arXiv subjects

P. K. Manoharan

Publications and source records attributed to P. K. Manoharan.

At least 19 recordsLinked to original sources

arXiv2026

Breakout/Interchange Reconnection as a driver of Jets, Fast CME, and Solar Energetic Particles

Understanding how energetic particles are accelerated and released from the low corona into the interplanetary medium during solar eruptions is crucial for space weather research. Here, we present multiwavelength observations of a solar eruption that are consistent with breakout reconnection playing an important role in driving a fast coronal mass ejection (CME) and the associated solar energetic particle (SEP) event. Extreme-ultraviolet and radio observations reveal evidence of breakout reconnection within a fan-spine topology. The filament eruption begins after the pre-eruption opening, accompanied by quasiperiodic jets and associated downflows near the null point, as well as recurrent faint Type III radio bursts during the ongoing slow breakout reconnection. Furthermore, the observations also reveal the formation of pre-eruption coronal rain via slow interchange reconnection near the null point. A large-scale circular ribbon, along with simultaneous four hard X-ray footpoint sources and intense Type III radio bursts, was observed during the explosive breakout reconnection that enabled opening of field lines and allowed energetic particles to escape into interplanetary space. In situ measurements by PSP and Wind confirm the prompt injection of electron beams consistent with the timing of the explosive breakout reconnection. A fast shock associated with the erupting flux rope during interchange reconnection played a major role in producing the gradual SEP event. These observations highlight the key role of breakout reconnection in producing a fast CME/shock with the SEP release and acceleration process. These results have broader implications for particle acceleration and release processes in multiscale null-point topologies, which produce a continuum of eruptions ranging from small-scale jets to large-scale CMEs.

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arXiv2026

e-CALLISTO FITS Analyzer: A Software Framework for CALLISTO Solar Radio Data

Solar radio bursts are important signatures of dynamic processes in the solar corona, including particle acceleration and shock propagation associated with solar flares and coronal mass ejections. Among the missions that report solar radio bursts within 24 hours, the e-CALLISTO archive is the largest, with more than 150 stations worldwide. The archive generates large volumes of FITS data that are often affected by radio-frequency interference and background noise. Irregular frequency setups in different stations are also a limitation of statistical analysis of SRBs. Each CALLISTO observation is a 15-minute frame, which often causes a single burst to split over multiple frames, making event-level analysis difficult. This work presents the e-CALLISTO FITS Analyzer, a unified, interactive, cross-platform application for processing and analyzing e-CALLISTO dynamic spectra on Windows, macOS, and Linux. The application supports time and frequency merging to produce a continuous spectrum, applies mean background subtraction with user-controlled threshold clipping, and isolates burst regions through an interactive polygon mask in the time-frequency plane. It also extracts the maximum-intensity backbone, allows interactive outlier removal, and performs power-law fitting to estimate drift rates and derive shock height and speed using the Newkirk model, including $n$-fold scaling. For a Type II burst observed by Arecibo Observatory on 2 March 2022, the analyzer yielded an average drift rate of $-0.0400 \pm 0.0003\, MHz/s$ and an average shock speed of $449 \pm 1\, km/s$ at a height of $1.715 \pm 0.002\, R_{\odot}$. The e-CALLISTO FITS Analyzer supports more reproducible, event-focused SRB analysis and improves access to physically meaningful measurements from e-CALLISTO FITS data.

