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Equatorial plasma irregularities in the ionospheric F‐region proliferate after sunset, causing the most apparent radio scintillation “hot‐spot” in geospace. These irregularities are caused by plasma instabilities, and appear mostly in the form of under‐densities that rise up from the F‐region's bottomside. After an irregularity production peak at sunset, the amplitude of the resulting turbulence decays with time. Analyzing a large database of irregularity spectra observed by one of the European Space Agency's Swarm satellites, we have applied a novel but conceptually simple statistical analysis to the data, finding that turbulence in the F‐region tends to decay with a uniform, scale‐independent rate, thereby confirming and extending the results from an earlier case study. We find evidence for two regimes, one valid post‐sunset (1.4 hr decay rate) and one valid post‐midnight (2.6 hr). Our results should be of utility for large‐scale space weather modeling efforts that are unable to resolve turbulent effects. Plain Language Summary After sunset in the equatorial region of Earth, GPS devices frequently experience service interruption due to space weather. The signal disruptions that cause these interruptions are in turn caused by plasma turbulence in Earth’s ionosphere, a layer of ionized gas that covers Earth like a blanket of electrical currents. The growth of such turbulence has been studied for decades, but little is still known about how such plasma irregularity structures decay with time. We elucidate the topic, showing that turbulence cause structures to decay at the same rate regardless of size. This important result will have consequences for large‐scale space weather modeling efforts, since such models rarely have the capability to resolve turbulence. Turbulence is an enigmatic chaotic behavior often that is often present in astrophysical processes, but also on Earth’s oceans and in its atmosphere. Key Points Turbulence forces equatorial irregularities to decay with a scale‐independent rate Equatorial irregularities of scale‐sizes between 500 m and 75 km are not dissipating by chemical recombination or perfect ambipolar diffusion Decay rates depend on solar local time, with post‐sunset decay rates around 1.4 hr, increasing to 2.6 hr post‐midnight

High-latitude irregularities can impair the operation of GPS-based devices by causing fluctuations of GPS signal amplitude and phase, also known as scintillation. Severe scintillation events lead to losses of phase lock, which result in cycle slips. We have used data from the Canadian High Arctic Ionospheric Network (CHAIN) to measure amplitude and phase scintillation from L1 GPS signals and total electron content (TEC) from L1 and L2 GPS signals to study the relative role that various high-latitude irregularity generation mechanisms have in producing scintillation. In the first year of operation during the current solar minimum the amplitude scintillation has remained very low but events of strong phase scintillation have been observed. We have found, as expected, that auroral arc and substorm intensifications as well as cusp region dynamics are strong sources of phase scintillation and potential cycle slips. In addition, we have found clear seasonal and universal time dependencies of TEC and phase scintillation over the polar cap region. A comparison with radio instruments from the Canadian GeoSpace Monitoring (CGSM) network strongly suggests that the polar cap scintillation and TEC variations are associated with polar cap patches which we therefore infer to be main contributors to scintillation-causing irregularities in the polar cap.

The January 15, 2010, solar annular eclipse crossed the magnetic equator in the middle of the day over India, in a region instrumented with several magnetometers, Total Electron Content stations using GPS data, and an ionosonde located very near the center of the eclipse. With the help of a one‐dimensional model appropriate for the region of interest we show that the ionosonde data was consistent with a lower F region plasma that was moving upwards with only modest velocities in the morning hours and moving resolutely downwards in the afternoon hours. This motion agreed well with the local magnetometer data which revealed a weakened electrojet taking place in the morning hours while a full‐blown counter‐electrojet was present in the afternoon hours. We show that the unusual solar eclipse‐induced electrodynamics resulted in a reduction in the Total Electron Content depletion not just at the magnetic equator but also, more markedly, in the Equatorial Ionization Anomaly (EIA) zone, a further 10 degrees to the north. This latter point clearly shows that the eclipse led to a cut‐off in the supply of plasma provided through the equatorial fountain, by altering a fundamental aspect of the equatorial electrodynamics. Key Points Depletion in plasma density at the dip equator in response to a solar eclipse Solar eclipse triggered a counter electrojet like condition near dip equator Zonal electric field is being surmised to reverse as a function of height

The path of maximum obscuration for the annular solar eclipse of January 15, 2010, crossed the magnetic equator at Trivandrum, India, in the early afternoon hours. A strong counter‐electrojet was observed shortly after maximum obscuration. Moreover, as the eclipse passed overhead, the F region density peak underwent a large amplitude vertical oscillation. At the same moment, there was an oscillation in the zonal electric field inferred from the magnetometer data. The electric field turned westward after the time of maximum obscurity, reaching its largest westward value one hour before the end of the local eclipse. We show that these data are consistent with a fast eastward moving local neutral wind dynamo generated by a low pressure system postulated to have been triggered by the cold temperatures in the region of maximum obscuration.

Coordinated observations of ionospheric variability near the geomagnetic pole using the Resolute Bay Incoherent Scatter Radar (RISR‐N), Super Dual Auroral Radar Network (SuperDARN) High Frequency (HF) radars, and all‐sky imagers have clarified the relative contribution of structuring mechanisms operating on polar plasma patches. From the multipoint RISR‐N observations, a three dimensional image can be constructed of the plasma parameters. The colocated coherent echoes from the SuperDARN radars provide information on field aligned irregularities, and from all‐sky imagers located in Resolute Bay, Canada and Qaanaaq, Greenland, information is obtained on the emission brightness at different wavelengths. A good correlation is found between the location of the coherent, incoherent and optical signals of patches. From the SuperDARN radar data it is evident that plasma irregularities seem to be present throughout the region of enhanced electron density. The patches are observed to be formed in the cusp region due to bursty flux transfer events and are then transported across the polar cap. During the time period of about 10 minutes when a patch drifted through the RISR‐N field of view, the patch seemed to undergo significant deformation in all three spatial dimensions, with density fluctuations of about 10% and spatial variations leading to stretching and tilting of the patch. The findings show that plasma structuring can likely occur within polar cap patches, which support previous suggestions that a patch is highly variable as it drifts across the polar cap, with a faster spread of irregularities throughout the patch as a result. Key Points HF, ISR, and optics data are compared to study plasma patch dynamics Field‐aligned irregularities are found to be distributed throughout the patch Variations within the patch indicate ongoing redistribution of plasma

