@article{LuoXiongGuetal.2019,
  author    = {Luo, Xiaomin and Xiong, Chao and Gu, Shengfeng and Lou, Yidong and Stolle, Claudia and Wan, Xin and Liu, Kangkang and Song, Weiwei},
  title     = {Geomagnetically conjugate observations of equatorial plasma irregularities from swarm constellation and ground-based GPS stations},
  series = {Journal of geophysical research: Space physics},
  volume    = {124},
  journal   = {Journal of geophysical research: Space physics},
  number    = {5},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {2169-9380},
  doi       = {10.1029/2019JA026515},
  pages     = {3650 -- 3665},
  year      = {2019},
  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.},
  language  = {en}
}
@article{RodriguezZuluagaStolle2019,
  author    = {Rodriguez-Zuluaga, Juan and Stolle, Claudia},
  title     = {Interhemispheric field-aligned currents at the edges of equatorial plasma depletions},
  series = {Scientific reports},
  volume    = {9},
  journal   = {Scientific reports},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2045-2322},
  doi       = {10.1038/s41598-018-37955-z},
  pages     = {8},
  year      = {2019},
  abstract  = {A comprehensive description of electromagnetic processes related to equatorial plasma depletions (EPDs) is essential for understanding their evolution and day-to-day variability. Recently, field-aligned currents (FACs) flowing at both western and eastern edges of EPDs were observed to be interhemispheric rather than anti-parallel about the dip equator, as suggested by previous theoretical studies. In this paper, we investigate the spatial and temporal variability of the FACs orientation using simultaneous measurements of electron density and magnetic field gathered by ESA's Swarm constellation mission. By using empirical models, we assess the role of the Pedersen conductance in the preference of the FACs to close either in the northern or southern magnetic hemisphere. Here we show that the closure of the FACs agrees with an electrostatic regime determined by a hemispherical asymmetry of the Pedersen conductance. That is, the EPD-related FACs close at lower altitudes in the hemisphere with the highest conductivity. The evidence of this conclusion stands on the general agreement between the longitudinal and seasonal variability of both the conductivity and the FACs orientation.},
  language  = {en}
}
@article{SoaresYamazakiMatzkaetal.2019,
  author    = {Soares, Gabriel and Yamazaki, Yosuke and Matzka, J{\"u}rgen and Pinheiro, Katia and Stolle, Claudia and Alken, Patrick and Yoshikawa, Akimasa and Uozumi, Teiji and Fujimoto, Akiko and Kulkarni, Atul},
  title     = {Longitudinal variability of the equatorial counter electrojet during the solar cycle 24},
  series = {Studia geophysica et geodaetica},
  volume    = {63},
  journal   = {Studia geophysica et geodaetica},
  number    = {2},
  publisher = {Springer},
  address   = {New York},
  issn      = {0039-3169},
  doi       = {10.1007/s11200-018-0286-0},
  pages     = {304 -- 319},
  year      = {2019},
  abstract  = {Ground and space-based geomagnetic data were used in the investigation of the longitudinal, seasonal and lunar phase dependence of the equatorial counter electrojet (CEJ) occurrence in the Peruvian, Brazilian, African, Indian and Philippine sectors during geomagnetically quiet days from the solar cycle 24 (2008 to 2018). We found that CEJ events occur more frequently during the morning (MCEJ) than in the afternoon (ACEJ). The highest MCEJ and ACEJ occurrence rates were observed for the Brazilian sector. Distinct seasonal dependence was found for each longitudinal sector under investigation. The lunar phase dependence was determined for the first time for the Philippine sector (longitude 125 degrees E), and it was shown to be less pronounced than in the Peruvian, Brazilian and African sectors. We demonstrate that differences in CEJ rates derived from ground-based and satellite data can arise from the longitudinal separation between low-latitude and equatorial stations that are used to determine the signal and its consequent time delay in their sunrise/sunset times at ionospheric heights.},
  language  = {en}
}
@article{ZhelayskayaVasileShpritsetal.2019,
  author    = {Zhelayskaya, Irina S. and Vasile, Ruggero and Shprits, Yuri Y. and Stolle, Claudia and Matzka, J{\"u}rgen},
  title     = {Systematic Analysis of Machine Learning and Feature Selection Techniques for Prediction of the Kp Index},
  series = {Space Weather: The International Journal of Research and Applications},
  volume    = {17},
  journal   = {Space Weather: The International Journal of Research and Applications},
  number    = {10},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {1542-7390},
  doi       = {10.1029/2019SW002271},
  pages     = {1461 -- 1486},
  year      = {2019},
  abstract  = {The Kp index is a measure of the midlatitude global geomagnetic activity and represents short-term magnetic variations driven by solar wind plasma and interplanetary magnetic field. The Kp index is one of the most widely used indicators for space weather alerts and serves as input to various models, such as for the thermosphere and the radiation belts. It is therefore crucial to predict the Kp index accurately. Previous work in this area has mostly employed artificial neural networks to nowcast Kp, based their inferences on the recent history of Kp and on solar wind measurements at L1. In this study, we systematically test how different machine learning techniques perform on the task of nowcasting and forecasting Kp for prediction horizons of up to 12 hr. Additionally, we investigate different methods of machine learning and information theory for selecting the optimal inputs to a predictive model. We illustrate how these methods can be applied to select the most important inputs to a predictive model of Kp and to significantly reduce input dimensionality. We compare our best performing models based on a reduced set of optimal inputs with the existing models of Kp, using different test intervals, and show how this selection can affect model performance.},
  language  = {en}
}