The Swarm satellites offer an unprecedented opportunity for improving our knowledge about polar cap patches, which are regarded as the main space weather issue in the polar caps. We present a new ...robust algorithm that automatically detects polar cap patches using in situ plasma density data from Swarm. For both hemispheres, we compute the spatial and seasonal distributions of the patches identified separately by Swarm A and Swarm B between December 2013 and August 2016. We show a clear seasonal dependency of patch occurrence. In the Northern Hemisphere (NH), patches are essentially a winter phenomenon, as their occurrence rate is enhanced during local winter and very low during local summer. Although not as pronounced as in the NH, the same pattern is seen for the Southern Hemisphere (SH). Furthermore, the rate of polar cap patch detection is generally higher in the SH than in the NH, especially on the dayside at about 77° magnetic latitude. Additionally, we show that in the NH the number of patches is higher in the postnoon and prenoon sectors for interplanetary magnetic field (IMF) By<0 and IMF By>0, respectively, and that this trend is mirrored in the SH, consistent with the ionospheric flow convection. Overall, our results confirm previous studies in the NH, shed more light regarding the SH, and provide further insight into polar cap patch climatology. Along with this algorithm, we provide a large data set of patches automatically detected with in situ measurements, which opens new horizons in studies of polar cap phenomena.
Key Points
New polar cap patch detection method based on Swarm in situ data provides an unprecedented data set for polar cap patch statistical studies
Polar cap patch occurrence rate is highest during local winter in both hemispheres; in the south it is also significant during local summer
There is a clear IMF By dependency in the spatial distribution of polar cap patches, consistent with the ionospheric flow pattern
We use an automated procedure to identify periods of enhanced dayside reconnection followed by enhanced nightside reconnection in measurements of the polar cap size by the Active Magnetosphere and ...Planetary Electrodynamics Response Experiment between January 2010 and December 2012; we find 490 such events. We investigate the dynamics of the spatial distributions of the total electron content (TEC) and phase scintillations of Global Positioning System (GPS) signals across the northern polar region and here report three important findings: (1) While a TEC enhancement (due to polar cap patches) propagates across the polar cap during these events, this enhancement is not associated with significant GPS phase scintillations. (2) Instead, a significant impact on GPS signal quality is first found when the TEC enhancements cross the nightside auroral boundary. (3) In combination with upward field‐aligned currents, these TEC enhancements cause the strongest GPS phase scintillations. We conclude that polar cap patches are not, as previously thought, a space weather threat inside the polar cap but instead reveal their biggest impact once they reach the nightside auroral oval, in particular when combined with upward field‐aligned currents.
Key Points
During polar cap expansion/contraction, TEC enhancements cross the polar cap
Inside the polar cap, they are not associated with GPS scintillations
Once they enter the auroral oval, they cause significant GPS scintillations
The electron density data from the ICI‐2 sounding rocket experiment in the high‐latitude F region ionosphere are analyzed using the higher‐order spectra and higher‐order statistics. Two regions of ...enhanced fluctuations are chosen for detailed analysis: the trailing edge of a polar cap patch and an electron density enhancement associated with particle precipitation. While these two regions exhibit similar power spectra, our analysis reveals that their internal structures are significantly different. The structures on the edge of the polar cap patch are likely due to nonlinear wave interactions since this region is characterized by intermittency and significant coherent mode coupling. The plasma enhancement subjected to precipitation, however, exhibits stronger random characteristics with uncorrelated phases of density fluctuations. These results suggest that particle precipitation plays a fundamental role in ionospheric plasma structuring creating turbulent‐like structures. We discuss the physical mechanisms that cause plasma structuring as well as the possible processes for the low‐frequency part of the spectrum in terms of plasma instabilities.
Key Points
Steep density gradients in the polar cap reveal nonlinear wave interactions and intermittency
Irregularities at the edge of polar cap patches are characterized by coherent mode coupling
Particle precipitation on plasma gradients gives turbulent‐like structures in high‐latitude F layer
We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all‐sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and ...Defense Meteorological Satellite Program (DMSP)‐17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low‐latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open‐closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet.
