Space weather (or Space Weather) refers to the conditions of the Sun and the solar wind, the magnetosphere, the ionosphere and the thermosphere which can affect the performance and reliability of systems technologies on the ground and in space and which can endanger life or human health. The Space Weather events that most affect the aerospatial operations are those that corrupt operating systems and those that increase the radiation level at flight altitudes. Among the others: the increased flux of Galactic Cosmic Rays (GCR), Coronal Mass Ejections (CME), Solar Proton Events (SPE), solar flares, geomagnetic storms and ionospheric storms.
HF (High Frequency) communication systems exploit the reflection properties of the ionosphere to transmit radio signals over long distances. However, in case of an ionospheric storm, these systems can become inoperable at any latitude (Cannon et al, 2003). This is the case, for instance, for a particular type of disturbance called Polar Cap Absorption (PCA), that can last several days. PCA events are due to solar emission of high energy protons which, penetrating the atmosphere of the polar regions, produce an extra ionization in the lower part of the ionosphere (50–100 km) inducing the absorption of transmitted radio signals making the HF communications difficult, if not impossible (Warrington et al., 2004). Due to the effect of the ionosphere, higher frequency signals (of the order of GHz) used for communications and satellite positioning (GNSS, Global Navigation Satellite Systems) are subject to refraction and diffraction, leading to rapid fluctuations in amplitude and phase of the signal itself and unexpected propagation paths. In case of solar flares or CME the effects of the ionosphere on the signal passing through is such to cause total loss of signal lock to the ground and, consequently, of the communication and positioning services.
Recently, the aviation industry is getting interested in the effects of the Space Weather on the systems supporting the precision positioning, such as SBAS (Satellite Based Augmentation Systems), GBAS (Ground Based Augmentation Systems) and also on navigation systems and landing systems based on GNSS that will provide support to navigation of aircraft from cruising altitude to landing. Indeed, during a geomagnetic storm, the ionospheric perturbations can introduce horizontal and vertical errors of several tens of meters in the positioning.
The equatorial ionosphere is characterized by the presence of the Equatorial Ionospheric Anomaly (EIA) that is represented by higher values of electron density, the so-called crests, in a band of magnetic latitude which, in conditions of high solar activity, is located between ±10° and ±20° off the magnetic equator. The EIA is due to the joint role of the circulation of thermospheric neutral winds at middle and low latitudes, of the electrodynamics induced from such circulation on the ionospheric plasma and of the Earth's magnetic field configuration at equatorial altitudes (Fejer et al, 2011). At the crests of the EIA and in the hours following local sunset, Equatorial Plasma Bubble (EPB) may form. EPBs are the main responsible for the formation of ionospheric irregularities that corrupt the functionality of GNSS signals (Kintner et al., 2009, Balan et al., 2019) and HF communications. Indeed, the ionospheric irregularities, presenting a broad spectrum of scales, even smaller than the typical Fresnel scale for L-band signals (hundreds of meters) and of the order of km in the HF band, cause fluctuations on the amplitude and phase of GNSS signals known as “ionospheric scintillations” (Wernik et al., 2003) and disturbance in the HF signal received at ground, known as Equatorial Spread F (ESF). The scenario is further complicated by the occurrence of geomagnetic storms due to phenomena originated on the Sun which, disturbing the quiet conditions of the equatorial ionosphere, lead to significant changes in the dynamics and in the morphology of the EIA crests and in the formation of ionospheric irregularities that produce scintillations (see Alfonsi et al., 2011; Astafyeva et al., 2016; Spogli et al., 2016; Tulasi Ram et al., 2016; Olwendo et al., 2017; Venkatesh et al., 2017). Although the mechanisms with which solar events modify the equatorial ionosphere are known, the large variability of induced effects opens new fields of research of international interest, especially in the development of low latitude ionosphere modeling and prediction tools. This is especially true for the African sector, still characterized by paucity of instruments designed to observe and characterize the ionosphere and its behavior under disturbed conditions. GNSS receivers are therefore a valuable tool for ionospheric studies, because the received signals, traveling through the entire ionosphere, provide information on the properties of the crossed medium. The main ionospheric parameter derived from GNSS measurements is the Total Electron Content (TEC), defined as the integral of electron density in the ionosphere in a cylindrical volume with a section of 1 m2 along the raypath between the satellite and the receiver. TEC is traditionally measured in TECU (TEC units) and 1 TECU is 1016 electrons / m2. Many articles in the recent scientific literature are addressed to the study of the TEC and in particular of its gradients - both spatial and temporal - from which the mechanisms of formation of the irregularities are studied ionospheric (see, for example, Cesaroni et al., 2015). Then there is one special class of receivers, so called “Ionospheric Scintillation Monitors Receiver (ISMR)” (Van Dierendonk et al., 1993; Bougard et al., 2011) which, thanks to high sampling rates, ultra-stable low noise oscillators, they are able to provide indices that are commonly used to characterize the ionospheric scintillation, that of phase e that of amplitude S4 (Van Dierendonk et al., 1993). These indices are used to define the intensity levels of the scintillation: from weak to strong (see, for example, Spogli et al., 2013). The day to day variability in the formation of ionospheric irregularities (Otsuka, 2018), the behavior under geomagnetic storm conditions (Spogli et al., 2019), their modeling and prediction (Yokoyama, 2017; Grzesiak et al., 2018) and the mitigation of their effects on GNSS positioning (Park et al., 2016) are an entirely open challenge to scientific community. On the other hand, the GNSS receivers only provide an integrated measure of free electron concentration at the line of sight between the receiver and the satellite. In order to obtain the information related to the vertical distribution of the ionospheric plasma there is the need to integrate the monitoring by GNSS signals with a different measurement, the one done by means of a spanning frequency HF radar
better known as ionosonde (Zuccheretti et al., 2003). The ionosonde allows the vertical sounding of the ionosphere through the use of signals in the HF band with frequency varying between 1 and 30 MHz which, under appropriate conditions, is backscattered from the ionosphere down to the ground carrying information on the reflection rate and on the electron density at that particular height. Therefore, through inversion algorithms (Pezzopane and Scotto 2007, Scotto 2009, Cesaroni et al., 2013), it is possible to pass from HF measurements to a vertical electron density profile on the survey station overhead. The joint use of GNSS and ionosonde measurements, as input to models empirics such as the NeQuick2 (Nava et al., 2008, Nava et al., 2011), becomes crucial in order to be able to provide a three-dimensional description of the ionospheric morphology useful for both scientific and space weather applications (mitigation of ionospheric effects on technological systems of positioning and communication).
Figure 2. Probability of moderate to strong amplitude scintillation on signals GNSS, estimated through the occurrence of the S4 index above the value of 0.25 measured over 3 years of accumulated data. This occurrence is given as the day changes of the year and local time and refers to a geographical sector which covers the expected position of the southern crest of the EIA in the Brazilian longitudinal sector. The blue lines represent sunrise and sunset times.