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High latitude ionospheric density structures such as polar cap patches and arcs are capable of deflecting high frequency (HF) radio waves to off‐great circle paths, and are likely detrimental to technologies dependent on HF radio propagation. In this study, nearly 2.5 years of 4.6–14.4 MHz data from a multi‐frequency HF radio link between Qaanaaq, Greenland and Alert, Canada are used to investigate high‐latitude off‐great circle propagation in the polar cap. After an example of HF radio propagation affected by polar cap patches is shown in detail, a statistical analysis of the occurrence and impacts of off‐great circle deflections in the polar cap is presented. Off‐great circle propagation is shown to be increasingly common with increasing frequency up to 11.1 MHz, such that averaged over 1 year, received 11.1 MHz signals experienced deflections >30° from the great circle direction 65.6% of the time. The occurrence of these deflections across the year is shown to be at a maximum in the winter, while occurrence across the day varies with season. Trends across both time of day and time of year for 11.1 and 14.4 MHz deflections are consistent with polar cap patch occurrence trends. Off‐great circle deflections are shown to be associated with increased time‐of‐flights, a larger range of positive and negative Doppler shifts, increased Doppler spreads, and lower signal‐to‐noise ratios. These results are discussed in the context of ionospheric phenomena in the polar cap, and implications for over‐the‐horizon radars operating at high latitudes. Plain Language Summary: High frequency (HF) radio waves can travel long distances by bouncing between the ground and the ionosphere; an upper layer of the Earth's atmosphere, allowing them to be used for communications and surveillance in remote locations. However, strong space weather can change the paths these radio waves take, causing problems for HF radio technologies. In this paper, radio transmissions sent between Qaanaaq, Greenland and Alert, Canada are studied in order to better understand how often HF radio waves are deflected by space weather at high latitudes, and what effect these deflections have on the signals themselves. After a brief example of what space weather deflections look like, transmissions sent over a whole year are studied to determine how deflections depend on radio frequency, time of day, and time of year. Then, the effects these deflections have on other signal properties is studied. It is found that deflections are very common at high latitudes, especially in the winter. Deflections tend to increase the travel time of signals, change their frequency, and reduce their signal strength. These results are shown to be connected to polar cap patches and arcs; which are common space weather phenomena near the poles. Key Points: Off‐great circle propagation occurring in the polar cap is studied using years of data from an HF radio link in the polar capLarge (greater than 30°) deflections from the great circle are shown to be very common, especially in the winterThese deflections are shown to affect HF radio signals in ways that are consistent with interactions with polar cap patches and arcs [ABSTRACT FROM AUTHOR]

In this study, the features of ionospheric total electron content (TEC) over four stations in the northern and southern hemispheres during the geomagnetic storm on November 4, 2021, were studied. This study was based on GPS-TEC data obtained from University NAVSTAR Consortium dual-frequency GPS devices over Tromso, Nyal, Riga, and Svetloe in the northern stations and Majak, Steward, Wilson, and Woollard in the southern stations. The impact of solar activity on the ionospheric TEC was analysed using the continuous wavelet transform and power spectral density analysis and found to have strong coherence. The results indicate that southern hemisphere stations have higher values of changes in TEC and power spectrum change than northern hemisphere stations. Also, a highly negative impact on TEC is measured predominantly at southern stations compared with northern stations. Interestingly, the midnight TEC became higher and more comparable to the daytime TEC over southern stations than over northern stations. This might be due to the ingestion of additional plasma from higher latitudes at the southern stations. However, the values of TEC were higher during the local daytime at the northern stations. On November 4, 2021, when the IMF Bz was moving southward and sharply incremental Kp values and plasma speed were observed, the enhancement of dTEC and TEC uncertainty were more pronounced at southern stations than at northern stations. In addition, during the main phase of the storm, the spatial gradient enhancement or depletion of TEC over two stations in the northern hemisphere was larger than that of the two stations in the southern hemisphere around the day-side hours. [ABSTRACT FROM AUTHOR]

