arXiv subjects

Dipankar Banerjee

Publications and source records attributed to Dipankar Banerjee.

At least 19 recordsLinked to original sources

arXiv2026

Evidence for CME--CME Interaction in a Magnetic-Cloud-Like Ejecta: Insights from Multipoint Observations and Polytropic Analysis

Using in-situ observations from Solar Orbiter, STEREO-A, and Wind, we investigate the heliospheric evolution of an interplanetary coronal mass ejection (ICME). The magnetic ejecta (ME) shows a magnetic-cloud-like (MCL) configuration: a front region in which the magnetic field rotates and its magnitude declines, followed by a weakly rotating "back region" of nearly constant field magnitude. Near-Sun EUV and white-light observations reveal two fast CMEs launched in rapid succession and interacting at low coronal heights, providing direct evidence that an MCL ejecta can arise from interaction between two closely spaced CMEs sampled near their apex. Using near-radially aligned Solar Orbiter and STEREO-A observations, we find that the sheath expands more rapidly than the ME, consistent with the "snow-plow" effect, while the ME properties broadly follow trends reported in previous ICME studies. Comparison of STEREO-A and Wind observations, separated by only 9.2 degrees in longitude, reveals measurable mesoscale variability within both the sheath and the ME. We identify dual polytropic behavior within both substructures, consistent with the merging of two interacting CMEs to form the observed MCL ejecta. This dual behavior in the ME is most pronounced in the inner heliosphere and weakens with heliocentric distance, indicating thermodynamic homogenization during radial expansion, whereas the sheath maintains thermodynamic contrasts from the inner heliosphere to 1 au. Our findings suggest that polytropic diagnostics can help reveal thermodynamically distinct plasma populations associated with interacting CMEs, providing additional evidence for CME-CME interaction in MCL ejecta where conventional in-situ signatures show no clear evidence of such interactions.

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arXiv2026

Multi-Wavelength Diagnostics of Pre-Flare Evolution with Aditya-L1: From the Solar Chromosphere to the Corona

The pre-flare phase of solar flares provides important insight into the processes that drive active regions toward instability. We investigate chromospheric pre-flare activity using observations from the Solar Ultraviolet Imaging Telescope (SUIT) onboard Aditya-L1, complemented with X-ray measurements from High Energy L1 Orbiting X-ray Spectrometer (HEL1OS) and Solar Low Energy X-ray Spectrometer (SoLEXS). We analyse seven M- and X-class flares, focusing on spatially resolved Mg II h (2803~\AA) observations from SUIT. We identify 102 pre-flare transients within regions of interest prior to flare onset. These transients are detected in the Mg II h channel, with no counterparts in continuum filters, confirming their chromospheric origin. In most cases, the transients are co-spatial with polarity inversion lines (PILs) and the eventual flaring region. Approximately 28~\% of transients have X-ray counterparts in HEL1OS (10-30~keV); The Spectrometer Telescope for Imaging X-rays (STIX) spectral analysis reveals non-thermal emission in a subset, indicating that some transients are small-scale flare-like events. A hot X-ray onset is identified in four cases. For the remaining three cases, the signal-to-noise ratio above the background is insufficient to determine whether a hot-onset phase is present. The peak-flux distribution of the transients follows a broken power law with indices $\alpha_1 = 1.64^{+0.59}_{-0.57}$ and $\alpha_2 = 3.12^{+0.64}_{-0.61}$, with the higher-energy slope consistent with the Ly-$\alpha$ flare distribution. These results suggest that chromospheric pre-flare transients represent small-scale magnetic energy-release events that contribute to the progressive destabilisation of active regions prior to major flare onset.

