We report, here, the warming of the mesosphere-lower thermosphere (MLT) region over the equatorial region in the Indian subcontinent during the geomagnetic superstorm of May 10–11, 2024 (Dst –412 nT and SYM-H –518 nT) using Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements. A comparative study of vertical temperature profiles between 90 and 110 km on 11 May with that of control profile and 09 May (a neighbouring quiet day) reveals an increase in temperature beyond 96 ± 2 km over 15° S–15° N. We found this warming to increase with increasing height, and the temperature enhancement exceeded 30–45 K near 108–109 km. Slight lowering of the mesopause altitude was noted. An extension of this comparative study over higher latitude bins showed similar warming of the MLT region. Supported with existing reports on auroral observations during this superstorm and expansion of auroral oval, the warming in latitudes greater than 30° appears to a direct consequence of Joule and auroral heating; whereas, the warming within ±30° latitude could be a manifestation of the superstorm-induced changes in global wind circulation and associated redistribution of deposited energy.

The Polar mesospheric summer echoes (PMSE) are Bragg-scale radar backscatter from the electron density irregularities mainly between 80 and 90 km altitude. The irregularities are generated by the turbulent transport of ice particles and aerosols in the polar summer mesosphere. PMSEs have been effectively used as a diagnostic tool for the state and dynamics of the mesosphere. This study presents the first results of PMSE observed by the Moveable Atmospheric Radar for Antarctica (MARA) operated between February 2014 and January 2024 at the Indian Antarctic research base, Maitri (70.77 S, 11.73 E). Over Maitri, the PMSEs on average start between 86 and 89 km, while the signal-to-noise ratio (SNR) peaks around 86 km. From the onset altitude, PMSEs spread to a few kilometers and last for hours. The diurnal variation of PMSE occurrence distinctly exhibits a maximum around local noon and a minimum around 17-18 h. This suggest that in the polar mesosphere, solar radiation, tides and transport of cold air during noon greatly affect the PMSE occurrence. The total length of the PMSE season at Maitri was observed to be between 81 and 97 days. (c) 2025 COSPAR. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

In 2023 April, a low-latitude aurora observed by the all-sky camera at Hanle, Ladakh, India (33 degrees 14'N geographic latitude), generated significant interest. This was the first such aurora recorded from the Indian region in the space era and occurred during a moderate solar storm. This study explores this low-latitude auroral sighting, which happened during the sheath-region passage of an interplanetary coronal mass ejection. We analyze in situ multispacecraft particle measurements and geomagnetic field observations from both ground-based and satellite-based magnetometers. The auroral observations at Hanle coincided with intense substorm activity. Our findings indicate that the aurora did not actually reach India; the equatorward boundary was beyond 50 degrees N geographic latitude. Enhanced electron fluxes with energies below 100 eV were detected at 54 degrees N geographic latitude at about 830 km altitude in the predawn sector (4-5 hr local time). In the midnight sector, the equatorward boundary is estimated to be around 52 degrees N geographic latitude, based on Hanle observations and considering emission altitudes of 600-650 km due to low-energy electrons. Thus, the low-latitude red aurora observed from India resulted from the emissions at higher altitudes due to low-energy electron precipitation in the auroral oval and a slight equatorward expansion of the auroral oval. The low-energy electrons likely originated from the plasma sheet and were precipitated due to enhanced wave-particle interactions from strong magnetosphere compression during high solar wind pressure. This study is crucial in understanding low-latitude auroras in the modern space era.

