Far-side active region emergence catalogue from Solar Orbiter/PHI

Solar Orbiter is able to cover the far-side of the Sun for almost 6 months every year. This allows us to detect far-side flux emergence or disappearance events that remain undetected at Earth for several days. We showed in a previous study that these events, although located on the other side of the Sun, can affect the modeling of the Sun-Earth chain and change space weather previsions (Perri et al. 2024).

Our aim is to scan Solar Orbiter/PHI data in order to provide a catalogue of the major far-side events undetected at Earth.

We scan data for the period 2022-2024, and for each year we look at the data between March and September where the far-side coverage is the best. We combine SO/PHI maps with SDO/HMI, and compare them with GONG-ADAPT synoptic maps used in space weather forecasts. We use a specific post-processing in order to make the data comparable, and find criteria and thresholds to help us detect major differences between day-side and far-side magnetic fields.

We find a list of 27 true flux-emergence events, and an additional list of 3 events where a decaying active region actually regained an intense magnetic field. The delay between the far-side and the Earth field of view detection ranges from 2 to 15 days, with a peak at 12. All these far-side emergence events take place at low latitudes (between -25 and 30) due to the fact that we are at the beginning of solar cycle 25. However, they appear at all longitudes (no active longitude for this kind of events). They all show a similar size (about 10 degrees in both latitudinal and longitudinal extent). We compare these observations with far-side maps from both GONG and HMI websites, and find an average delay of 4 days for the detection for HMI, and 7 days for GONG.

This catalogue can be used to improve space weather forecasts, and shows the need for synchronic views of the Sun.