@article{ParkStolleXiongetal.2015,
  author    = {Park, Jaeheung and Stolle, Claudia and Xiong, Chao and L{\"u}hr, Hermann and Pfaff, Robert F. and Buchert, Stephan and Martinis, Carlos R.},
  title     = {A dayside plasma depletion observed at midlatitudes during quiet geomagnetic conditions},
  series = {Geophysical research letters},
  volume    = {42},
  journal   = {Geophysical research letters},
  number    = {4},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {0094-8276},
  doi       = {10.1002/2014GL062655},
  pages     = {967 -- 974},
  year      = {2015},
  abstract  = {In this study we investigate a dayside, midlatitude 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 similar to 25 degrees-30 degrees. The DMLPD was also revealed in total electron content 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 degrees clockwise from the geomagnetic east-west direction). Ground-based airglow images at Arecibo showed 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.},
  language  = {en}
}
@article{SiddiquiYamazakiStolleetal.2018,
  author    = {Siddiqui, Tarique Adnan and Yamazaki, Yosuke and Stolle, Claudia and L{\"u}hr, Hermann and Matzka, J{\"u}rgen and Maute, Astrid and Pedatella, Nicholas},
  title     = {Dependence of Lunar Tide of the Equatorial Electrojet on the Wintertime Polar Vortex, Solar Flux, and QBO},
  series = {Geophysical research letters},
  volume    = {45},
  journal   = {Geophysical research letters},
  number    = {9},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {0094-8276},
  doi       = {10.1029/2018GL077510},
  pages     = {3801 -- 3810},
  year      = {2018},
  abstract  = {The lower atmospheric forcing effects on the ionosphere are particularly evident during extreme meteorological events known as sudden stratospheric warmings (SSWs). During SSWs, the polar stratosphere and ionosphere, two distant atmospheric regions, are coupled through the SSW-induced modulation of atmospheric migrating and nonmigrating tides. The changes in the migrating semidiurnal solar and lunar tides are the major source of ionospheric variabilities during SSWs. In this study, we use 55 years of ground-magnetometer observations to investigate the composite characteristics of the lunar tide of the equatorial electrojet (EEJ) during SSWs. These long-term observations allow us to capture the EEJ lunar tidal response to the SSWs in a statistical sense. Further, we examine the influence of solar flux conditions and the phases of quasi-biennial oscillation (QBO) on the lunar tide and find that the QBO phases and solar flux conditions modulate the EEJ lunar tidal response during SSWs in a similar way as they modulate the wintertime Arctic polar vortex. This work provides first evidence of modulation of the EEJ lunar tide due to QBO. Plain Language Summary This study focuses on the vertical coupling between the polar stratosphere and equatorial ionosphere during sudden stratospheric warmings (SSWs). Extreme meteorological events such as SSWs induce variabilities in the ionosphere by modulating the atmospheric migrating and nonmigrating tides, and these variabilities can be comparable to a moderate geomagnetic storm. Observations and modeling studies have found that the changes in the migrating semidiurnal solar and lunar tides are a major source of ionospheric variabilities during SSWs. The equatorial electrojet (EEJ) is a narrow ribbon of current flowing over the dip equator in the ionosphere and is particularly sensitive to tidal changes. Long-term ground-magnetometer recordings have been used in this study to estimate the variations induced in EEJ during SSWs due to the lunar semidiurnal tide in a statistical sense. The wintertime Arctic polar vortex and the occurrence of SSWs are modulated by solar flux conditions and the phases of quasi-biennial oscillation. In this work, we find the first evidence of lunar tidal modulation of EEJ due to quasi-biennial oscillation during SSWs. Our findings will be useful in providing improved predictions of ionospheric variations due to SSWs. The aeronomy community will be the most impacted by this paper.},
  language  = {en}
}
@misc{LuehrWichtGilderetal.2018,
  author    = {L{\"u}hr, Hermann and Wicht, Johannes and Gilder, Stuart A. and Holschneider, Matthias},
  title     = {General Introduction and Scientific Summary of the German Priority Program "PlanetMag"},
  series = {Magnetic Fields in the Solar System},
  volume    = {448},
  journal   = {Magnetic Fields in the Solar System},
  publisher = {Springer},
  address   = {Dordrecht},
