No abstract

Coordinated in situ measurements of high-resolution (spatial resolution 35 m) irregularity structures in the topside ionosphere using the retarding potential analyzer (RPA) on the AE-E satellite and scintillation measurements using the geostationary satellite Marisat transmissions at 257 MHz and 1.54 GHz from Ascension Island were made in December, 1979 during the recent solar maximum period. The in situ irregularity spectra could be classified into two categories: those having or not having spectral breaks. The first category of spectra is characterized by two spectral indices: one above and the other below a scale-length that lies typically between 500 m and 1 km. The spectral index in the long scale-length end (10 km to -,• 1 km) varies between -1 to -1.5, whereas the index in the short scale-length end(-,• 1 km to 70 m} is between -3 to -3.5. In the second category, the power law spectral index of the irregularities ranges between -2 to -2.5 over the entire scale-length range of 70 m to 10 km. The in situ measurements reveal that in the early evening hours, the E-W gradient scale-lengths can be as small as 35 m, the sampling interval of the RPA instrument. In the pre-midnight and post-midnight hours, the gradient scale length becomes larger, but scale lengths of several hundred meters are frequently encountered. The temporal variation of 1.54-GHz scintillation magnitudes are found to track the E-W bubble structure, scintillations being minimum when the ray traverses the center and large when it crosses the eastern and western walls of the bubble. On the other hand, scintillations at 257 MHz remain saturated during an encounter with a bubble and focussing is often observed, but the autocorrelation interval tracks the bubble structure, being large when the ray path is deep within the bubble where the perturbation is weaker. The power spectra of weak 1.54-GHz scintillations are found to be consistent with the predictions of weak scatter theory based on irregularities having the observed in situ spectra. Significant differences are noted in the spectra of strongly scattered 257-MHz scintillations. In this case the decorrelation bandwidth extends beyond 1 Hz, and the spectral slope of the transitional portion of the spectrum becomes as steep as -6. The moderate to strong GHz scintillations sometimes show a dual slope power spectrum with the lower frequencies exhibiting a slope of -1.5 and the higher frequencies showing a slope of-5.5. The observations of steep structures in irregularities with scale-length of a few hundred meters near midnight, implying relatively slow erosion of sharp gradients are discussed in terms of plasma processes in the topside ionosphere. pling interval) satellite in situ data have shown that in the early phase, the bottomside irregularity spectra exhibit shallow slopes due to steep spatial structures, whereas in the late phase erosion of these steep structures lead to spectra with steeper slopes [Basu et al., 1980]. High-resolution (--• 15 m) rocket data were the first to show that under...

No abstract

Polar cap ionospheric irregularities have been monitored for several years from Thule Air Base, Greenland using 250 MHz satellite beacon scintillation measurements. The seasonal pattern of the polar cap irregularities shows very high intensity levels during the winter and lower levels during the summer (sunlit) months. This behavior is similar to in‐situ polar cap electric field measurements which show larger fluctuations in the winter than in the summer, an effect which may be related to E layer conductivity changes. A striking contrast was noted between high scintillation levels observed during 1979/80, a year of high solar flux, and much lower levels observed during 1975, a year of low solar flux. This variation may be related to a corresponding solar cycle variation in polar cap F layer electron density. The data reveal little difference between periods of high and low Kp, and only a weak diurnal variation in any season. Direct optical and ionosonde measurements indicate that these scintillations are produced by ionospheric irregularities in the polar cap F‐region. Results of spaced receiver drift measurements indicate that the small scale of irregularity drift was antisunward. Intense irregularities are associated with discrete sun aligned F layer auroras. A weak background level of scintillation persisted in the high solar flux years.

A comparison of scintillation levels at 1.5 GHz made from the Appleton anomaly region of the magnetic equator and from the region close to the magnetic equator (termed the electrojet latitudes) showed increased F region irregularity intensity over the anomaly region during years of high sunspot number. Peak to peak fading greater than 27 dB was noted from Ascension Island (through a dip latitude of 17 ø) in the anomaly region while only 7-9 dB from Natal, Brazil, and Huancayo, Peru, were noted, the last two paths being close to the magnetic equator. The hypothesis advanced is that the dominant factor responsible for the intense gigahertz scintillation is the traversal of the propagation path through the anomaly region. During years of high sunspot numbers the high levels of/XN constituting the F region irregularity structure are due to (1) very high electron density in the anomaly region (compared to the electrojet region) and (2) the late appearance of these high electron densities (to 2200 local time) in the anomaly region. The patches or plumes of irregularities seen in the postsunset time period then produce high /XN; scintillation excursions are proportional to this parameter. The postulation of vertical irregularity sheets in the patches was examined to determine the possibility of this being an important factor in the difference between electrojet and anomaly scintillation levels. Older gigahertz data from the sunspot maximum years 1969-1970 were reanalyzed, and more recent observations from other studies were also reviewed. It was found •that through the anomaly region, high scintillation indices were noted at a variety of azimuths of the propagation path rather than just along a path closely aligned with the magnetic meridian. A more complete evaluation of the geometrical factor, which must be of considerable importance in determining the absolute value of the scintillation intensity, awaits further observations.