Coronal holes are regions in the hot solar atmosphere ("corona") where the plasma density at that temperature is very low compared to its surroundings, and thus they look like dark shapes in the corona. Linked to unipolar, open magnetic fields stretching into space, they are the source of the high-speed solar wind which can create geomagnetic disturbances. A relatively large equatorial coronal hole started to transit the Sun's central meridian on 5 March (see the SDO/AIA 193 imagery underneath).
The related Corotating Interaction Region (CIR - see this STCE newsitem) arrived early on 8 March, and was the source of a brief, moderate geomagnetic storm. It was followed by the High-Speed Stream (HSS) from the coronal hole, with the solar wind speed increasing to about 700 km/s by 9 March (yellow curve in the DSCOVR graph of solar wind data underneath). The wind speed remained above 500 km/s until late on 10 March. The greater than 2 MeV electron fluence reached moderate levels (GOES). These energetic particles are a pest for the satellites' electronics, and thus satellite operators keep a vigilant eye on their number. More on these highly energetic electrons is at the STCE's SWx Classification page and in these STCE newsitems (here and here).
Space weather forecasters also keep an eye on the Sun's poles. Indeed, the Sun's polar fields reversed their magnetic polarity in resp. January 2024 (North) and September 2023 (South; smoothed data from the Wilcox Solar Observatory), and so the polar coronal holes should gradually start reforming. This can be seen in the annotated STEREO-A imagery underneath. The image on the left dates from 2021 at the early stages of solar cycle 25 (SC25), with still reasonably well defined coronal holes at both poles. The "+" and "-" indicate the magnetic polarity. The middle picture dates from January 2024, so just after the polar field reversal, and shows the absence of the polar coronal holes. Early 2025 (picture on the right), the Sun has developed a small but clear northern polar coronal hole that has an opposite magnetic polarity than back in 2021. Near the Sun's south pole, no clearly defined polar coronal hole has developed yet. It is expected that this will happen later in 2025. As SC25 further progresses, the polar coronal holes are thought to increase in size with occasionally an extension towards equatorial latitudes. Numerous examples can be found in previous STCE newsitems, e.g. here, here and here. The clip shows the evolution of the coronal holes from 2021 to early 2025, with 1 image per week.
