Once again, there’s a space weather storm warning issued by NOAA’s Space Weather Prediction Center. There are accompanying news pieces, too; especially in states and areas that are more likely to see aurora. Then, of course, there are some rather colorful pieces, too, that do not help the reader understand what will happen.
Overall, it’s really hard to get a handle on how big a storm is going to be before it happens. This is true for scientists and the layperson as well. As we wait for this storm to hit, here’s some things to look for as the storm approaches and begins:
- Look at the size of the flare. Initial reports will give the “class” of the flare, which is a way of characterizing its strength. X-class flares are the ones that produce stronger events. Today’s storm is being driven by an M-class event, so don’t get too excited.
- Look for sudden changes in the solar wind speed and density. Real-time data taken from satellites positioned between the Earth and the Sun act as a first in-situ look into the conditions of the solar wind. It’s like a weather station that sees the thunderhead before you do and gives you a last-second warning. Storm onsets are associated with sudden, impulsive jumps in solar wind speed and solar wind density. The background values are 5ccm (yes, that’s five protons per cubic centimeter!) and 400 kilometers per second (km/s). If these values jump up suddenly, the storm is here! Not all storms have significant jumps, but it’s an easy tell for large events.
- Look for the solar wind’s magnetic field to turn and stay southward. This is the most important factor when trying to determine if a storm is going to drive strong aurora and other effects. When the interplanetary magnetic field (IMF) is southward oriented (it’s Z component is negative), it most effectively delivers energy to the Earth system. Look for IMF BZ to go negative for an hour or more. At -5nT (that’s nano-Tesla, a unit of magnetic flux), we’ll get a good storm, at -10nT or more, the light show will really begin.
- Watch the KP and DST indices to know storm strength. Magnetic indices are quantities that summarize the strength of space weather. KP and DST are the most important and easiest to read. KP ranges from 0 to 9 on a logarithmic scale (i.e., a storm of KP=7 is ten times stronger than a storm of KP=6, etc.). DST summarizes the energy in the Earth’s magnetosphere; a more negative number means a stronger storm. Look for DST to drop to -100nT or lower. A storm of -300nT is a very big storm!
There are many places to find these data. Real time KP can be found at SWPC’s website. You can find real time DST at the US Geological Survey’s website
Let’s put these in action for the conditions right now, before the storm hits. Going to SWPC’s space weather summary, we see that an M2 flare was observed on July 14th. Time to start paying attention! Here are the current (as of this writing) solar wind conditions as measured by the DSCOVR spacecraft:
Right now, IMF BZ (top frame, red line) is negative. That means that the solar wind is delivering electromagnetic energy to the Earth, and probably driving aurora! However, the value is only about -1nT, so it’s only delivering a little bit of energy. The solar wind is also pretty slow right now (purple line). Remember that the average speed is 400km/s, and we’re sitting at 300. The solar wind is a bit denser than usual, at 10ccm. Overall pretty boring. There are no big jumps yet, so the storm we’re expecting hasn’t started.
Next, let’s look at geomagnetic indices. Jumping over to the USGS Real Time DST page shows us that DST is currently… positive. Nothing yet. When a storm hits, expect a sudden jump in DST followed by a huge dip into negative numbers. Now, what about KP? Jumping over to the SWPC real time KP shows…
…nothing yet. We’ll watch for KP to jump during the storm. All of this is the calm before the storm, however. Note that KP is a 3-hour index, meaning that it’s slow to respond. It’s the last thing you’ll want to look for when watching a storm real time.
Of course, there’s a lot more to it. We haven’t touched on aurora watching, radiation belt intensifications, or any of the real fun stuff. But these are the basics for watching if a storm warning becomes a real space weather event.