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 - Rodríguez Zuluaga, Juan A1 - Stolle, Claudia A1 - Yamazaki, Yosuke A1 - Xiong, Chao A1 - England, Scott L. T1 - A synoptic-scale wavelike structure in the nighttime equatorial ionization anomaly JF - Earth and Space Science : ESS N2 - Both ground- and satellite-based airglow imaging have significantly contributed to understanding the low-latitude ionosphere, especially the morphology and dynamics of the equatorial ionization anomaly (EIA). The NASA Global-scale Observations of the Limb and Disk (GOLD) mission focuses on far-ultraviolet airglow images from a geostationary orbit at 47.5 degrees W. This region is of particular interest at low magnetic latitudes because of the high magnetic declination (i.e., about -20 degrees) and proximity of the South Atlantic magnetic anomaly. In this study, we characterize an exciting feature of the nighttime EIA using GOLD observations from October 5, 2018 to June 30, 2020. It consists of a wavelike structure of a few thousand kilometers seen as poleward and equatorward displacements of the EIA-crests. Initial analyses show that the synoptic-scale structure is symmetric about the dip equator and appears nearly stationary with time over the night. In quasi-dipole coordinates, maxima poleward displacements of the EIA-crests are seen at about +/- 12 degrees latitude and around 20 and 60 degrees longitude (i.e., in geographic longitude at the dip equator, about 53 degrees W and 14 degrees W). The wavelike structure presents typical zonal wavelengths of about 6.7 x 10(3) km and 3.3 x 10(3) km. The structure's occurrence and wavelength are highly variable on a day-to-day basis with no apparent dependence on geomagnetic activity. In addition, a cluster or quasi-periodic wave train of equatorial plasma depletions (EPDs) is often detected within the synoptic-scale structure. We further outline the difference in observing these EPDs from FUV images and in situ measurements during a GOLD and Swarm mission conjunction. KW - equatorial ionization anomaly KW - equatorial ionosphere KW - equatorial plasma bubbles KW - wave structure KW - forcing from below Y1 - 2021 U6 - https://doi.org/10.1029/2020EA001529 SN - 2333-5084 VL - 8 IS - 2 PB - American Geophysical Union CY - Malden, Mass. ER - TY - JOUR A1 - Sobhkhiz-Miandehi, Sahar A1 - Yamazaki, Yosuke A1 - Arras, Christina A1 - Miyoshi, Yasunobu A1 - Shinagawa, Hiroyuki T1 - Comparison of the tidal signatures in sporadic E and vertical ion convergence rate, using FORMOSAT-3/COSMIC radio occultation observations and GAIA model JF - Earth, planets and space : EPS N2 - Sporadic E or Es is a transient phenomenon where thin layers of enhanced electron density appear in the ionospheric E region (90-120 km altitude). The neutral wind shear caused by atmospheric tides can lead ions to converge vertically at E-region heights and form the Es layer. This research aims to determine the role of atmospheric solar and lunar tides in Es occurrence. For this purpose, radio occultation data of FORMOSAT-3/COSMIC have been used, which provide complete global coverage of Es events. Moreover, GAIA model simulations have been employed to evaluate the vertical ion convergence induced by solar tides. The results show both migrating and non-migrating solar tidal signatures and the semidiurnal migrating lunar tidal signature mainly in low and mid-latitude Es occurrence. The seasonal variation of the migrating solar tidal components of Es is in good agreement with those in the vertical ion convergence derived from GAIA at higher altitudes. Furthermore, some non-migrating components of solar tides, including semidiurnal westward wavenumbers 1 and 3 and diurnal eastward wavenumbers 2 and 3, also significantly affect the Es occurrence rate. KW - Sporadic E KW - Es KW - wind shear KW - solar tide KW - lunar tide KW - GAIA KW - radio occultation Y1 - 2022 U6 - https://doi.org/10.1186/s40623-022-01637-y SN - 1880-5981 VL - 74 IS - 1 PB - Springer CY - Heidelberg 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 - Matzka, Jürgen A1 - Stolle, Claudia A1 - Yamazaki, Yosuke A1 - Bronkalla, Oliver A1 - Morschhauser, Achim T1 - The geomagnetic Kp index and derived indices of geomagnetic activity JF - Space weather : the international journal of research and applications N2 - The geomagnetic Kp index is one of the most extensively used indices of geomagnetic activity, both for scientific and operational purposes. This article reviews the properties of the Kp index and provides a reference for users of the Kp index and associated data products as derived and distributed by the GFZ German Research Centre for Geosciences. The near real-time production of the nowcast Kp index is of particular interest for space weather services and here we describe and evaluate its current setup. Y1 - 2021 U6 - https://doi.org/10.1029/2020SW002641 SN - 1542-7390 VL - 19 IS - 5 PB - Wiley CY - New York ER - TY - JOUR A1 - Soares, Gabriel Brando A1 - Yamazaki, Yosuke A1 - Cnossen, Ingrid A1 - Matzka, Jürgen A1 - Pinheiro, Katia J. A1 - Morschhauser, Achim A1 - Alken, Patrick A1 - Stolle, Claudia