arXiv subjects

Jeffrey W. Reep

Publications and source records attributed to Jeffrey W. Reep.

35 records · Page 2Linked to original sources

arXiv2020

Simulating Solar Flare Irradiance with Multithreaded Models of Flare Arcades

Understanding how energy is released in flares is one of the central problems of solar and stellar astrophysics. Observations of high temperature flare plasma hold many potential clues as to the nature of this energy release. It is clear, however, that flares are not composed of a few impulsively heated loops, but are the result of heating on many small-scale threads that are energized over time, making it difficult to compare observations and numerical simulations in detail. Several previous studies have shown that it is possible to reproduce some aspects of the observed emission by considering the flare as a sequence of independently heated loops, but these studies generally focus on small-scale features while ignoring the global features of the flare. In this paper, we develop a multithreaded model that encompasses the time-varying geometry and heating rate for a series of successively-heated loops comprising an arcade. To validate, we compare with spectral observations of five flares made with the MinXSS CubeSat as well as light curves measured with GOES/XRS and SDO/AIA. We show that this model can successfully reproduce the light curves and quasi-periodic pulsations in GOES/XRS, the soft X-ray spectra seen with MinXSS, and the light curves in various AIA passbands. The AIA light curves are most consistent with long duration heating, but elemental abundances cannot be constrained with the model. Finally, we show how this model can be used to extrapolate to spectra of extreme events that can predict irradiance across a wide wavelength range including unobserved wavelengths.

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arXiv2020

Electron Beams Cannot Directly Produce Coronal Rain

Coronal rain is ubiquitous in flare loops, forming shortly after the onset of the solar flare. Rain is thought to be caused by a thermal instability, a localized runaway cooling of material in the corona. The models that demonstrate this require extremely long duration heating on the order of the radiative cooling time, localized near the footpoints of the loops. In flares, electron beams are thought to be the primary energy transport mechanism, driving strong footpoint heating during the impulsive phase that causes evaporation, filling and heating flare loops. Electron beams, however, do not act for a long period of time, and even supposing that they did, their heating would not remain localized at the footpoints. With a series of numerical experiments, we show directly that these two issues mean that electron beams are incapable of causing the formation of rain in flare loops. This result suggests that either there is another mechanism acting in flare loops responsible for rain, or that the modeling of the cooling of flare loops is somehow deficient. To adequately describe flares, the standard model must address this issue to account for the presence of coronal rain.

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arXiv2019

What determines the X-ray intensity and duration of a solar flare?

Solar flares are observed and classified according to their intensity measured with the GOES X-ray Sensors. We show that the duration of a flare, as measured by the full width at half maximum (FWHM) in GOES is not related to the size of the flare as measured by GOES intensity. The durations of X-class flares range from a few minutes to a few hours, and the same is true of M- and C-class flares. In this work, we therefore examine the statistical relationships between the basic properties of flares -- temperature, emission measure, energy, etc. -- in comparison to both their size and duration. We find that the size of the flare is directly related to all of these basic properties, as previously found by many authors. The duration is not so clear. When examining the whole data set, the duration appears to be independent of all of these properties. In larger flares, however, there are direct correlations between the GOES FWHM and magnetic reconnection flux and ribbon area. We discuss the possible explanations, finding that this discrepancy may be due to large uncertainties in small flares, though we cannot rule out the possibility that the driving physical processes are different in smaller flares than larger ones. We discuss the implications of this result and how it relates to the magnetic reconnection process that releases energy in flares.

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arXiv2018

Efficient Calculation of Non-Local Thermodynamic Equilibrium Effects in Multithreaded Hydrodynamic Simulations of Solar Flares