  isbn      = {978-3-319-64292-5},
  issn      = {0067-0057},
  doi       = {10.1007/978-3-319-64292-5_1},
  pages     = {1 -- 6},
  year      = {2018},
  abstract  = {This book aims at understanding the diversity of planetary and lunar magnetic fields and their interaction with the solar wind. A synergistic interdisciplinary approach combines newly developed tools for data acquisition and analysis, computer simulations of planetary interiors and dynamos, models of solar wind interaction, measurement of terrestrial rocks and meteorites, and laboratory investigations. The following chapters represent a selection of some of the scientific findings derived by the 22 projects within the DFG Priority Program Planetary Magnetism" (PlanetMag). This introductory chapter gives an overview of the individual following chapters, highlighting their role in the overall goals of the PlanetMag framework. The diversity of the different contributions reflects the wide range of magnetic phenomena in our solar system. From the program we have excluded magnetism of the sun, which is an independent broad research discipline, but include the interaction of the solar wind with planets and moons. Within the subsequent 13 chapters of this book, the authors review the field centered on their research topic within PlanetMag. Here we shortly introduce the content of all the subsequent chapters and outline the context in which they should be seen.},
  language  = {en}
}
@article{SiddiquiStolleLuehr2017,
  author    = {Siddiqui, Tarique Adnan and Stolle, Claudia and L{\"u}hr, Hermann},
  title     = {Longitude-dependent lunar tidal modulation of the equatorial electrojet during stratospheric sudden warmings},
  series = {Journal of geophysical research : Space physics},
  volume    = {122},
  journal   = {Journal of geophysical research : Space physics},
  number    = {3},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9380},
  doi       = {10.1002/2016JA023609},
  pages     = {3760 -- 3776},
  year      = {2017},
  abstract  = {The effects of coupling between different layers of the atmosphere during Stratospheric Sudden Warming (SSW) events have been studied quite extensively in the past fewyears, and in this context large lunitidal enhancements in the equatorial ionosphere have also been widely discussed. In this study we report about the longitudinal variabilities in lunitidal enhancement in the equatorial electrojet (EEJ) during SSWs through ground and space observations in the Peruvian and Indian sectors. We observe that the amplification of lunitidal oscillations in EEJ is significantly larger over the Peruvian sector in comparison to the Indian sector. We further compare the lunitidal oscillations in both the sectors during the 2005-2006 and 2008-2009 major SSW events and during a non-SSW winter of 2006-2007. It is found that the lunitidal amplitude in EEJ over the Peruvian sector showed similar enhancements during both the major SSWs, but the enhancements were notably different in the Indian sector. Independent from SSW events, we have also performed a climatological analysis of the lunar modulation of the EEJ during December solstice over both the sectors by using 10years of CHAMP magnetic measurements and found larger lunitidal amplitudes over the Peruvian sector confirming the results from ground magnetometer observations. We have also analyzed the semidiurnal lunar tidal amplitude in neutral temperature measurements from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) at 110km and found lesser longitudinal variability than the lunitidal amplitude in EEJ. Our results suggest that the longitudinal variabilities in lunitidal modulation of the EEJ during SSWs could be related to electrodynamics in the E region dynamo.},
  language  = {en}
}
@article{ParkLuehrKervalishvilietal.2017,
  author    = {Park, Jaeheung and L{\"u}hr, Hermann and Kervalishvili, Guram and Rauberg, Jan and Stolle, Claudia and Kwak, Young-Sil and Lee, Woo Kyoung},
  title     = {Morphology of high-latitude plasma density perturbations as deduced from the total electron content measurements onboard the Swarm constellation},
  series = {Journal of geophysical research : A, Space physics},
  volume    = {122},
  journal   = {Journal of geophysical research : A, Space physics},
  number    = {1},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9380},
  doi       = {10.1002/2016JA023086},
  pages     = {1338 -- 1359},
  year      = {2017},
