David R. Themens

A top to bottom evaluation of IRI 2007 within the polar cap

Monthly median values of ionospheric peak height (hmF2) and density (NmF2), derived from ionosonde measurements at four Canadian High Arctic Ionospheric Network (CHAIN) stations situated within the polar cap and Auroral Oval, are used to evaluate the performance of the International Reference Ionosphere (IRI) 2007 empirical ionospheric model during the recent solar minimum between 2008 and 2010. This analysis demonstrates notable differences between IRI and ionosonde NmF2 diurnal and seasonal behavior over the entire period studied, where good agreement is found during summer periods but otherwise errors in excess of 50% were prevalent, particularly during equinox periods. hmF2 is found to be marginally overestimated during winter and equinox nighttime, while also being underestimated during summer and equinox daytime by in excess of 25%. These errors are shown to be related to significant mismodeling of the M(3000)F2 propagation factor. The ionospheric bottomside thickness parameter (B0) is also evaluated using ionosonde measurements. It is found that both of the IRIs internal B0 models significantly misrepresent both seasonal and diurnal variations in bottomside thickness when compared to ionosonde observations, where errors at times exceed 40%. A comparison is also presented between IRI and Resolute (74.75N, 265.00E) Advanced Modular Incoherent Scatter Radar (AMISR)-derived topside thickness. It is found in this comparison that the IRI is capable of modeling ionospheric topside thickness exceptionally well during winter and summer periods but fails to represent significant diurnal variability during the equinoxes and seasonal variations.

Determining receiver biases in GPS-derived total electron content in the auroral oval and polar cap region using ionosonde measurements

Global Positioning System (GPS) total electron content (TEC) measurements, although highly precise, are often rendered inaccurate due to satellite and receiver differential code biases (DCBs). Calculated satellite DCB values are now available from a variety of sources, but receiver DCBs generally remain an undertaking of receiver operators and processing centers. A procedure for removing these receiver DCBs from GPS-derived ionospheric TEC at high latitudes, using Canadian Advanced Digital Ionosonde (CADI) measurements, is presented. Here, we will test the applicability of common numerical methods for estimating receiver DCBs in high-latitude regions and compare our CADI-calibrated GPS vertical TEC (vTEC) measurements to corresponding International GNSS Service IONEX-interpolated vTEC map data. We demonstrate that the bias values determined using the CADI method are largely independent of the topside model (exponential, Epstein, and α-Chapman) used. We further confirm our results via comparing bias-calibrated GPS vTEC with those derived from incoherent scatter radar (ISR) measurements. These CADI method results are found to be within 1.0 TEC units (TECU) of ISR measurements. The numerical methods tested demonstrate agreement varying from within 1.6 TECU to in excess of 6.0 TECU when compared to ISR measurements.

The nature of GPS differential receiver bias variability: An examination in the polar cap region

While modern GPS receiver differential code bias estimation techniques have become highly refined, they still demonstrate unphysical behavior, namely, notable solar cycle variability. This study investigates the nature of these seasonal and solar cycle bias variabilities in the polar cap region using single-station bias estimation methods. It is shown that the minimization of standard deviation bias estimation technique is linearly dependent on the users choice of shell height, where the sensitivity of this dependence varies significantly from 1 total electron content unit (1 TECU = 1016 el m−2) per 4000 km in solar minimum winter to in excess of 1 TECU per 90 km during solar maximum summer. Using an ionosonde, we find appreciable shell height variability resulting in bias variabilities of up to 2 TECU. Comparing northward face Resolute Incoherent Scatter Radar (RISR-N) measurements to a collocated GPS station, we find that RISR-derived GPS receiver biases vary seasonally but not with solar cycle. RMS differences between bias estimation methods and observation between 2009 and 2013 were found to range from 2.7 TECU to 3.4 TECU, depending on method. To account for the erroneous solar cycle variability of standard bias estimation approaches, we linearly fit these biases to sunspot number, removing the trend. RMS errors after sunspot detrending these biases are reduced to 1.91 TECU. Also, these ISR-derived and sunspot-detrended biases are fit to ambient temperature, where a significant correlation is found. By using these temperature-fitted biases we further reduce RMS errors to 1.66 TECU. These results can be taken as further evidence of temperature-dependent dispersion in the GPS cabling and antenna hardware.

