| The irregularities in the electron density distribution of the ionosphere
over the equatorial region frequently disrupt space-based communication and
navigation links by causing severe amplitude and phase scintillations of
signals. Development of a specification and forecast system for
scintillations is needed in view of the increased reliance on space-based
communication and navigation systems, which are vulnerable to ionospheric
scintillations. It has been suggested in recent years that a developed
equatorial anomaly in the afternoon hours, with a steep gradient of the
F-region ionization or Total Electron Content (TEC) in the region between the
crest and the trough, may be taken as a precursor to scintillations on
transionospheric links. Latitudinal gradient of TEC measured using Faraday
Rotation technique from LEO NOAA 12/14 transmissions during the afternoon
hours at Calcutta shows a highly significant association with L-band
scintillations recorded on the INMARSAT link, also from Calcutta, during the
equinoxes, August through October 2000, and February through April 2001.
The daytime equatorial electrojet is believed to control the development of
the equatorial anomaly and plays a crucial role in the subsequent
development of F-region irregularities in the post-sunset hours. The diurnal
maximum and integrated value (integrated from the time of onset of plasma
influx to off-equatorial latitudes till local sunset) of the strength of the
electrojet in the Indian longitude sector shows a significant association
with post-sunset L-band scintillations recorded at Calcutta during the two
equinoxes mentioned earlier.
Generation of equatorial irregularities over the magnetic equator in the
post-sunset hours is intimately related to the variation of the height of
the F-layer around sunset. Ionosonde data from Kodaikanal, a station situated
close to the magnetic equator, has been utilized to calculate the vertical
drift of the F-layer over the magnetic equator for the period August through
October 2000. The post-sunset F-region height rise over the magnetic equator
shows a remarkable correspondence with the occurrence of scintillations at
Calcutta located near the northern crest of the equatorial anomaly.
Existence of a flat-topped ionization distribution over the magnetic equator
around sunset has been suggested as a possible indication of occurrence of
post-sunset scintillations. Width of the latitudinal distribution of
ionization obtained from DMSP satellite shows some correspondence with
post-sunset L-band scintillations. During the period of observation of the
present study (August through October 2000, and February through April
2001), it has been observed that although the probability of occurrence of
scintillations is high on days with flat-topped ion density variation over
the equator, there are cases when no scintillations were observed even when
a pronounced flat top variation was recorded. |