The VLF amplitude does not show a symmetrical behavior over the year, which would be expected from its dependency on the solar position. The VLF amplitude rather shows a distinctive sharp decrease around October, which is hence called the “October effect”. This study is the first to systematically investigate this October effect, which shows a clear latitudinal dependency.

The upper part of the atmosphere, the ionosphere, is partially ionized, and it is being crossed by many multi-frequency signals of the Global Navigation Satellite System (GNSS) satellites. This unique source of data can be acquired in real time from hundreds of permanent GNSS receivers. The real-time processing providing the distribution of the ionospheric free electrons (Global Ionospheric Maps) can be done as well in real time. We present their updated real-time assessment and combination.

We focus on reconstructing the topside ionosphere and plasmasphere and assimilating the space-based Global Navigation Satellite System slant total electron content (STEC) measurements with an ensemble Kalman filter (EnKF). We present methods for realizing the propagation step without a physical model. We investigate the capability of our estimations to reconstruct independent STEC and electron density measurements. We compare the EnKF approach with SMART+ and the 3D ionosphere model NeQuick.

This paper presents a data-driven model of the ionospheric electron density that has been developed for the EUMETSAT Polar System - Second Generation mission, with the main goal to improve the inversion of radio occultations for neutral atmospheric sounding. This model has been developed using occultation data from the COSMIC/FORMOSAT-3 satellite, which has been inverted using a LMS-based mechanization of the Abel inversion that implements the separability hypothesis.