January 12, 2001
: I hear people say, I read that we don't know why the solar corona, the gaseous environment surrounding the Sun, is at a temperature of about a million degrees, while its surface is only at six thousand. This morning, an idea emerged.
One observed fact: the Sun periodically emits large arches of plasma. The points where these arches attach to the Sun's surface are sunspots. The plasma surrounding the Sun is in a condition, unless I'm mistaken, of "high magnetic Reynolds number." That is, magnetic field lines are "frozen in" to the plasma. Imagine women's hair in a swimming pool, and a comb smoothing it. The hair and the comb are tightly linked—each moves the other, and vice versa.
These new plasma arches extend far out, then break apart. In the right-hand figure, magnetic field lines are schematically represented. At every point in space, one can associate with the value of B a corresponding "magnetic pressure" value, indicated.
Magnetic flux is also conserved:
Thus, the magnetic field strength is maximum near sunspots and minimum at the farthest point of the arch's extension. This results in a magnetic field gradient. The arch then functions as a natural particle accelerator. Gas therefore lifts off from the solar surface, from each sunspot, and, propelled by this magnetic field gradient—far stronger than gravitational force—rises and accelerates toward the region of minimal field, where the arch is most extended and its cross-section largest. These masses of plasma could, it seems to me, collide. The result would be the conversion of magnetic energy (used to accelerate the two plasma masses) into thermal energy. The following diagram illustrates the concept. Acceleration of particles by a magnetic pressure gradient, especially over very large distances, is an extremely efficient process. All of this should be numerically modeled more precisely, but I believe this phenomenon could explain such heating of the corona. One must remember, however, that thermal agitation velocity varies as the square root of temperature, so going from 6,000 to a million degrees represents only a 12-fold increase in thermal agitation speed.
Still, where these plasma masses collide (the arches would then be sites of fascinating magneto-acoustic phenomena worth studying), the plasma pressure could become so high that it escapes confinement by the magnetic field lines. Hence, the arches break apart, releasing their hot contents. Afterward, two scenarios are possible. Moderate heating would feed the Sun's gaseous environment, its corona. The gas at the Sun's surface is effectively pinned down. At 6,000 degrees, thermal agitation velocity is significantly lower than the speed needed for a particle to move appreciably away from the solar surface. This is why the surface remains close to spherical. But faster particles, accelerated within the arches and then released upon their disintegration, form the Sun's "atmosphere," extending much farther out.
More violent solar eruptions (which are actually secondary effects of a magnetohydrodynamic (MHD) instability at high magnetic Reynolds number) produce... the stellar wind (in the case of the Sun, the solar wind). Indeed, it is known that a high abundance of sunspots correlates with intense bombardment of Earth by gas emitted from the Sun.
For non-specialists in plasma physics, this acceleration via magnetic field gradient may be somewhat difficult to grasp. But many people are familiar with Earth's "magnetosphere":
On the left, the Earth sphere, with its dipole magnetic axis tilted. The Earth's "magnetic north" is actually a south pole, since it attracts the north poles of compass needles. Charged particles (mostly electrons) emitted by the Sun (solar wind) become trapped in Earth's magnetic field line network. On the right: they bounce back and forth between regions of high field strength, spiraling around the field lines. These spiral trajectories illustrate how the plasma is bound to the magnetic field. This plasma, forming the "Van Allen belts" named after the astrophysicist who discovered them, travels back and forth between the northern and southern polar regions of Earth, being reflected like tennis balls (solely due to the magnetic pressure gradient effect). In plasma physics, this is known as a "magnetic mirror." On the left: the Earth's magnetospheric tail, opposite the Sun.
Under normal conditions, charged particles reverse direction at very high altitudes, beyond Earth's atmosphere—whose boundary we can define at about 80 km. When a particularly strong burst of solar wind reaches Earth, particles, despite the braking effect of the magnetic field gradient, manage to penetrate the upper atmosphere. All astronomers know this causes the phenomenon of the aurora borealis. Thus, a Van Allen belt is a structure that, at least in certain aspects, seems quite similar to the arches associated with solar eruptions.
Well, this seems like an idea worth exploring further. But I have so many other things to do...
See the dossier "The Sun's Anger," from September 16, 2005
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