Phase recordings at Atibaia, Brazil (23°S, 46°W), of 13.6 ‐kHz signal transmitted from Golfo Nuevo, Argentina (43°S, 65°W), a trajectory confined almost completely within the South Atlantic anomaly region, show significant perturbations, indicative of the lowering of the VLF reflection level, following the onset of magnetic disturbances. Simultaneous measurements of the Es layer parameters ftEs and fbEs over Cachoeira Paulista (22°S, 45°W) also show enhancements, with some delay with respect to the magnetic disturbance onset, as was found in our earlier work (Batista and Abdu, 1977). These results show magnetic storm associated ionization enhancements taking place in a height region from approximately 110 km down to 70 km, which we interpret as having been produced by precipitation of high‐energy charged particles in the South Atlantic magnetic anomaly. The results also suggest some degree of day to day variability in the abundance of metallic species and/or in the dynamics of the E region over this region.

[1] We present a study of solar flares that do not produce disturbances in the low terrestrial ionosphere, detectable in very low frequency (VLF) low ionosphere propagation as sudden phase advances (SPA). We selected only flares with larger optical H-a importance (equal or larger than 1), occurred near the cycle 22 solar maximum (1987)(1988)(1989), at times when VLF data were available for long-distance propagation paths entirely illuminated by the Sun. For the 463 optical solar flares selected, we found that 18.8% had no measurable effects on the lower ionosphere. Inversely, all measured SPAs did have a solar counterpart, optical (such a subflare) and/or X-ray fluxes. Among the H-a flares that do not produce measurable disturbances in the low terrestrial ionosphere, there is a surprising selection for events occurred at the solar limbs. On the other hand, the X-ray flux for the H-a flares selected, obtained from GOES for the bands 0.5-4 Å and 1-8 Å exhibited a rather scattered correlation with the SPAs amplitudes. GOES X-rays fluxes for H-a flares not producing SPAs extend over 2-3 orders of magnitude. These results may suggest real physical distinctions between events. Possible explanations suggest the existence of a directional trend for the soft X-ray produced in flares possibly combined with the blockage of ionizing X rays behind the solar limbs.

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