I checked a few sites about aurora: http://www.aurora-service.eu/aurora-school/all-about-the-kp-index/ https://www.swpc.noaa.gov/products/aurora-30-minute-forecast https://www.swpc.noaa.gov/products/3-day-forecast The 30 min forecast gives the probability of visible aurora on a grid, while the 3 day forecast gives only maximum Kp index. It is not clear what is the relationship between the brightness of the aurora and these values. For example if there is high probability for a certain coord in our grid, then does that mean that we will have a bright aurora with a good chance too, or does that mean that we will have a faint aurora, but we can be certain it will be there? The same is true for the Kp, they write that the higher the Kp the further we can see aurora from the poles, but they don't tell us if closer to the poles the brightness will raise with the Kp or it just stays the same. They don't tell anything about whether they calculated the daytime, moon phases, weather into the probability of visible aurora values or not (probably not). It looks really weird from scientific perspective that space agencies are using this kind of values without clear definition and units. Can anybody clarify what these values mean and what is the relationship between them? It would be nice to forecast both the brightness and the probability of the aurora with some sort of distribution for a certain coord, but maybe I ask too much...
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$\begingroup$ Looks like even scientists don't know what the Kp index means: sciencedirect.com/science/article/abs/pii/0032063363900746 $\endgroup$– inf3rnoCommented Dec 31, 2019 at 17:19
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1$\begingroup$ I think citing a paper from the 60s is a bit misleading to use as evidence that scientists do not understand what a Kp index means. This is especially true since space plasma physics is only ~50 years old... $\endgroup$– honeste_vivereCommented Jan 1, 2020 at 16:56
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$\begingroup$ @honeste_vivere You are right, I forgot to check the year. $\endgroup$– inf3rnoCommented Jan 1, 2020 at 18:50
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$\begingroup$ @honeste_vivere Thanks! I think I'll use the probability grid then and don't try to calculate the brightness. I guess only NOAA has the data to do that. $\endgroup$– inf3rnoCommented Jan 1, 2020 at 18:52
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$\begingroup$ There should be available Kp and AE indices through OMNIWeb via NASA's SPDF services. $\endgroup$– honeste_vivereCommented Jan 1, 2020 at 20:42
1 Answer
Both the Kp index 1, 2 and AE indices 3, 4, 5, 6 are indications of geomagnetic activity. However, auroral brightness depends upon the total number of oxygen and nitrogen atoms that are excited 7, 8, which depends upon the number of precipitating electrons in a certain energy range. The last part is loosely tied to AE index and, I think, even more loosely tied to the Kp index. Generally these things are not used alone or without context. For instance, a large Kp during a large geomagnetic storm can be associated with a very dim, red aurora seen down to the tropics e.g. Carrington event.
Of course the indices are not meant to fully describe the aurora. They were only ever meant as forecasting numerical values to give a rough description of the disturbance level of the geomagnetic field and auroral region.
It looks really weird from scientific perspective that space agencies are using this kind of values without clear definition and units.
The AE indices are given in units of nanotesla and are a measure of the deviation of the geomagnetic field from a nominal quiet time measured by multiple ground-based magnetometers at large geographic (and geomagnetic) latitudes.
The Kp index is a numerical latch-filter-like measure of the global disturbance of the magnetic field as a function of geomagnetic latitude measured by multiple ground-based magnetometers. That is, the further away from the geomagnetic poles a global disturbance is measured, the larger the Kp index.
Note that one can have large AE indices with a small Kp index if the disturbances are limited to the magnetic poles as is sometimes the case during substorms.
