The mechanisms triggering solar flares and driving coronal heating occur across wide temperature ranges on small spatial scales and short timescales, making them difficult to observe with current instrumentation. The upcoming SOLAR-C mission, launching in the late 2020s, will provide unprecedented plasma diagnostic capability with its high-throughput extreme-ultraviolet (EUV) spectroscopic telescope (EUVST), capable of similar to 0.'' 2 pix(-1) spatial sampling (similar to 0.'' 4 resolution), continuous temperature coverage from 0.02 to 15 MK, and exposure times down to 0.5 s. We present forward modelling of the spectrograph's short wavelength camera (170.0-212.3 & Aring;; SOLAR-C/EUVST-SW) and its response to log T similar to 6.2 coronal plasma in a three-dimensional MHD-simulated pre-flare active region. We compare this performance to that of the previous-generation EUV Imaging Spectrometer (EIS) on Hinode (SOLAR-B). Our results demonstrate that SOLAR-C/EUVST can distinguish individual flux tubes in simulated active region loops, which Hinode/EIS cannot resolve. In simulated pre-flare plasma, SOLAR-C/EUVST captures sharp velocity gradients between adjacent upflowing and downflowing plasma which Hinode/EIS is unable to resolve. Doppler velocity measurement accuracy will reach better than 1 km s(-1) in active regions. We show that this next-generation spectrograph can be expected to directly observe processes potentially related to flare triggering, such as plasma flows from low-altitude reconnection linked to emerging flux, and determine whether active region loops consist of a small number of strands or the hundreds predicted by magnetic reconnection-induced nanoflare heating models.

Solar Orbiter conducted a series of flare-optimised observing campaigns in 2024 using the Major Flare Solar Orbiter Observing Plan (SOOP). Dedicated observations were performed during two distinct perihelia intervals in March/April and October, during which over 22 flares were observed, ranging from B- to M-class. These campaigns leveraged high-resolution and high-cadence observations from the mission’s remote-sensing suite, including the High-Resolution EUV Imager (EUI/HRIEUV), the Spectrometer/Telescope for Imaging X-rays (STIX), the Spectral Imaging of the Coronal Environment (SPICE) spectrometer, and the High Resolution Telescope of the Polarimetric and Helioseismic Imager (PHI/HRT), as well as coordinated ground-based and Earth-orbiting observations. EUI/HRIEUV, operating in short-exposure modes, provided two-second-cadence, non-saturated EUV images, revealing structures and dynamics on scales not previously observed. Simultaneously, STIX captured hard X-ray imaging and spectroscopy of accelerated electrons, while SPICE acquired EUV slit spectroscopy to probe chromospheric and coronal responses. Together, these observations offer an unprecedented view of magnetic reconnection, energy release, particle acceleration, and plasma heating across a broad range of temperatures and spatial scales. These campaigns have generated a rich dataset that will be the subject of numerous future studies addressing Solar Orbiter’s top-level science goal: “How do solar eruptions produce energetic particle radiation that fills the heliosphere?”. This paper presents the scientific motivations, operational planning, and observational strategies behind the 2024 flare campaigns, along with initial insights into the observed flares. We also discuss lessons learned for optimizing future Solar Orbiter Major Flare campaigns and provide a resource for researchers aiming to utilize these unique observations.

Alfvénic fluctuations in the corona are regularly observed in EUV imaging and infrared spectroscopic data. We report here on the Doppler velocity fluctuations within the data from the Extreme-Ultraviolet Imaging Spectrometer (EIS) on board Hinode. The capabilities of EIS provide an opportunity to detect fluctuations in the solar atmosphere and confirm the previous results from ground-based spectroscopic data using EIS’s EUV spectra and in seeing-free conditions. We analyzed Doppler velocity time series for a particular date of observation, 2007 September 27, above the limb. The Fourier transform of the observed signals show the Doppler velocity has a broad spectrum distributed as a power law with a power enhancement around 3–4 mHz. This result is consistent with previous observational results obtained with Coronal Multi-Channel Polarimeter data, and hence we interpret the Doppler velocity fluctuations as Alfvénic in nature. We performed nonlinear regression on the power spectra to determine the index of the power law and to parameterize the properties of the enhancement. We find good agreement between the estimated values and those from previous studies. Furthermore, correlation analysis shows that the observed oscillations are spatially coherent perpendicular to the magnetic field on the length scale L _⊥ ∼ 8 Mm, providing an estimate on the energy injection scale of a bundle of coherently oscillating loops.

