Search

Using a large data set of ground‐based GNSS scintillation observations coupled with in situ particle detector data, we perform a statistical analysis of both the input energy flux from precipitating particles, and the observed occurrence of density irregularities in the northern hemisphere cusp. By examining trends in the two data sets relating to geomagnetic activity, we conclude that observations of irregularities in the cusp grows increasingly likely during storm‐time, whereas the precipitating particle energy flux does not. We thus find a weak or nonexistent statistical link between geomagnetic activity and precipitating particle energy flux in the cusp. This is a result of a previously documented tendency for the cusp energy flux to maximize during northward IMF, when density irregularities tend not to be widespread, as we demonstrate. At any rate, even though ionization and subsequent density gradients directly caused by soft electron precipitation in the cusp are not to be ignored for the trigger of irregularities, our results point to the need to scrutinize additional physical processes for the creation of irregularities causing scintillations in and around the cusp. While numerous phenomena known to cause density irregularities have been identified and described, there is a need for a systematic evaluation of the conditions under which the various destabilizing mechanisms become important and how they sculpt the observed ionospheric "irregularity landscape." As such, we call for a quantitative assessment of the role of particle precipitation in the cusp, given that other factors contribute to the production of irregularities in a major way. Plain Language Summary: The cusp ionosphere is a particularly turbulent part of the interaction between Earth and the solar wind. Here, magnetic field lines connect the ionosphere directly to the solar wind, and a range of plasma physical mechanisms can studied. "Soft" (low‐energy) electrons raining into Earth's atmosphere inside the cusp has been suggested several times in the past as an important contributor to the various energetic phenomena found in the cusp, but little evidence exists that can tell us how important they are. We present a large statistical study of the cusp's input energy (the soft electrons) and how it affects the cusp's turbulent output (widespread plasma irregularities). We find a discrepancy in that soft electrons largely exhibit opposite long‐term trends compared to the cusp plasma turbulence. Put simplistically, the energy budget provided by soft electrons might not be sufficient to explain the enormous energy that are at times pent up in the ionospheric cusp plasma turbulence. These results point to the fact that the community needs to address the role of particle precipitation (electrons, but also ions) in producing the observed ionosphere "irregularity landscape." Key Points: Cusp scintillation occurrence increases significantly with rising geomagnetic activityThe energy flux of soft electron precipitation in the cusp has a statistical tendency to decrease as geomagnetic activity increasesThe increase in cusp scintillation with geomagnetic activity must be caused by drivers other that soft electron precipitation [ABSTRACT FROM AUTHOR]

This paper uses a regional simulation of plasma convective instability in the postsunset equatorial ionosphere together with a global atmosphere/ionosphere/plasmasphere GCM (WAM‐IPE) to forecast irregularities associated with equatorial spread F (ESF) for 1–2 hr after sunset. First, the regional simulation is initialized and forced using ionosphere state parameters derived from campaign data from the Jicamarca Radio Observatory and from empirical models. The irregularities produced by these simulations are found to be quantitatively similar to those observed. Next, the aforementioned state parameters are replaced with parameters from WAM‐IPE, and the resulting departures between the simulated and observed irregularities are noted. In one of five cases, the forecast failed to accurately predict ESF irregularities due to the late reversal of the zonal thermospheric winds. In four of five cases, significant differences between the observed and predicted prereversal enhancement (PRE) of the background vertical drifts resulted in degraded forecast accuracy. This highlights the need for improved PRE forecasting in the global‐scale model. Key Points: Equatorial ionospheric F‐region stability can be reproduced quantitatively using a regional data‐driven direct numerical simulation (DNS)DNS model runs become forecasts when initialized and driven with parameters from a comprehensive GCM such as Whole Atmosphere Model with Ionosphere, Plasmasphere, ElectrodynamicThe post‐sunset background vertical plasma drift and zonal thermospheric wind profile are the most critical forecast parameters [ABSTRACT FROM AUTHOR]