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arXiv2026

Visualizing the Magnetic Structure in Interplanetary Coronal Mass Ejections with ATHARV

Interplanetary coronal mass ejections (ICMEs) are major drivers of space weather, and their geoeffectiveness is strongly governed by the structure and orientation of their internal magnetic field. However, in-situ observations provide only 1D sampling along a spacecraft trajectory, limiting direct inference of the ICME 3D magnetic structure. We introduce the Analysis Tool for Heliospheric Arrangement of Remapped Vectors (ATHARV), which remaps in-situ time-series measurements into spatial coordinates while accounting for ICME expansion and spacecraft motion. ATHARV assumes self-similar expansion with different expansion rates along three orthogonal directions, while more general cases use measured velocities as proxies for plasma motion. The framework also incorporates complementary diagnostics, including hodograms and magnetic-field orientation angles, to assess magnetic coherence and field rotation within ICMEs. We demonstrate ATHARV using multipoint observations of an ICME detected near 1 au by STEREO-A and Wind on 2023 April 23--24. The reconstructed sheath exhibits disordered and variable magnetic fields, whereas the magnetic ejecta (ME) shows a coherent rotation consistent with a right-handed SWN flux-rope configuration at both spacecraft. However, differences in magnetic-field magnitude profiles, rotation signatures, and inferred ME sizes indicate mesoscale inhomogeneity within the ICME magnetic structure, possibly associated with a writhed or distorted flux rope. This event highlights the limitations of interpreting ICME magnetic configurations from single-point measurements and demonstrates the importance of multipoint observations for investigating their 3D structure and evolution. ATHARV provides a consistent framework for interpreting in-situ ICME observations and investigating their spatial structure and evolution, and is publicly available to the heliophysics community.

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arXiv2026

Radio Signature of Higher Atmospheric Meridional Flow and Implications for Magnetic Trees in the Sun

The coupling between plasma flows and magnetic fields in the solar atmosphere governs the transport of angular momentum and the redistribution of magnetic flux, yet its manifestation in the magnetically dominated upper chromosphere remains uncertain. Using 27 years of 17 GHz full-disk solar radio imaging observations from the Nobeyama Radioheliograph, we report the first detection of a poleward flow signature at heights of $3000\pm500$ km, an altitude where plasma magnetohydrodynamics expects magnetic dominance ($\beta<1$). The derived latitudinal velocity profile ($5-15$ m/s) mirrors the established photospheric meridional circulation, displaying modulation with solar cycle parameters. Comparison with long-term synoptic magnetograms reveals that the motion of 17 GHz brightness features closely tracks poleward magnetic flux transport, implying a deep magnetic anchoring of these structures. This finding provides the first observational evidence that chromospheric flows at radio wavelengths reflect subsurface meridional dynamics, consistent with the "magnetic tree" hypothesis, which links high-altitude motion to deep-seated magnetic connectivity.

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arXiv2026

Turbulent Properties of Interplanetary Coronal Mass Ejections Observed by Solar Orbiter in the Inner Heliosphere

We investigate the turbulent properties of 12 interplanetary coronal mass ejections (ICMEs) observed by Solar Orbiter between 0.29 and 1.0 AU. We analyze fluctuation power, spectral indices, break scales, and correlations between magnetic and velocity fluctuations (v-b) to quantify differences between ICME substructures (sheath and magnetic ejecta (ME)) and the surrounding solar wind. The ICME sheath is consistently the most turbulent region at all distances. In the solar wind, Alfv\'enicity influences inertial-range scaling, resulting in either single power laws near f^-3/2 or f^-5/3, or a coexistence of both, whereas ICME substructures consistently exhibit Kolmogorov-like f^-5/3 spectra. Alfv\'enicity is reduced within ICMEs, particularly in the ejecta, indicating more balanced Alfv\'enic fluctuations than in the solar wind. Spectral breaks shift to higher frequencies in ICME regions, with average break frequencies of 0.53 +/- 0.35 Hz (solar wind), 1.87 +/- 1.46 Hz (sheath), and 1.46 +/- 1.28 Hz (ME), reflecting differences in underlying microphysical scales. Our findings highlight distinct turbulence regimes in ICMEs compared to the solar wind and support the use of fluctuation power, spectral breaks, and v-b correlations as effective diagnostics for identifying ICME boundaries.

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arXiv2025

Machine Learning Based Identification of Solar Disk and Plages in Kodaikanal Solar Observatory Historical Suncharts

Kodaikanal Solar Observatory (KoSO) is one of the oldest solar observatories, possessing an archive of multi-wavelength solar observations, including white light, Ca II K, and H-alpha images spanning over a century. In addition to these observations, KoSO has preserved hand-drawn suncharts (1904-2022), on which various solar features such as sunspots, plages, filaments, and prominences are marked on the Stonyhurst grid with distinct colour coding. In this study, we present the first comprehensive result that includes the entire data set from these suncharts using a supervised Machine Learning model called "Convolutional Neural Networks (CNNs)", firstly to identify the solar disks from the charts (1909-2007), secondly to identify the plages, spanning 9 solar cycles (1916-2007). We train the CNN with the manually identified solar disk and plage. We first detect the solar limb and the North-South line in the suncharts, which enables the extraction of disk centre coordinates, radius, and P-angle. Following that, we use a CNN similar architecture to achieve accurate image segmentation for the identification of plages. We compare plage areas derived from the suncharts with those obtained from Ca II K full-disk observations, and find good agreement that demonstrates the potential application of such an ML technique for historical data. The results of this study further demonstrate the potential application of sunchart data to fill the existing data gaps in the KoSO multi-wavelength observations and contribute toward constructing a composite series over the last century.

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arXiv2025

Observed Joys law of Bipolar Magnetic Region tilts at the emergence supports the thin flux tube model

Bipolar sunspots, or more generally, Bipolar Magnetic Regions, BMRs, are the dynamic magnetic regions that appear on the solar surface and are central to solar activity. One striking feature of these regions is that they are often tilted with respect to the equator, and this tilt increases with the latitude of appearance, popularly known as Joys law. Although this law has been examined for over a century through various observations, its physical origin is still not established. An attractive theory that has been put forward behind Joys law is the Coriolis force acting on the rising flux tube in the convection zone, which has been studied using the thin flux tube model. However, observational support for this theory is limited. If the Coriolis force is the cause of the tilt, then we expect BMRs to hold Joys law at their initial emergence on the surface. By automatically identifying the BMRs over the last two solar cycles from high resolution magnetic observations, we robustly capture their initial emergence signatures on the surface. We find that from their appearance, BMRs exhibit tilts consistent with Joys law. This early tilt signature of BMRs suggests that the tilt is developed underneath the photosphere, driven by the Coriolis force and helical convection, as predicted by the thin flux tube model. Considerable scatter around Joys law observed during the emergence phase, which reduces in the post emergence phase, reflects the interaction of the vigorous turbulent convection with the rising flux tubes in the near surface layer.

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arXiv2025

Polar Filaments Capture High Latitude Solar Poloidal Field Interactions and can Foretell the Future Sunspot Cycle Amplitude before Polar Field Precursors

Polar fields at the minimum of a sunspot cycle -- which are a manifestation of the radial component of the Sun's poloidal field -- are deemed to be the best indicator of the strength of the toroidal component, and hence the amplitude of the future sunspot cycle. However, the Sun's polar magnetic fields are difficult to constrain with ground-based or space-based observations from near the plane-of-ecliptic. In this context, polar filaments -- dark, elongated structures that overlie polarity inversion lines -- are known to offer critical insights into solar polar field dynamics. Through investigations of the long-term evolution of polar filament areas and length acquired from the Meudon Observatory and complimentary solar surface flux transport simulations, here we establish the common physical foundation connecting the Babcock-Leighton solar dynamo mechanism of solar polar field reversal and build-up with the origin and evolution of polar filaments. We discover a new relationship connecting the residual filament area of adjacent solar cycles with the amplitude of the next sunspot cycle -- which can serve as a new tool for solar cycle forecasts -- advancing the forecast window to earlier than polar field based precursors. We conclude that polar filament properties encapsulate the physics of interaction of the poloidal magnetic field of the previous and current sunspot cycles, the resultant of which is the net poloidal magnetic field at the end of the current cycle, thus encoding as a precursor the strength of the upcoming solar cycle.

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arXiv2025

Backtracking Bipolar Magnetic Regions to their emergence: Two groups and their implication in the tilt measurements

Bipolar Magnetic Regions (BMRs) that appear on the solar photosphere are surface manifestations of the Suns internal magnetic field. With modern observations and continuous data streams, the study of BMRs has moved from manual sunspot catalogs to automated detection and tracking methods. In this work, we present an additional module to the existing BMR tracking algorithm, AutoTAB, that focuses on identifying emerging signatures of BMRs. Specifically, for regions newly detected on the solar disk, this module backtracks the BMRs to their point of emergence. From a total of about 12,000 BMRs identified by AutoTAB, we successfully backtracked 3,080 cases. Within this backtracked sample, we find two distinct populations. One group shows the expected behavior of emerging regions, in which the magnetic flux increases significantly during the emerging phase. The other group consists of BMRs whose flux, however, does not exhibit substantial growth during their evolution, the instances where our algorithm fails to capture the initial emergence of the BMRs. We classify these as discarded BMRs and examine their statistical properties separately. Our analysis shows that these discarded BMRs do not display any preferred tilt angle distribution and do not show systematic latitudinal tilt dependence, in contrast to the trends typically associated with emerging BMRs. This indicates that including such regions in statistical studies of BMR properties can distort or mask the underlying physical characteristics. We therefore emphasise the importance of excluding the discarded population from the whole dataset when analysing the statistical behavior of BMRs.

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arXiv2025

Polarimeter to Unify the Corona and Heliosphere (PUNCH)

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1+3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5 degrees (6 R$_\odot$) to 8 degrees (32 R$_\odot$); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3 degrees (12 R$_\odot$) to 45 degrees (180 R$_\odot$) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single ``virtual instrument'' with a 90 degree wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

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arXiv2025

The effect of poloidal magnetic field and helicity injection on a breakout CME

Coronal mass ejections (CMEs), as crucial drivers of space weather, necessitate a comprehensive understanding of their initiation and evolution in the solar corona, in order to better predict their propagation. Solar Cycle 24 exhibited lower sunspot numbers compared to Solar Cycle 23, along with a decrease in the heliospheric magnetic pressure. Consequently, a higher frequency of weak CMEs was observed during Solar Cycle 24. Forecasting CMEs is vital, and various methods, primarily involving the study of the global magnetic parameters using datasets like Space-weather Helioseismic and Magnetic Imager Active Region Patches (SHARP), have been employed in earlier works. In this study, we perform numerical simulations of CMEs within a magnetohydrodynamics framework using Message Passing Interface - Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC) in 2.5 dimensions. By employing the breakout model for CME initiation, we introduce a multipolar magnetic field configuration within a background bipolar magnetic field, inducing shear to trigger the CME eruption. Our investigation focuses on understanding the impact of the background global magnetic field on CME eruptions. Furthermore, we analyze the evolution of various global magnetic parameters in distinct scenarios (failed eruption, single eruption, multiple eruptions) resulting from varying amounts of helicity injection in the form of shear at the base of the magnetic arcade system. Our findings reveal that an increase in the strength of the background poloidal magnetic field constrains CME eruptions. Furthermore, we establish that the growth rate of absolute net current helicity is the crucial factor that determines the likelihood of CME eruptions.

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arXiv2025

Unraveling the Secrets of the lower Solar Atmosphere: One year of Operation of the Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1

The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard Aditya--L1, the first solar space observatory of the Indian Space Research Organization (ISRO), India, launched on September 2, 2023. SUIT is designed to image the Sun in the 200--400 nm wavelength band in eight narrowband and three broadband filters. SUIT's science goals start with observing the solar atmosphere and large-scale continuum variations, the physics of solar flares in the NUV region, and many more. The paper elucidates the functioning of the instrument, software packages developed for easier calibration, analysis, and feedback, calibration routines, and the regular maintenance activity of SUIT during the first year of its operation. The paper also presents the various operations undergone by, numerous program sequences orchestrated to achieve the science requirements, and highlights some remarkable observations made during the first year of observations with SUIT.

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arXiv2025

Insights into Chromospheric Large-Scale Flows using Nobeyama 17 GHz Radio Observations I. The Differential Rotation Profile

Although the differential rotation rate on the solar surface has long been studied using optical and extreme ultraviolet (EUV) observations, associating these measurements to specific atmospheric heights remains challenging due to the temperature-dependent emission of tracers observed in EUV wavelengths. Radio observations, being primarily influenced by coherent plasma processes and/or thermal bremsstrahlung, offer a more height-stable diagnostic and thus provide an independent means to test and validate rotational trends observed at other EUV wavelengths. We aim to characterize the differential rotation profile of the upper chromosphere using cleaned solar full-disc 17 GHz radio imaging from the Nobeyama Radioheliograph (NoRH), spanning a little over two solar cycles (1992 - 2020). A tracer-independent method based on automated image correlation was employed on daily full-disc 17 GHz radio maps. Our results suggest that the upper chromosphere rotates significantly faster than the photosphere at all latitudes, with a relatively flatter latitudinal profile. A very weak anti-correlation between the equatorial rotation rate and solar activity is also observed. Our findings reaffirm the potential of radio observations to probe the dynamics of the solar chromosphere with reduced height ambiguity. The overlap of the equatorial rotation rate found in this study with that for $304$ {\AA} in the EUV regime lends additional support to the view that the equatorial rotation rates increase with height above the photosphere. Future coordinated studies at wavelengths with better-constrained height formation will be crucial for further understanding the complex dynamics of the solar atmosphere.

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arXiv2025

X-class flare on Dec 31, 2023, observed by the Solar Ultraviolet Imaging Telescope on board Aditya-L1

We present the multi-wavelength study of the ejection of a plasma blob from the limb flare SOL2023-12-31T21:36:00 from NOAA 13536 observed by the Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1. We use SUIT observations along with those from Atmospheric Imaging Assembly (AIA) on board SDO and Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter to infer the kinematics and thermal nature of the ejected blob and its connection to the associated flare. The observations show that the flare was comprised of two eruptions. The blob was ejected during the first eruption and later accelerated to velocities over 1500 km/s measured at a maximum projected height of ~ 178 Mm from the Sun's surface. The acceleration of the ejected plasma blob is co-temporal with the bursty appearance of the hard X-ray light curve recorded by STIX. Radio spectrogram observations from STEREO-A/WAVES and RSTN reveal type III bursts at the same time, indicative of magnetic reconnection. DEM analysis using AIA observations suggests the plasma blob is comprised of cooler and denser plasma in comparison to the ambient corona. To the best of our knowledge, this is the first observation of such a plasma blob in the NUV, providing crucial measurements for eruption thermodynamics.

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arXiv2025

Test and Calibration of the Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1

The Solar Ultraviolet Imaging Telescope (SUIT) on board the AdityaL1 mission observes the Sun in the 200-400 nm wavelength range. This paper presents the results of various on ground and on board tests and their comparison with the specifications. Moreover, we also present the scheme for data calibration. We demonstrate that the test results are compliant with the specified figures, except the spatial resolution. Such discrepancy will limit the photometric measurements only, at a scale of 2.2" instead of 1.4" as originally envisioned. The results obtained here show that SUIT observations open up a new window for solar observations.

