Pulsating describes a low‐intensity aurora that undergoes rapid alternating increases and decreases in luminosity. Extensive new data available from ground‐based low‐light‐level television cameras and satellite scanners have allowed a detailed study of the pulsating aurora phenomenon. Intensity variations in pulsating auroras may be repetitive, quasi‐periodic, or occasionally periodic with a time scale ranging from less than 1 s to several tens of seconds. The maximum intensity does not exceed approximately 10 kR in N2+ 1 NG (first negative group) 4278 Å, and a background luminosity is often observed in association with the pulsating forms. Pulsations occur in auroral arcs, arc segments, and patches of fixed and variable area. The temporal and spatial characteristics are highly variable over a broad and continuous spectrum; rapid changes from one set of characteristics to another frequently occur, as do reversible changes from pulsating to nonpulsating auroras. Diffuse, slowly pulsating arcs occur in the evening sector immediately before and after the passage of the westward traveling surge. In the midnight sector, both arc segments and patches occupy a broad region behind the westward traveling surge. Poleward stretching torchlike structures containing pulsating patches and arc segments are often observed at the poleward boundary of the diffuse auroral oval in the midnight sector. A narrow region of pulsating arcs and eastward drifting patches occupies the morning sector, the arcs generally being located at the poleward boundary. A 3 ± 1 Hz modulation appears in more than 50% of all pulsating auroras in the midnight and morning sectors, the amplitude of modulation ranging up to 20%.

Refractive index irregularities in the equatorial mesosphere have been investigated using both the Jicamarca VHF radar and a rocket‐borne Langmuir probe launched from Punta Lobos, Peru. On February 27, 1983, a single layer of turbulence was observed in the upper mesosphere by both experiments. There is very good agreement between the observed radar echo power and the radar scattering cross section calculated from the rocket data when these are interpreted in the context of isotropic turbulence. The inner and outer scales of turbulence have been calculated from both the radar and the rocket data, and good agreement is found. The radar data show indications of large‐scale vortices in the layer of irregularities. Rocket data show that the inner scale of turbulence in the upper mesosphere is a few tens of meters and that the Jicamarca radar Bragg wavelength (3 m) is well within the viscous subrange of turbulence in this altitude range. The spectral index in the inertial subrange is close to −5/3, changing to about −7 at higher wave numbers. Energy dissipation rate in the layer was calculated to be 0.05 W kg−1, in good agreement with previous estimates.

Ten hours of coherent scatter radar data were collected near 150-kin altitude above Jicamarca, Peru, on July 27, 1977. The antenna system probed three separate volumes horizontally separated by approximately 9 km at 150 kin. Measured vertical velocity varied between 10 and 20 m/s and is similar to the vertical ionization drift at these altitudes. However, the vertical velocity may change as much as 6 m/s over a 4-kin height interval. Ten-minute amplitude modulations in the scattered power are seen throughout the observation period and are probably related to evanescent gravity waves. Several layers of enhanced scattering are observed at times. The half width of the measured spectra varies between 1 and 3 Hz with the vertically pointing antenna showing a consistently broader spectrum than the other two antennas. This narrow spectral width indicates a nonthermal scattering owing to electron density fluctuations. INTRODUCTIONCoherent scattering of radio waves in the altitude region between 140 and 160 km has been observed by using a vertically pointing radar at the Jicamarca Radar Observatory near Lima, Peru. Returns from this altitude region have previously been observed by several users of the Jicamarca radar and were first reported by Balsley [1964], who found the received signal to be stronger than the expected incoherent scatter level [Bowles, 1961] by 20 dB or more.Balsley [1964] concluded that the behavior of these echoes is independent of the electrojet phenomenon at 105 to 115 km. He found that there typically is a multiple layer reflection region, with up to four layers simultaneously present, and that each layer is generally somewhat less than 3 km thick. A fading rate on the order of 5 fades per second or less was also one of the characteristics found, along with a maximum amplitude of the scattered power around local noon and a disappearance of all echoes by 1600 local time.The present work, based on ten hours of data, is mainly concerned with such parameters of the scattered returns as the variation in power, the width, and the Doppler shift of the spectra, all as functions of time, altitude, and horizontal distance. A more thorough statistical analysis and interpretation of the data is left until a broader data base has been collected. EXPERIMENTAL TECHNIQUESThe Jicamarca radar operates on a frequency of 49.92 MHz with a peak power of 1.0 MW. The antenna system is a crossed dipole array, with one polarization focused vertically, and the other polarization split into two (one phased 3.45 ø south, the other 3.45 ø west of vertical), giving three different antenna beams. Thus the scattered power in the vertical antenna beam receives 6 dB higher scattered power than each of the other two antennas. Vertical distances between the centers of the scattering volumes at the altitude of 150 km are 9 km between the vertical and each of the other two antenna beams and 13 km between the westward and southward-pointing antenna beams. The real and imaginary parts of the coherently detected signals from each of the a...

Recent observations at Jicamarca, Peru, of VHF radio signals coherently scattered from the upper equatorial E region during midday hours show that the corresponding irregularities in the electron density distribution are aligned along the earth's magnetic field. Vertical electron drift velocities in the altitude region between 120 km and 170 km show spatial and temporal variations of as much as 30%. These variations are attributed to small‐scale varying electric fields which are generated by the dynamo action of gravity waves in the neutral atmosphere.

Radio signals scattered from the equatorial mesosphere above Jicamarca, Peru have been studied. The scattered power level is dependent on look angle in the lower parts of the mesosphere. Indications are that this aspect sensitivity in the received signal cannot be adequately explained by specular reflection from stratified layers. Interferometer studies of the scattered signal have shown that there are structures in the mesosphere that have vertical velocity that varies in the horizontal direction within the antenna beam. The size of these structures is of the order of a few hundred meters. Considerations of vertical and horizontal scale sizes of these structures indicate that they are Kelvin‐Helmholtz billows drifting through the antenna beam.