Enigmatic solar wind disappearance events – Do we understand them?

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