arXiv subjects

Angelos Vourlidas

Publications and source records attributed to Angelos Vourlidas.

At least 19 recordsLinked to original sources

arXiv2026

Role of SKA in Advancing Remote Measurements of Magnetic Fields of Solar Coronal Mass Ejections

Coronal Mass Ejections (CMEs) are large expulsions of magnetized plasma from the Sun into interplanetary space and are the primary drivers of extreme space weather variations. The strength and topology of CME magnetic fields largely determine their impact on Earth. Although visible-light coronagraphs routinely observe CMEs and provide their geometric and kinematic properties, they cannot directly measure CME vector magnetic fields. These fields evolve from initiation through the inner heliosphere due to interactions with other CMEs, coronal structures, and the ambient solar wind, leading to significant structural deformation. Such evolution complicates predictions of the CME magnetic field at Earth. Accurate measurements of CME magnetic fields in the corona and heliosphere are therefore essential for advancing space weather forecasting. Radio observations spanning MHz to GHz frequencies provide a powerful remote-sensing approach for measuring CME magnetic fields from the ground. Recent observations with Square Kilometre Array (SKA) precursors and pathfinder instruments, as well as other new-generation facilities, have demonstrated the potential of these radio techniques for CME magnetic-field diagnostics. At the same time, these studies have highlighted several limitations of current instruments. The higher sensitivity, wider instantaneous bandwidth, and broader frequency coverage of the SKA will open a new observational window, enabling these techniques to be fully exploited for constraining SpWx models and improving predictive accuracy. However, such observations are non-standard and require special consideration in scheduling, calibration, and imaging. Developments achieved with SKA precursors and pathfinders are paving the way for robust CME magnetic-field measurements with the SKA.

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arXiv2026

Review of Machine Learning Models for Solar Energetic Particle Prediction

Solar energetic particle (SEP) events have attracted increasing attention due to their significant radiation hazards for aviation, spacecraft electronics, and human missions beyond Earth's magnetosphere. From a scientific perspective, SEP events are intriguing because they arise from a set of physical processes extending from the solar surface and corona through the heliosphere, offering insight into particle acceleration and transport mechanisms that are widely applicable across astrophysics. Therefore, advancing our ability to understand and predict SEP events is essential both for deepening our knowledge of such mechanisms and for safeguarding space technologies and exploration. Traditionally, researchers have modeled SEPs using physics-based simulations and empirical methods. More recently, machine learning (ML) has emerged as a new tool for understanding and predicting SEP events. The purpose of this manuscript is to review the currently available ML models for SEP prediction, identify the datasets used for training, compare their architectures, inputs, and outputs, and, based on these insights, outline good practices and recommendations for future research.

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arXiv2026

Sunrise III: Instrument, mission, data, and first results

Sunrise III is a stratospheric balloon-borne solar observatory with a 1-m diameter telescope and three post-focus instruments, along with an image stabilisation system, all within a protective gondola. It samples the lower solar atmosphere, from the solar surface to the middle chromosphere, at a resolution approaching 50~km on the Sun. Sunrise III flew successfully for 6.5 days suspended from a zero-pressure stratospheric balloon from northern Sweden to north-western Canada in July 2024, gathering around 200 TB of data. The present issue of ApJL focuses on the first scientific results from the data collected during that flight. This paper introduces this Focus Issue, providing a very brief overview of the capabilities of the instrumentation, the flight and of the gathered data. Challenges for the measurements, data reduction and interpretation are also briefly touched upon. The paper ends with an overview of the first set of science results obtained from these data, as presented in the current Focus Issue.

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arXiv2026

Understanding the complex morphology of a CME II: how pre-eruptive conditions shape CME evolution

The morphology and heliospheric impact of coronal mass ejections (CMEs) are strongly shaped by their preeruptive magnetic configuration and surrounding coronal environment, yet these influences remain difficult to constrain observationally. We analyze a complex CME that erupted on 2024 October 26 using multiviewpoint remote sensing observations and in situ measurements. Using the physics based CORHELCME magnetohydrodynamic model, we test multiple physically plausible realizations of the preeruptive magnetic flux rope (MFR) and background magnetic field, using agreement with the observed evolution as a constraint on the CMEs initial state. We find that modest changes in MFR footpoint location and force balance lead to substantially different coronal trajectories, enabling rapid discrimination among candidate initial states. While several configurations reproduce the CMEs large scale propagation, realistic small scale morphology is achieved only when a near dated background magnetic field is employed. The resulting simulation reproduces key morphologies observed from three viewpoints without fine tuning, indicating that the inferred preeruptive configuration represents a robust, global solution and provides a physically consistent interpretation of their magnetic origin. Comparison with in situ shock detections highlights the role of CME solar wind interactions in shaping heliospheric signatures, though shock arrival times remain uncertain at the 11 hr level. These results demonstrate that data informed, physics based modeling can meaningfully constrain CME preeruptive conditions and bridge remote and in situ observations, while emphasizing the need for timely magnetic field measurements to improve predictive capability.

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arXiv2026

Improving Solar Flare Soft X-ray Classification With FOXES: A Framework For Operational X-ray Emission Synthesis

The Geostationary Operational Environmental Satellite (GOES) solar soft X-ray (SXR) irradiance in the 1-8Å wavelength range is a long-standing measure of solar activity, used to define the classification of flare strengths. As a result, the flare class, along with the SXR light curves, are routinely used as a primary input for forecasting properties of space weather drivers, from coronal mass ejection speed to energetic particle output. However, the GOES SXR irradiance lacks spatial information, leading to known classification errors, such as misattributed flare locations during periods of high activity. Moreover, GOES only provides observations from Earth's orbit, hindering forecasting for other places in the heliosphere. Motivated by these limitations, we introduce the Framework for Operational X-ray Emission Synthesis (FOXES), a Vision Transformer-based approach for translating Extreme Ultraviolet (EUV) spatially-resolved observations into SXR irradiance predictions. The model produces two outputs: (1) a global 1-8Å SXR flux prediction and (2) per-patch flux contributions, which offer a spatially-resolved interpretation of where the model attributes SXR emission. Trained, validated, and tested on over 3200 hours of observations, FOXES has demonstrated a translational mean absolute error of 0.051 dex for integrated SXR measurements. FOXES has also shown promise in dissecting the solar background SXR flux during flaring and non-flaring events. Overall, this model paves the way for EUV-based spatially-resolved flare detection to be extended beyond Earth's line of sight. Such capabilities could lead to a more comprehensive flare catalog and enable a true multiviewpoint monitoring of solar activity.

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arXiv2026

The First Quantitative Study of Tail Regrowth of CME-Driven Disconnection in Comet C/2023 P1 Nishimura

Comet C/2023 P1 (Nishimura) was observed by the Solar Orbiter Heliospheric Imager (SoloHI), onboard the Solar Orbiter spacecraft, from 2023 September 1 to 14. During this period, the ion tail of the comet exhibited continual fluctuations and four tail disconnection events (TDEs), each coinciding with the passage of a coronal mass ejection (CME). In this work, we report on the ion tail dynamics of the best observed TDE, which occurred on September 11. The SoloHI white-light images reveal an abrupt bending, subsequent kinks, and severing of a downstream portion of the pre-existing ion tail. The onset of disconnection occurred $\sim$6.5 hours after the projected passage of the CME leading edge in the images, consistent with a CME flank encounter. After the disconnection, the ion tail reformed within $\sim$24 hours, with a regrowth rate of $\sim$86$\pm7~\mathrm{km\,s^{-1}}$, indicating the rate at which newly ionized material forms along the magnetic field draped around the comet's coma. After the TDE, the detached tail drifted anti-sunward at an estimated speed of $\sim$295$\pm20~\mathrm{km\,s^{-1}}$, comparable to the local CME flank's speed, suggesting that the severed plasma was most likely carried away from the comet by the CME. This study provides the first direct, quantitative characterization of comet-CME interactions and the subsequent regrowth phase of a cometary TDE. These measurements were achievable by SoloHI's unique inner-heliospheric coverage, thanks to a combination of high photometric sensitivity, short exposure times, and a wide field of view that preserves the fine-scale tail dynamics.

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arXiv2025

FOXES: A Framework For Operational X-ray Emission Synthesis

Understanding solar flares is critical for predicting space weather, as their activity shapes how the Sun influences Earth and its environment. The development of reliable forecasting methodologies of these events depends on robust flare catalogs, but current methods are limited to flare classification using integrated soft X-ray emission that are available only from Earth's perspective. This reduces accuracy in pinpointing the location and strength of farside flares and their connection to geoeffective events. In this work, we introduce a Vision Transformer (ViT)-based approach that translates Extreme Ultraviolet (EUV) observations into soft x-ray flux while also setting the groundwork for estimating flare locations in the future. The model achieves accurate flux predictions across flare classes using quantitative metrics. This paves the way for EUV-based flare detection to be extended beyond Earth's line of sight, which allows for a more comprehensive and complete solar flare catalog.

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arXiv2025

Solar Observation with MeerKAT: Demonstration of Technical Readiness and Initial Science Highlight

Solar radio emissions offer unique diagnostic insights into the solar corona. However, their dynamic and multiscale nature, along with several orders of magnitude variations in intensity, pose significant observational challenges. To date, at gigahertz frequencies, MeerKAT stands out globally with high potential of producing high-fidelity, spectroscopic snapshot images of the Sun, enabled by its dense core, high sensitivity, and broad frequency coverage. Yet, as a telescope originally designed for observing faint galactic and extragalactic sources, observing the Sun at the boresight of the telescope requires customized observing strategies and calibration methods. This work demonstrates the technical readiness of MeerKAT for solar observations at the boresight of the telescope in the UHF (580-1015 MHz) and L-band (900-1670 MHz) frequency ranges, including optimized modes, a dedicated calibration scheme, and a tailored, entirely automated calibration and imaging pipeline. The quality of solar images is validated through morphological comparisons with the solar images at other wavelengths. Several unique early science results showcase the potential of this new capability of MeerKAT. Once fully commissioned and operational, this will unlock novel solar studies, significantly expand the scientific portfolio of MeerKAT, and lay the groundwork for solar observations with the mid-frequency telescope of the upcoming Square Kilometre Array Observatory, for which MeerKAT serves as a precursor.

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arXiv2025

Modelling gyrosynchrotron emission from coronal energetic electrons in a CME flux rope

Solar flares and coronal mass ejections (CMEs) can accelerate electrons, causing bursts such as type IV emissions in the solar radio continuum. Although radio spectroscopy is a powerful diagnostic tool for the corona, the origin and mechanisms of type IV bursts remain uncertain. In situ measurements can occasionally shed some light on these mechanisms, but they are limited in space and time. Sophisticated numerical modelling offers the best approach to improve our understanding of the physical processes involved. This research examines type IV radio bursts, exploring the effects of various electron distribution properties and CMEs on their generation and characteristics. To transcend idealised assumptions, we employ realistic, anisotropic electron distributions - obtained from particle transport simulations within complex magnetohydrodynamic (MHD) environments - as input for radio emission models. We use the 3D MHD model COCONUT to generate coronal background configurations, including a CME modelled as a modified Titov-Démoulin magnetic flux rope (MFR). These MHD simulations are used by the PARADISE particle transport code, which injects energetic electrons into the MFR and tracks their evolution. Finally, we feed the electron distributions and solar wind parameters into the Ultimate Fast Gyrosynchrotron (GS) Codes to compute radio emission along lines of sight. Electrons injected close to the MFR's central axis remain largely confined, producing a GS emission spectrum resembling observed type IV characteristics. Varying observer positions, CME properties, and spectral indices of the electron energy distributions modify the intensities and durations of the observed bursts. The strongest GS emission is observed to originate from the CME flanks. Our results indicate that GS emission is the major component in type IV spectra, although additional contributors cannot be ruled out.

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arXiv2025

Understanding the complex morphology of a CME: multi-view analysis and numerical modeling

Although all coronal mass ejections (CMEs) that propagate into the heliosphere should contain a magnetic flux rope (MFR) component, the majority do not exhibit the expected white-light MFR morphology of a leading edge plus cavity. This different appearance could be the result of distortion of the internal magnetic structure, merging with other structures, or simply projection effects. These factors complicate the interpretation of CMEs. This complexity is exemplified by a CME observed on 28 March 2022. The event originated from a single eruption, evolving as a textbook CME in the low corona but appearing as a complex two-MFR structure in white-light observations. Why? To answer this question, we performed a multi-view data and modeling analysis to describe the CME coronal evolution. The thermodynamic MHD model, CORHEL-CME, helps reveal the magnetic configuration of this CME and also reveals that the ambient field plays a crucial role in shaping the complex structure of the CME during early evolution. Our research underscores the importance of integrating multiview observations with physics-based models to gain a deeper insight into the development of complex CMEs.

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arXiv2025

Coronal dimmings and what they tell us about solar and stellar coronal mass ejections

Coronal dimmings associated with coronal mass ejections (CME) from the Sun have gained much attention since the late 1990s when they were first observed in high-cadence imagery of the SOHO/EIT and Yohkoh/SXT instruments. They appear as localized sudden decreases of the coronal emission at extreme ultraviolet (EUV) and soft X-ray (SXR) wavelengths, that evolve impulsively during the lift-off and early expansion phase of a CME. Coronal dimmings have been interpreted as "footprints" of the erupting flux rope and also as indicators of the coronal mass loss by CMEs. However, these are only some aspects of coronal dimmings and how they relate to the overall CME/flare process. The goal of this review is to summarize our current understanding and observational findings on coronal dimmings, how they relate to CME simulations, and to discuss how they can be used to provide us with a deeper insight and diagnostics of the triggering of CMEs, the magnetic connectivities and coronal reconfigurations due to the CME as well as the replenishment of the corona after an eruption. In addition, we go beyond a pure review by introducing a new, physics-driven categorization of coronal dimmings based on the magnetic flux systems involved in the eruption process. Finally, we discuss the recent progress in studying coronal dimmings on solar-like and late-type stars, and to use them as a diagnostics for stellar coronal mass ejections and their properties.

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arXiv2025

A Formalism for Calibrating the Instrumental Polarization of Radio Interferometric Arrays at Meter Wavelengths using Unpolarized Sky: A Demonstration using the MWA Observations

Calibration of instrumental polarization is critical for measuring polarized radio emissions from astrophysical sources to extract the magnetic field information in astrophysical, heliospheric, and terrestrial plasmas. At meter wavelengths, calibration of radio polarimetric observations is particularly challenging because of the scarcity of bright polarized sources due to significant Faraday depolarization. Here, we present a novel formalism for polarization calibration using an unpolarized sky model. The formalism is specifically designed for wide-field, low-frequency instruments like the Murchison Widefield Array (MWA), the LOw Frequency ARray (LOFAR), New Extension in Nançay Upgrading LoFAR (NenuFAR), Owens Valley Radio Observatory - Long Wavelength Array (OVRO-LWA), low-frequency telescope of the Square Kilometre Array Observatory (SKAO-low), etc. By leveraging the apparent polarization of the unpolarized sky induced by the polarized primary beam of the radio telescope, this method avoids dependence on bright polarized calibrators. It is also immune to ionospheric Faraday rotation. The validation of the approach via MWA observations confirms the accuracy of the method. This formalism provides a robust framework for low-frequency polarization calibration. It addresses the longstanding calibration challenges and advances the field of low-frequency polarimetry by enabling polarization studies of astrophysical radio sources.

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arXiv2025

How the CME on 21 April 2023 Triggered the First Severe Geomagnetic Storm of Solar Cycle 25

The first severe (G4) geomagnetic storm of Solar Cycle 25 occurred on 23-24 April 2023, following the arrival of a Coronal Mass Ejection (CME) on 23 April. The characteristics of this CME, measured from coronagraphs (speed and mass), did not indicate that it would trigger such an intense geomagnetic storm. In this work, our aim is to understand why this CME led to such a geoeffective outcome. Our analysis spans from the source active region to the corona and inner heliosphere through 1 au using multiwavelength, multi-viewpoint remote sensing observations and in situ data. We find that rotation and possibly deflection of the CME resulted in an axial magnetic field nearly parallel to the ecliptic plane during the Earth encounter, which might explain the storm's severity. Additionally, we find that imaging away from the Sun-Earth line is crucial in hindcasting the CME Time-of-Arrival at Earth. The position (0.39 au) and detailed images from the SoloHI telescope onboard the Solar Orbiter mission, in combination with SOHO and STEREO images, helped decisively with the three-dimensional (3D) reconstruction of the CME.

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arXiv2025

The SOHO LASCO CME Catalog -- Version 2

This paper provides an update on the coronal mass ejection (CME) catalog maintained at the CDAW Data Center, NASA Goddard Space Flight Center (https://cdaw.gsfc.nasa.gov/CME_list). This is version 2 (v2) of the Catalog that has been made as the default version as of May 1, 2024. The new features of the Catalog v2 are (i) online measurement tool, (ii) combination JavaScript movies from the STEREO and Solar Dynamics Observatory (SDO) missions, and (iii) insertion of newly identified CMEs for the period 1996 to 2004. The CME identification was revisited resulting in a set of $\sim$3000 new CMEs added to the Catalog. A vast majority of these CMEs are weak and narrow. The resulting statistical properties of CMEs are not significantly different from those reported using version 1.

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arXiv2024

Connecting the Low to High Corona: Propagating Disturbances as Tracers of the Near-Sun Solar Wind

We revisit a quiet 14-day period of solar minimum during January 2008 and track sub-streamer propagating disturbances (PDs) from low heights in STEREO/EUVI to the extended corona through STEREO/COR1 and into STEREO/COR2 along nonradial paths that trace the structure of the underlying streamers. Using our recently developed method for generating nonradial Height-Time profiles of outward PDs (OPDs) and inward PDs (IPDs), we obtained their velocities along the radial and position angle directions. Our analysis of 417 unique OPDs revealed two classes: slow and fast OPDs. Slow OPDs form preferentially at $\approx$1.6 $R_\odot$ closer to the streamer boundaries, with asymmetric occurrence rates, and show speeds of $16.4_{-8.4}^{+26.6}km/s$ at 1.5 $R_\odot$ and accelerate up to $200.1_{-57.9}^{+71.1}km/s$ at 7.5 $R_\odot$. Fast OPDs form preferentially at $\approx$ 1.6 $R_\odot$ and at $\approx$3.0 $R_\odot$ both at the streamer boundaries and slightly more often within them. They show speeds of $87.8_{-24.8}^{+59.1}km/s$ at 1.5 $R_\odot$ up to $197.8_{-46.7}^{+61.8}km/s$ at 7.5 $R_\odot$. IPDs are observed forming at $\approx$1.8 $R_\odot$ with speeds of tens of $km/s$, mostly concentrated in the aftermath of a CME eruption. We present an example in which we show that periodic brightness variations related to OPDs remained in the range of 98 to 128 min, down to $\approx$2.0 $R_\odot$, well within the field of view of COR1. The velocity profiles of slow OPDs for heliocentric height below 3.0 $R_\odot$ show good agreement with speeds more closely related to the bulk solar wind obtained via interplanetary scintillation.

