Intense Scintillation Research Articles

Coordinated measurements of auroral zone plasma enhancements

The Air Force Geophysics Laboratory's Airborne Ionospheric Observatory (AIO) performed radio, optical, and scintillation measurements on a series of north‐south flight legs along the Chatanika radar magnetic meridian. The incoherent scatter radar was operated in a north‐south magnetic meridian scan mode to measure ionospheric parameters (electron density, temperature, and ion drift) from 600 km north to 600 km south of the radar and from ∼80 to 700 km altitude. Ionospheric structure measured by the radar is compared with remote optical and ionosonde measurements from the AIO and with precipitating electron characteristics measured by a Defence Meteorological Satellite Program satellite. Long‐lived F region plasma enhancements (plasma blobs) were observed during this experiment. From the simultaneous measurements it is shown that these enhancements were not locally produced by precipitating particle fluxes. F region electron concentrations, calculated from simultaneously observed precipitating electron fluxes, are significantly less than those observed in the plasma enhancements. These premidnight features (plasma blobs) were in fact observed to be convecting sunward (to the west) at a few hundred meters per second and are thus presumed to have been produced well upstream in the convection flow within a region of significantly greater production rate. Intense scintillation of satellite signals due to ionospheric irregularities is generally confined to these regions of enhanced F region density. Simultaneous measurements of independent parameters identifying the auroral E layer (1–20 keV precipitating electrons, 4278‐Å N2+ emission, f0E, and radar electron densities) all agree well as to location and the latitude profile of the auroral E layer and associated diffuse aurora.

Observations of cloud‐produced amplitude scintillation on 19‐ and 28‐GHz earth‐space paths

The amplitudes of satellite signals sometimes scintillate ± several decibels when heavy cumulus clouds pass through the radio path on hot summer days. These scintillations have been measured on a 19‐GHz and 28‐GHz earth‐space path using 7‐m and 0.6‐m antennas at Crawford Hill, New Jersey. Scintillation intensity at 28 GHz is 1.2 times that at 19 GHz, consistent with the ƒ7/12 frequency dependence produced by a thin turbulent layer. The scintillation process is polarization‐independent and has a low‐pass power spectrum with a cutoff frequency of about 0.3 Hz. Rain attenuation often accompanies the more intense scintillation. The mean duration of scintillation‐produced fades is short (about 1.3 s for fades greater than 1 dB), but over 1000 fades of over 1 dB at 28 GHz were observed in two summer months. Because of their weak frequency dependence these short but frequent events could produce repeated outages on 4‐ and 6‐GHz earth‐space links having low attenuation margins.

Microwave equatorial scintillation intensity during solar maximum

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°) 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 ΔN 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 ΔN; 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.

Intense daytime radio wave scintillations and sporadic E layer near the dip equator

The paper describes two cases of very intense scintillation activity (amplitude exceeding 10 dB) recorded at Huancayo during the daytime on radio beacons on 257 MHz from Marisat satellite and ATS 3 (137 MHz) and LES 6 (254 MHz) signals. Both these events are shown to be associated with an intense blanketing equatorial sporadic E layer at Huancayo. It is suggested that the bursts of strong daytime scintillations of VHF radio waves are due to densely ionized thin sheets of ionization over the equator and only normally weak (≈ 1 dB) daytime scintillations may be due to instabilities associated with the q type of equatorial Es.

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Multistation and multi-technique observation of equatorial ionospheric irregularities from locations over the magnetic equator through the northern crest of the Equatorial Ionization Anomaly (EIA) and beyond is one of the important objectives of the Indian CAWSES program. For this purpose, multistation observational campaign was conducted during September 2011, April 2012 and September 2012 involving GPS TEC and S4 from Calcutta situated virtually underneath the northern crest of the EIA, and GPS S4 measurements from Siliguri, located beyond the northern crest of the EIA. Intense amplitude scintillation observations have been noted around midnight and post-midnight hours on certain GPS links located north of Siliguri with no scintillation patches during pre-midnight period. Satellite links experiencing scintillations from Siliguri were unaffected at Calcutta. Day-to-day variability has been noted in the maximum northern observation of GPS scintillations at different S4 levels from both Calcutta and Siliguri. Simultaneous observations from Siliguri and Calcutta indicate that post-midnight GPS amplitude scintillations are possibly associated with interaction of traveling ionospheric disturbances from mid-latitudes towards the equator, with transionospheric GPS links contrary to early evening hours when such movement are usually from over the magnetic equator towards the northern and southern crests of EIA. The present paper reports three cases of ionospheric irregularity tracking using the above stations on September 25, 2012, April 12, 2012 and September 11, 2011, all magnetically quiet days. Information related to the variabilities of latitudinal extent of scintillation observations along 88°E meridian at different times during 13–19 UT and different levels of intensities of scintillation could provide useful information for transionospheric satellite link design and Satellite-based Augmentation System (SBAS).

Nighttime ionospheric radio scintillations and vertical drifts at the magnetic equator

The scintillation of VHF radio waves traversing the equatorial ionosphere is largely a nighttime phenomenon starting shortly after sunset and reaching maximum occurrence around 2100–2200 LT. Comparing the F-region vertical drift velocities (produced by the east-west electric field) at Jicamarca and the amplitude scintillation of VHF radio waves at Huancayo, it has been shown that besides the usual pre-midnight events, any abnormal reversal of the equatorial horizontal electric field to the eastward direction at night is followed by the occurrence of intense VHF scintillation with a delay in time of the order of an hour. Temporary reversals of the electric field lasting less than half an hour do not seem to develop irregularities of strong enough intensity to produce scintillations.

A Possible Interpretation of the Fast Regular Fading observed on Satellite Transmissions

Frihagen and Troim1 have already observed a deep and nearly sinusoidal type of fading on fast amplitude records at 108 Mc/s. The effect only lasted for a few periods and a time delay between similar features on records taken at two spaced aerials indicated that it was introduced in the ionosphere. They proposed that this phenomenon was caused by diffraction at a single ‘blob’ in the ionosphere and that it is similar to that which was observed in radio-star scintillation by Wild and Roberts2. Apparently the same kind of amplitude variations were observed on several occasions at Kiruna (67.8° N., 20.4° E.) (ref. 3) on records of Transit IV A at 54 Mc/s which showed intense scintillation. The phenomenon was usually more persistent than that observed by Frihagen and Troim and often lasted up to a hundred periods. The fading periods varied between 50 and 500 msec, but there was a predominance of periods of about 100 msec. Fig. 1 gives an example of persistent regular fading lasting about 10 sec. Some cases of the phenomenon of short duration (about 1 sec) which exhibit monotonically increasing (or decreasing) amplitude and period may be explained by diffraction at single ‘blob’ in the ionosphere. However, it is difficult to explain long lasting, regular fading in this way.