T1 - Evolution of the geomagnetic daily variation at Tatuoca, Brazil, From 1957 to 2019 BT - a transition from Sq to EEJ JF - Journal of geophysical research : Space physics N2 - The magnetic equator in the Brazilian region has moved over 1,100 km northward since 1957, passing the geomagnetic observatory Tatuoca (TTB), in northern Brazil, around 2013. We recovered and processed TTB hourly mean values of the geomagnetic field horizontal (H) component from 1957 until 2019, allowing the investigation of long-term changes in the daily variation due to the influence of secular variation, solar activity, season, and lunar phase. The H day-to-day variability and the occurrence of the counter electrojet at TTB were also investigated. Until the 1990s, ionospheric solar quiet currents dominated the quiet-time daily variation at TTB. After 2000, the magnitude of the daily variation became appreciably greater due to the equatorial electrojet (EEJ) contribution. The H seasonal and day-to-day variability increased as the magnetic equator approached, but their amplitudes normalized to the average daily variation remained at similar levels. Meanwhile, the amplitude of the lunar variation, normalized in the same way, increased from 5% to 12%. Within the EEJ region, the occurrence rate of the morning counter electrojet (MCEJ) increased with proximity to the magnetic equator, while the afternoon counter electrojet (ACEJ) did not. EEJ currents derived from CHAMP and Swarm satellite data revealed that the MCEJ rate varies with magnetic latitude within the EEJ region while the ACEJ rate is largely constant. Simulations with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model based on different geomagnetic main field configurations suggest that long-term changes in the geomagnetic daily variation at TTB can be attributed to the main field secular variation. Y1 - 2020 U6 - https://doi.org/10.1029/2020JA028109 SN - 2169-9380 SN - 2169-9402 VL - 125 IS - 9 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Park, Jaeheung A1 - Stolle, Claudia A1 - Yamazaki, Yosuke A1 - Rauberg, Jan A1 - Michaelis, Ingo A1 - Olsen, Nils T1 - Diagnosing low-/mid-latitude ionospheric currents using platform magnetometers BT - CryoSat-2 and GRACE-FO JF - Earth, planets and space N2 - Electric currents flowing in the terrestrial ionosphere have conventionally been diagnosed by low-earth-orbit (LEO) satellites equipped with science-grade magnetometers and long booms on magnetically clean satellites. In recent years, there are a variety of endeavors to incorporate platform magnetometers, which are initially designed for navigation purposes, to study ionospheric currents. Because of the suboptimal resolution and significant noise of the platform magnetometers, however, most of the studies were confined to high-latitude auroral regions, where magnetic field deflections from ionospheric currents easily exceed 100 nT. This study aims to demonstrate the possibility of diagnosing weak low-/mid-latitude ionospheric currents based on platform magnetometers. We use navigation magnetometer data from two satellites, CryoSat-2 and the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), both of which have been intensively calibrated based on housekeeping data and a high-precision geomagnetic field model. Analyses based on 8 years of CryoSat-2 data as well as similar to 1.5 years of GRACE-FO data reproduce well-known climatology of inter-hemispheric field-aligned currents (IHFACs), as reported by previous satellite missions dedicated to precise magnetic observations. Also, our results show that C-shaped structures appearing in noontime IHFAC distributions conform to the shape of the South Atlantic Anomaly. The F-region dynamo currents are only partially identified in the platform magnetometer data, possibly because the currents are weaker than IHFACs in general and depend significantly on altitude and solar activity. Still, this study evidences noontime F-region dynamo currents at the highest altitude (717 km) ever reported. We expect that further data accumulation from continuously operating missions may reveal the dynamo currents more clearly during the next solar maximum. KW - Platform magnetometers KW - CryoSat-2 KW - GRACE-FO KW - Inter-hemispheric KW - field-aligned currents KW - F-region dynamo currents Y1 - 2020 U6 - https://doi.org/10.1186/s40623-020-01274-3 SN - 1343-8832 SN - 1880-5981 VL - 72 IS - 1 PB - Springer CY - New York 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 - Siddiqui, Tarique Adnan A1 - Yamazaki, Yosuke A1 - Stolle, Claudia A1 - Lühr, Hermann A1 - Matzka, Jürgen A1 - Maute, Astrid A1 - Pedatella, Nicholas T1 - Dependence of Lunar Tide of the Equatorial Electrojet on the Wintertime Polar Vortex, Solar Flux, and QBO JF - Geophysical research letters N2 - 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. KW - lunar tide KW - equatorial electrojet KW - vertical coupling KW - SSW KW - QBO KW - planetary waves Y1 - 2018 U6 - https://doi.org/10.1029/2018GL077510 SN - 0094-8276 SN - 1944-8007 VL - 45 IS - 9 SP - 3801 EP - 3810 PB - American Geophysical Union CY - Washington ER -