No abstract

The complete genomic sequences of two geographically distinct isolates of rice tungro bacilliform virus (RTBV) from India were determined. Both the sequences showed equal divergence from previously reported Southeast Asian isolates. Numerous insertions, deletions and substitutions, mostly in the intergenic regions, were found. The genome sizes were 7907 and 7934 bp respectively, 95 and 68 residues short of an infectious clone reported earlier. Between them, both the isolates showed high homology all along the genome, except for a 30-nucleotide insertion/deletion close to the 3' end of ORF III in one of them. Both the isolates indicated an unconventional start codon in ORF I, similar to the type isolate. In addition, as novel features, both the Indian isolates showed an unconventional start codon for ORF IV. Considering the low amounts of genome variability noticed in other RTBV isolates, the Indian isolates show that they have diverged sufficiently from the rest and should be considered belonging to a distinct strain.

In this paper, measurements made by the Pioneer Venus orbiter ion mass spectrometer are used to study the temporal and spatial distribution of O ++ ion concentrations in the Venus ionosphere. The diurnal analysis shows an enhancement of O ++ around midnight. On the nightside the O ++ densities are highly variable but show a positive correlation with variations in the O + densities. Model calculations of O ++ concentrations in the nightside are made assuming that impact ionization by precipitating electrons and transport of ions from the dayside w ++ ere the sources for O ions. We find that in situ ionization by precipitating electrons contributes very little, whereas a downward 0 ++ flux in the range of 10 s to 10 • cm -2 s -1 can account for the measured values of O ++ densities near midnight. The high abundance of O ++ near midnight is related to the special characteristics of multiply charged ions. Because of the double charge, thermal diffusion and the polarization electric field are particularly found to be signifiicant factors in the momentum equations for O ++, and therefore the downward velocity for O ++ is small compared with that of other ions. High 0 ++ densities are observed to occur under low solar wind dynamic pressure conditions, consistent with the transterminator transport mechanism as the source of O ++ ions in the antisolar region.

Abstract. The ground based vertical radio wave propagation experiment was conducted at the frequency of 2.5 MHz to study variations in ionospheric radio wave absorption during partial solar eclipse that occurred on October 24, 1995 at Delhi (28.6øN, 77.2 ø E). It was observed that the minimum in absorption data was four minutes earlier than the ground level eclipse totality, and an anomaly was accom•ied by an abnormal sharp rise (shock-like wave structure) in absorption data around 30 minutes after the eclipse totality. Absorption during post-eclipse control days was higher compared to those on preeclipse control days.

No abstract