General overview
R-index is a local nowcast of auroral occurrence derived from geomagnetic measurements. The chance to see the aurora is displayed by a colour-coded approach where two aurora strength categories (determined by location-specific thresholds) are adopted:
- The first category (yellow) corresponds to a 50 % probability of weak auroras. Strong auroras are usually not seen at this value of R.
- The second category (red) corresponds to a 50 % probability of strong auroras. Probability of some (at least weak) auroras at this index value is considerably over 50 %.
The thresholds have been determined by statistical comparison with all-sky camera data. Note that "weak" auroras may be too dim to be seen by naked eye, or low on the horizon. Thus in practice, yellow values mean a lowish probability of auroras. Red values indicate that there is a good probability to see some auroras and a moderate probability for bright or extensive auroras.
Mathematical definition
R-index is calculated from a 10-minute interval of geomagnetic data with a time resolution of 10 seconds. For each component (X, Y, Z), the absolute values of the differences of 10-second averages are summed. The last 5 minutes of data are used with a weight factor 1 and the previous 5 minutes with a weight 0.5. The three components are also summed. After this, a station-specific constant is deducted from the result. This constant is chosen to correspond to the quiet-time value of the sum, i.e. its purpose is to take away the effect of instrument noise from the result.
The R-index could be interpreted as an average of the componentwise absolute value of the time derivative of the geomagnetic field. Dividing the index value by 1440 would put it in units of nT/s. However, we prefer to treat R as a dimensionless number, as its definition is tied to a specific time resolution of data, as well as orientation of components, and thus it is not a true time derivative.
How to see auroras with the help of R?
R-index is a nowcast, i.e. it correlates with the probability of auroras at the time of observation. It does not forecast the future. In practice, an auroral display often continues for 10-20 minutes and sometimes longer. So if one follows the index value inside, one needs to be prepared to go out immediately when high values occurr, and one needs to have a dark observation place with an unobstructed view of the sky very close.
Regions of auroral occurrence are typically elongated in east-west-direction, so it is advisable to look primarily at the station the latitude of which is closest to that of one's observation location.
As active auroral displays often propagate southward, one could also follow the next northward station to get a little earlier warning of an auroral show that might advance to one's observation location. Even if the auroral display does not expand southward, something could be visible near the northern horizon.