On 7 January, a strong geomagnetic storm (Kp=7; see the NOAA scales) was observed. The solar wind magnetic field turned southward with maximum values of -21 nT, which is even a bit stronger than the 12 September storm from last year (-18 nT; see this news item).
This geomagnetic storm took the space weather forecasters by surprize. Most expected active to quiet conditions (Kp=4 or less) for 7 January as the geomagnetic field was recovering from the effects of a high speed stream from an impressive coronal hole near the Sun's south pole.
Usually, examination of the ACE solar wind data already reveals some clues concerning the source of the disturbance such as a coronal mass ejection (CME) or the high speed stream from a coronal hole (CH). In this case however, there was no clear shock observed in the solar wind speed (yellow curve below), which was not in favor of a CME. The solar wind speed only gradually increased from about 420 km/s around midnight to a maximum of 480 km/s about eight hours later. This low speed is also not very typical for a CH, and on top of that, the low temperature (green curve) and rather high density (particles per volume unit; orange curve) seemed to further exclude a CH as the source for the disturbance.
Hence, the current consensus is that this was a CME after all, based on the evolution of the orientation of the magnetic field (red curve above). This started slightly positive (northward oriented) between 02:00UT and 05:30UT. Then, as the core of the disturbance passed, the field quickly became strongly negative for about 4 hours, gradually turning northward (+15 nT) again before returning to nominal levels around 20:00UT. This is indicative for the rotation of the magnetic field of the core of the disturbance, thus favoring a CME over a CH, despite the lack of a shock in the solar wind speed. This happens occasionally, and a similar case was already discussed in this news item.
So, the obvious question now became where this CME was coming from. From the observed solar wind speed at Earth, a CME would be expected to have lift off somewhere during the weekend. Also, no shock was observed, so it should be a relatively slow CME too. As it turns out, not many CMEs were observed during that time frame by SOHO's coronagraphs. In fact, there's only one serious candidate: A weak, diffuse CME directed to the southeast observed on 3 January, with a speed of only about 150 km/s (image above). Initially, forecasters thought this CME was not going to be directed to Earth, but now, after careful examination of difference images (one image subtracted from the previous), it seems there might have been a very faint earth-directed component. See the annotated difference image from CACTus underneath.
Linking this CME to some eruptive event on the solar surface was not so obvious. For example, the CME is not related to the M-class flare that took place on 3 January peaking at 09:47UT. Indeed, the CME was already visible several hours prior to the flare, and the flare itself was not associated with any of the CME-related features such as Type II radio-bursts, transient coronal holes (EUV dimmings), post-flare coronal loops,... (see these news letters for more info here and here). Interestingly, some EUV dimmings were observed in SDO/AIA imagery during the early morning hours on 3 January, indicating that some erupting event near the tip of the large southern polar coronal hole had punctured a temporal hole through the corona, as shown in this LMSAL movie. It is quite likely that this was the source of the observed CME early on 3 January. No flaring was associated to this dimming. This kind of CMEs, i.e. without a distinct association to solar surface activity, is called a stealth CME.
The above script is certainly not the final one. No space weather forecaster would predict such a strong storm based on the EUV and coronagraphic images, if any impact would have been predicted at all. Indeed, the most pressing question is how such a weak CME can have such intense magnetic fields at Earth, and cause such a strong geomagnetic storm. Maybe, the 3 January CME got scooped up by the fast stream of the nearby CH. Or the conditions were such that the erupting flux rope efficiently could compress the magnetic sheat that was in front of it as it lifted off the Sun. At any rate, the last word on this curious case has not been said. Events as these underscore that we still have no full understanding on the origin of heliospheric disturbances that may affect space weather conditions at Earth.
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