ALBERT

Description: In this study, we investigated the scale sizes of equatorial plasma irregularities (EPIs) using measurements from the Swarm satellites during its early mission and final constellation phases. We found that wit...

Description: In this study we investigated conditions for loss of GPS signals observed by the Swarm satellites during a two-year period, from December 2013 to November 2015. Our result shows that the Swarm satellites encountered most of the total loss of GPS signal at the ionization anomaly crests, between ±5°- ± 20° magnetic latitude, forming two bands along the magnetic equator; and these low-latitude events mainly appear around post-sunset hours from 1900-2200 local time. By further checking the in-situ electron density measurements of Swarm , we found that practically all the total loss of GPS signal events at low latitudes are related to equatorial plasma irregularities (EPIs), that show absolute density depletions larger than 10 × 10 11  m -3 ; Then, the Swarm satellites encountered for up to 95% a loss of GPS signal for at least one channel, and up to 45% tracked less than four GPS satellites (making kinematic navigation solutions impossible). For those EPIs with density depletions less than 10 × 10 11  m -3 , the chance of tracked GPS signals less than four reduces to only 1.0%. Swarm also observed total losses of all GPS signal at high latitudes, mainly around local noon; and these events are related to large spatial density gradients due to polar patches or increased geomagnetic/auroral activities. We further found that the loss of GPS signals were less frequent after appropriate settings of the Swarm GPS receivers had been updated. However, the more recent period of the mission, e.g., after the GPS receiver settings have been updated, also coincides with less severe electron density depletions due to the declining solar cycle, making GPS loss events less likely. We conclude that both, lower electron density gradients and appropriate GPS receiver settings, reduce the probability for loss of GPS signals.

Description: Previous studies suggested that electric and/or magnetic field fluctuations observed in the nighttime topside ionosphere at mid-latitudes generally originate from quiet-time nocturnal medium-scale traveling ionospheric disturbances (MSTIDs). However, decisive evidences for the connection between the two have been missing. In this study we make use of the multi-spacecraft observations of mid-latitude magnetic fluctuations (MMFs) in the nighttime topside ionosphere by the Swarm constellation. The analysis results show that the area hosting MMFs are elongated in the NW-SE (NE-SW) direction in the northern (southern) hemisphere. The elongation direction and the magnetic field polarization support that the area hosting MMFs are nearly field-aligned. All these properties of MMFs suggest that they have close relationship with MSTIDs. Expectation values of root-mean-square (RMS) field-aligned currents (FACs) associated with MMFs are up to about 4 nA/m 2 . MMF coherency significantly drops for longitudinal distances of ≥1°.

Description: The Swarm constellation has been used to estimate zonal currents in the topside F-region ionosphere at about 500 km. Near-simultaneous magnetic field measurements from two altitudes but the same meridian are used for the current density calculations. We consider the period 15 February to 23 June 2014 for deriving a full 24-hour local time coverage of the latitudinal distribution over ±50° in magnetic latitude. Intervals with close orbital phasing at the two heights are considered, which repeat every 6 days. From such days seven successive orbits are used where the epochs of equator crossings differ by less than 2 minutes. Deduced current densities are predominantly eastward (about 20 nA/m 2 ) on the dayside and westward (about 10 nA/m 2 ) on the nightside. A number of different drivers contribute to the observed total current. We identified the gravity-driven eastward current as the most prominent at low latitudes. Eastward currents in the northern hemisphere are clearly stronger than in the south. This is attributed to the proximity of our study period to June solstice, when the solar radiation is stronger in the north. In addition, inter-hemispheric winds from the northern (summer) to the southern (winter) hemisphere contribute. They cause eastward currents in the north and westward in the south. We find a relatively large variability of the zonal currents both in space and time. The standard deviation is at least twice as large as the mean value of current density. This large variability is suggested to be related to gravity wave forcing from below.

Description: Low Earth orbiting geomagnetic satellite missions, such as the Swarm satellite mission, are the only means to monitor and investigate ionospheric currents on a global scale and to make in situ measurements of ...

Description: In this study we investigate a dayside, mid-latitude plasma depletion (DMLPD) encountered on 22 May 2014 by the Swarm and GRACE satellites, as well as ground-based instruments. The DMLPD was observed near Puerto Rico by Swarm near 10 LT under quiet geomagnetic conditions at altitudes of 475–520 km and magnetic latitudes of ~25°−30°. The DMLPD was also revealed in TEC observations by the Saint Croix station and by the GRACE satellites (430 km)near 16 LT and near the same geographic location. The unique Swarm constellation enables the horizontal tilt of the DMLPD to be measured (35° clockwise from the geomagnetic east–west direction). Ground-based airglow images at Arecibo show no evidence for plasma density depletions during the night prior to this dayside event. The C/NOFS equatorial satellite showed evidence for very modest plasma density depletions that had rotated into the morningside from nightside. However, the equatorial depletions do not appear related to the DMLPD, for which the magnetic apex height is about 2500 km. The origins of the DMLPD are unknown, but may be related to gravity waves.

Description: Abstract The near‐polar orbit satellites of Swarm mission provide a good opportunity to investigate the conjugacy of equatorial plasma irregularities (EPIs) since their trajectories at low latitudes are basically aligned with fixed geographical longitude. However, the Swarm in situ electron density occasionally shows EPIs at only one hemisphere at this longitude. In this study, we provide detailed analysis of such EPI events from the in situ electron densities and onboard global positioning system (GPS) measurements of Swarm low pair satellites, and simultaneous GPS data from two geomagnetically conjugate ground stations at the Africa longitudes. The result indicates that when Swam in situ electron density sometime shows EPIs at only one hemisphere, the GPS scintillations are still observed from the Swarm onboard receiver and by the two conjugate ground stations. It implies that the EPIs should generally elongate along the geomagnetic flux tube. More than two‐year statistic results show that the onset time of scintillation in the northern station is on average 16 and 18 min earlier than that in the southern station for September equinox and December solstice in 2015, while for March equinox in 2016 the onset time of scintillation of northern station is about 11 min later than that of southern station, which indicates the asymmetry features of EPIs along the flux tube. Further analysis of nearly three‐year GPS data from two conjugate stations at the Asia longitudes, we find that during solar maximum years the local sunset time plays an important role for causing the difference of onset time of scintillation between two conjugate stations.

Description: In this study we investigate the three-dimensional structure of low-latitude plasma blobs using multi-instrument and multi-satellite observations of the Swarm constellation. During the early commissioning phase the Swarm satellites were flying at the same altitude with zonal separation of about 0.5° in geographic longitude. Electron density data from the three satellites constrain the blob morphology projected onto the horizontal plane. Magnetic field deflections around blobs, which originate from field-aligned currents near the irregularity boundaries, constrain the blob structure projected onto the plane perpendicular to the ambient magnetic field. As the two constraints are given for two non-coplanar surfaces, we can get information on the three-dimensional structure of blobs. Combined observation results suggest that blobs are contained within tilted shells of geomagnetic flux tubes, which are similar to the shell structure of equatorial plasma bubbles suggested by previous studies.

Description: Abstract 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.

Description: The daytime mid-latitude plasma depletions (DMLPDs) observed on 22 May 2014 and 20 May 2015 by the Swarm constellation are not explained by any known natural phenomena. The DMLPDs were detected after rocket launches, and the DMLPD traces converged to the launch station. The event in 2015, for which sufficient total electron content (TEC) data are available, is accompanied with TEC depletion lasting for about 6 hours. The persistence generally agrees with the lifetime expected for rocket exhaust depletions (REDs) which is determined by the recombination of the ionospheric oxygen ion with water molecules in the rocket exhaust. These results lead to the conclusion that DMLPDs are REDs in the topside. The RED characteristics identified from the observations on both days are: (1) enhancement in electron temperature, (2) reduction in electron pressure, and (3) absence of substructures down to scale sizes of about 8 kilometers (Nyquist's scale size).