The structure of mid‐latitude sporadic E layers has been investigated in a series of observations at the Arecibo Observatory. The basic experiment uses phase‐coded pulses to measure the electron density profiles with a range resolution of 600 m while scanning the radar beam. The data from incoherent scatter are compared with simultaneous ionosonde observations to determine the source of the partial transparency of sporadic E layers to radio waves at frequencies between fbEs and ftEs. It is concluded that sporadic E layers contain electron density irregularities of sufficient intensity to account for the partially transparent echoes observed by the ionosonde. A feature that becomes immediately obvious in examining the data taken with the incoherent scatter radar is that sporadic E layers are as variable in structure as they are unpredictable in occurrence. Some have simple vertical profiles and are uniform in the horizontal plane, consistent with rocket observations; others have complex vertical and horizontal structure, as has been implied by the radio experiments. Wavelike structure in the electron density is present in many of the layers observed. Evidence is shown of the limiting of peak ionization density in a layer by turbulence generated in a Kelvin‐Helmholtz instability. The possible proximity of a sporadic E layer to a critical level of a dominant gravity wave mode is discussed as a source of intense ionization irregularities. A possible irregular source of ionization from ablating meteors is also considered. It is shown that under specific conditions an ionized meteor trail can be converged by the action of the neutral wind into an irregularity of relatively small dimensions. It is concluded that the observations support the wind shear mechanism of formation of sporadic E layers when the combined effects of gravity waves, tides, and the irregular deposition of metallic ions are included.

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.

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Aims. We present the design and pre-launch performance of the Solar Orbiter Heliospheric Imager (SoloHI) which is an instrument prepared for inclusion in the ESA/NASA Solar Orbiter mission, currently scheduled for launch in 2020. Methods. The goal of this paper is to provide details of the SoloHI instrument concept, design, and pre-flight performance to give the potential user of the data a better understanding of how the observations are collected and the sources that contribute to the signal. Results. The paper discusses the science objectives, including the SoloHI-specific aspects, before presenting the design concepts, which include the optics, mechanical, thermal, electrical, and ground processing. Finally, a list of planned data products is also presented. Conclusions. The performance measurements of the various instrument parameters meet or exceed the requirements derived from the mission science objectives. SoloHI is poised to take its place as a vital contributor to the science success of the Solar Orbiter mission.

An investigation of the horizontal structure of sporadic-Elayers has been made using the incoherent scatter radar at the Arecibo Observatory. Data are presented for two observations, one with the radar beam held in a vertical position, and another with the beam scanning in azimuth across the sporadic-E layer. The first observation was made at sunset and shows the passage of a large region of ionization, about 150 km in extent, having little small-scale horizontal structure. The second, at midday, shows considerable variation in the value of the maximum electron density in the vertical cross section of an intense sporadic-E layer. The horizontal dimensions of the features range in size down to the resolution of the radar (300 m). By considering the data statistically it is shown that small patches, 300 m or less in horizontal extent, exist in the sporadic-E layer with densities great enough to account for the maximum frequency of the echo kecorded on the ionosonde located at the observatory. _ 271