Magnetic holes are sudden changes in the magnetic field intensity |B| from typical interplanetary values (about 10 nT) to less than 1 nT in a matter of seconds. The intensity then recovers ~30 seconds to ~30 minutes later. These |B| dropouts can be seen daily; less often observed, but even more dramatic, are magnetic holes that last for up to 1 hour. At present it is not known whether these holes originate in interplanetary space, or whether they are formed close to the Sun. The frequent observation of magnetic holes close to the heliospheric current sheet supports the former scenario, however, the holes might as well be associated with solar structures in the slow solar wind.
These scenarios have been tested with data from the SWICS instrument on ACE, which measures the composition of solar wind ions. Solar wind composition patterns depend strongly on the solar source region and the solar wind expansion in the corona. If magnetic holes were born in the corona, SWICS data would be likely to show compositional signatures of the plasma environment where they originate. The figure shows the solar-wind temperature (determined from O7+/O6+) and the Fe/O ratio during a typical long-lasting magnetic hole observed on April 9, 1998 by the MAG instrument on ACE. None of the compositional signatures show a significant change during the magnetic-hole period, suggesting that these holes form in interplanetary space and are not likely a solar phenomenon.
If magnetic holes originate in interplanetary space, they may provide an opportunity to investigate other physical processes relevant to the dynamics of particles in the heliosphere: Characteristic wave properties inside and outside the hole are expected to differ by an order of magnitude, which should affect the interaction of these waves with interplanetary particles. Such particle signatures could in turn provide more specific information on the physical processes relevant to the formation of magnetic holes.
Contributed by Chuck Smith of the University of Delaware, and Thomas Zurbuchen of the University of Michigan.
See The SWICS/SWIMS Home Page for more information about the SWICS instrument, and The MAG Home Page for more information about the MAG instrument.
Last modified 9 March, 1999,
Andrew Davis
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