Helium abundance enhancements in the solar wind

Publisher Website
Google Scholar
Abstract
Helium abundance (A(He)) enhancements observed with Los Alamos instruments on IMP 6, 7, and 8 during the 1972–1978 interval have been investigated. Statistical analysis of 73 large events with A(He) ≥ 10% (HAEs) provides evidence for a close link between helium enhancements at 1 A.U. and transient coronal mass ejections. HAE events are sporadic, sometimes clustered in time, and their frequency of occurrence is approximately in phase with the solar cycle. Nearly 50% of HAEs are associated with interplanetary shocks and/or geomagnetic activity sudden commencements, but the plasma pattern associated with A(He) enhancements is independent of shock occurrence. This pattern features high magnetic field strength, low alpha‐proton velocity difference, and low proton temperature. These plasma properties suggest that the enhancement is embedded in a ‘closed,’ magnetically dominated structure that expands adiabatically. In fact, HAEs are likely to be still evolving dynamically at 1 A.U. Evidence of a significant association between helium enhancements at 1 A.U. and type II and IV radio bursts in the corona is presented. We interpret these results to mean that most HAE events originate in transient coronal disturbances in which the magnetic field strength is enhanced.