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ACE News #49 - September 25, 2000

ACE News Archives

Revisiting the Day the Solar Wind Almost Disappeared

ACE News #40 described a remarkable 3-day period in May 1999 when the solar wind nearly disappeared from the near-Earth environment. Panels 3 and 4 from the figure above repeat the key aspect of that presentation: on day 131 (May 12) of 1999 the solar wind displayed a continuous drop in density, N_p, that lasted until early on day 132. The density depletion spanned 2 orders of magnitude from typical 1 AU values of about 10 p/cc to only 0.1 p/cc. The wind speed, V_R, decreases more slowly and shows an abrupt reduction late in the interval to levels consistent with a cool coronal source and little secondary heating or momentum transfer. The intensity of the interplanetary magnetic field (IMF), B, shown in the top panel, stays constant until late in the period when it increases slightly. It is noteworthy that B fails to show any depletion when the source of the interplanetary plasma is greatly diminished. Whatever causes the solar wind density to be depleted, there is ample source to carry the Sun's magnetic field into interplanetary space.

The RMS level of the IMF fluctuations shown in panel 2 undergoes a factor of 5 reduction during this time. Spectral analysis confirms that the IMF fluctuations are reduced throughout the measured frequency range 0.0001 to 1.5 Hz. The Alfven speed shown in panel 5 increases within the region of depleted density. So, Alfven waves attempting to propagate into the region will be refracted away, while waves propagating within the region will refract outward and toward the surrounding plasma, thereby depleting the fluctuation energy within the interval of low density.

One possible explanation for this interval is as follows: The source of solar wind plasma was temporarily diminished, thereby raising the Alfven speed within the plasma. There remained sufficient plasma to carry the magnetic field into space, which provides most of the observed pressure at 1 AU (not shown), thereby preventing adjacent material from pushing in and compressing the observed depletion region. Fast mode and possibly Alfven waves are depleted in the region early in its evolution due to refraction effects, thereby reducing secondary heating and acceleration processes and maintaining low proton temperatures and wind speeds. This continues with some adjustment due to interplanetary expansion and a reduced level of heating, neither of which are sufficient to overcome the deficit due to the early conditions. This is only one possible interpretation of this very interesting interval now under study by several groups.

Contributed by the MAG team of the Bartol Research Institute, University of Delaware.

See The MAG Home Page for additional information on ACE/MAG.

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Last modified 26 September 2000, by Andrew Davis