Formation of Midlatitude Sporadic E

It has been suggested that wind shears in the upper atmosphere are responsible for the formation of sporadic E at midlatitudes. We shall review this process briefly. It should be recalled from examination of photochemistry in the ionosphere that molecular ions such as those, which exist in the E region, introduce relatively rapid electron loss by recombination. At the same time it is recognized that an enormous number of meteors burn up in the E region. This meteoric debris is largely comprised of metallic ions, which are monatomic. Their presence has been confirmed by mass spectroscopy measurements using rockets, and these atomic species include iron, sodium, magnesium, etc. Since monatomic ions exhibit a small cross section for electron capture, the process by which atomic ions become concentrated in well-defined layers will lead to reduced loss rates for ambient free electrons in the interaction region.

Auroral Zone

Figure 3-17: Probability of Es occurrence as observed in the period 1951-52. It is representative of the global, seasonal, and diurnal variation of sporadic-E ionization. A form of sporadic E is virtually omnipresent near the auroral zone during nocturnal hours. Another form of Es ionization is evidenced during the midday period at (geomagnetic) equatorial latitides. At middle latitudes, sporadic E is most pronounced during the summer daytime hours. However, no portion of the diurnal cycle is completely immune from the effects of sporadic E. From Goodman [1991], after Davies [1965],

Figure 3-17: Probability of Es occurrence as observed in the period 1951-52. It is representative of the global, seasonal, and diurnal variation of sporadic-E ionization. A form of sporadic E is virtually omnipresent near the auroral zone during nocturnal hours. Another form of Es ionization is evidenced during the midday period at (geomagnetic) equatorial latitides. At middle latitudes, sporadic E is most pronounced during the summer daytime hours. However, no portion of the diurnal cycle is completely immune from the effects of sporadic E. From Goodman [1991], after Davies [1965],

The influx of this foreign mass of metallic ions when distributed over the whole of the E region is still insufficient to overwhelm the omnipresent molecular species (such as NO+), which are in a state of photochemical equilibrium were it not for a mechanism which preferentially concentrates the meteoric debris ions. Apparently wind shear is this mechanism. The basic wind shear theory was proposed by Whitehead [1970], but it remained for Gossard and Hooke [1975] to outline a process for meteoric ion concentration based upon the interaction of the meteoric debris with atmospheric gravity waves, the latter wave structures being responsible for the development of traveling ionospheric disturbances (TIDs) as well. The ultimate process involves a corkscrew propagation of atmospheric gravity waves and atmospheric tides that results in a rotation of wind velocity as a function of altitude. The velocity rotation effect can cause the wind to change direction over an altitude of only a kilometer or so, sufficient to trap meteoric ions at an intermediate point having zero velocity. This buildup in a narrow region is sufficient to generate an intense sporadic E patch.

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