CME Geostorm Kp index vs Solar Flare Intensity

[Drax Spacex]

NOAA swpc geomagnetic storm maximum Kp index predicted value of the Dec 10, 2020 CME shockwave arrival of the Dec 7, 2020 earth-facing C7.4 solar flare overestimated the actual value substantially.  Arrival time was predicted accurately, but the intensity was not.  Predicted maximum was G3=Kp7 (SEVERE).  Actual maximum was Kp4 (ACTIVE).

What was the basis for NOAA swpc to issue an alert to predict a severe G3 Geomagnetic storm from just a C solar flare?  I know there are many factors, many variables, many unknowns.  But isn't it true that the intensity of a solar flare with earth-facing CME has a reasonably high correlation to the expected geomagnetic storm intensity?

From the material I have referenced, it typically takes an X intensity solar flare to produce a CME resulting in a G3 or higher storm intensity.  That same material shows that an earth-facing C flare with CME can be expected to produce a MINOR to ACTIVE geomagnetic storm (which is exactly what we saw).  See FDIM81.pdf, figure 17.

https://www.swpc.noaa.gov/impacts/hf-radio-communications
https://www.qrparci.org/resource/FDIM81.pdf

If we were to review the historical data, what percentage of C solar flares with Earth-facing CME result in a geomagnetic storm a few days later that is G3 or higher?  I suspect it is a small number.

 

[Christopher S.]

Well, I'm on my phone so I wont go too in depth, but even solar wind from coronal holes can push us up to G1 or even G2 status. A CME is a lot more dense and fast moving, so logically it would potentially be more intense at the upper-end. Moreover, a flares intensity is not the same as a CME's intensity.

[stevenfraussen]

There are several reasons why NOAA predicted a G3 storm.  The problem in predicting a geomagnetic storm is that we don't know what happens to the CME when it's traveling through space. We can calculate the speed of a CME from coronagraph imagery, but we don't know what happens after that. Dr. Tamitha Skov had an interesting theory about what happened to the CME. She believes the northern coronal hole "pushed" the CME (coming from a southern sunspot) further towards the south. This resulted in the earth only getting a glancing blow.

Another factor to take into account is the Bz value. It's happened several times that NOAA predicts G3 storming, but the Bz value is still largely unpredictable. If the CME coming from this C-class flare hit us fully, and the Bz-value would have gone to -15 for example, we definitely would have seen a geomagnetic storm. Generally, whenever NOAA predicts a G1 magnetic storm, I'm always on the watch for bigger storming because you never know what the Bz will do. 

The CMEs coming from a C-class flare do cause geomagnetic storms, but it doesn't happen that often. In the archive of the 50 heaviest geomagnetic storms, there is one storm that was caused by a C-class flare. The maximum Kp-value during that storm was Kp8. So yes, it does happen, but generally the M-class and X-class flares are the really interesting ones for strong geomagnetic storming.

[Vancanneyt Sander]

I totally follow Steven here 😉 There's a distance between the Sun and Earth where still a lot is happening that we don't understand yet. With the CH being this close an interaction was possible and I do follow the theory that the bulk was pushed further south with only a minor blow. I even remember a G3 being predicted for three CME's that had to arrive, two slower ones and a big third one that catched up the other two CME's and did a kamikaze effect and when it arrived at Earth it was nothing anymore. 

And yes the Bz, from the moment of impact until later the Bz didn't go much south and if it did it was too short to initiate anything. If the field was southward when it arrived we could have stil seen a G1.

A good long duration event makes up a good storm. Even if it's a long duration C-class, it has to be long duration,n event with a good coronal dimming (and of course on a good spot on the solar disk). (But it's always better to have a higher flare class for that extra sauce   )

@stevenfraussenGood find in the archive 😉 

[Drax Spacex]

Thanks for your insights - I think we're all in agreement.  There are opportunities here for model refinement based on new theories (e.g. the coronal hole interaction with the CME).  Bz direction being unpredictable yet having such a large influence seems to be a crux to accurate geomagnetic storm Kp prediction.  More research should perhaps be allocated to predicting Bz.  Perhaps the magnetic polarity of the sunspots of the originating solar flare imparts some Bz polarity within an associated CME?  Who knows.   I do think they should consider increasing the weighting factor of the source solar flare intensity in their CME-impacted Kp prediction model (if it improves overall accuracy, though perhaps missing some outliers).

Edited December 11, 2020 by Drax Spacex

[helios]

CME interaction with coronal hole high speed stream is not a particularly new theory (paper), it's just very difficult or sometimes impossible to predict, despite all the instruments we have. Small uncertainties in time, velocity and angle can lead to entirely different situations.
 

And here's a little reading about the prediction of the magnetic configuration of a CME (yes, that is also taken into account by the people at NOAA 😉)
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016SW001458

 

Edited December 12, 2020 by helios

[Drax Spacex]

Ah yes, light reading indeed but interesting to see that relevant research is being conducted.

As for why NOAA/SWPC tends to overestimate the maximum Kp index predictions, I found the answer to that.

"The Government Performance and Results Act (GPRA) metrics for NOAA/SWPC are based on the performance of geomagnetic storm forecasts, which monitor the Kp index...  The 24 h geomagnetic storm forecast is considered accurate if a Kp = 5 (G1 storm) or greater storm event was correctly predicted."

To comply with legislated performance goals, it is better that NOAA/SWPC overestimate their Kp index predictions.

There is no penalty for false positives (predicting a storm when one does not occur).  There is a penalty for false negatives (not predicting a storm when one occurs).

https://www.swpc.noaa.gov/content/geomagnetic-activity-forecast-verification

Edited December 14, 2020 by Drax Spacex
Added SWPC verification link