Ionospheric plasma irregularities can be successfully studied with the Swarm satellites. Parameters derived from the in‐situ plasma measurements and from the topside ionosphere total electron content provide a comprehensive dataset for characterizing plasma structuring along the orbits of the Swarm satellites. The Ionospheric Plasma IRregularities (IPIR) data product summarizes these parameters and allows for systematic studies of ionospheric irregularities. IPIR has already been used in investigations of structuring and variability of ionospheric plasma. This report provides a detailed description of algorithms behind the IPIR data product and demonstrates its use for ionospheric studies. Key Points: Ionospheric Plasma IRregularities (IPIR) is a new Swarm data product that characterizes ionospheric plasma irregularitiesPlasma irregularity parameters are assigned to different geomagnetic regionsIPIR allows for both detailed case studies and global statistical studies of ionospheric plasma variability [ABSTRACT FROM AUTHOR]

Large amplitude plasma density irregularities have occasionally been detected at night in the midlatitude F region during geomagnetic storms. They are often interpreted in terms of equatorial plasma bubbles (EPBs) because midlatitude irregularities have the morphology of EPBs. This study assesses whether morphology can be a determining factor in ascribing the origin of such midlatitude ionospheric irregularities. We address this question by analyzing the observations of the First Republic of China satellite (ROCSAT‐1) and Defense Meteorological Satellite Program (DMSP)‐F14 and ‐F15 satellites during the geomagnetic storms on 12 February 2000 and 29 October 2003. On both days, ROCSAT‐1 detects plasma depletions at midlatitudes in broad longitude regions and DMSP satellites detect isolated severe plasma depletions whose widths and depths are much wider and deeper than those of typical EPBs. The distinguishing characteristics during the storms are the detection of midlatitude depletions only in the Southern Hemisphere and the occurrence of some of these depletions before 19 hr local time and at the longitudes where EPBs are absent in the equatorial region. These characteristics are not explained satisfactorily by the characteristics of EPBs. Considering the detection of some of the midlatitude depletions at the equatorward edge of ionospheric perturbations in midlatitudes, midlatitude depletions are likely ionospheric perturbations that originated from higher latitudes. Because midlatitude depletions can originate from different sources, the morphology alone is not a determining factor of their origin. Key Points: Large amplitude plasma density irregularities that resemble equatorial plasma bubbles (EPBs) develop at midlatitudes during geomagnetic stormsConnection of midlatitude plasma depletions to EPBs is assessed by examining their occurrence times and distributionBubble‐like plasma density irregularities develop in midlatitudes by sources other than EPBs [ABSTRACT FROM AUTHOR]

We present an investigation of polar cap plasma structure lifetimes. We analyze both simulated data from ionospheric models (International Reference Ionosphere model and Mass Spectrometer Incoherent Scatter model) and in situ data from the Swarm satellite mission (the 16 Hz Advanced Plasma Density dataset). We find that the theoretical prediction that E‐region conductance is a predictor of F‐region polar cap plasma structure lifetimes is indeed supported by both in situ‐based observations and by ionospheric models. In situ plasma structure lifetimes correlate well with the ratio of F‐ to E‐region conductance. We present explicit predictions of small scale (∼1 km) structure lifetimes, which range from less than 1 h during local summer to around 3 h during local winter. We highlight a large discrepancy between the observational and theoretical scale‐dependency of decay due to diffusion.Key Points: The ratio of E‐region to F‐region conductance is an accurate predictor of F‐region polar cap plasma diffusionThe observed scale‐dependency of the structure lifetimes deviates strongly from the theoretical predictionsF‐region small‐scale plasma structure lifetimes in the central polar caps range from 1 to 3 h [ABSTRACT FROM AUTHOR]

Type‐A (weaker and wider Doppler spectra) and Type‐B (stronger and narrower Doppler spectra) echoes are inherent to the puzzling 150‐km echoing phenomenon. In this paper, we investigate the characteristics and possible origin of the Type‐A and Type‐B 150‐km echoes using high‐resolution 53‐MHz radar and Digisonde observations from Gadanki. Results show that the Type‐A echoes dominate the echoing phenomenon and they always precede the Type‐B echoes. Intriguingly, the Type‐B echoes are fundamentally not narrower than the Type‐A echoes as was originally thought. The primary reason that distinguishes the Type‐B echoes from the rest is the confinement of a few high PSD values at the center of the spectra, resulting in narrow spectral width. Spectral spreads of both types of echoes are found to increase with echo signal‐to‐noise ratio (SNR), and in fact, the spectral spreads of the Type‐B echoes are higher than those of the Type‐A echoes. Observations also show spectral evolution displaying the occurrence of Type‐A and Type‐B echoes in tune with the quasiperiodicity in the echo strength (SNR). We propose that both types of echoes primarily are of common origin, presumably linked with naturally enhanced incoherent scattering. Looking at the simultaneously observed short period variations in the lower F region electron density, we propose a gravity wave induced pathway as a plausible mechanism accounting for echo intensity modulation as well as spectral transition. Key Points: Details of Type‐A and Type‐B 150‐km echoes including the spectral evolution are presentedType‐B echoes differ from Type‐A echoes in terms of high PSDs at the center of the spectraQuasi‐periodic variations in 150‐km echoes and F region electron density indicate the role of gravity waves [ABSTRACT FROM AUTHOR]

The formation of polar cap density enhancements, such as tongues‐of‐ionization (TOIs), are often attributed to enhanced dayside reconnection and convection due to solar wind changes. However, ionospheric poleward moving density enhancements can also form in the absence of changes in the solar wind. This study examines how TOI and patch events that are not triggered by solar wind changes relate to magnetospheric processes, specifically substorms. Based on total electron content and Super Dual Auroral Radar Network (SuperDARN) observations, we find substorms that occur at the same time as TOIs are associated with sudden enhancements in dayside poleward flows during the substorm expansion phase. Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations also show enhanced field‐aligned currents (FACs) that extend into the dayside ionosphere during this period. We suggest that the global enhancement of FACs and convection during these substorms are the drivers of these TOIs by enhancing dayside convection and transporting high‐density lower‐latitude plasma into the polar cap. However, we also find that not all substorms are coincident with polar cap density enhancements. A superposed epoch study showed that the AL index for TOIs during substorms is not particularly stronger than substorms without TOIs, but epoch studies of AMPERE observations do show events with TOIs to have a higher total FAC on both the dayside and nightside. Our results show the importance of TOI formation during substorms when solar wind drivers are absent, and the importance of considering substorms in the global current system. This work also shows the need to incorporate substorms into models of high‐latitude global convection and currents. Key Points: Enhanced dayside flows without interplanetary magnetic field changes can occur during substormsDayside plasma flows during substorms can transport photoionized plasma into the polar capSubstorms impact dayside currents and patches despite being typically associated with the nightside [ABSTRACT FROM AUTHOR]

The high‐latitude ionosphere is highly dynamical with significant irregularities and density gradients. However, the spatial and temporal distributions of density gradients and irregularities are very different between the Arctic and Antarctic. In this report, we study the interhemispheric asymmetry of the large‐scale (100 km) density gradients in both polar caps. Our results show that density gradients in the Arctic are enhanced during local winter (December solstice) with a peak around 19 UT. The UT and spatial distributions in the Antarctic local winter (June solstice) are similar to the Arctic except that they are reversed by 12 hr, which indicates a mirror symmetry between hemispheres. The 12‐hr difference in the peak density gradients can be explained by the displacements between the geographic and geomagnetic poles. The only asymmetry (anomaly) is the persistence of strong density gradients in the southern polar cap during local summer (December solstice). Plain Language Summary: The high‐latitude ionosphere is highly variable with significant fluctuations in plasma density. So far, the long‐term studies have been conducted in the Arctic due to better ground‐based data coverage, while the knowledge of the Antarctic is limited. We take advantage of the Swarm mission to study the long‐term behaviors in both the Arctic and Antarctic. New statistics reveal phenomena that we understand and an "anomaly" that do not understand. The results during local winter (December in the Arctic and June in the Antarctic) are similar and can be explained by the previous knowledge. However, the anomaly is the persistence of strong density fluctuations in the Antarctic during local summer (December solstice). Key Points: Density gradients (100 km) in the Arctic are enhanced from autumn to spring equinoxes with a significant "hole" during 3–12 UT near December solsticeThe variations of density gradients are mirror symmetric for two hemispheres during local winterDensity gradients in the Antarctic are strong and persistent during local summer (December solstice) [ABSTRACT FROM AUTHOR]