Electron density and electric field over Resolute Bay and F region ionospheric echo detection with the Rankin Inlet and Inuvik SuperDARN radars

Joint observations of the Rankin Inlet and Inuvik Super Dual Auroral Radar Network HF radars and Resolute Bay (RB) Canadian Advanced Digital Ionosonde are used to assess the electron density at the F region peak and the electric field magnitude as factors affecting echo detection over RB. We demonstrate that the radars show similar diurnal and seasonal variations in ionospheric echo occurrence. During nighttime and at radar frequencies of ~12 MHz, optimum densities for both radars are shown to be ∼ 1.4 × 105 cm− 3, ~1.8 × 105 cm− 3, and ~2.0 × 105 cm− 3 for winter, equinox, and summer, respectively. During daytime, optimum densities are larger by (0.2 − 0.3) × 105 cm− 3. Observations at lower radar frequencies of ~10 MHz show smaller required densities during nighttime, by ~0.3 × 105 cm− 3. Optimum electric fields for the moments of echo detection over RB are found to be 5–25 mV/m with no clear threshold effect and any seasonal dependence. The presented data suggest that for echo detection, favorable propagation conditions along the entire path of radio waves toward the scattering volume are important.

Comparison of observed and predicted MUF(3000)F2 in the polar cap region

The maximum usable frequency for a 3000 km range circuit (MUF(3000)F2), computed from ionosonde measurements at Resolute (74.75∘N, 265.00∘E) and Pond Inlet (72.69∘N, 282.04∘E), has been compared with values obtained from the Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC), Voice of America Coverage Analysis Program (VOACAP), and International Telecommunication Union Recommendation 533 (REC533) models over a 4 year period. Predictions and observations show diurnal and seasonal variations; however, the VOACAP and ICEPAC models fail to reproduce the diurnal variation trend observed in the measurements during the summer period. The performance of these models has been statistically analysed: REC533 gives a better performance in winter and equinox months, while VOACAP has a better performance for both equinox and summer months. ICEPAC shows poor performance during low solar activity.

Seasonal and diurnal variations of PolarDARN F region echo occurrence in the polar cap and their causes

F region echo occurrence rates for the Polar Dual Auroral Radar Network (PolarDARN) HF radars at Inuvik (INV), Rankin Inlet (RKN), and Clyde River (CLY) are compared for observations in 2013. The CLY radar shows somewhat smaller echo occurrence rates consistent with its more poleward geographic and geomagnetic location. For all three radars, the winter occurrence rates are roughly twice that of the corresponding summer rates. For observations in the midnight sector, strong equinoctial maxima are evident. In terms of season and local time, echo occurrence patterns are found to be roughly the same for all radars: seasonally, clear maxima are found near noon during both winter and summer, while, diurnally, enhancements are found during equinoctial dusk. A comparison of data from roughly the same scattering area shows that having strong electron density in the scattering volume is not sufficient for getting an HF echo: propagation conditions along the propagation path are also important. Diurnal variations in the F region electron density and electric field (both measured by Canadian Advanced Digital Ionosonde) are compared to those of RKN echo occurrence rates for observations over Resolute Bay (RB) located at a geomagnetic latitude of 83°N. These results show a reasonable correlation between occurrence and electron density for both winter and summer periods and correlation between occurrence and electric field during summer periods.

