The outstanding feature in the solar wind during this storm is the long period (near 18 hours!) of southward Interplanetary Magnetic Field (IMF). This is seen in the red line in the top frame starting shortly after Oct. 13th and lasting into the next day. Southward IMF conditions are when the solar wind most efficiently transfers energy to the magnetosphere and Earth’s atmosphere, so the storm looks promising on these terms alone. IMF BZ (the component of the field that is oriented north- and southward, shown in the red line in the plot) reaches a value of almost -30nT! Values of -10nT are good enough to give us a strong response on the ground; -30nT is approaching what we would expect during a very strong storm.
Now, let’s jump down to the bottom two frames, which show the geomagnetic activity indices KP and DST. The former is an index of the global space weather activity at Earth and ranges from 1 to 9. The latter is an index of how much energy is stored in the “ring current” of the magnetosphere, a region that extends out to around 8 Earth radii (around 28,000 miles altitude.) The more negative DST, the more energy stored. In my previous post, we had hoped that this storm would intensify from “G2” to “G3” status, or a KP of 7 from 5 or 6; DST had already reached -100nT as I frantically typed. Unfortunately, my post came during the maximum of activity: both DST and KP recovered after that point. These are moderate values; this is a typical and relatively uninspiring event. Browsing the community-driven aurora monitoring website, Aurorasaurus, there is evidence of a light show reaching the northern United States, but not much further south than that.
So what happened? This is a time where the southward IMF values needed a little more help from the solar wind. The solar wind was relatively slow for a space weather storm (yellow line), staying around 400-500 km/s. During strong events, we expect much faster values- 600, 800, or even 1000 km/s. Next, we want a dense solar wind- tens of particles per cubic centimeter. When the IMF was southward, the solar wind was the least dense: <10 per cubic centimeter. The IMF was just right, but it didn’t get the help it needed to make this a very exciting space weather storm.
Finally, in the bottom frame, we see a strange discontinuity in the predicted DST as it jumps from -125nT back to zero. We see a similar jump in the KP plot. What could’ve happened here? This is a cold restart of the Space Weather Modeling Framework- something went wrong, and the code needed to be restarted from scratch. The problem could have happened anywhere in the predictive chain: in the processing of real-time solar wind data, a hardware glitch in the supercomputing environment, or an issue with the SWMF software itself. This is disappointing, but not completely unexpected- the real-time simulations at SWPC are brand new, and we – the researchers and code developers at the University of Michigan as well as the team at NOAA – will use this event to make the entire system more robust for future storms.
As a scientist, this storm will still be important. Every storm gives us an opportunity to make new observations of the sun-Earth system. With our sparse and ever-changing fleet of satellites taking measurements, each storm has the potential for something new to be discovered, even if it is not a record-setting event. This storm is one of the few that has happened in the lifetime of the young Magnetosphere Multi-Scale (MMS) satellite mission, so I expect some exciting observations to come from that.