Most of the low‐latitude ionospheric radar observations in South America come from the Jicamarca Radio Observatory, located in the western longitude sector (∼75°W). The deployment of the 30 MHz FAPESP‐Clemson‐INPE (FCI) coherent backscatter radar in the magnetic equatorial site of São Luis, Brazil, in 2001 allowed observations to be made in the eastern sector (∼45°W). However, despite being operational for several years (2001–2012), FCI only made observations during daytime and pre‐midnight hours, with a few exceptions. Here, we describe an upgraded system that replaced the FCI radar and present results of full‐night F‐region observations. This radar is referred to as Measurements of Equatorial and Low‐latitude Ionospheric irregularities over São Luís, South America (MELISSA), and made observations between March 2014 and December 2018. We present results of our analyses of pre‐ and post‐midnight F‐region echoes with focus on the spectral features of post‐midnight echoes and how they compare to spectra of echoes observed in the post‐sunset sector. The radar observations indicate that post‐midnight F‐region irregularities were generated locally and were not a result of "fossil" structures generated much earlier in time (in other longitude sectors) and that drifted into the radar field‐of‐view. This also includes cases where the echoes are weak and that would be associated with decaying equatorial spread F (ESF) structures. Collocated digisonde observations show modest but noticeable F‐region apparent uplifts prior to post‐midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizing F‐region conditions leading to ESF development. Key Points: The MELISSA 30 MHz radar system is described and examples of F‐region observations are introducedResults of the spectral analysis of pre‐ and post‐midnight F‐region echoes are presented and comparedResults indicate the local development of F‐region irregularities in the post‐midnight sector [ABSTRACT FROM AUTHOR]

The uplooking total electron contents (TECs) from the GRACE, SWARM‐A, TerraSAR‐X, and MetOp‐A satellites and in situ electron density (Ne) from SWARM‐A were utilized to investigate the topside ionospheric conditions during the 7–8 September 2017 geomagnetic storm. The rate of TEC index (ROTI) and rate of density index (RODI), which are derivative indices of TEC and Ne, respectively, were also used to characterize the topside ionospheric irregularities. The main results of this study are as follows: (1) There were significant enhancements seen in the uplooking TEC during the first main phase of the storm. (2) The uplooking TEC did not show unusual enhancement at the morning and evening local times in the Asian‐Australian sector during the recovery phase of the storm. (3) Prominent TEC hemispheric asymmetry at the middle and high latitudes was observed at both day and night sectors. (4) Long‐duration recovery of topside TEC with respect to the prestorm condition was also detected in this event. (5) Nighttime ROTI enhancements were presented in a wide latitudinal range from the equator to the poles during the main phases of the storm. (6) The ionospheric electric field disturbances associated with IMF‐Bz fluctuations probably played a very important role in triggering ionospheric irregularities during the relatively weak geomagnetic activity on 7 September, which implies that ionospheric irregularities do not necessarily occur under the severe geomagnetic conditions only. Key Points: Long‐duration depletion of topside TEC recovery was observed during the morning and evening local times in this eventEnhancements of ionospheric irregularities were presented at night with wide latitudinal range during the two main phases of the stormIonospheric electric field disturbances due to Bz fluctuations probably triggered topside ionospheric irregularities before the main phases [ABSTRACT FROM AUTHOR]

