High time resolution observations of the drivers of Forbush decreases

The drivers of Forbush decreases in galactic cosmic ray (GCR) fluxes are thought to be magnetic turbulence in the sheath of an interplanetary coronal mass ejection (ICME) and the closed magnetic field lines in the ICME itself. This model, however, is the result of studies utilizing hourly or longer time averaging. Such averaging can smooth over important correlations between variabilities in the GCR flux and those in the interplanetary medium. To test the validity of the current model of Forbush decreases, we analyze a number of Forbush decreases using high time resolution GCR data from the High Sensitivity Telescope (HIST) on Polar and the Spectrometer for INTEGRAL (SPI). We seek causal correlations between the onset of the decrease and structures in the solar wind plasma and interplanetary magnetic field, as measured concurrently with ACE and/or Wind. We find evidence that planar magnetic structures in the sheath preceding the ICME may be a factor in driving the decrease in at least one event.