TY - JOUR A1 - Zhelayskaya, Irina S. A1 - Vasile, Ruggero A1 - Shprits, Yuri Y. A1 - Stolle, Claudia A1 - Matzka, Jürgen T1 - Systematic Analysis of Machine Learning and Feature Selection Techniques for Prediction of the Kp Index JF - Space Weather: The International Journal of Research and Applications N2 - 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. KW - Kp index KW - Predictive models KW - Feature selection KW - Machine learning KW - Validation Y1 - 2019 U6 - https://doi.org/10.1029/2019SW002271 SN - 1542-7390 VL - 17 IS - 10 SP - 1461 EP - 1486 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Park, Jaeheung A1 - Lühr, Hermann A1 - Stolle, Claudia A1 - Rodriguez-Zuluaga, Juan A1 - Knudsen, David J. A1 - Burchill, Johnathan K. A1 - Kwak, Young-Sil T1 - Statistical survey of nighttime midlatitude magnetic fluctuations: Their source location and Poynting flux as derived from the Swarm constellation JF - Journal of geophysical research : Space physics N2 - 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. Y1 - 2016 U6 - https://doi.org/10.1002/2016JA023408 SN - 2169-9380 SN - 2169-9402 VL - 121 SP - 11235 EP - 11248 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Xiong, Chao A1 - Stolle, Claudia A1 - Luehr, Hermann A1 - Park, Jaeheung A1 - Fejer, Bela G. A1 - Kervalishvili, Guram N. T1 - Scale analysis of equatorial plasma irregularities derived from Swarm constellation JF - Earth, planets and space N2 - 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 degrees, 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). KW - Equatorial plasma irregularities KW - Ionospheric scale lengths KW - Swarm constellation Y1 - 2016 U6 - https://doi.org/10.1186/s40623-016-0502-5 SN - 1880-5981 VL - 68 SP - 189 EP - 202 PB - Springer CY - Heidelberg ER - TY - JOUR A1 - Siddiqui, Tarique Adnan A1 - Luehr, H. A1 - Stolle, Claudia A1 - Park, J. T1 - Relation between stratospheric sudden warming and the lunar effect on the equatorial electrojet based on Huancayo recordings JF - Annales geophysicae N2 - 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. KW - Ionosphere KW - electric fields and currents KW - meteorology and atmospheric dynamics KW - waves and tides Y1 - 2015 U6 - https://doi.org/10.5194/angeo-33-235-2015 SN - 0992-7689 SN - 1432-0576 VL - 33 IS - 2 SP - 235 EP - 243 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Yamazaki, Yosuke A1 - Stolle, Claudia A1 - Matzka, Jürgen A1 - Alken, Patrick T1 - Quasi-6-Day Wave Modulation of the Equatorial Electrojet JF - Journal of geophysical research : Space physics N2 - The equatorial electrojet is an enhanced eastward current in the dayside E region ionosphere flowing along the magnetic equator. The equatorial electrojet is highly variable as it is subject to various forcing mechanisms including atmospheric waves from the lower layers of the atmosphere. There are occasionally times when the intensity of the equatorial electrojet at a fixed longitude shows an oscillatory variation with a period of approximately 6days. We present case studies of such events based on the equatorial electrojet measurements from the CHAMP and Swarm satellites. The spatial and temporal variability of the equatorial electrojet intensity during these events reveals characteristics of a westward propagating wave with zonal wavenumber 1, consistent with the effect of the quasi-6-day planetary wave. Analyses of the geopotential height data from the Aura satellite confirm the presence of the quasi-6-day planetary wave in the lower thermosphere during the events. The amplitude of the quasi-6-day variation in the equatorial electrojet intensity depends on longitude, but no systematic longitudinal dependence is found for different events. During the event of August 2010, quasi-6-day variations are also observed by ground-based magnetometers and a radar in the Peruvian sector. The effect of the quasi-6-day wave accounts for up to +/- 5.9m/s in the equatorial vertical plasma velocity at noon, which is much larger than previously predicted by a numerical model. These results suggest that the quasi-6-day planetary wave is an important source of short-term variability in the equatorial ionosphere. Y1 - 2018 U6 - https://doi.org/10.1029/2018JA025365 SN - 2169-9380 SN - 2169-9402 VL - 123 IS - 5 SP - 4094 EP - 4109 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Rodriguez-Zuluaga, Juan A1 - Stolle, Claudia A1 - Park, J. T1 - On the direction of the Poynting flux associated with equatorial plasma depletions as derived from Swarm JF - Geophysical research letters N2 - Magnetic and electric field observations from the European Space Agency Swarm mission are used to report the direction of electromagnetic energy flux associated with equatorial plasma depletions. Contrary to expectations, the observations suggest a general interhemispheric Poynting flux rather than concurrent flows at both hemispheres toward or away from the equator. Of high interest is a particular