ACE-ULEIS/STEREO-SIT Spectrograms
These are 1/ion-speed plots from the Suprathermal Ion Telescope (SIT) instrument on each of the two STEREO spacecraft (Ahead and Behind), and the ULEIS instrument on ACE.
Each plot contains 20 days of data, showing arrival times of ions in the mass range 10-70 AMU (C-Fe group). Each panel plots 1/ion speed in hour/AU vs day of the year. The top panel shows STEREO-B SIT data; the middle panel shows ACE ULEIS data, and the bottom panel shows STEREO-A SIT data. This arrangement of the panels is such that corotating features will appear first on the top panel, then on the middle and bottom panels. The approximate corotation time between the spacecraft is indicated on the middle panel with thick blue and red lines. The blue line length shows the approximate corotation time between STEREO-B and ACE, and red line length shows the approximate corotation time between ACE and STEREO-A. These line lengths indicate the offset between the panels for corotating features. Although the corotation time varies over the course of 20 days, the slow separation rate of the spacecraft does not change appreciably over the time interval shown by a plot (corresponding to ~0.1 day for each line).
These panels may be helpful in identifying corotating features vs. features such as shocks or SEP events that could appear simultaneously on each panel. Consulting other data such as solar wind speed can also be helpful in identifying features such as corotating interaction regions.
There is a nearly steady background in both STEREO instruments appearing in a band at 1/ion speed of ~2.5 hour/au. This is caused by high energy particles penetrating the telescope from the back side. STEREO-A SIT also has a smaller background near 200 keV/nucleon visible in the bottom panel; this is due to an effect in the instrument electronics. During high intensity periods, these backgrounds are small compared to legitimate events. These background bands in the STEREO instruments should be disregarded in the interpretation of these plots.
Early in the mission short increases seen in at all 3 spacecraft are upstream events. As the STEREO spacecraft move far from the magnetosphere these events are no longer seen on the STEREO panels.