Structure and Dynamics of Ionospheric Plasma

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Software-defined radio technology for GNSS scintillation analysis: bring Antarctica to the lab

Global navigation satellite systems (GNSSs) are widely used to support logistics, scientific operations, and to monitor the polar ionosphere indirectly, which is a region characterized by strong phase scintillation events that severely affect the quality and reliability of received signals. Professional commercial GNSS receivers are widely used for scintillation monitoring; on the contrary, custom-designed solutions based on data grabbers and software receivers constitute novelty. The latter enables a higher level of flexibility and configurability, which is important when working in remote and severe environments. We describe the scientific, technological, and logistical challenges of installing an ionospheric monitoring station in Antarctica, based on a multi-constellation and multi-frequency GNSS data grabber and a software-defined radio receiver. Having access to the full receiver chain and to intermediate signal processing stages allows a deep analysis of the impact of scintillation and, in turn, a better understanding of the physical phenomenon. The possibility to process high-resolution raw intermediate frequency samples of the signal enables not only the computation of scintillation indexes with the same quality as professional devices but also the design and test of innovative receiver architectures and algorithms. Furthermore, the record and replay approach offers the possibility to process in the lab the signals captured on site, with high fidelity level. It is like being in Antarctica again, but with an unlimited set of receivers and higher computational, storage, and bandwidth resources. The main advantages and disadvantages of this approach are analyzed. Examples of monitoring results are reported, confirming the monitoring capabilities, showing the good agreement with commercial receiver outputs and confirming the validity of post-processing and re-play operations

Space weather challenges of the polar cap ionosphere

This paper presents research on polar cap ionosphere space weather phenomena conducted during the European Cooperation in Science and Technology (COST) action ES0803 from 2008 to 2012. The main part of the work has been directed toward the study of plasma instabilities and scintillations in association with cusp flow channels and polar cap electron density structures/patches,which is considered as critical knowledge in order to develop forecast models for scintillations in the polar cap. We have approached this problem by multi-instrument techniques that comprise the EISCAT Svalbard Radar, SuperDARN radars, in-situ rocket, and GPS scintillation measurements. The Discussion section aims to unify the bits and pieces of highly specialized information from several papers into a generalized picture. The cusp ionosphere appears as a hot region in GPS scintillation climatology maps. Our results are consistent with the existing view that scintillations in the cusp and the polar cap ionosphere are mainly due to multi-scale structures generated by instability processes associated with the cross-polar transport of polar cap patches. We have demonstrated that the SuperDARN convection model can be used to track these patches backward and forward in time. Hence, once a patch has been detected in the cusp inflow region, SuperDARN can be used to forecast its destination in the future. However, the high-density gradient of polar cap patches is not the only prerequisite for high-latitude scintillations. Unprecedented high resolution rocket measurements reveal that the cusp ionosphere is associated with filamentary precipitation giving rise to kilometer scale gradients onto which the gradient drift instability can operate very efficiently... (continued

Scintillations climatology over low latitudes: statistical analysis and WAM modelling

