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This study investigates the impact on the Earth’s ionosphere of a severe geomagnetic storm (Dst ∼ −212 nT) that began on 23 April 2023 at around 17:37 UT according to very low-frequency (VLF, 3–30 kHz) or low-frequency (LF, 30–300 kHz) radio signals and ionosonde data. We analyze VLF/LF signals received by SuperSID monitors located in mid-latitude (Europe) and low-latitude (South America, Colombia) areas across nine different propagation paths in the Northern Hemisphere. Mid-latitude regions exhibited a daytime amplitude perturbation, mostly an increase, by ∼3–5 dB during the storm period, with a subsequent recovery after 7–8 days post April 23. In contrast, signals received in low-latitude regions (UTP, Colombia) did not show significant variation during the storm-disturbed days. We also observe that the 3-hour average of foF2 data declined by up to 3 MHz on April 23 and April 24 at the European Digisonde stations. However, no significant variation in foF2 was observed at the low-latitude Digisonde stations in Brazil. Both the VLF and ionosonde data exhibited anomalies during the storm period in the European regions, confirming that both D- and F-region ionospheric perturbation was caused by the severe geomagnetic storm.

Phase stability at low radio frequencies is severely impacted by ionospheric propagation delays. Radio interferometers such as the giant metrewave radio telescope (GMRT) are capable of detecting changes in the ionosphere’s total electron content (TEC) over larger spatial scales and with greater sensitivity compared to conventional tools like the global navigation satellite system (GNSS). Thanks to its unique design, featuring both a dense central array and long outer arms, and its strategic location, the GMRT is particularly well-suited for studying the sensitive ionospheric region located between the northern peak of the equatorial ionization anomaly (EIA) and the magnetic equator. In this study, we observe the bright flux calibrator 3C48 for ten hours to characterize and study the low-latitude ionosphere with the upgraded GMRT (uGMRT). We outline the methods used for wideband data reduction and processing to accurately measure differential TEC (δTEC) between antenna pairs, achieving a precision of< mTECU (1 mTECU = 10−3 TECU) for central square antennas and approximately mTECU for arm antennas. The measured δTEC values are used to estimate the TEC gradient across GMRT arm antennas. We measure the ionospheric phase structure function and find a power-law slope of β=1.72±0.07, indicating deviations from pure Kolmogorov turbulence. The inferred diffractive scale, the spatial separation over which the phase variance reaches 1rad2, is ∼6.66 km. The small diffractive scale implies high phase variability across the field of view and reduced temporal coherence, which poses challenges for calibration and imaging.

The annular solar eclipse of the previous decade that occurred on December 26, 2019, was mostly visible from most Asian countries, including India. In this manuscript, we present the variation of ionospheric plasma density profiles at different heights during the eclipse and non-eclipse days. We use both the ground- and space-based instruments to study the depletion in the electron density profile in the ionospheric D and F layers. We use the International GNSS Service (IGS)/Global Positioning System (GPS) stations and Swarm satellites outcomes to compute ionospheric Total Electron Content (TEC). We choose the IGS stations either on the path of the annularity or in the close vicinity of the annularity belt. For the first kind, we choose two IGS stations, GUUG in Guam island near the Western Pacific and CNMR in the Northern Mariana islands. We choose two Indian stations, IISC in Bangalore and HYDE in Hyderabad, that are close to the annularity belt. We observe ∼ 20–40% depletion in the diurnal profile of TEC, as estimated from the IGS stations. This depletion is validated by a NovAtel GPS station-6 instrument situated at Hyderabad. Significant depletion in the spatio-temporal profile of TEC and electron density profile as computed from Swarm satellite data. The depletion in TEC is also validated by NASA’s CDAWEB archive using Global Ionospheric Map (GIM). We try to estimate the TEC in the F layer and the electron density in the D layer using a numerical model. We compute the solar obscuration percentage by the geometrical configuration of the Sun and Moon. For TEC we use the solar irradiation model and calculate the delay of the maximum depletion compared to the maximum obscuration. To investigate the effects of the eclipse at the lower ionospheric heights (68–88 km), we use the GPI ion-chemistry model and observe depletion in the electron density profiles of around ∼ 58–73% for all the locations. We observe that the time between the maximum depletion in the electron density and the maximum obscuration decreases with increasing height, and it gets the minimum value at an altitude of 84 km. All methods give consistent outcomes with depletion of plasma density that corroborates the effects of the solar eclipse on the Earth’s ionosphere.

We wish to report our observations of ionospheric disturbances made by incident Gamma Rays from the Soft Gamma Repeater SGR J1550-5418 which took place on 22nd of January, 2009. The observations were made by a loop antenna and a Gyrator-II type receiver which was tuned to the 500KW VTX station transmitting Very Low Frequency (VLF) signal at 18.2 kHz. We looked for signatures of sudden ionospheric disturbances (SID) which commenced within seconds of the observations reported by various satellites. We used Long Wave Propagation Capability code to compute the changes in the ionospheric parameters due to the repeated passage of the high energy radiation. We detected seventy six events in our VLF detector which appear to be associated with the SGR J1550-5418. We found that 28 of them are within three seconds of the satellite observations and 38 (fifty percent) of them are within seven seconds of the satellite observations. We also compute the evolution of the electron number density of the ionosphere due to this events and found that the ionosphere was becoming increasingly charged due to repeated bombardment of the high energy radiations. The lower ionospheric height went down significantly. We found convincing evidence that the source SGR J1550-5418 repeatedly caused ionospheric disturbances on the 22nd of January, 2009. The electron number density went up significantly at all the heights. The height of the lower ionosphere went down by about 15 km due to these repeated events.