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arXiv2024

Electron Cyclotron Maser Emission and the Brightest Solar Radio Bursts

This paper investigates the incidence of coherent emission in solar radio bursts, using a revised catalog of 3800 solar radio bursts observed by the Nobeyama Radio Polarimeters from 1988 to 2023. We focus on the 1.0 and 2.0 GHz data, where radio fluxes of order 10 billion Jansky have been observed. Previous work has suggested that these bursts are due to electron cyclotron maser (ECM) emission. In at least one well studied case, the bright emission at 1 GHz consists of narrowband spikes of millisecond duration. Coherent emission at 1 GHz can be distinguished from traditional incoherent gyrosynchrotron flare emission based on the radio spectrum: gyrosynchrotron emission at 1 GHz usually has a spectrum rising with frequency, so bursts in which 1 GHz is stronger than higher frequency measurements are unlikely to be incoherent gyrosynchrotron. Based on this criterion it is found that, for bursts exceeding 100 sfu, three-quarters of all bursts at 1 GHz and half of all 2 GHz bursts have a dominant coherent emission component, assumed to be ECM. The majority of the very bright bursts at 1 GHz are highly circularly polarized, consistent with a coherent emission mechanism, but not always 100% polarized. The frequency range from 1 to 2 GHz is heavily utilized for terrestrial applications, and these results are relevant for understanding the extreme flux levels that may impact such applications. Further, they provide a reference for comparison with the study of ECM emission from other stars and potentially exoplanets.

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arXiv2024

SuNeRF: 3D reconstruction of the solar EUV corona using Neural Radiance Fields

To understand its evolution and the effects of its eruptive events, the Sun is permanently monitored by multiple satellite missions. The optically-thin emission of the solar plasma and the limited number of viewpoints make it challenging to reconstruct the geometry and structure of the solar atmosphere; however, this information is the missing link to understand the Sun as it is: a three-dimensional evolving star. We present a method that enables a complete 3D representation of the uppermost solar layer (corona) observed in extreme ultraviolet (EUV) light. We use a deep learning approach for 3D scene representation that accounts for radiative transfer, to map the entire solar atmosphere from three simultaneous observations. We demonstrate that our approach provides unprecedented reconstructions of the solar poles, and directly enables height estimates of coronal structures, solar filaments, coronal hole profiles, and coronal mass ejections. We validate the approach using model-generated synthetic EUV images, finding that our method accurately captures the 3D geometry of the Sun even from a limited number of 32 ecliptic viewpoints ($|\text{latitude}| \leq 7^\circ$). We quantify uncertainties of our model using an ensemble approach that allows us to estimate the model performance in absence of a ground-truth. Our method enables a novel view of our closest star, and is a breakthrough technology for the efficient use of multi-instrument datasets, which paves the way for future cluster missions.

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arXiv2023

The Critical Coronal Transition Region: A Physics-framed Strategy to Uncover the Genesis of the Solar Wind and Solar Eruptions

Our current theoretical and observational understanding suggests that critical properties of the solar wind and Coronal Mass Ejections (CMEs) are imparted within 10 Rs, particularly below 4 Rs. This seemingly narrow spatial region encompasses the transition of coronal plasma processes through the entire range of physical regimes from fluid to kinetic, and from primarily closed to open magnetic field structures. From a physics perspective, therefore, it is more appropriate to refer to this region as the Critical Coronal Transition Region (CCTR) to emphasize its physical, rather than spatial, importance to key Heliophysics science. This white paper argues that the comprehensive exploration of the CCTR will answer two of the most central Heliophysics questions, "How and where does the solar wind form?" and "How do eruptions form?", by unifying hardware/software/modeling development and seemingly disparate research communities and frameworks. We describe the outlines of decadal-scale plan to achieve that by 2050.

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