Zolesi, Bruno

Ionospheric Monitoring and Modelling for Space Weather: An Introduction to the Special Issue

This paper is a final review of the Simplified Ionospheric Regional Model (SIRM) devel- oped as a prototype in the early 1990s and improved in the following years. By means of an algorithm based on the Fourier synthesis, the SIRM model in its prototype version pro- vides predicted monthly median values of the main ionospheric characteristics such as: the ordinary wave critical frequencies (foE, foF1, and foF2) of the E, F1, and F2 ionospheric layers; the lowest virtual height (h’F) of the ordinary trace of the F region; the obliquity factor for a distance of 3000 km (M(3000)F2). Instead, the improved version focuses only on foF2 and M(3000)F2. The SIRM model has been largely employed in the framework of different international research projects as the climatological reference to output foF2 and M(3000)F2 monthly median predictions, but in its SIRMUP version is used also as a nowcasting model and as an intermediate step of complex procedures for a near real- time three-dimensional representation of the ionospheric electron density. In this regard, some results provided by both SIRM and SIRMUP for telecommunication applications are shown. Moreover, the mathematical treatment concerning both the phase correction of the Fourier synthesis and the fundamental steps carried out to define the SIRM algorithm in its final version, never published so far, will be described in detail in dedicated Appendices. Finally, for the first time the SIRM code is now downloadable for the benefit of users.

On 25 May 1928 the airship “Dirigibile Italia” during its return trip to the base in NyAlesund, after overflying the North Pole, shipwrecked on the ice‐pack in a region at about 400 km northeast of Svalbard Islands. Survivors, by using a portable high frequency (HF) radio transmitter, tried unsuccessfully to send SOS messages and to establish a radio link with the ship “Città di Milano” of the Italian Navy, closely anchored at King's Bay. Only after 9 days of repeated radio‐distress transmissions, a Russian radio amateur close to the town of Arkhangelsk about 1,900 km away was able to receive the messages launched by the survivors and raise the alarm. This paper aims at giving a retrospective analysis of the ionospheric and geomagnetic conditions of that epoch in order to explain the HF radio communications problems encountered by the survivors. The International Reference Ionosphere model has been applied, and early geomagnetic measurements have been evaluated, to come up with theories explaining the events. We assert the HF transmission difficulties were associated with the “radio silent” or “dead zones” associated with F‐region propagation. These may have been exacerbated by solar and geomagnetically disturbed conditions of the days immediately following the airship wreck.

This paper consists of a review of the important contributions of four COST (European Co-operation in Science and Technology) Actions in the period 1991–2009 to terrestrial ionospheric research, with applications in modern communication and navigation systems. Within this context, new ionospheric studies were initiated, leading to the development of a number of models, algorithms for prediction, forecasting, and real-time specification, as well as numerical programs. These were successfully implemented in different collaborative projects within EU instruments, promoting co-operation between scientists and researchers across Europe. A further outcome was to bring together more than a hundred researchers from around 40 scientific institutions, agencies, and academia in about 25 countries worldwide. They collaborated with enthusiasm in research, as briefly described in this paper, forming a lively ionospheric community and presenting a strong intellectual response to the rapidly growing contemporary challenge of space weather research

The SIRM model proposed by Zolesi et al. (1993, 1996) is an ionospheric regional model for predicting the vertical-sounding characteristics that has been frequently used in developing ionospheric web prediction services (Zolesi and Cander, 2014). Recently the model and its outputs were implemented in the framework of two European projects: DIAS (DIgital upper Atmosphere Server; http://www.iono.noa.gr/DIAS/) (Belehaki et al., 2005, 2015) and ESPAS (Near-Earth Space Data Infrastructure for e-Science; http://www.espas-fp7.eu/) (Belehaki et al., 2016). In this paper an updated version of the SIRM model, called SIRMPol, is described and corresponding outputs in terms of the F2-layer critical frequency (foF2) are compared with values recorded at the mid-latitude station of Rome (41.8°N, 12.5°E), for extremely high (year 1958) and low (years 2008 and 2009) solar activity. The main novelties introduced in the SIRMPol model are: (1) an extension of the Rome ionosonde input dataset that, besides data from 1957 to 1987, includes also data from 1988 to 2007; (2) the use of second order polynomial regressions, instead of linear ones, to fit the relation foF2 vs. solar activity index R12 ; (3) the use of polynomial relations, instead of linear ones, to fit the relations A0 vs. R12 , An vs. R 12 and Yn vs. R12 , where A0 , An and Yn are the coefficients of the Fourier analysis performed by the SIRM model to reproduce the values calculated by using relations in (2). The obtained results show that SIRMPol outputs are better than those of the SIRM model. As the SIRMPol model represents only a partial updating of the SIRM model based on inputs from only Rome ionosonde data, it can be considered a particular case of a single-station model. Nevertheless, the development of the SIRMPol model allowed getting some useful guidelines for a future complete and more accurate updating of the SIRM model, of which both DIAS and ESPAS could benefit.

The ionosphere affects the electromagnetic wave propagation and then its study is important for Earth-Earth, satellite-Earth, and satellite-satellite communication purposes. Diffractive and refractive processes due to irregular electron density structures cause signal fluctuations that can disrupt satellite-ground communications and represent a hazard for navigation systems. The study and the real-time monitoring of the ionosphere are important for Space Weather purposes. The ionospheric vertical sounding is described, together with the automatic scaling of the ionograms.

