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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