Solar Mission Research Articles

An Elemental Abundance Diagnostic for Coordinated Solar Orbiter/SPICE and Hinode/EIS Observations

Plasma composition measurements are a vital tool for the success of current and future solar missions, but density- and temperature-insensitive spectroscopic diagnostic ratios are sparse, and their underlying accuracy in determining the magnitude of the first ionization potential (FIP) effect in the solar atmosphere remains an open question. Here we assess the Fe viii 185.213 Å/Ne viii 770.428 Å intensity ratio that can be observed as a multispacecraft combination between Solar Orbiter/SPICE and Hinode/EUV Imaging Spectrometer (EIS). We find that it is fairly insensitive to temperature and density in the range of log (T/K) = 5.65–6.05 and is therefore useful, in principle, for analyzing on-orbit EUV spectra. We also perform an empirical experiment, using Hinode/EIS measurements of coronal fan loop temperature distributions weighted by randomly generated FIP bias values, to show that our diagnostic method can provide accurate results as it recovers the input FIP bias to within 10%–14%. This is encouraging since it is smaller than the magnitude of variations seen throughout the solar corona. We apply the diagnostic to coordinated observations from 2023 March and show that the combination of SPICE and EIS allows measurements of the Fe/Ne FIP bias in the regions where the footpoints of the magnetic field connected to Solar Orbiter are predicted to be located. The results show an increase in FIP bias between the main leading polarity and the trailing decayed polarity that broadly agrees with Fe/O in situ measurements from Solar Orbiter/Solar Wind Analyser. Multispacecraft coordinated observations are complex, but this diagnostic also falls within the planned wavebands for Solar-C/Extreme UltraViolet high-throughput Spectroscopic Telescope.

A magnetohydrodynamic mechanism for the formation of solar polar vortices

Polar vortices are ubiquitous features of planetary atmospheric flows, from the Earth-like rocky planets to Jupiter- and Saturn-like gas giant planets. Very little is known about their existence or dynamics on the Sun. What should be expected near the Sun's pole for the upcoming solar multi-viewpoint and polar missions? Here, we report the magnetohydrodynamic (MHD) nonlinear simulations for the formation and evolution of solar polar vortices using a near-surface MHD shallow-water model. Our findings indicate that the rush to the poles, the migration of magnetic fields toward the pole following the Sun's magnetic cycle, can positively contribute to the formation of polar vortices. The mechanism proposed here for the formation of polar vortices involves the role of magnetic fields and may be relevant to any star with a magnetic cycle. The Sun's polar vortices resulting from this mechanism are predominantly MHD, consisting of a tight pair of cyclonic and anticyclonic swirls. This mechanism is likely to operate during all solar cycle phases except the peak, when the polar field reverses. Polar vortices can impact dynamical evolution of global flows and polar fields, which seed the next activity cycle, hence better knowledge of physics of polar regions may lead to improved solar cycle and space weather forecasts.

SynCOM: An Empirical Model for High-resolution Simulations of Transient Solar Wind Flows

The Synthetic Corona Outflow Model (SynCOM), an empirical model, simulates the solar corona’s dynamics to match high-resolution observations, providing a useful resource for testing velocity measurement algorithms. SynCOM generates synthetic images depicting radial variability in total brightness and includes stochastic elements for plasma outflows and instrumental noise. It employs a predefined probability distribution for flow velocity and an adjustable signal-to-noise ratio to evaluate different data analysis methods for coronal flows. By adjusting parameters to match specific coronal and instrumental conditions, SynCOM offers a platform to assess these methods for determining coronal velocity and acceleration. Validating these measurements would help us to understand the origins of the solar wind and support missions such as the Polarimeter to Unify the Corona and Heliosphere (PUNCH). In this study, we demonstrate how SynCOM can be employed to assess the precision and performance of two different flow-tracking methods. By providing a ground truth based on observational data, we highlight the importance of SynCOM in confirming observational standards for detecting coronal flows.

New Results on the Onset of a Coronal Mass Ejection from 5303 Å Emission Line Observations with VELC/ADITYA-L1

We report on the onset of a coronal mass ejection (CME) using spectroscopic observations in the 5303 Å coronal emission line with the Visible Emission Line Coronagraph (VELC) onboard ADITYA-L1, the recently launched first Indian space solar mission. The CME was observed on 2024 July 16 in association with an X1.9 class soft X-ray flare from heliographic location S05W85. The VELC observations were near the west limb of the Sun during the CME. The results obtained helped to constrain the onset time of the CME. In addition, they indicate a ≈50% decrease in the coronal intensity near the source region of the CME due to mass depletion, a ≈15% enhancement in the emission line width, and a redshifted Doppler velocity of about ≈10 km s−1. The nonthermal velocity associated with the line broadening is ≈24.87 km s−1.

The Sun from space: Discoveries from space missions over the past fifty years

Abstract The Sun is a magnetically active star that displays many exciting phenomena when observed from space in X-rays, extreme ultraviolet wavelengths and visible light: coronal holes, active regions, magnetic loops, flares, huge mass eruptions and the solar wind. Observations from space have revealed incredible mysteries about the Sun’s atmosphere and its extended corona, and even the interior of the Sun. This paper reviews and illustrates the progress and achievements over the last fifty years, presenting selected instruments and observations from fifteen solar missions, starting from Skylab in 1973 up to the latest Solar Orbiter mission, giving unprecedented insight into how the Sun works.

Review of Solar Sail Propulsion Mechanisms and Missions for Space Travel

Since solar sailing employs the speed of sunlight to drive spacecraft, it is an efficient and environmentally friendly alternative to traditional propulsion technologies. The physics of photon compression, the significance of novel materials and systems that enhance sailing performance, and the appropriate testing of solar sailing systems are all covered in this research paper. The main solar sailing projects— LightSail, NEA Scout, and IKAROS—are examined in this paper, and their merits, demerits, and contributions to the field are discussed. Potential solar sailing technology for extended space travel are also covered, along with space sustainability. Consider how crucial it is for space exploration in the future. The goal of this research is to demonstrate the transformative potential of navigation and to spark further developments in the exciting field of space aerospace engineering by effectively integrating mission data and current research in the ever-changing landscape of space exploration. Since solar sailing employs the speed of sunlight to drive spacecraft, it is a practical and environmentally friendly alternative to traditional propulsion technologies. The physics of photon compression, the significance of novel materials and systems that enhance sailing performance, and the appropriate testing of solar sailing systems are all covered in this research paper. The main solar sailing projects—LightSail, NEA Scout, and IKAROS—are examined in this paper, and their merits, demerits, and contributions to the field are discussed. Potential solar sailing technology for extended space travel are also covered, along with space sustainability. Pay attention to how crucial it is for future space exploration. Through the efficient integration of mission data and current research, this project seeks to address the dynamic landscape of space exploration in the industry and encourage future advancements in the exciting field of space aeronautical engineering. Additionally, it aims to illustrate how navigation can have a transforming effect. Keywords: Space Exploration, Propulsion Technology, Interstellar Travel, Spacecraft Navigation, Attitude control, Trajectory optimization, Material Durability, Deep Space Missions, Interplanetary Travel, Solar Radiation Effect, Photon Pressure, Solar sail Deployment, Orbital Mechanics, Continues Thrust.

Trajectory & maneuver design of the NEA Scout solar sail mission

The Near-Earth Asteroid Scout (NEA Scout) mission aimed to deliver a 6U CubeSat to a slow flyby of a Near Earth Asteroid using a solar sail as the primary propulsion system. NEA Scout was launched as a secondary payload aboard Artemis I, on November 16, 2022, but communication with the spacecraft was not achieved. This paper describes the target asteroid selection and the innovative trajectory and maneuver design for the NEA Scout mission, including cold gas maneuvers, multiple lunar flybys, solar sail thrusting, and design margins for solar sailing. The resulting trajectory design tools, solutions, and analyses are potentially applicable to future solar sail missions.

Automatic Detection of Large-scale Flux Ropes and Their Geoeffectiveness with a Machine-learning Approach

Abstract Detecting large-scale flux ropes (FRs) embedded in interplanetary coronal mass ejections (ICMEs) and assessing their geoeffectiveness are essential, since they can drive severe space weather. At 1 au, these FRs have an average duration of 1 day. Their most common magnetic features are large, smoothly rotating magnetic fields. Their manual detection has become a relatively common practice over decades, although visual detection can be time-consuming and subject to observer bias. Our study proposes a pipeline that utilizes two supervised binary classification machine-learning models trained with solar wind magnetic properties to automatically detect large-scale FRs and additionally determine their geoeffectiveness. The first model is used to generate a list of autodetected FRs. Using the properties of the southward magnetic field, the second model determines the geoeffectiveness of FRs. Our method identifies 88.6% and 80% of large-scale ICMEs (duration ≥ 1 day) observed at 1 au by the Wind and the Solar TErrestrial RElations Observatory missions, respectively. While testing with continuous solar wind data obtained from Wind, our pipeline detected 56 of the 64 large-scale ICMEs during the 2008–2014 period (recall = 0.875), but also many false positives (precision = 0.56), as we do not take into account any additional solar wind properties other than the magnetic properties. We find an accuracy of 0.88 when estimating the geoeffectiveness of the autodetected FRs using our method. Thus, in space-weather nowcasting and forecasting at L1 or any planetary missions, our pipeline can be utilized to offer a first-order detection of large-scale FRs and their geoeffectiveness.

Interplanetary Type IV Solar Radio Bursts: A Comprehensive Catalog and Statistical Results

Decameter hectometric (DH; 1–14 MHz) type IV radio bursts are produced by flare-accelerated electrons trapped in postflare loops or the moving magnetic structures associated with the coronal mass ejections (CMEs). From a space weather perspective, it is important to systematically compile these bursts, explore their spectrotemporal characteristics, and study the associated CMEs. We present a comprehensive catalog of DH type IV bursts observed by the Radio and Plasma Wave Investigation instruments on board the Wind and Solar TErrestrial RElations Observatory spacecraft covering the period of white-light CME observations by the Large Angle and Spectrometric Coronagraph on board the Solar and Heliospheric Observatory mission between 1996 November and 2023 May. The catalog has 139 bursts, of which 73% are associated with a fast (>900 km s−1) and wide (>60°) CME, with a mean CME speed of 1301 km s−1. All DH type IV bursts are white-light CME-associated, with 78% of the events associated with halo CMEs. The CME source latitudes are within ±45°. Seventy-seven events had multiple-vantage-point observations from different spacecraft, letting us explore the impact of the line of sight on the dynamic spectra. For 48 of the 77 events, there were good data from at least two spacecraft. We find that, unless occulted by nearby plasma structures, a type IV burst is best viewed when observed within a ±60° line of sight. Also, bursts with a duration above 120 minutes have source longitudes within ±60°. Our inferences confirm the inherent directivity in the type IV emission. Additionally, the catalog forms a Sun-as-a-star DH type IV burst database.

Correcting Fabry-Pérot etalon effects in solar observations

Context. Data processing pipelines of Fabry–Pérot interferometers (FPI) must take into account the side effects these devices introduce in the observations. Interpretation of these observations without proper correction can lead to inaccurate or false results, with consequent impact on their physical interpretation. Corrections typically require prior knowledge of the properties of the etalon and the way they affect the incoming light in order to calibrate the data successfully. Aims. We have developed an algorithm to derive etalon properties from flat-field observations and tested its applicability and accuracy using simulated observations and real measurements. Methods. We employed analytical expressions of the transmission profiles for FPIs in collimated and telecentric configurations to derive their expected impact on the observations. These analytical expressions allowed us to develop a customized optimization algorithm capable of inferring the properties of the etalon from the observations. The algorithm’s performance has been tested on simulated observations with an etalon in collimated and telecentric setups employing various noise levels and spectral samplings. Additionally, we explored how tilting the etalon in a telecentric configuration influences the algorithm’s effectiveness. Lastly, we also applied the algorithm to a set of real flat-field observations taken with the high-resolution telescope of the Polarimetric and Helioseismic Imager on board the Solar Orbiter mission (HRT-SO/PHI). Results. The algorithm is able to retrieve the gain and etalon induced transmission velocity shifts (cavity map), with an average accuracy ranging between 0.4% and 0.1% for the former and between 120 m s−1 and 30 m s−1 for the latter. Both reducing the noise level and increasing the spectral sampling of the observations proved to greatly increase the algorithm’s performance, as expected. Results also suggest that determination of the observed object from the data is possible but an additional error between 40 m s−1 and 10 m s−1 is to be expected in the inferred cavity map. Furthermore, we show that neglecting the asymmetries arising from either tilts of the etalon or imperfections in the telecentrism can lead to large errors when determining the gain. Tests with HRT-SO/PHI data have verified the applicability of the algorithm in real cases. Conclusions. Our presented method enabled us to derive the transmission profile of FPIs from observations of collimated and telecentric configurations. It has proven to be robust against the presence of noise and limited spectral line sampling. The results reported here also show the importance of accounting for the asymmetries arising in real telecentric mounts when interpreting the results of real instruments.

Firehose instability in heat-conducting solar wind plasmas including FLR corrections and electrical resistivity

The effects of finite Larmor radius (FLR) corrections and heat-flux vector are studied on the pressure anisotropy-driven firehose instability in finitely conducting solar wind plasmas described by the double-adiabatic Chew, Goldberger and Low (CGL) fluid theory. The fluid description of collisionless plasmas is governed through modified adiabatic equations due to the heat-flux vector and finite ion Larmor radius corrections. The analytical dispersion relation of the firehose instability has been derived using the normal mode analysis and discussed in the solar wind plasmas. In the transverse mode, the dispersion relation of the Alfvénic mode is modified due to electrical resistivity and FLR corrections. In the longitudinal mode, the effects of the heat-flux parameter and electrical resistivity are observed separately. The dispersion relation of the firehose mode is modified due to the combined effects of FLR corrections and electrical resistivity. The graphical illustrations show that finite electrical resistivity and ion Larmor frequency destabilize the growth rate of the firehose instability. The results are useful for analyzing the solar mission data to study the firehose instability in the solar wind plasmas.

Optimal Guidance for Heliocentric Orbit Cranking with E-Sail-Propelled Spacecraft

In astrodynamics, orbit cranking is usually referred to as an interplanetary transfer strategy that exploits multiple gravity-assist maneuvers to change both the inclination and eccentricity of the spacecraft osculating orbit without changing the specific mechanical energy, that is, the semimajor axis. In the context of a solar sail-based mission, however, the concept of orbit cranking is typically referred to as a suitable guidance law that is able to (optimally) change the orbital inclination of a circular orbit of an assigned radius in a general heliocentric three-dimensional scenario. In fact, varying the orbital inclination is a challenging maneuver from the point of view of the velocity change, so orbit cranking is an interesting mission application for a propellantless propulsion system. The aim of this paper is to analyze the performance of a spacecraft equipped with an Electric Solar Wind Sail in a cranking maneuver of a heliocentric circular orbit. The maneuver performance is calculated in an optimal framework considering spacecraft dynamics described by modified equinoctial orbital elements. In this context, the paper presents an analytical version of the three-dimensional optimal guidance laws obtained by using the classical Pontryagin’s maximum principle. The set of (analytical) optimal control laws is a new contribution to the Electric Solar Wind Sail-related literature.

Unleashing India's Solar Potential: A Review of the National Solar Mission and the Path to Sustainable Energy in India

India possesses vast potential for generating clean energy from Renewable Energy Sources (RES), specifically hydro, wind, and solar. This potential has been appropriately recognized, indicating India's commitment to reducing its carbon footprint as a developing nation. The significance of this endeavor is becoming increasingly evident worldwide. The objectives of this review study are to provide an overview of the abundant solar energy resources in India, including availability, current status, strategies, perspectives, challenges, achievements, and future prospects. The review study examines the Government of India's Jawaharlal Nehru National Solar Mission (JNNSM), launched in 2009 to promote clean energy. This mission, initiated on January 11, 2010, is one of the eight missions under the National Action Plan on Climate Change (NAPCC–2008). The JNNSM aims to deploy 22,000 MW of solar power through grid-connected and off-grid solar power plants. The review results show that as of August 2023, India has achieved a cumulative installed solar power capacity of 63,000 MW, exceeding the initial target of 22,000 MW set under the JNNSM. The country has seen a significant increase in solar energy deployment, with an average annual growth rate of 18% over the past five years. India's solar energy potential is estimated to be around 750 GW, indicating vast untapped resources. The review study recommends that India should continue to strengthen its policy and regulatory frameworks, incentivize private sector participation, and invest in research and development to further harness its solar energy potential. Addressing challenges related to grid integration, storage, and financing will be crucial for the sustainable growth of the solar energy sector in India.

Multi-source connectivity as the driver of solar wind variability in the heliosphere

The ambient solar wind that fills the heliosphere originates from multiple sources in the solar corona and is highly structured. It is often described as high-speed, relatively homogeneous, plasma streams from coronal holes and slow-speed, highly variable, streams whose source regions are under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify solar wind sources and understand what drives the complexity seen in the heliosphere. By combining magnetic field modelling and spectroscopic techniques with high-resolution observations and measurements, we show that the solar wind variability detected in situ by Solar Orbiter in March 2022 is driven by spatio-temporal changes in the magnetic connectivity to multiple sources in the solar atmosphere. The magnetic field footpoints connected to the spacecraft moved from the boundaries of a coronal hole to one active region (12961) and then across to another region (12957). This is reflected in the in situ measurements, which show the transition from fast to highly Alfvénic then to slow solar wind that is disrupted by the arrival of a coronal mass ejection. Our results describe solar wind variability at 0.5 au but are applicable to near-Earth observatories.

SPICE connection mosaics to link the Sun’s surface and the heliosphere

Aims. We present an analysis of the first connection mosaic made by the SPICE instrument on board the ESA/NASA Solar Orbiter mission on March 2, 2022. The data will be used to map coronal composition that will be compared with in-situ measurements taken by SWA/HIS to establish the coronal origin of the solar wind plasma observed at Solar Orbiter. The SPICE spectral lines were chosen to have varying sensitivity to the first ionization potential (FIP) effect, and therefore the radiances of the spectral lines will vary significantly depending on whether the elemental composition is coronal or photospheric. We investigate the link between the behavior of sulfur and the hypothesis that Alfvén waves drive FIP fractionation above the chromosphere. Methods. We performed temperature diagnostics using line ratios and emission measure (EM) loci, and computed relative FIP biases using three different approaches (two-line ratio (2LR), ratios of linear combinations of spectral lines (LCR), and differential emission measure (DEM) inversion) in order to perform composition diagnostics in the corona. We then compared the SPICE composition analysis and EUI data of the potential solar wind source regions to the SWA/HIS data products. Results. Radiance maps were extracted from SPICE spectral data cubes, with values matching previous observations. We find isothermal plasma of around log T = 5.8 for the AR loops targeted, and that higher FIP-bias values are present at the footpoints of the coronal loops associated with two ARs. Comparing the results with the SWA/HIS data products encourages us to think that Solar Orbiter was connected to a source of slow solar wind during this observation campaign. We demonstrate FIP fractionation in observations of the upper chromosphere and transition region, emphasized by the behavior of the intermediate-FIP element sulfur.

Comparison of magnetic data products from Solar Orbiter SO/PHI-FDT and SDO/HMI

Context.The Polarimetric and Helioseismic Imager (SO/PHI), on board the Solar Orbiter mission, is the first photospheric magnetograph and tachograph to observe the Sun from outside the Sun-Earth line. The Full Disc Telescope (FDT) of SO/PHI, images the whole solar disk with a spatial resolution that varies with the distance between the Sun and the spacecraft.Aims.We check for consistency between the magnetic field strength (B), the field inclination (γ), the line-of-sight (LoS) magnetic component (BLoS) and the field azimuth (ϕ), inferred by SO/PHI-FDT and the Helioseismic and Magnetic Imager (HMI), on board Solar Dynamics Observatory (SDO), and obtain linear correlation coefficients among them.Methods.We use data from both instruments obtained on 8 March 2022, when the angle between SDO and Solar Orbiter was 3.4° and the solar disk showed four developed active regions. Before comparing the magnetic field products of both instruments we perform a precise alignment of the data, including a matching of the plate scale. Further, in order to improve the homogeneity of the compared data products, the SDO/HMI data were convolved with the SO/PHI-FDT point spread function (PSF). The linear correlation coefficients are obtained through a linear regression of SDO/HMI to SO/PHI-FDT.Results.The two instruments yield comparable magnetic field data products. The slope coefficients for a linear fit are 1.37 forB, 1.11 forγ, 1.35 forBLoSand 1 for the azimuth. The corresponding fit offsets are −94 G, −9.8°, 5.2 G and 0.1°, respectively. The agreement between both instruments is significantly better when we take into account the different spatial resolution of both instruments. The fitting results vary slightly depending on the analyzed active region except for one of the four active regions, which shows larger differences and has been excluded from the comparison. The comparison of the LoS magnetic field products from SDO/HMI at 45 s and 720 s with SO/PHI-FDT shows a slope value of 1.17, with the offset less than 6 G, in both cases.

Comprehensive modeling and characterization of the General-Purpose Heat Source Radioisotope Thermoelectric Generator for solar system missions

The radioisotope thermoelectric generator relies solely on radioactive decay for its energy and generates power through the thermoelectric effect. The General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG) represents the RTG with the largest power and highest conversion efficiency ever built, and its energy supply supported four daring interplanetary missions (Galileo, Ulysses, Cassini, New Horizons). To explore the use potential of the GPHS-RTG in the solar system, its conservative application range and performance should be determined. In this work, a comprehensive model of the GPHS-RTG was developed, and its calculated results agreed well with Lockheed Martin’s test report (temperature relative error < 1 %). The study demonstrates that within the Earth orbit, the distance of the mission area from the sun and the angle of incidence sunlight significantly affect the thermoelectric performance and hot-junctions’ temperature. Moreover, conservative application ranges and their extending methods (changing coating material, thermal loading, and operational voltage) for GPHS-RTG at different attitudes in the solar system were determined for the first time by comparing the proportion of the hot-junction over-temperature area. The temperature distribution and its cause of formation in the axial and circumferential directions of the GPHS-RTG during solar system missions were revealed in the end. This work is valuable for future RTG design and mission planning.

The Design of CubeSats for Outer Solar System Scientific Missions

In the past decade, CubeSats have emerged as a cost-effective solution for scientific missions beyond Earth’s orbit, though they have not yet gone further than orbiting Mars, as with NASA’s MarCO CubeSats. This paper discusses the technical challenges and solutions for designing CubeSats for Outer Solar System missions, specifically in the context of the proposed Astraeus Mission to Titan. These CubeSats, called the Mites, aim to measure heavy anions in Titan’s upper atmosphere and have undergone analysis to determine their optimal size and drag coefficient. A significant challenge addressed is the CubeSats’ power system, with solar panels being less effective at greater distances from the Sun. The paper proposes investigating Radioisotope Power Systems (RPS) as an alternative. However, there is the additional challenge of packaging the RPS in a sufficiently small form factor so that the upper atmospheric experiment can be completed. This must cover the greatest range of longitudes, latitudes, and ranges hence the orbital decay rate must be controlled to achieve this. The paper also explores the Mites’ long-duration exposure to space during transit and strategies to minimise cosmic radiation exposure. Whilst this is completed in the context of the Astraeus Mission, the data obtained can guide similar missions and aid others in overcoming the limitations of CubeSats so that they can be used more frequently for Outer Solar System science missions. Keywords: Titan, CubeSat, Radioisotope Thermoelectric Generator, Orbital Decay, Outer Solar System

Unraveling The Trifecta of Governance, Sustainability, And Technological Advancements in India

This paper delves into the complex interplay of innovation, governance, sustainability, and technological advancements within the unique context of India, aiming to unravel the intricate dynamics that define the nation's development landscape. As India positions itself on the global stage, understanding the synergies and challenges inherent in these three critical domains becomes imperative for sustainable socio-economic progress. The status paper commences by introducing the significance of the status paper, emphasizing the symbiotic relationships between innovation, governance, sustainability, and technological advancements. The multifaceted exploration encompasses historical perspectives, contemporary policies, and case studies that exemplify the nation's endeavors to create a robust innovation ecosystem. Examining the evolution of innovation and governance in India, the paper highlights pivotal policies such as "Make in India" and "Startup India," showcasing the government's commitment to fostering a culture of innovation. The review also scrutinizes sustainability challenges faced by India, underscoring environmental concerns and resource depletion. However, it reveals instances where sustainability has been seamlessly integrated into innovation ecosystems, citing examples like the National Solar Mission and waste management initiatives. The status paper concludes by summarizing the paper's key findings, emphasizing the need for an integrated approach to policymaking and industry practices. This research contributes valuable insights to policymakers, researchers, and industry stakeholders, guiding India toward a more sustainable and innovative future.

Variation of the electron flux spectrum along a solar flare loop as inferred from STIX hard X-ray observations

Context. Regularized imaging spectroscopy was introduced for the construction of electron flux images at different energies from count visibilities recorded by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). In this work we seek to extend this approach to data from the Spectrometer/Telescope for Imaging X-rays (STIX) on board the Solar Orbiter mission. Aims. Our aims are to demonstrate the feasibility of regularized imaging spectroscopy as a method for analysis of STIX data, and also to show how such an analysis can lead to insights into the physical processes affecting the nonthermal electrons responsible for the hard X-ray emission observed by STIX. Methods. STIX records imaging data in an intrinsically different manner from RHESSI. Rather than sweeping the angular frequency plane in a set of concentric circles (one circle per detector), STIX uses 30 collimators, each corresponding to a specific angular frequency. For this work, we derived an appropriate modification to the previous computational approach for the analysis of the visibilities observed by STIX. This approach also allows for the observed count data to be placed into nonuniformly spaced energy bins. Results. We show that the regularized imaging spectroscopy approach is not only feasible for an analysis of the visibilities observed by STIX, but also more reliable. The application of the regularized imaging spectroscopy technique to several well-observed flares reveals details of the variation of the electron flux spectrum throughout the flare sources. Conclusions. We conclude that the visibility-based regularized imaging spectroscopy approach is well suited for the analysis of STIX data. We also used STIX electron flux spectral images to track, for the first time, the behavior of the accelerated electrons during their path from the acceleration site in the solar corona toward the chromosphere.