Sunday, July 7, 2013

On Magnetic Reconnection and "Discharges"

There is a popular misconception, mostly among supporters of Electric Universe (EU) claims, that solar flares and similar eruptive events in plasmas are discharges much like terrestrial lightning or an arc furnace.

To support this claim, they often resort to papers by James Dungey, one of the pioneers in the study of the Earth's magnetosphere, who used the term 'discharge' when discussing one of his particular ideas of solar and magnetospheric eruptive events.  The popular papers to cite in this case are
If one actually takes the time to read these papers, one finds that the framework around the process Dungey describes for his use of the term 'discharge' actually fits our modern description of magnetic reconnection!  Dungey describes the event in terms of magnetic neutral points, AKA X-points!

In the abstract of the 1953 paper, Dungey writes:
It is shown that discharges are unlikely to occur anywhere except at neutral points of the magnetic field.
But digging through the text reveals even more the error of interpreting this paper as support for terrestrial electric discharges like lightning.  Dungey defines his term as
'discharge' as region where electrons accelerated to high energies by electric field so all electrons moving in same direction with high velocities.
Dungey points out the problems involved in considering the discharge as due to an externally applied electric field (such as what occurs in terrestrial electric discharges).  Figure 1 in Dungey's 1958 paper illustrates the classic magnetic reconnection configuration.

Dungey even uses the 'frozen-in' approximation, which I sure would infuriate EU advocates.

This is reinforced in Dungey's other papers on this topic:
Our early understanding of magnetic configuration associated with these eruptive events dated from less than a decade earlier (see Magnetic and Electric Phenomena in the Sun's Atmosphere associated with Sunspots) when Giovanelli began to examine the magnetic configuration around sunspots.  Dungey also cites this work.  What is amazing about the Giovanelli paper is that modern analyses solving the full Maxwell equations and plasma equations in this region have very little difference with the graphics in this early paper.   From these initial observations of magnetic fields, combined with application of electromagnetic theory to sunspots came the recognition at a null point (a location where the magnetic field magnitude drops to zero) can form between two magnetic fields.  By Maxwell equations, such null points can only form when the magnetic field around them forms a 'X' configuration. 

A Discharge By Any Other Name
To add to the confusion with Dungey's word choice, the term 'discharge' has a much older history from the early days of plasma physics which developed from laboratory experiments with gas tubes and arc furnaces as well as observations of terrestrial lightning.  In these cases, a discharge corresponds to a dielectric breakdown in a neutral gas under an externally applied electric field. 
For these discharges, we eventually understood the atomic processes involved, that a charge difference builds-up between two positions, and 'discharges' in an attempt to return to a more electrically neutral configuration.  The applied electric field from the charge build-up must be strong enough to accelerate free electrons to energies high enough to create ionization of the ambient gas atoms on their next collision, which provides more electrons to accelerate and ionize even more gas.

But solar flares cannot be a 'discharge' in this sense.  The solar atmosphere is almost completely ionized and therefore quickly shorts any strong electric field unless that field is created in the plasma configuration itself.  In this type of environment, such a 'discharge'  cannot occur.  Since solar flares and geomagnetic storms did not occur by this electrical breakdown process, the use of the term 'discharge' for these cases has been discouraged to improve clarity of the discourse.  When discussing Dungey's model, some researchers would write 'discharge' in quotes to clarify the distinction between Dungey's use of the term and the classical definition of a discharge (see The Motion of Magnetic Field Lines by D. Stern).

In spite of these differences, one occasionally encounters a researcher who uses the term 'discharge' when describing the current flow in the reconnection region.

"X" Marks the Spot
As mentioned above, one of the common characteristics in observations of solar flares was the existence of a magnetic null point near the location of the observed flare.  This null point would divide the region up into four zones, forming an 'X'-shaped configuration, as noted above.

If it were just a static magnetic field in a vacuum or in air, according to Maxwell's equations, it would be of little interest.  But let that magnetic field vary in time, and according to Maxwell's equations, you'll get an electric field.  Put this configuration in a plasma, even a neutral plasma, with equal numbers of positive ions and negative electrons, and all kinds of interesting things start to happen. Some Electric Universe supporters erroneously claim that magnetic reconnection occurs in a static magnetic field, which is incorrect.  Often, Electric Universe supporters don't even mention that reconnection with energy release can only occur when the field is imbedded in a plasma.

