The influence of Epoch Time selection when doing Superposed Epoch Analysis on ACE and MPA LANL data

The influence of the reference time selection in superposed epoch analysis was examined for intense storms at solar maximum. The events were selected according to the pressure-corrected Dst, Dst*, being less than -100 nT. Solar wind data from ACE was used, along with near-Earth data from the magnetospheric plasma analyzer (MPA) instruments on the LANL-operated geosynchronous spacecraft. Numerous choices for the zero epoch time were used, ranging from the storm sudden commencement (SSC), the peak of the ring current enhancement (maximum Dst* slope), to the time of the storm peak (minimum Dst* value). Our results suggest that when doing superposed epoch analysis, the choice of the time stamp can be very important; for different choices, different storm characteristics are revealed in the averaged data. In the ACE data we find that when using SSC as a time reference, the SSC-related jump in solar wind parameters is very well revealed, but near the storm peak, Bz does not follow the well-known criteria for intense storms (Bz<-10nT for more than 3 hours). When the zero epoch time is chosen near the storm peak, the jump in solar wind parameters is not as sharp (and eventually lost) but the criterion for Bz is met. Regarding the MPA data, there are certain parameters that require the choice of a certain epoch time in order to reproduce an accurate behavior and others that are less sensitive to the choice of the epoch time when doing the superposed epoch analysis, since they appear to be less distinct in their temporal and spatial location. For instance, the night-side and morning-side hot ion density along with the hot ion anisotropy are main phase traits and a zero epoch time near the peak of the ring current enhancement is required to make these features distinct. Conversely, the hot electron density exhibits less structure requiring a particular epoch time choice.