He+ and other unusual ions in the solar wind: A systematic search covering 1972–1980

Abstract

Following the recent discovery of large fluxes of He+ in the solar wind, a systematic search of solar wind energy/charge (E/Q) spectra was initiated, using plasma data from IMP 7, IMP 8, ISEE 1, and ISEE 3 that covered the time period from October 1972 to February 1980. A technique of vertically stacking successive E/Q spectra made it possible to detect systematic trends in the spectra that were indicative of ion species beyond the He++ peak down to the one count level. A search of nearly 8 years of data yielded only three distinct events with identifiable levels of He+. These events occurred on July 29, December 1, and December 4, 1977, all within 1 year and during the rising phase of solar cycle 21. We believe more events would have been detected if the instruments had been more sensitive and tracking more complete. Two of the events occurred within driver gas, following interplanetary shocks, while the third event occurred in a noncompressive density enhancement (NCDE). For all three the abundances of various minor ions, including He++, were enhanced in comparison to normal solar wind abundances. One unusual feature found in these three He+ events, in contrast to normal solar wind, was the appearance of ionization states corresponding to both warm and very cold coronal freezing‐in conditions within the same plasma. In addition to the unusual appearances of large fluxes of He+ ions, iron ions ranging from Fe+10 to Fe+5 were observed. Evidence was found for the appearance of C+4 ions, and in one event the data can be interpreted as containing O+2 and O+3 at detectable levels. One of the phenomenological results following from these observations is that certain unusual transient events eject both hot and unusually cold coronal plasma mixed and coexisting on the same magnetic field lines.