analytical and numerical modeling of coronal supra-arcade...

Roger B. ScottDepartment of Physics

Montana State University

Bozeman, MT 59717, USA

Phone: +1 406 994 6718

E-mail: [email protected]

URL: solar.physics.montana.edu/rscott

Selection CommitteeJack Eddy Postdoctoral Fellowship ProgramVisiting Scientist ProgramUniversity Corporation for Atmospheric Research

Dear members of the selection committee, January 14, 2016

I am writing to submit my application for the Jack Eddy Postdoctoral Fellowship. My application issupported by Professor Marco Velli at the University of California, Las Angeles.

As a graduate student in solar physics at Montana State University I have spent the last several yearsstudying magnetohydrodynamics (MHD) in the context of solar flares and coronal dynamics. In Octoberof 2015 I defended my dissertation, entitled Analytical and Numerical Modeling of Coronal Supra-Arcade FanStructures. I am currently awaiting conferment of my Ph.D., which is anticipated in April of 2016, and Iam now pursuing a postdoctoral position related to plasma physics in the solar corona.

My past research involves observations and modeling of supra-arcade fan structures, with a particularfocus on disambiguating observational features through theoretical and numerical modeling. To date Ihave focused on the apparent motion of plasma in the plane of the supra-arcade slab. This has been afruitful approach since supra-arcade structures appear to be nearly two-dimensional and can therefore betreated without consideration of variability along the line of sight. However, as it is broadly accepted thatthese supra-arcade structures are coincident with a current layer, I hope in future work to incorporate thedynamics of the current sheet itself and to treat these structures self consistently in three dimensions.

It is for this reason that Professor Velli and I have chosen to draft this application together. Wheremy expertise is in the non-di�usive regime of MHD, his is directly related to current sheet stability and theonset of fast reconnection. It is our hope that through our combined e�orts we can form a more completepicture of the dynamics of reconnecting current layers and the surrounding plasma.

Please find attached a detailed description of our intended avenue of research, as well as supportingmaterial for this application.

Kind Regards,

Roger B. Scott

Roger B. Scott Curriculum Vitae

Department of Physics Mobile: +1 808 652 2167Montana State University O�ce: +1 406 994 6718Bozeman, MT, 59717, United States E-mail: [email protected]

Contact

Montana State University, Bozeman, Montana, United StatesEducation

PhD - Physics – awaiting conferment: Spring 2016

M.S. - Physics – 2010

Reed College, Portland, Oregon, United States

B.A. - Physics – 2006

Montana State University, Bozeman, Montana, United StatesResearchExperience Solar Physics Group 2011 – Present

Graduate Research Assistant: Working with Professors David E. McKenzie and DanaW. Longcope to observe and model the dynamics of Supra-Arcade Fan Structures.

• Developed 2D analytical model of supra-arcade downflows in zero-� limit• Developed 2D non-di↵usive MHD simulation of supra-arcade downflows with arbitraryplasma �

• Developed data-assimilated model for evolution of supra-arcade fan structures with ap-plication to estimating plasma � and radiated Alfven spectrum

Montana State University, Bozeman, Montana, United StatesApplied Optics 2008 – 2011

Graduate Research Assistant: Worked with Professor Alan Craig on trapping and char-acterization of silicon nano-spheres for application to quantum memory systems.

• Developed interferometric optical trap using high-power parabolic reflector• Characterized dielectric potential and synthesize silicon nano-spheres

Skills : Linux System Administration, Hardware Assembly & ReplacementComputingLanguages : IDL, Octave, LATEX, PythonEnvironments : Emacs, VI, Linux, Unix, BSDFocus : PDE Solvers (Initial Value, Shooting, Root Finding, Stability, Convergence)

Scott, Roger B.; Longcope, Dana W.; McKenzie, David E., Peristaltic Pumping near Post -PublicationsCoronal Mass Ejection Supra-Arcade Current Sheets ApJ 776, 54 (2013).

