

Christopher Immanuel
424 posts

@NonlinearChris
Plasma physicist. "There is no prize to perfection. Only an end to pursuit."







Why this was a truly high-end CME (a near Carrington event if you will). The solar wind speed was extreme - the shock front going at about 1200-1300 km/s, crossing the distance between L1 and the Earth in only about 20 minutes. This is exceptional, we have not seen such a CME hit the Earth in two decades. But the most exceptional is the IMF (Bt/Bz). Bt peaked at a whopping 91 nT (21:24 UTC) in the late part of the sheath or early part of the flux rope. This is more than during the 10-11 May 2024 G5 Gannon storm (74 nT), 10-11 October 2024 G4 storm (46 nT) and also more than during the 2003 Halloween storms (63 nT & 40 nT). Unfortunately, Bz remained strongly positive, up to 81 nT. It only turned negative after 5h UTC, but never reached below -26 nT. Imagine if the Bz curve was inverted or equal to -Bt. This would have resulted in Bz in -50 to -70 nT range for up to 5 hours. Bz was strongly negative, in the -30 to -50 nT range for about 5 hours during the 10-11 May 2024 G5 storm, with periods of positive Bz during this period. Solar wind speed at this time was 700-750 km/s, far lower than last night. It reached -47 nT and was below -35 nT for about two hours during the 10-11 October 2024 G4 storm, while solar wind speed reached 700 km/s. These two events were lower-speed, but strong and persistent Bt and -Bz. The 2003 Halloween storms, two G5 storms in two days, were caused by super fast CMEs with less favourable Bt/Bz. The first storm reached G5 during a short (15 min) dip to -54 nT in the sheath region, but a solar wind speed likely close to 2000 km/s. The second reached G5 during a 3-hour dip below -20 nT down to -35 nT in the flux rope, with solar wind speed likely in the 1200-1300 km/s range. The big CME last night combined both extreme solar wind speed and extreme Bt. There is little doubt in my mind that had Bz been strongly negative in the CME flux rope last night, we would have seen a G5 storm significantly stronger than the Gannon storm or the 2003 Halloween storms. Likely the strongest since at least the 13 March 1989 G5 event. Alas we got a taste of what a brutally strong & persistent Bz is like. Ugh.