Understanding the dynamics of the solar chromosphere is crucial to understanding energy transport across the solar atmosphere. The chromosphere is optically thick at many wavelengths and described by non-local thermodynamic equilibrium (NLTE), making it difficult to interpret observations. Furthermore, there is considerable evidence that the atmosphere is filamented, and that current instruments do not sufficiently resolve small scale features. In flares, it is likely that multithreaded models are required to describe the heating. The combination of NLTE effects and multithreaded modeling requires computationally demanding calculations, which has motivated the development of a model that can efficiently treat both. We describe the implementation of a solver in a hydrodynamic code for the hydrogen level populations that approximates the NLTE solutions. We derive an accurate electron density across the atmosphere, that includes the effects of non-equilibrium ionization for helium and metals. We show the effects on hydrodynamic simulations, which are used to synthesize light curves using a post-processing radiative transfer code. We demonstrate the utility of this model on IRIS observations of a small flare. We show that the Doppler shifts in Mg II, C II, and O I can be explained with a multithreaded model of loops subjected to electron beam heating, so long as NLTE effects are treated. The intensities, however, do not match observed values very well, which is due to assumptions about the initial atmosphere. We briefly show how altering the initial atmosphere can drastically alter line profiles, and therefore derived quantities, and suggest that it should be tuned to pre-flare observations.

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arXiv2018

The Duration of Energy Deposition on Unresolved Flaring Loops in the Solar Corona

Solar flares form and release energy across a large number of magnetic loops. The global parameters of flares, such as the total energy released, duration, physical size, etc., are routinely measured, and the hydrodynamics of a coronal loop subjected to intense heating have been extensively studied. It is not clear, however, how many loops comprise a flare, nor how the total energy is partitioned between them. In this work, we employ a hydrodynamic model to better understand the energy partition by synthesizing Si IV and Fe XXI line emission and comparing to observations of these lines with IRIS. We find that the observed temporal evolution of the Doppler shifts holds important information on the heating duration. To demonstrate this we first examine a single loop model, and find that the properties of chromospheric evaporation seen in Fe XXI can be reproduced by loops heated for long durations, while persistent red-shifts seen in Si IV cannot be reproduced by any single loop model. We then examine a multi-threaded model, assuming both a fixed heating duration on all loops, and a distribution of heating durations. For a fixed heating duration, we find that durations of 100 -- 200 s do a fair job of reproducing both the red- and blue-shifts, while a distribution of durations, with a median of about 50 -- 100 s, does a better job. Finally, we compare our simulations directly to observations of an M-class flare seen by IRIS, and find good agreement between the modeled and observed values given these constraints.

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arXiv2017

A Hydrodynamic Model of Alfvénic Wave Heating in a Coronal Loop and its Chromospheric Footpoints

Alfvénic waves have been proposed as an important energy transport mechanism in coronal loops, capable of delivering energy to both the corona and chromosphere and giving rise to many observed features, of flaring and quiescent regions. In previous work, we established that resistive dissipation of waves (ambipolar diffusion) can drive strong chromospheric heating and evaporation, capable of producing flaring signatures. However, that model was based on a simplified assumption that the waves propagate instantly to the chromosphere, an assumption which the current work removes. Via a ray tracing method, we have implemented traveling waves in a field-aligned hydrodynamic simulation that dissipate locally as they propagate along the field line. We compare this method to and validate against the magnetohydrodynamics code Lare3D. We then examine the importance of travel times to the dynamics of the loop evolution, finding that (1) the ionization level of the plasma plays a critical role in determining the location and rate at which waves dissipate; (2) long duration waves effectively bore a hole into the chromosphere, allowing subsequent waves to penetrate deeper than previously expected, unlike an electron beam whose energy deposition rises in height as evaporation reduces the mean-free paths of the electrons; (3) the dissipation of these waves drives a pressure front that propagates to deeper depths, unlike energy deposition by an electron beam.

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arXiv2017

Spectroscopic Observations of Current Sheet Formation and Evolution

We report on the structure and evolution of a current sheet that formed in the wake of an eruptive X8.3 flare observed at the west limb of the Sun on September 10, 2017. Using observations from the EUV Imaging Spectrometer (EIS) on Hinode and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), we find that plasma in the current sheet reaches temperatures of about 20 MK and that the range of temperatures is relatively narrow. The highest temperatures occur at the base of the current sheet, in the region near the top of the post-flare loop arcade. The broadest high temperature line profiles, in contrast, occur at the largest observed heights. Further, line broadening is strong very early in the flare and diminishes over time. The current sheet can be observed in the AIA 211 and 171 channels, which have a considerable contribution from thermal bremsstrahlung at flare temperatures. Comparisons of the emission measure in these channels with other EIS wavelengths and AIA channels dominated by Fe line emission indicate a coronal composition and suggest that the current sheet is formed by the heating of plasma already in the corona. Taken together, these observations suggest that some flare heating occurs in the current sheet while additional energy is released as newly reconnected field lines relax and become more dipolar.

