Energy deposition in the lower auroral ionosphere through energetic particle precipitation

Ground-based imaging and broad beam riometers are used in conjunction with ionospheric radars and satellite instruments to investigate high-energy precipitation in the auroral zone. There are two dominant precipitation regimes in the auroral zone which lead to enhanced high frequency radio absorption; high energy electrons (> keV) from closed field lines, and protons (> MeV) penetrating from the solar wind following solar flares. Much of the work in this thesis uses data from riometers in Fennoscandia to measure the extent and movement of energetic precipitation from both sources. A case study of dayside absorption combines data from the imaging riometer with radar and satellite observations leading to an estimation of the energy of precipitation based on the riometer data. Two separate precipitation mechanisms were identified in the case study through the use of satellite particle measurements and ground-based observations of geomagnetic pulsations. The riometer showed varying movements of the absorption patches through the case study and a determination of different dominating particle drift regimes was possible through comparison with coherent HF radar. A statistical analysis of absorption in the imaging riometer field of view is carried out. The absorption is linked to both Kp and solar wind velocity using linear and quadratic fits of the data. The daily variation and distribution of absorption is investigated along with seasonal effects which are shown to be reliant on geomagnetic activity. A study of the large number of solar proton events from 1995 to 2001 inclusive is carried out with particular reference to those that produce significant absorption in the northern hemisphere polar cap (polar cap absorption –PCA). The occurrence of the absorption events is investigated and a simple empirical relationship between the integral proton flux and the absorption observed during geomagnetically undisturbed PCA conditions is developed