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Magnetospheric ULF wave activity monitoring based on the ULF-index. OLGA KOZYREVA and N. Kleimenova. Institute of the Earth Physics, RAS. ULF-index.
OLGA KOZYREVA and N. Kleimenova
Institute of the Earth Physics, RAS
210 Magnetic Meridian Chain
Russian Arctic magnetic stations
DMI WDC observatories
O.Kozyreva,V. Pilipenko, M. Engebretson et al.,Planetary and Space Science, 2007, 55, pp.755–769.
Geomagnetic pulsations at frequencies of ~2–7 mHz are the most typical oscillations in the geomagnetic field. They are represented by quasi-sinusoidal Pc5 in the morning–daytime and impulsive bursts of Pi3 in the evening–nighttime sector. The amplitude of these pulsations is maximal in the auroral zone and shifts toward lower latitudes with increasing magnetic activity. In the initial phase of strong magnetic storms maximum of ULF activity is mainly observed in the morning-dayside polar cap.
03 -15 MLT
DF = 60-70°
50-60° 60-70° 70-90°
We upgraded the base version of ULF-index and applied it to Pc5/Pi3 pulsations study in different MLT sectors and different ranges of geomagnetic latitudesduring the different phases of strong magnetic storms:
-150 nT < Dstmin < -100 nT
We used the superposed epoch technique. A “zero” epoch is the UT when Dst-index reached minimal value during main phase.
We have analyzed the time interval of 48 hours for the each selected storms, i.e. 24 hours before and 24 hours after the Dst minimum.
In polar latitudes the ULF-index is maximal in the morning in all phases of magnetic storms.
In auroral latitudes the ULF-index demonstrates a strong maximum in the morning in the main phases of magnetic storms. In the initial and recovery phases, the morning ULF activity (Pc5) is comparable with the night ULF activity (Pi3).
In subauroral latitudes the enhancement of night-side ULF activity could be a result of the auroral magnetic activity shifting to lower latitudes in the main phase of storms.
As the typical example, we consider a strong magnetic storm on 14-16 May 1997.
The morning ULF activity in polar and auroral zonesabruptly increases with the beginning of the main phase of storm (~07 UT). The ULF-index values are much higher in the morning than in the night.
The second burst of ULF activity is observed at storm development maximum.
The top map is plotted for 07-09 UT corresponding to the first peak of the ULF-index, the bottom map (14-16 UT) – to the second one.
The maps confirm that in the both intervals the ULF pulsations were most intense in the morning.
In the first case the strongest ULF waves were observed at latitudes of 65°–72°, which corresponded to our definition of auroral and polar zones.
In the second case the ULF enhancement was at 59°–68°.
We constructed the maps of the ULF activity distribution in the main phase of this storm when the maximal values of the ULF-index were obtained.
The magnetograms from several auroral stations (MEA, NAQ, and LRV), located at different longitudes, shows the strong ULF wave in the local geomagnetic morning in both intervals: 07-09 UT (NAQ and LRV) and 14-16 UT (MEA).
Morning ULF pulsations are registered near the substorm maximum.
We compare the variations in the ULF index in the auroral zone during strong magnetic storms with Dstmin varying from –100 to –150 nT and moderate storms with Dstmin varying from –50 to –100 nT.
The general regularities in the geomagnetic pulsation variations during strong and moderate storms are similar and differ in only the level of amplitudes.
We upgraded the base version of ULF-index and applied it to Pc5/Pi3 pulsations study in different MLT sectors and different ranges of geomagnetic latitudes during the different phases of strong magnetic storms. It was found: