05/31/2013 15:30:00 UTC

Comment Section

• This is a varSITI campaign event

• M. Temmer >> question: CME? or rather CIR which caused the intense geomagnetic storm of -125 nT?

Jie Zhang: I believe that the slow/gradual May 27 CME is the cause of the intense geomagnetic storm. The CME in LASCO is not even close to a typical halo CME (JAVA movie in SEEDS: http://spaceweather.gmu.edu/seeds/dailymkmovie_ql.php?cme=20130527 ). The extremely faint arm across the equator at a later time indicates a partial halo nature. But as pointed out by Nariaki, it would have been regarded as a backside event, due to the lack of surface signature.

Dave Webb: I agree this looks like a great candidate for ISEST study. One reason for the lack of consensus may be that the CME/ICME would have passed mostly north of the ecliptic as evidenced by the COR movies. Note that the ACE data show only brief, but intense southward field after the weak shock. Bernie’s IPS and the HI data should help nail this down. Remember also the slow June 2008 event which was deemed a “stealth” or problem CME even with the STEREO obs. (it hit ST-B, not L1).

Manuela Temmer: There was a big CH on May 29, 2013 from which a solar wind flow at 1AU of 750km/s was estimated (see [1] and [2]). The high-speed stream from the CH might have increased the geo-effectiveness of the CME.

Bothmer Volker: I have been reluctant to comment on this earlier, but the in-situ data Show a classic CIR with sector boundary. No Need for CME search. Textbook CIR storm. Forward Shocks from HSS at 1Au are not so frequent but they occur.

Dave Webb: I tend to agree with Volker B.; I had noted the distinct sector boundary right away. However, I am somewhat reluctant to put this to bed yet. First, the Dst>-100 nT is at the outer limits of what CIRs by themselves can produce (see, eg, Richardson, I.G. et al., JGR, 111, A07S09, 2006). The Dst is mostly driven by the very narrow, brief, strong Bsouth spike at early on June 1. And as earlier posts noted, there are several CME candidates, albeit weak, during this period that could have been compressed by the HSS. CMEs can erupt through the HCS and get caught up at sector boundaries. Is there evidence of ICME flow around this time (I again attach the ACE data plot)? There is the shock, and enhanced density and low T. There are also some rotations in the IMF which was enhanced for >1 day. What do others think about this?

Janet Luhmann: [...] The key to the larger storm is of course the big -Bz that came with the June passage. This is about as big as a CIR storm can get from what I have read. Also although there may not be a CME involved there could of course be some small transients (blobs or slow CME for ex.) that help make the Bz large-though some of it must arise from the standard CIR velocity deflections.

Ian Richardson: I tend to agree with Volker that this is a classic CIR with what looks like forward (5/31~18 UT) and reverse (6/2~02 UT) shocks (or developing shocks; I haven't looked at any high resolution data), and evidence of a stream interface (e.g., the increase in speed, temperature, decrease in density at 6/1~08 UT) within the CIR , preceded by the already noted sector boundary. The slow-fast stream speed difference is quite large ~300-800 km/s, so this may account for the well formed CIR. Here is a schematic of a CIR after Belcher and Davis 1971, with additions [3].

Image Data

In-Situ Data

A combination of SWEPAM and MAG data from the ACE Satellite:

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The blue lines are an approximation of the CME cloud and the red line denotes the shock.