The first simultaneous observations of the Fe xviii 974.86 Å and Fe xx 721.56 Å forbidden lines from the Spectral Imaging of the Coronal Environment (SPICE) spectrograph on Solar Orbiter are presented. The lines were observed from the postflare loops of an M2.5 class solar flare that peaked at 23:49 UT on 2024 March 23. The Fe xx /Fe xviii ratio is used to derive a temperature spacetime map for the flaring period, with values ranging from 8 to 20 MK. The map reveals repeated episodes of heating at the SPICE slit location over a 30 minute period. For one location with the brightest emission, the plasma cooled from 10 to 8 MK in 5 minutes, which is longer than the expected conductive cooling time of 170 s, suggesting continued background heating during the cooling period. Doppler shifts of between 0 and +10 km s ^−1 were obtained with precisions of 1–4 km s ^−1 , but the accuracies are lower due to uncertainties over the absolute wavelength calibration; hence, we cannot conclude there are plasma flows in the flare loops. The widths of the two lines were found to be close to the instrumental widths with no evidence of nonthermal broadening although this result is limited by the instrument resolution. The Fe xviii and Fe xx lines have high signal-to-noise with only a 5 s exposure time, demonstrating that the lines will be valuable for high-cadence flare studies with SPICE.

HelioIndex is a directory of authors who are active in solar and heliospheric physics (SHP). It is available at the webpage HelioIndex.org, and it includes several derived products such as publication lists, country and institute data, journal data, and lists of the most cited articles in the field. HelioIndex is built from ORCID identifiers and publication data obtained from the Astrophysics Data System and ORCID. Selection criteria have been chosen to approximately correspond to a researcher having completed a PhD and published original research in a refereed journal. HelioIndex is intended to be a comprehensive directory of SHP authors that is generated and maintained through software procedures, with minimal human intervention. At the time of writing, 1910 SHP researchers are listed in HelioIndex and they belong to 55 countries. The countries with the largest numbers of researchers are the US, China and the UK, with 29%, 15%, and 8% of the total, respectively. HelioIndex authors average 0.69 first author papers per year over their careers, and the median citations for a paper is 15. Based on journal keyword data, it is estimated that 57% and 28% of HelioIndex authors belong to solar physics and heliospheric physics, respectively, with the remainder overlapping both disciplines.

Context. The quiet solar atmosphere is populated with plasma loops that are typically observed in the ultraviolet (UV) and extreme-ultraviolet (EUV) wavelengths. The coronal counterparts of these loops are traditionally referred to as coronal bright points. Although they are very compact, bright points reveal a high degree of multithermal complexity through different layers in the solar atmosphere. Aims. We investigate the thermal structuring of these loop systems to gain further insights into the physical mechanisms that heat the plasma. To this end, we report on the multithermal characteristics of bright points in the quiet solar atmosphere through the transition region and the corona. Methods. We combined spectral data from the EUV spectrometer SPICE on board Solar Orbiter and imaging data from AIA on board SDO to cover a broad temperature range (log T [K] ≈ 4.6–6.5). The bright points were observed simultaneously in all available spectral and imaging channels. We analyzed 14 features in total, computed their differential emission measure (DEM) distribution, and compared them with the emission measure from the (average) quiet Sun. Results. We found common characteristics of the DEM in the bright points. In the upper transition region, above temperatures of log T [K] ≈ 5.2, the slope of the DEM toward higher temperatures (i.e., towards the corona) is significantly shallower than in the quiet Sun. The situation is different in the lower transition region, below log T [K] ≈ 5.2: The negative slope of the DEM is similar to that of the quiet Sun there, which implies that in response to the additional heating, the density in the bright point increases by the same factor at all temperatures. Conclusions. Our finding of the very shallow gradient of the DEM toward higher temperatures is relevant for coronal heating models. Based on earlier studies, a shallower DEM slope would imply fewer heating events in the bright points than in the quiet Sun. The apparent dichotomy between the plasma at the lower and at the higher temperatures might also imply distinct heating mechanisms or thermally disconnected loops in the two temperature ranges. To confirm these, however, a more detailed analysis is required. In particular, UV and EUV spectroscopic time series combined with co-temporal imaging data are required to better capture the thermal evolution of bright points, which in turn will shed further light on the nature of thermal structuring and plasma heating in the corona.

Much of the dynamics of the ambient solar wind (SW) manifests at mesoscales—scales larger than kinetic but smaller than global structures. Mesoscale structures often form at the Sun, survive to 1 au, and are geoeffective. Identifying their origins, release, and acceleration mechanisms is critical to advancing heliophysics. We investigate a 6 day interval (2022 March 4 00:00:00 UT–2022 March 9 23:59:59 UT) in which Solar Orbiter (SO) was radially aligned with Earth. We characterize mesoscale structures observed in SO Heavy Ion Sensor (HIS) Fe/O and O ^7+ /O ^6+ in situ measurements, and confirm their survival to L1. A subset of these structures are periodic, suggesting that they formed at the Sun. We use the ADAPT-WSA model to connect in situ measurements to their remotely observed sources at the Sun. We characterize the SW sources using model-determined proxies for SW formation (e.g., the S-web). We compare Fe/O observed at SO/HIS, with S/O abundances derived by the Spectral Imaging of the Coronal Environment (SO/SPICE), at the WSA-determined SW sources. We find (1) SO and ACE/Wind observe similar many-hour structures, linking mesoscale structures from their source to two well-separated locations in situ for the first time. (2) Observed periodicities are consistent with those observed in prior studies of mesoscale structures throughout the inner heliosphere. (3) SW originating from only one of the four SPICE rasters can be interpreted as plasma originating from open fields. The SW from the other three require a different interpretation, and we suggest interchange reconnection as the most natural solution.

The Sun is a standard reference object for Astrophysics and also a fascinating subject of study in its own right. X-ray and extreme ultraviolet movies of the Sun's atmosphere show an extraordinary diversity of plasma phenomena, from barely visible bursts and jets to coronal mass ejections that impact a large portion of the solar surface. The processes that produce these phenomena, heat the corona and power the solar wind remain actively studied and accurate atomic data are essential for interpreting observations and making model predictions. For the Sun's interior intense effort is focused on resolving the "solar problem," (a discrepancy between solar interior models and helioseismology measurements) and atomic data are central to both element abundance measurements and interior physics such as opacity and nuclear reaction rates. In this article, topics within solar interior and solar atmosphere physics are discussed and the role of atomic data described. Areas of active research are highlighted and specific atomic data needs are identified.

Version 11 of the CHIANTI database and software package is presented. Advanced ionization equilibrium models have been added for low charge states of seven elements (C, N, O, Ne, Mg, Si and S), and represent a significant improvement especially when modelling the solar transition region. The models include the effects of higher electron density and charge transfer on ionization and recombination rates. As an illustration of the difference these models make, a synthetic spectrum is calculated for an electron pressure of 7$\times 10^{15}$ cm$^{-3}$ K and compared with an active region observation from HRTS. Increases are seen of factors of two to five in the predicted radiances of the strongest lines in the UV from Si IV, C IV, and N V, compared to the previous modelling using the coronal approximation. Much better agreement (within 20\%) with the observation is found for the majority of the lines. The new atomic models better equip both those who are studying the transition region and those who are interpreting emission from higher density astrophysical and laboratory plasma. In addition to the advanced models, several ion datasets have been added or updated, and data for the radiative recombination energy loss rate have been updated.

Previously-unexplored diagnostics of O IV in the extreme ultraviolet region 260-280 A are used to derive a temperature and density for a solar flare kernel observed on 2012 March 9 with the Extreme ultraviolet Imaging Spectrometer on the Hinode satellite. Seven lines from the 2s 2p^2 - 2s 2p 3s transition array between 271.99 and 272.31 A are both temperature and density sensitive relative to the line at 279.93 A. The temperature, T, is constrained with the 268.02/279.93 ratio, giving a value of log (T/K)=5.10 +/- 0.03. The ratio 272.13/279.93 then yields an electron number density, N_e, of log (N_e/cm^-3) = 12.55 with a lower limit of 11.91, and an upper limit of 14.40. The O IV emitting volume is estimated to be 0.4 arcsec (300 km) across. Additional O IV lines at 196, 207 and 260 A are consistent with the derived temperature and density but have larger uncertainties from the radiometric calibration and blending. Density diagnostics of O V and Mg VII from the same spectrum are consistent with a constant pressure of 10^17.0 K cm^-3 through the transition region. The temperature derived from O IV supports recent results that O IV is formed around 0.15 dex lower at high densities compared to standard "zero-density" ionization balance calculations.

The center-to-limb variations (CLV) of transition region line Gaussian fit parameters in solar plage are reported for the first time. The Si iv 1402.77 & Aring; line observed by the Interface Region Imaging Spectrograph is used. The spectral intensity increases linearly from the disk center to the solar limb. Similarly, the nonthermal velocity also increases linearly from 23.6 (at the disk center) to 30.9 km s-1 (at the solar limb). On the other hand, the Doppler velocity decreases from 8.9 +/- 1.0 at the disk center to 0.0 km s-1 at the limb. This CLV pattern in solar plages is consistent with the CLV pattern reported in the quiet Sun (QS). However, the average values of the parameters in the solar plage are significantly higher than in the QS. The intensity and nonthermal velocity increase linearly with the magnetic field at the disk center, while the Doppler velocity does not depend on the magnetic field. Due to the line-of-sight effect, the plasma column depth increases toward the solar limb, which leads to a linear increase in the spectral intensity. Further, the increasing plasma column depth toward the solar limb adds more and more unresolved motions, and as a result, the nonthermal velocity increases from the disk center to the solar limb. In the solar plages, the higher plasma density due to the strong magnetic field leads to higher intensity and nonthermal velocity compared to the QS and coronal hole.

Explosive transient events occur throughout the solar atmosphere. The differing manifestations range from coronal mass ejections to Ellermann bombs. The former may have negligible signatures in the lower atmosphere, and the latter may have negligible nonthermal emissions such as hard X-radiation. A solar flare generally involves a broad range of emission signatures. Using a suite of four space-borne telescopes, we report a solar event that combines aspects of simple UV bursts and hard X-ray emitting flares at the same time. The event is a compact C-class flare in active region AR11861, SOL2013-10-12T00:30. By fitting a combined isothermal and nonthermal model to the hard X-ray spectrum, we inferred plasma temperatures in excess of 15 MK and a nonthermal power of about 3 x 10(27) erg s(-1) in this event. Despite these high temperatures and evidence for nonthermal particles, the flare was mostly confined to the chromosphere. However, the event lacked clear signatures of UV spectral lines, such as the Fe XII 1349 & Aring; and Fe XXI 1354 & Aring; emission lines, which are characteristic of emission from hotter plasma with a temperature over 1 MK. Moreover, the event exhibited very limited signatures in the extreme-UV wavelengths. Our study indicates that a UV burst - hard X-ray flare hybrid phenomenon exists in the low solar atmosphere. Plasma that heats to high temperatures coupled with particle acceleration by magnetic energy that is released directly in the lower atmosphere sheds light on the nature of active region core heating and on inferences of flare signatures.

This entry contains input files for the LOPT atomic physics code. The results are described in the article "Updated reference wavelengths for Si VII and Mg VII lines in the 272--281 Angstrom range" by Dr. Peter R. Young (NASA Goddard Space Flight Center) that was submitted to The Astrophysical Journal. There are two sub-directories called mg7 and si7 for the two ions. In each there are three input files of the form: Mg7_FixLEV_EIS.txt - List of fixed levels in the atomic model.Mg7_lin_EIS.prn - List of wavelengths and uncertainties for the lines of the model.Mg7_lopt_EIS.par - The parameter input file for LOPT You will need to install the LOPT code to use these files. See Kramida, A. E. 2011, Computer Physics Communications,236182, 419. In each directory you will see two additional files that are the output wavelength and energy files generated by the author.

The CHIANTI atomic database provides sets of assessed data that are used for simulating spectral observations of astrophysical plasmas. This article describes updates that will be released as version 10.1 of the database. A key component of CHIANTI is the provision of ionization and recombination rates that are used to compute the ionization balance of a plasma over a range of temperatures. Parameters for calculating the ionization rates of all stages of ions from H through Zn were compiled and inserted into the CHIANTI database in 2009. These were based on all measurements that were available at the time and supplemented with distorted wave calculations. Since then, there have been a number of new laboratory measurements for ions that produce spectral lines that are commonly observed. Parameters have been fit to these new measurements to provide improved ability to reproduce the ionization cross sections and rate coefficients, and these are added to the database. CHIANTI 10.1 also includes new recombination rates for the phosphorus isoelectronic sequence, and the updated ionization and recombination rates have been used to calculate a new ionization equilibrium file. In addition, CHIANTI 10.1 has new electron collision and radiative data sets for eight ions in the nitrogen and oxygen isoelectronic sequences and updated energy level and wavelength data for six other ions.

We discuss spectroscopic capabilities needed to resolve two important unresolved questions in solar physics, namely, "What heats the solar corona?" and "What causes the sudden, rapid release of energy that produces flares?"Spectroscopic capabilities needed to answer these questions include:(1) high spectral resolution, to enable measurement of unblended profiles of numerous emission lines; (2) high sensitivity with a wide dynamic range, to enable detection of faint line emission as well as weak wing components; (3) comprehensive temperature coverage, observing lines formed at log T separated by 0.1 or 0.2 dex between at least 20,000 K and 20 MK; (4) an absolute wavelength scale, to enable Doppler velocity measurements accurate to within 1 km/s or better; (5) simultaneous measurements over spatial area comparable in size to active regions (4 ′ × 4 ′ ); (6) spatial resolution sufficient to resolve loop strands and the immediate vicinities of reconnection sites (< 1 ′′ ); (7) cadence short enough to monitor evolution of the solar atmosphere during periods of most rapid change (< 1 s).

Contains derived data for the article "Updated reference wavelengths for Si VII and Mg VII lines in the 272-281 Angstrom range" submitted to The Astrophysical Journal and authored by Dr. Peter Young (NASA Goddard Space Flight Center). The zip file contains 13 sub-directories corresponding to the 13 datasets studied in the article. In each directory there are two files: an IDL save file, and an ASCII text file. Upon restoring the IDL save file into IDL, you will see four quantities (the names may vary slightly between datasets): MAP: an IDL map containing the Si VII 275.37 image from which the pixel mask was selected.MASK: a pixel mask generated with the routine pixel_mask_gui.pro using the input MAP. The pixels in the map are used to create the spectrum.SWSPEC: A spectrum structure generated with the routine eis_mask_spectrum.pro containing the EIS short-wavelength channel spectrum.LWSPEC: A spectrum structure generated with the routine eis_mask_spectrum.pro containing the EIS long-wavelength channel spectrum. LWSPEC was input to the Gaussian-fitting routine spec_gauss_eis.pro and fits were generated for the emission lines listed in the article. The routine writes the fit parameters to the ASCII text file. The Gaussian fit parameters can be read into IDL using the routine read_line_fits.pro. You can also use the routine line_fits_summary.pro to generate a printout of the fit parameters. The IDL software described here is available with the Solarsoft repository (https://sohoftp.nascom.nasa.gov/solarsoft/).

This paper outlines the necessity for the availability, accessibility, and expansion of atomic physics data and analysis tools for the meaningful interpretation of spectroscopic and polarimetric observations. As we move towards observing the Sun at higher spatio-temporal resolutions, and near-continuously at a range of wavelengths, it becomes critical to develop the appropriate atomic data and physics tools to facilitate scientific progress. We recommend the continued improvement and expansion of current databases to support the development of optically-thick/radiative transfer models, evaluate non-thermal and non-equilibrium ionization effects, and quantify uncertainties in atomic and molecular values. A critical long-term goal will require extending and strengthening collaborations across the atomic, solar/heliospheric, and laboratory plasma physics communities through the participation and training of early career scientists. We also recommend establishing funding for a centralized atomic physics resource made up of a comprehensive and user-oriented atomic database and modeling framework.

New reference wavelengths for atomic transitions of Mg vii and Si vii in the 272–281 Å wavelength range are derived using measurements from the Extreme-ultraviolet Imaging Spectrometer (EIS) on board the Hinode spacecraft. Mg vii and Si vii are important ions for measuring plasma properties in the solar transition region at around 0.6 MK. The six Si vii wavelengths are 13–21 mÅ and 7–11 mÅ longer than the values in the NIST Atomic Spectra Database (ASD) and the compilations of B. Edlén, respectively. The four Mg vii wavelengths are shorter than the values in the ASD by 8–12 mÅ but show reasonable agreement with the Edlén values. The new wavelengths will lead to more accurate Doppler shift measurements from the EIS instrument and will be valuable for spectral disambiguation modeling for the upcoming Multi-Slit Solar Explorer mission.

Coronal mass ejection spray plasma associated with the M1.5-class flare of 16 February 2011 is found to exhibit a Doppler blue-shift of 850 km/s - the largest value yet reported from ultraviolet (UV) or extreme ultraviolet (EUV) spectroscopy of the solar disk and inner corona. The observation is unusual in that the emission line (Fe XII 193.51 A) is not observed directly, but the Doppler shift is so large that the blue-shifted component appears in a wavelength window at 192.82 A, intended to observe lines of O V, Fe XI and Ca XVII. The Fe XII 195.12 A emission line is used as a proxy for the rest component of 193.51 A. The observation highlights the risks of using narrow wavelength windows for spectrometer observations when observing highly-dynamic solar phenomena. The consequences of large Doppler shifts for ultraviolet solar spectrometers, including the upcoming Multi-slit Solar Explorer (MUSE) mission, are discussed.

The transition region between the Sun’s corona and chromosphere is important to the mass and energy transfer from the lower atmosphere to the corona; consequently, this region has been studied intensely with ultraviolet and extreme ultraviolet (EUV) observations. A major result of these studies is that the amount of plasma at low temperatures, <10 ^5 K, is far too large to be compatible with the standard theory of thermal conductivity. However, it is not clear whether the disagreement lies with a problem in the observations or a problem in the theory. We address this issue by analyzing high–spatial and temporal resolution EUV observations from an X1.6-class flare, taken with the Interface Region Imaging Spectrograph and the Solar Dynamic Observatory/Atmospheric Imaging Assembly (AIA). These data allow us to isolate the emission of flare loops from that of surrounding structures. We compare the emission measures (EMs) derived from the C ii 1334.525 Å and Si iv 1402.770 Å transition region spectral lines, the Fe xxi 1354.066 Å flare line, and the AIA 171 Å coronal images. We find that the EM ratios are incompatible with a standard conduction-dominated transition region model. Furthermore, the large increases in the EM magnitudes due to flare heating make it highly unlikely that the disagreement between data and theory is due to observational uncertainties in the source of the emission. We conclude that the standard Spitzer–Härm thermal conductivity must be invalid for, at least, flare loops. We discuss the possibility that turbulent suppression of thermal conduction can account for our results.