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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.

The distinctive features of the ion gyro‐harmonic discrete structures in the narrowband stimulated electromagnetic emission (NSEE) spectra (within ±1 kHz of the heater frequency), excited by an extraordinary (X‐mode) polarized high frequency (HF) pump wave in the F‐region of the high latitude ionosphere, are reported. Results were obtained from three sets of experiments at the EISCAT (European incoherent Scatter) HF Heating facility at pump frequencies of 5.423, 6.77 and 7.953 MHz, which are below the fourth, fifth, and sixth electron gyro‐harmonics. High power HF radio waves were radiated in the magnetic field‐aligned direction with an effective radiated power of 238–740 MW. Discrete spectral features, ordered by the ion gyro frequency (for O+ ions), were recorded at a large distance (1,200 km) away from the Heating facility. Up to 10 downshifted discrete ion gyro‐harmonic structures paired with the upshifted spectral components (Stokes and anti‐Stokes modes) in the NSEE spectra have been excited in the course of these experiments. It was found that preconditioning effects have a significant impact on the temporal evolution of the discrete spectral structures. The ion gyro‐harmonic structures coexisted with field‐aligned artificial irregularities (FAIs). Comparing the NSEE and EISCAT incoherent scatter radar observational results, it was concluded that the electron accelerations along and across the geomagnetic field coexist during X‐mode heating. The magnetized stimulated Brillouin scatter and stimulated ion Bernstein scatter are discussed as relevant processes for the observed multiple down‐ and upshifted ion gyro‐harmonic structures in the NSEE spectra. Plain Language Summary: Experiments in the Earth's ionosphere (part of the upper atmosphere) using high power high frequency (HF, or shortwave) radio waves may be used to study a wide variety of fundamental phenomena. We discuss recent advances in observations of stimulated radio emissions generated by a powerful HF radio transmitter at the European incoherent Scatter observatory in northern Norway. We found that the high power HF wave is able to generate radio emissions that can be observed more than one thousand kilometers away, in Saint Petersburg, Russia. We found and investigated distinctive frequencies of the radio emissions, and we believe, based on past results, that such emissions are not observable from locations close to the radio transmitter. The emissions we observed can be exploited for diagnostics of the Earth's environment, and they are similar to emissions seen in laser interactions with ionized gases such as occur in laser fusion experiments, and also in piezoelectric semiconductor devices, in fiber optics, and in many biological systems. This opens a window for utilizing our results to assist in these and other related areas. Key Points: We found a preconditioning effect on the temporal evolution of X‐mode ion gyro‐harmonic structures observed far away from the Heating facilityX‐mode ion gyro‐harmonic spectral structures and field‐aligned artificial irregularities coexist during high frequency (HF) pumping below electron gyro‐harmonicsElectron accelerations along and across the magnetic field coexist during X‐mode HF pumping at frequencies below electron gyro‐harmonics [ABSTRACT FROM AUTHOR]

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.

The solar cycle variation and seasonal changes significantly affects the ionization process of earth's ionosphere and required to be monitored in real time basis for regional level refinement of existing models. In view of this, the present study has been carried out by using the ionospheric Total Electron Content (TEC) data observed with the help of Global Ionospheric Scintillation and TEC monitoring (GISTM) system installed at Indian Antarctic Research Station, "Maitri" [70°46′00″S 11°43′56″E] during the ascending phase of 24th solar cycle. The daily values of solar extreme ultraviolet (EUV) flux (0.1–50 nm wavelength), 10.7 cm radio flux F 10.7 and Sunspot number (SSN) has been taken as a proxy to represent the solar cycle variation to correlate with TEC. The linear regression results revels better correlation of TEC with EUV flux rather than F 10.7 and SSN. Also, the EUV and TEC show better agreement during summer as compared to winter and equinox period. Correlation between TEC and EUV appears significantly noticeable during ten internationally defined quiet days of each month (stable background geophysical condition) as compared to the overall days (2010–2014). Further, saturation effect has been observed on TEC values during the solar maxima year 2014. The saturation effects are more prominent during the night hours of winter and equinox season due to transportation losses manifested by the equator-ward direction of meridional wind. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that energy can enter the high-latitude regions of the Ionosphere-Thermosphere (IT) system on open field lines. To assess the extent of high-latitude energy input, we have carried out a study of Poynting flux measured by the Defense Meteorological Satellite Program (DMSP) satellites during magnetic storms. We report sporadic intense Poynting fluxes measured by four DMSP satellites at polar latitudes during two moderate magnetic storms which occurred in August and September 2011. Comparisons with a widely used empirical model for energy input to the IT system show that the model does not adequately capture electromagnetic (EM) energy at very high latitudes during storms. We have extended this study to include more than 30 storm events and find that intense EM energy is frequently detected poleward of 75° magnetic latitude. [ABSTRACT FROM AUTHOR]

