geomagnetic field is thought to be the dominant process for mass, energy, and momentum transfer from the magnetosheath into the magnetosphere. Downward precipitating ions from the reconnection site are observed in the cusp by polar orbiting satellites and exhibit sudden changes in their ion-energy distributions, forming distinctive structures. These structures have been identified as temporal structures, most likely caused by variations of the reconnection rate at the magnetopause, as well as spatial structures caused by spatially separated flux tubes. Comparisons of spatial cusp structures observed by Cluster with simultaneously observed ionospheric convection pattern derived from SuperDARN radar observations showed that spatial cusp structures are linked with separated ionospheric convection cells. It has been suggested that these convection cells and their related spatial cusp structures are driven by multiple reconnection lines at the magnetopause. This study revisits a spatial cusp structure event and shows that the two dispersion events are indeed coming from separated reconnection lines located in different hemispheres.