A document by Gareth Dorrian. Click on the document to view its contents.

A document by Benjamin Reid. Click on the document to view its contents.

The lunar ionosphere is a ~100 km thick layer of electrically charged plasma surrounding the moon. Despite knowledge of its existence for decades, the structure and dynamics of the lunar plasma remain a mystery due to lack of consistent observational capacity. An enhanced observational picture of the lunar ionosphere and improved understanding of its formation/loss mechanisms is critical for understanding the lunar environment as a whole and assessing potential safety and economic hazards associated with lunar exploration and habitation. To address the high priority need for observations of the electrically charged constituents near the lunar surface, we introduce a concept study for the Radio Instrument Package for Lunar Ionospheric Observation (RIPLIO). RIPLIO would consist of a multi-CubeSat constellation (at least two satellites) in lunar orbit for the purpose of conducting “crosslink” radio occultation measurements of the lunar ionosphere, with at least one satellite carrying a very high frequency (VHF) transmitter broadcasting at multiple frequencies, and at least one satellite flying a broadband receiver to monitor transmitting satellites. Radio occultations intermittently occur when satellite-to-satellite signals cross through the lunar ionosphere, and the resulting phase perturbations of VHF signals may be analyzed to infer the ionosphere electron content and high- resolution vertical electron density profiles. As demonstrated in this study, RIPLIO would provide a novel means for lunar observation, with the potential to provide long-term, high-resolution observations of the lunar ionosphere with unprecedented pan-lunar detail.

Prior to use in operational systems, it is essential to validate ionospheric models in a manner relevant to their intended application to ensure satisfactory performance. For Over-the-Horizon radars (OTHR) operating in the high-frequency (HF) band (3-30 MHz), the problem of model validation is severe when used in Coordinate Registration (CR) and Frequency Management Systems (FMS). It is imperative that the full error characteristics of models is well understood in these applications due to the critical relationship they impose on system performance. To better understand model performance in the context of OTHR, we introduce an ionospheric model validation technique using the oblique ground backscatter measurements in soundings from the Super Dual Auroral Radar Network (SuperDARN). Analysis is performed in terms of the F-region leading edge (LE) errors and assessment of range-elevation distributions using calibrated interferometer data. This technique is demonstrated by validating the International Reference Ionosphere (IRI) 2016 for January and June in both 2014 and 2018. LE RMS errors of 100-400 km and 400-800 km are observed for winter and summer months, respectively. Evening errors regularly exceeding 1,000 km across all months are identified. Ionosonde driven corrections to the IRI-2016 peak parameters provide improvements of 200-800 km to the LE, with the greatest improvements observed during the nighttime. Diagnostics of echo distributions indicate consistent underestimates in model NmF2 during the daytime hours of June 2014 due to offsets of -8° being observed in modelled elevation angles at 18:00 and 21:00 UT.

Radio interferometers used to make astronomical observations, such as the LOw Frequency ARray (LOFAR), experience distortions imposed upon the received signal due to the ionosphere as well as those from instrumental errors. Calibration using a well-characterised radio source can be used to mitigate these effects and produce more accurate images of astronomical sources, and the calibration process provides measurements of ionospheric conditions over a wide range of length scales. The basic ionospheric measurement this provides is differential Total Electron Content (TEC, the integral of electron density along the line of sight). Differential TEC measurements made using LOFAR have a precision of <1 mTECu and therefore enable investigation of ionospheric disturbances which may be undetectable to many other methods. We demonstrate an approach to identify ionospheric waves from these data using a wavelet transform and a simple plane wave model. The noise spectra are robustly characterised to provide uncertainty estimates for the fitted parameters. An example is shown in which this method identifies a wave with an amplitude an order of magnitude below those reported using GNSS TEC measurements. Artificially generated data are used to test the accuracy of the method and establish the range of wavelengths which can be detected using this method with LOFAR data. This technique will enable the use of a large and mostly unexplored dataset to study travelling ionospheric disturbances over Europe.

