Deriving the normalised ion-neutral collision frequency from EISCAT observations
Common programme observations by the EISCAT UHF radar revealed an extended interval, post geomagnetic local noon on 03 April 1992, during which the F-region ion velocity orthogonal to the geomagnetic field was significantly enhanced, to values exceeding 2 km s−1 corresponding to a perpendicular electric field of some 100 mV m−1. Observations from this interval are used to illustrate a method by which estimates of the E-region ion-neutral collision frequency may be derived in the presence of enhanced electric field. From both the rotation of the ion velocity vector and the reduction in the ion velocity magnitude relative to that in the F-region, independent estimates of the normalised ion-neutral collision frequency are made at the UHF E-region tristatic altitudes; the derived values are, in general, lower than model predictions. Although initial calculations assume a stationary neutral atmosphere, first-order estimates of the E-region neutral wind are subsequently employed to calculate revised estimates of the normalised ion-neutral collision frequency; these neutral winds are derived by attributing the difference between predicted and observed enhancements in field-parallel ion temperature to thermospheric motion. The inclusion of neutral winds, which are themselves not inconsiderable, appears to have only a limited effect on the normalised collision frequencies derived.
Meso-scale observations of Joule heating near an auroral arc and ion-neutral collision frequency in the polar cap E-region
We report on the first meso-scale combined ionospheric and thermospheric observations, partly in the vicinity of an auroral arc, from Svalbard in the polar cap on 2 February 2010. The EISCAT Svalbard radar employed a novel scanning mode in order to obtain F- and E-region ion flows over an annular region centred on the radar. Simultaneously, a co-located Scanning Doppler Imager observed the E-region neutral winds and temperatures around 110 km altitude using the 557.7 nm auroral optical emission. Combining the ion and neutral data permits the E-region Joule heating to be estimated with an azimuthal spatial resolution of ∼64 km at a radius of ∼163 km from the radar. The spatial distribution of Joule heating shows significant meso-scale variation. The ion-neutral collision frequency is measured in the E-region by combining all the data over the entire field of view with only weak aurora present. The estimated ion-neutral collision frequency at ∼113 km altitude is in good agreement with the MSIS atmospheric model.
Shape and surface texture of molten droplets deposited on cold surfaces
When a molten droplet impacts on a cold surface, the interaction between solidification and fluid dynamics can lead to different shapes and surface textures of deposited droplets. This article presents an experimental study on deposition of small droplets (39 μm diameter) of molten wax ink on solid surfaces. Our results show that both the final shape and the surface texture of deposited droplets are extremely sensitive to deposition conditions, which include initial temperature of droplets, type of substrate surface and distance between substrate and printhead. Different surface textures are observed and shown to depend on the thermal and flow histories of ink droplets during the process of impact. A test using crystalline wax reveals crystallization as one of the major factors in determining the final texture.