Ionospheric Response To Solar Flares

Now we shall take note of a special class of effects called Sudden Ionospheric Disturbances (SID). These constitute those events that arise as a result of the atmospheric interaction with electromagnetic flux from solar flares. A book by Mitra [1974] is an excellent treatise on the ionospheric effects of solar flares.

We recognize that the sun is the ultimate source for a large variety of ionospheric and magnetospheric effects. Figure 3-24 exhibits the hierarchy of solar-induced ionospheric effects. There are many types of SID observed, and one of the most important is the Short-Wave-Fade (SWF) which affects HF communication circuits on the sunlit side of the earth. The source of the enhanced D-region ionization responsible for the SWF is typically an impulse burst of x-ray energy from within an active region on the sun (generally a sunspot). An x-ray flare generates a significant increase in D layer ionization with a temporal pattern that mimics the flare itself. This results in an increase in the product of the electron density and the collision frequency. It is the growth of this product that accounts for the absorption of HF signals passing through the D-region. Flares tend to be more prevalent during the peak in sunspot activity, and the individual flare duration distribution ranges between a few seconds to roughly an hour.

Electromagnetic Radiatirjn

Magnetosphere

Geomagnetic Sturm

Hlgfi-ertsrgy Solar Particles

Solar-wind Impact on the .Magnetosphere

Short wa\ie Fades (SWF)

Polar-Cap Absorption {PGA)

Ionosphere Storm Effecîs

Enhance Auroral Effects

Geomagnetic Sturm

Electromagnetic Radiatirjn

Hlgfi-ertsrgy Solar Particles

Solar-wind Impact on the .Magnetosphere

Short wa\ie Fades (SWF)

Polar-Cap Absorption {PGA)

Magnetosphere

Figure 3-24: Hierarchy of solar-terrestrial effects. From Goodman [1991].

Figure 3-25 below is an example of a short wave fade. As indicated previously, the fade pattern mimics the pattern exhibited by the 1-8 Angstrom x-ray flux. Refer to Chapter 5 for more information about nowcasting these effects. (Figure 5-7 derived from the NOAA-SEC web site depicts a nowcast for global D-region absorption based upon the observation of an x-ray event. An x-ray flare, also extracted from the SEC web site is given in Figure 5-9, albeit for a separate event.) Any forecasting of short wave fade events is problematic, since only the tendencies can be predicted from other observables (i.e., active regions, etc.).

1300 | 1500 | 1700 | 1900 | 2100 | 2300 | 1400 1600 1800 2000 2200 2300

SWF Association with a Solar Flare

1300 | 1500 | 1700 | 1900 | 2100 | 2300 | 1400 1600 1800 2000 2200 2300

SWF Association with a Solar Flare

Figure 3-25: Sample short wave fade (SWF). From US Navy sources.

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