We present an overview of the ionospheric conditions during the launch of the Investigation of Cusp Irregularities 3 (ICI‐3) sounding rocket. ICI‐3 was launched from Ny‐Ålesund, Svalbard, at 7:21.31 UT on 3 December 2011. The objective of ICI‐3 was to intersect the reversed flow event (RFE), which is thought to be an important source for the rapid development of ionospheric irregularities in the cusp ionosphere. The interplanetary magnetic field was characterized by strongly negative Bz and weakly negative By. The EISCAT Svalbard radar (ESR) 32‐m beam was operating in a fast azimuth sweep mode between 180° (south) and 300° (northwest) at an elevation angle of 30°. The ESR observed a series of RFEs as westward flow channels that were opposed to the large‐scale eastward plasma flow in the prenoon sector. ICI‐3 intersected the first RFE in the ESR field of view and observed flow structures that were consistent with the ESR observations. Furthermore, ICI‐3 revealed finer‐scale flow structures inside the RFE. The high‐resolution electron density data show intense fluctuations at all scales throughout the RFE. The ionospheric pierce point of the GPS satellite PRN30, which was tracked at Hornsund, intersected the RFE at the same time. The GPS scintillation data show moderate phase scintillations and weak amplitude scintillations. A comparison of the power spectra reveals a good match between the ground‐based GPS carrier phase measurements and the spectral slope of the in situ electron density data in the lower frequency range. It demonstrates the possibility of modelling GPS scintillations from high‐resolution in situ electron density data. Key Points: RFE is associated with sharp flow shear and particle precipitation which result in GPS scintillationsThe spectral slopes between scintillation and in situ measurements are consistent with the phase screen theoryThe study suggests that it is possible to model GPS scintillations from high‐resolution in situ electron density data [ABSTRACT FROM AUTHOR]

We present a new quantitative theory for the rate of change of total electron content index (ROTI) by noting its straightforward relationship to the phase structure function of ionospheric turbulence. The theory provides the dependence of ROTI on the sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of the irregularities. We also present useful approximations to the full theory that elucidate the principal dependencies. We show, for example, that ROTI varies with the effective scan velocity (Veff) to the power ν − 1/2, where 2ν + 1 is the spectral index of the 3‐D irregularities. This dependence on Veff persists in the ratio of ROTI to the intensity scintillation index (S4), which thus depends on the particular viewing geometry for each satellite. While irregularity strength cancels when forming this ratio, the dependence on Fresnel scale remains. We validate the theory by comparing predictions of irregularity strength (CkL) and S4 from 1‐Hz ROTI measurements with calculations derived from 20‐Hz intensity samples collected at Ascension Island. The theory explains the observations well, except when the scan is directed nearly along the field‐aligned irregularities (within 20° of meridional). For cross‐field scans the standard deviations of the error in predicting S4 from 1‐Hz ROTI were 0.05, 0.11, and 0.14 for weak, moderate, and strong scintillation, respectively. These encouraging results suggest the possibility of using dense networks of inexpensive total electron content monitors to provide quantitative diagnostics of ionospheric scintillation and the irregularities that cause them. Plain Language Summary: We present a new theoretical model to relate a widely used statistical measure of fluctuations in total electron content to statistical measures of ionospheric irregularities and the scintillations they produce. This model includes several effects not previously accounted for quantitatively, including the sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of magnetic field‐aligned ionospheric irregularities. We validate the theory using measurements collected at Ascension Island, an equatorial station, during solar maximum conditions. Key Points: We present a new theory for the rate of change of total electron content index (ROTI)ROTI varies with sampling interval, satellite motion, propagation geometry, and the spectral shape, strength, anisotropy, and drift of field‐aligned irregularitiesThe new theory enables quantitative prediction of intensity scintillation (S4) and irregularity strength (CkL) from measurements of ROTI [ABSTRACT FROM AUTHOR]