Scott, Roger B., Gyroscopically Coupled Mechanical Oscillators, Reed College Library, ThesisArchive (2006)

Scott, Roger B.; McKenzie, David E.; Longcope, Dana W., Inferring the MagnetohydrodynamicManuscripts inPreparation Structure of Solar Flare Supra-Arcade Plasmas from a Data Assimilated Field Transport

Model, (submitted to ApJ, Dec. 2015)

Scott, Roger B.; McKenzie, David E.; Longcope, Dana W., Numerical Simulations of PlasmaDynamics in the Vicinity of a Retracting Flux Tube, (submitting to ApJ, Spring 2016)

Scott, Roger B., Analytical and Numerical Modeling of Coronal Supra-Arcade Fan Structures,Dissertation(UMI publishing, 2016), preprint

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Observations and MHD Modeling of Supra-Arcade Fan Structures – talkPosters andPresentations Ninth Annual Hinode Science Meeting, Belfast, North Ireland, UK, 2015

On the Magnetohydrodynamics of Supra-Arcade Fan Structures – talkTriennial Earth-Sun Summit / SPD, Indianapolis, Indiana, USA, 2015

Inferring Magnetic Evolution in Supra-Arcade Fan Structures – posterCombined Hinode VIII, LWS, SDO, IRIS Workshop, Portland, Oregon, USA, 2014

Inferring the Evolution of Supra-Arcade Magnetic Fields – talkRelativity and Astrophysics Colloquium, Bozeman, Montana, USA, 2014

Advection of Magnetic Field Lines in Supra-Arcade Fan Structures – posterAnnual Meeting of the AAS: SPD, Boston, Massachusetts, USA, 2014

Peristaltic Pumping near Post-CME Supra-Arcade Current Sheets – talkAnnual Meeting of the AAS: SPD, Bozeman, Montana, USA, 2013

Peristaltic Pumping in Post-CME Supra-Arcade Current Sheets – posterCoronal Loops Workshop VI, La Roche-en-Ardenne, Belgium, 2013

Peristaltic Shocks in Post-CME Unreconnected Field – posterSixth Annual Hinode Science Meeting, Saint Andrews, Scotland, UK, 2012

Nozzle Driven Shocks in Post-CME Plasma – posterAnnual Meeting of the AAS: SPD, Anchorage, Alaska, USA, 2012

SPD Studentship Travel Award (2014)Honors andAwards

Sixth Coronal Loops Workshop Student Travel Award (2013)

Montana State University, Bozeman, Montana, United StatesTeachingExperience Department of Physics

Introductory Physics Course Instructor Summer, 2011

• Developed curriculum and course schedule• Prepared and delivered daily course lectures• Supervised graduate teaching assistants• Graded bi-weekly examinations

Introductory Physics Graduate Teaching Assistant 2008 – 2009

• Supervised introductory physics laboratory experiments and inquiry sessions• Graded daily work assignments and quarterly examinations

Reed College, Portland, Oregon, United StatesPhysics Department

Department Associate / Physics 100 Laboratory Supervisor 2006 – 2008

• Supervised students and student teaching assistants in introductory laboratory sections• Assisted with demonstrations for introductory physics lectures• Graded quarterly physics projects and reports

Physics 100 Laboratory Teaching Assistant 2004 – 2006

• Assisted students with introductory laboratory experiments• Graded weekly laboratory reports

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Roger B. Scott List of References

Department of Physics Mobile: +1 808 652 2167

Montana State University O�ce: +1 406 994 6718

Bozeman, MT, 59717, United States E-mail: [email protected]

Contact

David E. McKenzie

245 EPS Building Collaborator, Graduate Committee Chair

Department of Physics [email protected]

Montana State University +1 406 994 7843

Bozeman, MT, 59717, United States

References

Dana W. Longcope

260D EPS Building Collaborator, Graduate Committee Member




Charles Kankelborg

260C EPS Building Graduate Committee Member




Jiong Qiu

221 EPS Building Graduate Committee Member




Ph.D. Dissertation Information

In October of 2015 I defended my Ph.D. dissertation, which has now been accepted into the library at Mon-tana State University. The title and abstract of my dissertation are as follows:

Analytical and Numerical Modeling of Coronal Supra-Arcade Fan Structures

Abstract

Among the myriad of interesting phenomenon in the solar corona is the highly dynamic region

above active region arcades, commonly referred to as the “supra-arcade” region. In the minutes and

hours following the formation of an arcade of post-flare loops, we commonly observe the development

of a curtain like structure, with spiny rays of enhanced emission in X-Ray and extreme ultra-violet.

