Climatology of GNSS ionospheric scintillation at high latitudes

Abstract

The ionosphere is characterized by a highly variable degree of ionization maintained by a wide range of solar radiation and by electrons and protons originating from Sun. This plasma is under the permanent solar forcing, and interacts with the geomagnetic and interplanetary magnetic fields. The ionosphere shows diurnal and seasonal variations, together with a 11-year period variability related to the solar cycle. Sporadic events due to the intermittent behaviour of the Sun are superimposed to these quasi-periodic trends: coronal mass ejections, particle and radiation bursts (flares) yield impulsive perturbations in the Sun-Earth environment and to magnetic storms and substorms in the near-Earth region. By consequence, under these perturbed conditions coming from the outer space, the ionosphere may become highly turbulent and the probability of irregularities formation, typically enhancements or depletions of the electron density embedded in the ambient ionosphere, increases. Such irregularities cause diffraction effects, mainly due to the random fluctuations of the refractive index of the ionosphere, on the satellites signals passing through them and consequent perturbations may cause GNSS navigation errors and outages, abruptly corrupting its performance. Due to the morphology of the geomagnetic field, whose lines are almost vertical at high latitude, polar areas are characterized by the presence of significant ionospheric irregularities having scale sizes ranging from hundreds of kilometres down to a few centimetres and with highly dynamic structures. The understanding and consequent mitigation of the effect of such phenomena is important, in preparation for the next solar cycle (24), whose maximum is expected in 2012. We analyse the fluctuations in the carrier frequency of the radio waves received on the ground, commonly referred to as ionospheric amplitude and phase scintillations, to investigate the physical processes causing them and, conversely, to understand how these processes affect the operational capabilities of GNSS receivers under different geomagnetic conditions. The phase scintillations on GNSS signals are likely caused by ionospheric irregularities of scale size of hundreds of meters to few kilometers. The amplitude scintillations on GNSS signals are caused by ionospheric irregularities of scale size smaller than the Fresnel radius, which is of the order of hundreds of meters for GNSS signals, typically embedded into the patches. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Institute of Engineering Surveying and Space Geodesy (IESSG) of the University of Nottingham manage the same kind of GISTM (GPS Ionospheric Scintillation and TEC Monitor) receivers over the European high and mid latitude regions and over Antarctica. The GISTM receivers consist of NovAtel OEM4 dual-frequency receivers with special firmware specifically able to compute in near real time the amplitude and the phase scintillation from the GPS L1 frequency signals, and the ionospheric TEC (Total Electron Content) from the GPS L1 and L2 carrier phase signals. From this ground-based network, we are able to capture the dynamics of ionospheric plasma in a wide latitudinal range, from auroral to cusp/cap regions, considering the contribution of both hemispheres, in a bi-polar framework. In particular, the stations considered in our analysis are located at Ny-Ålesund (78.9°N, 11.9°E), Hammerfest (70.7°N, 23.7°E), Brønnøysund (65.5°N, 12.2°E) in the Northern hemisphere and at Mario Zucchelli Station (74.7°S, 164.1°E) and Concordia Station (75.1°S, 123.2°E) in Antarctica. The data collection started in 2001 and is still in progress. The results, obtained by statistically analyzing a large data sample over a wide period, show the effect of ionospheric disturbances on the GNSS signals, evidencing the different contributions of the auroral and the cusp/cap ionosphere and highlighting possible scintillation scenarios over polar regions.