Abstract
Abstract. We have presented a comparison between the modeled NmF2 and hmF2, and NmF2 and hmF2 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, EΛ, from 16:30 UT to 21:00 UT on 25 August bring the modeled and measured hmF2 into reasonable agreement. In both hemispheres, the meridional neutral wind, W, taken from the HWW90 wind model and the NRLMSISE-00 neutral temperature, Tn, and densities are corrected so that the model results agree with the ionospheric sounders and MU radar observations. The geomagnetic latitude variations in NmF2 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 north-south asymmetry in hmF2 and NmF2 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, Tn, and neutral densities relative to the geomagnetic equator are responsible for the north-south asymmetry in NmF2 and hmF2 on 26 August. A theory of the primary mechanisms causing the morning and evening peaks in the electron temperature, Te, is developed. An appearance, magnitude variations, latitude variations, and a disappearance of the morning Te peaks during 25-27 August are caused by variations in EΛ, thermospheric composition, Tn, and W. The magnitude of the evening Te 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 Te peak is caused by variations in W, the thermospheric composition, Tn, and EΛ.
Highlights
The ionosphere at the geomagnetic equator and low geomagnetic latitudes is the site of important ionospheric phenomena, which include the equatorial electrojet, equatorial plasma fountain, equatorial (Appleton) anomaly, additional layers, plasma bubbles, and spread F
During 25 and 27 August, the model produces the onset of the equatorial anomaly crest formation near 01:00–01:30 UT and the crests disappear close to 14:00 UT, while a geomagnetic latitude electron density profile with two equatorial anomaly crests is distinguished from 01:00 UT to 04:00 UT on 26 August
We have presented a comparison between the modeled and experimental N mF 2 and hmF 2 at the 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 during the 25–27 August 1987 geomagnetically storm-time period at low solar activity near approximately the same geomagnetic meridian of 201◦
Summary
The ionosphere at the geomagnetic equator and low geomagnetic latitudes is the site of important ionospheric phenomena, which include the equatorial electrojet, equatorial plasma fountain, equatorial (Appleton) anomaly, additional layers, plasma bubbles, and spread F. We continue to investigate the role of horizontal neutral winds in the ionization distribution, plasma dynamics, structuring, and thermal balance of the low-latitude ionosphere in the present case study, in which N mF 2 and hmF 2 are observed simultaneously close to the same geomagnetic meridian at the geomagnetic longitudes of 201◦± 11◦ by the Akita, Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper atmosphere (MU) radar at Shigaraki (34.85◦ N, 136.10◦ E, Japan) during the 25–27 August 1987 geomagnetically storm-time period at solar minimum. Ionospheric models are valuable for investigating the changes that would result, in observed quantities, from changes in individual input parameters, and, the theoretical study of the ionospheric storm response features is a highly complex task in the absence of the measurements of the disturbed thermospheric wind, electric field, neutral composition, and neutral temperature for the studied time period at low-latitudes close to 201◦ geomagnetic longitude. The reliability of the conclusions is based on the comparison between the measured MU radar and modeled Te, and the use of the updated electron cooling rates (Pavlov, 1998a, b; Pavlov and Berrington, 1999) in the model
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