Additionally, these structures often exhibit dynamics over a variety of length scales, from large-amplitude

coherent transverse oscillations, to the appearance of low-emission columns that seem to descend toward

the solar limb. The wealth of dynamical aspects that are present in the supra-arcade seems to indicate

that the plasma there is subject to a complex balance of influencing factors, which makes it di�cult to

develop a self-consistent hypothesis for describing all of the various features simultaneously. In this work

we undertake to explain one such behavior as an isolated phenomenon. We argue that the descending

low-emission voids, sometimes called Supra-Arcade Downflows (SADs) are consistent with the formation

of a particular kind of shock in the vicinity of a retracting element of reconnected magnetic flux. We then

use numerical simulations to expand this result to a broader parameter space, as well as investigating the

details of a variety of other behavioral regimes. Finally, in an e�ort to understand the broader dynamics of

the supra-arcade region, we undertake a study that incorporates imaging data into a numerical simulation,

which can then be used to estimate the ambient plasma parameters in the supra-arcade region. In this

way we show that the balance of influencing factors in the supra-arcade is indeed highly dynamic and

that the simplifications o�ered in certain extremes of magnetohydrodynamics are ill-applied in this case.

Project Proposal

Current Sheets and Plasma Dynamics Above Active Region Arcades:

Exploring the Interconnectedness of Bulk Plasma Evolution and the Transition to Fast Reconnection

Motivation

Many of the longstanding questions about solar energetic events can be couched in terms of the followingquery: How is it that long-lived, slowly evolving coronal magnetic structures are able to undergo spontaneousreconfiguration in the context of solar energetic events? To many, the answer is given in terms of the onsetof fast reconnection. And while the accepted view is that Sweet-Parker reconnection (Parker, 1957) is tooslow to account for the reconfiguration of magnetic fields in a solar energetic event, there are a variety ofproposed mechanisms that could speed up the reconnection process; Petschek reconnection (Petschek, 1964)allows for an increased reconnection rate due to the presence of a standing magnetosonic shock, while theplasmoid instability (Biskamp, 1986) allows for turbulent enhancement of the “macroscopic” resisitivity andcan thereby lead to increased reconnection rates. And yet, while these models allow for reconciliation ofobservations with the general theory of reconnection, instrumental limitations continue to prohibit directdetection of current sheets and measurements of their internal structure.

In recent years much attention has been given to the di�use emission above post-CME flare loop arcades,in the so called supra-arcade region (see Fig. 1). Interest in these structures, which are qualitatively similarto helmet streamers, has been stimulated by a combination of two key observations. The first is that, likehelmet streamers, the enhanced emission above the flare loop arcade is thought to coincide with a rotationaldiscontinuity in the open magnetic field, i.e. a current sheet. And since currents in the solar corona cannot be viewed directly, observations of these plasma structures are thought to be indicative of the dynamicsof the coincident current sheet. The second reason for studying the supra-arcade region is that, while theenergy distribution in the underlying active region is typically understood to be magnetically dominated,the behavior of the plasma in the supra-arcade o�ers evidence of a much more subtle energy balance –recent work by McKenzie (2013) and Scott et al. (accepted: 2016) suggests that — Ø 1 is a likely possibility.Thus, supra-arcade structures are ideal laboratories for studying the transition from low to high — and theirnearly-2D configuration allows for comparison of simplified analytical and numerical models, without theneed to disambiguate geometric e�ects due to variation along the line of sight.

Figure 1: An example of a flare loop arcade is shown in the 131 A and 193 A channels of the AtmosphericImaging Assembly (Warren et al., 2011). The supra-arcade ‘fan’ can be seen in the 131 A channel on theleft. This event occurred on the North-West limb on 2011 October 22.

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Ongoing Research

Research related to the supra-arcade region has largely centered around Supra-Arcade Downflows (SADs),which were first observed by McKenzie & Hudson (1999). To date, several authors have o�ered plausibleexplanations for the existence of SADs, the first of which came from McKenzie (2000) and was later refinedby Savage et al. (2012). In their interpretation, SADs are understood to be a sort of wake that forms inthe region behind a ‘shrinking magnetic flux tube’, whose existence is posited as the result of an isolatedreconnection event that occurs far above the limb, perhaps at the upper ‘Y-point’, just below the expandingCME.

