Coronal holes (CHs) 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 when viewed in extreme ultraviolet (EUV). Linked to unipolar (i.e. "open") magnetic fields stretching into space, they are the source of the high-speed solar wind which can create geomagnetic disturbances. Space weather forecasters at the SIDC of the Royal Observatory of Belgium assign a number to each CH that appears on the Sun (link). Returning CHs get the same number. SIDC CH 116 was first numbered on 23 May 2025 and started its fifth transit over the solar disk on 6 September. This was indeed already the fifth appearance of this CH. It had a negative magnetic polarity, meaning that the open magnetic field lines were pointing towards the Sun. The EUV images underneath were taken by GOES/SUVI 193 and show how the CH evolved during its 5 passages over the Sun's central meridian (CM): from a narrow, elongated shape in May over a grouping of irregular, smaller CHs in July to the current single, supersized entity.
The area of the current CH is over 250 times the total surface area of the Earth. It was by far the largest of the 5 transits, but in the past there have been many other and larger CHs (see these STCE newsitems here and here for some references). The table underneath gives for each of the transits the date of the CM passage, the area of the CH in billion km2 (from SIDC's SolarMap), the maximum speed (km/s) of the associated high-speed wind stream (HSS) and the time (in hours) that the speed remained above 500 km/s (DSCOVR). The last two columns give the Kp index (GFZ Potsdam) and the electron fluence, i.e. the number of highly energetic electrons (energies of 2 or more MeV) per cm2 and per steradian, accumulated over 24 hours at geostationary orbit (GEO). More info on the Kp index and the electron fluence can be found on the STCE's Space Weather Classifications page.
The effects of the HSS associated with the first passage of SIDC CH 116 were clipped short due to the passage of a fast and potent coronal mass ejection on 1 June (STCE newsitem). But even then, this was still the strongest of the first four high-speed streams. The electron fluence reached moderate levels (more than 5 . 107 electrons / cm2 sr day during each of the 4 first passages, but now with the ongoing wind stream there's a chance that the electron fluence will increase to high levels over the next few days. As the current solar cycle continues its (gradual) decline, the number and the size of coronal holes will increase. The associated wind streams may then drive the number of energetic electrons at GEO to even higher levels, to the point that these "killer electrons" pose a genuine threat to satellites. Note that fluence values greater than 5 . 107 electrons / cm2 sr day are already indicative of adverse space weather conditions hazardous to GEO satellites. As these charged particles accumulate around the electronics of the satellite, an electrostatic discharge may occur (ESD ; see image underneath) that may affect the component and even cripple the satellite. A notable example of satellite damage due to ESD is the consecutive outage of Telesat Canada's Anik-E1 and Anik-E2 geostationary communication satellites on 20 January 1994 that interrupted telecommunication and data transmission services across Canada for a considerable time.
