SWPC: G2 geomagnetic storm reached Oct 4 as coronal-hole stream lingers

Note

Update, Monday morning: G1 (Minor) returned overnight, as SWPC had expected. At 11:52 PM EDT Sunday SWPC issued alert ALTK05 Serial 2054, two minutes after a new K5 warning (WARK05 Serial 2269) that runs to 11 AM EDT Monday. The estimated planetary Kp reached 5.33 for 11 PM to 2 AM EDT, then fell to 2.33 for 2 to 5 AM EDT. No K6 or higher alert has been issued since the G2 on Sunday afternoon.

Update, Sunday evening: G2 (Moderate) was reached. At 4:58 PM EDT SWPC issued alert ALTK06 Serial 726: the K-index reached 6 at 4:57 PM EDT, in the 1800 to 2100 UT period. It came about two hours after the G2 warning (WARK06 Serial 671, issued 2:47 PM EDT), which expired at 7:59 PM EDT without extension. Earlier, at 2:40 PM EDT, SWPC had issued its third K5 alert of the day (ALTK05 Serial 2052), and a fourth (Serial 2053) followed at 7:57 PM EDT. No K7 alert or warning has been issued. SWPC’s 3-Day Forecast issued at 8:30 PM EDT records a greatest observed Kp of 6 over the past day and expects G1 early on Oct 5, then quieter levels; a K4 warning runs to 7:59 PM EDT Oct 5. A G2 storm can cause power grid fluctuations and voltage alarms on high-latitude systems, add drag on low-orbit satellites and fade HF radio at higher latitudes; SWPC says aurora may be seen as low as New York, Wisconsin and Washington state.

NOAA’s Space Weather Prediction Center issued ALERT ALTK05, Serial 2050, at 1123 UTC on Oct 4 (7:23 AM EDT). The geomagnetic K-index reached 5 at 1122 UTC during the 0900–1200 UT period, which is a G1 (Minor) storm on NOAA’s scale. SWPC had issued warning WARK05, Serial 2267, at 1107 UTC, expecting K5 conditions from 1110 UTC until 2100 UTC, or 5 PM EDT.

The storm built through the night. SWPC warned at 0613 UTC that K4 was expected, and the index reached 4 at 0757 UTC (3:57 AM EDT). SWPC attributes the activity to a high-speed stream of solar wind from a coronal hole. Its three-day forecast issued at 0520 UTC said G1 levels were likely on Oct 4, after an 0030 UTC issue had briefly moved them to Oct 5.

The watch verified. SWPC’s WATA20 Serial 1128, issued on Oct 1, predicted G1 on Oct 2 and again on Oct 4. It was never cancelled, and the K5 alert came on the predicted day. As of 1216 UTC (8:16 AM EDT), SWPC had issued no K6 or G2 warning or alert and no extended warning.

G1 is the lowest level on NOAA’s scale. SWPC says effects are mainly poleward of 60 degrees geomagnetic latitude: weak power-grid fluctuations can occur, satellite operations may see minor impacts, and aurora may be visible as far south as northern Michigan and Maine. The Sun itself is quiet, with very low solar activity and no Earth-directed CMEs. On this lane it is a monitor reading, not evidence for ECDO.