This interpretation has been largely accepted for its seeming plausibility, but was not initially groundedin magnetohydrodynamics, and there are those who are critical of the model – the most common criticismbeing the fact that the dark lanes behind SADs are so well collimated and so long lived, neither of whichis consistent with the common perception of a “wake”. Subsequently, Scott et al. (2013) I developed amodel for SADs in which the retracting magnetic loop (whose origins are outside of the model) deformsthe surrounding magnetic field as it passes, which deformation constrains the velocity of the plasma alongindividual field lines and ultimately leads to the formation shocks and the appearance of a column of rarifiedplasma. They argued that the rarified, shocked column could be interpreted as a SAD and went on to o�er afew preliminary estimates of the density ratio and plasma velocity that might be expected within the SAD,as well as characterizing the pressure and, thus, the drag force that the surrounding plasma might exert onthe retracting magnetic loop.

While the model of Scott et al. (2013) o�ers an explanation for the formation of dark lanes in the supra-arcade, it su�ers from two major limitations, the first being that it is predicated on the assumption that —is very low in the supra-arcade region; in the model, — is taken to be zero. Second, the model makes noclaim regarding the formation of the retracting magnetic loop; its existence is merely posited. Thus, thefundamental result of the model is predicated on the existence of a feature that is not, itself, universallyaccepted in the solar community – an isolated, retracting magnetic element that pierces the supra-arcadestructure would require an instance of ‘patchy reconnection’, which may or may not be plausible in thesupra-arcade environment.

In order to address the first of these limitations, the same authors recently completed a numerical studyof the dynamics of a magnetized plasma as it flows around a rigid (circular) boundary (see Scott et al., inpreparation: 2016). This study su�ers from the same limitation as its predecessor in that it presupposes theexistence of a retracting magnetic element; however, it relaxes the zero-— assumption through the inclusionof an ideal induction equation and an evolving magnetic field, and therefore allows arbitrary values of —.Through this study, the authors showed that the shocked column predicted in Scott et al. (2013) couldpersist even in cases of — . 1, provided that the speed of the retracting flux tube is not in excess of theAlfven speed.

With the extension of the model of Savage et al. (2012) to energy distributions approaching — ≥ 1there are now four distinct models that each o�er a mechanism to describe the formation of Supra-ArcadeDownflows. In addition to the interpretation of Savage et al. (2012) there is the work of Cassak et al.(2013), who suggest that a continous reconnection process could lead to the formation of a column throughwhich the reconnection outflow proceeds with successive magnetic elements stacking on top of one anotherto maintain a cohesive structure. A third interpretation comes from Cecere et al. (2012), who suggest thata high-pressure feature might lead to the formation of shocks, which reflect and interfere with each other toform a low density feature. And, more recently, Guo et al. (2014) have performed simulations of supra-arcadereconnection with uniform resistivity in which they observed the formation of tear-drop like structures, whichthey attribute to the Raleigh-Taylor instability.

Each of these has its own merits and is likely to exist in some form in the solar corona but it remainsto be seen which, if any is a viable candidate to describe the apparent motion of supra-arcade plasmas. Inparticular, while the model of Scott et al. (2013) has been demonstrated for — . 1, with the collimatedshocks transforming into traditional shock cones as the speed of the retracting loops approaches and exceedsthe sound speed, the other three models each assumes — ≥ 0.5, and it is not clear how their results dependon this choice of —. For this reason, it is important to continue to explore the viability of each of these

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mechanisms in the context of the supra-arcade region.The extension of these models to arbitrary values of — is particularly important in light of the recent

work of McKenzie (2013), who performed a PFSS extrapolation and compared the magnetic fiend strengthto published estimates of the temperature and density in the supra-arcade, and thereby estimated — in thesupra-arcade to be somewhere in the range of 4-12, well above the assumed value of — ≥ 0.5. In orderfurther explore the possibility of a high-— supra-arcade environment, Scott et al. (accepted: 2016) recentlyundertook another, parallel study in which they used EUV observations and DEM inversion techniques tofirst infer the electron density variations and associated velocity field, and then subsequently to estimatethe magnetic structure and associated energy density. Through this technique they estimated — to be oforder 10 in the supra-arcade region, though this estimate may be systematically high due to an assortmentof assumptions whose impacts cannot be easily quantified.

