Student project at IRF Uppsala
| INSTITUTET FÖR RYMDFYSIK
| Swedish Institute of Space Physics
|| (59°50.272′N, 17°38.786′E)
Project work (30 c)/Examensarbete (30 hp)
Far comet ion tails
Student: Ilona Alexandersson,
On may 1, 1996, Ulysses spacecraft crossed the ion tail of comet Hyakutake, revealing an ion tail length of more than 3 times the Sun-Earth distance. The signatures of an ion tail, especially the ion tail far from the nucleus, are not well explored and many question marks remain. This report summarizes previous observations of spacecraft - ion tail crossings and what signatures that can be expected, as well as signatures of other known solar wind structures. A data analysis is made of possible ion tail encounters from Rosetta spacecraft measurements, Ulysses spacecraft measurements and Earth-orbiting spacecraft measurements. A search from Venus Express data to detect ion tails of sungrazing comets is presented.
Rosetta is an ESA (European Space Agency) spacecraft, launched March 2, 2004 for arrival at its target comet in 2014. To be able to catch up with the comet, Rosetta has to take a long route through the planetary system, including three flybys of Earth and one of March. At the Swedish Institute of Space Physics in Uppsala, we have built an instrument called LAP (Langmuir probe) to study the ionized gas (plasma) close to the comet. En route, we may possibly encounter the tail of some other comet. The chance of crossing a tail close to a comet is small, but comet tails have been identified at very large distances from the nucleus -- more than 3 astronomical units, in the case of the very distant interceptions of the Ulysses spacecraft with the tail of comet Hyakutake (Jones et al., Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures, Nature, 404, 574-576, 2000; Gloeckler et al., Interception of comet Hyakutake's ion tail at a distance of 500 million kilometres, Nature, 404, 576-578, 2000). In a previous project, it was shown that Rosetta has some chance of crossing some ion tails during its cruise to the target comet, so there may possibly be comet tail signatures hiding in the data we have already acquired. But to find them, one needs a clear understanding of what one is looking for, and what other phenomena could possibly give similar signatures in the data.
The project started with a
survey of recent (post-2000) literature on far comet ion tails and solar wind plasma structures.
This let to some criteria for comet tail idenfication from density and magnetic field data, which
were applied to data from some interplanetary and Earth-orbiting spacecraft including Rosetta,
Ulysses, Wind, ACE and Venus Express, to look for ion tail signatures. Analysis of Venus Express
data were done during a stay at IRF Kiruna with Gabriella Stenberg and Hans Nilsson.
Comet tail sketch. Credit: NASA