MESSENGER observations of cusp plasma filaments at Mercury

Abstract

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft while in orbit about Mercury observed highly localized, ~3‐s‐long reductions in the dayside magnetospheric magnetic field, with amplitudes up to 90% of the ambient intensity. These magnetic field depressions are termed cusp filaments because they were observed from just poleward of the magnetospheric cusp to midlatitudes, i.e., ~55° to 85°N. We analyzed 345 high‐ and low‐altitude cusp filaments identified from MESSENGER magnetic field data to determine their physical properties. Minimum variance analysis indicates that most filaments resemble cylindrical flux tubes within which the magnetic field intensity decreases toward its central axis. If the filaments move over the spacecraft at an estimated magnetospheric convection speed of ~35 km/s, then they have a typical diameter of ~105 km or ~7 gyroradii for 1 keV H+ ions in a 300 nT magnetic field. During these events, MESSENGER's Fast Imaging Plasma Spectrometer observed H+ ions with magnetosheath‐like energies. MESSENGER observations during the spacecraft's final low‐altitude campaign revealed that these cusp filaments likely extend down to Mercury's surface. We calculated an occurrence‐rate‐normalized integrated particle precipitation rate onto the surface from all filaments of (2.70 ± 0.09) × 1025 s−1. This precipitation rate is comparable to published estimates of the total precipitation rate in the larger‐scale cusp. Overall, the MESSENGER observations analyzed here suggest that cusp filaments are the magnetospheric extensions of the flux transfer events that form at the magnetopause as a result of localized magnetic reconnection.

Mercury's Solar Wind Interaction as Characterized by Magnetospheric Plasma Mantle Observations With MESSENGER

“…However, there are limitations to this conclusion. The supporting work only superposed 16 cusp events (Poh et al, ). Although this gives a good presentation of the cusp, the number of events used is not large enough to give a more statistical idea of Mercury's cusp ions.…”

“…If we assume that reconnection at Mercury's dayside magnetopause is occurring near continuously, then this means that most of the particles injected into the cusp are lost to the planet (and do not mirror to form the plasma mantle). Superposed cusp ion observation analysis shows that ions are mainly observed to be flowing toward the planet with pitch angles of 0–90° measured in the northern cusp (Poh et al, ). These observations took place close to the mirror point, so we assume that most of these particles did not mirror and would not be able to form the mantle, supporting our idea that the mantle is not a permanent feature of Mercury's magnetosphere.…”

MESSENGER Observations and Global Simulations of Highly Compressed Magnetosphere Events at Mercury

“…In particular, as indicated by the close correspondence between the colors along the trajectory and those in the XZ plane, the spatial variations in the modeled field associated with crossings of key magnetospheric boundaries, including the tail current sheet (as indicated by the polarity reversal of B x ) and the dayside magnetopause (as indicated by the sharp discontinuity in the magnitude of B z ), all match the observations very well. Furthermore, Figure d shows a comparison of the simulated plasma pressure P sim with the observed magnetic field strength | B obs | along the MESSENGER trajectory, from which one can identify the location of the cusp where a field depression is expected due to enhanced plasma density (Poh et al, ; Raines et al, ). It can be seen that the simulated plasma pressure exhibits a localized enhancement (white colors in Figure ) in the northern high‐latitude region, which colocates with the portion of the spacecraft trajectory where strong magnetic field depressions (green and blue colors) were observed.…”

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

“…Under more typical conditions the spacecraft would have crossed the magnetopause in the~1,200-1,500-km altitude region. Again, for a typical pass this is the region where the spacecraft would be expected to transition from the closed dayside magnetosphere into the northern cusp (e.g., Poh et al, 2016 ;Raines et al, 2014 ;Winslow et al, 2012). At this point the spacecraft entered the high-latitude magnetosphere at an altitude of 410 km and a local time of 13:00.…”

“…The newly opened Mercury field lines carried tailward by the solar wind all map to the north or south magnetospheric cusps and pivot about the cusps as the high-altitude end is incorporated into the tail lobes. Detailed analyses by Poh et al (2016) have shown that it is very likely that these filaments are the low-altitude extensions of flux transfer events. Solar wind particles generally have a significant component of their velocity parallel to the interplanetary magnetic field.…”