Tarun Kumar Pant

Additional stratifications in the equatorial F region at dawn and dusk during geomagnetic storms: Role of electrodynamics

[1] The role of electrodynamics in producing additional stratifications in the equatorial F region (F 3 layer) at dawn and dusk during geomagnetic storms is discussed. Two cases of F 3 layer at dawn (0600-0730 LT on 5 October 2000 and 8 December 2000) and one case of F 3 layer at dusk (1600-1730 LT on 5 October 2000) are observed, for the first time, by the digital ionosonde at the equatorial station Trivandrum (8.5°N; 77°E; dip ∼ 0.5°N) in India. The unusual F 3 layers occurred during the geomagnetic storms and are associated with southward turning of interplanetary magnetic field B z , suggesting that eastward prompt penetration electric field could be the main cause of the F 3 layers. The dawn F 3 layer on 5 October is modeled using the Sheffield University Plasmasphere-Ionosphere Model by using the E x B drift estimated from the real height variation of the ionospheric peak during the morning period. The model qualitatively reproduces the dawn F 3 layer. While the existing F 2 layer rapidly drifts upward and forms the F 3 layer and topside ledge, a new layer forming at lower heights develops into the normal F 2 layer.

A comparative study of daytime mesopause temperatures obtained using unique ground based optical and meteor wind radar techniques over the magnetic equator

[1] This paper presents the first ever comparison of daytime Mesopause OH rotational temperatures as determined using Meinel (8-3) dayglow emissions with those obtained using the recently established and collocated, meteor radar (SKiYMET) over the magnetic equator in India. The measurements were made over Thiruvananthapuram (8.5°N, 76.5°E, 0.5°N diplat.) for January-May 2005 period. Overall agreement had been good (±20 K) with all the gross features coming out fairly well. Further, the temperatures estimated using these two techniques, exhibit significant wavelike modulations with periods spanning ∼3-65 days with varying phase differences, presumably due to their interaction with the longer period waves like the semiannual oscillation (SAO). The consistency of the temperature variabilities at two different altitudes viz. 87 and 94 km enables investigation of the mesosphere lower thermosphere dynamic coupling.

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

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; 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.

Daytime wave characteristics in the mesosphere lower thermosphere region: Results from the Balloon‐borne Investigations of Regional‐atmospheric Dynamics experiment

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−1. 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.

Planetary wave-tidal interactions over the equatorial mesosphere-lower thermosphere region and their possible implications for the equatorial electrojet

[1] Optically measured daylight mean mesopause temperatures over a dip equatorial station, Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), have been analyzed in conjunction with simultaneously measured equatorial electrojet (EEJ)-produced magnetic field at the surface. The signature of planetary wave-tidal interactions in the mesosphere-lower thermosphere (MLT) region has been observed for the first time in the day-to-day variability in the EEJ, i.e., the time of its peaking and the duration, as inferred from the EEJ-produced magnetic field on the ground. The present study shows that the planetary wave of quasi 16 day periodicity plays an important role in causing these variabilities, especially during the winter months. The quasi 16 day wave is found to be modulating the mesopause temperature (MT), duration, and time of the maximum EEJ intensity (D EEJ and T EEJ ). During positive excursions of the planetary wave, T EEJ showed a shift toward evening, while the MT showed an increase and D EEJ showed a broadening. Similarly, all these parameters exhibited an opposite trend during negative excursions. The planetary wave-tidal interactions and subsequent modification of the tidal components have been shown to be responsible for the observed variations. This study presents a new perspective addressing the day-to-day variability of the EEJ.

Gravity wave signatures in the dip equatorial ionosphere‐thermosphere system during the annular solar eclipse of 15 January 2010

The present work pertains to the eclipse-induced gravity wave modulations in the ionosphere-thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. Electron density and neutral wind rocket payload measured horizontal winds and electron densities at E region altitudes, and ionosonde-derived foF1 and foF2 parameters are used to analyze the characteristics of the eclipse-induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30–100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave-induced winds in generating blanketing Es over the equator is also examined.