  abstract  = {In this study, we investigate the climatology of high-latitude total electron content (TEC) variations as observed by the dual-frequency Global Navigation Satellite Systems (GNSS) receivers onboard the Swarm satellite constellation. The distribution of TEC perturbations as a function of geographic/magnetic coordinates and seasons reasonably agrees with that of the Challenging Minisatellite Payload observations published earlier. Categorizing the high-latitude TEC perturbations according to line-of-sight directions between Swarm and GNSS satellites, we can deduce their morphology with respect to the geomagnetic field lines. In the Northern Hemisphere, the perturbation shapes are mostly aligned with the L shell surface, and this anisotropy is strongest in the nightside auroral (substorm) and subauroral regions and weakest in the central polar cap. The results are consistent with the well-known two-cell plasma convection pattern of the high-latitude ionosphere, which is approximately aligned with L shells at auroral regions and crossing different L shells for a significant part of the polar cap. In the Southern Hemisphere, the perturbation structures exhibit noticeable misalignment to the local L shells. Here the direction toward the Sun has an additional influence on the plasma structure, which we attribute to photoionization effects. The larger offset between geographic and geomagnetic poles in the south than in the north is responsible for the hemispheric difference.},
  language  = {en}
}
@article{ParkLuehrKervalishvilietal.2015,
  author    = {Park, Jaeheung and L{\"u}hr, Hermann and Kervalishvili, Guram N. and Rauberg, Jan and Michaelis, Ingo and Stolle, Claudia and Kwak, Young-Sil},
  title     = {Nighttime magnetic field fluctuations in the topside ionosphere at midlatitudes and their relation to medium-scale traveling ionospheric disturbances: The spatial structure and scale sizes},
  series = {Journal of geophysical research : Space physics},
  volume    = {120},
  journal   = {Journal of geophysical research : Space physics},
  number    = {8},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9380},
  doi       = {10.1002/2015JA021315},
  pages     = {6818 -- 6830},
  year      = {2015},
  abstract  = {Previous studies suggested that electric and/or magnetic field fluctuations observed in the nighttime topside ionosphere at midlatitudes 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 multispacecraft observations of midlatitude magnetic fluctuations (MMFs) in the nighttime topside ionosphere by the Swarm constellation. The analysis results show that the area hosting MMFs is 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 is nearly field aligned. All these properties of MMFs suggest that they have close relationship with MSTIDs. Expectation values of root-mean-square field-aligned currents associated with MMFs are up to about 4nA/m(2). MMF coherency significantly drops for longitudinal distances of 1 degrees.},
  language  = {en}
}
@article{SiddiquiMautePedatellaetal.2018,
  author    = {Siddiqui, Tarique Adnan and Maute, Astrid and Pedatella, Nick and Yamazaki, Yosuke and L{\"u}hr, Hermann and Stolle, Claudia},
  title     = {On the variability of the semidiurnal solar and lunar tides of the equatorial electrojet during sudden stratospheric warmings},
  series = {Annales geophysicae},
  volume    = {36},
  journal   = {Annales geophysicae},
  number    = {6},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {0992-7689},
  doi       = {10.5194/angeo-36-1545-2018},
  pages     = {1545 -- 1562},
  year      = {2018},
  abstract  = {The variabilities of the semidiurnal solar and lunar tides of the equatorial electrojet (EEJ) are investigated during the 2003, 2006, 2009 and 2013 major sudden stratospheric warming (SSW) events in this study. For this purpose, ground-magnetometer recordings at the equatorial observatories in Huancayo and Fuquene are utilized. Results show a major enhancement in the amplitude of the EEJ semidiurnal lunar tide in each of the four warming events. The EEJ semidiurnal solar tidal amplitude shows an amplification prior to the onset of warmings, a reduction during the deceleration of the zonal mean zonal wind at 60 degrees N and 10 hPa, and a second enhancement a few days after the peak reversal of the zonal mean zonal wind during all four SSWs. Results also reveal that the amplitude of the EEJ semidiurnal lunar tide becomes comparable or even greater than the amplitude of the EEJ semidiurnal solar tide during all these warming events. The present study also compares the EEJ semidiurnal solar and lunar tidal changes with the variability of the migrating semidiurnal solar (SW2) and lunar (M2) tides in neutral temperature and zonal wind obtained from numerical simulations at E-region heights. A better agreement between the enhancements of the EEJ semidiurnal lunar tide and the M2 tide is found in comparison with the enhancements of the EEJ semidiurnal solar tide and the SW2 tide in both the neutral temperature and zonal wind at the E-region altitudes.},
