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  • 1
    Publication Date: 2016-07-17
    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...
    Print ISSN: 1343-8832
    Electronic ISSN: 1880-5981
    Topics: Geosciences
    Published by SpringerOpen
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  • 2
    Publication Date: 2016-08-02
    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.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
    Publication Date: 2015-07-08
    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°.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Publication Date: 2015-12-16
    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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
    Publication Date: 2016-07-09
    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 ...
    Print ISSN: 1343-8832
    Electronic ISSN: 1880-5981
    Topics: Geosciences
    Published by SpringerOpen
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  • 6
    Publication Date: 2015-01-25
    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.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
    Publication Date: 2012-06-01
    Description: Standing fast mode waves known as global modes, or cavity/waveguide modes, have been extensively studied as a potential driver of monochromatic shear Alfvén waves in the Earth's magnetosphere via the field line resonance (FLR) mechanism. However, their existence outside of the plasmasphere remains controversial. We present here a global mode observation outside the plasmasphere, using simultaneous multispacecraft observations in the solar wind, the magnetosheath and the outer magnetosphere, as well as ground magnetometer data. Broadband solar wind dynamic pressure fluctuations are the most likely drivers of the Pc5 frequency range (2–7 mHz) global mode. The global mode transfers energy toward the plasmapause, where it drives localized shear Alfvén wave activity in the Pc5 frequency range. Global modes are thus a viable mechanism for converting broadband energy sources to monochromatic, radially localized shear Alfvén waves in the Pc5 frequency range.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
    Publication Date: 2019
    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.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
    Publication Date: 2014-11-28
    Description: In this study we present for the first time the nonmigrating tidal spectrum for the electron density in the topside ionosphere on global scale for different seasons at both solar maximum and minimum conditions. The electron density observations from the CHAMP (CHAllenging Minisatellite Payload) satellite provide evidence for prominent nonmigrating tides at different latitude regions. At middle and high latitudes the most prominent diurnal tides are DE1, D0, and DW2, as well as semidiurnal tides SW1 and SW3 preferably during equinox seasons. DE1 is only found in the northern middle and high latitudes, while D0 and DW2 are much stronger in the southern hemisphere. These tides are believed to be driven by the thermospheric winds through ion-neutral interactions. At equatorial and low latitude regions the most prominent diurnal tides are DE2 and DE3. DE2 is found to be present at low latitude regions throughout the whole year with larger amplitudes in the southern hemisphere, while DE3 shows largest amplitudes (symmetric above the dip equator) at the EIA crest region during September solstice. A general anti-phase behavior between the EIA crest and trough is observed for the tides DE3, DE2, DW2 and SW3. We consider this as strong evidence for the modulation of the EIA electron density by the E-layer zonal electric field via the ion fountain effect. An exception is the tidal component D0, which exhibits anti-phase variations between the two hemispheres. The phase value at the equatorial trough is half way between those of the two hemispheres. Presently we cannot give an explanation for it, and study concerning special model effort is further needed.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
    Publication Date: 2015-04-04
    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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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