The polar ionosphere is often characterized by irregularities and fluctuations in the plasma density. We present a statistical study of ionospheric plasma irregularities based on the observations from the European Space Agency's Swarm mission. The in situ electron density obtained with the Langmuir probe and the total electron content from the onboard global positioning system receiver are used to detect ionospheric plasma irregularities. We derive the irregularity parameters from the electron density in terms of the rate of change of density index and electron density gradients. We also use the rate of change of total electron content index as the irregularity parameter based on the global positioning system data. The background electron density and plasma irregularities are closely controlled by the Earth's magnetic field, with averaged enhancements close to the magnetic poles. The climatological maps in magnetic latitude/magnetic local time coordinates show predominant plasma irregularities near the dayside cusp, polar cap, and nightside auroral oval. These irregularities may be associated with large‐scale plasma structures such as polar cap patches, auroral blobs, auroral particle precipitation, and the equatorward wall of the ionospheric trough. The spatial distributions of irregularities depend on the interplanetary magnetic field (IMF). By filtering the irregularity parameters according to IMF By, we find a clear asymmetry of the spatial distribution in the cusp and polar cap between the Northern (NH) and Southern Hemispheres (SH). For negative IMF By, irregularities are stronger in the dusk (dawn) sector in the NH (SH) and vice versa. This feature is in agreement with the high‐latitude ionospheric convection pattern that is regulated by the IMF By component. The plasma irregularities are also controlled by the solar activity within the current declining solar cycle. The irregularities in the SH polar cap show a seasonal variation with higher values from September to April, while the seasonal variation in the NH is only obvious around solar maximum during 2014–2015. Key Points: The high‐latitude ionospheric irregularities are enhanced in the dayside cusp, polar cap, and nightside auroral ovalThe spatial distribution of irregularities in plasma density depends on the IMF ByThe seasonal variations of plasma irregularities in the Northern and Southern Hemispheres are different with respect to the local seasons [ABSTRACT FROM AUTHOR]

Since the 1950s, high frequency and very high frequency radars near the magnetic equator have frequently detected strong echoes caused ultimately by the Farley‐Buneman instability (FBI) and the gradient drift instability (GDI). In the 1980s, coordinated rocket and radar campaigns made the astonishing observation of flat‐topped electric fields coincident with both meter‐scale irregularities and the passage of kilometer‐scale waves. The GDI in the daytime E region produces kilometer‐scale primary waves with polarization electric fields large enough to drive meter‐scale secondary FBI waves. The meter‐scale waves propagate nearly vertically along the large‐scale troughs and crests and act as VHF tracers for the large‐scale dynamics. This work presents a set of hybrid numerical simulations of secondary FBIs, driven by a primary kilometer‐scale GDI‐like wave. Meter‐scale density irregularities develop in the crest and trough of the kilometer‐scale wave, where the total electric field exceeds the FBI threshold, and propagate at an angle near the direction of total Hall drift determined by the combined electric fields. The meter‐scale irregularities transport plasma across the magnetic field, producing flat‐topped electric fields similar to those observed in rocket data and reducing the large‐scale wave electric field to just above the FBI threshold value. The self‐consistent reduction in driving electric field helps explain why echoes from the FBI propagate near the plasma acoustic speed. Plain Language Summary: Since the 1950s, radars near the Earth's geomagnetic equator have frequently seen their signals reflected from electrically charged structures in the upper atmosphere. A tremendous amount of research has gone into understanding what creates charged structures on different size scales, as well as how they evolve individually, but we understand relatively little about how those differently sized structures interact. This paper presents results of numerical simulations that leverage modern supercomputing power to model a few sections of the electrically charged gas of the upper atmosphere. The simulations were designed to test the hypothesis that a wave 1 km long can create electric fields strong enough to produce waves of only a few meters which move up and down the larger wave's trough and crest. The result? Yes they can. Not only do the smaller waves' motions explain decades of radar reflections—they also shed light on some strange rocket observations from the 1980s and tie both components together to answer a longstanding question about how fast these waves travel. Key Points: Kilometer‐scale primary wave drives meter‐scale secondary waves via polarization electric fieldMeter‐scale waves transport plasma across the magnetic fieldAnomalous plasma transport reduces wave electric field, creating flat‐top waveform and reducing wave speed [ABSTRACT FROM AUTHOR]

