Reconnection Guide Field and Quadrupolar Structure Observed by MMS on 16 October 2015 at 1307 UT

We estimate the guide field near the X point, B(sub M0), for a magnetopause crossing by the Magnetospheric Multiscale (MMS) spacecraft at 1307 UT on 16 October 2015 that showed features of electron-scale reconnection. This component of the magnetic field is normal to the reconnection plane L-N containing the reconnection magnetic field, B(sub L), and the direction e(sub N) normal to the current sheet. The B(sub M) field component appears to approximately have quadrupolar structure close to the X point. Using several different methods to estimate values of the guide field near the X point, some of which use an assumed quadrupolar symmetry, we find values ranging between -3.1 nT and -1.2 nT, with a nominal value of about -2.5 nT. The rough consistency of these values is evidence that the quadrupolar structure exists.