Rajesh Singh

Atmospheric Gravity Waves (AGWs) play a crucial role in atmosphere‐ionosphere coupling. Tropical cyclones (TCs) are considered one of the potential sources of AGWs. This study investigates four extreme category tropical cyclones from the last two decades over the Indian subcontinent: VSCS Phailin (2013), ESCS Fani (2019), SuCS Amphan (2020) over the Bay of Bengal, and ESCS Tauktae (2021) in the Arabian Sea. The meteorological observations confirmed the presence of deep convective activity during these TCs, which reached higher tropospheric altitudes during the TC's duration. The low cloud‐top temperatures exhibited a positive correlation with the increasing lightning activity. Further, a notable increase in lightning activity was found during the intensification phases of the storm, particularly within the wind‐field regions in all four cases, which could have led to an intensification of TCs. Simultaneously, an enhancement in AGW activity occurred in the mesosphere‐lower thermosphere (MLT) region. Possibly, mesoscale convective system associated with the TC led to the formation of AGWs that propagated in both the vertical and horizontal domains to higher altitudes in the MLT region. The propagation of vertical and horizontal AGWs induced by TCs at higher altitudes has been validated and quantified by analyzing vertical temperature profiles obtained from SABER and day‐night band DNB imagery from VIIRS satellites. Both vertical and horizontal wavelengths of AGWs showed peak power of waves during the maximum intensity stages of TCs. Both vertical and horizontal components of AGWs are quantified for the first time over the Indian region.

We present lightning-induced ionospheric perturbations in narrowband very-low-frequency (VLF) signals from the transmitters NWC (21.82° S, 114.17° E, 19.8 kHz) and VTX (8.4° N, 77.8° E, 18.6 kHz) recorded at the low-latitude station Dehradun (DDN; 30.3° N, 78.0° E) over a 12-month period from September 2020 to October 2021. Early/slow VLF events, VLF LOREs, and step-like VLF LOREs associated with lightning were analyzed for their onset and recovery times. This study utilized data from the World Wide Lightning Location Network (WWLLN), which provides lightning locations and energy estimates. The results show that early/slow VLF events occur most frequently, accounting for approximately 68% of cases, followed by VLF LOREs at 12%, and step-like VLF LOREs at 10%. Furthermore, we observed that 100% of the VLF perturbing events occurred during the nighttime, which is not entirely consistent with previous studies. Moreover, more than 60% of VLF LOREs were associated with lightning energies of approximately 1 kJ, and about 40% were associated with lightning energies of ~10 kJ. Step-like VLF LOREs were linked to WWLLN energies between 1 and 5 kJ. The observed WWLLN energy range is somewhat lower than the energies reported in previous studies. Scattering characteristics revealed that 87.3% of events were associated with wide-angle scattering, while approximately 12.6% were linked to narrow-angle scattering. LWPC version 2.1 was used to simulate these perturbing events and to estimate the reflection height (H′, in km) and the exponential sharpness factor (β, in km⁻¹) corresponding to changes in D-region electron density. The reflection height (H′, in km) and the exponential sharpness factor (β, in km⁻¹) of the D-region varied from 83 to 87 km and from 0.42 to 0.79 km⁻¹ for early/slow VLF events, from 83 to 85 km and from 0.5 to 0.75 km⁻¹ for step-like VLF LOREs, and from 81 to 83 km and from 0.75 to 0.81 km⁻¹ for VLF LOREs, respectively.

D‐region effect of intense storm of 23 March (D_st = −163 nT) and super storm of 23 April 2023 (D_st = −212 nT) have been investigated using VTX and NWC transmitter signals recorded at low latitude station Dehradun (DDN; 30.31°N, 78.03°E), India. During March storm on 23 March, amplitude anomalies of +2.92 (NWC) and +1.91 dB (VTX) were observed. April storm showed amplitude anomalies of −9.41 (NWC) and −2.68 dB (VTX) on 23 April. Long Wave Propagation Capability code (LWPC) has been used to model signal anomalies to obtain D‐region Very low frequency (VLF) reference height (H′) and electron density gradient (β). During March storm, NWC‐Dehradun path showed a decrease in H′ by 12.04 km and an increase in β by 0.301 km⁻¹, while VTX‐Dehradun path showed an increase in H′ by 0.07 km and β by 0.032 km⁻¹ on 23 March. During April storm, VTX‐Dehradun path showed increase in H′ by 0.315 km and an increase in β by 0.024 km⁻¹, while H′ increased by 2.91 km and β by 0.23 km⁻¹ for NWC—Dehradun path on 23 April. Wavelet analysis of VLF anomalies showed signatures of atmospheric gravity waves of periods between 30 and 100 min during both storms associated with joule heating in the high/auroral latitudes. Results suggest that storm time prompt penetration (PP) of electric fields has contributed to VLF signal anomalies during April storm's main phase, which is also supported by PP Equatorial Electric Field Model run for the location of DDN station.

One of the most intense geomagnetic storms of recent times occurred on 10–11 May 2024. With a peak negative excursion of Sym‐H below −500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously ∼6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow‐shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (≥15 nPa) with an intense eastward interplanetary electric field (IEF_Y ≥ 2.5 mV/m) within a super‐dense Interplanetary Coronal Mass Ejection lasted for 409 min–is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2–12 MHz frequency band due to strong D‐ and E‐region ionization resulting from a solar flare prior to this storm.

