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Auroral displays generally become more and more impressive with increasing geomagnetic activity. The measure of the activity level applied in this service is the rate of change of the ground magnetic field (dB/dt). When it exceeds a given level at a specific site, auroras are likely seen there. For the present time, such an area is visualised by a green zone on the map.
Concerning the near future up to 12 hours from the present, the expected region for auroras is estimated based on the latest measurements of the ground magnetic field and space weather alerts by NOAA. If there is an effective alert of sufficiently high level and geomagnetic activity is also enhanced then auroras are probable within the next hours, and their expected location is visualised on the map by green colour.
A typical situation is that there are no space weather alerts. However, it is still possible to see auroras at higher latitudes, since there is a persistent auroral oval around the poles. Its estimated location is determined by a statistical model, which relies on NOAA's prediction of global geomagnetic activity. To distinguish from the case based on effective alerts, grey colour is used for the auroral region.
Auroras are visible for the human eye only in the night time. Therefore the service provides sunset and sunrise times at selected sites. Moonlight can also disturb the visual show, although it does not fully hide bright auroras. Consequently, moonrise and moonset times are also given with the information of the nearest days of full and new Moon.
Cloudiness maps are derived from weather forecasts. Note that thin clouds do not completely hide auroras, and there can also be gaps in the cloud cover even if the forecast indicates uniform cloudiness.
The three lamps follow the occurrence of NOAA space weather alerts, SIDC Halo CME alerts and one-hour statistical forecasts of the hourly range of magnetic field north component (RX) provided by FMI. From NOAA alerts the service uses the ones which are associated with X-ray fluxes and Kp levels of 4 or higher. The RX forecast for the next hour is based on in-situ solar wind observations by the NOAA L1 satellite.
If the NOAA or SIDC alerts have been issued during the previous 72 hours or the RX forecast shows strong enough activity, information about favourable conditions will be presented with the three "lamps" and explanatory text strings. The lamps are in "on" (red) or "off" (black) state according to the following principles:
The first lamp is "on" if NOAA X-ray alert or Halo CME issuance has taken place during the previous 72 hours (no matter the alert level). The text beside the lamp tells how many hours have elapsed from the alert issuance time. If both X-ray and Halo CME have been issued, the lamp text tells about the most recent alert.
The second lamp is "on" if NOAA alert of Kp 4 or higher has taken place. For Kp=4 the lamp is "on" if the issuance has taken place during two previous hours. For Kp=5 the lamp is "on" if the issuance has taken place during three previous hours and for Kp=6 or larger the lamp is "on" if the issuance has taken place during 20 hours. The text beside the lamp tells how many hours have elapsed from the alert issuance time.
The third lamp is "on" if in the FMI forecast service the estimates of forthcoming min(RX) or max(RX) are equal or above certain station specific threshold values. The thresholds are 326 nT, 303 nT, 167 nT, 77 nT, and 64 nT for the stations KEV, MUO, OUJ, HAN, and NUR, respectively. The forecast service adjusts the parameters min(RX) and max(RX) according to NOAA solar wind observations so that RX of the next hour will be between min(RX) and max(RX) with 90% probability. The lamp is "on" if either one station in Lapland (KEV or MUO) or two stations in middle or southern Finland (OUJ, HAN, or NUR) have exceeded the threshold. The explanatory text nearby the lamp tells where activity is anticipated to occur.
Previous studies comparing auroral and magnetic activity have shown that for each camera station it is possible to determine an empirical threshold value for the magnetic disturbance level. When this threshold is exceeded, auroras can be seen in the area with higher than 80% probability. On the map for the present situation ("Now"), those latitude bands are shown with green where the near-real-time magnetic recordings show disturbance levels above the given threshold.
The forecast service is based on space weather alerts issued by the US NOAA and Belgium SIDC. Ten year archives of these alerts have been used to determine conditional probabilities which link the different alert types with the regional geomagnetic disturbance levels recorded in Fennoscandia. The probability functions derived from the statistical analysis can answer to questions such as: "What is the probability to exceed in geomagnetic actity the threshold for auroral displays at geomagnetic latitude 65 N when NOAA has issued an alert of solar flare 20 hours ago?"