Key Points
Unique coordinated observations by imagers, radars, and DMSP are presented
A day‐night coupling via a localized flow channel was identified
Coupling between dayside and nightside reconnection was suggested
The characteristics of turbulent plasma in the winter cusp ionosphere is studied based on in‐situ data from the Investigation of Cusp Irregularities (ICI) sounding rockets. The electron density ...fluctuations from ICI‐2 and ICI‐3 missions have been analyzed for the whole flight, using advanced time‐series analysis techniques. The analysis of the autocorrelation scale is indicative of the turbulence integral scale and marks the onset of the turbulent cascade from large to small scales in the power spectrum density of the rocket data. The power spectrum is typical for a turbulent field. The turbulent structures are persistent in the data, which are quantified by analyzing the Probability Distribution Functions (PDFs) and their deviations from the Gaussian distribution. The kurtosis analysis indicates the presence of intermittency. These results are compared with the Local Intermittence Measure ‐ LIM, which confirms the presence of small scale intermittent structures. The turbulence measured by rockets appears well developed and covers frequencies between 1 Hz and several hundred Hz. There is a good agreement with the previous results, suggesting that density fluctuations in the ionospheric cusp agree with the turbulence framework in which intermittent processes transfer energy across different scales.
Key Points
Sounding rocket in situ observations show a complex behavior similar to a turbulent field in the ionospheric cusp
The integral scale of the turbulence exhibits large‐scale structures of the order of tens of kilometers
Density fluctuations agree with the turbulence framework in which intermittent processes transfer energy across different scales
We examined the source region of dayside large‐scale traveling ionospheric disturbances (LSTIDs) and their relation to cusp energy input. Aurora and total electron content (TEC) observations show ...that LSTIDs propagate equatorward away from the cusp and demonstrate the cusp region as the source region. Enhanced energy input to the cusp initiated by interplanetary magnetic field (IMF) southward turning triggers the LSTIDs, and each LSTID oscillation is correlated with a TEC enhancement in the dayside oval with tens of minutes periodicity. Equatorward‐propagating LSTIDs are likely gravity waves caused by repetitive heating in the cusp. The cusp source can explain the high LSTID occurrence on the dayside during geomagnetically active times. Poleward‐propagating ΔTEC patterns in the polar cap propagate nearly at the convection speed. While they have similar ΔTEC signatures to gravity wave‐driven LSTIDs, they are suggested to be weak polar cap patches quasiperiodically drifting from the cusp into the polar cap via dayside reconnection.
Key Points
The cusp region in the dayside auroral oval has been found to be the source region of dayside LSTIDs
Enhanced energy input to the cusp initiated by IMF southward turning triggers the dayside LSTIDs
Midlatitude LSTIDs are thermospheric gravity waves; polar cap TEC perturbations are weak polar cap patches
We report direct observations of the double‐slope power spectra for plasma irregularities in the F layer of the polar ionosphere. The investigation of cusp irregularities ICI‐2 sounding rocket, which ...was launched into the polar cusp ionosphere, intersected enhanced plasma density regions with decameter‐scale irregularities. Density measurements at unprecedented high resolution with multi‐Needle Langmuir Probes allowed for a detailed study of the plasma irregularities down to kinetic scales. Spectral analysis reveals double‐slope power spectra for regions of enhanced fluctuations associated mainly with density gradients, with the steepening of the spectra occurring close to the oxygen gyrofrequency. These findings are further supported with the first results from the ICI‐3 rocket, which flew through regions with strong precipitation and velocity shears. Previously, double‐slope spectra have been observed in the equatorial ionosphere. The present work gives a direct evidence that the double‐slope power spectra can be common in the high‐latitude ionosphere.
Key Points
Double‐slope spectra of plasma irregularities observed in the polar ionosphere
The spectrum steepening related to the onset of kinetic plasma phenomena
Spectral characteristics of irregularities similar to equatorial ionosphere
The Swarm mission represents a strong new tool to survey polar cap patches and plasma structuring inside the polar cap. In the early commissioning phase, the three Swarm satellites were operated in a ...pearls‐on‐a‐string configuration making noon‐midnight transpolar passes. This provides an unparalleled opportunity to examine the potential role of the gradient drift instability (GDI) process on polar cap patches by systematically calculating GDI growth times during their transit across the pole from day to night. Steep kilometer‐scale gradients appeared in this study as initial structures that persisted during the approximate 90 min it took a patch to cross the polar cap. The GDI growth times were calculated for a selection of the steep density gradients on both the dayside and the nightside. The values ranged from 23 s to 147 s, which is consistent with recent rocket measurements in the cusp auroral region and provides a template for future studies. Growth times of the order of 1 min found both on the dayside and on the nightside support the existing view that the GDI may play a dominant role in the generation of radio wave scintillation irregularities as the patches transit the polar cap from day to night.
