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

TL;DR

The paper investigates two rapid downflow episodes in the X9-class flare of 2024-10-03 using IRIS Si IV spectroscopy, complemented by hard X-ray, Ly-α, and magnetic-flux data. It demonstrates a first stage tied to impulsive energy release and chromospheric condensation, and a second stage consistent with flare-induced coronal rain, despite near-zero reconnection activity. A persistent ~50 s quasi-periodicity in Doppler velocities across both stages suggests an underlying MHD oscillation mechanism, possibly a magnetic tuning fork, rather than bursty reconnection. Machine-learning clustering further reveals evolving, complex line-profile behavior, underscoring rich sub-structure in flare ribbon dynamics and highlighting future multi-slit observations for disentangling the driving processes.

Abstract

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.

Figures (6)

• Figure 1: Overview of the X9.0 flare observed on 2024 October 3rd: Top row: SJI 1330 Å snapshots (full FOV) of the flare ribbon evolution. The first four snapshots are sequentially 90 seconds apart during the impulsive phase, and final snapshot later in the gradual phase. The vertical black line is the location of the IRIS slit. The horizontal dashed line marks the lower extent of the FOV of subsequent panels. Second row: Sit-and-stare Si4 1402.77 Å intensity map (along a sub-range of the slit), computed by line integration. The dark green line outlines the southern flare ribbon mask. Green circles highlight high redshift pixels examined in Figure \ref{['fig:spectra']}. Third row: Same as above, showing Si4 1402.77 Å Doppler velocity determined from line moment centroid. Fourth row: Progression of IRIS Si4 1402.77 Å exposure time throughout the raster, alongside the number of saturated pixels per time-step. Fifth row: Reconnection flux rate (magnitude) for the southern (AIA 1600 Å and SJI 1330 Å) and northern (AIA 1600 Å) flare ribbons). Bottom row: Time-series of GOES 0.5--4 Å soft X-rays, and ASO-S HXI 10--20, 20--50, 50--100, 100--300 keV hard X-rays. ASO-S HXI measurements are multiplied by 1e--8 for comparison to GOES. Vertical dashed gray lines mark the location of SJI snapshots in the top row.
• Figure 2: A: Max and mean ($\times5$) Si4 1402.77 Å Doppler velocity across ribbon mask. Black line marks smoothed time series. B: Max and mean Si4 1402.77 Å intensity across ribbon mask. Black line marks smoothed time series. C: ASO-S HXI HXR time series. D: LYRA Lyman-$\alpha$ time series. Vertical dashed lines mark the locations of local peaks in the smoothed maximum Si4 Doppler velocity (black line). We use yellow and blue background shading to highlight our two stages of rapid Si4 Doppler velocities -- the impulsive ($\sim$12:15--12:19 UT) and non-impulsive ($\sim$12:23--12:28 UT) phases.
• Figure 3: Top: 120-second detrended Si4 Doppler velocity within the southern flare ribbon (detrended version of the red maximum velocity curve in Figure \ref{['fig:time_series']}. Bottom-left: Wavelet power spectrum of time-series plotted in top panel (on a log color scale). Orange lines mark the cone of influence. Bottom-right: Global wavelet power for the same dataset. Red line marks the 95% confidence threshold.
• Figure 4: Top: Hard X-ray spectra (black markers) from ASO-S/HXI, within the time windows of 12:15:00--12:15:30 UT (left panel) and 12:25:16--12:25:46 UT (right panel). This time periods encapsulate part of our two stages of interest, with the former focusing on the peak of the HXR emission, and latter centered around the strongest Si4 downflow observed in the later stage. In the second panel, we label in the data the locations of residual background emission from $^{241}$Am (see text), and the residual background noise level. The red curve in each panel presents the total spectral fit, consisting of two isothermal components and a non-thermal component (cyan, blue and orange, respectively), with key parameters as noted in the legend; a photospheric albedo correction (gray curve) is also included. Bottom: The fit residuals (the differences between black markers and red curves) normalized by respective uncertainties in each energy bin.
• Figure 5: Si4 1402.77 Å line profile and triple-Gaussian fit for a selection of the fastest red shift pixels across the sit-and-stare raster (plotted as green circles in Figure \ref{['fig:IRIS_context']}). Black line plots the spectral data (with error bars), green the sum of the triple Gaussian fit, and yellow/orange/red the individual Gaussian fit profiles. Pixel position and velocity of the red-most Gaussian are noted in the top-right corner of each panel.