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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Nanoflare Diagnostics from Magnetohydrodynamic Heating Profiles  

Jeffrey Reep   Submitted: 2020-09-03 15:08

The nanoflare paradigm of coronal heating has proven extremely promising for explaining the presence of hot, multi-million degree loops in the solar corona. In this paradigm, localized heating events supply enough energy to heat the solar atmosphere to its observed temperatures. Rigorously modeling this process, however, has proven difficult, since it requires an accurate treatment of both the magnetic field dynamics and reconnection as well as the plasma's response to magnetic perturbations. In this paper, we combine fully 3D magnetohydrodynamic (MHD) simulations of coronal active region plasma driven by photospheric motions with spatially-averaged, time-dependent hydrodynamic (HD) modeling of coronal loops to obtain physically motivated observables that can be quantitatively compared with observational measurements of active region cores. We take the behavior of reconnected field lines from the MHD simulation and use them to populate the HD model to obtain the thermodynamic evolution of the plasma and subsequently the emission measure distribution. We find the that the photospheric driving of the MHD model produces only very low-frequency nanoflare heating which cannot account for the full range of active region core observations as measured by the low-temperature emission measure slope. Additionally, we calculate the spatial and temporal distributions of field lines exhibiting collective behavior, and argue that loops occur due to random energization occurring on clusters of adjacent field lines.

Authors: Kalman J. Knizhnik, Will T. Barnes, Jeffrey W. Reep, Vadim M. Uritsky
Projects: None

Publication Status: Published in ApJ
Last Modified: 2020-09-05 01:05
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Simulating Solar Flare Irradiance with Multithreaded Models of Flare Arcades  

Jeffrey Reep   Submitted: 2020-03-25 16:05

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.

Authors: Jeffrey W. Reep, Harry P. Warren, Christopher S. Moore, Crisel Suarez, Laura A. Hayes
Projects: None

Publication Status: Under review by ApJ
Last Modified: 2020-03-26 12:25
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The Distribution of Time Delays Between Nanoflares in Magnetohydrodynamic Simulations  

Jeffrey Reep   Submitted: 2020-02-13 08:53

Nanoflares are thought to be an important energy source for heating the solar corona. The exact amount of heating contributed by nanoflares depends, in part, on the frequency of the heating and the time interval between nanoflares. Several numerical models have attempted to constrain the frequency of the heating by fitting to observed emission measures. To date, however, no physically motivated value for the time interval between nanoflares has been obtained. In this paper, we calculate a physically motivated distribution of time intervals between successive "nanoflare" reconnection events in driven magnetohydrodynamic simulations. We show that this distribution follows a power law with a slope near -1, much shallower than previously inferred from observations and determined from loop model comparisons with observations. We show that the energy flux injected into the corona in our model is of order 107 erg/cm2/s, and that the heating rate due to these reconnection events reaches values of order 1 - 10 erg/cm3/s. Additionally, we show that this power law slope is dependent only weakly on the amount of magnetic helicity injected into the solar magnetic field by the photospheric motions, but the rate of reconnection is about 45% higher if no magnetic helicity is injected.

Authors: Kalman J. Knizhnik & Jeffrey W. Reep
Projects: None

Publication Status: Published in Solar Physics
Last Modified: 2020-02-13 11:50
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What determines the X-ray intensity and duration of a solar flare?  

Jeffrey Reep   Submitted: 2019-03-27 14:00

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.

Authors: Jeffrey W. Reep & Kalman J. Knizhnik
Projects: None

Publication Status: In press, ApJ
Last Modified: 2019-03-29 03:16
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On the Synthesis of GOES Light Curves from Numerical Models  

Jeffrey Reep   Submitted: 2018-06-26 08:48

Numerical simulations of solar flares are often used to produce synthetic GOES light curves that can be compared against data (e.g., Warren 2006; Reep & Toriumi 2017; Zhu et al. 2018). There are a few different methods that have been used in the past to synthesize these light curves, but the literature does not generally discuss advantages and disadvantages of each method. In this note, we briefly discuss three different methods that can be used.