Peculiar features of ionospheric F3 layer during prolonged solar minimum (2007–2009)

We present the seasonal and local time occurrence of ionospheric F 3 layer over Tirunelveli (geographic longitude 77.8 ◦ E, geographic latitude 8.7 ◦ N, dip 0.7 ◦ ) during extremely low and prolonged solar activity period (2007-2009). Canadian Advanced Digital Ionosonde observations from this station are used in the present study. We find that the occurrence of F 3 layer is nearly 3 times higher during 2009 (∼ 48%) as compared to that during 2007 (∼ 16%). The increase of this order just within the low solar activity period is unusual. In earlier studies similar increase in F3 occurrence has been reported when solar activity changes from high (F10.7 = 182 )t o low (F10.7 = 72). The other important feature is the presence of postnoon F3 layers which are observed dominantly during summer solstice of 2009. Such occurrence of postnoon F3 layers was nearly absent during summer solstice of the previous solar minimum (1996) over nearby dip equatorial station Trivandrum. We take equatorial electrojet (EEJ) as a proxy for eastward electric field. It is noticed that the EEJ strength and the maximum rate of change of EEJ are higher for F3 days as compared to those on non-F3 days. We find that the peak occurrence of prenoon F3 layer closely coincides with the time of maximum rate of change of EEJ. It is in general accordance with the theory proposed by Balan et al. (1998) that suggests the formation of F3 through vertically upward E × B drift in presence of equatorward neutral wind. The present study reveals that the rate of change of eastward electric field (dE/dt) as well plays an important role in the formation of F3 layer.

Variability of mesopause temperature derived from two independent methods using meteor radar and its comparison with SABER and EOS MLS and a collocated multi-wavelength dayglow photometer over an equatorial station, Thumba (8.5° N, 76.5° E)

Two independent methods for deriving mesopause temperature using meteor radar installed at an equatorial station, Thumba (8.5° N, 76.5° E), are discussed in this article. This meteor radar-derived mesopause temperature is then compared with two different types of spaceborne measurement, namely (i) Sounding the Atmosphere using Broadband Emission Radiometry (SABER) and (ii) the Earth Observing System Microwave Limb Sounder (EOS MLS), and a collocated multi-wavelength dayglow photometer (DGPM). The meteor radar-derived temperature is in fairly good agreement with all the three measurement techniques, with an uncertainty of ±10°. This study focuses on a detailed evaluation and inter-comparison of mesopause temperature derived from different measurement techniques. An attempt is also made to compare the suitability of these observations to study planetary waves and other oscillation activities in the mesospheric region.

Morphology and seasonal characteristics of low latitude E‐region quasiperiodic echoes studied using large database of Gadanki radar observations

[1] In this paper we study the morphology, seasonal features, and statistics of low latitude quasiperiodic (QP) echoes using an unprecedented data set of 147 nights of observations spanning over all seasons made using the Gadanki radar. The QP echoes have periods 2-20 min and occur on more than 50% nights. They are often found embedded in descending echoing layers whose characteristics resemble to those of tidal winds. Their occurrence shows strong seasonal dependence with 48% of the time in summer, 26-32% in equinox, and 14% in winter. The QP echoes have a tendency to appear first during 21-00 LT, which could last for 2-8 h, leading to their maximum occurrence rate during postmidnight hours. Height-time occurrences of radar echoes and QP structures both show common descending behavior, which agree extremely well with the observed descending behavior of the Es layers. We also find that the seasonal variations of QP echo occurrence are in very close agreement with that of the Es activity, tidal activity in the same height region, and also gravity wave activity in the mesosphere. Interestingly, the occurrence statistics are much more than their midlatitude counterpart; and time of first appearance, time of maximum occurrence, and duration of QP echoes all strongly contrast the midlatitude features. These observations have been discussed in detail in the light of current understanding of QP echoes.

Impact of the perturbation zonal velocity variation on the spatio/temporal occurrence pattern of L band scintillation—A case study

The earlier evolved method for the forecast of the spatiotemporal variation of L band scintillation based on the expected variation of the perturbations, under favorable ionospheric/thermospheric conditions, has been refined by duly accounting for the local time variation of the zonal velocity of the perturbations. The unique combination of the two geostationary satellites (GSAT-8 and GSAT-10) over the Indian zone has been used to estimate the typical local time dependence of the perturbation velocities by closely following identifiable features in the scintillation pattern. The measured velocities, that registered a steady decrease with the progression of night, had been shown to significantly alter the forecast pattern of the scintillations with respect to longitude and local time. The significant improvement in the forecast pattern has been demonstrated through a case study putting the forecast method on a firmer footing.