Theoretical and experimental studies have shown that the increase of the intensity of the ring current has the consequence to produce intense and persistent disturbances in the geomagnetic field that lead to a decrease of the intensity of the geomagnetic field horizontal component (H) observed on the Earth surface. During these magnetically perturbed periods, defined as magnetic storms, it is possible to observe some variations in the geomagnetic field observed on the surface that, although irregular, have systematic features in their time behaviour. Generally, the magnetic storm starts with a sudden increase, called SSC (from Storm Sudden Commencement), of the intensity of the geomagnetic field horizontal component H. Although SSC is a global phenomenon, it can vary in latitude and local time. Immediately after the SSC (within an hour) there is the storm initial phase that begins with a rapid increase of the intensity of the horizontal component H that within 2-3 minutes can reach tens of nT.
Afterwards the H component, although fluctuating, can maintain a high value for some hours (1 to 10) and then abruptly decrease, thus reaching a value much smaller than the starting one (main phase). At this point the storm recovery phase; during this phase the field horizontal component increases again, initially with a time scale of some hours, then of some days, to reach again the pre-storm value. This evolution is statistically defined; while examining the single magnetic storms it is possible to find a wide variety of behaviours. In fact, there are perturbations during which the behaviour of H is strictly similar to the one described above and other during which not all the phases are clearly distinguishable.
This figure shows the behaviour of the horizontal component (H) of the geomagnetic field, as observed at L'Aquila geomagnetic observatory during one of the last intense geomagnetic storms (i.e. from 29/10/2003 to 30/10/2003).(Background picture was taken on 6th of November 1999 by TRACE mission).
INGV - Istituto Nazionale di Geofisica e Vulcanologia provides real time recordings of the behaviour of the geomagnetic field components measured at L'Aquila permanent geomagnetic observatory (AQU).
Moreover it is possible to view K index values, particularly: