Ryan J. French

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 the Solar Orbiter SPICE and Extreme Ultraviolet Imager (EUI), Hinode Extreme Ultraviolet Imaging Spectrometer (EIS), and Chandrayaan-2 Solar X-ray Monitor (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 September 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 postflare looptops 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 looptops seen with SPICE show coronal abundances due to coronal material deposited near the looptops.

Supra-arcade downflows (SADs) are sunward-traveling features routinely observed in hot fan structures above magnetic loop arcades during eruptive solar flares. We manually compiled a catalog (SADCat) of 178 SAD-productive flares imaged by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) during solar cycle 24 as a resource for the wider solar flare community. We conducted a preliminary analysis of the SADCat, comparing the flare X-ray and CME properties between eruptive solar flares with and without SADs. We found that peak GOES X-ray flux, flare duration, CME speed, and CME mass have a strong influence on whether a flare produces visible SADs, whereas flare impulsivity and CME acceleration have little effect.

Recent three-dimensional flare models suggest that flare ribbon substructure is linked to the fragmentation of the reconnecting current sheet in the corona. Flare ribbon substructure can therefore potentially serve as a unique diagnostic tool for physical processes in the flare current sheet. In this paper, we describe a new method to quantify the evolution of ribbon substructure that first extracts the ribbon’s bright leading edge and then quantifies its morphology using the box-counting dimension and correlation dimension mapping (CDM). We first test our method using synthetic observations. We then apply it to an M6.5-class solar flare on 2015 June 22 observed by the Interface Region Imaging Spectrograph (IRIS) 1330 Å slit-jaw imager. We find that when the flare ribbon boundary has more multiple-spatial-scale features (a higher box-counting dimension), hard X-ray emission and magnetic reconnection rates are the strongest. We also find that the flare ribbon complexity characterized by CDM has a moderate correlation with the IRIS Si IV 1402.77 Å nonthermal velocity (in the negative polarity ribbon) and reconnection flux rates (in ribbons of both magnetic polarities). We conclude that the buildup of the spatial complexity of the ribbons at multiple spatial scales can serve as an observational proxy for current-sheet fragmentation in the corona.

Recent three-dimensional flare models suggest that flare-ribbon substructure is linked to the fragmentation of the reconnecting current sheet in the corona. Flare-ribbon substructure can therefore potentially serve as a unique diagnostic tool for physical processes in the flare current sheet. In this paper, we describe a new method to quantify the evolution of ribbon substructure, which first extract the ribbon's leading bright front and the quantifies its morphology using the box-counting dimension and Correlation Dimension Mapping (CDM). We first test our method using synthetic observations. We then find that when the flare ribbon boundary has more multi-spatial-scale features (higher box-counting dimension), hard X-ray (HXR) emission and magnetic reconnection rates are the strongest. We also find that the flare-ribbon complexity characterized by CDM has moderate correlation with the IRIS Si IV 1402.77 Å non-thermal velocity (in the negative-polarity ribbon) and reconnection flux rates (in ribbons of both magnetic polarities). We conclude that the build-up of the spatial complexity of the ribbons at multiple spatial scales can serve as an observational proxy for current-sheet fragmentation in the corona.

We detect rapid downflows of 150–217 km s ⁻¹ in Infrared Interferometer in Space (or IRIS) Si iv 1402.77 nm measurements of an X9-class solar flare on 2024 October 3. 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 is synchronized with peaks in emission from the Advanced Space-based Solar Observatory Hard X-ray Imager (or ASO-S HXI) and Large Yield Radiometer (or LYRA) Ly α measurements, indicating that the chromospheric downflows (with a maximum redshift of 176 km s ⁻¹ ) 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 ⁻¹ ), despite the magnetic flux rate falling to zero and high-energy hard X-ray (HXR) and Ly α measurements returning to background levels. This is reflective of downflows in the flare ribbon footpoints of flare-induced coronal rain. HXR spectral analysis supports this scenario, revealing strong nonthermal 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 quasiperiodic pulsations with a constant ∼50 s period across the 15 minutes of flare evolution (independent 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.

We present fast-cadence and high-resolution observations of flare ribbons from the Solar Orbiter Extreme Ultraviolet Imager (EUI). Utilizing the short-exposure observations from the EUI High Resolution Imager in the extreme-ultraviolet, we find small-scale blob/bead-like kernel structures propagating within a hook at the end of a flare ribbon, during the impulsive phase of a C9.9-class solar flare. These bead structures are dynamic, with well-resolved spatial separations as low as ≈420–840 km (3–6 pixels) below the observable limit of full-disk solar imagers. We analyze the evolution of the plane-of-sky apparent velocity and separation of the flare ribbon structures, finding evidence for multiple processes occurring simultaneously within the flare ribbon. These processes include quasiperiodic pulsation–like brightenings, slow back-and-forth zigzag motions along the ribbon, rapid apparent motions along the ribbon (600+ km s ⁻¹ ), and stationary blob-like structures. Finally, we conduct fast Fourier transform analysis and analyze the start times of exponential growth in the power spectrum at different spatial scales across the flare ribbon. Our analysis reveals that the ribbon beads form with a key spatial separation of 1.7–1.9 Mm before developing into more complex structures at progressively larger and smaller spatial scales. This observation is consistent with predictions of the tearing mode instability.

We present fast cadence and high resolution observations of flare ribbons from the Solar Orbiter Extreme Ultraviolet Imager (EUI). Utilizing the short-exposure observations from the EUI High Resolution Imager in EUV (HRIEUV), we find small-scale blob/bead-like kernel structures propagating within a hook at the end of a flare ribbon, during the impulsive phase of a C9.9-class solar flare. These bead structures are dynamic, with well-resolved spatial separations as low as ~420-840 kilometers (3-6 pixels) - below the observable limit of full-disk solar imagers. We analyze the evolution of the plane-of-sky apparent velocity and separation of the flare ribbon structures, finding evidence for multiple processes occurring simultaneously within the flare ribbon. These processes include - quasi-periodic pulsation (QPP)-like brightenings, slow back-and-forth zig-zag motions along the ribbon, rapid apparent motions along the ribbon (600+ km/s), and stationary blob-like structures. Finally, we conduct Fast Fourier Transform analysis and analyze the start times of exponential growth in the power spectrum at different spatial scales across the flare ribbon. Our analysis reveals that the ribbon beads form with a key spatial separation of 1.7-1.9 Mm, before developing into more complex structures at progressively larger and smaller spatial scales. This observation is consistent with predictions of the tearing mode instability.

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.