arXiv subjects

Amir Caspi

Publications and source records attributed to Amir Caspi.

At least 19 recordsLinked to original sources

arXiv2026

Elemental Composition Evolution during the 2024 September 30 Solar Eruption: A Comparison of Hot and Cool Plasma Components with Solar Orbiter/SPICE, Hinode/EIS, and Chandrayaan-2/XSM

Solar plasma composition differs between the photosphere and corona over a range of timescales, with preferential enhancement of elements with low first ionization potential (FIP). However, the physical origin of the FIP fractionation remains incompletely understood. Furthermore, during flares, the FIP bias also exhibits rapid changes, associated with fast transport of material with different FIP biases. We present novel observations from Solar Orbiter SPICE and EUI, Hinode/EIS, and Chandrayaan-2 XSM instruments, finding rapid abundance changes in the emitting plasma, on timescales of minutes, during the eruptive M7.6-class solar flare observed on 2024 Sept 30. These instruments have wide temperature coverage and find contrasting abundance-evolution patterns between the hotter and cooler plasma components. 3D reconstruction of the active region and additional observations from the Solar Orbiter STIX X-ray telescope show how the hot and cool plasma components, emitting in different spectral regions and observed with the various instruments, sample the plasma composition evolution in distinct locations within the observed flaring plasma. The bright post-flare loop tops observed by SPICE show coronal FIP bias, while the hot plasma observed with XSM exhibits FIP-bias decreasing from coronal to photospheric during the impulsive phase. We interpret these observations as evidence of the X-ray diagnostics seeing hot coronal reconnection outflows mixing with chromospheric plasma as flare loops sequentially energize and relax, explaining why the FIP bias decreases from coronal to a hybrid; and the cool loop tops seen with SPICE show coronal abundances due to coronal material deposited near the looptops.

astro-ph.SR↗

arXiv2026

Cross-Calibration of Chandrayaan-2 XSM with INSPIRESat-1 DAXSS and GOES-16 XRS

X-ray spectroscopic observations of the solar corona and flares provide crucial diagnostics of plasma properties and are essential for understanding the physical processes responsible for coronal heating and solar eruptive activity. The Chandrayaan-2 Solar X-ray Monitor (XSM) provides disk-integrated spectra of the Sun in the 1--15~keV soft X-ray band, enabling modeling of the thermal X-ray emission from the corona across quiet phases to intense solar flares. XSM has been operational for about seven years, starting from the last solar minimum and covering the maximum of the current Solar Cycle. The Dual-zone Aperture X-ray Solar Spectrometer (DAXSS) instrument on board INSPIRESat-1 covers the solar X-ray spectra in a similar energy range as XSM and was operational during 2022--2026. With multiple instruments simultaneously observing the Sun in X-rays, there is scope to compare measurements across instruments. Here, we present the cross-calibration of XSM with DAXSS and a broadband X-ray flux monitor, the GOES-16 X-ray Sensor (XRS). Comparisons of XSM and DAXSS spectra reveal an unaccounted attenuation in the XSM low-energy response. Supported by laboratory measurements, we attribute this difference to the effective detector beryllium window thickness being 25 microns rather than the previously assumed 8 microns. Incorporating this revision into the XSM calibration significantly improves the agreement between the two instruments, with flux measurements agreeing within ~10% in the 1--8 Angstrom band. Comparison with GOES-16 XRS measurements over a broad range of solar activity levels further demonstrates consistency, with a median flux difference of less than 10%.

astro-ph.SR↗

arXiv2026

UK White Paper on Space-based total solar eclipse observations: structure and dynamics of the solar atmosphere

Our Sun is uniquely placed to enable a detailed study of astrophysical plasmas and how they are governed by the magnetic fields that thread through them. On the one hand, magnetic fields confine plasma and determine plasma heating, flows, and energisation. On the other hand, magnetic fields and their evolution give rise to the most violent eruptions in the Solar System. Understanding the details of how energy is built up and released, and the impact of these physical processes on the plasma, remain key open questions that directly map to UKRI's science strategy through the STFC Solar System Advisory Panel's roadmap for Solar System research goals: What are the causes, consequences and predictability of solar magnetic variability and the solar cycle? What are the structures, dynamics and energetics of the Sun? What are the underlying processes that drive Sun-planet connections? And what are the fundamental processes at work in the Solar System? As laid out in this White Paper, the Moon-Enabled Sun Occultation Mission (MESOM) directly addresses these questions and in doing so delivers several Pillars of the National Space Strategy.

astro-ph.IM↗

arXiv2025

Citizen CATE 2024: Extending Totality During the 8 April 2024 Total Solar Eclipse with a Distributed Network of Community Participants

The Citizen CATE 2024 next-generation experiment placed 43 identical telescope and camera setups along the path of totality during the total solar eclipse (TSE) on 8 April 2024 to capture a 60-minute movie of the inner and middle solar corona in polarized visible light. The 2024 TSE path covered a large geographic swath of North America and we recruited and trained 36 teams of community participants ("citizen scientists") representative of the various communities along the path of totality. Afterwards, these teams retained the equipment in their communities for ongoing education and public engagement activities. Participants ranged from students (K12, undergraduate, and graduate), educators, and adult learners to amateur and professional astronomers. In addition to equipment for their communities, CATE 2024 teams received hands-on telescope training, educational and learning materials, and instruction on data analysis techniques. CATE 2024 used high-cadence, high-dynamic-range (HDR) polarimetric observations of the solar corona to characterize the physical processes that shape its heating, structure, and evolution at scales and sensitivities that cannot be studied outside of a TSE. Conventional eclipse observations do not span sufficient time to capture changing coronal topology, but the extended observation from CATE 2024 does. Analysis of the fully calibrated dataset will provide deeper insight and understanding into these critical physical processes. We present an overview of the CATE 2024 project, including how we engaged local communities along the path of totality, and the first look at CATE 2024 data products from the 2024 TSE.

astro-ph.SR↗

arXiv2025

Dual Origins of Rapid Flare Ribbon Downflows in an X9-class Solar Flare

We detect rapid downflows of 150-217 km/s in IRIS Si IV 1402.77 nm measurements of an X9-class solar flare on 2024 October 3rd. The fast redshift values persist for over 15 minutes from flare onset, and can be split into two distinct stages of behavior, suggesting that multiple mechanisms are responsible for the downwards acceleration of flare ribbon plasma. The first stage of rapid downflows are synchronized with peaks in emission from the Advanced Space-based Solar Observatory Hard X-ray Imager (ASO-S/HXI) and Large Yield Radiometer (LYRA) Lyman-alpha measurements, indicative that the chromospheric downflows (with a maximum redshift of 176 km/s) result from chromospheric condensations associated with impulsive energy release in the solar flare. Later in the event, strong Si IV flare ribbon downflows persist (to a maximum value of 217 km/s), despite the magnetic flux rate falling to zero, and high-energy HXR and Lyman-alpha measurements returning to background levels. This is reflective of downflows in the flare ribbon footpoints of flare-induced coronal rain. Hard X-ray spectral analysis supports this scenario, revealing strong non-thermal emission during the initial downflow stage, falling near background levels by the second stage. Despite these distinct and contrasting stages of ribbon behavior, Si IV Doppler velocities exhibit quasi-periodic pulsations with a constant ~50 s period across the 15-minute flare evolution (independently of loop length). We deduce that these pulsations are likely caused by MHD oscillations in the magnetic arcade. Finally, we utilize machine learning K-means clustering methods to quantify line profile variations during the stages of rapid downflows.

astro-ph.SR↗

arXiv2025

Engaging and Educating Eclipse Observers Through Workshops, Media, Planetarium shows and Citizen Science

The Heliophysics Big Year was an extended year where major solar events engaged the public. It included two eclipses (annular on October 14, 2023 and total on April 8, 2024), plus solar max and the Parker Solar Probe perihelion December 24, 2024. After the eclipse of 2017, many millions more Americans planned to view the solar corona. We expanded our eclipse website with activities, citizen science projects, resources, training videos, equipment, and external links. We were the Southwest Regional Coordinator for Citizen CATE 2024 project, training the state coordinators and their teams with the equipment and procedures. We trained teachers at local, regional, national, and international workshops, providing eclipse viewing cards, lenses to make solar cup projectors, a safe viewing screen pattern, and access to the training materials. We made presentations to the media and hosted public events to demonstrate safe eclipse viewing techniques. HMNS hosted live viewing for the annular and total plus solstice and equinox events, reaching tens of thousands of people. HMNS also secured a grant to provide 100 eclipse viewing cards for every public school (8,800+) in Texas. We distributed another 57,000 eclipse viewers to teachers and the public. We appeared in media both in advance of the eclipses and as live commentators. The most lasting and impactful product was our planetarium show Totality, which was given away free and shown in various formats (flatscreen, fisheye, or prewarped). Over 180,000 views of the show and its animations have been documented. We continued to improve our space weather forecasting site, which correctly predicted the major solar storms of May 10-11 and October 8-10, 2024. In total, we reached nearly two million learners.

physics.space-ph↗

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.

astro-ph.SR↗

arXiv2025

The Integrating Miniature Piggyback for Impulsive Solar Hard X-rays (IMPISH): a spectrometer for the GRIPS-2 balloon campaign

The Integrating Miniature Piggyback for Impulsive Solar Hard X-rays (IMPISH) is a piggyback mission originally designed for the second flight of the Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS-2) Antarctic balloon. IMPISH will take measurements of collimated, full-Sun X-ray spectra with the goal of detecting sub-second variations (order of tens of milliseconds) of nonthermal X-ray emission during the impulsive phase of large solar flares to probe particle acceleration mechanisms driven by magnetic reconnection. The IMPISH detector system, made up of four identical detectors totaling 64 cm$^2$ effective area, is capable of measuring from ~10 keV to over 200 keV through the use of silicon photomultipliers (SiPMs) and LYSO scintillators. At the stratospheric altitude of GRIPS-2, the effective lower energy limit is ~20-30 keV. The geometry of the LYSO crystal has been optimized to balance the light collection efficiency with the effective area required for stratospheric X-ray measurements. Development of the IMPISH detectors has introduced a path for a low-cost solution to fast solar X-ray measurements across a large energy range utilizing commercially available components. The payload has a 3U form factor and has been designed so that both the electronics and detectors may be easily adaptable for space-based missions.

physics.ins-det↗

arXiv2025

Optimized scintillation configuration for IMPISH hard x-ray detection

The Integrating Miniature Piggyback for Impulsive Solar Hard X-rays (IMPISH) is a solar X-ray spectrometer that features large-area scintillators, fast readout electronics, and good energy resolution in the hard X-ray band. IMPISH is a low-cost spectrometer designed to measure subsecond variation in hard X-ray time profiles from solar flares, with the goal of constraining particle acceleration timescales. To meet these requirements, we carried out a systematic optimization of the scintillation design, focusing on maximizing photon collection, reducing noise level, and improving energy resolution. We tested two high-yield scintillating crystals (LYSO, lutetium yttrium oxyorthosilicate, and GAGG, gadolinium aluminum gallium garnet), two reflector types (specular and lambertian), two kinds of surface finishes (all sides polished and readout face only polished), different optical coupling materials and thicknesses, multiple geometries, and readout face angles. Our studies show that light collection improves with increasing the crystal's effective area, breaking its symmetry, and reducing the photon's travel length. These minimize photon loss due to self-absorption and total internal reflection. Through combined simulation and laboratory tests, we achieved an optimal energy resolution with the LYSO trapezoid-shaped crystal coupled to a Broadcom NUV-MT SiPM with an approximately 0.25 mm thin Sylgard 184 optical pad.

astro-ph.IM↗

arXiv2025

Evidence of Extreme Ultraviolet Resonant Excitation in the Middle Corona During A Solar Flare

We present observations of an eruptive solar flare on 2016 January 6 that occurred behind the solar limb from the perspective of the Earth, but was well observed by STEREO and the Solar Extreme Ultraviolet Monitor on the Mars Atmosphere and {Volatile} EvolutioN (MAVEN) mission. Light curves showing the evolution of the flare's irradiance as a function of time taken by MAVEN are well correlated with the brightness evolution of fan structures observed in the PROBA2 SWAP 174 {\AA} passband, suggesting that the radiance of structures near the flare site was influenced by emission from the flare. Because SWAP did not directly observe the flare itself, this event represents a rare opportunity to study the evolution of emission near a flare without the risk of instrumental scattered light contaminating the observations. We analyze this evolution and implement a simple model to explore the possibility that resonant excitation (or resonant scattering) plays an important role in driving coronal EUV emission during flaring events. Our modeling shows that for a large flare, resonant excitation could increase emission from nearby structures by about 45%, consistent with our findings that the involved structures observed by SWAP increased in brightness by about 60% during the flare. We conclude that resonant excitation may play an important role in driving coronal EUV emission under certain circumstances and should be accounted for in models and emission-based analysis tools.

astro-ph.SR↗

arXiv2024

CATEcor: an Open Science, Shaded-Truss, Externally-Occulted Coronagraph

We present the design of a portable coronagraph, CATEcor, that incorporates a novel "shaded truss" style of external occultation and serves as a proof-of-concept for that family of coronagraphs. The shaded truss design style has the potential for broad application in various scientific settings. We conceived CATEcor itself as a simple instrument to observe the corona during the darker skies available during a partial solar eclipse, or for students or interested amateurs to detect the corona under ideal non-eclipsed conditions. CATEcor is therefore optimized for simplicity and accessibility to the public. It is implemented using an existing dioptric telescope and an adapter rig that mounts in front of the objective lens, restricting the telescope aperture and providing external occultation. The adapter rig, including occulter, is fabricated using fusion deposition modeling (FDM; colloquially "3D printing"), greatly reducing cost. The structure is designed to be integrated with moderate care and may be replicated in a university or amateur setting. While CATEcor is a simple demonstration unit, the design concept, process, and trades are useful for other more sophisticated coronagraphs in the same general family, which might operate under normal daytime skies outside the annular-eclipse conditions used for CATEcor.

astro-ph.IM↗

arXiv2024

Observations of the Polarized Solar Corona during the Annular Eclipse of October 14, 2023

We present results of a dual eclipse expedition to observe the solar corona from two sites during the annular solar eclipse of 2023 October 14, using a novel coronagraph designed to be accessible for amateurs and students to build and deploy. The coronagraph "CATEcor" builds on the standardized eclipse observing equipment developed for the Citizen CATE 2024 experiment. The observing sites were selected for likelihood of clear observations, for historic relevance (near the Climax site in the Colorado Rocky Mountains), and for centrality to the annular eclipse path (atop Sandia Peak above Albuquerque, New Mexico). The novel portion of CATEcor is an external occulter assembly that slips over the front of a conventional dioptric telescope, forming a "shaded-truss" externally occulted coronagraph. CATEcor is specifically designed to be easily constructed in a garage or "makerspace" environment. We successfully observed some bright features in the solar corona to an altitude of approximately 2.25 R$_\odot$ during the annular phases of the eclipse. Future improvements to the design, in progress now, will reduce both stray light and image artifacts; our objective is to develop a design that can be operated successfully by amateur astronomers at sufficient altitude even without the darkened skies of a partial or annular eclipse.

astro-ph.SR↗

arXiv2023

A Chromatic Treatment of Linear Polarization in the Solar Corona at the 2023 Total Solar Eclipse

The broadband solar K-corona is linearly polarized due to Thomson scattering. Various strategies have been used to represent coronal polarization. Here, we present a new way to visualize the polarized corona, using observations from the 2023 April 20 total solar eclipse in Australia in support of the Citizen CATE 2024 project. We convert observations in the common four-polarizer orthogonal basis (0°, 45°, 90°, & 135°) to -60°, 0°, and +60° (MZP) polarization, which is homologous to R, G, B color channels. The unique image generated provides some sense of how humans might visualize polarization if we could perceive it in the same way we perceive color.

physics.pop-ph↗

arXiv2023

The existence of hot X-ray onsets in solar flares

It is well known among the scientific community that solar flare activity often begins well before the main impulsive energy release. Our aim is to investigate the earliest phase of four distinct flares observed by Solar Orbiter/STIX and determine the relationships of the newly heated plasma to flare structure and dynamics. The analysis focuses on four events that were observed from both Earth and Solar Orbiter, which allows for a comparison of STIX observations with those of GOES/XRS and SDO/AIA. The early phases of the events were studied using STIX and GOES spectroscopic analysis to investigate the evolution of the physical parameters of the plasma, including the isothermal temperature and emission measure. Furthermore, to determine the location of the heated plasma, STIX observations were combined with AIA images. The events with clear emission prior to the impulsive phase show elevated temperatures ($> 10\,\mathrm{MK}$) from the very beginning, which indicates that energy release started before any detection by STIX. Although the temperature shows little variation during the initial phase, the emission measure increases by about two orders of magnitude, implying a series of incrementally greater energy releases. The spectral analysis of STIX and GOES from the very first time bins suggests that the emission has a multi-thermal nature, with a hot component of more than $10\,\mathrm{MK}$. This analysis confirms the existence of "hot onsets," with STIX detecting the hot onset pattern even earlier than GOES. These elevated temperatures imply that energy release actually begins well before any detection by STIX. Therefore, hot onsets may be significant in the initiation, early development, or even prediction of solar flares.

astro-ph.SR↗

arXiv2023

High-fidelity 3D Reconstruction of Solar Coronal Physics with the Updated CROBAR Method

We present an extension of the Coronal Reconstruction Onto B-Aligned Regions (CROBAR) method to Linear Force Free Field (LFFF) extrapolations, and apply it to the reconstruction of a set of AIA, MDI, and STEREO EUVI data. The results demonstrate that CROBAR can not only reconstruct coronal emission structures, but also that it can help constrain the coronal field extrapolations via the LFFF's helicity $\alpha$ parameter. They also provide a real-world example of how CROBAR can easily incorporate information from multiple perspectives to improve its reconstructions, and we also use the additional perspectives to help validate the reconstructions. We furthermore touch on the use of real-world emission passbands rather than idealized power-law type ones using DEMs. We conclude with a comparison of CROBAR generated emission to observed emission and those produced with idealized DEM based power-laws. These results further illustrate the promise of CROBAR for real-world applications, and we make available a preliminary release of the software available for download.

astro-ph.SR↗

arXiv2023

Reimagining Heliophysics: A bold new vision for the next decade and beyond

The field of Heliophysics has a branding problem. We need an answer to the question: ``What is Heliophysics\?'', the answer to which should clearly and succinctly defines our science in a compelling way that simultaneously introduces a sense of wonder and exploration into our science and our missions. Unfortunately, recent over-reliance on space weather to define our field, as opposed to simply using it as a practical and relatable example of applied Heliophysics science, narrows the scope of what solar and space physics is and diminishes its fundamental importance. Moving forward, our community needs to be bold and unabashed in our definition of Heliophysics and its big questions. We should emphasize the general and fundamental importance and excitement of our science with a new mindset that generalizes and expands the definition of Heliophysics to include new ``frontiers'' of increasing interest to the community. Heliophysics should be unbound from its current confinement to the Sun-Earth connection and expanded to studies of the fundamental nature of space plasma physics across the solar system and greater cosmos. Finally, we need to come together as a community to advance our science by envisioning, prioritizing, and supporting -- with a unified voice -- a set of bold new missions that target compelling science questions - even if they do not explore the traditional Sun- and Earth-centric aspects of Heliophysics science. Such new, large missions to expand the frontiers and scope of Heliophysics science large missions can be the key to galvanizing the public and policymakers to support the overall Heliophysics program.

physics.space-ph↗

arXiv2023

First Results for Solar Soft X-ray Irradiance Measurements from the Third Generation Miniature X-Ray Solar Spectrometer

Three generations of the Miniature X-ray Solar Spectrometer (MinXSS) have flown on small satellites with the goal "to explore the energy distribution of soft X-ray (SXR) emissions from the quiescent Sun, active regions, and during solar flares, and to model the impact on Earth's ionosphere and thermosphere". The primary science instrument is the Amptek X123 X-ray spectrometer that has improved with each generation of the MinXSS experiment. This third generation MinXSS-3 has higher energy resolution and larger effective area than its predecessors and is also known as the Dual-zone Aperture X-ray Solar Spectrometer (DAXSS). It was launched on the INSPIRESat-1 satellite on 2022 February 14, and INSPIRESat-1 has successfully completed its 6-month prime mission. The INSPIRESat-1 is in a dawn-dusk, Sun-Synchronous Orbit (SSO) and therefore has 24-hour coverage of the Sun during most of its mission so far. The rise of Solar Cycle 25 (SC-25) has been observed by DAXSS. This paper introduces the INSPIRESat-1 DAXSS solar SXR observations, and we focus the science results here on a solar occultation experiment and multiple flares on 2022 April 24. One key flare result is that the reduction of elemental abundances is greatest during the flare impulsive phase and thus highlighting the important role of chromospheric evaporation during flares to inject warmer plasma into the coronal loops. Furthermore, these results are suggestive that the amount of chromospheric evaporation is related to flare temperature and intensity.

astro-ph.SR↗

arXiv2023

Next-Generation Comprehensive Data-Driven Models of Solar Eruptive Events

Solar flares and coronal mass ejections are interrelated phenomena that together are known as solar eruptive events. These are the main drivers of space weather and understanding their origins is a primary goal of Heliophysics. In this white paper, we advocate for the allocation of sufficient resources to bring together experts in observations and modeling to construct and test next generation data-driven models of solar eruptive events. We identify the key components necessary for constructing comprehensive end-to-end models including global scale 3D MHD resolving magnetic field evolution and reconnection, small scale simulations of particle acceleration in reconnection exhausts, kinetic scale transport of flare-accelerated particles into the lower solar atmosphere, and the radiative and hydrodynamics responses of the solar atmosphere to flare heating. Using this modeling framework, long-standing questions regarding how solar eruptive events release energy, accelerate particles, and heat plasma can be explored. To address open questions in solar flare physics, we recommend that NASA and NSF provide sufficient research and analysis funds to bring together a large body of researchers and numerical tools to tackle the end-to-end modeling framework that we outline. Current dedicated theory and modeling funding programs are relatively small scale and infrequent; funding agencies must recognize that modern space physics demands the use of both observations and modeling to make rapid progress.

astro-ph.IM↗