Attempts of reconstructing the spatial and temporal distribution of the ionospheric irregularities have been conducted developing a scintillation “climatology” technique, which was very promising in characterizing the plasma conditions triggering L-band scintillations at high latitudes ([1.],[2.]) and further analysis on bipolar high sampling rate (50 Hz) GPS data are currently in progress for deeper investigations. The core of the scintillation climatology technique is represented by the maps of percentage of occurrence of the scintillation indices above a given threshold. The maps at high latitude are expressed in terms of geomagnetic coordinates (Magnetic Latitude vs. Magnetic Local Time) and their fragmentation depends on the available statistics. Typically the selected thresholds are 0.25º for the phase scintillation index σΦ and 0.25 for the amplitude one S4, which represent a good compromise between the need of a meaningful sample in each map bin and the necessity to distinguish moderate/strong scintillations. The scintillation climatology technique has been very useful in identifying the main areas of the ionosphere (from mid to cusp/cap latitudes) in which plasma irregularities could lead to scintillation phenomena on GPS signals and their dependence on different geomagnetic conditions of the ionosphere and on different level of the solar activity. As the promising results achieved, we propose to apply the same approach to draw a first raw representation of the scintillations climatology over the Latin America sector. In the development of the study, it will be considered that, at low latitudes, scintillations effects are most severe around the magnetic equator and around the crests of the equatorial anomaly in the early evening hours. Moreover, the morphology of the ionosphere is different from that at other latitudes, because the magnetic field B is nearly parallel to the Earth’s surface, leading to different configurations, dimensions and dynamics of the ionosphere irregularities causing scintillation. Scintillation climatology in geographic coordinates will be performed on scintillation data collected at the site of Presidente Prudente (Brazil, 22.12ºS, 51.41ºW) via a SCINTMON receiver [3.]. The SCINTMON receiver is developed by the space plasma physics group from Cornell University and designed to monitor the amplitude scintillations at the L1 frequency (1.575 MHz). The SCINTMON is capable of logging the signal intensity at 50 samples per second for up to 11 visible satellites simultaneously, then the data collected are post-processed via software, and for each 60 s interval of data the S4 scintillation index is computed for all satellites tracked during the observation nights (0900–2100 UT). In relation with the aforementioned climatology, here we also discuss the extension to low latitudes of the empirical Wernik-Alfonsi-Materassi (WAM) [4.] model. This is a simple phase screen model of propagation of a plane wave through the irregular ionosphere. It ingests the electron density in situ satellite data to reproduce empirically the irregular medium. WAM was originally developed to model high latitude irregularities, and now it is going to be extend to lower latitudes. The concept of such extension is here described. The low latitude scintillation climatology will be used for understanding the key points to be carefully explored to concretely envisage a reliable modelling. The main innovative idea of the WAM model [4.] is that the statistics of the medium, giving rise to the irregular pattern formation called “scintillation” when crossed by an electromagnetic wave, should be constructed from in situ data instead of being assumed a priori. This is because the ionization fluctuations, due to a form of “dirty plasma” turbulence, are expected to show non-trivial statistics, often non Gaussian ones, due to the strong gradients possibly occurring in the ionosphere. WAM was constructed as a phase screen model, good for climatological use, with the statistics of the phase fluctuations δφ directly calculated from the in situ data of the ionization fluctuations δN collected by the DE2 mission in the years 1981-1983. The S4 scintillation index is predicted, along an assigned satellite-ground radio link, via the analytical formulæ for the weak scattering due to Rino [5.]. The location and thickness of the phase screen, and the value of the ionization maximum, all enter in Rino’s formulæ, and these are given in WAM by matching the background ionization as measured by the DE2 satellite with the ionospheric profile provided by some ionospheric background model. In its original form, WAM uses the IRI95 as a profiler [6.]. In its first release, described in [4.], the model predicts the S4 climatology within high invariant latitudes (larger than 50°), and may calculate the most likely S4 along a given radio link of identified geometry, time and geomagnetic conditions (represented through the Kp index). The choice of high latitudes was due to some elements: being DE2 a polar orbiting satellite, its passes form a denser network around poles; real scintillation measurements to compare with are more abundant in the polar regions; the IRI95 profiler is an excellent tool for mid-high latitudes (with some suitable corrections for the topside at high latitudes). In order to extend the WAM model to low latitudes as well, some changes to it must be done. First of all, low latitude in situ observations from DE2 are included, plus other similar data of a low latitude orbiting satellite (in the future, possibly ROCSAT data [7.]). The background ionosphere must be represented via some model which turns out to be more reliable than IRI95 to represent the so Equatorial Anomaly, which is the main feature of the low latitude ionosphere. The successive developments of IRI95 represent improvements of the low latitude background, among the other things, but the choice here was to use the further development referred to as NeQuick model [8.], in its ITU-R version [9.]. Once the WAM model has been expanded to ±40° of latitude thanks to further in situ data and the NeQuick background model, it will be possible to predict a climatology of S4 that will be tested against the real data of the scintillation climatology: this comparison will allow for operation of finer tuning in the low latitude extended WAM model