In this study, changes according to invariant magnetic latitude of the hourly foF2 data that are taken from seven ionosondes stations are situated at between 60°-90° invariant magnetic latitude as known high latitude region are investigated for different geomagnetic activity conditions during December 1971-September 1972. According to the results obtained from the analyses, it was found that the foF2 values in the daytime are larger than those in the nighttime and while the foF2 values in these hours increase between about 68o and 72o invariant magnetic latitude, the foF2 values in larger and lower latitudes than these latitudes decrease with increasing magnetic latitude. At night time, it was observed that the foF2 values increases between about 66o and 72o invariant magnetic latitude, while the foF2 values exhibit a trough structure at larger and smaller latitudes than these latitudes. It is also observed that seasonal and geomagnetic activity changes have a significant influence on the latitudinal changes of the foF2 values. The foF2 values in the geomagnetic quiet condition are larger than the foF2 values in the geomagnetic active conduction. Especially in the geomagnetic active conduction of the December, the polar cap and polar oval zones are more prominent and have a larger latitudinal structure. [ABSTRACT FROM AUTHOR]

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]

Abstract: A brief review is provided of recent progress in understanding the ionospheric Alfvén resonator (IAR) at high latitude. Firstly, naturally occurring resonances of the IAR as detected by pulsation magnetometers in the auroral zone at Sodankylä and in the polar cap at Barentsburg are considered. The characteristics of the IAR in the two regions are broadly similar, although the effects of solar illumination are less clear at the higher latitudes. Secondly we review recent attempts to stimulate the IAR through high-power radio frequency experiments both in the auroral zone at Tromsø with the European Incoherent SCATter (EISCAT) heater, and within the polar cap at Longyearbyen with the Space Plasma Exploration by Active Radar (SPEAR) facility. In the auroral zone at, Tromsø the stimulated IAR has been observed by ground-based magnetometers, and through electron acceleration observed on the FAST spacecraft. At SPEAR in the polar cap, the stimulated IAR has been investigated, with ground magnetometers, with the first results indicative of a positive detection. [Copyright &y& Elsevier]

Abstract: Early results are presented from high power ionospheric modification experiments with the SPEAR (Space Plasma Exploration by Active Radar) HF facility, which is used to generate artificial field-aligned irregularities (AFAI) in the polar cap ionosphere. These AFAIs are detected both through HF coherent backscatter radar and bistatic HF Doppler experiments. The AFAIs created with SPEAR are demonstrated to provide an important geophysical diagnostic of the polar cap ionospheric electric field, and may also scatter radio waves out of the ionospheric waveguide, even when they have propagated over very considerable distances. [Copyright &y& Elsevier]

Morphological features of the quiet/disturbed time variations in the Total Electron Content (TEC) at the polar cusp station Bharati (76.69°S MLAT) during a period of 5 years starting from February 2013 to December 2017 has been studied using GPS TEC measurements. The TEC at Bharati follows a diurnal pattern with its peak appearing close to local noon/magnetic noon during the summer/winter months. A nighttime enhancement in the TEC is seen around the magnetic midnight during winter. The plasma density at Bharati also exhibits semi-annual variation and a strong dependence on solar activity. A comparison of the IRI 2016 model derived TEC and the GPS TEC at Bharati shows significant differences with large underestimation of TEC especially during the nighttime period of the winter months. A two fold difference in magnitude between the GPS and modeled TEC is also observed in the summer months of the high solar activity period of 2013–2015. The response of the TEC to geomagnetic storms is found to depend on the onset time of the storm. We show that the morphological features in the temporal evolution of the plasma density at Bharati vary as the location of Bharati changes from being inside the polar cap, to the auroral region, and to the polar cusp in quick succession in a day. Our results highlight the fact that the dynamic nature of the location of Bharati with respect to the position of the polar cap plays an important role in deciding the plasma distribution at the polar cusp station. • TEC peaks at local noon/magnetic noon during summer/winter months at the polar cusp station Bharati. • Variation in the TEC at Bharti exhibits a semiannual anomaly. • The response of the TEC at Bharati to geomagnetic storms depends on the onset time of the storm. [ABSTRACT FROM AUTHOR]