Key Points
New technique to assess GDI/polar cap plasma structuring using Swarm
Internal kilometer‐scale structures persist as patches convect across the polar cap
The GDI can act quickly on several kilometer‐scale gradients within polar cap patches
Global Navigation Satellite Systems (GNSS) are subject to disturbances caused by plasma irregularities in the ionosphere. Studies have suggested that in addition to the gradient drift and ...Kelvin‐Helmholtz instabilities, electron precipitation may be important for phase scintillations in the dayside auroral region. This study combines in situ Swarm data with ground GNSS observations to investigate the potential role of filamentary field‐aligned currents (FACs) on phase scintillations in the dayside auroral region by analyzing 22 events with phase scintillations exceeding 0.45 radians. We observe colocation between regions of severe phase scintillations and highly filamented FACs with fluctuations measured in the spacecraft frame of the order of 20 Hz. The observations indicate that filamentary FACs are crucial drivers for irregularities responsible for creating severe phase scintillations measured in the dayside auroral region and are thus of significant importance in the context of space weather impact on satellite communication.
Plain Language Summary
Satellite‐based navigation systems such as Global Positioning System (GPS) are known to be affected by dynamic phenomena in the upper atmosphere. The signals are subject to disturbances, resulting in reduced position accuracy or even a loss of signal reception. Several processes have been suggested as being responsible for these disturbances in the dayside auroral region, but their relative importance is still unclear. Thus, we used Swarm data as well as ground‐based instrumentation to investigate this issue. We identified 22 events with severe disturbances that were located where we could compare data from the satellite and from ground‐based instruments. We find that the occurrences of severe phase scintillation coincide with structured field‐aligned currents (FACs), making them potentially a primary driver for signal disturbances in the daytime auroral region.
Key Points
Severe GNSS phase scintillations in the winter dayside auroral ionosphere coincide with filamentary FACs
Filamentary FACs appear to be essential for the creation of severe phase scintillations
On December 08, 2018 the Twin Rocket Investigation of Cusp Electrodynamics 2 (TRICE 2) mission was successfully launched. The mission consisted of two sounding rockets, each carrying a payload ...capable of measuring electron and ion distributions, electric and magnetic fields, and plasma waves occurring in the northern magnetospheric cusp. This study highlights the ion and wave observations obtained by TRICE 2 in the cusp and observations from the magnetospheric multiscale (MMS) spacecraft at the low‐latitude magnetopause two hours prior to the TRICE 2 traversal of the cusp. Within the cusp, typical ion cusp features were observed, that is, energy‐latitude dispersion of injected magnetosheath plasma. However, a lower energy population was also measured near the equatorward edge of the cusp on open field lines. Pitch‐angle distributions of the low‐energy ions suggest that this population was magnetospheric in origin, and not from ionospheric upflows. Data from MMS show that counterstreaming ions were present in the outer magnetosphere and low‐latitude boundary layer at similar energies to those observed by TRICE 2 in the cusp. Correlations between the low‐energy ions within the cusp and broadband extremely low frequency waves suggest that the low‐energy magnetospheric ions that filled the flux tube may have undergone wave‐particle interactions. These interactions may cause pitch‐angle scattering of low‐energy magnetospheric ions closer to the loss cone, thereby allowing them to precipitate into the cusp and be measured by the TRICE 2 sounding rockets.
Key Points
Low‐energy ions measured near the equatorward edge of the cusp by TRICE 2 are locally mirroring and propagating toward the ionosphere
A similar counter‐streaming ion population is observed at the magnetopause by MMS two hours prior to the TRICE 2 cusp traversal
Ion and wave data in the cusp suggest wave‐particle interactions are pitch angle scattering low‐energy magnetospheric ions
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