Expert: Philip A. Stahl - 2/14/2009

Question
Hello. I recently downloaded a paper you wrote on "SID flares" and am still confused as to what causes them. What makes a flare an "SID" flare? Are there special astrophysical criteria that apply which don't for normal flares? Can we predict them?

Answer
Hello,

Verey interesting that you were able to find and DL my paper. Anyway, let's get to your questions in the order asked:

In my original paper (Solar Phys., 1983, Vol. 88, p. 137) I noted that the theoretical basis for these flares (like normal H-alpha optical flares)is likely the rapid release of free magnetic energy (MFE) in major active regions that has been accumulated during the shearing of the associated force-free field.

a force -free field is one for which the conditions apply:

1) Curl B = alpha (B)

2) (B*grad) alpha = 0

where alpha is a scale factor, and B the magnetic induction.


More fundamentally, the force-free assumption reuqires that we have for the Lorentz force:

J X B = 0

where J is the current density. Thus, the current density is essentially parallel to the magnetic induction.

Typically for plasma regions in which this holds, one finds that the plasma beta << 1.

since the plasma beta is defined by the ratio:

(B^2/ 2u)/ (0.5 rho v^2)

[u = magnetic permeability, rho = plasma density, v = fluid velocity]

where the numerator is the magnetic energy density and the denominator the kinetic energy density, then it means the behavior is dominated by the former.

The result of this is that the plasma's effects can be separated from those of the magnetic field making a full and detailed MHD (magneto-hydrodynamic) description unnecessary.

Once all these conditions hold, the stage is set for the triggering of the flare. This may occur by any number of immediate causes, including impact of a Moreton wave (google it!) with the magnetically stressed arcade or loop, or the sudden emergence of new magnetic flux in the existing region, or perhaps a kink instability unique to the loop.

To transform specifically into an SID flare (and recall 'SID' means sudden ionospheric disturbance) the emitted soft x-ray flux needs to surpass the threshold of: 2 x 10^-3 erg cm^-2 s^-1. (A nice exercise here is for you to be able to convert these units to J m^2 s^-1!). This was pointed out in my original paper.

My paper was mostly concerned with D-region ionizing flares, which generate SIDs in the form of: SWFs, or shortwave fadeouts (as the words imply actual fadeouts of high frequency radio waves occurring at the same time as the flare), SPAs, or sudden phase anomalies, generally caused by hard x-rays in the 0.5A - 8A band that engender a reduction in the reflection height for the incoming waves, and SEA or sudden enhancement of atmospherics- specifically, enhanced intensity of VLF or very low frequency waves.

The D-region tie-in to solar flares is of interest because the D-region is the primary seat of radio absorption. So measurements of the degree of absorption (of radio waves) in the region are a good proxy way of monitoring flare effects.

In general, absorption in the D-region varies with the solar zenith angle (Z) as:

A ~   cos( Z)^n

where:

0.7  <  n  <  1.0

As for special astrophysical criteria which apply that don't for normal flares, I am currently working on this problem in a book I have 90%  completed. My preliminary finding is that there can be low frequency turbulent fluctuations in a tiny resonant cavity in the high corona (of a solar loop) that can, under certain conditions, attain overall resonance with the whole loop. When this happens, eruption of a loop flare is possible (via 'two-stream instability'- see prior answers to do with it)  with intensity large enough to cause SID flares. But bear in mind this is still a work in progress and I am in the process of doing more numerical simulations.

As for predictions, clearly, until and unless a model such as the one I've tentatively proposed (above) can show a temporal or causal link between the synchronous resonance and occurrence of D-region disturbances on Earth - we will remain in the realm of mostly conjecture and hypothesis.

Hope this helps!