Lightning has been declared as a new Essential Climate Variable by the World Meteorological Organization. Schumann resonance is a valuable parameter to monitor the global lightning activity, thus, the Atmospheric Observation Panel for Climate accepted Schumann resonance (SR) measurements as an emerging tool for studying lightning-related large-scale processes in the atmosphere. Previous studies showed a clear extraterrestrial influence on the SR parameters at different time scales (e.g., solar cycle). For all these reasons, a growing new interest arises in the scientific community to exploit the potential of SR better in gaining more information on electrodynamic coupling mechanisms taking place in the atmosphere. This has motivated the installation of new instruments worldwide to monitor SR measurements.We performed a multi-station spectral analysis of the SR parameters (frequency and intensity) by using wavelet transformation. SR records from different monitoring sites around the globe were analyzed simultaneously for the first time: Hornsund (~12 years of data) and Belsk (~7 y.) managed by Poland, Rovaniemi and Ivalo in Finland (~16 y.), Eskdalemuir in Scotland (~10 y.), Nagycenk in Hungary (~22 y.), Boulder Creek in USA (~4 y.) and Shillong in India (~9 y.). For all SR sites, the periodicities of 0.5, 1, ~180 and 365-day appeared both in the frequency and the intensity of SR modes. Evidence was also found for the ~27- and ~45-day periods at specific time intervals. Cross-wavelet transform and wavelet coherence analyses were made between SR frequencies and the Kp index, and between SR intensities and Madden-Julian Oscillation index. Time periods of highly coherent 27-day as well as 45-day periodicities were found in the time series of these parameters intermittently. These preliminary results suggest that these periodicities are likely related to the solar rotation and Madden-Julian Oscillation, respectively. A detailed analysis about our findings will be presented and discussed.

During geomagnetic quiet conditions, Indian Antarctic stations are considered to be located outside the auroral oval: Maitri (CGM coordinates: 63.3°S, 54.2°E) is equatorward and Bharati (CGM coordinates: 74.8°S, 98.4°E) is poleward of the auroral oval. Simultaneous observations of magnetic field variations at these two locations for 10-years (2013–2022) provide an opportunity to study quiet-time magnetic field patterns, if any. Geomagnetic quiet days with ΣKp ≤ 3 are selected, during which the lower values of solar wind and interplanetary parameters are also confirmed. Maitri station exhibits clear southern hemispheric solar quiet (Sq) type of magnetic field variation on geomagnetic quiet days in all seasons, indicating the influence of ionospheric dynamo due to thermospheric winds. Interestingly, Bharati station also displays regular and systematic magnetic field variations in all three components. The D-component at Bharati exhibits very strong variation at early morning hours (7–8 MLT), which is ∼2–4 times stronger than that of H-component, driving strong equatorward/northward currents during all seasons. Both stations show annual type of seasonal variation with peak amplitude during summer and least during winter. The schematic illustration of global and polar Sqs proposed here explains the results obtained through 10-years' statistical study.

The importance of lightning has long been recognized from the point of view of climate-related phenomena. However, the detailed investigation of lightning on global scales is currently hindered by the incomplete and spatially uneven detection efficiency of ground-based global lightning detection networks and by the restricted spatio-temporal coverage of satellite observations. We are developing different methods for investigating global lightning activity based on Schumann resonance (SR) measurements. SRs are global electromagnetic resonances of the Earth-ionosphere cavity maintained by the vertical component of lightning. Since charge separation in thunderstorms is gravity-driven, charge is typically separated vertically in thunderclouds, so every lightning flash contributes to the measured SR field. This circumstance makes SR measurements very suitable for climate-related investigations. In this study, 19 days of global lightning activity in January 2019 are analyzed based on SR intensity records from 18 SR stations and the results are compared with independent lightning observations provided by ground-based (WWLLN, GLD360 and ENTLN) and satellite-based (GLM, LIS/OTD) global lightning detection. Daily average SR intensity records from different stations exhibit strong similarity in the investigated time interval. The inferred intensity of global lightning activity varies by a factor of 2-3 on the time scale of 3-5 days which we attribute to continental-scale temperature changes related to cold air outbreaks from polar regions. While our results demonstrate that the SR phenomenon is a powerful tool to investigate global lightning, it is also clear that currently available technology limits the detailed quantitative evaluation of lightning activity on continental scales.

The long-lasting, persistence electromagnetic ion cyclotron (EMIC) waves can have a profound effect on the dynamics of the relativis-tic electrons in the Earth's radiation belt. EMIC waves observed on the ground are the manifestation of their generation in the Earth's inner magnetosphere. With the spacecraft measurements, it is difficult to determine their spatio-temporal extent and sustainability. In this context, in addition to the spacecraft observations the ground observations can give better insights into these features. We have identified the long-duration EMIC wave events observed at the subauroal station Maitri during 2011-2017. We have found a total of 12 EMIC wave events that are steadily observed for >10 h. The start times of these events are identified between 1.4-9.3 LT hours, and they lasted for 10-18 h. All these events are dominantly seen during the recovery phase of the weak-moderate geomagnetic storm. Comparison with spacecraft observations indicate the simultaneous EMIC waves in the magnetosphere within L-shell 4-6 with wider longitudinal cover-age, and sufficiently long time presence in the magnetosphere. Simultaneous differential electron flux observations from Van Allen Probes indicate that approximately 90% of these EMIC waves were accompanied by a decrease in relativistic (2.6-4.2 MeV) electrons in the outer radiation belts. (c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

