<i>F</i>-region ionospheric perturbations in the low-latitude ionosphere during the geomagnetic storm of 25-27 August 1987

Abstract

Abstract.We have presented a comparison between the modeled N mF 2 and hmF 2, and N mF 2 and hmF 2 which were observed at the equatorial anomaly crest and close to the geomagnetic equator simultaneously by the Akita, Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper atmosphere (MU) radar (34.85 • N, 136.10 • E) during the 25-27 August 1987 geomagnetically storm-time period at low solar activity near 201 • , geomagnetic longitude. A comparison between the electron and ion temperatures measured by the MU radar and those produced by the model of the ionosphere and plasmasphere is presented. The corrections of the storm-time zonal electric field, E , from 16:30 UT to 21:00 UT on 25 August bring the modeled and measured hmF 2 into reasonable agreement. In both hemispheres, the meridional neutral wind, W, taken from the HWW90 wind model and the NRLMSISE-00 neutral temperature, T n , and densities are corrected so that the model results agree with the ionospheric sounders and MU radar observations. The geomagnetic latitude variations in N mF 2 on 26 August differ significantly from those on 25 and 27 August. The equatorial plasma fountain undergoes significant inhibition on 26 August. This suppression of the equatorial anomaly on 26 August is not due to a reduction in the meridional component of the plasma drift perpendicular to the geomagnetic field direction, but is due to the action of storm-time changes in neutral winds and densities on the plasma fountain process. The asymmetry in W determines most of the northsouth asymmetry in hmF 2 and N mF 2 on 25 and 27 August between about 01:00-01:30 UT and about 14:00 UT when the equatorial anomaly exists in the ionosphere, while asymmetries in W, T n , and neutral densities relative to the geomagnetic equator are responsible for the north-south asymmetry in N mF 2 and hmF 2 on 26 August. A theory of the primary mechanisms causing the morning and evening peaks in the electron temperature, T e , is developed. An appearCorrespondence to: A. V. Pavlov (pavlov@izmiran.rssi.ru) ance, magnitude variations, latitude variations, and a disappearance of the morning T e peaks during 25-27 August are caused by variations in E , thermospheric composition, T n , and W. The magnitude of the evening T e peak and its time location are decreased with the lowering of the geomagnetic latitude due to the weakening of the effect of the plasma drift caused by W on the electron density. The difference between 25 August and 26-27 August in an appearance, magnitude and latitude variations, and a disappearance of the evening T e peak is caused by variations in W, the thermospheric composition, T n , and E .

A modeling study of ionospheric F2-region storm effects at low geomagnetic latitudes during 17-22 March 1990

“…Pavlov et al, 2004b), and, therefore, storm-time changes in these number densities in one of the crest regions or in both crest regions or in the trough region contribute to changes in the equatorial anomaly development. As far as the authors know, the relative role of these mechanisms in the equatorial anomaly suppression is studied only by Pavlov et al (2004b) for the 25-26 August 1987 geomagnetic storm. Pavlov et al (2004b) found that weak differences between disturbed and quiet E×B plasma drifts cannot be responsible for the suppression of the equatorial anomaly on 26 August, and this suppression is due to the action of storm-time changes in neutral winds and densities on the equatorial anomaly development. In this paper, we continue the study of the complex problem of the low-latitude ionospheric response to variations in the thermospheric wind, electric field, neutral composition, and neutral temperature during 17-22 March 1990 at solar maximum in the present case study, in which the electron density and temperature are measured simultaneously by the MU and Arecibo radars, and NmF2 and hmF2 are observed by the Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders.…”

“…Pavlov et al (2004b) have presented a comparison of the modeled F2-layer peak densities, NmF2, and altitudes, hmF2, with NmF2 and hmF2, which were observed at low geomagnetic latitudes simultaneously by the Akita, Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and by the MU radar during the 25-27 August 1987 geomagnetically storm, taking into account the storm-time changes in the thermospheric wind, the electric field, the neutral composition, and the neutral temperature. It is found by Pavlov et al (2004b) that the geomagnetic latitude variations in NmF2, calculated for 26 August 1987 (the recovery phase of the geomagnetic storm), differ significantly from those calculated for 25 August 1987 (before SSC and during the main phase of the geomagnetic storm) and for 27 August 1987 (after the geomagnetic storm). The Arecibo radar observations of the ionospheric F-region during the 1-5 May 1995 geomagnetic storm period have shown that there is the possibility of poleward expansion of the equatorial anomaly zone with the northern anomaly crest location close to the 29 • dip latitude (Buonsanto, 1999).…”

“…As far as the authors know, the relative role of these mechanisms in the equatorial anomaly suppression is studied only by Pavlov et al (2004b) for the 25-26 August 1987 geomagnetic storm. Pavlov et al (2004b) found that weak differences between disturbed and quiet E×B plasma drifts cannot be responsible for the suppression of the equatorial anomaly on 26 August, and this suppression is due to the action of storm-time changes in neutral winds and densities on the equatorial anomaly development. In this paper, we continue the study of the complex problem of the low-latitude ionospheric response to variations in the thermospheric wind, electric field, neutral composition, and neutral temperature during 17-22 March 1990 at solar maximum in the present case study, in which the electron density and temperature are measured simultaneously by the MU and Arecibo radars, and NmF2 and hmF2 are observed by the Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders. The use of data from the MU radar and the Arecibo radar, in combination with the ionosonde measurements, allows us to investigate the differences and similarities between the storm effects in the ionosphere in the different longitude sectors at low geomagnetic latitudes.…”

“…The value of NmF2 is a function of [O], [N 2 ], and [O 2 ] at hmF2 at low geomagnetic latitudes (e.g. Pavlov et al, 2004b), and, therefore, storm-time changes in these number densities in one of the crest regions or in both crest regions or in the trough region contribute to changes in the equatorial anomaly development. As far as the authors know, the relative role of these mechanisms in the equatorial anomaly suppression is studied only by Pavlov et al (2004b) for the 25-26 August 1987 geomagnetic storm.…”

“…V. Pavlov et al: A modeling study of ionospheric F2-region storm effects geomagnetic storms of 6-8 February 1986, 20-21 January 1989, 20-23 October 1989, and 25-27 August 1987 (Oliver et al, 1988;Reddy et al, 1990;Oliver et al, 1991;Pavlov et al, 2004b). The changes in F-layer electron density observed by the MU radar in the 6-8 February 1986 storm were explained by changes in an influx of ionization from the plasmasphere, modulated by the passage of a large-scale, southward traveling gravity wave (Oliver et al, 1988).…”