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On ionospheric phenomena during pre-storm and main phase of a very intense geomagnetic storm

Chukwuma V.U.

Acta Geophysica

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2010

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Vol. 58, no. 6

1164-1192

EN

An investigation to elucidate the mechanisms responsible for the pre-storm and main phase ionospheric phenomena during November 20- 21, 2003, is presented using heliophysical, interplanetary, geomagnetic, and global ionospheric data. The results show that the ionospheric responses in the main phase do not indicate prompt penetration electric fields as the main ionospheric driver. The results also show that the prestorm phenomena do not originate from a local time effect. The simultaneous occurrence of foF2 enhancements at two widely separated longitudinal zones appeared to suggest a role played by the magnetospheric electric field. However, the analysis of hmF2 at the stations could not confirm the notion that these fields are the main drivers of pre-storm phenomena. An investigation of flare effects on the pre-storm phenomena also revealed that solar flares are not the main drivers. The present results appear to suggest that the pre-storm ionospheric phenomena could be a result of some underlying mechanisms that are working together with varying degree of importance.

2

On heliophysical and geophysical phenomena during October-November 2003

Chukwuma V.U.

Acta Geophysica

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2009

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Vol. 57, no. 3

778-800

EN

A study of the geomagnetic storm of November 20-21, 2003, is presented using Solar X-rays data, solar wind parameters and magnetic index, Dst. The results suggest that very large X class flares may not cause very intense geomagnetic storms, as flares of M importance would do. Furthermore, the results suggest that the solar wind structure that was responsible for this storm is of the shock-driver gas configuration in which the sheath is the most geoeffective element. Presently it is shown that an intense storm can be driven by two successive southward Bz structures without a resultant "double dip" at the boundary of these structures within the corresponding interval of the main phase. Furthermore, this study confirms earlier results that show that pressure enhancement does not cause the direct injection of new particles into the ring current region; rather it causes a local adiabatic energization of the particles already within the ring current region.

3

On the geomagnetic and ionospheric responses to an intense storm associated with weak IMF Bz and high solar wind dynamic pressure

Chukwuma V.U.

Acta Geophysica

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2007

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Vol. 55, no. 4

469-489

EN

A study of the geomagnetic storm of July 13-14, 1982, and its ionospheric response is presented using the low-latitude magnetic index, Dst, and interpreted using solar wind interplanetary data: proton number density, solar wind flow speed, interplanetary magnetic field southward component BZ, and solar wind dynamic pressure. The F2 region structure response to the geomagnetic storm was studied using foF2 data obtained during the storm from a network of various ionosonde stations. Our results appear to show simultaneous abrupt depletion of foF2 that occurred at all latitudes in both the East Asian and African/European longitudinal zone during the period: 18:00–19:00 UT on July 13 and is as result of an abrupt in-crease in the dynamic pressure between 16:00 and 17:00 UT. The dynamic pressure increased from 3.21 to 28.07 nPa within an hour. The aforementioned abrupt depletion of foF2 simultaneously resulted in an intense negative storm with peak depletion of foF2 at about 19:00 at all the stations in the East Asian longitudinal zone. In the African/European longitudinal zone, this simultaneous abrupt depletion of foF2 resulted in intense negative storm that occurred simultaneously at the low latitude stations with peak depletion at about 20:00 UT on July 13, while the resulting negative storm at the mid latitude stations recorded peak depletion of foF2 simultaneously at about 2:00 UT on July 14. The present results indicate that most of the stations in the three longitudinal zones showed some level of simultaneity in the depletion of foF2 between 18:00 UT on July 13 and 2:00 UT on July 14. The depletion of foF2 during the main phase of the storm was especially strongly dependent on the solar wind dynamic pressure.