The modulation of electromagnetic ion cyclotron (EMIC) waves by different geomagnetic pulsations is known to us from both ground and satellite observations. However, their dependence on the EMIC wave characteristics is not well explored. We report a statistical analysis of modulation of EMIC waves by short and long periodicities at the Indian Antarctic station, Maitri ( L ≈ 5). We have analyzed the induction coil magnetometer data for the period of 2011–2017 and identified 6,845 EMIC wave events, out of which 5,502 events (80%) clearly showed the presence of short period modulation. These short period modulations are associated with repetitive rising tone EMIC wave emissions. Among these 5,502 EMIC wave events only 2,413 events showed presence of long period modulation, in addition to the short period modulation. Next we have examined the characteristics like start time, end time, peak frequency, frequency extent, maximum power, and dominant short and long periodicities present in each EMIC wave event. Based on the statistical analysis, we found that the dominant short and long periodicities in the EMIC waves are in the range of 1.5–3 min and 10–60 min, respectively. These short period decreases with an increase in the peak frequency of the EMIC wave. It is attributed to the decrease in magnetic field line oscillation period at lower L‐shells. Additionally, we noticed that the stronger EMIC wave events are likely to have a higher peak frequency. All these observed tendencies are examined in light of nonlinear theory, and they are found to be in good agreement.

The impulse-induced super substorm of 21st January 2005 has been reported by Hajra and Tsurutani (2018) and they have discussed the abnormal morphology of the auroral dynamics mainly examining the auroral images and currents. This event is revisited here with additional datasets of global Cosmic Noise Absorption (CNA) by Riometers, auroral images by a space-based Imager, plasma flux data from geostationary satellites and magnetic field measurements of ground stations covering almost all latitudinal and local time sectors. We have observed some peculiar characteristics of this event: (1) No southward component of IMF Bz prior to the substorm onset. (2) Westward electrojet current peaks in the dawn sector, and not in the midnight sector, during the substorm interval. (3) Presence of dayside shock-auroras: The electron & proton auroras are stronger in the pre and post-midnight sectors and strongest on the dayside. (4) The time of CNA onset and substorm onset coincide in the narrow belt of magnetic latitude near 65°. The time delay between these two increases away from this belt on both-poleward and equatorward latitudes. The delay is small (~ 2 minutes) on the dayside and significant (~11 minutes) on the night side. (5) The percentage of CNA is found to vary with MLT as well as with latitudes. CNA absorption is lowest on the dayside (~20%) whereas near dawn and dusk the percentage of CNA is nearly equal (~40-50%). (6) Considering the model proposed by Sorathia et al. (2019) we can say that the dawn-dusk symmetry observed in the CNA indicates the entry of particles from the flanks facilitated by Kelvin-Helmholtz instability.

The study aims to find out the ionic and isotopic compositions of some lakes in the Schirmacher Oasis in east Antarctica. The water samples were collected from 12 lakes near the Indian Antarctic station, Maitri. The chemistry of lake water indicates contribution from glacial melt water and lithology of the area. Evaporation from shallow lakes was also found to be controlling the lake water chemistry. The isotopic concentration of the lake waters also indicated input from the melt water from adjacent glaciers. delta D varied between-167.2 parts per thousand and -250.2 parts per thousand and delta O-18 from -18.0 parts per thousand to -31.2 parts per thousand. It was found that diffusion controlled kinetic effect at the liquid-ice interface for different water isotopologues and evaporative enrichment of heavier isotopes from open water bodies determined the slope of the regression line of the lake water.

