@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{PickKorteThomasetal.2019,
  author    = {Pick, Leonie and Korte, Monika and Thomas, Yannik and Krivova, Natalie and Wu, Chi-Ju},
  title     = {Evolution of Large-Scale Magnetic Fields From Near-Earth Space During the Last 11 Solar Cycles},
  series = {Journal of Geophysical Research: Space Physics},
  journal   = {Journal of Geophysical Research: Space Physics},
  publisher = {Union},
  address   = {Washington, DC},
  issn      = {2169-9402},
  doi       = {10.1029/2018JA026185},
  pages     = {2527 -- 2540},
  year      = {2019},
  abstract  = {We use hourly mean magnetic field measurements from 34 midlatitude geomagnetic observatories between 1900 and 2015 to investigate the long-term evolution and driving mechanism of the large-scale external magnetic field at ground. The Hourly Magnetospheric Currents index (HMC) is derived as a refinement of the Annual Magnetospheric Currents index (HMC, Pick \& Korte, 2017, https://doi.org/10.1093/gji/ggx367). HMC requires an extensive revision of the observatory hourly means. It depends on three third party geomagnetic field models used to eliminate the core, the crustal, and the ionospheric solar-quiet field contributions. We mitigate the dependency of HMC on the core field model by subtracting only nondipolar components of the model from the data. The separation of the residual (dipolar) signal into internal and external (HMC) parts is the main methodological challenge. Observatory crustal biases are updated with respect to AMC, and the solar-quiet field estimation is extended to the past based on a reconstruction of solar radio flux (F10.7). We find that HMC has more power at low frequencies (periods = 1 year) than the Dcx index, especially at periods relevant to the solar cycle. Most of the slow variations in HMC can be explained by the open solar magnetic flux. There is a weakly decreasing linear trend in absolute HMC from 1900 to present, which depends sensitively on the data rejection criteria at early years. HMC is well suited for studying long-term variations of the geomagnetic field.},
  language  = {en}
}