Sixto joined the staff in October 1993. His principal research interest is in upper atmospheric physics, in particular, the composition and dynamics of the topside ionosphere a region that couples the F-region ionosphere to the plasmasphere. Recently, he has been active in studies that combine data from multiple radars, together with computer simulations of this region to understand the mechanisms for the observed variations in altitude distributions of the principal species: oxygen, hydrogen and helium ions. In addition, he has participated in studies of the neutral atmosphere using Fabry-Perot interferometers, radar studies of ionospheric electric fields at low latitudes, and studies of the low and mid latitude ionosphere using data from spacecraft. Currently, he is working on developing high altitude incoherent scatter radar experiments to measure accurate temperatures densities and compositions at both Arecibo and Jicamarca ISR. These measurements can be used to study the effects of geomagnetic storms on the ionosphere, establish metrics for the space weather applications and study long term changes in the upper atmosphere. In addition he is interested in applying to telescience techniques to these radar 'chain' studies of the ionosphere.