Multi-point Cluster Observations of VLF Risers, Fallers, and Hooks at and near the Plasmapause.

The four Cluster Wideband (WBD) plasma wave receivers occasionally observe electromagnetic triggered wave emissions at and near the plasmapause. We present the remarkable cases of such observations. These triggered emissions consist of very fine structured VLF risers, fallers and hooks in the frequency range of 1.5 to 3.5 kHz with frequency drifts for the risers on the order of 1 kHz/s. They appear to be triggered out of the background whistler mode waves (hiss) that are usually observed in this region, as well as from narrowband, constant frequency emissions. Occasionally, identical, but weaker, emissions are seen to follow the initial triggered emissions. When all the Cluster spacecraft are relatively close (<800 km, with interspacecraft separations of around 100–200 km), the triggered emissions are correlated across all the spacecraft. The triggered emissions reported here are observed near the perigee of the Cluster spacecraft (around 4–5 RE) within about 20 degrees, north or south, of the magnetic equator at varying magnetic local times and generally at times of low to moderate Kp. In at least one case they have been observed to be propagating toward the magnetic equator at group velocities on the order of 5−9×107 m/s. The triggered emissions are observed in the region of steep density gradient either leading up to or away from the plasmasphere where small-scale density cavities are often encountered. Through analysis of images from the EUV instrument on board the IMAGE spacecraft, we provide evidence that Cluster may sometimes be immersed in a low density channel or other complex structure at the plasmapause when it observes the triggered emissions. Examples of the various types of triggered emissions are provided which show the correlations across spacecraft. Supporting density data are included in order to determine the location of the plasmapause. A nonlinear gyroresonance wave-particle interaction mechanism is discussed as one possible generation mechanism

Cloud condensation nuclei production associated with atmospheric nucleation: a synthesis based on existing literature and new results

This paper synthesizes the available scientific information connecting atmospheric nucleation with subsequent cloud condensation nuclei (CCN) formation. We review both observations and model studies related to this topic, and discuss the potential climatic implications. We conclude that CCN production associated with atmospheric nucleation is both frequent and widespread phenomenon in many types of continental boundary layers, and probably also over a large fraction of the free troposphere. The contribution of nucleation to the global CCN budget spans a relatively large uncertainty range, which, together with our poor understanding of aerosol-cloud interactions, results in major uncertainties in the radiative forcing by atmospheric aerosols. In order to better quantify the role of atmospheric nucleation in CCN formation and Earth System behavior, more information is needed on (i) the factors controlling atmospheric CCN production and (ii) the properties of both primary and secondary CCN and their interconnections. In future investigations, more emphasis should be put on combining field measurements with regional and large-scale model studies.</p