Observations are reported of enhancements of the incoherent scatter spectrum excited by strong high frequency (HF) radio waves. A number of enhancement characteristics, observed during an ionospheric heating experiment in January at Arecibo, are described. The enhancement spectrum includes: a strong narrow line displaced below the HF (5.62 MHz) by the frequency of the ion acoustic waves /• in the plasma (about 4 kHz); a broader line (about 30-kc width) and other weak lines near the HF; and a line near •. Enhancements above thermal levels range above a factor of 10 .4 and may be taken as verification of the HF excitation of parametric instabilities in the ionosphere. Significant time variations over scales of tens of •sec through hours were seen. Upshifted and downshifted enhanced plasma lines display asymmetries in intensity, width, power dependence, and decay rates (of the order of msec). The O-mode (but not the X-mode) excitation of these enhancements is of significance to heating experiments. A new phase of plasma physics studies in the ionosphere has been opened with the observation of enhancements of the incoherent scatter spectrum excited by strong, high frequency radio waves. An ionospheric heating experiment in January at Arecibo has indicated a number of features of the enhancements. While transmitting (O-mode) 100 kw powers at 5.62 MHz, the nominal 430-MHz incoherent scatter diagnostic echo showed strong enhancements from ranges near the O-mode reflection level. The enhancement spectrum includes: the plasma fine, which is a strong line, a few kIIz wide that is displaced below the I-IF frequency f.• by about 4 kItz, which is the frequency f• of the ion acoustic waves; and a line near f•. The enhancement of the plasma line component of the incoherent spectrum [Salpeter, 1960a, b] above thermal level is variable, but it ranges above 104 . These observations may be taken as verification of the generation of parametric instabilities in the ionospheric plasma by the HF radio waves. A broader line, about 30 kItz wide, and weaker narrow lines are also seen near fH• --ft. Significant time variations were seen over all time scales observed from tens of •sec through seconds and minutes to hours. Nonetheless, meaningful upshifted versus downshifted plasma line asymmetries, power dependences, and decay rates were found. The presence of

Intense plasma waves are generated by an HF pump wave in an ionospheric heating experiment at the Arecibo Observatory. These plasma waves can be observed as enhancements to the ion and plasma lines of the incoherent backscatter echo. The enhancements can be 3 or 4 orders of magnitude more intense than the unenhanced lines and tend to fluctuate wildly. Both the purely growing and the decay mode parametric instabilities are present. When the pump wave is turned on abruptly, the enhancements develop in time in a repeatable manner. A rather remarkable feature on time scales of seconds is an overshoot in instability power. Overshoots occur frequently but not universally and last for 1-6 s. They can have a magnitude from 10 to hundreds of times the average instability level. The growth of field-aligned irregularities may be the cause of the overshoots. The overshoots appear to be definitely related to an unusually rapid rise in measured electron temperature that cannot be understood in terms of ohmic energy deposition. On time scales of milliseconds th.•re is a 'miniovershoot' before the growth of the instability to a large value. The spectral details also cl•ange in a striking manner. The instabilities can first be detected 2-4 ms after pump wave turn-on. The decay mode is present as well as a broad featureless 'noise bump,' which partially sharpens into a line as time progresses. These changes of the spectra in time seem to run counter to the currently accepted theories of plasma wave saturation.

A method for measuring the critical frequencies of the ionospheric layers to accuracies better than 1 kHz is presented. The method depends on the fact that the intensity of the incoherent backscatter plasma line is a function of the electron number density scale height. At the peak or valley of a layer, more electrons are resonant within a specific frequency resolution cell. When fine-frequency measurements are made of the echo from a long radar pulse, the abrupt signal intensity variations as a function of the frequency permit accurate determination of the critical frequencies. Measurements with the Arecibo Observatory 430-MHz radar of the peak densities show that fluctuations of the order of 0.005% can be detected. The integration time must not be longer than a few seconds or frequency smearing caused by ionospheric changes occurs. Comparison of the upshifted and downshifted plasma line frequencies demonstrates the effect of different reception wavelengths on the resonant frequency and also permits determination of the electron drift velocity.

ABSTRACT.A small CCD photometer dedicated to the detection of extrasolar planets has been developed and put into operation at Mount Hamilton, California. It simultaneously monitors 6000 stars brighter than 13th magnitude in its 49 deg 2 field of view. Observations are conducted all night every clear night of the year. A single field is monitored at a cadence of eight images per hour for a period of about 3 months. When the data are folded for the purpose of discovering low-amplitude transits, transit amplitudes of 1% are readily detected. This precision is sufficient to find Jovian-size planets orbiting solar-like stars, which have signal amplitudes from 1% to 2% depending on the inflation of the planet's atmosphere and the size of the star. An investigation of possible noise sources indicates that neither star field crowding, scintillation noise, nor photon shot noise are the major noise sources for stars brighter than visual magnitude 11.6.Over one hundred variable stars have been found in each star field. About 50 of these stars are eclipsing binary stars, several with transit amplitudes of only a few percent. Three stars that showed only primary transits were examined with high-precision spectroscopy. Two were found to be nearly identical stars in binary pairs orbiting at double the photometric period. Spectroscopic observations showed the third star to be a high mass ratio single-lined binary. On 1999 November 22 the transit of a planet orbiting HD 209458 was observed and the predicted amplitude and immersion times were confirmed. These observations show that the photometer and the data reduction and analysis algorithms have the necessary precision to find companions with the expected area ratio for Jovian-size planets orbiting solar-like stars.

In an ionospheric modification experiment at Arecibo in July 1976 it was discovered that the height of the enhanced plasma line due to photoelectrons does not agree with the height of the enhanced plasma line due to the HF heating wave. The properties of the Barker decoder, used in the experiment, indicate that the photoelectron enhanced plasma line occurs at the height expected by theory for a uniformly varying ionosphere, whereas the observed HF enhanced plasma line occurs a few kilometers above this height. The Langmuir waves responsible for the observed HF plasma line at Arecibo probably exist near the largest or first maximum of the Airy function, which describes the standing HF electric field. This is about 200 m below the heater wave reflection height. This observation requires that the Langmuir waves responsible for the HF‐induced plasma line be generated in ionization irregularities and subsequently propagate in the irregularities to the appropriate plasma frequency for detection by the Arecibo radar.