Clearly, an improved understanding of the observable aspects of the supra-arcade region requires contin-ued e�ort both in refining the current theoretical models and in improving what few estimates exist regardingthe ambient conditions of supra-arcade plasmas. And while there are many ways to proceed, any serioustreatment of the supra-arcade region should consider the following criteria:

• Extension of each current models to arbitrary plasma-—:

The interpretation of Savage et al. (2012) has recently been shown to be valid for — Æ 1 given certainconditions, however, the models of Guo et al. (2014), Cecere et al. (2012), and Cassak et al. (2013)have not been extensively explored with a variable magnetic field strength as is necessary in order tounderstand their range of applicability.

• Analytical description of each e�ect:

Just as Scott et al. (2013) was first explored analytically before being confirmed numerically, theremaining three models should each be discussed in the context of a simplified analytical description sothat the fundamental driving mechanism behind each can be made more clear. For example, Guo et al.(2014) et al. site Rayleigh-Taylor type instabilities to explain the formation of tear-drop like features intheir reconnection simulation. For this claim to be validated, a Rayleight-Taylor type analysis shouldbe applied to the conditions of their model to see if this is, in fact, the driving mechanism behind theirresult.

• Extension of each model to more complex geometries:

While the models of Cassak et al. (2013) and Cecere et al. (2012) have been seen in 3D, Guo et al.(2014) et al. relied on a 2.5D arrangement and Scott et al. (2013) relied on an explicitly 2D arrang-ment. For these models to be fully corroborated, it is important that each be imbedded in a fully 3Dsimulation in order to see whether the results of each stand up to the more complex geometry that isinherent in coronal plasmas

• Reconciliation with reconnection models:

Of these models, only Guo et al. (2014) uses actual reconnection processes in their explanation ofsupra-arcade dynamics. Since the supra-arcade is expected to coincide with a magnetic discontinuityand an associated current sheet, it is vital that each of these models should ultimately be reconciled withthe dynamics of such a current sheet. Scott et al. (2013), for example, rely on an instance of ‘patchyreconnection’; however, until that e�ect is seen in the context of a simulation in which reconnection isaccounted for, the results of the model will remain suspect.

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Proposed Research

While it is important that the community proceed with several avenues of research simultaneously, I intendto focus my e�orts on two specific aspects: the expansion of the analytical and numerical models of Scottet al. (2013, in preparation: 2016) and then reconciliation of this model with ongoing research in magneticreconnection. Through these e�orts I hope to further our understanding of plasma dynamics both in thesupra-arcade and in other areas by way of inference. And I further hope to help make the study of supra-arcade plasma and the details of current sheet dynamics more cohesive. Not long ago these were consideredone and the same. More recently there has been a divergence; the current sheet and the surrounding plasmaare understood to be distinct and are often treated separately. It is my hope that we can begin to appreciatethat these two aspects of MHD are distinct and yet fundamentally linked and that only by treating thesystem self consistently can we begin to make real progress.

While the findings of Scott et al. (2013) are suggestive of SADs (see Fig 2, upper panel) in the regimewhere — Æ 1 and flow speeds below the Alfven speed, the current implementation assumes an isothermalplasma and it remains to be seen whether these e�ects remain valid after the inclusion of viscous heating andthermal conduction. Further, in the current implementation the retracting flux tube descends at a constantspeed relative to the solar limb and the ambient plasma density is assumed to be uniform (atmosphericstratification and gravitational e�ects are ignored). Clearly there is room to extend this model throughthe inclusion of acceleration terms and gravitational e�ects, and while early e�orts have been made to thisend (see Fig. 2, lower panel), a more sophisticated treatment is needed. In this way, the interpretation ofthese results can be more carefully considered in the context of the supra-arcade environment, where SADsinteract with a non-uniform atmosphere and are known to decelerate as they approach the limb.

