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Ann. Geophys., 22, 3513-3522, 2004
www.ann-geophys.net/22/3513/2004/
© European Geosciences Union 2004
Multi-technique investigations of storm-time ionospheric irregularities over the São Luís equatorial station in Brazil
E. R. de Paula1, K. N. Iyer1,3, D. L. Hysell2, F. S. Rodrigues1,2, E. A. Kherani1, A. C. Jardim1, L. F. C. Rezende1, S. G. Dutra1, and N. B. Trivedi1
1National Institute for Space Research (INPE), C.P. 515, 12.245-970, São José dos Campos, SP, Brazil
2Cornell University, 2108 Snee Hall, Ithaca, NY, 14853, USA
3Department of Physics, Saurashtra University, Rajkot, 360 005, India
Abstract. On 11 April 2001, a large magnetic storm occurred with SSC at 13:43 UT, and
Dst reached below -200nT after two southward Bz excursions. The Kp index
during this storm reached 8 and remained high (>4) for about 21h,
and the São Luís magnetometer H component presented simultaneous
oscillations and decreased substantially relative to the previous
magnetically quiet days. This storm triggered strong ionospheric
irregularities, as observed by a recently installed 30MHz coherent scatter
radar, a digisonde, and a GPS scintillation receiver, all operating at the
São Luís equatorial station (2.33° S, 44° W, dip latitude
1.3° S). The ionospheric conditions and the characteristics of the
ionospheric irregularities observed by these instruments are presented and
discussed. The VHF radar RTI (Range Time Intensity) echoes and their power
spectra and spectral width for the storm night 11-12 April 2001, were used
to analyse the nature and dynamics of the plasma irregularities and revealed
the coexistence of many structures in the altitudinal range of 400-1200km,
some locally generated and others that drifted from other longitudinal
sectors. The radar data also revealed that the plumes had periodic eastward
and westward zonal velocities after 22:20 UT, when well-developed quiet-time
plumes typically drift eastward. Another interesting new observation is that
the F-layer remained anomalously high throughout the 11-12 April 2001 storm
night (21:00 UT to 09:00 UT next day) (the LT at São Luís is UT
-3h), as indicated by the digisonde parameters hmF2 and h'F, which is a
condition favourable for spread F generation and maintenance. The AE auroral
index showed enhancements (followed by decreases) that are indicative of
magnetospheric convection enhancements at about 15:00 UT, 20:00 UT and 22:00 UT
on 11 April 2001 and at 00:20 UT (small amplitude) on 12 April 2001,
associated with many Bz fluctuations, including clear two southward
incursions that gave rise to large and long lasting Kp values and large
negative Dst values. This intense auroral activity generated disturbance
dynamo and prompt penetration electric fields that were responsible for the
maintenance of the F-layer at a high altitude along the night
of 11-12 April 2001. The short-lived F-region height rise seen between 16:00 to 18:00 UT on
11 April 2001 is probably due to the prompt penetration eastward electric
fields of magnetospheric origin during the first IMF Bz turning to south
around 15:00 UT.
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