On a geomagnetically disturbed night (7 January 2005), an equatorial spread F (ESF) event was captured during premidnight hours by the Indian MST radar (operated in ionospheric mode) at Gadanki (13.5°N, 79.2°E, dip angle 12.5°N). The base height of the ionospheric F region over dip equator and a low‐latitude station showed similar variation during most of the ESF interval except mainly during 2145–2200 IST (Indian standard time, IST = universal time, UT + 5.5 hours). The zonal electric field variation over dip equator responded to a “prompt penetration” event at ∼2000 IST by gradually changing its polarity from westward to eastward after ∼2015 IST leading to the initiation of ESF at ∼2035 IST. The linear growth rate analysis supports the generation of irregularities only when the eastward electric field owing to the “prompt penetration” effect is taken into account. The zonal electric field became westward again after 2100 IST. However, it ephemerally turned eastward shortly after ∼2145 IST (1615 UT) that is believed to be associated with an “overshielding” condition. A plasma plume got resurrected after ∼30 min indicating the association of the development of this plume structure with the overshielding electric field. The OI 630.0 nm airglow intensity variations observed by a collocated airglow photometer, simultaneously operated in a bidirectional (zenith and east) mode, corroborated well with the ESF structure and dynamics during the disturbed period. These observations evince the active role of interplanetary electric field (IEF) in the development of ESF in the premidnight hours.

[1] On a geomagnetically disturbed and non-spread

Coordinated 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; A p = 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 (B z ); 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.

Technological innovations in optics in the form of a spiral mask system and in electronics in the form of on-line gate scanning of Fabry-Pérot fringes and critical balancing of gate widths for complete background rejection have allowed unambiguous detection of faint dayglow emission features of multiple wavelengths buried in the bright daytime background continuum. The technical details of this unique multiwavelength daytime photometer (MWDPM) and its performance characteristics together with its potential application for the investigation of several geophysical phenomena are presented and discussed.

Results obtained from a joint INDO-US experiment on the investigations of mesosphere/lower thermosphere wave dynamics using balloon-borne optical dayglow measurements in combination with ground-based optical, radio, and magnetometer data are presented. Ultraviolet OI 297.2 nm dayglow emissions that originate at~120 km were measured from low-magnetic latitudes from onboard a balloon on 8 March 2010. This paper describes the details of a new spectrograph that is capable of making high spectral resolution (0.2 nm at 297.2 nm) and large (80°) field of view ultraviolet dayglow emission measurements and presents the first results obtained from its operation onboard a high-altitude balloon. Waves of scale sizes ranging from 40 to 80 km in the zonal direction were observed in OI 297.2 nm emissions. Meridional scale sizes of similar waves were found to be 200 km as observed in the OI 557.7 nm emissions that originate from~100 km. Periodicities were also derived from the variations of equatorial electrojet strength and ionospheric height on that day. Common periodicities of waves (in optical, magnetic, and radio measurements) were in the range of 16 to 30 min, which result in intrinsic horizontal wave speeds in the range of 21 to 77 m s À1. It is argued that gravity waves of such scale sizes and speeds at these heights are capable of propagating well into the thermosphere because the background wind directions were favorable. These waves were potentially capable of forming the seeds for the generation of equatorial plasma irregularities which did occur on that night.