Authors: Jeffrey W. Reep and Harry P. Warren
Projects: None

Publication Status: Published in RNAAS (unrefereed)
Last Modified: 2018-06-26 11:51
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Efficient Calculation of Non-Local Thermodynamic Equilibrium Effects in Multithreaded Hydrodynamic Simulations of Solar Flares  

Jeffrey Reep   Submitted: 2018-06-26 08:45

Understanding the dynamics of the solar chromosphere is crucial to understanding the transport of energy across the atmosphere, especially in impulsive heating events. The chromosphere is optically thick and described by non-local thermodynamic equilibrium (NLTE), often making observations difficult to interpret. There is also considerable evidence that the atmosphere is filamented and that current instruments do not sufficiently resolve small scale features. In flares, for example, it is likely that multithreaded models are required to describe and understand the heating process. 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 chromosphere and corona that includes the effects of non-equilibrium ionization for helium and metals. We show the effects of this solver on simulations, which we then use to synthesize light curves and Doppler shifts of spectral lines, with a post-processing radiative transfer code. We demonstrate the utility of this model on multithreaded simulations, where we simulate IRIS observations of a small flare. We show that observed velocities in Mg II, C II, and O I can be explained with a multithreaded model of loops subject to electron beam heating, so long as NLTE effects are treated. The synthesized intensities, however, do not match observed ones very well, which we suggest is primarily due to assumptions about the initial atmosphere. We briefly show how altering the initial atmosphere can drastically alter line profiles and derived quantities, and suggest that it should be tuned to preflare observations for better agreement.

Authors: Jeffrey W. Reep, Stephen J. Bradshaw, Nicholas A. Crump, Harry P. Warren
Projects: None

Publication Status: Submitted to ApJ
Last Modified: 2018-06-26 11:52
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The Duration of Energy Deposition on Unresolved Flaring Loops in the Solar Corona  

Jeffrey Reep   Submitted: 2018-02-27 09:15

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.

Authors: Jeffrey W. Reep, Vanessa Polito, Harry P. Warren, Nicholas A. Crump
Projects: None

Publication Status: Accepted to ApJ
Last Modified: 2018-02-28 14:00
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A Hydrodynamic Model of Alfvénic Wave Heating in a Coronal Loop and its Chromospheric Footpoints  

Jeffrey Reep   Submitted: 2017-12-19 07:50

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.

Authors: Jeffrey W. Reep, Alexander J.B. Russell, Lucas A. Tarr, & James E. Leake
Projects: None

Publication Status: Accepted to ApJ
Last Modified: 2017-12-20 10:33
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The direct relation between the duration of magnetic reconnection and the evolution of GOES light curves in solar flares  

Jeffrey Reep   Submitted: 2017-11-06 08:09

GOES soft X-ray light curves are used to measure the timing and duration of solar flare emission. The timing and duration of the magnetic reconnection and subsequent energy release which drives solar flares are unknown, though the light curves are presumably related. It is therefore critical to understand the physics which connects the two: how does the time scale of reconnection produce an observed GOES light curve? In this work, we model the formation and expansion of an arcade of loops with a hydrodynamic model, which we then use to synthesize GOES light curves. We calculate the FWHM and the e-folding decay time of the light curves and compare them to the separation of the centroids of the two ribbons which the arcade spans, which is representative of the size scale of the loops. We reproduce a linear relation between the two, as found observationally in previous work. We show that this demonstrates a direct connection between the duration of energy release and the evolution of these light curves. We also show that the cooling processes of individual loops comprising the flare arcade directly affect the measured time scales. From the clear consistency between the observed and modeled linearity, we conclude that the primary factors that control the flare time scales are the duration of reconnection and the loop lengths.

Authors: Jeffrey W. Reep & Shin Toriumi
Projects: GOES X-rays

Publication Status: Accepted to ApJ
Last Modified: 2017-11-06 11:34
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Transition Region and Chromospheric Signatures of Impulsive Heating Events. II. Modeling  

Jeffrey Reep   Submitted: 2016-08-08 06:29

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 ≈-1.6; (4) the multi-stranded model reproduces the observed line profiles, peak temperatures, differential emission measure distributions, and densities.

Authors: Jeffrey W. Reep, Harry P. Warren, Nicholas A. Crump, Paulo J.A. Simões
Projects: IRIS,RHESSI

Publication Status: Accepted to ApJ
Last Modified: 2016-08-10 16:05
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Amended results for hard X-ray emission by non-thermal thick target recombination in solar flares  

Jeffrey Reep   Submitted: 2016-04-18 07:50

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 δ ≳ 6 (E0c/4 keV)0.4.

Authors: Reep, J.W. & Brown, J.C.
Projects: RHESSI

Publication Status: Accepted to ApJ
Last Modified: 2016-04-18 21:09
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Alfvénic Wave Heating of the Upper Chromosphere in Flares  

Jeffrey Reep   Submitted: 2016-01-11 11:08

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.

Authors: Reep, J.W., & Russell, A.J.B.
Projects: None

Publication Status: Accepted to ApJL
Last Modified: 2016-01-13 12:46
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X-ray Source Heights in a Solar Flare: Thick-target versus Thermal Conduction Front Heating  

Jeffrey Reep   Submitted: 2016-01-04 07:31

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.

Authors: Reep, J.W., Bradshaw, S.J., Holman, G.D.
Projects: RHESSI

Publication Status: Accepted to ApJ
Last Modified: 2016-01-06 10:49
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Optimal Electron Energies for Driving Chromospheric Evaporation in Solar Flares  

Jeffrey Reep   Submitted: 2015-07-06 07:38

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.

Authors: Reep, J.W., Bradshaw, S.J., & Alexander, D.
Projects: None

Publication Status: Accepted to ApJ
Last Modified: 2015-07-06 13:04
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On the sensitivity of the GOES flare classification to properties of the electron beam in the thick target model  

Jeffrey Reep   Submitted: 2013-10-16 13:13

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 for in the low energy passband (1-8 Å) and in the high energy passband (0.5-4 AA); (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.

Authors: Reep, J.W., Bradshaw, S.J., McAteer R.T.J.
Projects: None

Publication Status: ApJ (accepted)
Last Modified: 2013-10-16 18:26
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Abstracts by Author
Nanoflare Diagnostics from Magnetohydrodynamic Heating Profiles
Simulating Solar Flare Irradiance with Multithreaded Models of Flare Arcades
The Distribution of Time Delays Between Nanoflares in Magnetohydrodynamic Simulations
What determines the X-ray intensity and duration of a solar flare?
On the Synthesis of GOES Light Curves from Numerical Models
Efficient Calculation of Non-Local Thermodynamic Equilibrium Effects in Multithreaded Hydrodynamic Simulations of Solar Flares
The Duration of Energy Deposition on Unresolved Flaring Loops in the Solar Corona
A Hydrodynamic Model of Alfv?nic Wave Heating in a Coronal Loop and its Chromospheric Footpoints
The direct relation between the duration of magnetic reconnection and the evolution of GOES light curves in solar flares
Transition Region and Chromospheric Signatures of Impulsive Heating Events. II. Modeling
Amended results for hard X-ray emission by non-thermal thick target recombination in solar flares
Alfv?nic Wave Heating of the Upper Chromosphere in Flares
X-ray Source Heights in a Solar Flare: Thick-target versus Thermal Conduction Front Heating
Optimal Electron Energies for Driving Chromospheric Evaporation in Solar Flares
On the sensitivity of the GOES flare classification to properties of the electron beam in the thick target model

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