Observations of Mercury's Northern Cusp Region with MESSENGER's Magnetometer

We have identified Mercury's northern cusp region from orbital observations with the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The cusp is identified from the dayside depression in the total magnetic field after removing the field predicted by a paraboloid magnetospheric model from the data. The model includes long-wavelength fields due to the internal dipole, the magnetopause, and the magnetotail, parameterized by a dipole moment of 195 nT-R_M³ (where R_M is Mercury's radius), offset northward from the planetary center by 484 km and aligned with the planet's spin axis, a paraboloidal magnetopause with a subsolar standoff distance of 1.4 R_M, a distance to the inner edge of the tail current sheet of 1.43 R_M, a tail current sheet half-width of 0.1 R_M, and tail lobe field of 100 nT. We have confirmed that the cusp identification is robust with respect to changes in the parameters in the baseline magnetospheric model. An increase in the high-frequency (1-10 Hz) variability of the magnetic field is also observed on each pass at these times. A superposed epoch analysis, in which individual profiles are aligned in time on their respective cusp midpoints and stacked, indicates that on average the latitudinal extent of the cusp region is 11°, centered on 71.2° N. Cusp observations extend in local time from 7.1 hr to 17.1 hr. The minimum southerly latitude of the cusp observed to date is 56.5° N, and the maximum is 83.1° N. The general location and dimensions of the high-latitude cusp region are in agreement with those indicated by observations with MESSENGER'S Fast Imaging Plasma Spectrometer. To date, no clear correlation is observed between the cusp traversal time or the magnitude of the magnetic field depression and the interplanetary magnetic field (IMF) direction, solar wind density, or solar wind velocity. However, the largest-amplitude magnetic field depressions associated with the cusp are observed during low solar wind ram pressure. On average, the magnitude of the magnetic depression and the magnetic fluctuation amplitude are correlated with the cusp traversal time. The extent to which the longitudinal length of the cusp varies with IMF and solar wind conditions will become clearer with additional observations. Identifying the cusp region has important implications not only for internal field modeling but also for space weathering and the exosphere. Due to the weakened magnetic field, Mercury's surface is more susceptible to solar wind bombardment and associated space weathering processes in the cusp region.