Expert: Philip A. Stahl - 1/26/2009

Question
Hello. I've been reading the answers to the questions about Earth "flares" and find them intriguing. Isn't it true that for bona fide Earth flares there'd ave to be some kind of magnetic reconnection occurring? It wouldn't be just an electrical thing. Also can't magnetic substorms on Earth already be considered a kind of Earth flare? Just wondering!

Answer
Hello,

Technically speaking one need not have magnetic reconnection to qualify for a flare. However, one would need to have at least a magnetic mirror system in place, according to one paper by Lennartson (in 'Planetary Science' I believe, 1979). The paper, as I recall, showed that It is known, for example, that *both magnetic mirrors and double layers* (go back to the previous questions you asked on these and the answer I gave) are active simultaneously with the former presence necessarily leading to double layers in which a voltage drop is produced.

If the voltage drop is large enough, one can observe flare conditions.

According to Alfven ('Cosmic Plasma', 1981), any double layers within region "separators" will “explode” before a saturation level of current is reached such that

I_s  =    [V(b) – V(D)] / R


where V(b) =  L(dI/dt) + RI

with L the inductance, and I the current, R the resistance.

and the flare power (P) = I V(D)

where V(D) is the voltage across the double layer.

The problem with the Earth flare hypothesis is that it is impossible to see how a magnetic mirror system would operate for it, given the low altitudes (?) at which the phenomenon appeared to be seen by the questioner ('David'). No magnetic mirrors, no likely Earth flares! (Since no double layers would form to enable an explosive release independent of reconnection- and the mere existence of plasma is no signature for a flare.)

As to your next question, yes, in a way, magnetic substorms can be considered a kind of Earth flare - though I doubt our questioner would see it that way.

The ongoing interest in the solar flare process has incepted a rivalry between two paradigms: unloading and driven. The former has received most prominence in the Space Physics domain, with Magnetosphere-Ionosphere (M-I) coupling models explored by Akasofu (1979), Kan et al (1983, 1994) that invoke such auroral substorm mechanisms as V-, S-potentials, double layers and field-aligned potential drops.  

The general emphasis is on currents and current systems, namely the “field aligned” current density and how it can configure an energy balance in solar flare descriptions. This has also been labeled the “E-J paradigm”  after the electric field (E) and current density (J)  that is emphasized.

V-potentials (or V-shaped potential structures or inverted- V events), actually refer to discrete auroral arcs in the ionosphere, at energies of several keV or more, obtained by space-based observations.   However, the role of these sub-storm artifacts is entirely vague in the context of the solar situation and moreover unobserved. Up to now no solar team or researcher has unambiguously identified a specific solar structure or process that might be tied to these.

They are rather uniquely auroral structures because the energy vs. time plots (of particle fluxes associated with auroras made by space borne detectors) roughly mimic the shape of an inverted V. No similar solar flare energy vs. time plots do this. So, on the one hand, the term "Earth flare" in respect of magnetic substorm processes is *not* appealing!

One implication of the E-J paradigm models is that adequate power for flares can be provided once sufficiently large field-aligned potential drops can occur (as in double layers) arising from sufficiently large longitudinal (J‖) current densities. (cf. Kan et al, Solar Phys. Vol. 84, 1983).

The Kan et al model is intriguing if for no other reason it invokes a model ("dynamo") peculiar to auroral substorms in order to account for flares - without having to use magnetic reconnection.

In the dynamo model of Kan et al (1983), the “neutral wind” acts perpendicularly to the field –aligned ( J ‖ ) and cross-field (J ⊥ ) current (see Fig. 1 of Kan et al, 1983). The wind is described in their paper as a “shear flow” (p. 154).

This conjecture is then much further developed into a “dynamo region” that generates flares, and which features “neutral winds” and V-, S-potentials. A double layer process is also invoked. The core equation that may be said to govern detailed energy balance in such a dynamo may be written (e.g. Kan et al, 1983):

- DIV•S = - DIV• (E x b/ u_o ) = E• J

which states that the power density delivered by the Poynting vector S  to the double layer potential  (-DIV*S ) is balanced everywhere by the power density removed by the current –carrying particles J•E.  (Note here that – J•E  = E• J).

In detailed auroral models it can be shown that the "dynamo currents" in such a process flow earthward on the morning side of the magnetic pole and spaceward on the evening side. The circuit can be visualized completed by connecting the two flows across the polar ionosphere, from the morning side, to the evening side. This is exactly what Kan et al have done in arriving at their “dynamo solar flare” model. That is, taken a mechanism that might be justified in the case of the aurora – and transferred it to the solar flare situation.

Bottom line? If the model for flares proposed by Kan et al is taken seriously, and in its context, then indeed, magnetic substorms are a form of Earth flare"!

My problem with the above inheres in the fact that circuits on the Sun – given unidirectional current flows – need not be driven by any “dynamo action” – or be part of any dynamo. Further, one doesn’t require a dynamo to have a conservative energy system to account for solar flares. It is possible to use the conservation of magnetic helicity in a more general context to explain energy balance – especially for large, two-ribbon flares.

Given the fact that we can actually observe magnetic helicity buildup (cf. Demoulin et al, Solar Phys. 2006) in terms of the angles made by solar arcade footpoints, it stands to reason that this is a superior proxy for assessing energy accumulation in the pre-flare stage. Indeed, a heuristic “fast dynamo” has been developed (Vainshtein and Zel’ dovich, Ap. J. 2005) predicated on a “stetch-twist-fold” cycle that generates helicity. In this case, the field is maintained by the motion of the MHD fluid, rather than currents at infinity.

I do hope this all makes some kind of sense! If not, feel free to send a follow-up.