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 - Rodriguez-Zuluaga, Juan A1 - Stolle, Claudia A1 - Yamazaki, Yosuke A1 - Lühr, H. A1 - Park, J. A1 - Scherliess, L. A1 - Chau, J. L. T1 - On the balance between plasma and magnetic pressure across equatorial plasma depletions JF - Journal of geophysical research : Space physics N2 - In magnetized plasmas such as the ionosphere, electric currents develop in regions of strong density gradients to balance the resulting plasma pressure gradients. These currents, usually known as diamagnetic currents decrease the magnetic pressure where the plasma pressure increases, and vice versa. In the low‐latitude ionosphere, equatorial plasma depletions (EPDs) are well known for their steep plasma density gradients and adverse effect on radio wave propagation. In this paper, we use continuous measurements of the magnetic field and electron density from the European Space Agency's Swarm constellation mission to assess the balance between plasma and magnetic pressure across large‐scale EPDs. The analysis is based on the magnetic fluctuations related to diamagnetic currents flowing at the edges of EPDs. This study shows that most of the EPDs detected by Swarm present a decrease of the plasma pressure relative to the ambient plasma. However, EPDs with high plasma pressure are also identified mainly in the vicinity of the South Atlantic magnetic anomaly. From the electron density measurements, we deduce that such an increase in plasma pressure within EPDs might be possible by temperatures inside the EPD as high as twice the temperature of the ambient plasma. Due to the distinct location of the high‐pressure EPDs, we suggest that a possible heating mechanism might be due to precipitation of particle from the radiation belts. This finding corresponds to the first observational evidence of plasma pressure enhancements in regions of depleted plasma density in the ionosphere. KW - equatorial plasma depletions KW - spread F KW - plasma pressure KW - magnetic pressure KW - diamagnetic currents Y1 - 2019 U6 - https://doi.org/10.1029/2019JA026700 SN - 2169-9402 VL - 124 IS - 7 SP - 5936 EP - 5944 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 - 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 -