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The width of the spectra of signals incoherently scattered from 85 to 93 km at Arecibo shows significant variations during sunrise and sunset periods. After sunset, the width decreases substantially for all these heights, indicating a 20–30% decrease in the quotient of neutral temperature to neutral number density T N ). Some increase in effective positive ion mass could also be possible. Somewhat opposite situations are evidenced during sunrise for heights above 87.5 km. At 85 km, on the other hand, the width decreases during sunrise. This is due to the disappearance of negative ions, which were formed during the night hours.

The Arecibo 430 MHz incoherent scatter radar was used to observe the diurnal variation of electron concentration in the 6–100 km altitude region on 14 August 1977. This report is an evaluation of the technique and includes a fairly complete discussion of errors involved. Although interference remains a serious problem, the results are useful down to about 60 km altitude and a minimum density of about 50 electrons cm−3. Characteristic statistical plus systematic errors indicate that an observed 100 electrons cm−3 value actually lies between 50 and 180 electrons cm−3 assuming no interference. Observed variations of electron concentration include not only those due to basic solar control but also at least one wavelike feature characterized by phase shift with altitude. These results should prove particularly useful as constraints to time dependent models of the D-region chemistry.

For the first time a ground based technique, i.e. incoherent scatter observation of the mesospheric spectra has been utilized in identifying the ‘transition height’ of simple molecular ions to complex cluster ions around 80–90 km in the D-region. This transition height also matches with the electron density ledge. A substantial diurnal variation of this height is observed. The transition height varies by about 10km during the course of a day, the lowest being 80 km, near noon. There is also a strong likelihood that both the neutral temperature and effective positive ion mass vary during the course of a day.

The results obtained with ionospheric modification experiments over the three years preceding the XXI General Assembly of URSI in 1984 at Florence are reviewed. The topics discussed include weak electromagnetic sidebands observed using a single pump frequency, the HF-induced plasma line at 3.175 MHz and its similarity to the plasma lines observed using higher HF frequencies near Tromso, the HF-enhanced plasma line observed with the 50 MHz radar, the HF-induced plasma line with a doublehumped spectrum below threshold. HF-induced plasma line spectra with height discrimination using a new technique, the HF-induced plasma line and ion line spectra obtained with two pumps differing in frequency by a few kHz, narrow features such as the OTSI in the HF-enhanced plasma line and ion line spectra observed by a new technique, the use of such narrow features for measuring the line-of-sight electron drift velocity, the discovery of a radical qualitative change in the spectrum of the HF-induced plasma line as the HF power (CW) is increased or as the duty cycle is changed while pulsing, observations of the temporal development of the enhancement of the thermal plasma line at the peak of the F 2-layer by electrons accelerated during ionospheric heating studies of artificial density stratification resulting from the standing wave nature of the heating wave and strong electromagnetic sidebands generated by two Powerful HF radio waves differing from each other by some tens of Hz. Theoretical work on soliton formation and on VLF generation by HF heating is briefly mentioned, as well as experimental studies of self-focusing by observing the scintillation of extraterrestrial radio sources, direct conversion and studies of short scale field-aligned irregularities by VHF radar backscatter during ionospheric modification.

Incoherent scatter observations of mesospheric spectra at Arecibo have shown the presence of zones of large negative ions in the altitude range 85–90 km. These large negative ion regions are somewhat sporadic in nature and are observed on roughly about one third of the evenings. On at least one occasion, somewhat similar observations have been made during the noon period.