Abstract
One of the major questions about magnetic reconnection is how specific solar wind and interplanetary magnetic field conditions influence where reconnection occurs at the Earth’s magnetopause. There are two reconnection scenarios discussed in the literature: a) anti-parallel reconnection and b) component reconnection. Early spacecraft observations were limited to the detection of accelerated ion beams in the magnetopause boundary layer to determine the general direction of the reconnection X-line location with respect to the spacecraft. An improved view of the reconnection location at the magnetopause evolved from ionospheric emissions observed by polar-orbiting imagers. These observations and the observations of accelerated ion beams revealed that both scenarios occur at the magnetopause. Improved methodology using the time-of-flight effect of precipitating ions in the cusp regions and the cutoff velocity of the precipitating and mirroring ion populations was used to pinpoint magnetopause reconnection locations for a wide range of solar wind conditions. The results from these methodologies have been used to construct an empirical reconnection X-line model known as the Maximum Magnetic Shear model. Since this model’s inception, several tests have confirmed its validity and have resulted in modifications to the model for certain solar wind conditions. This review article summarizes the observational evidence for the location of magnetic reconnection at the Earth’s magnetopause, emphasizing the properties and efficacy of the Maximum Magnetic Shear Model.
Highlights
Collisionless magnetic reconnection occurs between the shocked solar wind plasma in the magnetosheath and the terrestrial magnetospheric plasma in the magnetosphere
Notable exceptions are the Flux Transfer Event (FTE) concept of reconnection localized in both time and space (Russell and Elphic 1978, 1979) and the postulation that the low-latitude boundary layer may be produced by random reconnection at the magnetopause for northward interplanetary magnetic field (IMF) (Nishida 1989)
With Magnetospheric Multiscale (MMS) located in the magnetopause boundary layer in the subsolar region, the spacecraft are far enough away from the predicted X-line that they observe both the accelerated ion beam from the X-line and the return beam mirrored from the ionosphere, before the newly opened magnetic field line convects over the spacecraft and cuts the return beam off
Summary
Collisionless magnetic reconnection occurs between the shocked solar wind plasma in the magnetosheath and the terrestrial magnetospheric plasma in the magnetosphere. For a pure southward IMF and for zero-degree tilt angle of the Earth’s dipole magnetic field, magnetic reconnection could take place all along the dayside geomagnetic equator where the magnetic fields internal and external to the magnetopause are anti-parallel to one another This geometry is depicted as an extended reconnection line ( often described as a continuous “X-line”). Investigations of the effect of a nonzero IMF BY-GSM component during southward IMF on the location of anti-parallel magnetic reconnection at the magnetopause were conducted by Crooker (1979) and Luhmann et al (1984) (again, for a zero-degree tilt angle of Earth’s dipole magnetic field).
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