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arXiv2017

Science Objective: Understanding Energy Transport by Alfvénic Waves in Solar Flares

Solar flares are driven by the release of magnetic energy from reconnection events in the solar corona, whereafter energy is transported to the chromosphere, heating the plasma and causing the characteristic radiative losses. In the collisional thick-target model, electrons accelerated to energies exceeding 10 keV traverse the corona and impact the chromosphere, where they deposit their energy through collisions with the much denser plasma in the lower atmosphere. While there are undoubtedly high energy non-thermal electrons accelerated in flares, it is unclear whether these electron beams are the sole mechanism of energy transport, or whether they only dominate in certain phases of the flare's evolution. Alfvénic waves are generated during the post-reconnection relaxation of magnetic field lines, so it is important to examine their role in energy transport.

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arXiv2016

Properties and Modeling of Unresolved Fine Structure Loops Observed in the Solar Transition Region by IRIS

Recent observations from the Interface Region Imaging Spectrograph (IRIS) have discovered a new class of numerous low-lying dynamic loop structures, and it has been argued that they are the long-postulated unresolved fine structures (UFS) that dominate the emission of the solar transition region. In this letter, we combine IRIS measurements of the properties of a sample of 108 UFS (intensities, lengths, widths, lifetimes) with 1-D non-equilibrium ionization simulations using the HYDRAD hydrodynamic model to examine whether the UFS are now truly spatially resolved in the sense of being individual structures rather than composed of multiple magnetic threads. We find that a simulation of an impulsively heated single strand can reproduce most of the observed properties suggesting that the UFS may be resolved, and the distribution of UFS widths implies that they are structured on a spatial scale of 133km on average. Spatial scales of a few hundred km appear to be typical for a range of chromospheric and coronal structures, and we conjecture that this could be an important clue to the coronal heating process.

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arXiv2016

Transition Region and Chromospheric Signatures of Impulsive Heating Events. II. Modeling

Results from the Solar Maximum Mission showed a close connection between the hard X-ray and transition region emission in solar flares. Analogously, the modern combination of RHESSI and IRIS data can inform the details of heating processes in ways never before possible. We study a small event that was observed with RHESSI, IRIS, SDO, and Hinode, allowing us to strongly constrain the heating and hydrodynamical properties of the flare, with detailed observations presented in a previous paper. Long duration red-shifts of transition region lines observed in this event, as well as many other events, are fundamentally incompatible with chromospheric condensation on a single loop. We combine RHESSI and IRIS data to measure the energy partition among the many magnetic strands that comprise the flare. Using that observationally determined energy partition, we show that a proper multi-threaded model can reproduce these red-shifts in magnitude, duration, and line intensity, while simultaneously being well constrained by the observed density, temperature, and emission measure. We comment on the implications for both RHESSI and IRIS observations of flares in general, namely that: (1) a single loop model is inconsistent with long duration red-shifts, among other observables; (2) the average time between energization of strands is less than 10 seconds, which implies that for a hard X-ray burst lasting ten minutes, there were at least 60 strands within a single IRIS pixel located on the flare ribbon; (3) the majority of these strands were explosively heated with energy distribution well described by a power law of slope $\approx -1.6$; (4) the multi-stranded model reproduces the observed line profiles, peak temperatures, differential emission measure distributions, and densities.