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arXiv2025

X-CME: From In Situ Flux-Rope Reconstruction to CME Propagation Forecasting

Accurate forecasts of Coronal Mass Ejection (CME) arrival times and impact geometry remain a major challenge for space-weather operations. Coronagraph-based techniques typically achieve mean absolute errors of order ten hours, while in situ measurements at L1 provide excellent magnetic-field information but only tens of minutes of warning. In this work we introduce X-CME, a framework that links in situ flux-rope reconstructions at intermediate heliocentric distances with a physics-based CME propagation model. The internal magnetic structure is obtained with an elliptical cylindrical, radial poloidal flux-rope model and embedded into a tapered torus CME geometry. The subsequent propagation is computed by solving a drag-based equation of motion in a Parker solar-wind background, including gravitational deceleration, self-similar expansion of the cross section, and an explicit calculation of the CME wetted area and swept area in the ecliptic plane. We apply X-CME to two events observed by the Parker Solar Probe and Solar Orbiter spacecraft, respectively, and propagate the reconstructed structures to Earth and Mars. For both cases, the model reproduces the observed in situ signatures at L1 and predicts the CME arrival time at Earth with errors of a few hours (typically about 2-4 hours), while correctly distinguishing between central encounters and glancing blows. These results demonstrate that combining intermediate-distance magnetic reconstructions with a geometrically consistent propagation scheme can substantially improve CME arrival-time forecasts and impact assessment in the inner heliosphere.

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arXiv2025

Generalization of Elliptical-Cylindrical Flux Rope Models for ICME Reconstruction

We present a generalized elliptical cylindrical flux rope model for interplanetary coronal mass ejections (ICMEs) that allows for a non zero poloidal component in the internal magnetic field. We introduce a two step reconstruction algorithm that decouples the geometric configuration from the magnetic field fitting in order to improve numerical stability and physical consistency. Applied to Parker Solar Probe data, the new radial poloidal model preserves the global flux rope geometry while achieving a substantially better fit to the internal magnetic field than the traditional radial model, offering a more accurate and realistic description of ICME structures.

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arXiv2025

Anomalous scattering of pulsars towards the Gum Nebula

We report wideband scatter-broadening estimates of 14 pulsars towards the Gum nebula region using the Band-3 of the upgraded GMRT. This work increases the measurements of frequency scaling index of scatter-broadening ($\alpha$) across the nebula by more than 3 times. A strong correlation between the distance and the scattering strength is observed for pulsars behind the nebula. It is also observed that for distant pulsars ($> 2 kpc$), the effect of the Gum nebula in DM and scattering strength is not substantial. We also report a much flatter $\alpha$ for the Vela pulsar and argue that its scattering is not caused by the Gum nebula, but the Vela supernova remnant.

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arXiv2025

Imaging and Radio Signatures of Shock-Plasmoid Interaction

Understanding how shocks interact with coronal structures is crucial for understanding the mechanisms of particle acceleration in the solar corona and inner heliosphere. Using simultaneous radio and white-light observations, we investigate the interaction between a CME-driven shock and a plasmoid. LASCO and STEREO-A COR-2 white-light images are analyzed to track the evolution of the plasmoid, CME and its associated shock, while the Wind/WAVES and STEREO/WAVES dynamic spectra provide complementary radio signatures of the shock-plasmoid interaction at $\approx$7 R$_\odot$. An interplanetary Type II radio burst was detected as the shock propagated through the plasmoid. The merging of the plasmoid into the CME was accompanied by interplanetary Type III radio bursts, suggesting escaping electron beams during the reconnection process. These observations clearly demonstrate that shock-plasmoid interactions can enhance the efficiency of particle acceleration associated with CMEs, with implications for electron acceleration in flare and heliospheric current sheets as well.

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arXiv2025

Arecibo Multi-frequency IPS Observations: Solar Wind Density Turbulence Scale Sizes and their Anisotropy

We present an analysis of interplanetary scintillation (IPS) observations conducted with the Arecibo 305-m radio telescope during the minimum phase at the end of solar cycle 24 and the onset of solar cycle 25. These observations span a broad frequency range of ~300 to 3100 MHz, encompassing the P-, L-, and S-bands, and covered heliocentric distances from ~5 to 200 solar radii. The dynamic spectrum of the scintillations obtained at L-band shows a systematic decrease in the scintillation index from the lowest to the highest frequency, offering valuable insight into the influence of the solar wind density microstructures responsible for scintillation. Analyses of the scintillation index ($m$) for multiple sources at L-band, along with near-simultaneous observations of selected sources covering the P-, L-, and S-bands, clearly demonstrate a wavelength dependence of $m \propto λ^ω$, which inherently leads to a dependence of $m$ on the Fresnel scale, when considering the effective distance to the scattering screen, $z$. The index $ω$ ranges between $\sim$1 and 1.8. The average $ω$ value of a source, determined from observations made on different days, exhibits variability across sources. The results on the radial dependence of scintillation agree with earlier IPS measurements. The temporal power spectra obtained over the wide frequency range exhibit a power-level evolution in accordance with the wavelength dependence, and a broadening with increasing observation frequency. Furthermore, the increased temporal-frequency rounding of the `Fresnel knee' in the spectrum with the observing frequency suggests a novel phenomenon: an increase in anisotropy as the scale size of the density-turbulence structure decreases.

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arXiv2023

A Cryogenic Wideband (2.5-14 GHz) Receiver system for the Arecibo 12m Telescope

In this paper we present details of the construction of a wideband, cryogenic receiver and its successful commissioning on the Arecibo 12m telescope. The cryogenic receiver works in the 2.5-14 GHz frequency range. The telescope is operated by the Arecibo Observatory, and is located within the premises of the Observatory. We upgraded the current narrow band, room temperature receivers of the telescope with the new wideband receiver. The current receiver is built around a Quadruple-Ridged Flared Horn (QRHF) developed by Akgiray et al. (2013). To mitigate strong radio frequency interference (RFI) below 2.7 GHz, we installed a highpass filter before the first stage low noise amplifier (LNA). The QRHF, highpass filter, noise coupler and LNA are located inside a cryostat and are cooled to 15 K. The measured receiver temperature is 25 K (median value) over 2.5 GHz to 14 GHz. The system temperature measured at zenith is about 40 K near 3.1 and 8.6 GHz and the zenith antenna gains are 0.025 and 0.018 K/Jy at the two frequencies respectively. In the next stage of the development, we plan to upgrade the highpass filter in order to achieve better RFI rejection near 2.5 GHz, improve the aperture efficiency at 8.6 GHz and upgrade the IF system to increase the upper frequency of operation from 12 GHz to 14 GHz.

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arXiv2023

CME Propagation Through the Heliosphere: Status and Future of Observations and Model Development

The ISWAT clusters H1+H2 have a focus on interplanetary space and its characteristics, especially on the large-scale co-rotating and transient structures impacting Earth. SIRs, generated by the interaction between high-speed solar wind originating in large-scale open coronal magnetic fields and slower solar wind from closed magnetic fields, are regions of compressed plasma and magnetic field followed by high-speed streams that recur at the ca. 27 day solar rotation period. Short-term reconfigurations of the lower coronal magnetic field generate flare emissions and provide the energy to accelerate enormous amounts of magnetised plasma and particles in the form of CMEs into interplanetary space. The dynamic interplay between these phenomena changes the configuration of interplanetary space on various temporal and spatial scales which in turn influences the propagation of individual structures. While considerable efforts have been made to model the solar wind, we outline the limitations arising from the rather large uncertainties in parameters inferred from observations that make reliable predictions of the structures impacting Earth difficult. Moreover, the increased complexity of interplanetary space as solar activity rises in cycle 25 is likely to pose a challenge to these models. Combining observational and modeling expertise will extend our knowledge of the relationship between these different phenomena and the underlying physical processes, leading to improved models and scientific understanding and more-reliable space-weather forecasting. The current paper summarizes the efforts and progress achieved in recent years, identifies open questions, and gives an outlook for the next 5-10 years. It acts as basis for updating the existing COSPAR roadmap by Schrijver+ (2015), as well as providing a useful and practical guide for peer-users and the next generation of space weather scientists.