The Earth’s ionosphere is a magnetoionic medium imbedded in a background neutral atmosphere, exhibiting very interesting refractive properties, including anisotropy, dispersion, and dissipation. As such, it poses a challenge for several radio systems that make use of signal transmission through all or some portion of the medium. It is important therefore to develop prediction systems able to inform the operators of such systems about the current state of the ionosphere, about the expected effects of forthcoming space weather disturbances and about support long-term planning of operations and data post-processing projects for improving modelling and mitigation techniques. The European Space Agency (ESA) in the framework of the Space Situational Awareness (SSA) Programme has supported the development of the European Ionosonde Service (EIS) that releases a set of products to characterise the bottomside and topside iono- sphereoverEurope.The Service is based on a set of prediction models driven by data from ground based ionosondes and supportive data from satellites and spacecraft. The service monitors the foF2 and the electron density profile up to the height of the Global Navigation Satellite System (GNSS) at European middle and high latitudes and provides estimates for forth coming disturbances mainly triggered by geoeffective Coronal Mass Ejections (CMEs).The model’s performance has been validated and based on these results ,it was possible to issue together with the products, quality metrics characterizing the product’s reliability. The EIS products meet the requirements of various SSA service domains, especially the transionospheric radio link and the spacecraft operations. Currently, the service is freely available to all interested users, and access is possible upon registration.

This paper investigates possible use of middle latitude daytime COSMIC and CHAMP ionospheric radio occultation (IRO) electron density profiles (EDPs) to retrieve thermospheric parameters, based on the Mikhailov et al. (2012) method. The aim of this investigation is to assess the applicability of this type of observations for the routine implementation of the method. According to the results extracted from the analysis presented here, about half of COSMIC IRO EDP observed under solar minimum (2007–2008) conditions gave neutral gas density with an inaccuracy close to the declared absolute inaccuracy ±(10–15)% of CHAMP observations, with the results being better than the empirical models JB-2008 and MSISE-00 provide. For the other half of IRO EDP, either the solution provided by the method had to be rejected due to insufficient accuracy or no solution could be obtained. For these cases, the parameters foF2 and hmF2 extracted from the corresponding IRO profiles have been found to be inconsistent with the classic mid-latitude daytime F2-layer formalism that the method relies on, and they are incompatible with the general trend provided by the IRI model. For solar maximum conditions (2002) the method was tested with IRO EDP from CHAMP and it is indicated that its performance is quite stable in the sense that a solution could be obtained for all the cases analyzed here. However available CHAMP EDP are confined by ~ 400 km in altitude and this might be the reason for the 20% bias of the retrieved densities toward larger values in respect to the observed densities. IRO observations up to 600 km under solar maximum are required to confirm the exact performance of the method.

The present paper proposes to discuss the ionospheric absorption, assuming a quasi-flat layered ionospheric medium, with small horizontal gradients. A recent complex eikonal model [Settimi et al., 2013b] is applied, useful to calculate the absorption due to the ionospheric D-layer, which can be approximately characterized by a linearized analytical profile of complex refractive index, covering a short range of heights between h1= 50 km and h2= 90 km. Moreover, Settimi et al. [2013c] have already compared the complex eikonal model for the D-layer with the analytical Chapman’s profile of ionospheric electron density; the corresponding absorption coefficient is more accurate than Rawer’s theory [1976] in the range of middle critical frequencies. Finally, in this paper, the simple complex eikonal equations, in quasi-longitudinal (QL) approximation, for calculating the non-deviative absorption coefficient due to the propagation across the D-layer are encoded into a so called COMPLEIK (COMPLex EIKonal) subroutine of the IONORT (IONOspheric Ray-Tracing) program [Azzarone et al., 2012]. The IONORT program, which simulates the three-dimensional (3-D) ray-tracing for high frequencies (HF) waves in the ionosphere, runs on the assimilative ISP (IRI-SIRMUP-P) discrete model over the Mediterranean area [Pezzopane et al., 2011]. As main outcome of the paper, the simple COMPLEIK algorithm is compared to the more elaborate semi-empirical ICEPAC formula [Stewart, undated], which refers to various phenomenological parameters such as the critical frequency of E-layer. COMPLEIK is reliable just like the ICEPAC, with the advantage of being implemented more directly. Indeed, the complex eikonal model depends just on some parameters of the electron density profile, which are numerically calculable, such as the maximum height.

In this paper we analyze the behaviour of the critical frequency of the F2 region of the ionosphere(foF2)and the height of the maximum density of free electrons in F2 region(hmF2)overTucumán(26.91S,294.61E), during the deep solar minimum occurred in 2008–2009. Data used were compared with those obtained at solar minimum observed in 1975–1976. In addition, we check the validity of theI nternational Reference Ionosphere model(IRI), in the version 2012, to predict the maximum free electron density in the ionosphere(NmF2)above the mentioned station, for very low solar activity. The results show that: (a) Ionization was lowest for recent solar minimum.(b)The semmianual anomaly which are present in the behaviour of foF2 at times of increased solar activity, was not clearly observed during the period 2008–2009. This phenomenon could be related with the very low solar activity for that period, confirming the relationship of the amplitude of this anomaly with the solar activity reported by other authors.(c)In most cases, the values of hmF2 recorded in the deep solar minimum are lower than those observed in the period 1975–1976, suggesting a decrease in the height of the ionosphere in the course of time, which could be related to the greenhouse effect in the atmosphere and the anomalously low solar extreme-ultraviolet irradiance.(d)IRI predictions show significant deviations from the experimental values, indicating the need for improvements in the model.