Magnetospheric ULF Magnetospheric ULF wave activity monitoring wave activity monitoring based on the ULF-indexbased on the ULF-index

OLGA KOZYREVAOLGA KOZYREVA and N. Kleimenova and N. Kleimenova

Institute of the Earth Physics, RAS Institute of the Earth Physics, RAS

Various geomagnetic indices (Kp, Ap, Dst, Various geomagnetic indices (Kp, Ap, Dst, SYM-H, AE, AU, AL, PC, etc.) characterize SYM-H, AE, AU, AL, PC, etc.) characterize the level of the electrodynamics of the near-the level of the electrodynamics of the near-Earth environment. However, there was no Earth environment. However, there was no index for the wave activity monitoring. index for the wave activity monitoring.

We developed the special index of the We developed the special index of the dayside ULF wave activity dayside ULF wave activity in order to in order to estimate estimate the level of low-frequency the level of low-frequency turbulence, which we called turbulence, which we called ““ULF-indexULF-index”.”.

The most promising frequency range for the The most promising frequency range for the index definition is the Pc5 band (~ 2-7 mHz) index definition is the Pc5 band (~ 2-7 mHz) with the most intense fluctuationswith the most intense fluctuations is of key is of key importance.importance.

ULF-indexULF-index

The wave index as a proxy The wave index as a proxy of global ULF activity is of global ULF activity is reconstructed from the 1-reconstructed from the 1-min data of the magnetic min data of the magnetic stations arrays in Northern stations arrays in Northern hemispherehemisphere::• INTERMAGNET INTERMAGNET • Greenland ArraysGreenland Arrays• MACCS MACCS • 210 Magnetic Meridian Chain210 Magnetic Meridian Chain• Russian Arctic magnetic Russian Arctic magnetic

stations stations • DMI WDC observatories DMI WDC observatories

ULF-indexULF-index

Nk

ij Tif

NkAT

1

1 fH

fL

SN dffFTi )(

O.O. Kozyreva,Kozyreva, V. Pilipenko, M. Engebretson et V. Pilipenko, M. Engebretson et al.,al., Planetary and Space Science, 2007, 55, pp.755–769.Planetary and Space Science, 2007, 55, pp.755–769.

BASE VERSION fmHz

03 -15 MLT

Pc5 / Pi3

50-60° 60-70° 70-90°

03-12 MLT

12-18 MLT

NEW

21-03 MLT

ftpftp://://spacespace..augsburgaugsburg..eduedu//MACCSMACCS//ULFULF__IndexIndex//

We upgraded the base version of ULF-We upgraded the base version of ULF-index and applied it to Pc5/Pi3 pulsations index and applied it to Pc5/Pi3 pulsations study in different MLT sectors and study in different MLT sectors and different ranges of geomagnetic latitudesdifferent ranges of geomagnetic latitudes

during the different phases of strong during the different phases of strong magnetic storms:magnetic storms:

-150 nT < Dst-150 nT < Dstminmin < -100 nT < -100 nT

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.

KozyrevaKozyreva et al., Geomagnetism and et al., Geomagnetism and AeronomyAeronomy, 2004, No.1, pp.37, 2004, No.1, pp.37--4848

Initial phase

Main phase

For an analysis we selected 19 strong isolated storms (in 1995-2002) that continued for not more than two days.

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.

RESULTS of statistical RESULTS of statistical analysisanalysis

In polar latitudesIn polar latitudes the ULF- the ULF-index is maximal in the index is maximal in the morningmorning in all phases of in all phases of magnetic storms.magnetic storms.

In auroral latitudesIn auroral latitudes the the ULF-index demonstrates a ULF-index demonstrates a strong maximum in the strong maximum in the morning morning in the main phases in the main phases of magnetic storms. In the of magnetic storms. In the initial and recovery phases, initial and recovery phases, the morning ULF activity the morning ULF activity (Pc5) is comparable with (Pc5) is comparable with the night ULF activity (Pi3).the night ULF activity (Pi3).

In subauroral latitudesIn subauroral latitudes the the enhancement of night-side enhancement of night-side ULF activity could be a ULF activity could be a result of the auroral result of the auroral magnetic activity shifting magnetic activity shifting to lower latitudes in the to lower latitudes in the main phase of storms.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 zones

abruptly 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.

15.05.199715.05.1997

- MIDNIGHT

NOON

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.

17-19.04.2001

25-27.03.1995

Strong StromsModerate Storms

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.

CONCLUSIONCONCLUSION We upgraded the base version of ULF-index and applied We upgraded the base version of ULF-index and applied it to Pc5/Pi3 pulsations study in different MLT sectors and it to Pc5/Pi3 pulsations study in different MLT sectors and different ranges of geomagnetic latitudes during the different ranges of geomagnetic latitudes during the different phases of strong magnetic storms. It was found:different phases of strong magnetic storms. It was found:

In polar latitudesIn polar latitudes the greatest ULF activity is observed the greatest ULF activity is observed in the in the morning morning sector in all phases of magnetic storms.sector in all phases of magnetic storms.

In auroral latitudesIn auroral latitudes the ULF-index demonstrates a the ULF-index demonstrates a strong maximum in the strong maximum in the morningmorning in the main phases of in the main phases of magnetic storms. In the initial and recovery phases, magnetic storms. In the initial and recovery phases, the morning ULF activity (Pc5) is comparable with the the morning ULF activity (Pc5) is comparable with the night ULF activity (Pi3).night ULF activity (Pi3).

In subauroral latitudesIn subauroral latitudes the enhancement of night-side the enhancement of night-side ULF activity could be a result of the auroral magnetic ULF activity could be a result of the auroral magnetic activity shifting to lower latitudes during the activity shifting to lower latitudes during the development of the storm main phase.development of the storm main phase.

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