Equatorial Ionosphere-thermosphere System Research Articles

Three different types of electric field disturbances affecting equatorial ionosphere during a long‐duration prompt penetration event

AbstractCoordinated digisonde and OI 630.0 nm airglow observations from Thumba (TVM), an Indian dip equatorial station, in conjunction with magnetic and geosynchronous particle flux measurements, reveal three different types of electric field disturbances in the equatorial ionosphere‐thermosphere system (ITS) occurring in succession over a period of 6 h on a single night (22–23 January,2012; Ap = 24). These include (1) westward electric field perturbations owing to a pseudo‐breakup and a substorm event, each lasting for about 30 min; (2) eastward electric field perturbations continuing for about an hour, owing to the southward excursion of Z component of interplanetary magnetic field (Bz); and (3) DP2‐type fluctuating (period ∼40 min) electric field perturbation sustaining for about 4 h. The pseudo‐breakup and the fully grown substorm events are found to be longitudinally localized and different in terms of response in the westward auroral electrojet index (AL) as well as geosynchronous electron/proton injections. The polarity of the prompt penetration of interplanetary electric field that affects the equatorial ionosphere is observed to be eastward during 2100–2200 IST (Indian Standard Time) which is observationally sparse but consistent with modeling studies. Interestingly, on the same night, DP2‐type electric field fluctuations with ∼40 min periodicity and occasional eastward polarity (akin to daytime) are also found to affect the equatorial ITS for about 4 h (2200–0200 IST). The case study, thus, brings out different processes that constitute a long duration prompt penetration event which, otherwise, would have been categorized as a single event.

Signatures of Sudden Stratospheric Warming on the Equatorial Ionosphere-Thermosphere System

The effect of Sudden Stratospheric Warming (SSW) event of January 2008 on the Equatorial Ionosphere-Thermosphere System (EITS) over Trivandrum (8.5°N, 77°E, 0.5°N diplat.), a geomagnetic dip equatorial station in India has been investigated. It has been observed that the entire EITS exhibits significant variability during the SSW period. EITS parameters like Equatorial Electrojet (EEJ), F2 layer critical frequency (foF2), F1 layer base height (h'F), Total Electron Content (TEC) and 630nm thermospheric dayglow showed clear cut signatures of the SSW induced effects. It is suggested that dynamical perturbations associated with the SSW and subsequent modifications in the tidal components through wave–tidal interactions are responsible for such changes in the equatorial electrodynamics. The present study comprehensively brings out the quiet time equatorial ionosphere/thermosphere variability, during both night and daytimes, vis-à-vis the SSW event of January 2008.

Equatorial ionosphere–thermosphere system: Electrodynamics and irregularities

The equatorial ionosphere and thermosphere constitute a coupled system, with its electro dynamical and plasma physical processes being responsible for a variety of ionospheric phenomena peculiar to the equatorial region. The most important of these phenomena are: the equatorial electrojet (EEJ) current system and its instabilities, the equatorial ionization anomaly (EIA), and the plasma instabilities/irregularities of the night ionosphere (associated with the plasma bubble events – ESF). They constitute the major topics of investigations having both scientific and practical objectives. The tidal wind interaction with the geomagnetic field is responsible for the atmospheric dynamo electric fields, that together with the wind system, drives the major phenomena, under quiet conditions. Drastic modifications of these phenomena can occur due to magnetospheric forcing under solar-, interplanetary- and magnetospheric disturbances. They can also undergo significant modifications due to forcing by atmospheric waves (such as planetary- and atmospheric gravity waves) propagating upward or from extra tropics. This article will focus on the ambient conditions of the ionosphere–thermosphere system and the electro dynamics and plasma instability processes that govern the plasma irregularity generation. Major emphasis is given to problems related to the structuring of the equatorial night ionosphere through plasma bubble/ESF irregularity processes. Specific topics to be covered will include: equatorial electric fields, thermospheric winds, sunset electrodynamic processes, plasma drifts, EEJ plasma instability/irregularity generation, nighttime/post sunset plasma bubble irregularity generation, and very briefly, disturbance electric fields and winds and their effect on the ionization anomaly, the TEC and ESF/plasma bubble irregularities.