The solar radio flux at 10.7 cm, known as F10.7, is a critical operational space weather index. However, without a clear backup, any interruption to the service can result in substantial errors in model outputs. In this paper we show the impact of one such outage in March 2022 and present a number of alternative solutions for any future outages. The approach resulting in the smallest reconstruction error of F10.7 uses the solar radio flux observations at alternative wavelengths (the best giving a percentage error of 3.1%). Alternatively, use of Sunspot Number, a regular, robust alternative observation, results in a mean percentage error of 8.2% and is also a reliable fallback solution. Additionally, analysis of the error on the use of the conversion between the 12-month rolling sunspot number (R12) and its conversion to F10.7 as used by the IRI is included.

Ground scatter (GS) echoes in Super Dual Auroral Radar Network (SuperDARN) observations have been always expected to occur under high-enough electron density in the ionosphere providing sufficient bending of HF radio wave paths toward the ground. In this study we provide direct evidence statistically supporting this notion by comparing the GS occurrence rate for the Rankin Inlet SuperDARN radar and the F region peak electron density NmF2 measured at Resolute Bay by the CADI ionosonde and incoherent scatter radars RISR-N/C. We show that the occurrence rate increases with NmF2 roughly linearly up to about ~4·1011 m-3, and the trend saturates at larger NmF2. One expected consequence of this relationship is correlation in seasonal and solar cycle variations of the GS echo occurrence rate and NmF2. GS occurrence rates for a number of SuperDARN radars at middle latitudes, in the auroral zone and in the polar cap are considered separately for daytime and nighttime. The data indicate that the daytime occurrence rates are maximized in winter and nighttime occurrence rates are maximized in summer for middle latitude and auroral zone radars in the Northern Hemisphere, consistent with the Winter Anomaly (WA) phenomenon. The effect is most evident in the North American and Japanize sectors, and the quality of WA signatures deteriorates in the European and, especially, in the Australian sectors. The effect does not exist in the South American sector and in the polar caps of both hemispheres.

The Empirical-Canadian High Arctic Ionospheric Model (E-CHAIM) has been shown to reasonably reproduce important general features of the electron density distribution in the high-latitude ionosphere such as solar cycle and seasonal variations of the F layer peak and the location of the ionospheric trough. The utility of the model for practical applications, such as predictions of HF radio wave propagation, is less clear. In this study, we consider ground-scatter (GS) data for three SuperDARN radars at various latitudes to compare the observed skip distance against ray tracings based on E-CHAIM predictions. Monthly-averaged GS band locations in 2010-2018 are computed for every half-hour of the day, and are correlated with modelled skip distances. We show that the agreement is better during summer months and we discuss the quality of predictions at various latitudes.

The large scale morphology and finer sub-structure within a slowly propagating traveling ionospheric disturbance (TID) are studied using wide band trans-ionospheric radio observations with the LOw Frequency ARray (LOFAR: van Haarlem, et al., 2013). The observations were made under geomagnetically quiet conditions, between 0400-0800 UT on 7 January 2019, over the UK. In combination with ionograms and Global Navigation Satellite System (GNSS) Total Electron Content (TEC) anomaly data we estimate the TID velocity to ~65ms-1, in a NorthWesterly direction. Clearly defined substructures with oscillation periods of ~300 seconds were identified within the LOFAR observations of the TID, corresponding to scale sizes of ~ 20 km. At the geometries and observing wavelengths involved, the Fresnel scale is between 3-4 km, hence these substructures contribute significant refractive scattering to the received LOFAR signal. The refractive scattering is strongly coherent across the LOFAR bandwidth used here (25-64 MHz). The size of these structures distinguishes them from previously identified ionospheric scintillation with LOFAR in Fallows et al., 2020 where the scale sizes of the plasma structure varied from ~500 m – 5 km.