Rapid changes in solar wind dynamic pressure can produce a transient geomagnetic response in the high latitude ionosphere. In this study we carry out a superposed epoch analysis of the geomagnetic response based on 2,058 events. The events are divided into 12 groups based on interplanetary magnetic field clock angle and dipole tilt and the magnetic perturbation field is modeled using spherical harmonics. We find that the high latitude transient current vortices associated with a sudden commencement are most clearly observed when the interplanetary magnetic field is northward during equinox and winter in the northern hemisphere. The high latitude geomagnetic response during northward interplanetary magnetic field is decomposed into a preliminary and main impulse. The preliminary impulse onset is 1–2 min prior to the onset of the low/mid latitude geomagnetic response and its rise time is 4–6 min. The main impulse onset is around 2 min after the low/mid latitude geomagnetic response and has a rise time of 6–11 min. When examining the change relative to pre-onset conditions a coherent transient geomagnetic response emerges for all IMF clock and dipole tilt angles. The current vortex associated with the main impulse on the dawnside appears at (9.3 ± 0.5 mlt, 64.8° ± 1.5° mlat) and moves westward with a velocity of 5 ± 1.4 km/s. The vortex on the duskside appears at (15.3 ± 0.9 mlt, 65.8° ± 2.5° mlat) and does not move significantly. In addition, the models were used to recreate the SMR index showing a significant mlt dependence on the magnetic perturbation above 40° mlat and below 10° mlat. The former is thought to be caused by high latitude ionospheric currents. The latter is potentially a combination of the event occurrence probability being skewed toward certain UT ranges for large dipole tilt angles and a UT dependence of the equatorial electrojet magnitude caused by the south atlantic magnetic anomaly.

We report experimental results from multi-instrument diagnostic tools related to phenomena in the high latitude ionosphere F-region, induced by the simultaneous action of powerful extraordinary (X-mode) and ordinary (O-mode) polarized HF radio waves at EISCAT (European Incoherent SCATter) during magnetically quiet background geophysical conditions. HF pump waves were injected towards the magnetic zenith at a frequency of 5.423 MHz, which was just below the critical frequency of the F2 layer when the excitation of X- and O-mode phenomena was possible. In the course of the experiment, the effective radiated power (ERP) of the X-mode wave was 130.5 MW, while the portion of the O-mode ERP was 29.9 MW (about 20% of the full ERP). It was found that simultaneous action of the X- and O-mode pump waves lead to the excitation of effects typical for both X- and O-mode HF pump waves. Namely, the excitation of small-scale field-aligned irregularities due to the thermal parametric (resonance) instability and a downshifted maximum (DM) component in the wideband stimulated electromagnetic emission (WSEE) spectra, typical for the O-mode HF pumping, was observed, together with multiple ion gyro-harmonic structures in the narrowband stimulated electromagnetic emission (NSEE) spectra. These latter results were recorded at a large (more than 1000 km) distance from the EISCAT/heating facility, typical only for an X-mode HF pumping. The results obtained are compared with “pure” O-mode heating and discussed in detail.

We report experimental results from multi-instrument diagnostic tools related to phenomena in the high latitude ionosphere F-region, induced by the simultaneous action of powerful extraordinary (X-mode) and ordinary (O-mode) polarized HF radio waves at EISCAT (European Incoherent SCATter) during magnetically quiet background geophysical conditions. HF pump waves were injected towards the magnetic zenith at a frequency of 5.423 MHz, which was just below the critical frequency of the F2 layer when the excitation of X- and O-mode phenomena was possible. In the course of the experiment, the effective radiated power (ERP) of the X-mode wave was 130.5 MW, while the portion of the O-mode ERP was 29.9 MW (about 20% of the full ERP). It was found that simultaneous action of the X- and O-mode pump waves lead to the excitation of effects typical for both X- and O-mode HF pump waves. Namely, the excitation of small-scale field-aligned irregularities due to the thermal parametric (resonance) instability and a downshifted maximum (DM) component in the wideband stimulated electromagnetic emission (WSEE) spectra, typical for the O-mode HF pumping, was observed, together with multiple ion gyro-harmonic structures in the narrowband stimulated electromagnetic emission (NSEE) spectra. These latter results were recorded at a large (more than 1000 km) distance from the EISCAT/heating facility, typical only for an X-mode HF pumping. The results obtained are compared with "pure" O-mode heating and discussed in detail. [ABSTRACT FROM AUTHOR]