Overestimated importance of Kp index? How to interprete geomagnetic storm indicators?

[kzez1986]

I try to observe the aurora in Poland and I observed situation during many geomagnetic storms. From my observations I think that there is no clear correlation between Kp index and real visibility of the aurora. For example: in the 27th of February 2014 the aurora was photographed in southern Poland, although finalized Kp index from Potsdam was only 5+ (estimated Kp from SWPC was 6). In the 15th of Septemper 2014 there was no aurora in Poland (many people observed during the almost whole night (also I), but nobody saw the aurora, although finalized Kp index was 6+ (estimated by SWPC 7). I observed the sky also during the geomagnetic storm from the 17th March 2015, but I observed only one curtain of light short after sunset, but before it became completely dark. Watchers reported that it was not possible to observe the aurora in my location by eyes but only by camera. Finalized Kp was finalized 8-, estimated 8. I saw video from the 17th March 2013 from the aurora in my location when Kp finalized was 7-, estimated was 6. It was visual aurora because people observed the comet that dat and the aurora was a surprise. As a result, I can't see a relation between Kp index and the aurora in Poland.

 

So I'd like to know more about the geomagnetic storm factors. E.g. before Easter 2015, there's geomagnetic conditions with Kp=3 and Bt=15 and in the 18th March 2015 - Kp=6 and Bt=5. If Bt is a value that shows total power ot the geomagnetic activity, which conditions was stronger? I know that Bz should be below zero. The lower Bz (certainly, lower minus, higher number) is, the better conditions for aurora are. Is Kp related to Bt/Bz in some way?

 

THere's some maps in the Internet showing relation between Kp and aurora visibility (different maps have significant differences). E.g. I saw that the aurora during Kp 8 should shine in Poland over heads, not only low on the horizon (can be visible even in northern Italy low on the horizon). On the another map I saw that Kp 9 is needed for the aurora in the whole Poland. So is it good to use Kp index as a aurora indicator? I think, it's not good.

 

I observed that there's better view of the aurora the the solar wind speed is higher and when geomagnetic storm is caused by stronger solar flare even with the same or even lower Kp. So, I'd like to read any article which explain real impact of different indicators (Kp, Bt, Bz, solar wind speed, solar wind density and so on) on visibility of the aurora. Observing only Kp isn't enough, I think but I don't know what's the exact impact and what values are good for the aurora.

 

Is Kp index, and K-indicies the same?

[Acidus]

Kp-index and K-Indices are the same thing if i understand it correctly, but the most accurate way to predict visible aurora is magnetometer stations. I don't know of any in poland, but for me (I live in Norway) the magnetometer datas are most accurate

[Vancanneyt Sander]

The difference of K-indeces and Kp is that K-indices are for a local value and the Kp is for a global value. Read our help article about the Kp index which gives you more detail on what the K-indice is and what it's based on. 

 

There are three commonly used K-indices:

• Estimated 3h global K by Wing
• Estimated predicted global K with lead time according to the solar wind speed and measurements at ACE (15 minute cadence)
• Potsdam global K-indice
• SAM local magnetic K-indice (SAM magnetometers)
• magnetometer stations with their local K-indice

When you read our article about the Kp index you'll learn that SWPC Kp is based on a few magnetometer stations. Their measurements in local K-indices are then re-evaluated to get a global K-index value. But Potsdam Kp indices are mostly used because they are more precise and i guess they have more stations that give data and is widely known as the official Kp index. And yes SWPC is often way of the charts and is often totally not correct or exaggerating. With the introduction of Wing-Kp this would totally replace SWPC's measurements but they stil keep using it... Wing Kp is an improved model of what previously was used and is now the standard for current predictions (3h K-indices) and for quick in the future (predicted K). Wing Kp has it's faults also but is way more reliable.

The predicted Kp-index of the Wing model uses solar wind parameters and IMF parameters combined with some other ingredients to obtain a more reliable Kp-index that's more in line of what you really can expect.

 

Oh and a sidenote: the 3h Kp gives the activity of the past 3 hours, so you could already missed the storm.

 

As always we use the predicted K of the Wing model for a fast view and most of all we only look at solar wind data and IMF data and last but not least the magnetometer data. If you really want to chase aurora, than you are not much with just the Kp index.

[Guest Priit Retsep]

Look real-time magnetometer and live-cameras

[Vancanneyt Sander]

Last night was again a perfect example, the north of the Netherlands (middle latitude) got photographic aurora but the estimated Kp was only at G1. But the magnetometers did show a very strong deflection of about 800nT making it photographic possible to catch it on camera. The magnetometers won again

[Waldo Hazeleger]

Agree, yesterday I saw that the magnetometer of Dombas (in the middle of Norway) had a maximum of -638 nT, which is equivalent to a local K-index of 8. Other magneto's had much smaller deflections. This made the "official" Kp smoothend, because it is an average of about 13 magneto stations, most of them in the US, not in Europe.

Noaa's Kp-index was 6 (G2-scale), the WingKp Est-Kp only 5.33

So, to get an estimate, I use Noaa's three-day forecast, and the solar wind and Bt,Bz values from ACE, but to really know if it is active enough for my location, I look at the magnetometers in Tromso, Dombas, Brorfelde and Kiruna.

[kzez1986]

Thank you for your answers. I understand it better. But, is it a method to calculate local K-index for Poland using magnetometers. How to convert magnetometers data to local K-index and which magnetometer is best for Poland?

[Vancanneyt Sander]

Every official magnetometer station has it's own unique K-values. On the Kiruna magnetometer site you can see an example of that. You can alywas inform at a closer magnetometer station to you what values of deflection they use.