Low Density Periods in the Solar Wind

A long period of low solar wind density was observed between 5/24/0530 and 5/25/1200 UT of 2002. The density was below 0.1/cc for part of this time, and below 2.0/cc for all of it, making the event similar to the examples given in "Electrons in the Low Density Solar Wind", Ogilvie, Fitzenreiter and Desch; J. Geophys. Res., 105, 2000. In the present paper we investigate the solar and interplanetary conditions at the time of this event. A CME at 5/21/2338, perhaps associated with a flare at 5/21/2322 UT, drove a large discontinuity that passed Wind at 5/23/1200. At this time the solar wind speed rose to 850-900 km s-1 and the pressure to 1.1 x 10-8 ergs/cm3. The rarefaction following this discontinuity appeared to be responsible for the long period of low density. These occurrences will be discussed in detail, and compared with similar earlier events, to support the notion that periods of abnormally low density are rarefactions following high speed discontinuities driven by CME material. The low probability for the occurrence of a high speed CME sending its associated disturbance in the earth's direction, accounts for the rarity of these long duration low density events.