Diurnal Variation of Scintillation

Scintillation varies diurnally, and it is principally a nighttime phenomenon. It exhibits a climatology that is unique to four geographical regions: equatorial, midlatitude, auroral, and the polar cap. The most virulent zone corresponds to the post-sunset equatorial region, and the least active zone is at middle latitudes. Scintillation at equatorial latitudes peaks shortly following sunset and may persist throughout the evening but with decreasing virulence after local midnight. At high latitudes, scintillation is also more intense during the night in concert with the natural growth in oval intensity, but midlatitude scintillation may arise during storms as the scintillation boundary descends equatorward. Moreover, the normal diurnal variation of scintillation may become distorted by the superposition of storm-time influences.

Daytime scintillation events may also arise at midlatitudes as a result of sporadic E, and these events are sometimes termed "ringing irregularities" as a result of the quasi-periodic nature of the fading events [Goodman, 1967]. Figure 4-20 illustrates daytime scintillation occurrence at VHF (i.e., the geosynchronous Early Bird satellite downlink) compared to sporadic E (i.e., Es) and spread-F. Figure 4-21 is an example of the quasi-periodic disturbances caused by the interference of direct rays and refracted rays from the sharp edges of sporadic E patches.

"Worst Case" Fading Depths at L-Band

Solar Maximum L-Band Solar Minimum

■20 dB ■ 15 dB E310dB ea 5dB n 2dB . G3 1 dB>

Figure 4-19: Representation of ionospheric scintillation for solar maximum and minimum at L-band. From Goodman and Aarons [1990], courtesy of Santimay Basu.

Figure 4-19: Representation of ionospheric scintillation for solar maximum and minimum at L-band. From Goodman and Aarons [1990], courtesy of Santimay Basu.

1 1 1 1 1 1 1 0 2 4 6 8 10 12

i i 14 16

18

I I I 20 22 24

Occurrences of Spread F

Fort Belvoir lonosonde

~ « I ] I iítT I i i

i I

I

I

III!

0 2 4 6 8 10 12

14 16

18

20 22 24

Occurrences of Scintillation Disturbances

Early Bird III I

I

I

,Dat,a 7*rl 11 I

I ll

I

J,

I I

Figure 4-20: VHF scintillation occurrences near Chesapeake Beach Maryland and compared to the 8-day averaged jo Es and instances of spread-F measured at Fort Belvoir, Virginia. The experiment ran from May 21 - May 28,1965. While this evidence is circumstantial. Figure 4-21 shows the correlation between scintillation and Es more clearly. From Goodman, [1967].

Figure 4-21: Depiction of noisy and quasi-periodic scintillation thought to be the result of sporadic E edge refraction. It is noteworthy that the sporadic E "blanketing" frequency fbEs reached high levels during the period of time the disturbances were observed. The distance between the Ft. Belvoir ionosonde and the relevant ionospheric piercing point for the radio path was ~ 300 km (with the Es height taken to be 100 km). Hence we would not expect perfect correlation. From Goodman [1967],

Figure 4-21: Depiction of noisy and quasi-periodic scintillation thought to be the result of sporadic E edge refraction. It is noteworthy that the sporadic E "blanketing" frequency fbEs reached high levels during the period of time the disturbances were observed. The distance between the Ft. Belvoir ionosonde and the relevant ionospheric piercing point for the radio path was ~ 300 km (with the Es height taken to be 100 km). Hence we would not expect perfect correlation. From Goodman [1967],

Was this article helpful?

0 0

Post a comment