Ionospheric Activity

Free electrons in the ionosphere perturb the propagation of radio waves. Indeed, the ionosphere is defined as “the atmospheric layer where the free electron concentration is sufficient to affect radio wave propagation”.

The electron concentration in space and time is the main parameter which describes the state of the ionosphere. The ionospheric activity strongly depends on Solar activity. Indeed, extreme UV and X rays emitted by the Sun are the main source of ionization in the ionosphere. For this reason, Space Weather is the main driver of ionospheric disturbances.

SWANS provides users with real-time information about ionospheric activity (Total Electron Content, Slab thickness, electron concentration profile at Dourbes).

The purpose of the LIEDR (Local Ionospheric Electron Density Reconstruction) system is to acquire and process data from simultaneous ground-based GNSS TEC and digital ionosonde measurements, and subsequently to deduce the vertical electron density distribution in the local ionosphere (Stankov et al., 2009). LIEDR is primarily designed to operate in real time for service applications, and, if sufficient data from solar and geomagnetic observations are available, to provide short-term forecast as well. For research applications and further development of the system, a post-processing mode of operation is also envisaged. In essence, the reconstruction procedure consists in the following. The high-precision ionosonde measurements are used for directly obtaining the bottom part of the electron density profile. The ionospheric profiler for the lower side (i.e. below the density peak height, hmF2) is based on the Epstein layer functions using the known values of the critical frequencies, foF2 and foE, and the propagation factor, M3000F2. The corresponding bottom-side part of the total electron content is calculated from this profile and is then subtracted from the GPS TEC value in order to obtain the unknown portion of the TEC in the upper side (i.e. above the hmF2). Ionosonde data, together with the simultaneously-measured TEC and empirically obtained O+/H+ ion transition level values, are all required for the determination of the topside electron density scale height. The topside electron density is considered as a sum of the constituent oxygen and hydrogen ion densities with unknown vertical scale heights. The latter are calculated by solving a system of transcendental equations that arise from the incorporation of a suitable ionospheric profiler (Chapman, Epstein, or Exponential) into formulae describing ionospheric conditions (plasma quasi-neutrality, ion transition level). Once the topside scale heights are determined, the construction of the vertical electron density distribution in the entire altitude range is a straightforward process. As a by-product of the described procedure, the value of the ionospheric slab thickness can be easily computed. To be able to provide forecast, additional information about the current solar and geomagnetic activity is needed. For the purpose, observations available in real time – at the Royal Institute of Meteorology (RMI), the Royal Observatory of Belgium (ROB), and the US National Oceanic and Atmospheric Administration (NOAA) – are used. Recently, a new hybrid model for estimating and predicting the local magnetic index K has been developed. This hybrid model has the advantage of using both, ground-based (geomagnetic field components) and space-based (solar wind parameters) measurements, which results in more reliable estimates of the level of geomagnetic activity – current and future. The described reconstruction procedure has been tested on actual measurements at the RMI Dourbes Geophysics Centre (coordinates: 50.1°N, 4.6°E) where a GPS receiver is collocated with a digital ionosonde (code: DB049, type: Lowell DGS 256). Currently, the nominal time resolution between two consecutive reconstructions is set to 15 minutes with a forecast horizon for each reconstruction of up to 60 minutes. Several applications are envisaged. For example, the ionospheric propagation delays can be estimated and corrected much easier if the electron density profile is available at a nearby location on a real-time basis. Also, both the input data and the reconstruction results can be used for validation purposes in ionospheric models, maps, and monitoring services.

The ionospheric slab thickness, the ratio of the Total Electron Content (TEC) to the maximum ionospheric F2-layer electron density (NmF2), offers substantial information on the shape of the electron density profile, the neutral and ionospheric temperatures/gradients, on the ionospheric composition and dynamics, etc. The ionospheric monitoring capabilities of the slab thickness remain largely unexplored, despite the fact that, operationally, it is a very useful parameter as it allows a simple conversion between foF2 and TEC and additionally, it closely relates to other important ionospheric characteristics. From this aspect, various possibilities exist for utilising the ionospheric slab thickness modelling/monitoring efforts (Stankov and Warnant, 2009). For example, if having instantaneous access to data from regional/global digital ionosonde and GNSS reference networks, it would be possible to provide regional/global monitoring of the slab thickness in real time. If available in real time, over a region of interest, the operational slab thickness monitoring can be used for characterizing and eventually predicting the ionospheric density distribution/gradients, the extent of ionospheric density anomalies and their propagation characteristics. From this aspect, it is believed that the permanent ionospheric monitoring (incl. slab thickness) can assist in various GNSS applications, such as improving the integrity and performance of network RTK positioning services, the ‘ionospheric threat’ identification/estimation in aircraft navigation, etc.

References:

Stankov, S.M., Warnant, R., Stegen, K. (2009): Local ionospheric electron density reconstruction from simultaneous ground-based GNSS and ionosonde measurements. Geophysical Research Abstracts, Vol. 11, Abs.No. EGU2009-10956 (Proc. EGU General Assembly, 19-24 April 2009, Vienna).

Stankov, S.M., Warnant, R. (2009): Ionospheric slab thickness - Analysis, modelling and monitoring. Advances in Space Research, Vol.44, No.11, pp.1295-1303.