William Ashfield

Apparent slipping motions of flare ribbon kernels and the formation of hard X-ray (HXR) footpoints are important signatures of magnetic reconnection in solar flares. Ultraviolet (UV) and HXR ribbon emission can show quasiperiodic pulsations (QPPs), but the link between HXR QPP sources and slipping reconnection remains poorly understood. In this work, we analyze high-cadence IRIS and STIX observations of the 2022 March 31 M9.6 flare. STIX detected nonthermal QPPs with periods of ≈35 s from two relatively stationary footpoints. The majority of the HXR QPPs were correlated with UV pulsations observed by the IRIS Slit Jaw Imager in ribbon regions encompassing these footpoints. In one such region observed under the IRIS slit, the Si iv 1402.77 Å line exhibited pulsations in intensity, Doppler shift, and width, some of which were coincident with HXR QPPs. Apparent slipping motions of the UV kernels were also observed, but their locations, timing, and UV intensity variability showed lower correlation with the HXR QPPs. The ribbons along which the slipping kernels were found correspond well to footprints of quasi-separatrix layers, regions of high magnetic connectivity gradients, identified using a nonlinear force-free field extrapolation. Our multi-instrument analysis suggests that strong deposition of energy by nonthermal electrons was concentrated in a specific loop system within a large-scale 3D reconnection structure. Slipping kernels were subjected to weaker, if any, energization by nonthermal electrons, offering new constraints on 3D reconnection and flare energy release.

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 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.

Solar and stellar flares often exhibit oscillations, or quasi-periodic pulsations (QPPs), across the electromagnetic spectrum. While magnetic field reconnection drives these events, it remains to be determined whether oscillatory reconnection causes the quasi-periodicity, or if waves drive or mediate this process. Exploiting coordinated observations from NASA’s Interface Region Imaging Spectrograph and the Swedish 1m Solar Telescope, this work presents novel spectroscopic observations of QPPs in a solar flare at high-temporal (< 1s) and high-spatial (≈60 km) resolution. Downward velocities in the flare ribbon show synchronized oscillations at different atmospheric layers with a period of ≈35s. These velocities correlate with hard X-ray emissions, indicating a modulated deposition of accelerated electrons in the chromosphere as the driver. By negating MHD sausage modes as the modulator, we demonstrate that repeated reconnection drives the QPPs. These results advance our understanding of magnetic reconnection in flares and its role in driving oscillatory signals.

The excess broadening of high-temperature spectral lines, long observed near the tops of flare arcades, is widely considered to result from magnetohydrodynamic turbulence. According to different theories, plasma turbulence is also believed to be a candidate mechanism for particle acceleration during solar flares. However, the degree to which this broadening is connected to the acceleration of nonthermal electrons remains largely unexplored outside of recent work, and many observations have been limited by limited spatial resolution and cadence. Using the Interface Region Imaging Spectrometer, we present spatially resolved observations of loop-top (LT) broadenings using hot (≈11 MK) Fe xxi 1354.1 Å line emission at ≈9 s cadence during the 2022 March 30 X1.3 flare. We find nonthermal velocities upward of 65 km s⁻¹ that decay linearly with time, indicating the presence and subsequent dissipation of plasma turbulence. Moreover, the initial Fe xxi signal was found to be cospatial and cotemporal with microwave emission measured by the Expanded Owens Valley Solar Array, placing a population of nonthermal electrons in the same region as the LT turbulence. Evidence of electron acceleration at this time is further supported by hard X-ray measurements from the Spectrometer/Telescope for Imaging X-rays on board Solar Orbiter. Using the decay of nonthermal broadenings as a proxy for turbulent dissipation, we found the rate of energy dissipation to be consistent with the power of nonthermal electrons deposited into the chromosphere, suggesting a possible connection between turbulence and electron acceleration.

The excess broadening of high-temperature spectral lines, long observed near the tops of flare arcades, is widely considered to result from magnetohydrodynamic (MHD) turbulence. According to different theories, plasma turbulence is also believed to be a candidate mechanism for particle acceleration during solar flares. However, the degree to which this broadening is connected to the acceleration of non-thermal electrons remains largely unexplored outside of recent work, and many observations have been limited by limited spatial resolution and cadence. Using the Interface Region Imaging Spectrometer (IRIS), we present spatially resolved observations of loop-top broadenings using hot (11MK) Fe XXI 1354.1 Å line emission at ~9s cadence during the 2022 March 30 X1.3 flare. We find non-thermal velocities upwards of 65km/s that decay linearly with time, indicating the presence and subsequent dissipation of plasma turbulence. Moreover, the initial Fe XXI signal was found to be co-spatial and co-temporal with microwave emission measured by the Expanded Owens Valley Solar Array (EOVSA), placing a population of non-thermal electrons in the same region as the loop-top turbulence. Evidence of electron acceleration at this time is further supported by hard X-ray measurements from the Spectrometer/Telescope for Imaging X-rays (STIX) aboard Solar Orbiter. Using the decay of non-thermal broadenings as a proxy for turbulent dissipation, we found the rate of energy dissipation to be consistent with the power of non-thermal electrons deposited into the chromosphere, suggesting a possible connection between turbulence and electron acceleration.

Whitepaper #049 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: solar physics.

Whitepaper #048 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: solar physics; space-based missions/projects.

Whitepaper #361 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research; infrastructure/workforce/other programmatic. Additional topics: solar physics; research tools and infrastructure.