@article{XiongStollePark2018,
  author    = {Xiong, Chao and Stolle, Claudia and Park, Jaeheung},
  title     = {Climatology of GPS signal loss observed by Swarm satellites},
  series = {Annales geophysicae},
  volume    = {36},
  journal   = {Annales geophysicae},
  number    = {2},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {0992-7689},
  doi       = {10.5194/angeo-36-679-2018},
  pages     = {679 -- 693},
  year      = {2018},
  abstract  = {By using 3-year global positioning system (GPS) measurements from December 2013 to November 2016, we provide in this study a detailed survey on the climatology of the GPS signal loss of Swarm onboard receivers. Our results show that the GPS signal losses prefer to occur at both low latitudes between +/- 5 and +/- 20 degrees magnetic latitude (MLAT) and high latitudes above 60 degrees MLAT in both hemispheres. These events at all latitudes are observed mainly during equinoxes and December solstice months, while totally absent during June solstice months. At low latitudes the GPS signal losses are caused by the equatorial plasma irregularities shortly after sunset, and at high latitude they are also highly related to the large density gradients associated with ionospheric irregularities. Additionally, the high-latitude events are more often observed in the Southern Hemisphere, occurring mainly at the cusp region and along nightside auroral latitudes. The signal losses mainly happen for those GPS rays with elevation angles less than 20 degrees, and more commonly occur when the line of sight between GPS and Swarm satellites is aligned with the shell structure of plasma irregularities. Our results also confirm that the capability of the Swarm receiver has been improved after the bandwidth of the phase-locked loop (PLL) widened, but the updates cannot radically avoid the interruption in tracking GPS satellites caused by the ionospheric plasma irregularities. Additionally, after the PLL bandwidth increased larger than 0.5 Hz, some unexpected signal losses are observed even at middle latitudes, which are not related to the ionospheric plasma irregularities. Our results suggest that rather than 1.0 Hz, a PLL bandwidth of 0.5 Hz is a more suitable value for the Swarm receiver.},
  language  = {en}
}
@article{SiddiquiLuehrStolleetal.2015,
  author    = {Siddiqui, Tarique Adnan and Luehr, H. and Stolle, Claudia and Park, J.},
  title     = {Relation between stratospheric sudden warming and the lunar effect on the equatorial electrojet based on Huancayo recordings},
  series = {Annales geophysicae},
  volume    = {33},
  journal   = {Annales geophysicae},
  number    = {2},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {0992-7689},
  doi       = {10.5194/angeo-33-235-2015},
  pages     = {235 -- 243},
  year      = {2015},
  abstract  = {It has been known for many decades that the lunar tidal influence in the equatorial electrojet (EEJ) is noticeably enhanced during Northern Hemisphere winters. Recent literature has discussed the role of stratospheric sudden warming (SSW) events behind the enhancement of lunar tides and the findings suggest a positive correlation between the lunar tidal amplitude and lower stratospheric parameters (zonal mean air temperature and zonal mean zonal wind) during SSW events. The positive correlation raises the question whether an inverse approach could also be developed which makes it possible to deduce the occurrence of SSW events before their direct observations (before 1952) from the amplitude of the lunar tides. This study presents an analysis technique based on the phase of the semi-monthly lunar tide to determine the lunar tidal modulation of the EEJ. A statistical approach using the superposed epoch analysis is also carried out to formulate a relation between the EEJ tidal amplitude and lower stratospheric parameters. Using these results, we have estimated a threshold value for the tidal wave power that could be used to identify years with SSW events from magnetic field observations.},
  language  = {en}
}
@article{ParkLuehrStolleetal.2015,
  author    = {Park, J. and Luehr, H. and Stolle, Claudia and Malhotra, G. and Baker, J. B. H. and Buchert, Stephan and Gill, R.},
  title     = {Estimating along-track plasma drift speed from electron density measurements by the three Swarm satellites},
  series = {Annales geophysicae},
  volume    = {33},
  journal   = {Annales geophysicae},
  number    = {7},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {0992-7689},
  doi       = {10.5194/angeo-33-829-2015},
  pages     = {829 -- 835},
  year      = {2015},
  abstract  = {Plasma convection in the high-latitude ionosphere provides important information about magnetosphere-ionosphere-thermosphere coupling. In this study we estimate the along-track component of plasma convection within and around the polar cap, using electron density profiles measured by the three Swarm satellites. The velocity values estimated from the two different satellite pairs agree with each other. In both hemispheres the estimated velocity is generally anti-sunward, especially for higher speeds. The obtained velocity is in qualitative agreement with Super Dual Auroral Radar Network data. Our method can supplement currently available instruments for ionospheric plasma velocity measurements, especially in cases where these traditional instruments suffer from their inherent limitations. Also, the method can be generalized to other satellite constellations carrying electron density probes.},
  language  = {en}
}