We investigate the Global Positioning System (GPS) amplitude and phase scintillations during a severe geomagnetic storm on 17 March 2015. The auroral oval expanded significantly due to a strongly southward interplanetary magnetic field (Bz was −25 nT). When the auroral oval was over Skibotn in northern Norway, significant enhancements in total electron content (TEC) fluctuations, amplitude, and phase scintillation were observed. The strongest amplitude and phase scintillations were observed when a TEC blob propagated across the field of view. Strong amplitude and phase scintillations were observed near the edges of the TEC blob. The European Incoherent SCATter ultrahigh frequency radar observed significant enhancement of electron density (from 0.8 × 1011 to 1.6 × 1011 m−3) near the edge of the TEC blob in the F2 region, while the E region was only slightly enhanced. This indicates that the plasma processes and instability modes, which accounted for the strong GPS scintillations, should involve the F2 region ionosphere. We also analyzed the tracking performance of the GPS receiver during strong ionospheric scintillation condition. While the receiver maintained tracking of the GPS L1 signal, the strong amplitude scintillation resulted in a power fade up to 12 dB‐Hz. Losses of lock occurred in the GPS L2 band. Both the power fade and rapid phase fluctuation should contribute to losses of lock. Plain Language Summary: We study the disturbance of satellite navigation signals (Global Navigation Satellite Systems, GNSS) during a severe geomagnetic storm on 17 March 2015. Substantial changes in the ionosphere caused degraded performance of the GNSS service through degrading the signal quality and/or failed signal tracking. The study of the GNSS receiver tracking performance under strong ionospheric scintillations provides important guidelines for future research and development of mitigation techniques. Key Points: Significant GPS amplitude and phase scintillations were observed in association with a TEC blob in the polar ionosphereThe geomagnetic storm resulted in losses of lock in the GPS L2 band and a 12‐dB‐Hz signal power drop in the L1 bandIonospheric irregularities cause signal power fades and rapid phase fluctuations, which both contribute to losses of lock for GPS signals [ABSTRACT FROM AUTHOR]

Knowledge of Mars's ionosphere has been significantly advanced in recent years by observations from Mars Express and lately Mars Atmosphere and Volatile EvolutioN. A topic of particular interest are the interactions between the planet's ionospheric plasma and its highly structured crustal magnetic fields and how these lead to the redistribution of plasma and affect the propagation of radio waves in the system. In this paper, we elucidate a possible relationship between two anomalous radar signatures previously reported in observations from the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument on Mars Express. Relatively uncommon observations of localized, extreme increases in the ionospheric peak density in regions of radial (cusp‐like) magnetic fields and spread echo radar signatures are shown to be coincident with ducting of the same radar pulses at higher altitudes on the same field lines. We suggest that these two observations are both caused by a high electric field (perpendicular to B) having distinctly different effects in two altitude regimes. At lower altitudes, where ions are demagnetized and electrons magnetized, and recombination dominantes, a high electric field causes irregularities, plasma turbulence, electron heating, slower recombination, and ultimately enhanced plasma densities. However, at higher altitudes, where both ions and electrons are magnetized and atomic oxygen ions cannot recombine directly, the high electric field instead causes frictional heating, a faster production of molecular ions by charge exchange, and so a density decrease. The latter enables ducting of radar pulses on closed field lines, in an analogous fashion to interhemispheric ducting in the Earth's ionosphere. Key Points: MARSIS on MEX observed both echoes from distant ionospheric irregularities as well as locally ducted echoes in close successionBoth effects are consistent with the presence of extended, small‐scale field‐aligned density irregularitiesWe suggest that these may be the direct result of strong electric fields present at ionospheric altitudes [ABSTRACT FROM AUTHOR]

