Investigating potential benefits of future sub-L1 missions with STEREO-A

Abstract

We present the first statistical study of geomagnetic storm forecasting using in situ data from the STEREO-A spacecraft as a sub-L1 monitor. Between November 2022 and June 2024, STEREO-A crossed the Sun-Earth line, covering longitudinal and radial separations of +/-15° from the Sun-Earth line and 0.01-0.06 au from Earth. This passage provides a unique opportunity to assess future sub-L1 mission concepts by ESA, such as HENON and SHIELD. We identify 32 coronal mass ejections (CMEs) observed by both STEREO-A and L1 spacecraft. Eight of these 32 CME events are first detected at L1, indicating that radial spacecraft separations of up to ~0.05 au do not always yield lead time advantages. Furthermore, we find greater (smaller) gains in lead time when STEREO-A is east (west) of the Sun-Earth line. We develop a baseline methodology for the use of future sub-L1 in situ data to enable time-shifting and real-time modeling of the geomagnetic SYM-H index. This is run continuously over the entire time period, therefore modeling the geomagnetic response of all solar wind structures. Our methodology is empirically motivated and should be considered a first approach in addressing the use of sub-L1 data. Following this methodology, 26 of 47 observed geomagnetic storms are correctly identified from STEREO-A data. Intense events (82%, SYM-H<-100 nT) are well detected, most of which are also associated with an identified CME event. Most SYM-H minima are predicted later (72%) and stronger (58%) than those observed due to biases introduced by our methodology.

Figures (8)

• Figure 1: Differences in heliocentric distance for the different spacecraft. (a) Heliocentric distance, $r$, of STEREO-A (blue), ACE (magenta), DSCOVR (orange) and Earth (green). The solid black vertical line indicates the Sun-Earth line crossing of STEREO-A. Periods where STEREO-A and Earth are separated by $\Delta r>0.05$ au are shaded in green. Minimum and maximum radial separations between STEREO-A and Earth are indicated as dashed vertical black lines. (b) Differences in heliocentric distance $\Delta r$, for STEREO-A and Earth (green) and STEREO-A and ACE/DSCOVR (magenta/orange) in au.
• Figure 2: Differences in arrival time ($\Delta t_{s}$) between STEREO-A and L1 for the 32 CME events listed in Table \ref{['tab:CME_events']}. (a) STEREO-A (color-coded), ACE (magenta), and Earth (green) positions are plotted with the coordinates given in Heliospheric Earth Equatorial (HEEQ) coordinates. The colored horizontal dashed lines indicate the heliospheric distance range for spacecraft at L1 (magenta) and Earth (green) from November 2022 to June 2024. At STEREO-A positions, the difference in event start time for the identified 32 CME events are color-coded corresponding to the gain/loss in lead time. The shaded green areas indicate regions where STEREO-A and Earth are radially separated by more than 0.05 au. (b) $\Delta t_s$ as a function of radial separation from STEREO-A to L1. The colorbar indicates the arrival speed of the CME events at STEREO-A. (c) $\Delta t_s$ is plotted against the arrival speed of the 32 CME events. The gray area represents the expected time shift for spacecraft separations of 0.01--0.05 au, assuming a purely radially propagating CME front. The colors indicate the longitude of STEREO-A in HEEQ coordinates. (d) $\Delta t_s$ as a function of STEREO-A's longitude, given in HEEQ. As in panel b, the colorbar indicates the arrival speed of the CME events at STEREO-A.
• Figure 3: Differences in properties between STEREO-A and L1 for the 32 CME events. (a) Relative change in mean magnetic field strength $B_{tot}$ from STEREO-A to L1 plotted against the start time of the event at STEREO-A. Data points marked as stars indicate a gain in lead time ($\Delta t_s > 0$), while circles indicate a loss in lead time ($\Delta t_s < 0$), where the lead time is again defined via the shock/start of a MO as in Figure \ref{['fig:diff_arrivals']}a. (b) Time difference in measured minimum $vB_z, \Delta t_{vB_z}$, between STEREO-A and L1. The symbols indicate again, whether the start of the event was measured first at STEREO-A (stars) or L1 (circles).
• Figure 4: Calculated time shift for STEREO-A data when shifted to Earth. The blue line shows the expected shift when the parameters $V$ and $n$ in Equation \ref{['eq:time_shift_1']} are set to 444 km s$^{-1}$ and 0.2, respectively, with the corresponding $1\sigma$ spread from the ensemble. The gray line corresponds to the time shift when inserting the actual measured speed instead of the averaged into Equation \ref{['eq:time_shift']}. The actual difference in arrival of the 32 CME events between STEREO-A and Earth is given by the blue data points. For comparison, the dashed yellow line indicates the mean expected time shift for L1 data as approximately one hour.
• Figure 5: Overview of hits, misses, and false alarms when using STEREO-A data to model the geomagnetic response. SYM-H minima of 47 observed geomagnetic storms from November 2022 to June 2024 are shown. Events that are detected by indices modeled from STEREO-A data are indicated as yellow stars. Missed geomagnetic storms are marked as empty circles, whereas filled blue circles correspond to false alarms, i.e., geomagnetic storms that are forecasted but not observed.