Solar activity reached high levels on December 4, following an isolated M6.0 flare that erupted from Region 4300 (beta-gamma) at 02:50 UTC. The event started at 02:36 and ended at 02:59 UTC.
The flare was observed in GOES/SUVI 284Å imagery and in coronagraph data, with preliminary analysis indicating that the associated coronal mass ejection (CME) is not Earth-directed. A Type II radio sweep was detected near the flare peak by two USAF/RSTN stations, with estimated shock speeds of 345 km/s and 484 km/s.
Radio frequencies were forecast to be most degraded over Australia at the time of the flare.
Multiple radio bursts and noise storms were recorded throughout the period, including a Type IV sweep at 08:32 UTC.
Additionally, a filament eruption was observed in the southwest quadrant at 11:06 UTC on December 3, but is not expected to affect Earth. Coronagraph imagery showed no additional CMEs.
Geomagnetic activity was predominantly at minor storming levels (Kp=5) during the past 24 hours, with two active periods (Kp=4) recorded from 15:00–18:00 UTC on December 3 and 09:00–12:00 UTC on December 4.
A G3 – Strong geomagnetic storming interval (Kp=7) occurred from 18:00–21:00 UTC on December 3, likely driven by the combined influence of the arriving negative coronal hole high speed stream (CH HSS) and the CME produced by X1.9 solar flare on December 1.
Solar wind parameters showed clear signatures of mixed CH HSS and CME influence over the past 24 hours. The IMF phi angle turned predominantly negative after 19:15 UTC on December 3, consistent with a negative-polarity high-speed stream.
Solar wind density increased to roughly three times nominal levels, peaking around 19:45 UTC. The IMF total field reached 20 nT, with the Bz component dropping to −15 nT. Afterward, solar wind speed increased to about 700 km/s, while density and magnetic field strength decreased to values typical of a high-speed stream.
The greater than 2 MeV electrons returned to moderate levels around 18:45 UTC on December 3 after reaching a high-level peak of 6 340 particle flux units near 13:50 UTC.
Fluxes are expected to remain moderate on December 4–5 and return to high levels on December 6 as Earth crosses an IMF sector boundary and exits the ongoing negative CH HSS regime.
Greater than 10 MeV protons remained at background levels, with a slight 15% probability of enhancement above 10 pfu through December 6 due to the flare potential of the eight active regions currently on the visible disk.
Forecasts indicate isolated to occasional M-class flares (75% probability) through December 6, with a 30% chance of X-class flares. The combination of multiple magnetically complex active regions, including Regions 4300, 4301, and 4296, supports the elevated flare outlook.
Solar wind conditions are expected to remain disturbed through December 6, with a return to nominal levels on December 7.
References:
1 Forecast Discussion – NOAA/SWPC – Issued at 12:30 UTC on December 4, 2025