We discuss theoretically the excitation of artificial plasma irregularities in the auroral ionosphere by high‐frequency X‐mode radio wave. This is done via a two‐step process. As a first step we adopt the thermal self‐focusing instability excited in the F region of the ionosphere under the action of a strong high‐frequency (HF) radio wave. This instability causes the formation of perturbations of the electron temperature and plasma concentration across the magnetic field. In addition, the plasma becomes depleted in the regions of the electron temperature enhancements and vice versa, since the gradients of plasma concentration and the electron temperature have opposite signs. In such conditions the temperature gradient instability comes into play. As a second step we consider plasma and electron temperature inhomogeneities that appear due to this instability to be responsible for the generation of irregularities with transverse sizes smaller than the typical scales of the self‐focusing instability. Alternative mechanisms such as excitation of the gradient‐drift and the current‐convective instabilities, which are often attributed to the generation of plasma irregularities in the F region and can contribute to the formation of artificial irregularities in the case of X‐mode heating, are also discussed. Key Points: We have investigated F region irregularities produced by high power HF X‐mode radio wavesPlasma instabilities responsible for decametric irregularities in the F region are described via a two‐step processOur theoretical characteristic time for development of decametric irregularities is of the order of 1 min [ABSTRACT FROM AUTHOR]

Abstract: We examine the density structure of equatorial depletions referred to here as equatorial plasma bubbles (EPBs). Data recorded by the Ion Velocity Meter as part of the Coupled Ion Neutral Dynamics Investigation (CINDI) aboard the Communication/Navigation Outage Forecasting System (C/NOFS) satellite are used to study EPBs from 1600 to 0600 h local time at altitudes from 350 to 850 km. The data are taken during the 7 years from 2008 to 2014, more than one half of a magnetic solar cycle, that include solar minimum and a moderate solar maximum. Using a rolling ball algorithm, EPBs are identified by profiles in the plasma density, each having a depth measured as the percent change between the background and minimum density (ΔN/N). During solar moderate activity bubbles observed in the topside postsunset sector are more likely to have large depths compared to those observed in the topside postmidnight sector. Large bubble depths can be observed near 350 km in the bottomside F region in the postsunset period. Conversely at solar minimum the distribution of depths is similar in the postsunset and postmidnight sectors in all longitude sectors. Deep bubbles are rarely observed in the topside postsunset sector and never in the bottomside above 400 km in altitude. We suggest that these features result from the vertical drift of the plasma for these two solar activity levels. These drift conditions affect both the background density in which bubbles are embedded and the growth rate of perturbations in the bottomside where bubbles originate. [ABSTRACT FROM AUTHOR]

Abstract: Previous studies have suggested that weakening downward plasma drifts can produce favorable conditions for the ionospheric Generalized Rayleigh‐Taylor (GRT) instability and explain the occurrence of postmidnight equatorial spread F (ESF). We evaluated this hypothesis using numerical simulations aided by measurements and attempted to explain ESF events observed in the American sector during June solstice, low solar flux conditions. We analyzed plasma drifts and ESF measurements made by the incoherent scatter radar of the Jicamarca Radio Observatory (11.95° S, 76.87° W, ∼1° dip). We found adequate measurements during a prototypical, quiet time event on 4–5 June 2008 when the downward drifts weakened and a fully developed ESF appeared. The measured drifts were used as input for the SAMI2 model. SAMI2 reproduced an “apparent” uplift of the ionosphere based on h ′F measurements that was consistent with expectations and observations. SAMI2 also provided parameters for estimation of the flux tube linear growth rates of GRT instability associated with the weakening drift event. We found that the weakening drifts did produce unstable conditions with positive growth rates. The growth rates, however, were slower than those obtained for typical, premidnight ESF events and those obtained for similar drift conditions in other longitude sectors. We show, however, that departures in the wind pattern, from climatological model predictions, can produce favorable conditions for instability development. Following the hypothesis of Huba and Krall (2013) and using SAMI2 simulations, we show that equatorward winds, when combined with weakening drifts, could have contributed to the unstable conditions responsible for the postmidnight ESF events. [ABSTRACT FROM AUTHOR]