Dropouts of Fully Stripped Ions in the Solar Wind: A Diagnostic for Wave Heating versus Reconnection

The SWICS instrument on board the ACE satellite has detected frequent intervals in the slow solar wind and interplanetary coronal mass ejections in which C6+ and other fully stripped ions are strongly depleted, though the ionization states of elements such as Si and Fe indicate that those ions should be present. It has been suggested that these "outlier" or "dropout" events can be explained by the resonant cyclotron heating process, because these ions all have the same cyclotron frequency as He2+. We investigate the region in the corona where these outlier events form. It must be above the ionization freeze-in height and the transition to collisionless plasma conditions, but low enough that the wind still feels the effects of solar gravity. We suggest that the dropout events correspond to relatively dense blobs of gas in which the heating is reduced because local variations in the Alfven speed change the reflection of Alfven waves and the turbulent cascade. As a result, the wave power at the cyclotron frequency of the fully stripped ions is absorbed by He2+ and may not be able to heat the other fully stripped ions enough to overcome solar gravity. If this picture is borne out, it may help to discriminate between resonant cyclotron heating and stochastic heating models of the solar wind.