behavior noticed over the region with the largest variation in the magnetic declination. This is a Poynting flux flowing mainly into the southern magnetic hemisphere about between 60 degrees W and 30 degrees E and into the northern magnetic hemisphere between 110 degrees W and 60 degrees W. The abrupt change in the flow direction at 60 degrees W is suggested to be caused by an asymmetry between the hemispheres on the ionospheric conductivity, likely due to the influence of thermospheric winds and the presence of the South Atlantic Anomaly. Y1 - 2017 U6 - https://doi.org/10.1002/2017GL073385 SN - 0094-8276 SN - 1944-8007 VL - 44 SP - 5884 EP - 5891 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Park, Jaeheung A1 - Lühr, Hermann A1 - Kervalishvili, Guram N. A1 - Rauberg, Jan A1 - Michaelis, Ingo A1 - Stolle, Claudia A1 - Kwak, Young-Sil T1 - 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 JF - Journal of geophysical research : Space physics N2 - 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. KW - midlatitude nighttime magnetic fluctuation KW - nighttime MSTID KW - Swarm constellation Y1 - 2015 U6 - https://doi.org/10.1002/2015JA021315 SN - 2169-9380 SN - 2169-9402 VL - 120 IS - 8 SP - 6818 EP - 6830 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Prokhorov, Boris E. A1 - Förster, Matthias A1 - Lesur, Vincent A1 - Namgaladze, Alexander A. A1 - Holschneider, Matthias A1 - Stolle, Claudia T1 - Modeling of the ionospheric current system and calculating its JF - Magnetic Fields in the Solar System: Planets, Moons and Solar Wind Interactions N2 - The additional magnetic field produced by the ionospheric current system is a part of the Earth’s magnetic field. This current system is a highly variable part of a global electric circuit. The solar wind and interplanetary magnetic field (IMF) interaction with the Earth’s magnetosphere is the external driver for the global electric circuit in the ionosphere. The energy is transferred via the field-aligned currents (FACs) to the Earth’s ionosphere. The interactions between the neutral and charged particles in the ionosphere lead to the so-called thermospheric neutral wind dynamo which represents the second important driver for the global current system. Both processes are components of the magnetosphere–ionosphere–thermosphere (MIT) system, which depends on solar and geomagnetic conditions, and have significant seasonal and UT variations. The modeling of the global dynamic Earth’s ionospheric current system is the first aim of this investigation. For our study, we use the Potsdam version of the Upper Atmosphere Model (UAM-P). The UAM is a first-principle, time-dependent, and fully self-consistent numerical global model. The model includes the thermosphere, ionosphere, plasmasphere, and inner magnetosphere as well as the electrodynamics of the coupled MIT system for the altitudinal range from 80 (60) km up to the 15 Earth radii. The UAM-P differs from the UAM by a new electric field block. For this study, the lower latitudinal and equatorial electrodynamics of the UAM-P model was improved. The calculation of the ionospheric current system’s contribution to the Earth’s magnetic field is the second aim of this study. We present the method, which allows computing the additional magnetic field inside and outside the current layer as generated by the space current density distribution using the Biot-Savart law. Additionally, we perform a comparison of the additional magnetic field calculation using 2D (equivalent currents) and 3D current distribution. Y1 - 2018 SN - 978-3-319-64292-5 SN - 978-3-319-64291-8 U6 - https://doi.org/10.1007/978-3-319-64292-5_10 SN - 0067-0057 SN - 2214-7985 VL - 448 SP - 263 EP - 292 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Soares, Gabriel A1 - Yamazaki, Yosuke A1 - Matzka, Jürgen A1 - Pinheiro, Katia A1 - Stolle, Claudia A1 - Alken, Patrick A1 - Yoshikawa, Akimasa A1 - Uozumi, Teiji A1 - Fujimoto, Akiko A1 - Kulkarni, Atul T1 - Longitudinal variability of the equatorial counter electrojet during the solar cycle 24 JF - Studia geophysica et geodaetica N2 - 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. KW - geomagnetism KW - equatorial ionosphere KW - geomagnetic observatories KW - satellite data Y1 - 2019 U6 - https://doi.org/10.1007/s11200-018-0286-0 SN - 0039-3169 SN - 1573-1626 VL - 63 IS - 2 SP - 304 EP - 319 PB - Springer CY - New York ER - TY - JOUR A1 - Siddiqui, Tarique Adnan A1 - Stolle, Claudia A1 - Lühr, Hermann T1 - Longitude-dependent lunar tidal modulation of the equatorial electrojet during stratospheric sudden warmings JF - Journal of geophysical research : Space physics N2 - 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. KW - SSW KW - vertical coupling KW - equatorial electrojet KW - lunar tide of EEJ Y1 - 2017 U6 - https://doi.org/10.1002/2016JA023609 SN - 2169-9380 SN - 2169-9402 VL - 122 IS - 3 SP - 3760 EP - 3776 PB - American Geophysical Union CY - Washington ER -