An important role of electric field reversals for the initiation of gigahertz scintillations at midlatitude during geomagnetic storms.

Abstract

Large-scale features of the ionospheric disturbances which lead to the occurrence of gigahertz scintillation events at midlatitudes are re-examined using h'Fdata at five ionosonde stations and HF Doppler measurements at Kokubunji. From these analyses, it is recognized that ionospheric dynamics during the events are generally characterized by a two-stage process, despite case-dependent quantitative differences in the appearance of each event. In this construction, an increase and subsequent decrease of virtual heights occur simultaneously at five stations in the first stage, with the rapid fluctuations of HF Doppler around the apex of h'F. On the contrary, virtual heights increase again in the second stage with time shifts that indicate a motion from north to south. It is shown that the second stage is responsible for the plasma instabilities which cause the gigahertz scintillation event at midlatitudes, whereas the first stage which corresponds to the enhancement stage of equatorial anomalies, is not unstable. From these features, important conclusions are derived that electric field reversals are expected during the events and westward electric fields seem more important for gigahertz scintillations at midlatitudes. Some other pieces of evidence are also shown which support the electric field reversals. As the cause of the two-stage process, a push-pull mechanism between ionized and neutral atmospheres is proposed.At midlatitude regions around Japan, severe gigahertz scintillations are very rare phenomena and appear only during geomagnetic storms. In a previous paper (hereinafter referred to as paper I), TANAKA (1981) has clarified characteristic features of magnetospheric and ionospheric disturbances which lead to the occurrence of midlatitude gigahertz scintillations. In paper I, the following picture was constructed for the condition to generate severe gigahertz scintillations at midlatitudes. During the main phase of geomagnetic storms, an eastward electric field of magnetospheric origin occasionally penetrates toward low latitudes in the dusk meridian, associated with the development of low-latitude asymmetric disturbance fields. This electric field causes sudden enhancements of equatorial anomalies to create strong positive ionospheric storms at subequatorial regions and negative ionospheric storms at the equator. After these intervals, some plasma instabilities are triggered in the midlatitude ionosphere as the results of modifications in ionospheric

A large‐scale traveling ionospheric disturbance during the magnetic storm of 15 September 1999

“…Most of the simulations show dominant enhancement of equatorward wind and associated upward motion of the ionosphere in the midlatitudes during the LSTID passage. The rise of the F layer has been confirmed by observations [e.g., Tanaka , 1987; Hajkowicz and Hunsucker , 1987; Hajkowicz , 1990]. The event shown in this paper, however, is characterized by a drastic F layer descent and an associated airglow enhancement.…”

Thermospheric wind during a storm‐time large‐scale traveling ionospheric disturbance

“…Most of these calculations show dominant enhancement of “equatorward wind” and associated upward motion of the ionosphere in the midlatitudes during LSTID passage. The rise of the F layer has been confirmed by observations [e.g., Ogawa and Kumagai , 1985; Tanaka , 1987; Hajkowicz and Hunsucker , 1987; Hajkowicz , 1990; Lee et al , 2002].…”

Ionospheric Features Associated with the Fine Structure Recorded on Some Mid-Latitude Spread-F Ionograms.

“…As a result of these considerations KELLEY and FUKAO (1991) state "The most likely process seems to be the instability of the equilibrium which occurs when the mid-latitude plasma is supported against gravity either by an eastward electric field or by a southward neutral wind ..." Electric fields as possible m echanisms have also been proposed by other workers. TANAKA (1987) states "westward electric fields seem more important for gigahertz scintillations at mid-latitudes", while for equatorial latitudes RASTOGI (1989) suggests that during post-sunset hours spread-F irregularities are generated due to the presence of an eastward electric field. In the middle atmosphere (from about 80km to 150km) at which levels stratification of the ionosphere is well documented, the breaking of atmospheric gravity waves (AGWs) is a strong candidate to explain these stratifications (FRITTS, 1984).…”