Additionally, there are many models that invoke the interaction of a moving magnetic loop with thesurrounding plasma that fail to account for the details of this interaction – the thin flux tube model ofLinton & Longcope (2006) is one such. In that model, a reconnected flux element evolves under the influenceof its own magnetic tension but with no regard for the interaction with the surrounding plasma. By extendingthe numerical model that was developed in Scott et al. (in preparation: 2016) to account for an acceleratingframe (which can be made proportional to the pressure exerted on the surface of the magnetic loop) it ispossible to characterize the drag force and kinematic evolution of such a flux tube and the result of thisinvestigation can be made general so as to be made applicable to other models.

To further support this investigation I intend to perform a number of parallel studies. Since the TearingMode instability in current sheets with large Lundquist number is already a topic of great interest, partic-ularly of late, it seems only appropriate that other types of instabilities should also be considered in thiscontext, specifically the Rayleigh Taylor (RT) and Kelvin Helmholtz (KH) instabilities, both of which havebeen cited in reference to supra-arcade downflows. In particular, the analysis of Verwichte et al. (2005)suggested that the dark lanes that form the tail of SADs support transverse, surface mode oscillations andthey characterized their amplitude and phase velocity. If these dark lanes are the result of strong densitygradients (as opposed to a thermal e�ect), and if there is a shear velocity (as suggested by Scott et al. (2013))then these features should be susceptible to the Kelvin-Helmholtz instability.

I intend to generalize previous results regarding KH growth rates to the case of field aligned shear flowswith density gradients perpendicular to the magnetic field. In this way, the results of Verwichte et al. (2005)can be reconciled with estimates of the field strength and plasma density in the supra-arcade region andhopefully used to better constrain our understanding of the associated energy distribution. Likewise, Guoet al. (2014), have indicate Rayleigh-Taylor instabilities as the mechanism behind the formation of SADs intheir simulations. Since the supra-arcade is not typically characterized by density inversions, as is usuallythe case in RT analysis, the instability must be a result of stratified tension in the di�using magnetic fieldas it reconnects. However, a detailed analysis of instability formation has not been presented in this caseand is sorely needed if the interpretation of Guo et al. (2014) is to be upheld.

Following the completion of this study, I intend to undertake an investigation of the dynamics of re-tracting flux ropes in three dimensions. This study, much like that of Guo et al. (2014), will consider asingle, volume filling magnetic field, as opposed to the topologically distinct interior boundary employedin Scott et al. (2013). However, unlike Guo et al. (2014), I intend to generalize the reconnection process

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0.50 0.75 1.00 1.25 1.50 1.75 2.00

β = 0.1, Ms = 1.0 β = 0.1, Ms = 1.5 β = 0.5, Ms = 1.0 β = 0.5, Ms = 1.5

-10 -5 0 5 10

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t = 0 ts t = 20 ts t = 40 ts t = 60 ts t = 80 ts

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Figure 2: Simulation results for plasma flow in the vicinity of a retracing flux tube are shown in the upperpanel, with field lines traced in black. The lower panel demonstrates a preliminary study of a retracting fluxtube in a stratified atmosphere. Both simulations were performed with uniform, constant temperature. Thecolor table indicates normalized plasma density.

through the inclusion of spatially variable resistivity in order to allow for instances of ‘patchy reconnection’,as described in Linton & Longcope (2006). In this way, the occurrence of a reconnected flux tube thatretracts under its own tension while simultaneously interacting with the surrounding field and plasma canbe treated self-consistently. The details of this model will incorporate the results of recent advancements inour understanding of current sheet evolution and turbulent resistivity – it is not enough to merely posit theexistence of a region of anomalous resistivity; the magnetic di�usivity must be related to the fundamentalnature of the current sheet in a way that is consistent with contemporary results.

