INVESTIGATION OF GNSS SCINTILLATIONS UNDER DIFFERENT CONFIGURATIONS OF THE MAGNETOSPHERE-IONOSPHERE COUPLING

Abstract

This work presents an interhemispheric, multi-instrument study of the response of the high latitude ionosphere to strong perturbation coming from the Sun. In particular, the ionospheric response is evaluated in terms of scintillations on L-band signals, which are rapid and random fluctuations in the received amplitude and/or phase of radio waves passing through the ionosphere. This phenomenon occurs when a radio frequency signal (typically in a range of frequencies between 100 MHz and 4 GHz) crosses an ionospheric electron density irregularity, which is a delimited ionospheric region (in space and time) with an uneven electron density distribution. In fact, in crossing an irregularity the radio signal experiences diffractive and refractive effects that cause the signal scintillation analogously to what is observed for the brightness of stars in the optical band. In the case of Global Navigation Satellite Systems (GNSS), such as GPS, GLONASS and the nascent European GALILEO, scintillation may reduce the accuracy of the pseudorange and phase measurements. Consequently, the positioning errors increase and, in extreme cases, the service can become unavailable. Therefore, scintillation events may affect the use of modern technology causing economic loss. Since scintillations are due to both the presence and dynamics of plasma irregularities in the ionosphere, the understanding of the physical mechanisms that regulate the formation and dynamics of such irregularities is crucial to develop reliable prediction models and mitigation techniques to tackle the effect on GNSS-reliant services. In recent years, the investigation of the links between external drivers and GNSS scintillations has raised considerable interest in the space physics community. However, the identification of the physical properties characterising the solar wind-magnetosphere-ionosphere coupling has not yet reached the levels required to develop prediction models and mitigation techniques able to perform a safe support service. The physical mechanisms regulating such coupling, indeed, show a high degree of complexity related to the high variability of the geospace environment conditions including those of the Sun, the interplanetary medium, and the magnetosphere-ionosphere-thermosphere system. The understanding of the physical mechanisms regulating the solar wind-magnetosphere-ionosphere coupling motivates the necessity for an original contribution to the current knowledge in the sector. Therefore, this work aims to achieve a broader understanding of the existing link between the external origin geomagnetic field perturbations and ionospheric scintillations, under strong disturbance conditions of the circumterrestrial environment. In particular, this work provides new insights for predicting the effects of the perturbed ionosphere on technological systems thus contributing to studies of space weather. Consequently, the topic choice has been motivated both by the need to contribute to the study of physical processes that regulate the highly-ionized upper atmosphere, and by an awareness that the technological systems, sensitive to ionospheric changes, may have a development only if supported by a deep knowledge of the geospace environment. The main objective of this work is to understand how different configurations of the Interplanetary Magnetic Field (IMF)-magnetosphere-ionosphere coupling are translated into different structuring and dynamics of the high latitude ionosphere. For an in-depth study of this topic, it is necessary to reproduce the concatenation of the events triggering the generation of ionospheric irregularities. For this reason, the present PhD project presents an unprecedented combination of diverse data sets to explain GNSS scintillation. In particular, observations provided by satellites orbiting the first Lagrangian point (L1), by geostationary satellites, by GNSS constellation, by satellites flying in the magnetosphere and in the ionosphere and by ground-based devices were analysed and interpreted. The analysis and interpretation of such observations imply the integration of broad spectrum of information necessary to characterize the ionospheric disturbances at different time scales (from milliseconds to days) and spatial scales (from millimetres to hundreds meters/kilometres), in accordance with the evolution of the storms drivers. Starting from the scintillations recorded by selected GNSS receivers at ground, the proposed multi-instrumental and multi-parametric approach allows investigating the origin of irregularities: by evaluating the ionospheric background under different interplanetary conditions; by studying the magnetosphere configuration during the main phase of intense geomagnetic storms; by reconstructing the ionospheric scenarios resulting in the observed scintillations. The reconstruction of the spatial-temporal environment resulting in scintillation on L-band signals focuses on the study of the major storms of the 24th solar cycle and in particular on their main phase that occurred on 2015 March 17th, 2015 June 22nd and 2017 September 08th, respectively. The choice of such case studies was motivated not only by the intensity (that ensured a strong perturbation of the polar ionosphere) but also because those storms, having been caused by different solar phenomena and occurring in different seasons, have given rise to geomagnetic disturbances that are useful for the irregularities characterization under different helio-geophysical conditions. The achieved results can be summarized as follows. The investigation of the selected case study confirms the non-linear relationship between geo-spatial disturbances and the response of the circumterrestrial environment, showing how strong (or weak) solar perturbations do not necessarily correspond to strong (or weak) ionospheric perturbations in terms of scintillations. An important result for the modelling is that the intensification of the field-aligned currents due to the arrival of the interplanetary perturbation causes the concurrent scintillation triggers in the cusp/auroral regions. Still for the purpose of the forecast, the results obtained by the comparative study of three cases suggest the Bz,IMF southward condition as the most favourable to trigger scintillations, even of great intensity. In particular, during Bz,IMF southward condition, it is possible to observe irregularities enhancement due to both ionization increase and conductibility increase. Differently, Bz,IMF northward condition leads to a lower occurrence of scintillations. The comparison among the variations of the scintillation parameters measured at ground and of the in-situ electron density observations allows the identification of the polar cap as the region most exposed to the perturbations coming from the Sun, especially during Bz,IMF southward conditions. The investigation of in-situ particle precipitation fluxes allows supporting the hypothesis that the large exposition of polar cap regions is caused by particles precipitating in the polar caps following those magnetic field lines that, reconnecting on dayside, move towards the magnetotail crossing the polar caps. The multi-instrument investigation adopted in this work, covering both polar regions, improved the understanding of the response of the two hemispheres to the same interplanetary perturbation. The comparison among the three cases shows that the reconfiguration of the magnetospheric field lines, following interplanetary shocks, identifies the main cause of the interhemispheric asymmetry in the ionospheric response in terms of scintillation on L-band signals. The multi-observational reconstruction of the ionospheric scenario caused by the interplanetary perturbations reveals the size of the irregularities causing scintillation and allows speculating on the diffractive/refractive nature of the observed scintillation. In particular, the comparative study of different geomagnetic storms suggests that irregularities having scale size up to the radius of the first Fresnel’s zone, hence diffractive in nature, form in a highly ionized ionosphere exposed to rapid variations of the geomagnetic field induced by interplanetary perturbation. These results show how the combined use of data from in-situ and ground-based sensors allows a detailed characterization of ionospheric dynamics during a geomagnetic storm. Given that ionospheric scintillations are difficult to predict, the study of the ionosphere at different heights and with different time scales can provide information useful to better characterize the high latitude ionospheric scintillation, thus advancing the current understanding in the field. In the space weather context, this can pave the way to new approaches in developing predictive scintillation models. This study has demonstrated, with an unprecedented detail, the power of a multi-instruments approach to describe the ionospheric conditions triggering scintillations on GNSS signals at high latitude. The results, even though achieved with a limited number of cases, clearly identify for the first time the direct link between the magnetospheric field configuration and the formation of irregularities causing scintillations at high latitudes of both hemispheres.