"A Turning Point in Auroral Physics"
by Duncan Bryant, Eos, 26 September 2006, vol 87, no 39,.
Duncan Bryant, a British researcher on auroral physics, has long held that the polar aurora is caused by plasma waves excited in the Earth's magnetosphere. In the above article he lays out arguments in favor of this view.
First, he notes that the energy distribution of auroral electrons often has a distinctive peak. He then briefly describes the "standard theory today"--namely, that the aurora is accelerated by an electric potential drop along the magnetic field lines which guide its electrons. This is labeled here as "The Electrostatic Theory" and is dismissed by arguing that such a field cannot serve as a steady source of energy.
He then describes various wave processes in the plasma of the Earth's magnetic environment--Landau damping, lower hybrid waves and others, arguing that such waves can energize electrons by losing energy to them. He then proposes that the aurora is caused by wave turbulence in the geotail, originating in the positive ions of the plasma sheet as they interact with the plasma sheet flank at the boundary layer, or perhaps the boundary between plasma sheet and the high latitude lobe.
"Eos" limits comments to 300 words. At first, it seemed as if this would be waived here, but in the end (after a delay) the editor insisted on the comment to be pared down. The shorter comment appears in the journal, where it (and the original article) can be read in the on-line version of "Eos."
Below is the version as originally submitted.
Origin of Polar Aurora
An "Eos" article (26 September 2006, p. 403, vol. 87) claimed "a turning point in auroral physics," ascribing the aurora to a hypothetical wave process. Both data and theory suggest otherwise. Unsolved questions about auroral arcs remain, but auroral acceleration is now understood much better (and quite differently) from what is implied there. In particular, we now know more about the role of Birkeland currents and field aligned electric fields (E//), as well as about the altitude profile of E// and the evidence of upward beams of O+ ions.
Dr. Bryant starts by citing an "electrostatic theory" for the existence of E// aligned with auroral field lines. Actually, there was more. Persson  showed that a parallel voltage Φ along magnetic field lines could exist in a simple plasma, as long as Φ was proportional to B; in a way, E// was balanced by the mirror force. Such electric fields became known as "quasi-neutral" ones, because the plasma essentially maintained charge neutrality, deviating only slightly from neutrality to maintain E// (which fluctuates, because the charge density involves rapidly moving particles).
Is that electric field static? It can be, at least in principle. In that case, as noted in the article, it cannot be the source of the aurora, because it lacks a steady supply of energy. But an electric field needs not be static. If two points A and B on a magnetic field line have a potential difference of 1000 volts, that could be due to static charges, but it could also be due to a field-aligned electric current overcoming some dissipation, e.g. some ohmic resistance. It is then still possible for Φ and B to be (roughly) proportional, with plasma particles undergoing guiding center motion.
In a static quasi-neutral equilibrium, if all particles have the same total energy, Φ is proportional to the magnetic intensity B. With the aurora and field-aligned currents, deviations from such proportionality may exist, including possible discontinuities [Boström, 2003, 2004] . Such a discontinuity could also reflect, for instance, the boundary between a dense cool ionosphere and a hot rarefied magnetosphere [Stern, 1981] . The process has been successfully modeled on a computer [Ganguli et al., 1994].
One possible energy-dissipating process is parallel acceleration into a loss cone. Imagine a static upward E//. Electrons are accelerated downwards, then get mirrored back, and as they rise again they give up all the energy they have gained, and a quasi-neutral equilibrium, like Persson's, may exist. But now add an absorbing ionosphere, at the level where some of those electrons are expected to mirror. For them, only the descending part of the orbit exists. The current density then is non-zero (electrons descend but do not come back), the ion density, of course, must adjust, and up-flowing O+
beams affect it, too, creating a more complex profile of Φ.
Indeed, acceleration is a likely process, if the physical process demands a current density greater than the one usually reaching the loss cone. In such case (it may be shown) E// magnifies the effective aperture of the loss cone, allowing a greater current to reach ionospheric levels. Since dissipation occurs, a voltage is needed to drive the circuit, and it presumably comes from some MHD dynamo process, like the one expected if the magnetosphere were a conducting obstacle in the path of the solar wind. The dynamo process can also provide the energy, and may well explain why Region 1 Birkeland currents (the primary circuit) peak around MLT 3 and 18, or even closer to noon. (although the electrojets and Region 2 deform the pattern further, and substorm currents, of different origin, are closer to midnight). Bryant's wave mechanism, in contrast, lacks a well-defined energy source.
It needs to be stressed that although this explanation works well for bright auroral arcs, including the ones of substorms, the diffuse aurora, cusp aurora and poleward arcs have different sources.
Extensive evidence supports this explanation. The association between auroral arcs and magnetic disturbances (in turn associated with Birkeland currents) is well known, but perhaps the most striking observation (first pointed out by David Evans [1974,1976] ) is that most of the potential drop occurs close to Earth. If Φ were to be strictly proportional to B, 7/8 of the voltage drop should occur within about one Earth radius (starting at the bottom of the acceleration region). That was completely unexpected: since aurora originated on field lines stretching far from Earth, most people originally expected its acceleration to occur far from Earth, too. Evans even noted the existence (at low altitudes) of a population of secondary electrons, trapped (briefly) between a magnetic mirror below and a reflecting electric field above.
Add to this a prevalence of upward flowing O+ beams (O+ "conics" in contrast involve secondary processes), as well as the energy peak pointed out by Bryant, which E// explains well, too. All these suggest that the "turning point" proposed by the Eos article is not justified by either theory or observations.
David P. Stern
Kinetic and space charge control of current flow and voltage drops along magnetic flux tubes: J. Geophys. Res.,A4, 108, A09295, 2003
Kinetic and space charge control of current flow and voltage drops along magnetic flux tubes: 2. Space charge effects, J. Geophys. Res. 109, A01208, 2004
Evans, David S.
Precipitating electron fluxes formed by a magnetic field aligned potential difference, J. Geophys. Res., 79, 2853-58, 1974.
Evans, David S.
The acceleration of charged particles at low altitudes, p. 730-739 in Physics of Solar Planetary Environments, D.J. Williams, ed., Amer. Geophys. Union, Washington, DC 1976.
Ganguli, Supriya B., H. G. Mitchell and P. J. Palmadesso
Auroral plasma transport processes in the presence of kV potential structures
J. Geophys. Res., 99,. A4, p. 5761-5770, 1994
Electric field along a magnetic line of force in a low-density plasma, Phys. Fluids, 6, 1756-1759, 1963.
Stern, David P., One-dimensional models of quasi-neutral parallel electric fields J. Geophys. Res., 86, 5839-5860, 1981.
Author: David P. Stern, 31 Lakeside Drive, Greenbelt, Maryland 20770
Emeritus, Goddard Space Flight Center, Greenbelt, Maryland 20771
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