Recently there has been a renewed interest in the initiation and evolution of fast reconnection and itstriggering, namely the so-called ‘plasmoid instability’ and more specifically the concept of ‘ideal’ tearing, asdeveloped by the group of Prof. Marco Velli at UCLA (Pucci & Velli, 2014). The latter approach has led to anew scenario which is able not only to explain the observed fast reconnection rates, but also to account for thetrigger mechanism, in opposition to the scenarios that have emerged over the years relying essentially on theparadigmatic Sweet-Parker model. The extension of this model to realistic coronal configurations includingcoronal arcades and SADs has not yet been attempted, though it is in some sense a crucial component ofthe work that I have proposed to do here.In addition, Prof. Velli at UCLA has done work related to my ownon flows and their acceleration and stability, with expertise complementary to my own work on flows aroundan obstacle. For this reason I am encouraged to propose and to carry out the research of this proposal, ifgranted, at UCLA with the supervision of Prof. Velli.

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ReferencesBiskamp, D. 1986, Physics of Fluids, 29, 1520

Cassak, P. A., Drake, J. F., Gosling, J. T., et al. 2013, ApJ, 775, L14

Cecere, M., Schneiter, M., Costa, A., Elaskar, S., & Maglione, S. 2012, ApJ, 759, 79

Guo, L.-J., Huang, Y.-M., Bhattacharjee, A., & Innes, D. E. 2014, ApJ, 796, L29

Linton, M. G., & Longcope, D. W. 2006, ApJ, 642, 1177

McKenzie, D. E. 2000, Sol. Phys., 195, 381

—. 2013, ApJ, 766, 39

McKenzie, D. E., & Hudson, H. S. 1999, ApJ, 519, L93

Parker, E. N. 1957, J. Geophys. Res., 62, 509

Petschek, H. E. 1964, NASA Special Publication, 50, 425

Pucci, F., & Velli, M. 2014, ApJ, 780, L19

Savage, S. L., McKenzie, D. E., & Reeves, K. K. 2012, ApJ, 747, L40

Scott, R. B., Longcope, D. W., & McKenzie, D. E. 2013, ApJ, 776, 54

—. in preparation: 2016, ApJ

Scott, R. B., McKenzie, D. E., & Longcope, D. W. accepted: 2016, ApJ

Verwichte, E., Nakariakov, V. M., & Cooper, F. C. 2005, A&A, 430, L65

Warren, H. P., O’Brien, C. M., & Sheeley, Jr., N. R. 2011, ApJ, 742, 92

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Relevance to the Living With a Star Program

As stated in the program description, the “Living [W]ith a Star [Program] emphasizes the science necessaryto understand those aspects of the Sun and space environment that most directly a�ect life and society,”to which end the program attempts to cultivate research to address the following questions: How and whydoes the Sun vary? How does the Earth respond? What are the impacts to humanity?

Perhaps the most immediate causal relationship, beyond the steady balance of absorption and reradiatingof electromagnetic energy, is the constant and erratic bombardment of the Earth by Solar Energetic Particles(SEPs), i.e. Solar Storms. These are often related to solar energetic events, e.g. flares and CMEs, whosetriggering mechanisms are not well understood. Coronal currents and compact current layers are obviouslyconnected to the temporal evolution of active regions, and play a role in regulating the build up and release ofmagnetic energy, but the transition from slow to fast reconnection (e.g. the onset of plasmoid instabilities)remains a much debated topic of great interest and importance. As coronal currents cannot be detecteddirectly, continued e�orts toward modeling the evolution of plasmas in the vicinity of assumed currents andinstances of magnetic reconnection is crucial to improving our understanding of these processes. And sincethe region above a flare loop arcade is assumed to be host to compact current layer, or current sheet, animproved understanding supra-arcade dynamics, i.e. SADs, as well as current sheet dynamics, and how theyinteract, is intimately related to advancing our knowledge of reconnection in general.

For these reasons, I feel that the study of supra-arcade dynamics is an important one, with implicationsnot only to reconnection models but also to our understanding of the interaction between, and responseof the plasma to, the various forcing terms in magnetohydrodynamics. And while some models currentlyexist to explain observational aspects of the supra-arcade region, none is currently able to treat the bulkplasma evolution and the interior dynamics of the current sheet simultaneously. Therefore, a self consistentmodel that draws on contemporary research into current sheet formation and dynamics in order to motivatesuch macroscopic parameters as magnetic di�usivity, is critical for the advancement of our understanding ofreconnection, and thereby our broader understanding of solar energetic events and space weather as a whole.

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