Negative polarity CH HSS triggers G3 – Strong geomagnetic storm

Geomagnetic K-index of 5 (G1 – Minor) threshold was reached at 01:22 UTC on May 29, with initial impacts primarily poleward of 60 degrees Geomagnetic Latitude. This was followed by a K-index of 6 (G2 – Moderate) at 01:30 UTC and a K-index of 7 (G3 – Strong) by 02:16 UTC.

G3 – Strong geomagnetic storms primarily affect regions poleward of 50 degrees geomagnetic latitude, but impacts can extend to mid-latitudes during significant events. Power systems in affected areas may experience voltage irregularities due to induced currents, with the possibility of false alarms on some protection devices.

Spacecraft systems may be subject to surface charging, and low Earth-orbit satellites can encounter increased atmospheric drag and potential orientation issues. Satellite navigation systems, including GPS, may experience intermittent disruptions, such as loss of signal lock and increased range errors. This can impact navigation reliability for aviation and ground-based users.

High frequency (HF) radio communications may become intermittent during strong geomagnetic storms. Aurora may be visible at much lower latitudes than usual, with sightings possible as far south as Pennsylvania, Iowa, and Oregon.

Solar wind parameters in 24 hours to 00:30 UTC on May 29 were indicative of a smaller, negative polarity coronal hole high-speed stream (CH HSS). Solar wind speed reached approximately 645 km/s at 02:00 UTC on May 28, before gradually decreasing to below 400 km/s. The total interplanetary magnetic field strength increased to 14–18 nT, with the Bz component ranging between +9 and -8 nT. The phi angle was predominantly negative during this period.

According to the Space Weather Prediction Center (SWPC), enhanced solar wind conditions are expected to persist through May 31 as CH HSS activity continues. A transition into a larger trans-equatorial coronal hole high-speed stream is likely by May 29, with solar wind speeds possibly exceeding 600–700 km/s, based on recent recurrent trends.

Active to G1 – Minor geomagnetic storm levels are likely through May 30 as the transition into another negative polarity CH HSS continues, while unsettled to active geomagnetic conditions are expected on May 31.

Solar activity was low over the past 24 hours, with most flaring limited to the C-class range. The majority of this activity originated from Regions 4099 (S13E29, Dkc/beta-delta) and 4100 (N08E43, Eki/beta-gamma). Region 4100 exhibited separation in its larger leading sunspots, while slight growth was noted in Region 4099. One new region, 4103 (S17W01, Bxo/beta), was numbered during the period.

At approximately 12:00 UTC on May 28, a filament structure estimated at 22 degrees in length, centered near N40W30, was observed lifting off in SUVI 304 imagery. Further analysis will be conducted as additional coronagraph imagery becomes available.

Solar activity is expected to remain low, with a good chance for M-class flares (50% probability) and a slight chance for an X-class flare through May 29–31.

The greater than 2 MeV electron flux remained at normal to moderate levels over the past 24 hours, while the greater than 10 MeV proton flux persisted at background values.

The greater than 2 MeV electron flux is expected to remain at normal to moderate levels on May 29, increasing to moderate to high levels on May 30–31 as high-speed solar wind persists.

There is a slight chance for an S1 – Minor solar radiation storm between May 29 and 31.