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arXiv2023

Solar Radio Imaging at Arecibo: The Brightness Temperature and Magnetic Field of Active Regions

Strong solar magnetic fields are the energy source of intense flares and energetic coronal mass ejections of space weather importance. The key issue is the difficulty in predicting the occurrence time and location of strong solar eruptions, those leading to high impact space weather disturbances at the near-Earth environment. Here, we report regular solar mapping made at X-band (8.1 -- 9.2 GHz) with the Arecibo 12-m radio telescope. This has demonstrated its potential for identifying active regions, about one half to a day in advance, when they rotate on to the central meridian of the Sun, and predicting the strongest flares and coronal mass ejections directed towards the Earth. Results show (i) a good correlation between the temporal evolution of brightness temperature of active regions and their magnetic configurations; (ii) the ability of the mapping data to provide a better picture of the formation sites of active regions and to accurately track their evolution across the solar disk, giving forewarning of intense solar eruptions leading to severe space weather consequences; (iii) the importance of long-term monitoring of the Sun at X-band for understanding the complex three-dimensional evolution of solar features as a function of solar activity. The key point in this study is the identification of the magnetic properties of active regions on the solar disk to aid in improving forecast strategies for extreme space-weather events.

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arXiv2023

The Next Generation Arecibo Telescope: A preliminary study

The Next Generation Arecibo Telescope (NGAT) was a concept presented in a white paper Roshi et al. (2021) developed by members of the Arecibo staff and user community immediately after the collapse of the 305 m legacy telescope. A phased array of small parabolic antennas placed on a tiltable plate-like structure forms the basis of the NGAT concept. The phased array would function both as a transmitter and as a receiver. This envisioned state of the art instrument would offer capabilities for three research fields, viz. radio astronomy, planetary and space & atmospheric sciences. The proposed structure could be a single plate or a set of closely spaced segments, and in either case it would have an equivalent collecting area of a parabolic dish of size 300 m. In this study we investigate the feasibility of realizing the structure. Our analysis shows that, although a single structure ~300 m in size is achievable, a scientifically competitive instrument 130 to 175 m in size can be developed in a more cost effective manner. We then present an antenna configuration consisting of one hundred and two 13 m diameter dishes. The diameter of an equivalent collecting area single dish would be ~130 m, and the size of the structure would be ~146 m. The weight of the structure is estimated to be 4300 tons which would be 53% of the weight of the Green Bank Telescope. We refer to this configuration as NGAT-130. We present the performance of the NGAT-130 and show that it surpasses all other radar and single dish facilities. Finally, we briefly discuss its competitiveness for radio astronomy, planetary and space & atmospheric science applications.

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arXiv2022

The Indian Pulsar Timing Array: First data release

We present the pulse arrival times and high-precision dispersion measure estimates for 14 millisecond pulsars observed simultaneously in the 300-500 MHz and 1260-1460 MHz frequency bands using the upgraded Giant Metrewave Radio Telescope (uGMRT). The data spans over a baseline of 3.5 years (2018-2021), and is the first official data release made available by the Indian Pulsar Timing Array collaboration. This data release presents a unique opportunity for investigating the interstellar medium effects at low radio frequencies and their impact on the timing precision of pulsar timing array experiments. In addition to the dispersion measure time series and pulse arrival times obtained using both narrowband and wideband timing techniques, we also present the dispersion measure structure function analysis for selected pulsars. Our ongoing investigations regarding the frequency dependence of dispersion measures have been discussed. Based on the preliminary analysis for five millisecond pulsars, we do not find any conclusive evidence of chromaticity in dispersion measures. Data from regular simultaneous two-frequency observations are presented for the first time in this work. This distinctive feature leads us to the highest precision dispersion measure estimates obtained so far for a subset of our sample. Simultaneous multi-band uGMRT observations in Band 3 and Band 5 are crucial for high-precision dispersion measure estimation and for the prospect of expanding the overall frequency coverage upon the combination of data from the various Pulsar Timing Array consortia in the near future. Parts of the data presented in this work are expected to be incorporated into the upcoming third data release of the International Pulsar Timing Array.