  language  = {en}
}
@misc{SiddiquiMautePedatellaetal.2018,
  author    = {Siddiqui, Tarique Adnan and Maute, Astrid and Pedatella, Nick and Yamazaki, Yosuke and L{\"u}hr, Hermann and Stolle, Claudia},
  title     = {On the variability of the semidiurnal solar and lunar tides of the equatorial electrojet during sudden stratospheric warmings},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1075},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-46838},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-468389},
  pages     = {1545 -- 1562},
  year      = {2018},
  abstract  = {The variabilities of the semidiurnal solar and lunar tides of the equatorial electrojet (EEJ) are investigated during the 2003, 2006, 2009 and 2013 major sudden stratospheric warming (SSW) events in this study. For this purpose, ground-magnetometer recordings at the equatorial observatories in Huancayo and F{\´u}quene are utilized. Results show a major enhancement in the amplitude of the EEJ semidiurnal lunar tide in each of the four warming events. The EEJ semidiurnal solar tidal amplitude shows an amplification prior to the onset of warmings, a reduction during the deceleration of the zonal mean zonal wind at 60∘ N and 10 hPa, and a second enhancement a few days after the peak reversal of the zonal mean zonal wind during all four SSWs. Results also reveal that the amplitude of the EEJ semidiurnal lunar tide becomes comparable or even greater than the amplitude of the EEJ semidiurnal solar tide during all these warming events. The present study also compares the EEJ semidiurnal solar and lunar tidal changes with the variability of the migrating semidiurnal solar (SW2) and lunar (M2) tides in neutral temperature and zonal wind obtained from numerical simulations at E-region heights. A better agreement between the enhancements of the EEJ semidiurnal lunar tide and the M2 tide is found in comparison with the enhancements of the EEJ semidiurnal solar tide and the SW2 tide in both the neutral temperature and zonal wind at the E-region altitudes.},
  language  = {en}
}
@misc{LuehrWichtGilderetal.2018,
  author    = {L{\"u}hr, Hermann and Wicht, Johannes and Gilder, Stuart A. and Holschneider, Matthias},
  title     = {Preface},
  series = {Magnetic Fields in the Solar System: Planets, Moons and Solar Wind Interactions},
  volume    = {448},
  journal   = {Magnetic Fields in the Solar System: Planets, Moons and Solar Wind Interactions},
  publisher = {Springer},
  address   = {Dordrecht},
  isbn      = {978-3-319-64292-5},
  issn      = {0067-0057},
  pages     = {V -- VI},
  year      = {2018},
  language  = {en}
}
@misc{XiongStolleLuehretal.2016,
  author    = {Xiong, Chao and Stolle, Claudia and L{\"u}hr, Hermann and Park, Jaeheung and Fejer, Bela G. and Kervalishvili, Guram N.},
  title     = {Scale analysis of equatorial plasma irregularities derived from Swarm constellation},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1112},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-43184},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-431842},
  pages     = {14},
  year      = {2016},
  abstract  = {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 with longitudinal separation between Swarm satellites larger than 0.4°, no significant correlation was found any more. This result suggests that EPI structures include plasma density scale sizes less than 44 km in the zonal direction. During the Swarm earlier mission phase, clearly better EPI correlations are obtained in the northern hemisphere, implying more fragmented irregularities in the southern hemisphere where the ambient magnetic field is low. The previously reported inverted-C shell structure of EPIs is generally confirmed by the Swarm observations in the northern hemisphere, but with various tilt angles. From the Swarm spacecrafts with zonal separations of about 150 km, we conclude that larger zonal scale sizes of irregularities exist during the early evening hours (around 1900 LT).},
  language  = {en}
}
@article{DzhanoevSpahnYaroshenkoetal.2015,
  author    = {Dzhanoev, Arsen R. and Spahn, Frank and Yaroshenko, Victoriya and L{\"u}hr, Hermann and Schmidt, J{\"u}rgen},
