The first CME reached the NOAA’s DSCOVR and ACE spacecraft at the L1 Lagrange point (approximately 1.5 million km / 930 000 miles away from Earth) at 15:55 UTC on December 31 and impacted our planet at 16:21 UTC, with a deviation of 17 nT recorded at the Boulder station.
Bt measured by DSCOVR jumped from nominal background levels up to around 20 nT at the time of impact, Bz jumped to -16 nT, and wind speed jumped up to around 450 km/s.
After the shock arrival, Bt reached a peak of 27 nT at 17:35 UTC and then held largely steady at ~23 nT. Bz was largely negative, with a maximum of -18 nT at 18:59 UTC, while wind speeds ranged from 350 km/s to 489 km/s.
Total field slowly diminished to 5 nT by 01:30 UTC on January 1 before increasing once again to 14 – 26 nT. The Bz component reached a maximum of -23 nT at 10:20 UTC.
Solar wind speed increased around the same time to between 490 – 570 km/s, possibly associated with the influence of a second CME. Bz had a sustained period of around negative/southward 22 nT from around 10:00 UTC to 16:45 UTC on January 1, after which is flipped to largely positive/northward values.
Total field diminished from 25 nT to between 6 – 13 nT by 21:00 UTC on January 1 as effects from the CMEs slowly waned. Solar wind speed fluctuated between 456 km/s to 562 km/s.
A slow return to nominal levels is expected by late on January 2. Late on January 3 to early on January 4, a potential glancing blow from the January 1 CME is likely to cause another enhancement.
Geomagnetic filed responded with G1 – Minor geomatnetic storm periods starting at 05:00 UTC on January 1, followed by G2 – Moderate levels at 10:44 UTC and G3 – Strong (K-index of 7) at 14:10 UTC.
The G4 – Severe Geomagnetic Storm Alert threshold was reached at 17:41 UTC on January 1.
A G4 – Severe geomagnetic storm can cause significant disruptions across multiple systems. Power grids may experience widespread voltage control issues, with some protective mechanisms mistakenly disconnecting critical infrastructure. Spacecraft operations are also at risk, with surface charging and tracking problems potentially requiring corrective measures to maintain orientation. Additionally, induced currents in pipelines may interfere with preventive measures, increasing the likelihood of damage over extended periods.
Communication and navigation systems are heavily impacted during a G4 storms. High-frequency (HF) radio propagation can become sporadic, particularly in polar regions, while low-frequency radio navigation may be disrupted entirely. Satellite navigation systems, including GPS, often suffer degraded accuracy for hours.
During such events, auroras are visible at unusually low latitudes, such as Alabama and northern California, near geomagnetic latitudes of 45°. The Kp index during these storms typically reaches 8, with occasional spikes to 9-, and such storms occur about 100 times per solar cycle (approximately 60 days per cycle).
G3 to G4 storming lasted for more than 9 hours, with northern lights observed descending as far south as Mexico.
Among the more striking phenomena during this event was the widespread appearance of SAR arcs, a distinct atmospheric feature often mistaken for auroras.
“This is a beautiful SAR arc,” explains photographer Paul Martin. “It stretched across the sky after the main aurora show earlier in the evening. What a way to kick off 2025!”
Unlike typical auroras, which form when charged particles from solar storms collide with the atmosphere, SAR arcs are a signature of heat energy escaping from Earth’s ring current system—a massive, donut-shaped flow of electric current encircling the planet. First discovered in 1956, these arcs were named “Stable Auroral Red arcs,” though researchers later realized they are not auroras at all.
On January 1, the intense geomagnetic storming energized Earth’s ring current, causing energy to leak into the upper atmosphere and generate SAR arcs.
Observers across Europe and North America captured the arcs’ unique pure red glow, a color that can be difficult for the human eye to detect due to its wavelength of 630 nm. However, cameras, including those on smartphones, can easily capture this phenomenon. During future geomagnetic storms, pointing a camera at the sky could reveal these rare and stunning atmospheric features.
References:
1 Forecast discussions – SWPC – December 31, 2024 – January 2, 2025
2 Two CMEs to impact Earth, G3 – Strong Geomagnetic Storm Watch in effect for December 31 – The Watchers – December 30, 2024
3 The Storm is Over – SpaceWeather – January 2, 2025