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arXiv2025

Study of Wind pattern at the incursion site of Pangong Tso near Merak Village

This study analyzes twelve years of wind speed and direction data collected at the proposed National Large Solar Telescope (NLST) site near Pangong Tso, Merak village, Leh-Ladakh. A weather station from Campbell Scientific Instruments, installed in 2008, has been continuously monitoring meteorological parameters, including wind speed and direction. The data reveals a consistent pattern of predominantly northwest winds, particularly during morning hours, with speeds generally below 5 m/s. While seasonal variations influence wind speed and direction, the overall trend remains stable. To assess the site's suitability for astronomical observations, we compared high-altitude wind speeds at various renowned astronomical sites using reanalysis data from 2008 to 2020. Strong correlations were observed between surface and high-altitude wind speeds at 10~m, 50~m, and 500~m. Statistical analysis of 200-mbar pressure level wind speeds identified La Palma as the most favorable site with a wind speed of 18.76~m/s. La Silla, on the other hand, exhibited the highest wind speed at 34.76~m/s. Merak's estimated wind speed of 30.99~m/s, coupled with its favorable wind direction and low surface wind speeds, suggests its potential as a promising site for astronomical observations.

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arXiv2025

Near and Mid UltraViolet Observations of X-6.3 flare on 22nd February 2024 recorded by the Solar Ultraviolet Imaging Telescope on board Aditya-L1

Solar flares are regularly observed in extreme ultraviolet (EUV), soft X-rays (SXR), and hard X-rays (HXR). However, those in near and mid-UV are sparse. The Solar Ultraviolet Imaging Telescope (SUIT) onboard the Aditya-L1, launched on 2nd September, 2023 provides regular observations in the 200-400 nm wavelength range through eleven filters. Here, we report the observation of the X6.3 flare on Feb 22, 2024 using eight narrow band (NB) filters of SUIT. We have also used co-spatiotemporal observations from SDO/AIA, Solar Orbiter/STIX, GONG H$\alpha$, Aditya-L1/SoLEXS and GOES. We obtained light curves over the flaring region from AIA 1600, 1700 \r{A} and GONG H$\alpha$ and compared them with the disk-integrated lightcurve obtained from GOES and SoLEXS SXR and STIX HXR. We find that the flare peaks in SUIT NB01, NB03, NB04, and NB08 filters simultaneously with HXR, 1600, and 1700 \r{A} along with the peak temperature obtained from SoLEXS. In contrast, in NB02 and NB05, the flare peaks $\sim$ 2 minutes later than the HXR peak, while in NB06 and NB07, the flare peaks $\sim$ 3 minutes after the GOES soft X-ray peak. To the best of our knowledge, this is the first observation of a flare in these wavelengths (except in NB03, NB04 and NB05). Moreover, for the first time, we show the presence of a bright kernel in NB02. These results demonstrate the capabilities of SUIT observations in flare studies.

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arXiv2025

Estimation of Polar Magnetic Fields using Ca II K Polar Network as a Proxy

The polar magnetic field plays a crucial role in the solar dynamo model and contributes to predicting future solar cycles. However, continuous and direct measurements of this polar field have been available only since 1976, with data provided by the Wilcox Solar Observatory (WSO). Recent findings suggest that the Ca ii K Polar Network Index (PNI) can serve as a promising proxy for estimating the polar field of the Sun. In this study, we aim to reconstruct the polar field for the pre-1976 period by leveraging Ca ii K data from the Kodaikanal Solar Observatory (KoSO; 1904-2007) and modern Ca ii K observations from the Rome Precision Solar Photometric Telescope (Rome-PSPT; 2000-2022). We employ an automatic adaptive threshold technique to detect polar networks and calculate PNI values. Then, we calibrate these PNI values with the WSO polar field to reconstruct the polar field over 119 years.

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