  title     = {Secondary electron emission from surfaces with small structure},
  series = {Physical review : B, Condensed matter and materials physics},
  volume    = {92},
  journal   = {Physical review : B, Condensed matter and materials physics},
  number    = {12},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {1098-0121},
  doi       = {10.1103/PhysRevB.92.125430},
  pages     = {5},
  year      = {2015},
  abstract  = {It is found that for objects possessing small surface structures with differing radii of curvature the secondary electron emission (SEE) yield may be significantly higher than for objects with smooth surfaces of the same material. The effect is highly pronounced for surface structures of nanometer scale, often providing a more than 100\% increase of the SEE yield. The results also show that the SEE yield from surfaces with structure does not show a universal dependence on the energy of the primary, incident electrons as it is found for flat surfaces in experiments. We derive conditions for the applicability of the conventional formulation of SEE using the simplifying assumption of universal dependence. Our analysis provides a basis for studying low-energy electron emission from nanometer structured surfaces under a penetrating electron beam important in many technological applications.},
  language  = {en}
}
@article{ParkLuehrStolleetal.2016,
  author    = {Park, Jaeheung and L{\"u}hr, Hermann and Stolle, Claudia and Rodriguez-Zuluaga, Juan and Knudsen, David J. and Burchill, Johnathan K. and Kwak, Young-Sil},
  title     = {Statistical survey of nighttime midlatitude magnetic fluctuations: Their source location and Poynting flux as derived from the Swarm constellation},
  series = {Journal of geophysical research : Space physics},
  volume    = {121},
  journal   = {Journal of geophysical research : Space physics},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9380},
  doi       = {10.1002/2016JA023408},
  pages     = {11235 -- 11248},
  year      = {2016},
  abstract  = {This is the first statistical survey of field fluctuations related with medium-scale traveling ionospheric disturbances (MSTIDs), which considers magnetic field, electric field, and plasma density variations at the same time. Midlatitude electric fluctuations (MEFs) and midlatitude magnetic fluctuations (MMFs) observed in the nighttime topside ionosphere have generally been attributed to MSTIDs. Although the topic has been studied for several decades, statistical studies of the Poynting flux related with MEF/MMF/MSTID have not yet been conducted. In this study we make use of electric/magnetic field and plasma density observations by the European Space Agency's Swarm constellation to address the statistical behavior of the Poynting flux. We have found that (1) the Poynting flux is directed mainly from the summer to winter hemisphere, (2) its magnitude is larger before midnight than thereafter, and (3) the magnitude is not well correlated with fluctuation level of in situ plasma density. These results are discussed in the context of previous studies.},
  language  = {en}
}
@article{XiongStolleLuehr2016,
  author    = {Xiong, Chao and Stolle, Claudia and L{\"u}hr, Hermann},
  title     = {The Swarm satellite loss of GPS signal and its relation to ionospheric plasma irregularities},
  series = {Space Weather: The International Journal of Research and Applications},
  volume    = {14},
  journal   = {Space Weather: The International Journal of Research and Applications},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {1542-7390},
  doi       = {10.1002/2016SW001439},
  pages     = {563 -- 577},
  year      = {2016},
  abstract  = {In this study we investigated conditions for loss of GPS signals observed by the Swarm satellites during a 2 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 degrees and +/- 20 degrees magnetic latitude, forming two bands along the magnetic equator, and these low-latitude events mainly appear around postsunset hours from 19: 00 to 22: 00 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 x 10(11) m(-3); then, the Swarm satellites encountered for up to 95\% loss of GPS signal for at least one channel and up to 45\% tracked less than four GPS satellites (making precise orbit determination impossible). For those EPIs with density depletions less than 10 x 10(11) m(-3), the chance of tracked GPS signals less than four reduces to only 1.0\%. Swarm also observed total loss 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 Swarm satellites loss of GPS signals.},
  language  = {en}
}