SIDs recorded from assorted distinct generating flares (denoted by flare class) over Feb. 18.
SIDs (Sudden Ionospheric Disturbances) and the flares that generate them appear to be on the increase again as solar cycle 24 continues toward maximum. The latest news: the fast-growing sunspot complex 1161-1162 erupted on Feb. 18th, producing an M6.6-class solar flare. The almost-X category blast was one of the strongest flares in 4 years and continued the week-long trend of high solar activity. NOAA forecasters estimate a 75% chance of more M-flares during the next 24 hours.
Any time such flares erupt they are almost certainly likely to spawn "SID flares", or which I first defined in my 1983 research as "SID-generating flares" which appear to be associated with a particular (complex) morphology of sunspots. In addition, this translates into much more powerful and energetic aurorae over larger geographical areas as the auroral oval (see graphic) expands. See the papers:
http://adsabs.harvard.edu/full/1987SoPh..112..387S
and
http://www.springerlink.com/content/k5168403190u1386/
As the flares occur, highly ionizing radiation also rushes toward our planet at the speed of light, reaching it in about 8 1/3 minutes. Subsequently, waves of ionization ripple through Earth's upper atmosphere in response to the onslaught of solar flares. This affects the propagation of radio signals--suppressing some frequencies and boosting others. By monitoring distant transmitters at a frequency of 23.4 kHz, Rudolf Slosiar of Bojnice, Slovakia detected nearly a dozen sudden ionospheric disturbances (SIDs) on Feb. 18th, as seen in the accompanying graph.
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:
Beta (much less) than (B^2/ 2u_o)/ (0.5 rho v^2)
where u_o = 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 0.5Å - 8Å 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.
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