Solar eclipse is a unique phenomenon that produces an orderly disturbance in the ionosphere within a specific time frame. It provides us an opportunity to understand the ionospheric response due to its systematic variation during an eclipse. The amplitude and phase of a Very Low Frequency (VLF) signal carry the response of lower ionospheric perturbation due to the varying solar radiation impinging on Earth. During the Annular Solar Eclipse on December 26, 2019, (ASE2019), Indian Centre for Space Physics (ICSP), Kolkata, India conducted a nationwide VLF radio signal monitoring campaign to obtain the amplitude and phase variations of propagating VLF signal from fourteen different locations across India. The signal amplitude and phase profile exhibit unique profiles at these locations. These profiles in the VLF signal during ASE2019 are explained using the Long Wavelength Propagation Capability (LWPC) code and the modeled variation of solar disk obscuration by the moon. Furthermore, the lower ionospheric electron density (Ne) computed from the model is in agreement with the observed lower ionospheric conditions.

Very Low Frequency (VLF) radio waves propagate through the Earth-ionosphere waveguide. Irregularities caused by excess or deficient extreme ultra-violet and X-rays, which otherwise sustain the ionosphere, change the waveguide properties and hence the signals are modified. We report the results of monitoring of the NWC transmitter (19.8 kHz) by a receiver placed at Khukurdaha (22°27′N, 87°45′E) during the partial solar eclipse (75 %) of 15th January, 2010. The propagation path from the transmitter to the receiver crosses the annular eclipse belt. We got a clear depression in the data during the period of the eclipse. Most interestingly, there was also a X-ray flaring activity in the sun on that day which reached its peak (C-type) right after the time when the eclipse reached its maximum. We saw the effects of the occultation of this flare in our VLF signal since a part of the X-ray active region was clearly blocked by the moon. We quantitatively compared by using analogies with previous observations and found best fitting parameters for the time when the flare was occulted. We then reconstructed the VLF signal in the absence of the occulted flare. To our knowledge, this is the first such incident where the solar flare was observed through lunar occultation and that too during a partial eclipse.

We present a list of candidate tailed radio galaxies using the TIFR GMRT Sky Survey Alternative Data Release 1 (TGSS ADR1) at 150 MHz. We have visually examined 5336 image fields and found 264 candidates for the tailed galaxy. Tailed radio galaxies are classified as Wide Angle Tailed (WAT) galaxies and Narrow-Angle Tailed (NAT) galaxies, based on the angle between the two jets of the galaxy. We found a sample of tailed radio galaxies which includes 203 `WAT' and 61 `NAT' type sources. These newly identified tailed sources are significant additions to the list of known tailed radio galaxies. The source morphology and luminosity features of the different candidate galaxies and their optical identifications are presented in the paper. Other radio properties and general features of the sources are also discussed.

A small sub-class of radio galaxies that exhibit a pair of secondary low surface brightness radio lobes oriented at an angle to the primary high surface brightness lobes are known as X-shaped radio galaxies (XRGs). In cases, it is seen that the less luminous secondary lobes emerge from the edges of the primary high brightened lobes and give a Z-symmetric morphology. These are known as Z-shaped radio galaxies (ZRGs). From the TIFR GMRT Sky Survey at 150 MHz, we present a systematic search result for XRGs and ZRGs. We identified a total of 58 radio sources, out of which 40 are XRGs and 18 are ZRGs. Taking advantage of the large sample size of the XRGs and ZRGs reported in the current paper, different properties of XRGs and ZRGs are studied. Out of 58 XRGs and ZRGs presented in the current paper, 19 (32 per cent) are FR-I and 33 (57 per cent) are FR-II radio galaxies. For four XRGs and three ZRGs, the morphology is so complex that they could not be classified. We have estimated radio luminosity and spectral index of newly discovered winged radio galaxies and made a comparative study with previously detected XRGs and ZRGs. Most of the XRGs show a steep spectral index between 150 MHz and 1400 MHz and only 14 per cent of the sources show a flat spectrum but for ZRGs, a good fraction of the sources (36 per cent) show a flat spectrum.

One of the greatest astrophysical radio structures in the Universe is giant radio sources (GRSs) which have a linear projected size of nearly 0.7 Mpc or more. We systematically search for giant radio galaxies from the TIFR GMRT Sky Survey Alternative Data Release 1 (TGSS ADR1) at 150 MHz. Our search area covers 36,900 square degrees of the sky between \\(-\\)53 deg and +90 deg DEC, which is 90 per cent of the total sky. We have identified 53 GRSs with a linear size range between 0.7 Mpc to 1.82 Mpc. We studied characteristic parameters like identification of candidates, angular and projected linear size, redshift, spectral index, radio power, and black hole mass of GRSs. We study optical and mid-IR properties of discovered GRSs. We also classify newly discovered GRSs into giant radio galaxies (GRGs) and giant radio quasar and also classify GRGs into low-excitation giant radio galaxies (LEGRGs) and high-excitation giant radio galaxies (HEGRGs).

The response of electric field intensity of VLF radio atmospherics during two tropical cyclones Fani (May 2019) and Amphan (May 2020) has been presented in this paper. VLF radio atmospherics (or VLF sferics) received at Coochbehar (CHB) and Kolkata (CUB) at three discrete frequencies (4 kHz, 7 kHz, and 9 kHz) showed clear amplitude anomalies with respect to the reference level during the two cyclonic storms. This is explained using the electrical structure and distribution of cloud-to-ground lightning associated with the cyclones. Effects of 'local lightning' and 'distant lightning' have been identified for both the CUB and CHB receivers. Field intensity of VLF sferics at CUB station was found to get enhanced for both types of lightning events. But the intensity of VLF sferics at CHB station was found to be reduced for 'distant lightning' and enhanced for 'local lightning', possible reasons of which are also explained.