An asymmetric full halo Coronal Mass Ejection (CME) produced by the M5.5 solar flare at 20:33 UTC on September 4, 2017 is traveling toward Earth and is expected to arrive late on September 6. G3 - Strong geomagnetic storm watch is in effect for September 6 and 7.
Region responsible for this eruption - AR 2673 - grew vigorously from September 2 to 4, quickly becoming one of the largest sunspots of the year. It now has 'beta-gamma-delta' magnetic configuration and harbors energy for both M-class (71% chance) and X-class (25% chance) solar flares over the next three days.
The region is now moving away from the center of the Earth-facing Sun but Earth-directed CMEs are still very much likely.
M5.5 solar flare on September 4, 2017. Credit: NASA/SDO
Initial available SOHO/LASCO coronagraph imagery indicated a likely asymmetric full halo CME was associated with the M5.5 flare. In addition, Type-II (1 472 km/s) and Type-IV radio sweeps were observed around the time of the flare, beginning at 20:42 UTC, as well as 10cm radio bursts.
Asymmetric full halo CME observed September 4, 2017 after M5.5 solar flare. Credit: ESA/NASA SOHO/LASCO C3
Initial analysis of WSA/Enlil model output determined the CME to arrive mid-to-late September 6. However, additional analysis of the CME is in progress to gather a consensus arrival time/intensity forecast.
WSA/Enlil model for September 4th CME. Credit: NOAA/SWPC
SWPC initially issued G2 - Moderate storm watch for September 6 and 7 but later upgraded to G3 - Strong watch.
G3 storms occur approximately 200 times per one solar cycle. Voltage corrections may be required and false alarms triggered on some protection devices, under G3 conditions. Surface charging may occur on satellite components, drag may increase on low-Earth-orbit satellites, and corrections may be needed for orientation problems. Intermittent satellite navigation and low-frequency radio navigation problems may occur, HF radio may be intermittent, and aurora has been seen as low as Illinois and Oregon (typically 50° geomagnetic lat.).
Solar radiation storm
Meanwhile, the greater than 2 MeV electron flux reached high levels with a maximum flux of 15800 pfu observed at 18:05 UTC on September 4. The greater than 10 MeV proton flux values observed an enhancement that was associated with the M5 flare from Region 2673, reaching S2 - Moderate levels (peak flux 106 pfu at 07:20 UTC).
S2 - Moderate solar radiation storms occur approximately 25 times per one solar cycle. Under S2 conditions, passengers and crew in high-flying aircraft at high latitudes may be exposed to elevated radiation risk. Infrequent single-event upsets are possible in satellite operations. Small effects are possible on HF propagation through the polar regions and navigation at polar cap locations.
The greater than 2 MeV electron flux is expected to continue at high levels for the next three days. The greater than 10 MeV proton flux is expected to continue at S1 levels.
Solar wind parameters
Solar wind parameters remain elevated due to influences from a polar-connected, positive polarity coronal hole high speed stream (CH HSS). Although the trend is now towards background levels, enhanced levels are again expected on September 6 and 7 due to CME impact.
Geomagnetic field forecast
The geomagnetic field was at quiet to unsettled levels over the past 24 hours with an isolated G1 - Minor geomagnetic storming period at the end of September 4, likely associated with sustained -Bz.
The field is expected to be at quiet to active levels, with isolated periods of G1 storming as CH HSS influence diminishes today. Elevated levels are expected on September 6 and 7 due to the arrival of the abovementioned CME. G2 - Moderate conditions are expected, with isolated G3 - Strong conditions likely on both days.
Featured image: WSA ENLIL model for September 4th asymmetric full halo CME. Impact exected late September 6. Credit: NOAA/SWPC