Geomagnetic activity nowcast - short explanation - suomeksi

This is an empirical method based on near-real time solar wind observations and ground magnetometer data.

Solar wind data

The US Advanced Composition Explorer (ACE) observes solar wind parameters the Lagrange 1 libration point which is a point of Earth-Sun gravitational equilibrium about 1.5 million km from Earth and 148.5 million km from the Sun. We use near-real time data of ACE provided by the NOAA/Space Weather Prediction Center.

We use 1-minute data of the latest hour of the interplanetary magnetic field (IMF) Bz (northward component) and solar wind velocity and proton density. We apply the following criteria to estimate solar wind activity based on the hourly average:

activity description	%	IMF Bz (nT)	velocity (km/s)	density (1/cm³)
extreme	2.8	-7.4		18.2
very high	5.3	-4.9		12.6
high	9.3	-3.4		9.1
increased	40.5	-1.2		4.1
quiet	42.1	larger than -1.2	smaller than 393	smaller than 4.1

This is a simplified version of the corresponding table for the ground magnetic field. The parameter limits are based on data of 1996-2011. Example: if the average value of the solar wind velocity of the latest hour is 500 km/s then activity is at the "increased" level. Note: we consider that only negative IMF Bz may indicate activity. So all positive Bz values correspond to a quiet level. However, if the velocity of the solar wind is high then there may be geomagnetic activity even with positive Bz.

Since we use near-real time data in an automatic routine, there is no strict control of possibly erroneous values. The number of available 1-minute values of the latest hour may be less than 60.

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