NOPR Collection:

Interpretation of higher harmonic tweeks recorded at Agra

1995-08-01T00:00:00Z

Title: Interpretation of higher harmonic tweeks recorded at Agra Authors: Singh, Birbal Abstract: Higher harmonic tweeks (higher than the first harmonic) recorded for the first time at the low latitude station Agra (geomagn. lat., 17°N) are reported. The analysis of the data shows that higher harmonic tweeks are usually not associated with whistlers, and occur when the ionization in the lower ionosphere varies exponentially with height, giving the waveguide cut-off frequency f_c = 1.6 kHz. On the other hand, normal tweeks or those with first harmonic only, are often associated with whistlers and occur when f_c = 1.8 kHz. The higher harmonic tweeks have travelled distances between 4000 and 21000 km in the earth-ionosphere waveguide. The tweek activity is found to increase during periods of magnetic disturbances. Page(s): 205-208

Solar cycle dependence of thermospheric neutral winds at Arecibo, Puerto Rico

1995-08-01T00:00:00Z

Title: Solar cycle dependence of thermospheric neutral winds at Arecibo, Puerto Rico Authors: Kane, R P Abstract: The 12-month running averages of the meridional wind velocities obtained at Arecibo Observatory, Puerto Rico, using a Fabry-Perot interferometer, shoed transitions during the night which were of different natures during sunspot cycle maximum and minimum. The meridional component at 18-21 AST (Atlantic Standard Time) shifted from large southward at sunspot maximum to small northward at sunspot minimum. The meridional and zonal components at other local times showed small variations. The F-layer height obtained from incohered scatter radar measurements decreased from about 400 km to about 300 km from sunspot maximum to sunspot minimum in a manner compatible with the variations of neutral wind. Page(s): 199-204

Study of electron production and loss rates in low latitude ionosphere using geostationary satellite signals

1995-08-01T00:00:00Z

Title: Study of electron production and loss rates in low latitude ionosphere using geostationary satellite signals Authors: Rao, P V S Rama; Prasad, D S V V D; Nru, D Abstract: Ionospheric electron content (IEC) measurements made a low latitude station, Waltair (17.7°N, 83.3°E), using the radio beacon signals of ETS-II (March 1978 – February 1979) have been used to calculate the integrated production rate (Q₀) and the effective loss rate (´). Both these parameters exhibit seasonal and semi-annual variations. The overall average values of Q₀ and ´ are 4.3 x 10¹⁴ el. m^-2s^-1 and 0.66 x 10^-4s^-1 respectively. Monthly mean values of Q₀ show good correlation with 10.7 cm solar flux intensity. The results of Q₀and ´ at Waltair have been compared with those obtained at equatorial, low- and mid-latitude stations. An attempt has also been made to explain qualitatively the observed variations with the neutral composition changes in the atmosphere. Page(s): 193-198

Electron densities in the equatorial lower ionosphere over Thumba and SHAR

1995-08-01T00:00:00Z

Title: Electron densities in the equatorial lower ionosphere over Thumba and SHAR Authors: Sinha, H S S; Prakash, Satya Abstract: The paper presents the structures of the electron density profiles and the associated irregularities up to about 150 km over Thumba, India (8°52´ N, 76°87´E, dip 0°47´S) and up to 340 km over SHAR, India (13°42´ N, 80°E, dip 14°N). These profiles were obtained using rocket-borne Langmuir probes and resonance probes. In all, profiles of 27 rocket flights conducted from Thumba and 6 from SHAR are presented. During the daytime normal electrojet (NEJ), the electron density profiles over Thumba were very smooth with only positive electron density gradients (i.e. the density profiles over Thumba were very smooth with only positive electron density gradients (i.e. the density increasing with increasing altitude). Increase in the density above 100 km altitude was much slower as compared to that below 100 km. During the daytime counter electrojet (CEJ), sharp layers of electron density, having a vertical extent of a few kilometers, were observed over Thumba. The nighttime profiles over Thumba show pronounced structures in the form of layers which are believed to be due to shears in the gravity wave winds. Morning and evening profiles over Thumba (around the twilight periods) show structures similar to the nighttime ones except that the peak densities are slightly smaller as compared to the nighttime values. The single daytime electron density profile available over SHAR during the NEJ period, shows the presence of a sharp layer around 90 km. The morning, evening and nighttime profiles over SHAR show a lot more structures as compared to the profiles over Thumba. These structures are believed to be due to the gravity waves. Page(s): 184-192