The Astrophysical Journal 677, 798−807, 2008
© The American Astronomical Society
Analysis of Plasma-Tail Motions for Comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) Using Observations from SMEI
A. Buffington, M.M. Bisi, J.M. Clover, P.P. Hick and B.V. Jackson
Center for Astrophysics and Space Sciences, University of California San Diego,
La Jolla, CA 92093-0424
T.A. Kuchar
Institute for Scientific Research Boston College Chestnut Hill, MA 02147
Abstract
Comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) passed within 0.3 AU of Earth
in April and May of 2004. Their tails were observed by the Earth-orbiting
Solar Mass Ejection Imager (SMEI) during this period. A time series of
photometric SMEI sky maps displays the motions and frequent disruptions
of the comet plasma tails. Ephemerides are used to unfold the observing
geometry; the tails are often seen to extend 0.5 AU from the comet nuclei.
Having selected 12 of the more prominent motions as "events" for
further study, we introduce a new method for determining solar wind radial
velocities from these SMEI observations. We find little correlation
between these and the changing solar wind parameters as measured close
to Earth, or with coarse three-dimensional reconstructions using
interplanetary scintillation data. A likely explanation is that the
transverse sizes of the solar wind perturbations responsible for these
disruptions are small, ≤ 0.05 AU. We determine the radial velocities of
these events during the disruptions, using a technique only possible
when the observed comet tails extend over a significant fraction of an
AU. We find typical radial velocities during these events of 50-100
km s−1 lower than before or afterward. Time durations of such
events vary, typically from 3 to 8 hr, and correspond to comet
traversal distances 106 km (0.007 AU). We conclude that these large
disturbances are primarily due to ubiquitous solar wind flow variations,
of which these measured events are a subset.