The Assimilative Canadian High Arctic Ionospheric Model (A-CHAIM) is an operational ionospheric data assimilation model that provides a 3D representation of the high latitude ionosphere in Near-Real-Time (NRT). A-CHAIM uses low-latency observations slant Total Electron Content (sTEC) from ground-based Global Navigation Satellite System (GNSS) receivers, ionosondes, and vertical TEC from the JASON-3 altimeter satellite to produce an updated electron density model above $45^o$ geomagnetic latitude. A-CHAIM is the first operational use of a particle filter data assimilation for space environment modeling, to account for the nonlinear nature of sTEC observations. The large number (>10^4) of simultaneous observations creates significant problems with particle weight degeneracy, which is addressed by combining measurements to form new composite observables. The performance of A-CHAIM is assessed by comparing the model outputs to unassimilated ionosonde observations, as well as to in-situ electron density observations from the SWARM and DMSP satellites. During moderately disturbed conditions from September 21st, 2021 through September 29th, 2021, A-CHAIM demonstrates a 40% to 50% reduction in error relative to the background model in the F2-layer critical frequency (foF2) at midlatitude and auroral reference stations, and little change at higher latitudes. The height of the F2-layer (hmF2) shows a small 5% to 15% improvement at all latitudes. In the topside, A-CHAIM demonstrates a 15% to 20% reduction in error for the Swarm satellites, and a 23% to 28% reduction in error for the DMSP satellites. The reduction in error is distributed evenly over the assimilation region, including in data-sparse regions.

Over-the-horizon radar (OTHR) systems operating in the high-frequency (HF) band (3-30MHz) are unique in their ability to detect targets at extreme ranges, offering cost-effective large area surveillance. Due to their reliance on the reflective nature of the ionosphere in this band, OTHR systems are extremely sensitive to ionospheric conditions and can expect significant variations in operational performance. At high latitudes, the presence of auroral enhancements in the E-Region electron density can substantially modify the coverage area and frequency management of OTHR systems. In this study, HF raytracing is utilized to investigate these impacts for a hypothetical radar under different auroral conditions simulated using the Empirical Canadian High Arctic Ionospheric Model (E-CHAIM). Aurora were seen to increase maximum useable frequency (MUF) from 8.5 MHz to 26 MHz whilst also reducing median available target range from 2541 km to 1226 km, for the greatest differences. Target interception showed large variations in path coverage of between 33-115% and 0-107% for two flight paths tested with precipitation toggled. Two distinct propagation modes were observed with aurora, noted as the F-E ducted and Auroral E-modes. Long-range coverage provided by the auroral F-E ducted mode was of limited capacity with low solar activity due to the reduced NmF2. F-mode propagation transitioned to the dominating Auroral E-mode between Auroral Electrojet (AE) index values of 50- and 200-nT. The significant variations in both frequency and coverage observed within this study highlight some aspects of the importance of considering aurora in OTHR modelling and design.

Here we assess to what extent the Empirical Canadian High Arctic Ionospheric Model (E-CHAIM) can reproduce the climatological variations of vertical Total Electron Content (vTEC) in the Canadian sector. Within the auroral oval and polar cap, E-CHAIM is found to exhibit Root Mean Square (RMS) errors in vTEC as low 0.4 TECU during solar minimum summer but as high as 5.0 TECU during solar maximum equinox conditions. These errors represent an improvement of up to 8.5 TECU over the errors of the International Reference Ionosphere (IRI) in the same region. At sub-auroral latitudes, E-CHAIM RMS errors range between 1.0 TECU and 7.4 TECU, with greatest errors during the equinoxes at high solar activity. This represents an up to 0.5 TECU improvement over the IRI during summer but worse performance by up to 2.4 TECU during the winter. Comparisons of E-CHAIM performance against in situ measurements from the European Space Agency’s Swarm mission are also conducted, ultimately finding behaviour consistent with that of vTEC. In contrast to the vTEC results, however, E-CHAIM and the IRI exhibit comparable performance at Swarm altitudes, except within the polar cap, where the IRI exhibits systematic underestimation of electron density by up to 1.0e11 e/m^3. Conjunctions with mid-latitude ionosondes demonstrate that E-CHAIM’s errors appear to result from compounding same-signed errors in its NmF2, hmF2, and topside thickness at these latitudes. Overall, E-CHAIM exhibits strong performance within the polar cap and auroral oval but performs comparably to the IRI at sub-auroral latitudes.