Embedded in an electric field, the positive ions and negative electrons begin to move in opposite directions (the electrons much faster due to their smaller mass).  The magnetic field also imparts an additional gyroscopic motion on the particles as they move, electrons with a very small orbit radius, ions with a much larger orbit radius.  But these motions create a feedback system…

According to Maxwell's equations, distributions of charges and currents create electric and magnetic fields.  According to the Lorentz equation, electric and magnetic fields accelerate charges, changing their motion.  This acceleration is the source of the high energy charged-particles detected from reconnection sites.

Now there's a problem, because it is very difficult to mathematically model the behavior of lots of charged particles in electric and magnetic fields, especially when the particles themselves are contributing to the fields controlling their motion.  Theorists try to make the problem manageable by abstracting the more complex, small-scale motions into approximations that can be described with some simple parameters.

One of the few methods which can generate an electric field and current in a predominantly neutral plasma is a null-point in a magnetic field - AKA, an X-point or "magnetic reconnection" (Wikipedia, from Plasma Physics Lectures at UT, Scholarpedia).  Without reconnection, one must face the problem of how to generate the charge-separation in a neutral plasma to generate an electric field.

Some problems were found with the details of Dungey's description and mathematical model with observations, which are summarized by Heikkila in some papers:
While some of the details were found to be incorrect, the overall picture of the magnetic field environment remained intact.  Because the classical use of the term 'discharge' more correctly fit descriptions of lightning and this did not fit the environment for reconnection, the term 'discharge' was discouraged for solar and magnetospheric eruptive events.

What's in a Name?
This is also an example of how scientific terminology (like regular language) changes with time.  When we discovered that chemical elements could change with the emission or absorption of some radiation or particle, we didn't understand what was going on, so we called it "transmutation of the elements".  As we learned about the inner constituents of the atoms, and how the nucleus was the important component of these processes, the terminology changed to 'nuclear decay', 'nuclear reactions', and nucleosynthesis.  "Transmutation of the elements" is today considered archaic and generally used in historical writing.

Yet some scientists can get really hung up on the terminology, forgetting that it is the underlying physical process that is important - not what we call it.   Some researchers prefer the use of 'magnetic merging' (see Theoretical models of magnetic field line merging. I), those this term has not caught on in the research community.

Others put 'magnetic reconnection' in quotes to signify the analogy of reconnecting lines is not meant to be taken too literally.  Either way, the physical framework of the magnetic field configuration has remained pretty much unchanged for over fifty years.

"Magnetic reconnection" is another example of a 'place holder' term which we use while we try to learn the details of what's going on.  Historically, the neutrino was a place-holder from 1933 when it was first proposed, to 1954 when it was actually detected (Wikipedia).  Today, "Dark Matter" and "Dark Energy" are place-holders in cosmology while we learn more about the phenomena.  (ArsTechnica: Insert here: the role of placeholders in science).  Some of these place-holders get renamed as their underlying process is better understood, as happened for 'transmutation of the elements' described above.  Other disappear into obscurity as they are identified as data collection biases, such as the K-effect in stellar radial velocities from the early 1900s.

When physicists use the term 'magnetic reconnection', it covers a wide variety of conditions.  The magnetic field configuration and the presence of a plasma are common components, but many of the details, such as the plasma's resistivity/conductivity, diffusivity, composition, etc. become the details addressed in different reconnection models.  These differences determine the speed and energetics of the reconnection event.

The bottom line is that Dungey's paper is NOT support for solar and magnetospheric energetic events being physically analogous to terrestrial lighting or arc discharges.


Unknown said...

Another aspect of MR that some Electric Universe supporters deny is that it does not need a plasma to happen. Of course the interesting stuff happens in plasma but MR in a vacuum is just as valid.
You imply this with "But let that magnetic field vary in time," but maybe should make it explicit, e.g. "But let that magnetic field vary in time so that reconnection happens,".

W.T."Tom" Bridgman said...

To Unknown,

When they speak of magnetic reconnection in space physics, they are speaking of an energetic event that converts field energy into particle kinetic energy. Without the plasma, the event you describe cannot transfer energy to particles.