Solar Flares

One of the more well-known solar events responsible telecommunication disturbances is the solar flare. This is certainly true for HF communications. These events trigger many of the short duration ionospheric events called Sudden Ionospheric Disturbances (SID), and are closely related to other solar phenomena. Sunspot occurrence is closely associated with the observation of solar flares. In general the number of flares observed per solar rotation NF is proportional to the sunspot number.

where R is the smoothed sunspot number over a 27-day rotation period, and a is a constant which ranges between 1.5 and 2. Thus for R = 110 (near solar maximum), the value for NF is about 200. This implies that 7 flares/day will be observed on a global basis.

Flares have been classified in terms of the solar surface area that is enclosed as observed in the hydrogen-a line of the solar spectrum. Subflares cover ~ < 2 square degrees but the largest class of flares may cover about ~ 25 square degrees of the solar surface.

Another optical designation provides a qualitative indication of the brightness of the flare: F=faint; N=normal; B=bright. The most important flare classification for association with ionospheric effects is the flare strength as measured in the x-ray band. Table 2-5 shows the x-ray classifications in the 1-8 Angstrom band. This is a useful scale; in general only those flares with M and X classifications have any practical significance (i.e., enhanced electron production in the D-layer) leading to radiowave absorption. The process of D-layer absorption is covered in Chapter 4.

Table 2-5: Classification of x ray flares

Class of Flare

X-Ray Energy Output E at Earth (Watt/m2)


E> lO"4


10"5<E< 10"


lO^ E < 10"5


E< 10"6

An important illustration of the relationship between the sunspot number, ionospheric storms, and sudden ionospheric disturbances (SIDs) is given in Figure 2-12. The SIDs are directly related to x-ray flares, and the ionospheric storm variation is directly proportional to the incidence of magnetic storm activity. It is evident from Figure 2-12 that SIDs tend to favor the ascending phase of sunspot activity, whereas ionospheric storms favor the descending phase (see Section 2.2.8).

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