A coordinated rocket program to study winter variability in D-region ionization was conducted at White Sands Missile Range, New Mexico, during the winter of 1970–1971. Parachute-borne blunt probes that measure positive-ion and electron conductivities were flown, in conjunction with other experiments, on January 22 and 26 and February 1, 1971. Both Al and A3 radiowave absorption techniques were used to indicate anomalous winter days. The two rocket shots in January occurred on such days of high absorption. The first of these was one of quiet solar activity, whereas the second was preceded by a major solar proton event on January 24. The third flight occurred on a normal winter day. Unusual meteorological conditions preceded January 22, but not January 26 and February 1. Although the enhancement in electron densities was about the same on the two anomalous days (the radiowave absorption relating primarily to electron densities above 80 km and the blunt probe showing an enhancement of about 2 to 3 over the control day below 80 km), the positive-ion measurements showed that they were in fact quite different in character. The positive ions in the range 50–60 km on January 26 were almost precisely the same as those on the control day. This indicates that there was no change in the ion production or loss processes and implies that the enhanced electron density was due to a chemical change (but possibly related to the preceding solar event) in the attachment-detachment balance. By contrast, on January 22 the positive ions showed a factor of 1.5 or more increase in the altitude range 50–60 km. The fact that this enhancement occurred at an altitude where the ionization is cosmic ray rather than solar controlled indicates that the enhancement was due to a decrease in the loss rate rather than an increase in production. We tentatively conclude that we have observed two days of ‘anomalous’ winter absorption of very different character, one possibly related to a solar disturbance, and one probably entirely induced by meteorological effects. The former day was probably characterized by inhibition of the formation of negative ions due to increased detachment and the latter by decreased ionization loss rate.
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