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 - Yamazaki, Yosuke A1 - Wendt, Vivien A1 - Miyoshi, Y. A1 - Stolle, Claudia A1 - Siddiqui, Tarique Adnan A1 - Kervalishvili, Guram N. A1 - Laštovička, J. A1 - Kozubek, M. A1 - Ward, W. A1 - Themens, D. R. A1 - Kristoffersen, S. A1 - Alken, Patrick T1 - September 2019 Antarctic sudden stratospheric warming BT - Quasi-6-Day wave burst and ionospheric effects JF - Geophysical Research Letters N2 - An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. Ionospheric data from European Space Agency's Swarm satellite constellation mission show prominent 6-day variations in the dayside low-latitude region at this time, which can be attributed to forcing from the middle atmosphere by the Rossby normal mode "quasi-6-day wave" (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard National Aeronautics and Space Administration's Aura satellite reveal a burst of global Q6DW activity in the mesosphere and lower thermosphere during the SSW, which is one of the strongest in the record. The Q6DW is apparently generated in the polar stratosphere at 30-40 km, where the atmosphere is unstable due to strong vertical wind shear connected with planetary wave breaking. These results suggest that an Antarctic SSW can lead to ionospheric variability through wave forcing from the middle atmosphere. Plain Language Summary: A sudden stratospheric warming (SSW) is an extreme wintertime polar meteorological phenomenon occurring mostly over the Arctic region. Studies have shown that Arctic SSW can influence the entire atmosphere. In September 2019, a rare SSW event occurred in the Antarctic region, providing an opportunity to investigate its broader impact on the whole atmosphere. We present observations from the middle atmosphere and ionosphere during this event, noting unusually strong wave activity throughout this region. Our results suggest that an Antarctic SSW can have a significant impact on the whole atmosphere system similar to those due to Arctic events. KW - Rossby-normal modes KW - nonumiform background configuration KW - total electron-content KW - large-scale KW - planetary-waves KW - 5-day waves KW - equatorial electrojet KW - lower thermosphere KW - symmetric modes KW - 6.5-Day wave Y1 - 2020 U6 - https://doi.org/10.1029/2019GL086577 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 1 SP - 1 EP - 12 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Yamazaki, Yosuke A1 - Stolle, Claudia A1 - Matzka, Jürgen A1 - Siddiqui, Tarique Adnan A1 - Luehr, Hermann A1 - Alken, Patrick T1 - Longitudinal Variation of the Lunar Tide in the Equatorial Electrojet JF - Journal of geophysical research : Space physics N2 - The atmospheric lunar tide is one known source of ionospheric variability. The subject received renewed attention as recent studies found a link between stratospheric sudden warmings and amplified lunar tidal perturbations in the equatorial ionosphere. There is increasing evidence from ground observations that the lunar tidal influence on the ionosphere depends on longitude. We use magnetic field measurements from the CHAMP satellite during July 2000 to September 2010 and from the two Swarm satellites during November 2013 to February 2017 to determine, for the first time, the complete seasonal- longitudinal climatology of the semidiurnal lunar tidal variation in the equatorial electrojet intensity. Significant longitudinal variability is found in the amplitude of the lunar tidal variation, while the longitudinal variability in the phase is small. The amplitude peaks in the Peruvian sector (similar to 285 degrees E) during the Northern Hemisphere winter and equinoxes, and in the Brazilian sector (similar to 325 degrees E) during the Northern Hemisphere summer. There are also local amplitude maxima at similar to 55 degrees E and similar to 120 degrees E. The longitudinal variation is partly due to the modulation of ionospheric conductivities by the inhomogeneous geomagnetic field. Another possible cause of the longitudinal variability is neutral wind forcing by nonmigrating lunar tides. A tidal spectrum analysis of the semidiurnal lunar tidal variation in the equatorial electrojet reveals the dominance of the westward propagating mode with zonal wave number 2 (SW2), with secondary contributions by westward propagating modes with zonal wave numbers 3 (SW3) and 4 (SW4). Eastward propagating waves are largely absent from the tidal spectrum. Further study will be required for the relative importance of ionospheric conductivities and nonmigrating lunar tides. Y1 - 2017 U6 - https://doi.org/10.1002/2017JA024601 SN - 2169-9380 SN - 2169-9402 VL - 122 SP - 12445 EP - 12463 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Yamazaki, Yosuke A1 - Stolle, Claudia A1 - Matzka, Jürgen A1 - Liu, Huixin A1 - Tao, Chihiro T1 - Interannual variability of the daytime equatorial ionospheric electric field JF - Journal of geophysical research : Space physics N2 - Understanding the variability of the ionosphere is important for the prediction of space weather and climate. Recent studies have shown that forcing from the lower atmosphere plays a significant role for the short-term (day-to-day) variability of the low-latitude ionosphere. The present study aims to assess the importance of atmospheric forcing for the variability of the daytime equatorial ionospheric electric field on the interannual (year-to-year) time scale. Magnetic field measurements from Huancayo (12.05 degrees S, 75.33 degrees W) are used to augment the equatorial vertical plasma drift velocity (V-Z) measurements from the Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere radar during 