At Earth, we will focus on the auroral particle forcing. We will highlight the importance of the ion component of the precipitation as a source of ionization and of excitation of the upper atmosphere. We will explain how characteristics of this component can be retrieved from optical measurements. We will illustrate the approach with observations from Svalbard and from Tromso. The Saturnian moon, Titan, will be the other solar system body of focus. It is the only moon in the Solar System to sustain a dense, permanent atmosphere. We will highlight similarities and differences between the Earth's and Titan's atmospheres. We will then focused on the on-going Cassini mission which has been providing a rich dataset of in situ measurements of Titan's ionosphere. We will compare ionospheric observations (such as from CAPS/ELS) against predictions from modeling. Our analysis highlights the important role of the magnetic field line configuration for aeronomic studies at Titan.