Coupling and energetics of the equatorial ionosphere-thermosphere system: advances during the STEP period

The equatorial ionosphere-thermosphere system (EITS) investigations during the STEP period (1990-1997) were focused on improving our understanding of the dymanic and electrodynamic-coupling process and energetics that govern the climatology of the system, as well as the variabilities of the system arising from forcing by magnetospheric and high latitude processes and by atmospheric waves from below. Thermosphere dynamics and dynamo electric fields serving as drivers of the coupling process are responsible for the major EITS phenomena and their variabilities at different time scales. Especially, the day-to-day variabilities of the equatorial spread F (ESF) have recieved specific attention because of interest in space application areas as well Significant advances were achieved in our understanding and representation of quiet and disturbed electric fields. The STEP period also marked notable improvement in the experimental diagnostic facilities available in the equatorialregions as well as in theoretical modeling of the interactive process that control the major EITS phenomena. Data from coordinated observational campaigns have contributed to a better understanding of the EITS global responses to magnetospheric disturbances. The advances to be briefly discussed in this paper concern most of the major phenomena and the inherent characteristics of the EITS: the equatorial electric field and its sunset enhancement; thermospheric winds and temperature including the MTM (midnight temperature maximum); peculiar features of the ionosphere in the immediate vicinity of the magnetic equator; equatorial spread F/plasma bubble erregularities, including the energy requirement for large scale field aligned irregularities; equatorial anomalies in ion/neutral densities and temperatures. Also presented briefly is an overview of the results on the disturbance characteristics of the key EITS parameters and driving forces under magnetospheric disturbances and atmospheric wave forcing.

Major phenomena of the equatorial ionosphere-thermosphere system under disturbed conditions

We present here a brief review of recent results related to the response of the major phenomena of the equatorial ionosphere-thermosphere system (EITS) to magnetospheric disturbances. Modifications to the equatorial ionization anomaly (EIA), equatorial spread F (ESF) and equatorial electrojet (EEJ) are produced by (a) disturbance electric fields resulting from prompt equatorward penetration of magnetospheric/high latitude electric fields, (b) disturbance dynamo driven by enhanced global thermospheric circulation resulting from energy input at high latitude, and (c) disturbance winds (zonal and meridional) modifying the equatorial thermospheric dynamics. While the disturbance electric fields could cause enhanced development or inhibition of these phenomena the disturbance winds seem to always inhibit them, especially the EIA and ESF. New results discussed concern the role of meridional winds in the disturbance electric field induced F layer height response over low latitudes, evidence on ESF/plasma bubble inhibition by disturbance meridional/trans-equatorial wind, EIA expansion in latitude reaching spectacular magnitude during intense storms, longitudinal/local time dependence of EIA enhancements; and common sources of ESF and EIA response features and related aspects, among others. We advance, further, suggestions on specific studies to be pursued for better understanding of the problems presented.

Equatorial ionospheric electric fields during magnetospheric disturbances: local time/longitude dependences from recent EITS campaigns

Data sets collected during a few coordinated Equatorial Ionosphere-Thermosphere System (EITS) observational campaign periods, mainly from the Brazilian and Asian longitude sectors are analysed in this paper. Ionosonde magnetometer and Ionospheric Electron Content (IEC) data from the EITS-1 and -2 campaigns (during March and December 1991) are complemented by interplanetary magnetic field and some ground based data sets from other campaigns. The analysis focuses on the response of the equatorial ionospheric heights and ionization anomaly to disturbance electric fields, identified as a direct penetration electric field associated with IMF B z changes and development of the ring current (especially the asymmetric component), and that produced by a disturbance zonal neutral wind. New evidence on the local time and longitudinal dependences of these electric fields constitute the main results of this paper. Especially, a large eastward electric field (associated with the asymmetric ring current) in the dusk-dawn sector causes significant expansion of the EIA in this sector, and amplification of the evening prereversal uplift of the F-layer over Brazil. Significant inhibition of the evening prereversal electric field enhancement seems to be produced by the disturbance zonal wind associated with the magnetic disturbances prevailing several hours earlier. Some tentative evidence on the Brazilian dusk sector disturbance field being larger than that of the Asian dusk sector support the existence of a longitude asymmetry in the intensity of the disturbance electric field.