2001-2016. V-Z can be regarded as a measure of the zonal electric field. After removing the seasonal variation of similar to 10m/s, midday values of V-Z show an interannual variation of similar to 2m/s with an oscillation period of 2-3years. No evidence of solar cycle influence is found. The Ground-to-topside Atmosphere-Ionosphere model for Aeronomy, which takes into account realistic atmospheric variability below 30km, reproduces the pattern of the observed interannual variation without having to include variable forcing from the magnetosphere. The results indicate that lower atmospheric forcing plays a dominant role for the observed interannual variability of V-Z at 1200 local time. KW - interannual variability KW - plasma drifts KW - equatorial electrojet (EEJ) KW - vertical coupling KW - JULIA KW - GAIA Y1 - 2018 U6 - https://doi.org/10.1029/2017JA025165 SN - 2169-9380 SN - 2169-9402 VL - 123 IS - 5 SP - 4241 EP - 4256 PB - American Geophysical Union CY - Washington 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 - Yamazaki, Yosuke A1 - Matzka, Jürgen A1 - Stolle, Claudia A1 - Kervalishvili, Guram N. A1 - Rauberg, Jan A1 - Bronkalla, Oliver A1 - Morschhauser, Achim A1 - Bruinsma, Sean L. A1 - Shprits, Yuri Y. A1 - Jackson, David R. T1 - Geomagnetic activity index Hpo JF - Geophysical research letters N2 - The geomagnetic activity index Kp is widely used but is restricted by low time resolution (3-hourly) and an upper limit. To address this, new geomagnetic activity indices, Hpo, are introduced. Similar to Kp, Hpo expresses the level of planetary geomagnetic activity in units of thirds (0o, 0+, 1-, 1o, 1+, 2-, horizontal ellipsis ) based on the magnitude of geomagnetic disturbances observed at subauroral observatories. Hpo has a higher time resolution than Kp. 30-min (Hp30) and 60-min (Hp60) indices are produced. The frequency distribution of Hpo is designed to be similar to that of Kp so that Hpo may be used as a higher time-resolution alternative to Kp. Unlike Kp, which is capped at 9o, Hpo is an open-ended index and thus can characterize severe geomagnetic storms more accurately. Hp30, Hp60 and corresponding linearly scaled ap30 and ap60 are available, in near real time, at the GFZ website (https://www.gfz-potsdam.de/en/hpo-index). KW - Hpo KW - Hp30 KW - Hp60 KW - apo KW - ap30 KW - ap60 Y1 - 2022 U6 - https://doi.org/10.1029/2022GL098860 SN - 0094-8276 SN - 1944-8007 VL - 49 IS - 10 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Xiong, Chao A1 - Stolle, Claudia A1 - Park, Jaeheung T1 - Climatology of GPS signal loss observed by Swarm satellites JF - Annales geophysicae N2 - 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. KW - Ionosphere KW - equatorial ionosphere KW - ionospheric irregularities KW - radio science KW - radio wave propagation Y1 - 2018 U6 - https://doi.org/10.5194/angeo-36-679-2018 SN - 0992-7689 SN - 1432-0576 VL - 36 IS - 2 SP - 679 EP - 693 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Xiong, Chao A1 - Stolle, Claudia A1 - Lühr, Hermann T1 - The Swarm satellite loss of GPS signal and its relation to ionospheric plasma irregularities JF - Space Weather: The International Journal of Research and Applications N2 - 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. Y1 - 2016 U6 - https://doi.org/10.1002/2016SW001439 SN - 1542-7390 VL - 14 SP - 563 EP - 577 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 - Xiong, Chao A1 - Stolle, Claudia A1 - Alken, Patrick A1 - Rauberg, Jan T1 - Relationship between large-scale ionospheric field-aligned currents and electron/ion precipitations BT - DMSP observations JF - Earth, planets and space N2 - In this study, we have derived field-aligned currents (FACs) from magnetometers onboard the Defense Meteorological Satellite Project (DMSP) satellites. The magnetic latitude versus local time distribution of FACs from DMSP shows comparable dependences with previous findings on the intensity and orientation of interplanetary magnetic field (IMF)B(y)andB(z)components, which confirms the reliability of DMSP FAC data set. With simultaneous measurements of precipitating particles from DMSP, we further investigate the relation between large-scale FACs and precipitating particles. Our result shows that precipitation electron and ion fluxes both increase in magnitude and extend to lower latitude for enhanced southward IMFBz, which is similar to the behavior of FACs. Under weak northward and southwardB(z)conditions, the locations of the R2 current maxima, at both dusk and dawn sides and in both hemispheres, are found to be close to the maxima of the particle energy fluxes; while for the same IMF conditions, R1 currents are displaced further to the respective particle flux peaks. Largest displacement (about 3.5 degrees) is found between the downward R1 current and ion flux peak at the dawn side. Our results suggest that there exists systematic differences in locations of electron/ion precipitation and large-scale upward/downward FACs. As outlined by the statistical mean of these two parameters, the FAC peaks enclose the particle energy flux peaks in an auroral band at both dusk and dawn sides. Our comparisons also found that particle precipitation at dawn and dusk and in both hemispheres maximizes near the mean R2 current peaks. The particle precipitation flux maxima closer to the R1 current peaks are lower in magnitude. This is opposite to the known feature that R1 currents are on average stronger than R2 currents. KW - field-aligned currents KW - aurora KW - particle precipitation KW - DMSP Y1 - 2020 U6 - https://doi.org/10.1186/s40623-020-01286-z SN - 1880-5981 VL - 72 IS - 1 PB - Springer CY - New York ER -