Super-substorms(SSS) are long duration events with SML <;-2500 nT, which are not so frequently observed. Characteristics of such sub-storms have not been studied in details so far. The case study of two shock induced substorms which occurred on 2005 January 21 and 2010 April 5 had been studied earlier by Hajra and Tsurutani (2018) 1 , wherein it was observed that auroras associated with these events did not have the typical midnight onset characteristic. At the time of the SML peak, intense auroras were observed during premidnight and post-midnight local time sector instead of midnight. Thus, it becomes important to have a case-to-case study for the better understanding of such substorms phenomena and examine their general statistical behaviour. In the present study, an attempt has been made to study the sub-auroral characteristics of these events using multi-technique observations at Indian Antarctic station Maitri (geographic 70.75°S, 11.75°E) and other relevant corroborating satellite observations as well as ground observations from both the hemispheres. Low latitude characteristics of SSS will also be examined from the available ground stations relevant to those observed events. Depending upon the availability of data, we will also try to decipher the longitudinal and latitudinal extent of electrojet currents for these events.

Electromagnetic ion cyclotron (EMIC) wave activity observed on the ground is occasionally structured in the time domain, showing either well developed wave packets consisting of rising tone frequency emissions or more chaotic bursts. We present the ground observation of the modulation of electromagnetic ion cyclotron (EMIC) emissions by short and long periodicities at Indian Antarctic station, Maitri. The signatures of these waves were evident in the magnetic field variations recorded by an induction coil magnetometer during the interval 4.7-7.2 UT on 17 September 2011, a moderately disturbed day. These waves preceded by a gradual increase in the solar wind dynamic pressure, which started at 3.88 UT. The discrete rising tone EMIC waves were observed in the Pc1 frequency band (0.5-0.9 Hz). The investigation of the periodicities of the observed wave spectrogram shows the presence of short (2.9-3.2 minute) and long (42-83 minute) periodicities. Our analysis shows that the short periodicities are associated with the Pc5 ULF waves generated by magnetic field line oscillations, while long periodicities are associated with the ring current drifting ions. We computed the sweep rates of the discrete EMIC rising tones in the observed EMIC band. For a given EMIC discrete emission, a sweep rate is defined as the rate of change of frequency in a given time interval. A new method, based on the cross-correlation technique is adopted to determine sweep rates of the discrete rising tones. The average sweep rates estimated in the range of 0.44-1.9 mHz/s are relatively low as compared to the past reports of sweep rates derived from the satellite observations of EMIC waves (tens-hundreds mHz/s), which is attributed to the propagational effect. We found that the sweep rate varies with time, and the higher sweep rates are associated with the stronger EMIC waves on the ground, which is in agreement with the theoretical studies. This suggests that the theoretically proposed dependence of sweep rate on strength of EMIC wave in the generation region is retained even during the propagation of these waves on the ground. To investigate the role of modulated EMIC emissions in particle precipitation, we examined the ground observations at Maitri in conjunction with the electron count observations from GOES-13 (74.8°E) and GOES-15 (89.6°E). It is observed that the electron count in the MeV range decreased during the period of EMIC wave activity. In view of the potential role of EMIC rising tone emissions in the loss of MeV electrons from the radiation belts, the ICM observations from the Indian Antarctic station Maitri are crucial for deciphering the inner magnetospheric dynamics.

We present a ground observation of modulations of strong electromagnetic ion cyclotron (EMIC) waves by short and long periodicities at Indian Antarctic station, Maitri. The signatures of these waves were evident in the magnetic field variations recorded by an induction coil magnetometer during the interval 4.7–7.2 UT on 17 September 2011, a moderately disturbed day. These waves were preceded by a gradual increase in the solar wind dynamic pressure, which started at 3.88 UT. The discrete rising tone EMIC waves were observed in the Pc1 frequency band (∼0.5–0.9 Hz). The investigation of the periodicities of the observed wave spectrogram shows the presence of short (≈2.4 min) and long (≈39–69 min) periodicities. Our analysis shows that the short periodicities are associated with the Pc5 Ultra Low Frequency (ULF) waves generated by magnetic field line oscillations, while long periodicities might be associated with the ring current drifting ions. A new method, based on the cross‐correlation technique, is adopted to determine sweep rates of the discrete rising tones. The average sweep rates estimated in the range of 0.44–1.9 mHz/s are relatively low as compared to the past reports of sweep rates derived from the satellite observations of EMIC waves. We found that the higher sweep rates are associated with the stronger EMIC waves on the ground, which is in agreement with the theoretical studies. This suggests that the theoretically proposed dependence of sweep rate on strength of EMIC wave in the generation region is retained even during the propagation of these waves to the ground.