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arXiv2016

Transition Region and Chromospheric Signatures of Impulsive Heating Events. I. Observations

We exploit the high spatial resolution and high cadence of the Interface Region Imaging Spectrograph (IRIS) to investigate the response of the transition region and chromosphere to energy deposition during a small flare. Simultaneous observations from RHESSI provide constraints on the energetic electrons precipitating into the flare footpoints while observations of XRT, AIA, and EIS allow us to measure the temperatures and emission measures from the resulting flare loops. We find clear evidence for heating over an extended period on the spatial scale of a single IRIS pixel. During the impulsive phase of this event the intensities in each pixel for the Si IV 1402.770, C II 1334.535, Mg II 2796.354 and O I 1355.598 emission lines are characterized by numerous, small-scale bursts typically lasting 60s or less. Red shifts are observed in Si IV, C II, and Mg II during the impulsive phase. Mg II shows red-shifts during the bursts and stationary emission at other times. The Si IV and C II profiles, in contrast, are observed to be red-shifted at all times during the impulsive phase. These persistent red-shifts are a challenge for one-dimensional hydrodynamic models, which predict only short-duration downflows in response to impulsive heating. We conjecture that energy is being released on many small-scale filaments with a power-law distribution of heating rates.

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arXiv2016

Amended results for hard X-ray emission by non-thermal thick target recombination in solar flares

Brown & Mallik 2008 and the Brown et al. 2010 corrigendum of it presented expressions for non-thermal recombination (NTR) in the collisionally thin- and thick-target regimes, claiming that the process could account for a substantial part of hard X-ray continuum in solar flares usually attributed entirely to thermal and non-thermal bremsstrahlung (NTB). However, we have found the thick-target expression to become unphysical for low cut-offs in the injected electron energy spectrum. We trace this to an error in the derivation, derive a corrected version which is real-valued and continuous for all photon energies and cut-offs, and show that, for thick targets, Brown et al. over-estimated NTR emission at small photon energies. The regime of small cut-offs and large spectral indices involve large (reducing) correction factors but in some other thick-target parameter regimes NTR/NTB can still be of order unity. We comment on the importance of these results to flare and to microflare modeling and spectral fitting. An empirical fit to our results shows that the peak NTR contribution comprises over half the hard X-ray signal if delta > 6 (E_0c/4 keV)^(0.4).

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arXiv2016

Alfvénic Wave Heating of the Upper Chromosphere in Flares

We have developed a numerical model of flare heating due to the dissipation of Alfvénic waves propagating from the corona to the chromosphere. With this model, we present an investigation of the key parameters of these waves on the energy transport, heating, and subsequent dynamics. For sufficiently high frequencies and perpendicular wave numbers, the waves dissipate significantly in the upper chromosphere, strongly heating it to flare temperatures. This heating can then drive strong chromospheric evaporation, bringing hot and dense plasma to the corona. We therefore find three important conclusions: (1) Alfvénic waves, propagating from the corona to the chromosphere, are capable of heating the upper chromosphere and the corona, (2) the atmospheric response to heating due to the dissipation of Alfvénic waves can be strikingly similar to heating by an electron beam, and (3) this heating can produce explosive evaporation.

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arXiv2015

X-ray Source Heights in a Solar Flare: Thick-target versus Thermal Conduction Front Heating

Observations of solar flares with RHESSI have shown X-ray sources traveling along flaring loops, from the corona down to the chromosphere and back up. The 28 November 2002 C1.1 flare, first observed with RHESSI by Sui et al. 2006 and quantitatively analyzed by O'Flannagain et al. 2013, very clearly shows this behavior. By employing numerical experiments, we use these observations of X-ray source height motions as a constraint to distinguish between heating due to a non-thermal electron beam and in situ energy deposition in the corona. We find that both heating scenarios can reproduce the observed light curves, but our results favor non-thermal heating. In situ heating is inconsistent with the observed X-ray source morphology and always gives a height dispersion with photon energy opposite to what is observed.

