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During an interval spanning May 10 to 12, 1999, the SWEPAM instrument on ACE detected an extended region of unusually low-density solar wind. The solar wind proton density, which has a mean value of 8.7 cm-3 and is <3 cm-3 only 5% of the time, remained below 1.0 cm-3 from late on May 10th to early on May 12th. While rare excursions below 1 cm-3 sometimes occur, they have not previously been observed to last so long. These low densities occurred together with very low speeds (<300 km/s on May 11th), resulting in a solar-wind ram pressure more than 50 times lower than is typical. As a result, Earth's bow shock was observed at unusually large upstream distances by several spacecraft (e.g., at 52 Re from Earth by Wind).
Early on May 12th ACE observed the solar wind speed swinging between ~350 km/s and ~500 km/s. Although such large speed changes are ordinarily associated with the interaction of fast and slow-speed solar wind streams, these extremely unusual speed swings were of a different nature. While the plasma was quite rarified, the magnetic field magnitude remained at typical values (~5 nT) throughout. As a result, the Alfven speed, typically ~50 km/s, reached a peak value near 300 km/s, and remained above 100 km/s for a considerable period. This allowed the velocity of Alfvenic fluctuations to be very large. It can be seen that these tremendous speed swings are in fact Alfvenic fluctuations from their correlation with the magnetic field fluctuations.
Electrons with >100 eV in this event streamed both parallel and anti-parallel to the magnetic field, a behavior that generally indicates closed-loop field lines associated with coronal mass ejections (CMEs). It appears that the "disappearing solar wind" interval originated as an extremely slow-moving, low-density CME. Because it was preceded by faster solar wind, a large rarefaction region formed ahead of the CME, helping to draw out its leading edge, thereby further reducing the already low density. Because this event was being overtaken by faster, higher-density flow, it probably did not survive much longer. In coronagraph and interplanetary observations "slow" CMEs are normally accelerated up to the speed of the surrounding solar wind. It is very surprising to find that a portion of this CME was moving at <300 km/s.
Contributed by John Steinberg of Los Alamos National Laboratory.
See The SWEPAM Home Page for more information on ACE SWEPAM.
Last modified December 9, 1999.
Steve Sears