How Does a Coronal Loop’s Geometry Relate to its Other Properties? An Exploration of Non-Thermal Particle Effects on Simulated Loop Structures Chester CurmeDr. Henry (Trae) Winter

Outline Introduction – Coronal Heating Problem – The HyLoop Suite and non-thermal particles – The relevance of loop geometry Tapering a loop The heating function Preliminary results Future work

What is a coronal loop? The sun’s magnetic field penetrates its surface and loops around in its atmosphere. Plasma, bound to magnetic field lines, conforms to these tubes of magnetic flux to produce beautiful structures like the one on the right. s/9/93/Traceimage.jpg

THE BIG QUESTION: What heats the solar corona?

Temperature of the corona is in excess of 10 6 K. Temperature of the underlying photosphere is approximately 5800 K, several hundred times cooler than the corona. This is a mystery.

One Idea for the Heating Mechanism: Buchlin, E., Aletti, V., Galtier, S., Velli, M. Einaudi, G., and Vial, J.-C., "A simplified numerical model of coronal energy dissipation based on reduced MHD," A&A 406, (2003).

A nanoflare storm also produces “shrapnel” in the form of a population of high-energy non- thermal particles (electrons). This shrapnel further heats the plasma.

Nanoflare heating is a popular candidate for the heating of coronal loops. – Hot (T > 2 MK) loops tend to be under-dense with respect to density at static equilibrium, whereas warm (T ≈ 1 MK) loops tend to be over-dense (Klimchuk 2006). – This suggests that the plasma in coronal loops is not in static equilibrium, a phenomenon explained by the nanoflare theory. Most who simulate loops in nanoflare events employ 1D magnetohydrodynamic (MHD) models, and treat the NT electrons analytically (if at all).

HOWEVER

Only Dr. Henry (Trae) Winter III models the evolution of NT particles in nanoflare heating events! Dr. Winter at the Harpoon Barbeque Championships, 2009 Code is called the “HyLoop” suite (“Hy” for hybrid) and involves two interacting programs: SHrEC, a 1D MHD model which treats the thermal plasma PaTC (Particle Tracking Codes), which tracks the evolution of NT particle beams.

The HyLoop Suite Loop coordinate, s In each volume cell, at each timestep, HyLoop solves the differential equations that govern the plasma and NT particles, tracking the evolution of the loop.

Why are NT particles important to model? Non-thermal (NT) particles may leave an important observable signature: NT particles may deposit their energies in unexpected places as they travel along the loop. – There exists a “mirroring force” proportional to the tapering of the magnetic field (and hence the loop itself, since plasma is contained by the field lines): Where is the KE of the particle transverse to – This force causes high-energy particles to “bounce around” in a tapered loop. Note that both the radiative signatures and behavior of NT particles vary strongly with loop geometry. – SHrEC energy MHD equation:

Why are NT particles important to model? Non-thermal (NT) particles may leave an important observable signature: NT particles may deposit their energies in unexpected places as they travel along the loop. – There exists a “mirroring force” proportional to the tapering of the magnetic field (and hence the loop itself, since plasma is contained by the field lines): Where is the KE of the particle transverse to – This force causes high-energy particles to “bounce around” in a tapered loop. Note that both the behavior of NT particles and their radiative signatures vary strongly with loop geometry. – SHrEC energy MHD equation:

This brings us to our question: After taking into account the effects of NT particles, how is a coronal loop’s geometry related to its other properties?  Maximum temperature  Apex density  Average power at apex How geometry correlates (if at all) with these properties can shed light on the validity of the nanoflare theory, once compared with observations.

We define a loop’s geometry in terms of the tapering ratio Construct a semi-circular loop of circular cross-section and linearly taper loop radius by Γ. Study of 43 soft X-ray loops by Yohkoh reported median Γ of ≈ 1.30 (Klimchuk 1999).

The Heating Function First few minutes constitute a stabilization period: constant flux, uniform heating. After stabilization: Each simulated flare has the same amount of total energy. Energy is divided into a series of nanoflare storms. At a pre-defined timestep, a storm of nanoflares occurs. Each nanoflare injects a beam of test particles into the loop. Nanoflares in each storm are of equal power and distributed randomly along the loop coordinate s.

Volumetric Heating Rate

Preliminary Results

Mean r: 0.973

Mean r: 0.977

Mean r: 0.743

Mean r: 0.542

Conclusions Tapering loops has an appreciable effect on loop maximum temperature, apex density, and (weakly) apex power. – Contribution of NT particles to this circumstance is unquantified. Including NT particles in simulations generally lowers the loop’s maximum temperature and apex power, while raising its apex density.

Future Work PaTC treats NT particles stochastically; average results over multiple runs and calculate statistical error (Monte Carlo methods). Use emission data from simulated loops to produce images as would be viewed by satellites: Perform similar correlation studies, varying: – Location of nanoflares – Pitch-angle distribution – Ratio of thermal to non-thermal energies Simulate a multi-stranded loop

Special Thanks Dr. Winter Dr. Korreck Dr. Davey NSF And many more!

Questions: – Not hydrostatic eq.  use simulations? – Gaussians in heat function more physical? – No e_h from NT part. Collisions in previous codes?

What are non-thermal particles? A population of particles that follows a Maxwell- Boltzmann distribution can be said to have a temperature; the particles are thermal. Nanoflares may inject high- energy non-thermal (NT) particles into a loop. These particles are said to be non- thermal because they lie on the outer tail of the distribution, which follows a power-law; it is meaningless to ascribe a temperature to a population of NT particles. Energy Number of Particles Maxwellia n Power-law NT Tail

How is HyLoop stochastic? PaTC treats the NT particle beam as a series of test particles, and tracks their evolution through the thermal plasma. Test particles are drawn randomly from probability distributions that correspond to certain types of NT beam. In addition, the equations that govern the particles involve stochastic (random) terms. Results of simulations, then, will vary with each run. Monte Carlo methods are thus used to average over runs and compute statistical errors.

Most ideas for the heating mechanism can be grouped into one of two categories: Dissipation of waves, or “AC Heating” Dissipation of magnetic stresses, or “DC Heating.”

AC Heating Some propose that the dissipation of wave energy in the corona (acoustic, Alfvén, magnetosonic, torsional…) is responsible for its extreme temperatures. Presumably, however, only a small fraction of waves’ energy flux is able to pass through the steep density and temperature gradients in the chromosphere and transition region. Waves aren’t well-understood (energy flux, spectrum, etc.).

DC Heating Magnetic flux tubes extend up from photosphere and widen as they pass through the chromosphere, transition region and corona. The footpoints of these flux tubes are displaced via plasma convection in the photosphere, tangling the field and building magnetic stress energy. The magnetic stress energy is somehow converted to heat in the corona. We concerned ourselves with DC Heating. Picture of tangled field lines goes here.

DC Heating Magnetic flux tubes extend up from photosphere and widen as they pass through the chromosphere, transition region and corona. The footpoints of these flux tubes are displaced via plasma convection in the photosphere, tangling the field and building magnetic stress energy. The magnetic stress energy is somehow converted to heat in the corona. We concerned ourselves with DC Heating. Picture of tangled field lines goes here.

Heating mechanisms that are impulsive can explain these observations. E.g. flux tubes are tangled; current sheets form; reconnection events occur and energy is released as a storm of nanoflares which accelerate coronal particles. – We concerned ourselves with this theory. A nanoflare storm also produces “shrapnel” in the form of a population of high-energy non- thermal particles (electrons). This shrapnel further heats the plasma.