High frequency (HF, 10–20 MHz) radars routinely utilize Bragg scatter from plasma fluctuations to monitor ionospheric turbulence. While the propagation of the probing HF radio waves is also strongly affected by the regular structure of the ionosphere, this information is rarely extracted from the data. In the present work we present and test a new technique for estimating the F‐region peak electron densities, NmF2, using information about the vertical refraction of HF backscatter echoes that is readily available in the Super Dual Auroral Radar Network (SuperDARN) data set. Key Points A technique allows to extract new ionospheric information from existing data

In addition to being scattered by the ionospheric field‐aligned irregularities, HF radar signals can be reflected by the ionosphere toward the Earth and then scattered back to the radar by the rugged ground surface. These ground scatter (GS) echoes are responsible for a substantial part of the returns observed by HF radars making up the Super Dual Auroral Radar Network (SuperDARN). While a GS component is conventionally used in studying ionosphere dynamics (e.g., traveling ionospheric disturbances, ULF waves), its potential in monitoring the state of the scattering surface remains largely unexploited. To fill this gap, we investigated diurnal and seasonal variation of the ground echo occurrence and location from a poleward‐looking SuperDARN radar at Rankin Inlet, Canada. Using colocated ionosonde information, we have shown that seasonal and diurnal changes in the high‐latitude ionosphere periodically modulate the overall echo occurrence rate and spatial coverage. In addition, characteristics of GS from a particular geographic location are strongly affected by the state of the underlying ground surface. We have shown that (1) ice sheets rarely produce detectable backscatter, (2) mountain ranges are the major source of GS as they can produce echoes at all seasons of the year, and (3) sea surface becomes a significant source of GS once the Arctic sea ice has melted away. Finally, we discuss how the obtained results can expand SuperDARN abilities in monitoring both the ionosphere and ground surface.

This paper shows that, contrary to previous explanations, the apparent undulating motion of the equatorial F region peak at sunrise is produced by photochemistry rather than dynamics. Our study is based on an investigation of the behavior of the early morning ionosphere observed by a Digital Ionosonde at Trivandrum, India. The phenomenon is rooted in the production of new plasma at the upper F region altitudes soon after sunrise. As the peak photoproduction rate moves down in altitude and increases in magnitude the newly formed plasma follows a similar trend. Once the density becomes large enough to be detected by an ionosonde, a jump is observed in the F region peak altitude. The jump is followed by a quick downward motion of the increasingly strong F peak. Chemistry causes the downward motion of the F peak to end near 250 km. Electrodynamics is not responsible for the sunrise undulation, but plays an indirect role in the detection of the sunrise effect by simultaneously lowering during the night the peak height and decreasing the density. When detectable, the remnant plasma introduces a lower peak height that facilitates the observation of the initial increase in peak height, while the lower background density allows the relatively small initial density increase from photoionization to be observed. Key Points The F peak at the dip equator undulates rapidly at sunrise The effect looks like the sunset PRE but is driven by different processes Electrodynamics matters for the removal of plasma from the previous day

In past calculations of convective velocities from Super Dual Auroral Radar Network (SuperDARN) HF radar observations, the refractive index in the scattering region has not been taken into account, and therefore the inferred ionospheric velocities may be underestimated. In light of the significant contribution by SuperDARN to ionospheric and magnetospheric research, it is important to refine the velocity determination. The refractive index in the ionosphere at SuperDARN observation F region altitudes has typical values between 0.8 and close to unity. In the scattering region, where conditions are more extreme, the index of refraction may be much lower. A simple application of Snell's law in spherical coordinates (Bouguer's law) suggests that a proxy for the index of refraction at the scattering location can be determined by measuring the elevation angle of the returned ionospheric radar signal. Using this approximation for refractive index, the Doppler velocity calculation can be refined for each SuperDARN ionospheric echo, using the elevation angles obtained from the SuperDARN interferometer data. A velocity comparison of DMSP and SuperDARN observations has revealed that the SuperDARN speeds were systematically lower than the DMSP speeds. A linear regression analysis of the velocity comparisons found a best fit slope of 0.74. When the elevation angle data were used to estimate refractive index, the best fit slope rose 12% to 0.83. As most SuperDARN radars employ an interferometer antenna array for elevation angle measurements, the improvement in velocity estimates can be done routinely using the method outlined in this paper.

Simultaneous two‐dimensional observations of airglow enhancement and radar backscatter from field‐aligned irregularities (FAIs) associated with polar cap patches were conducted. The spatial structure of 630 nm airglow from polar cap patches was imaged using an all‐sky airglow imager at Resolute Bay, Canada, while backscatter echoes from decameter‐scale FAIs were observed using the newly constructed HF Polar Dual Auroral Radar Network (PolarDARN) radar at Rankin Inlet, Canada. Both the airglow enhancement and the radar backscatter appeared within a structured region with the spatial extent of about 500–1000 km. The decameter‐scale FAIs were found to extend over the entire region of airglow enhancement associated with polar cap patches, indicating that the polar patch plasma became almost fully structured soon after initiation (within approximately 20–25 min). These findings imply that some rapid structuring process of the entire patch area is involved in addition to the primary gradient‐drift instabilities.