Incoherent scatter radar studies of electron precipitation

The ionosphere is a partially ionized part of the Earth’s upper atmosphere at 60-1000 km altitudes, and it is characterized by the spatial and temporal variations of its plasma parameters such as electron density, electron and ion temperatures and ion velocity. The geomagnetic field connects the ionosphere to the surrounding magnetosphere at high latitudes. This connection allows electrons to precipitate from the magnetosphere down to the ionosphere and cause the spectacular phenomena of auroral displays. The precipitating electrons also ionize and heat the neutral atmospheric particles through collisions. Electron precipitation is characterized by the total energy flux (auroral power), number flux and average energy of the precipitating electrons.



Incoherent scatter radars (ISR) are among the most efficient ground-based instruments to study electron precipitation. Unlike fast moving satellite instruments, radar methods allow one to observe temporal evolution of auroral electron precipitation events for extended periods of time. The international EISCAT scientific association, of which Finland is a founding member, operates incoherent scatter radars north of the arctic circle in Finland, Norway and Sweden. EISCAT is also building the next-generation EISCAT_3D radar system.



In this PhD project, we developed a new ISR data analysis tool called Bayesian Filtering Module (BAFIM) that enabled us to calculate plasma parameters from ISR data with much better temporal and spatial resolutions than the EISCAT’s standard data analysis package. By analyzing the high-resolution electron density data from BAFIM with another method called ELSPEC, we derive total energy fluxes (auroral powers), number fluxes and peak energies of a rapidly varying auroral electron precipitation event with high time resolution (4 s). Previously, such high time resolution studies were possible only using the so-called raw electron density, which we show to be biased during some electron precipitation events. The results of the combined BAFIM-ELSPEC analysis method show a very good agreement with an independent but simultaneous and co-located optical observation of the same electron precipitation event.



Using more than 20 years of incoherent scatter radar data measured by the EISCAT UHF radar in Tromsø, we study statistical characteristics of auroral electron precipitation. The radar data allows us to include not only the 1–30 keV but also 30–100 keV energies, which have been poorly covered in previous satellite-based studies. We find that the radar observes auroral electron precipitation most frequently during morning hours (5–6 magnetic local time), and during September and March equinox months. In addition, electron precipitation occurs most frequently during years immediately after the sunspot maximum years. Electrons that have higher energies precipitate more commonly during morning hours than evening hours, and large energy and number fluxes occur more frequently in the evening and pre- midnight hours than in post-midnight and morning hours.



The analysis tools developed and used in this work can be applied to data analysis of the next-generation EISCAT_3D radar to investigate the aurora with unprecedented resolutions. The auroral occurrence rates obtained in this study are valuable information when planning EISCAT and EISCAT_3D radar experiments for auroral studies.