In a previous news item, it was already discussed how some CMEs use unconventional techniques to get to Earth undetected and create all of a sudden a geomagnetic disturbance.
Guess what? Those sneaky CMEs found a new way to surprise the space weather forecasters!...
It all happened last week when disturbances in the solar wind caused minor to major geomagnetic storming on 19 and 20 February, as can be seen in this movie. The last storming period (see ACE and NOAA data above; event 3) could easily be identified in the solar wind parameters (shock early on 20 February), and its source region traced back to a filament eruption early on 18 February.
Images underneath show a PROBA2/SWAP image of the post-eruption coronal loops (top left), and a STEREO-A/EUVI image at about the same time showing no flaring activity on the Sun's backside (top right). The big image at the bottom is an image from the STEREO-A/COR2 coronagraph showing that this event was a halo coronal mass ejection (CME) as seen from STEREO-A. For STEREO-A it's a backside event, as the location of the eruption was almost diametrically opposite compared to STEREO-A's location.
Things get a little fuzzier when it comes to the first storming period. A shock can clearly be observed in the morning hours of 19 February. It was probably related to material ejection from an event in the afternoon of 16 February, which can be seen in AIA 304 images as a dark cloud having a boomerang shape. Note that another candidate may be an eruptive event a few hours earlier.
In SOHO/LASCO images, no CME can be seen, in part because around the same time, there was an eruption with CME on the backside of the Sun near old region NOAA 1968. This camouflages any signs of a weak earth-directed CME. Luckily, STEREO-A provides a side view and does show a weak CME in the ecliptic plane.
Interestingly, the CME from event 2 arrived when the geomagnetic storm was already in progress. Here, it is really problematic to trace the source of the responsible CME (event 1). The best guess is that the CME is related to a M2-flare in NOAA 1974 early on 14 February. Again, no CME can be seen, but the AIA 193 images clearly show a temporary coronal hole to the north of NOAA 1974. This means that something escaped from the Sun puncturing a temporary hole in the corona.
Unfortunately, any CME signature is again camouflaged by an event with CME just a few hours later on the backside of the Sun, where good old NOAA 1967 produced a very strong flare with associated halo event as seen from STEREO-A. As (the flank of) the CME arrived at Earth on 18 February without much of an increase in solar wind speed, its sustained southwardly oriented magnetic field eventually resulted in a geomagnetic storm starting early on 19 February, just prior to the arrival of the CME from event 2.
The life of a space weather forecaster is not getting easier with all those malicious tricks that CMEs apply to get undetected to Earth. Do they have yet another trick up their sleeve? As the maximum of the ongoing solar cycle unfolds, we'll soon have our answer.
Credits - Data and imagery were taken from SDO, STEREO, SOHO/LASCO, PROBA2, ACE, and Helioviewer.
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