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arXiv2015

Flare Footpoint Regions and a Surge Observed by the Hinode/EUV Imaging Spectrometer (EIS), RHESSI, and SDO/AIA

The Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft observed flare footpoint regions coincident with a surge for a M3.7 flare observed on 25 September 2011 at N12 E33 in active region 11302. The flare was observed in spectral lines of O VI, Fe X, Fe XII, Fe XIV, Fe XV, Fe XVI, Fe XVII, Fe XXIII and Fe XXIV. The EIS observations were made coincident with hard X-ray bursts observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Overlays of the RHESSI images on the EIS raster images at different wavelengths show a spatial coincidence of features in the RHESSI images with the EIS upflow and downflow regions, as well as loop-top or near-loop-top regions. A complex array of phenomena was observed including multiple evaporation regions and the surge, which was also observed by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) telescopes. The slit of the EIS spectrometer covered several flare footpoint regions from which evaporative upflows in Fe XXIII and Fe XXIV lines were observed with Doppler speeds greater than 500 km s$^{-1}$. For ions such as Fe XV both evaporative outflows (~200 km s$^{-1}$) and downflows (~30-50 km s$^{-1}$) were observed. Non-thermal motions from 120 to 300 km s$^{-1}$ were measured in flare lines. In the surge, Doppler speeds are found from about 0 to over 250 km s$^{-1}$ in lines from ions such as Fe XIV. The non-thermal motions could be due to multiple sources slightly Doppler-shifted from each other or turbulence in the evaporating plasma. We estimate the energetics of the hard X-ray burst and obtain a total flare energy in accelerated electrons of $\geq7\times10^{28}$ ergs. This is a lower limit because only an upper limit can be determined for the low energy cutoff to the electron spectrum. We find that detailed modeling of this event would require a multi-threaded model due to its complexity.

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arXiv2013

Diagnosing the time-dependence of active region core heating from the emission measure: II. Nanoflare trains

The time-dependence of heating in solar active regions can be studied by analyzing the slope of the emission measure distribution cool-ward of the peak. In a previous study we showed that low-frequency heating can account for 0% to 77% of active region core emission measures. We now turn our attention to heating by a finite succession of impulsive events for which the timescale between events on a single magnetic strand is shorter than the cooling timescale. We refer to this scenario as a "nanoflare train" and explore a parameter space of heating and coronal loop properties with a hydrodynamic model. Our conclusions are: (1) nanoflare trains are consistent with 86% to 100% of observed active region cores when uncertainties in the atomic data are properly accounted for; (2) steeper slopes are found for larger values of the ratio of the train duration $Δ_H$ to the post-train cooling and draining timescale $Δ_C$, where $Δ_H$ depends on the number of heating events, the event duration and the time interval between successive events ($τ_C$); (3) $τ_C$ may be diagnosed from the width of the hot component of the emission measure provided that the temperature bins are much smaller than 0.1 dex; (4) the slope of the emission measure alone is not sufficient to provide information about any timescale associated with heating - the length and density of the heated structure must be measured for $Δ_H$ to be uniquely extracted from the ratio $Δ_H/Δ_C$.

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arXiv2012

Diagnosing the time-dependence of active region core heating from the emission measure: I. Low-frequency nanoflares

Observational measurements of active region emission measures contain clues to the time-dependence of the underlying heating mechanism. A strongly non-linear scaling of the emission measure with temperature indicates a large amount of hot plasma relative to warm plasma. A weakly non-linear (or linear) scaling of the emission measure indicates a relatively large amount of warm plasma, suggesting that the hot active region plasma is allowed to cool and so the heating is impulsive with a long repeat time. This case is called {\it low-frequency} nanoflare heating and we investigate its feasibility as an active region heating scenario here. We explore a parameter space of heating and coronal loop properties with a hydrodynamic model. For each model run, we calculate the slope $α$ of the emission measure distribution $EM(T) \propto T^α$. Our conclusions are: (1) low-frequency nanoflare heating is consistent with about 36% of observed active region cores when uncertainties in the atomic data are not accounted for; (2) proper consideration of uncertainties yields a range in which as many as 77% of observed active regions are consistent with low-frequency nanoflare heating and as few as zero; (3) low-frequency nanoflare heating cannot explain observed slopes greater than 3; (4) the upper limit to the volumetric energy release is in the region of 50 erg cm$^{-3}$ to avoid unphysical magnetic field strengths; (5) the heating timescale may be short for loops of total length less than 40 Mm to be consistent with the observed range of slopes; (6) predicted slopes are consistently steeper for longer loops.

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