Design and implementation of an oblique ionosonde receiver. For studies of spatial and temporal ionospheric structures

Abstract

The study of the ionosphere has been of central interest since its discovery in the early 20th century. In later decades there has been an increasing appreciation and need for understanding the spatial and temporal structure of the ionosphere and how the structure is affected by various processes in the atmosphere. An important process in this context is that of atmospheric gravity waves (AGWs). AGWs in the lower region of the atmosphere instigate propagating fluctuations of the electron density in the ionosphere, the signatures of which are called travelling ionospheric disturbances (TIDs). A series of methods that can be used for investigating TIDs are already in existence, but between complex techniques and expensive hardware there is still a need for access to simple and inexpensive methods of inquiry. Oblique ionosonde receivers that work with existing transmission infrastructure are a simple and inexpensive way to address this need. In this project, we developed an oblique ionosonde receiver using commercially available and/or easily manufactured components. The receiver is based on software defined radio technology and utilizes a magnetic loop antenna as the driven element operating with frequencies 1-30 MHz. The receiver system is designed to be a sub-system in a network of oblique ionosonde receivers that are synchronized using GPS. Four receiver systems were built according to the design and deployed in a geographically distributed network around an existing transmitter at Sodankylä Geophysical Observatory. The fluctuation period of TIDs is governed by the buoyancy period defined by the Brunt-Väisälä frequency. The buoyancy period varies with altitude but is rarely below 5 minutes. The radar receivers built in this project are capable of operating with a better temporal resolution than 1 minute, but the resolution is limited by the transmitter antenna in Sodankylä as it performs a sweep once every minute. A temporal resolution of 1 minute allows for Nyquist sampling the Brunt-Väisälä frequency, meaning that operating the receivers in a network makes for a suitable tool to resolve the signatures of TIDs. Both the receiver design and the software used for operating the receiver are made available to the public.