[1] The new Digisonde-4D, while preserving the basic principles of the Digisonde family, introduces important hardware and software changes that implement the latest capabilities of new digital radio frequency (RF) circuitry and embedded computers. The ''D'' refers to digital transmitters and receivers in which no analog circuitry is used for conversion between the baseband and the RF. In conjunction with the new hardware design, new software solutions offer significantly enhanced measurement flexibility, enhanced signal selectivity, and new types of data, e.g., the complete set of time domain samples of all four antenna signals suitable for independent scientific analysis. With the new method of mitigating in-band RF interference, the ionogram running time can be made as short as a couple of seconds. The h 0 (f) precision ranging technique with an accuracy of better than 1 km can be used on a routine basis. The 4D model runs the new ARTIST-5 ionogram autoscaling software which reports in real time the required data for assimilation in ionospheric models. The paper highlights technical advances of the new Digisonde for research and monitoring applications.

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[1] We study the plasma sheath surrounding an antenna that transmits whistler mode waves in the inner magnetosphere in order to investigate the feasibility of conducting controlled experiments on the role of wave-particle interactions in the pitch angle diffusion of relativistic radiation belt electrons. We propose a model for an electrically short antenna-sheath-plasma system with transmission frequencies below the electron characteristic frequencies and much higher than the ion characteristic frequencies. The ion current is neglected. We analytically solve a time-dependent one-dimensional situation by neglecting the effects of the wave's magnetic field. In our model, the antenna is charged to a large negative potential during a steady transmission. Positive charge occurs in the sheath and the sheath is free of electrons and conduction current. The net charge on the antenna and in the sheath is zero. The volume, or the radius in a cylindrical case, of the sheath varies in response to the charge/voltage variation on the antenna. The oscillating radius of the sheath translates to a current in the plasma, which radiates waves into the plasma. A whistler wave transmission experiment conducted by the RPI-IMAGE has shown that the model may describe the most important physical processes occurring in the system. The system response is predominately reactive, showing no evidence for significant sheath current or sheath resistance. The negligibly small sheath conduction electron current can be understood if the antenna is charged to a substantial negative potential, as described by the model. Quantitatively, the model may underestimate the sheath capacitance by about 20%.

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