The mesosphere and lower thermosphere (MLT) region between 60 and 110 km forms the boundary between the lower atmosphere and space. This region is dominated by atmospheric dynamics including planetary waves, tides, gravity waves, and stratified turbulence. The main sources of these dynamics lie mainly in the lower atmosphere. Similarly, neutral dynamics and electrodynamics at higher altitudes can be modified by locally generated MLT dynamics or by perturbations propagating from below and interacting with the MLT region (e.g., Vincent, 2015, and references therein).

[1] The second generation SOUSY MST radar at 78°N, 16°E on Svalbard has recently completed its inaugural year of combined troposphere and mesosphere observations. Here, troposphere observations have been processed using a robust detection algorithm designed for obtaining tropopause climatology by automatic data processing, and the resulting monthly statistics have been compared with corresponding surface air temperatures. As our main objective, we describe the new radar system, present the tropopause detection method, and validate the results using radiosonde and surface temperature data. The tropopause height depends on the temperature of the underlying atmosphere but is also influenced by downward control from the stratosphere. We find that the climatological tropopause height is correlated with the surface temperature but with the former lagging the latter by approximately 1 month.

Abstract. In recent years, more and more radar systems with multiple-receiver antennas are being used to study the atmospheric and ionospheric irregularities with either interferometric and/or imaging configurations. In such systems, one of the major challenges is to know the phase offsets between the different receiver channels. Such phases are intrinsic to the system and are due to different cable lengths, filters, attenuators, amplifiers, antenna impedance, etc. Moreover, such phases change as function of time, on different time scales, depending on the specific installation. In this work, we present three approaches using natural targets (radio stars, meteor-head and meteor trail echoes) that allow either an absolute or relative phase calibration. In addition, we present the results of using an artificial source (radio beacon) for a continuous calibration that complements the previous approaches. These approaches are robust and good alternatives to other approaches, e.g. self-calibration techniques using known data features, or for multiple-receiver configurations constantly changing their receiving elements. In order to show the good performance of the proposed phase calibration techniques, we present new radar imaging results of equatorial spread F (ESF) irregularities. Finally we introduce a new way to represent range-time intensity (RTI) maps color coded with the Doppler information. Such modified map allows the identification and interpretation of geophysical phenomena, previously hidden in conventional RTI maps, e.g. the time and altitude of occurrence of ESF irregularities pinching off from the bottomside and their respective Doppler velocity.

A network of high‐frequency (HF) transmitters and receivers used for ionospheric specification is being installed in Peru. The HF transmitters employ multiple frequencies and binary phase coding with pseudorandom noise, and the observables provided by the receivers include group delay, Doppler shift, amplitude, bearing (from interferometry), and polarization. Statistical inverse methods are used to estimate F region density in a volume from the data regionally. The method incorporates raytracing based on the principles of Hamiltonian optics in the forward model and involves an ionospheric parametrization in terms of Chapman functions in the vertical and bicubic B‐spline interpolation in the horizontal. Regularization is employed to minimize the global curvature of the reconstructions. HF beacon data for two nights in January 2018 are presented. We use the reconstructions to investigate why plasma irregularities associated with equatorial spread F formed on one occasion and not the other. The data indicate that the background ionospheric flow is not simply frozen in, that is, that longitude and local time variations cannot be equated, even at regional scales. This has ramifications for equatorial spread F forecasting strategies that assume equivalence.

We describe a new class of nonthermal plasma density irregularities observed in the postmidnight topside equatorial ionosphere under low solar flux conditions. They are distinct from irregularities associated with equatorial spread F (ESF) in terms of their morphology and because they exhibit strong spectral sidebands at the lower‐hybrid frequency. The coherent echoes were observed in a series high‐altitude radar experiments performed at Jicamarca utilizing long‐ and coded double‐pulse modes and a dual‐beam mode. The coded double‐pulse mode was used to measure the low‐frequency characteristics of the echoes with fine range resolution. Doppler shifts of the main backscatter line were observed to fall between ±150 m/s. The long‐pulse mode was employed for high‐frequency spectral analysis which revealed the presence of strong spectral sidelobes at the lower‐hybrid frequency. A dual‐beam mode was used to investigate the horizontal structure of the echoes. Zonal drift speeds of 50–70 m/s were inferred with this mode, and longitudinal dimensions of approximately 270 km were estimated. The study summarizes with a discussion of different mechanisms that may be responsible for the phenomenon and the lower‐hybrid sidebands in particular.