Browsing by Author "Yumoto, K."
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Item Restricted Can a nightside geomagnetic Delta H observed at the equator manifest a penetration electric field?(American Geophysical Union, 2013-02-12) Wei, Y.; Fraenz, M.; Dubinin, E.; He, M.; Ren, Z.; Zhao, B.; Liu, J.; