Producing a longitude-dependent ring current index for geomagnetic secular variation studies

Secular variation studies based on measurements recorded on ground and in space often rely on global geomagnetic indices such as the Disturbance storm time index (Dst) or the ring current (RC) index to select periods with minimal magnetospheric activity and separate internal and external magnetic fields. Both these indices represent the global symmetric part of the ring current. However, it is known that the ring current exhibits local time asymmetries, even during geomagnetically quiet periods, which cannot be fully captured by Dst or RC. In this study, we develop a new index which tracks magnetospheric field activity in different local time sectors. We follow the basic methodology used to construct the RC index, with modifications to incorporate local-time dependencies in the fitted data. We utilize data from the ground observatory network and, thus, are able to extend our index back to the 1950s.