arXiv subjects

Jeffrey Reep

Publications and source records attributed to Jeffrey Reep.

5 recordsLinked to original sources

arXiv2026

Chromospheric dynamics and turbulence regulate the solar FIP effect

Elemental abundance variations in the solar corona, commonly characterised by First Ionisation Potential (FIP) bias, provide crucial diagnostics of chromospheric processes. The ponderomotive force model, which attributes fractionation to Alfv\'en wave propagation, has successfully reproduced the observed fractionation patterns in various solar features. However, existing theoretical implementations rely on a static quiet Sun chromosphere, leaving the influence of chromospheric dynamics largely unexplored. We address this limitation by combining hydrodynamic simulations from HYDRAD with ponderomotive force calculations through FIPpy, a new open-source code. Comparing predictions between an initial VAL-C chromosphere and a heated chromosphere following impulsive nanoflare-like events, we show that the ponderomotive force model remains consistent under dynamic chromospheric conditions, while stronger changes in fractionation behaviour arise from variations in acoustic flux and turbulence. Most significantly, when acoustic wave flux drops below $\sim5\times10^6$ erg cm$^{-2}$ s$^{-1}$, mass-dependent thermal velocities dominate the fractionation process, producing counterintuitive patterns where Fe exceeds Ca in FIP bias, while high-FIP Ar shows fractionation. We demonstrate that any source of chromospheric turbulence will act to suppress fractionation. For flares, our results predict that the increased turbulence will suppress FIP bias, potentially explaining the observed abundance variations during flares. These findings suggest that coronal abundances and composition encode a sensitive balance between dominant mechanisms, determined by the ratio of ponderomotive acceleration to turbulent velocity.

astro-ph.SR↗

arXiv2025

Systematic non-thermal velocity increase preceding soft X-ray flare onset: A large-scale Hinode/EIS study

Non-thermal velocities, derived from spectral line broadening, can provide crucial insights into plasma dynamics before and during solar flares. To systematically study the pre-flare phase, we constructed a Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS) flare catalog of 1,449 flares from 2011--2024. This enabled a large-scale analysis of flare loop footpoint non-thermal velocity evolution across different flare magnitudes (C, M, X-classes). Analyzing Fe VIII--Fe XXIV emission lines formed at $\log(T/K) \sim 5.7-7.3$ with piecewise linear fits in the pre-flare period, we find that non-thermal velocities consistently increase 4--25 minutes before GOES soft X-ray start in C and M-class flares. Onset timing patterns vary with flare magnitude: smaller flares show temperature-dependent progression, while larger flares exhibit more compressed, near-simultaneous onsets across temperatures. While our limited X-class sample ($N=18$) also show onset before GOES, larger statistics are needed to confirm its behavior. M-class flares show a systematic precursor non-thermal velocity onset $\sim$30--60 minutes before GOES peak. In a subset of M-class flares (2011--2018), CME-associated events show earlier and more uniform precursor onsets (45--74 minutes before peak) than non-CME events, of which only some lines display a precursor, suggesting a strong link between extended pre-flare non-thermal broadening and successful eruptions. This large scale study establishes pre-flare non-thermal velocity increase at footpoints as a common precursor observable before any X-ray signature.

astro-ph.SR↗

arXiv2023

Fundamentals of impulsive energy release in the corona

It is essential that there be coordinated and co-optimized observations in X-rays, gamma-rays, and EUV during the peak of solar cycle 26 (~2036) to significantly advance our understanding of impulsive energy release in the corona. The open questions include: What are the physical origins of space-weather events? How are particles accelerated at the Sun? How is impulsively released energy transported throughout the solar atmosphere? How is the solar corona heated? Many of the processes involved in triggering, driving, and sustaining solar eruptive events -- including magnetic reconnection, particle acceleration, plasma heating, and energy transport in magnetized plasmas -- also play important roles in phenomena throughout the Universe. This set of observations can be achieved through a single flagship mission or, with foreplanning, through a combination of major missions (e.g., the previously proposed FIERCE mission concept).

astro-ph.IM↗

arXiv2015

Optimal Electron Energies for Driving Chromospheric Evaporation in Solar Flares

In the standard model of solar flares, energy deposition by a beam of electrons drives strong chromospheric evaporation leading to a significantly denser corona and much brighter emission across the spectrum. Chromospheric evaporation was examined in great detail by Fisher, Canfield, & McClymont (1985a,b,c), who described a distinction between two different regimes, termed explosive and gentle evaporation. In this work, we examine the importance of electron energy and stopping depths on the two regimes and on the atmospheric response. We find that with explosive evaporation, the atmospheric response does not depend strongly on electron energy. In the case of gentle evaporation, lower energy electrons are significantly more efficient at heating the atmosphere and driving up-flows sooner than higher energy electrons. We also find that the threshold between explosive and gentle evaporation is not fixed at a given beam energy flux, but also depends strongly on the electron energy and duration of heating. Further, at low electron energies, a much weaker beam flux is required to drive explosive evaporation.

astro-ph.SR↗

arXiv2013

On the sensitivity of the GOES flare classification to properties of the electron beam in the thick target model

The collisional thick target model, wherein a large number of electrons are accelerated down a flaring loop, can be used to explain many observed properties of solar flares. In this study, we focus on the sensitivity of GOES flare classification to the properties of the thick target model. Using a hydrodynamic model with RHESSI-derived electron beam parameters, we explore the effects of the beam energy flux (or total non-thermal energy), the cut-off energy, and the spectral index of the electron distribution on the soft X-rays (SXRs) observed by GOES. We conclude that (1) the GOES class is proportional to the non-thermal energy $E^α$ for $α\approx 1.7$ in the low energy passband (1-8 Å) and $\approx 1.6$ in the high energy passband (0.5-4 Å); (2) the GOES class is only weakly dependent on the spectral index in both passbands; (3) increases in the cut-off will increase the flux in the 0.5-4 Å passband, but decrease the flux in the 1-8 Å passband, while decreases in the cut-off will cause a decrease in the 0.5-4 Å passband and a slight increase in the 1-8 Å passband.

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