In the present paper, the perception of the seasonal variation of the Sq focus position is re-examined during low solar activity period (2006–2010). Equivalent current vectors are plotted for each geomagnetic quiet day (Ap⩽5), using diurnal variations of H and D components measured at the magnetic observatories located in a narrow longitudinal belt of the Indo-Russian region. On the formation of well-defined Sq current loop, the information about the Sq focus is extracted by identifying a pair of neighboring stations with opposite zonal currents and nearby local times with opposite meridional currents. Thus, the method employed here is different from the methods used in earlier studies. Prominent seasonal variations in the Sq focus latitude, as well as in the local time of Sq focus, are observed. It is observed that the Sq focus is located at ∼30deg in March equinox, but it moves to lower latitudes in the month of September. In winter, it shows large variability and also the formation of clear Sq current loop is less frequent. The local time of Sq focus is at ∼12 LT in March and shifts to ∼10 LT during September. It is clearly evident from the present analysis that the March and September equinoxes behave differently. The dominance of DE3 and semidiurnal waves in the September equinox could be the reason for the observed disparity.

The present study has focused on the intense production of cosmic noise absorption (CNA) at Maitri, Antarctica (L = 5; CGM −62°S, 55°E) during the early recovery phase of the largest storm of the current solar cycle commenced on 17 March 2015 St. Patrick's Day. The enhancement of CNA during 15–18 UT(14–17 magnetic local time (MLT); MLT = UT − 1 at Maitri) was as large as the CNA enhancement occurred during the main phase of the storm. During this time the CNA pattern also exhibits oscillation in the Pc5 (2–7 mHz) range and is in simultaneity with geomagnetic pulsations in the same frequency range. We observed the amplitude of CNA pulsation is well correlated with the level of CNA production. High‐amplitude Pc5 oscillations were observed in the vicinity of auroral oval near Maitri. Absence of electromagnetic ion cyclotron (EMIC) waves is marked suggesting the possible role of VLF waves in precipitation. The reason for the intense CNA production is found to be the precipitation caused mainly by hiss‐driven subrelativistic electrons. The CNA enhancement event is located well inside the dusk plasmaspheric bulge region as suggested by Tsurutani et al. (2015). Signature of enhanced eastward electrojet at Maitri during 14–17 MLT could be an additional factor for such large CNA. In order to establish the cause and effect relationship between the geomagnetic and CNA oscillations at Maitri, transfer entropy method has been used, which confirmed the modulation of CNA by geomagnetic pulsations.

On 02 April 2011, a couple of cosmic noise absorption (CNA) events were detected at Maitri, Antarctica (L = 5; CGM 63.14 degrees S, 53.69 degrees E) confining to nighttime and daytime. One of the two events that occurred during night hours was caused due to auroral substorm onset. The current study focuses on the later CNA event, which was recorded during daytime (10:00-13:00 magnetic local time (MLT), MLT = UT-1, at Maitri, Antarctica). We refer to this CNA event as dayside CNA (DCNA) event. Absence of westward electrojet during DCNA confirms its dissimilarity from auroral substorm absorption events. A comparison has been made between the DCNA event of 02 April 2011 with that of 14 July 2011, a day with substorm activity when Maitri is in dayside but without DCNA event. The comparison has been made in the light of interplanetary conditions, imaging riometer data, ground magnetic signatures, GOES electron flux density, and associated pulsations. The study shows that stronger prolonged eastward interplanetary electric field favors the occurrence of DCNA event. It is concluded that DCNA event is due to the gradient curvature drift of trapped nonrelativistic electrons in the equatorial plane. Estimated energy of trapped electrons using azimuthal drift time for a set of ground stations within the auroral oval confirms the enhancement in electron fluxes in the same energy band as recorded by geostationary satellites GOES 13 and GOES 15. The reason for precipitation of electrons is expected to be the loss cone scattering caused by wave-particle interaction triggered by ULF waves.