On estimating the phase scintillation index using TEC provided by ISM and IGS professional GNSS receivers and machine learning

Amplitude and phase scintillation indexes (S4 and sigma_phii) provided by Ionospheric Scintillation Monitoring (ISM) receivers are the most used GNSS-based indicators of the signal fluctuations induced by the presence of ionospheric irregularities. These indexes are available only from ISM receivers which are not as abundant as other types of professional GNSS receivers, resulting in limited geographic distribution. This makes the scintillation indexes measurements rare and sparse compared to other types of ionospheric measurements available from GNSS receivers. Total Electron Content (TEC), on the other hand, is an ionospheric parameter available from a wide range of multi-frequency GNSS receivers. Many efforts have worked on establishing scintillation indicators based on TEC, and geodetic receiver in general, introducing various metrics, including the Rate of TEC change (ROT) and ROT Index (ROTI)

Climatology of GPS scintillations over Antarctica under solar minimum conditions

We analyse GNSS ionospheric scintillation data recorded in Antarctica to investigate the conditions of the near- Earth environment leading to scintillation scenarios, producing a “scintillation climatology” over a large geomagnetic quiet period.Within this scope we realize maps of scintillation occurrence as a function of the magnetic local time (MLT) and of the altitude adjusted corrected geomagnetic coordinates (AACGM). The maps are realized merging observations of two GISTMs (GPS Ionospheric Scintillation and TEC Monitor) located at Mario Zucchelli Station (74.7°S, 164.1°E) and Concordia Station (75.1°S, 123.2°E) in Antarctica during 2008. The results highlight the possibility to investigate the impact of ionospheric irregularities on the phase and amplitude of GNSS signals, evidencing the cusp/cap and auroral contributions. This works aims to contribute to the development of nowcasting and forecasting tools for GNSS ionospheric scintillation

GPS ionospheric scintillation and HF radar backscatter – A comparison between GISTM network and SuperDARN at high latitudes

The occurrence of GPS ionospheric scintillation at high latitudes over Scandinavia in 2003 and 2008 is compared with the occurrence of HF radar backscatter from field-aligned irregularities as a function of magnetic local time and geomagnetic latitude for the same two years. The scintillation was observed using GPS Ionospheric Scintillation and TEC Monitors (GISTM) included in a network extending from high to mid latitudes. Both the HF radar backscatter and GPS scintillation predominantly occur in the night portion of the auroral oval and the ionospheric footprint of the cusp. Data subsets for geomagnetically quiet and disturbed periods show the expected shift in latitude of the ionospheric regions both in the occurrence of phase scintillation and the HF radar backscatter from ionospheric irregularities

The response of high latitude ionosphere to the 2015 June 22 storm

This work investigates physical mechanisms triggering phase scintillations on L-band signals under strong stormy conditions. Thanks to selected ground-based Global Navigation Satellite Systems (GNSS) receivers, located both in Antarctica and in the Arctic, an interhemispheric comparison between high latitude ionospheric observations in response to the peculiar solar wind conditions occurred on June 22, 2015 is here shown. To trace back the observed phase scintillations to the physical mechanisms driving it, we combine measurements from GNSS receivers with in-situ and ground-based observations. Our study highlights the ionospheric scenario in which irregularities causing scintillation form and move, leveraging on a multi-observation approach. Such approach allows deducing that scintillations are caused by the presence of fast-moving electron density gradients originated by particle precipitation induced by solar wind variations. In addition, we show how the numerous and fast oscillations of the north-south component of the interplanetary magnetic field (Bz,IMF) result to be less effective in producing moderate/intense scintillation events than during period of long lasting negative values. Finally, we also demonstrate how the in-situ electron density data can be used to reconstruct the evolution of the ionospheric dynamics, both locally and globally

Intrinsic Mode Cross Correlation: A Novel Technique to Identify Scale-Dependent Lags Between Two Signals and Its Application to Ionospheric Science

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