Sporadic E

General Characteristics. Even though the normal E region is Chapman-like in nature, isolated forms of ionization are often observed in the E-region, having a variety of shapes and sizes. These ionization forms have been termed sporadic E, because they appear quasirandomly from day to day, and they generally defy deterministic prediction methods. Sporadic E (Es) ionization has been observed during rocket flights and with incoherent backscatter radar, and a layer thickness of the order of 2 km has been observed. It generally takes the form of large-scale structures, having horizontal dimensions of hundreds of kilometers at middle latitudes.

Fig. 12. Variations in the ionosphere thought to be associated with traveling ionospheric disturbances. The foF2 variations shown here are of the order of ±2% and have periods of ~20 min. The NmaxF2 variations are ~ ±1%. [From Paul (63).]

Fig. 13. Effect of a large geomagnetic storm on Nmax. [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11).]

T f Г I 1———————————— 1——- 1——— г

1944 46 48 50 52 54

Year

Fig. 14. Variation in R12, foF2, foE, and 4 MHz absorption at noontime. The seasonal effects are clearly evident, the foE and D-layer variations being out of phase with the foF2 variations (i. e., seasonal anomaly). [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11).]

Polar and equatorial forms have different structures and causal mechanisms. Although sporadic E consists of an excess of ionization (against the normal E-region background), it does not appear to be strongly tied to solar photoionization processes. Still, midlatitude Es occurs predominantly during summer days. Sporadic E does exhibit seasonal and diurnal tendencies, which have been examined statistically, and at least three different types of sporadic-E ionization have been discovered with distinct geographical regimes: low-latitude

Fig. 15. Long-term variation inRi2, foF2, and foE at noontime. Since running 12-month averages were taken, the seasonal effects observed in Fig. 14 are smoothed out. [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11).]

(or equatorial), midlatitude (or temperate), and high-latitude ionization. Figure 16 depicts the probability of Es occurrence.

Formation of Midlatitude Sporadic E. It has 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 the examination of photochemistry in the ionosphere that molecular ions such as those that exist in the E region introduce 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 they 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.

The influx of this foreign mass of metallic ions, when distributed over the whole of the E region, would be insufficient to overwhelm the omnipresent molecular species (such as NO+), which are in a state of photo­chemical equilibrium, were it not for a mechanism that preferentially concentrates the meteoric debris ions. Apparently wind shear is this mechanism. The basic wind shear theory was proposed by Whitehead (26), but it remained for Gossard and Hooke (27) 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 TIDs as well. The ultimate process involves a corkscrew propagation of atmospheric gravity waves and atmospheric tides, which results in a rotation of wind velocity as a function of altitude. This

Auroral zone

High temperature zone

1951 1952

Fig. 16. Probability of Es occurrence as observed in the period 1951-1952. It is representative of the global, seasonal, and diurnal variation of sporadic-E ionization. [From Davies (1).]

effect can cause the wind to change direction over an altitude of only a kilometer or so, so as 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.

Sporadic E at Non temperate Latitudes. The high-latitude sources are evidently of two types, depending upon whether the observation is made in the neighborhood of the auroral oval or poleward of it

Anchorage Fairbanks

55 Є0 * j 65

Invariant latitude (deg)

Fig. 17. Idealized picture of ionospheric plasma frequencies in a north-south plane through Fairbanks and Anchorage, Alaska. E, region equatorward of trough; B, equatorward edge of trough: C, plasma frequencies (MHz); D, trough minimum; E, plasmapause field line; F, poleward edge of trough; G, F-region blobs; H, enhanced D-region absorption; I, E-region irregularities. [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11), after Hunsucker (28).]

(i. e., in the polar cap region). It has been found that auroral Es is basically a nocturnal phenomenon; it is associated with the optical aurora and is due to auroral electron precipitation. Because of its proximity to the seat of auroral substorm activity, it is not surprising to find some correlation between auroral Es and some appropriate magnetic index. Indeed, it has been found that auroral Es is positively correlated with magnetic activity. On the other hand, polar-cap Es may be relatively weak, and is negatively correlated with substorm activity.

Turning equatorward, it has been found that equatorial Es is most pronounced during daylight hours, and evidence points to the formation of ionization irregularities within the equatorial electrojet as the responsible agent at low latitudes.

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