The Empirical Canadian High Arctic Ionospheric Model (E‐CHAIM): NmF2 and hmF2

We present here the Empirical Canadian High Arctic Ionospheric Model (E-CHAIM) quiet NmF2, perturbation NmF2, and quiet hmF2 models. These models provide peak ionospheric characteristics for a domain above 50°N geomagnetic latitude. Model fitting is undertaken using all available ionosonde and radio occultation electron density data, constituting a dataset of over 28 million observations. A comprehensive validation of the model is undertaken and performance is compared to that of the International Reference Ionosphere (IRI). In the case of the quiet NmF2 model, the E-CHAIM model provides a systematic improvement over the IRI URSI maps. At all stations within the polar cap, we see drastic RMS error improvements over the IRI by up to 1.3MHz in critical frequency (up to 60% in NmF2). These improvements occur primarily during equinox periods and at low solar activities, decreasing somewhat as one tends to lower latitudes. Qualitatively, the E-CHAIM is capable of representing auroral enhancements in NmF2, as well as the location and extent of the Main Ionospheric Trough (MIT), not reproduced by the IRI. The included NmF2 storm model demonstrates improvements over the IRI by up to 35% and over the quiet-time E-CHAIM model by up to 30%. In terms of hmF2, over the validation periods used in this study, we found overall RMS errors of ~13km for E-CHAIM, with IRI2007 overall hmF2 errors ranging between 16km and 22km. The E-CHAIM performs comparably to or slightly better than the IRI within the polar cap; however, significant improvements are found within the auroral oval.

A Neural Network based foF2 model for a single station in the polar cap

A Neural Network (NN) model has been developed for the critical frequency of the F2 layer (foF2) at Resolute (74.70∘ N, 265.10∘ E) using data obtained from the Space Physics Interactive Data Resource, SPIDR (no longer available), for the period between 1975 - 1995. This model is a first step towards addressing the discrepancies of the International Reference Ionosphere (IRI) foF2 or peak electron density (NmF2) at high latitudes recently presented by Themens et al. [2014]. The performance of the NN model has been evaluated using foF2 data obtained from the Canadian Advanced Digital Ionosonde (CADI) at Resolute (74.75∘ N, 265.00∘ E) for the period between 2009 - 2013, in comparison with the IRI predictions. The 2012 version, and the International Union of Radio Science (URSI) option of IRI has been used. The NN nighttime monthly median foF2 variation demonstrates good agreement with observations compared to the IRI. The NN model is able to reproduce the enhancements in foF2 during the equinoxes, and also shows an improvement during disturbed days. Root Mean Square Errors (RMSE) were computed between hourly and monthly median model predictions and observations, and on the whole, the NN model seems to perform better during low solar activity and the equinoxes. The NN model shows an improvement in performance on average by 26.638% for hourly foF2 and 32.636% for monthly median foF2, compared to 7.877% obtained for the same station by the most recent NN model that attempted to predict foF2 at a polar cap station [Oyeyemi 24 and Poole, 2005].

Polar cap hot patches: Enhanced density structures different from the classical patches in the ionosphere

Based on in situ and ground-based observations, a new type of “polar cap hot patch” has been identified that is different from the classical polar cap enhanced density structure (cold patches). Comparing with the classical polar cap patches, which are transported from the dayside sunlit region with dense and cold plasma, the polar cap hot patches are associated with particle precipitations (therefore field-aligned currents), ion upflows, and flow shears. The hot patches may have the same order of density enhancement as classical patches in the topside ionosphere, suggesting that the hot patches may be produced by transported photoionization plasma into flow channels. Within the flow channels, the hot patches have low-energy particle precipitation and/or ion upflows associated with field-aligned currents and flow shears. Corresponding Global Navigation Satellite System (GNSS) signal scintillation measurements indicate that hot patches may produce slightly stronger radio signal scintillation in the polar cap region than classical patches.

Receiver bias estimation and validation of e-POP GAP-O ionospheric radio occultation measurements

This paper presents validation of ionospheric Global Positioning System (GPS) radio occultation (RO) measurements of the GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP-O). The primary source of uncertainty impacting GAP-O data products is the receiver differential code bias (rDCB). A minimization of standard deviations (MSD) technique for rDCB estimate has shown the most promise, and resulted in estimates ranging from −39 to −29 TECU, including a steady, long term decrease in rDCB magnitude. MSD estimates agree well with the “assumption of zero topside TEC” method at satellite apogee in the polar cap. Bias-corrected topside TEC of GAP-O was validated by statistical comparison with topside TEC obtained from ground-based GPS TEC and ionosonde measurements. GAP-O and ground-based topside TEC had similar variability, however GAP-O consistently underestimated the ground-derived topside TEC by up to 8 TECU. Ionospheric electron density profiles obtained from Abel inversion of GAP-O occultation TEC showed consistently good agreement with F-region densities of incoherent scatter radar measurements, however RO-derived E-region densities were not as reliable at high latitudes.