Enigmatic solar wind disappearance events – Do we understand them?

At the Sun-Earth distance of one astronomical unit (1 AU), the solar wind is known to be strongly supersonic and super Alfvenic with Mach and Alfven numbers being on average 12 and 9 respectively. Also, solar wind densities (average ∼10cm^-3) and velocities (average ∼450kms^-1) at 1AU, are known to be inversely correlated with low velocities having higher than average densities andvice versa. However, on May 11 and 12 1999 the Earth was engulfed by an unusually low density (< 0.1cm^-3) and low velocity (< 350km s^-1) solar wind with an Alfven Mach number significantly less than 1. This was a unique low-velocity, low-density, sub-Alfvénic solar wind flow which spacecraft observations have shown lasted more than 24 hours. One consequence of this extremely tenuous solar wind was a spectacular expansion of the Earth’s magnetosphere and bow shock. The expanding bow shock was observed by several spacecraft and reached record upstream distances of nearly 60 Earth radii, the lunar orbit. The event was so dramatic that it has come to be known asthe solar wind disappearance event. Though extensive studies of this event were made by many authors in the past, it has only been recently shown that the unusual solar wind flows characterizing this event originated from a small coronal hole in the vicinity of a large active region on the Sun. These recent results have put to rest speculation that such events are associated with global phenomenon like the periodic solar polar field reversal that occurs at the maximum of each solar cycle. In this paper we revisit the 11 May 1999 event, look at other disappearance events that have ocurred in the past, examine the reasons why speculations about the association of such events with global phenomena like solar polar field reversals were made and also examine the role of transient coronal holes as a possible solar source for such events.