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arXiv2022

Nanohertz Gravitational Wave Astronomy during the SKA Era: An InPTA perspective

Decades long monitoring of millisecond pulsars, which exhibit highly stable rotational periods, in pulsar timing array experiments is on the threshold of discovering nanohertz stochastic gravitational wave background. This paper describes the Indian Pulsar timing array (InPTA) experiment, which employs the upgraded Giant Metrewave Radio Telescope (uGMRT) for timing an ensemble of millisecond pulsars for this purpose. We highlight InPTA's observation strategies and analysis methods, which are relevant for a future PTA experiment with the more sensitive Square Kilometer Array (SKA) telescope. We show that the unique multi-sub-array multi-band wide-bandwidth frequency coverage of the InPTA provides Dispersion Measure estimates with unprecedented precision for PTA pulsars, e.g., ~ 2 x 10{-5} pc-cm{-3} for PSR J1909-3744. Configuring the SKA-low and SKA-mid as two and four sub-arrays respectively, it is shown that comparable precision is achievable, using observation strategies similar to those pursued by the InPTA, for a larger sample of 62 pulsars requiring about 26 and 7 hours per epoch for the SKA-mid and the SKA-low telescopes respectively. We also review the ongoing efforts to develop PTA-relevant general relativistic constructs that will be required to search for nanohertz gravitational waves from isolated super-massive black hole binary systems like blazar OJ 287. These efforts should be relevant to pursue persistent multi-messenger gravitational wave astronomy during the forthcoming era of the SKA telescope, the Thirty Meter Telescope, and the next-generation Event Horizon Telescope.

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arXiv2022

Long-term meter wavelength variability study of Blazar J1415+1320 using the Ooty Radio Telescope

J1415+1320 is a well-studied blazar that exhibits strong flux density variability at a wide range of radio frequencies (2.4 $-$ 230~GHz). In this letter, we present a variability study of this source at 327~MHz using the data obtained with the Ooty Radio Telescope taken during the period 1989 to 2018. Two significant flares are detected at epochs 2007.6 and 2008.6. These flares are also seen in the publicly available 15 and 37~GHz light curves but with a lead time of a few months. The fractional changes in the flux densities are larger at frequencies $>$ 15~GHz compared to those at 327~MHz and the spectral indices of the increased flux densities are flatter compared to the quiescent spectrum during these flares. These observed features are consistent with a model of uniformly expanding cloud of relativistic electrons or shock-in-jet model. Our 327~MHz dataset also overlaps with a rare form of variability $-$ symmetric achromatic variability (SAV) $-$ seen at higher frequencies ($>$ 15~GHz) toward the source. SAV is possibly due to gravitational milli-lensing of the core emission by an intervening massive object and is expected to be detected at all frequencies \cite{Vedantham2017}. No variability in association with the SAV events is seen in the 327~MHz dataset; however, if SAV is due to the lensing of core emission alone then the expected variability is less than 3$σ$ uncertainty in our measurements.

astro-ph.GA↗

arXiv2021

Search for fast radio transients using Arecibo drift-scan observations at 1.4 GHz

We conducted a drift-scan observation campaign using the 305-m Arecibo telescope in January and March 2020 when the observatory was temporarily closed during the intense earthquakes and the initial outbreak of the COVID-19 pandemic, respectively. The primary objective of the survey was to search for fast radio transients, including Fast Radio Bursts (FRBs) and Rotating Radio Transients (RRATs). We used the 7-beam ALFA receiver to observe different sections of the sky within the declination region $\sim$(10$-$20) deg on 23 nights and collected 160 hours of data in total. We searched our data for single-pulse transients, covering up to a maximum dispersion measure of 11 000 pc cm$^{-3}$ at which the dispersion delay across the entire bandwidth is equal to the 13 s transit length of our observations. The analysis produced more than 18 million candidates. Machine learning techniques sorted the radio frequency interference and possibly astrophysical candidates, allowing us to visually inspect and confirm the candidate transients. We found no evidence for new astrophysical transients in our data. We also searched for emission from repeated transient signals, but found no evidence for such sources. We detected single pulses from two known pulsars in our observations and their measured flux densities are consistent with the expected values. Based on our observations and sensitivity, we estimated the upper limit for the FRB rate to be $<$2.8$\times10^5$ sky$^{-1}$ day$^{-1}$ above a fluence of 0.16 Jy ms at 1.4 GHz, which is consistent with the rates from other telescopes and surveys.

