Structure and dynamics of Mercury's magnetospheric cusp: MESSENGER measurements of protons and planetary ions

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has observed the northern magnetospheric cusp of Mercury regularly since the probe was inserted into orbit about the innermost planet in March 2011. Observations from the Fast Imaging Plasma Spectrometer (FIPS) made at altitudes  10 cm−3) that are exceeded only by those observed in the magnetosheath. These high plasma densities are also associated with strong diamagnetic depressions observed by MESSENGER's Magnetometer. Plasma in the cusp may originate from several sources: (1) Direct inflow from the magnetosheath, (2) locally produced planetary photoions and ions sputtered off the surface from solar wind impact and then accelerated upward, and (3) flow of magnetosheath and magnetospheric plasma accelerated from dayside reconnection X-lines. We surveyed 518 cusp passes by MESSENGER, focusing on the spatial distribution, energy spectra, and pitch-angle distributions of protons and Na+-group ions. Of those, we selected 77 cusp passes during which substantial Na+-group ion populations were present for a more detailed analysis. We find that Mercury's cusp is a highly dynamic region, both in spatial extent and plasma composition and energies. From the three-dimensional plasma distributions observed by FIPS, protons with mean energies of 1 keV were found flowing down into the cusp (i.e., source (1) above). The distribution of pitch angles of these protons showed a depletion in the direction away from the surface, indicating that ions within 40° of the magnetic field direction are in the loss cone, lost to the surface rather than being reflected by the magnetic field. In contrast, Na+-group ions show two distinct behaviors depending on their energy. Low-energy (100–300 eV) ions appear to be streaming out of the cusp, showing pitch-angle distributions with a strong component antiparallel to the magnetic field (away from the surface). These ions appear to have been generated in the cusp and accelerated locally (i.e., source (2) above). Higher-energy (≥1 keV) Na+-group ions in the cusp exhibit much larger perpendicular components in their energy distributions. During active times, as judged by frequent, large-amplitude magnetic field fluctuations, many more Na+-group ions are measured at latitudes south of the cusp. In several cases, these Na+-group ions in the dayside magnetosphere are flowing northward toward the cusp. The high mean energy, pitch-angle distributions, and large number of Na+-group ions on dayside magnetospheric field lines are inconsistent with direct transport into the cusp of sputtered ions from the surface or newly photoionized particles. Furthermore, the highest densities and mean energies often occur together with high-amplitude magnetic fluctuations, attributed to flux transfer events along the magnetopause. These results indicate that high-energy Na+-group ions in the cusp are likely formed by ionization of escaping neutral Na in the outer dayside magnetosphere and the magnetosheath followed by acceleration and transport into the cusp by reconnection at the subsolar magnetopause (i.e., source 3 above).