Remote Sensing of Neptune's Bow Shock: Evidence for Large-Scale Shock Motions
I. H. Cairns, C. W. Smith, W. S. Kurth, D. A. Gurnett and S. Moses
Journal of Geophysical Research, A96, 19,153-19,169 (1991)
The Voyager 2 spacecraft observed high levels of Langmuir waves before the inbound crossing of Neptune's bow shock, thereby signifying magnetic connection to the bow shock. The Langmuir waves occurrred in multiple bursts throughout two distinct periods separated by an 85-min absence of wave activity. We use the times of onset, stable maxima, and disappearances of the waves, together with the magnetic field direction and spacecraft position, to perform a ``remote-sensing'' analysis of the shape and location of Neptune's bow shock prior to the inbound bow shock crossing. The bow shock is assumed to have a parabaloidal shape (symmetric about the Sun-Neptune line) with a standoff distance and flaring parameter determined independently for pairs of wave events. Our analyses show that published static models for the location of Neptune's bow shock cannot explain the wave data. Instead, the nose of Neptune's bow shock is located near 38-43 RN during the first wave period and moves monotonically planetward from 38.5 RN to 34.5 RN during the second wave period. The remote-sensing analysis gives a shock position consistent with the time of the inbound shock crossing and an average flaring parameter consistent with the inbound and outbound shock crossings. Location of the shock's nose near 38-45 RN during the first wave period is consistent with the observed variations in the solar wind ram pressure. Measured variations in the magnetic field direction and ram pressure explain the magnetic disconnection of the spacecraft from the shock during the period without observed Langmuir waves. However, the analysis indicates that the shock's nose moved continuously planetward during the second wave period while the ram pressure remained effectively constant. As normalized by the ram pressure, the remotely sensed shock moves sunward during the first wave period and planetward in the second wave period. The maximum standoff distance occurs while the dipole's axis is close to being perpendicular to the Sun-Neptune direction. One interpretation of these results is that the location of Neptune's bow shock is controlled by the rotation phase of Neptune's oblique, tilted dipole. Our analyses should permit testing of detailed theoretical models for changes in the position and shape of Neptune's bow shock with rotation phase and solar wind ram pressure variations.
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