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arXiv2021

High Precision Measurements of Interstellar Dispersion Measure with the upgraded GMRT

Pulsar radio emission undergoes dispersion due to the presence of free electrons in the interstellar medium (ISM). The dispersive delay in the arrival time of pulsar signal changes over time due to the varying ISM electron column density along the line of sight. Correcting for this delay accurately is crucial for the detection of nanohertz gravitational waves using Pulsar Timing Arrays. In this work, we present in-band and inter-band DM estimates of four pulsars observed with uGMRT over the timescale of a year using two different template alignment methods. The DMs obtained using both these methods show only subtle differences for PSR 1713+0747 and J1909$-$3744. A considerable offset is seen in the DM of PSR J1939+2134 and J2145$-$0750 between the two methods. This could be due to the presence of scattering in the former and profile evolution in the latter. We find that both methods are useful but could have a systematic offset between the DMs obtained. Irrespective of the template alignment methods followed, the precision on the DMs obtained is about $10^{-3}$ pc cm$^{-3}$ using only BAND3 and $10^{-4}$ pc cm$^{-3}$ after combining data from BAND3 and BAND5 of the uGMRT. In a particular result, we have detected a DM excess of about $5\times10^{-3}$ pc cm$^{-3}$ on 24 February 2019 for PSR J2145$-$0750. This excess appears to be due to the interaction region created by fast solar wind from a coronal hole and a coronal mass ejection (CME) observed from the Sun on that epoch. A detailed analysis of this interesting event is presented.

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arXiv2021

A Transient Coronal Sigmoid in Active Region NOAA 11909: Build-up Phase, M-class Eruptive Flare, and Associated Fast Coronal Mass Ejection

In this article, we investigate the formation and disruption of a coronal sigmoid from the active region (AR) NOAA 11909 on 07 December 2013, by analyzing multi-wavelength and multi-instrument observations. Our analysis suggests that the formation of `transient' sigmoid initiated $\approx$1 hour before its eruption through a coupling between two twisted coronal loop systems. A comparison between coronal and photospheric images suggests that the coronal sigmoid was formed over a simple $β$-type AR which also possessed dispersed magnetic field structure in the photosphere. The line-of-sight photospheric magnetograms also reveal moving magnetic features, small-scale flux cancellation events near the PIL, and overall flux cancellation during the extended pre-eruption phase which suggest the role of tether-cutting reconnection toward the build-up of the flux rope. The disruption of the sigmoid proceeded with a two-ribbon eruptive M1.2 flare (SOL2013-12-07T07:29). In radio frequencies, we observe type III and type II bursts in meter wavelengths during the impulsive phase of the flare. The successful eruption of the flux rope leads to a fast coronal mass ejection (with a linear speed of $\approx$1085 km s -1 ) in SOHO/LASCO field-of-view. During the evolution of the flare, we clearly observe typical "sigmoid-to-arcade" transformation. Prior to the onset of the impulsive phase of the flare, flux rope undergoes a slow rise ($\approx$15 km s -1 ) which subsequently transitions into a fast eruption ($\approx$110 km s -1 ). The two-phase evolution of the flux rope shows temporal associations with the soft X-ray precursor and impulsive phase emissions of the M-class flare, respectively, thus pointing toward a feedback relationship between magnetic reconnection and early CME dynamics.

astro-ph.SR↗

arXiv2021

pinta: The uGMRT Data Processing Pipeline for the Indian Pulsar Timing Array

We introduce pinta, a pipeline for reducing the upgraded Giant Metre-wave Radio Telescope (uGMRT) raw pulsar timing data, developed for the Indian Pulsar Timing Array experiment. We provide a detailed description of the workflow and usage of pinta, as well as its computational performance and RFI mitigation characteristics. We also discuss a novel and independent determination of the relative time offsets between the different back-end modes of uGMRT and the interpretation of the uGMRT observation frequency settings, and their agreement with results obtained from engineering tests. Further, we demonstrate the capability of pinta to generate data products which can produce high-precision TOAs using PSR J1909-3744 as an example. These results are crucial for performing precision pulsar timing with the uGMRT.

astro-ph.IM↗