Intervals of Radial Interplanetary Magnetic Fields at 1 AU, Their Association with Rarefaction Regions, and Their Apparent Magnetic Foot Points at the Sun

Abstract

We have examined 226 intervals of nearly radial interplanetary magnetic field orientations at 1 AU lasting in excess of 6 hr. They are found within rarefaction regions as are the previously reported high-latitude observations. We show that these rarefactions typically do not involve high-speed wind such as that seen by Ulysses at high latitudes during solar minimum. We have examined both the wind speeds and the thermal ion composition before, during and after the rarefaction in an effort to establish the source of the flow that leads to the formation of the rarefaction. We find that the bulk of the measurements, both fast- and slow-wind intervals, possess both wind speeds and thermal ion compositions that suggest they come from typical low-latitude sources that are nominally considered slow-wind sources. In other words, we find relatively little evidence of polar coronal hole sources even when we examine the faster wind ahead of the rarefaction regions. While this is in contrast to high-latitude observations, we argue that this is to be expected of low-latitude observations where polar coronal hole sources are less prevalent. As with the previous high-latitude observations, we contend that the best explanation for these periods of radial magnetic field is interchange reconnection between two sources of different wind speed.