The solar corona is a strange place. For the last few decades solar physicists have been trying to understand why it is so hot. Yes, it’s the Sun, and yes, it’s hot, but the corona is too hot. There are many possible solutions to the “coronal heating phenomenon”, but physicists are generally in agreement that this extreme heating is down to waves propagating along magnetic fields, interacting with coronal plasma, or by reconnection events (small explosions). In a study published earlier this year, scientists suggest that to account for the temperatures and densities observed in the corona, chaotic forces may be at work, regulating the scales of reconnection in the coronal plasma.
To put this problem into perspective, the Suns atmospheric gases can be hotter than a million Kelvin (or degrees Celsius). The solar “surface” (or photosphere) is about 6000 K. How can the atmosphere of a solar body be hotter than the Sun itself? It is like finding that the temperature of the air surrounding a light bulb is actually hotter than the bulbs glass surface, it’s an impossible situation, breaking all sorts of physical laws, principally the second law of thermodynamics:
“Heat cannot spontaneously flow from a material at lower temperature to a material at higher temperature.” – Rudolf Clausius (1822-1888)
So, since the mid-1900s solar physicists have been searching for an answer to this puzzling phenomenon. The two main contenders to the mechanism which supports coronal heating are:
- Magnetohydrodynamic (MHD) wave heating
- Small-scale reconnection events (i.e. nanoflares)
Both mechanisms have their advantages and disadvantages, but ultimately neither theory can explain why the corona is the temperature it is or the density it is. Generally, it is agreed that observational instrumentation currently may be of insufficient resolution to detect small-scale waves or flares, but this is cold comfort to solar physicists who continue to observe and model the solar corona.
In a recent paper, a group from the Bartol Research Institute (University of Delaware, USA) believe the corona may be governed by self-organized critical dynamics, put simply: chaos. By studying other stars, and assuming the coronal dynamics is similar in all Sun-like stars, a picture can be formed of how stellar coronae are driven.
The Bartol research group calculates the dynamical processes that convert stored magnetic energy into kinetic energy. Basically, they assume the vast amount of energy stored in the stars magnetic field is somehow liberated to go on and heat coronal gas particles. But how can this energy be released? This is where extreme plasma physics steps in, when magnetic field lines get too close.
Should two field lines be forced together (a common occurrence in the solar corona), they may “reconnect” (i.e. snap apart and reconnect). Magnetic energy may then be liberated, heating the surrounding solar plasma. But, there are two different types of reconnection event: collisional (slow) and collisionless (fast). Both are critical to ensuring energy can be explosively released.
In its “energy storage state” the magnetic field lines coexist with coronal plasma. Field line and plasma are “frozen in”, where the plasma flows, the magnetic field line follows. Should magnetic flux cross and reconnection begin, collisional reconnection may occur (it is collisional as the plasma particles are close enough to “feel” each other – they are colliding). Now, collisional reconnection, also known as Sweet-Parker reconnection, is very slow and insufficient at generating a rapid release of energy. It is however useful at making the reconnection region more and more collisionless, speeding up the reaction.
Now this is where it gets exciting. As the coronal reconnection region slowly reconnects in the Sweet-Parker regime, coronal plasma slowly diffuses across the crossed magnetic field lines and is ejected, but this is too slow for an explosive event. More and more field lines are dragged into the diffusion region as the reaction progresses, feeding the reconnection. The diffusion region becomes thinner and thinner as more and more magnetic field lines are dragged in, forcing plasma out of the region. When the diffusion region becomes thin enough, non-linear processes are sparked causing a catastrophic collapse. Reconnection goes from slow (Sweet-Parker regime) to fast (Hall regime), causing the region to erupt.
This transition is made possible through the non-linear process known as bifurcation, where any small change to the coronal conditions can kick-start a massive explosive event. As these events are driven by a chaotic process, they become hard to predict, but exhibit a pattern nonetheless. What’s more, due to the self-sustaining build-up of stored energy in the incoming magnetic field, this process can be repeated over and over until the magnetic energy has dissipated. So long as magnetic field energy is forced into the reconnection region, and the diffusion region is forced so thin non-linear processes are allowed to be kick-started, Hall reconnection will occur, rapidly releasing energy in the form of a flare, heating the stars corona. As magnetic energy is lost through reconnection, the corona will relax into a cooler state as less reconnection events become possible. Magnetic energy therefore becomes the regulator of coronal temperature.
Reconnection can directly heat coronal plasma, but it may also generate waves that propagate through the solar atmosphere and heat the corona. Also, there may be the possibility that these waves may destabilize magnetic fields so much that secondary reconnection events may occur.
Although the implication of non-linear dynamics and coronal heating is not new, this recent paper compares its theoretical findings with analysis of power spectra of our Sun and other stars and finds that the reconnection model outlined here agrees well with observation.
So, the jury is still out on deciding what mechanism heats the solar corona. But these results, combining so called self-organized criticality (SOC) models of the solar corona with known reconnection models to trigger flare events, will be encouraging to solar physicists still tracking down the causes of the coronal heating problem.
Reference: “From Solar and Stellar Flares to Coronal Heating: Theory and Observations of How Magnetic Reconnection Regulates Coronal Conditions” – P. A. Cassak, D. J. Mullan and M. A. Shay, arXiv:0710.3399