The study presents detailed meteorological characteristics of extremely severe cyclonic storm (ESCS) Fani, and subsequent Atmospheric Gravity Waves (AGWs) induced D‐region ionospheric perturbations and the role of lightning activity in it. The cyclone shaped as a weak disturbance over the north Indian Ocean (2.7°N, 89.7°E) on 25 April 2019. The disturbance intensified and evolved into ESCS Fani over Bay of Bengal (BoB) on 30 April, had landfall on 03 May, and dissipated after 04 May 2019. What makes Fani unique is its long life span of ∼10 days, and only ESCS to occur after ∼30 years over the BoB. Fani attained a minimum cloud top temperature of about −80°C, and a corresponding maximum cloud top altitude of about ∼17 km. Such meteorological conditions presented a strong convection process in the towering cumulonimbus in inner and outer rain bands, resulting in intense lightning activity. The peak lightning flash rate observed was ∼375 min⁻¹. SABER observations confirmed the coupling of atmosphere with ionosphere with strong AGWs in middle atmosphere during Fani. NWC (19.8 kHz) Very Low Frequency signal intersecting the track of Fani is used to decipher D‐region ionospheric perturbations induced by AGWs from Fani. The results show the presence of increased AGW activity in D‐region during the cyclone period when compared to that of the pre‐ and post‐cyclone periods. The periods of the observed gravity waves are between ∼13 and 20 min, highlighting the important role of lightning activity and AGWs in atmosphere‐ionosphere coupling.

A comprehensive analysis of subionospheric Very Low Frequency (VLF) signals from transmitters with call‐signs NWC, NPM, and NLK monitored at a low‐latitude station, Suva, Fiji, has been carried out to determine the D‐region effects of the intense geomagnetic storms for a period of 7 years (2011–2016 and 2018). The periods of 2011–2016 and 2018 fall under the 24th solar cycle with yearly mean sunspot numbers 80.8–39.8 and 7.0, respectively. Seven out of 12 geomagnetic storms revealed VLF anomalies during the storm main phase day and storm recovery phase days. A pronounced decrease in the VLF amplitude and phase for the storms having main phase onset in the nighttime was found. Out of seven geomagnetic storms, a storm of St Patrick's Day of 17–18 March 2013 is presented as a case study and the same analysis was done for all the storms. The anomalies in the VLF amplitude and phase have been modeled using Long‐Wave Propagation Capability code V2.1 to obtain the changes in Wait parameters (reference height, H′ and sharpness factor, ꞵ) and changes in the D‐region electron density. In most cases of storms, an increase in the H′ and a decrease in the ꞵ during the storm main and recovery phases were obtained which for the 17–18 March 2013 storm increased by 7.4–7.6 km and decreased from 0.03 to 0.07 km⁻¹, respectively. Wavelet analysis of signal anomalies showed clear wave‐like spectra (0.05–0.18 mHz) of atmospheric gravity waves indicating traveling ionospheric disturbances propagating toward low latitudes due to storm‐associated Joule heating at high latitudes.

We report rare simultaneous observations of columniform sprites and associated gravity waves (GWs) using the Transient Luminous Events (TLEs) camera and All-sky imager at Prayagraj (25.5° N, 81.9° E, geomag. lat. ~ 16.5° N), India. On 30 May 2014, a Mesoscale Convective System generated a group of sprites over the north horizon that reached the upper mesosphere. Just before this event, GWs (period ~ 14 min) were seen in OH broadband airglow (emission peak ~ 87 km) imaging that propagated in the direction of the sprite occurrence and dissipated in the background atmosphere thereby generating turbulence. About 9–14 min after the sprite event, another set of GWs (period ~ 11 min) was observed in OH imaging that arrived from the direction of the TLEs. At this site, we also record Very Low Frequency navigational transmitter signal JJI (22.2 kHz) from Japan. The amplitude of the JJI signal showed the presence of GWs with ~ 12.2 min periodicities and ~ 18 min period. The GWs of similar features were observed in the ionospheric Total Electron Content variations recorded at a nearby GPS site. The results presented here are important to understand the physical coupling of the troposphere with the lower and upper ionosphere through GWs.

A Mesoscale Convective System (MCS), consisting of three Super Cellsformed over South-east Indian, is assessed in detail with satellite and ground baseddata-sets. The MCS under investigation generated a total of Ten (10) upwardelectrical discharges (9 Sprites and 1 Gigantic Jet) commonly named as TransientLuminous Events (TLEs). The TLEs were recorded from TLE observation stationlocated at Allahabad, India. The event occurred in the Post-Monsoon period of 2013on October 7, during 15-23 UT hours. The MCS was spread over a region of 25000 sq.Kilometers. A lowest cloud top temperature value of -84.7 0 C was observed in themature stage of the MCS, during 2130 UT hours, and the cloud top altitude wasreaching 17.6 km. The coldest cloud top region was covering an average area of13000 sq. Km. The measured Convective Available Potential Energy (CAPE) value was606.9 J/kg at 00 UT on 7 th October which dropped to 211 J/kg at 00 UT on 8 thOctober. The mean lightning flash rate during the formation and maturity stages ofthe MCS was around 46.03 min -1 . During the entire lifespan of the thunderstorm,peak currents were found to be reaching ±400 kA. Such high electric currents,extreme cold temperature and towering altitudes of the convective complexes showhow much a MCS is dynamically active and the TLEs which it produced are known toelectrically connect the lower atmosphere to the upper space environment.