03/17/2015 04:00:00 UTC

Comment Section

• varSITI campaign event
• Largest geomagnetic storm at Earth for solar cycle 24, this event registered a Dst peak of -228 nT.
• Based on both the in-situ signature of the event and the ENLIL solar wind prediction for this date, I think it is likely a CIR played a role in making it so strong. There is a strong coronal hole at the South Pole and the ENLIL simulation ([[1]]) shows a fairly fast stream that interacts with the CME, and this fast speed stream (~600 km/s) shows up in ACE data as well. Based on the C2 and C3 images for the day, it appears there is a slow CME launching around noon on the 14th with a small but visible filament. On the morning of the 15th a partial halo CME, associated with a long duration flare that fell just short of M class (C9.1) and from the same active region (AR 12297), launched propagating to the East of the Sun Earth line. I think it is likely that an interaction between the CME+shock of this event and the previous blob CME, as well as the added energy from the CIR and fast speed stream behind the CME caused the severity of the geomagnetic activity at the Earth (Hess)
• This super storm is produced through a combination of effects: (1) strong magnetic field in the sheath region (> 25 nT at peak)) and ejecta (>30 nT at peak, (2) Bs field encompasses the entire duration of the ejecta, due to that the axis of the flux rope is highly inclined toward the north-south direction, (3) the interaction with CIR, and almost contained in a CIR region. Such containment by CIR prevents the expansion of the flux rope, thus makes the flux rope small in size by strong in magnetic field (Jie Zhang).
• This may be a kind of CME-CME interaction event. We have a large filament, embedded in a magnetic flux rope, close to the AR which released this highly geoeffective CME. Part of the filament (or flux rope) erupted - or at least, left the low corona - already on March 14 (around 12UT). The final and major eruption on March 15 seems to interact with the first disturbance. The interacting sectors might propagate close to Earth direction. This might be a reason for the complex in-situ signatures (two flux ropes?) as well as the increased geoeffectiveness (Manuela Temmer).
• With the ElEvo model results for the March 15 04:00 UT CME shock propagation from Sun to Earth, I need a quite low value of gamma to get the Wind speed and arrival time right, which reflects that this CME did not seem to experience much drag during interplanetary propagation. If the CME apex is really about 40° away from the Earth (as indicated by the source region position), I think its very surprising that Earth is hit by the flux rope. I think this is only possible if the flux rope had a very low inclination to the ecliptic, or as said before that there was some interaction with the CME on March 14. Maybe the drag parameter is low because the CIR was pushing from behind, adding an additional force? (Christian Moestl)

Image Data

In-situ data

• These are in-situ plots based on the ACE daily text files, I will update them when the cdf data becomes available. In these plots the shock is very clear, but beyond that any ejecta signature is weak and there does not appear to be any strong Magnetic cloud. But there are two clear and distinct periods of strong -Bz. (Hess)

LASCO/Kanzelhöhe

Image collection of white light and chromospheric data, showing two disturbances and the partly erupted filament which is related to the CME producing AR: [link http://www.uni-graz.at/~temmerma/download/varsiti/20150315.pdf]

GOES Plot

SOHO/LASO measurement

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• Height-Time plot based on SOHO/LASCO measurement

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• Velocity-Time plot from SOHO/LASCO H-T measurement
• Height-Time measurement data from SOHO/LASCO: Hess_measurement.docx

Interplanetary Propagation

Christian Möstl and Tanja Rollett: ElEvo results (parameters already tweaked so it matches Wind arrivals): shock arrival at Wind: March 17 03:50 UT arrival speed 665 km/s

Wind observations (taken from the Wu et al. draft): shock arrival March 17 03:59 UT arrival speed of the sheath is 500- 600 km/s, about 100 km/s less than the ElEvo arrival speed.

This model/plot can be adjusted very easily if you think the CME initial speed, direction and launch time should be different.

initial CME parameters: inital speed at 15 Rs: 1120 km/s, at time 2015 March 15 04:00 UT, direction to Earth west 39° the speed was taken from Kevin Schenk real time email, consistent with Gopalswamy et al. proceeding; same for direction. Thus I assume that the source region position is similar to the CME direction. Because the flare happens inside the AR and there are no large coronal holes nearby, it should be relatively safe to assume this direction as the CME propagation direction. The asymmetric halo with more material to the west of the Sun also supports this. Other Parameters: background wind: 400 km/s, gamma: 0.1, ellipse aspect ratio 1.6, full width: 100° in heliospheric longitude.

Video Data

AIA 171 movie
AIA 193 movie
AIA 304 movie
AIA 1600 movie
HMI movie
C2 movie
C3 movie

References

ElEvo model: Möstl et al. 2015 Nature Communications, open access: http://www.nature.com/ncomms/2015/150526/ncomms8135/full/ncomms8135.html