Low-latitude Pi2 pulsations in the topside ionosphere are investigated using vector magnetic field measurements from LEO satellite, CHAMP, and underneath ground station. Substorm-associated Pi2s are initially identified using high-resolution data from Indian station Shillong, during 2007-2009, and are further classified into three subgroups of Pi2 band (6-25mHz), based on its frequency. During nighttime, coherent in-phase oscillations are observed in the compressional component at satellite and horizontal component at underneath ground station for all the Pi2 events, irrespective of the Pi2 frequency. We observe that the identification of daytime Pi2s at CHAMP (compressional component) depends on the frequency of Pi2 oscillation; i.e., 40%, 45%, and 100% of Pi2 events observed in dayside ground station with frequency between 6-10mHz, 10-15mHz, and 15-25mHz were identified at satellite, respectively. At CHAMP during daytime, the presence of a dominant power in the lower frequencies of Pi2 band, which is unique to satellite, is consistently observed and can modify the Pi2 oscillations. Pi2s having frequency >15mHz are less affected by these background frequencies, and a clear signature of daytime Pi2s at CHAMP is possible to observe, provided that contribution from non-Pi2 frequencies at satellite from the lower end of Pi2 band is eliminated. Daytime Pi2s identified in the topside ionosphere showed coherent but mostly opposite phase oscillations with underneath ground station, and satellite-to-ground amplitude ratio is, in general, found to be less than 1. Present results indicate that a combination of fast cavity-mode oscillations and an instantaneous transmission of Pi2 electric field from high- to low-latitude ionosphere is responsible for the observation of daytime Pi2s.

Every lightning flash contributes energy to the TEM mode of the natural global waveguide that contains the Earth’s Schumann resonances. The modest attenuation at ELF (0.1 dB/Mm) allows for the continuous monitoring of the global lightning with a small number of receiving stations worldwide. In this study, nine ELF receiving sites (in Antarctica (3 sites), Hungary, India, Japan, Poland, Spitsbergen and USA) are used to provide power spectra at 12-minute intervals in two absolutely calibrated magnetic fields and occasionally, one electric field, with up to five resonance modes each. The observables are the extracted modal parameters (peak intensity, peak frequency and Q-factor) for each spectrum. The unknown quantities are the geographical locations of three continental lightning ‘chimneys’ and their lightning source strengths in absolute units (C2 km2/sec). The unknowns are calculated from the observables by the iterative inversion of an evolving ‘sensitivity matrix’ whose elements are the partial derivatives of each observable for all receiving sites with respect to each unknown quantity. The propagation model includes the important day-night asymmetry of the natural waveguide. To overcome the problem of multiple minima (common in inversion problems of this kind), location information from the World Wide Lightning Location Network has been used to make initial guess solutions based on centroids of stroke locations in each chimney. Results for five consecutive days in 2009 (Jan 7-11) show UT variations with the African chimney dominating on four of five days, and America dominating on the fifth day. The amplitude variations in absolute source strength exceed that of the ‘Carnegie curve’ of the DC global circuit by roughly twofold. Day-to-day variations in chimney source strength are of the order of tens of percent. Examination of forward calculations performed with the global inversion solution often show good agreement with the observed diurnal variations at individual receiving sites, lending confidence to the 3-chimney model for global lightning.

Cosmic noise absorption (CNA) measurred by imaging riometer, is an excellent tool to passively study the high latitude D-region ionospheric conditions and dynamics. An imaging riometer has been installed at Indian Antarctic station Maitri (geographic 70.75°S, 11.75°E; corrected geomagnetic 63.11°S, 53.59°E) in February 2010. This is the first paper using the imaging riometer data from Maitri. The present paper introduces the details of this facility, including its instrumentation, related CNA theory and its applications. Sidereal shift of around 2 hours in the diurnal pattern validates the data obtained from the newly installed instrument. Moreover, the strength of cosmic noise signal on quiet days also varies with months. This is apparently due to solar ionization of D-region ionosphere causing enhanced electron density where collision frequency is already high. The main objective of installing the imaging riometer at Maitri is to study magneotspheric–ionospheric coupling during substorm processes. In the current study, we present two typical examples of disturbed time CNA associated with storm-time and non-storm time substorm. Results reveal that CNA is more pronounced during storm-time substorm as compared to non-storm time substorm. The level of CNA strongly depends upon the strengthening of convectional electric field and the duration of southward turning of interplanetary magnetic field before the substorm onset.