|
|
 |
|
|
Ann. Geophys., 25, 855-861, 2007
www.ann-geophys.net/25/855/2007/
© European Geosciences Union 2007
On the seasonal variations of the threshold height for the occurrence of equatorial spread F during solar minimum and maximum years
G. Manju, C. V. Devasia, and R. Sridharan
Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum – 695 022, Kerala, India
Abstract. A study has been carried out on the occurrence of bottom side equatorial
spread F (ESF) and its dependence on the polarity and magnitude of the
thermospheric meridional wind just prior to ESF occurrence during summer,
winter and equinox seasons of solar maximum (2002) and minimum years (1995),
using ionosonde data of Trivandrum (8.5° N, 76.5° E, dip=0.5° N)
and SHAR (13.7° N, 80.2° E, dip ~5.5° N) in the Indian
longitude sector. In this study, we have examined the changes in the
threshold height of the base of the F layer for the triggering of ESF,
irrespective of the magnitude and polarity of the meridional winds during
the above periods. The study indicates that the threshold height above which
ESF triggering is entirely controlled only by the collisional R-T
instability is least for summer months, with higher values for winter and
equinox, during the solar minimum period, whereas for the solar maximum period the
threshold height is least for winter, with higher values for summer and
equinox. But the range over which the threshold height varies is very narrow
(<15 km) for solar minimum in relation to the large range of variation
(>50 km) in the solar maximum epoch. Further to this, the study also
reveals a clear-cut increase in threshold height with solar activity for all
seasons. Clear-cut seasonal variability is also observed in the threshold
height, especially for solar maximum. The study quantifies the level of the base of
the F layer below which neutral dynamical effects play a decisive role in
the triggering of ESF during different seasons and solar epochs.
Full Article in PDF (500 KB) |
|
|
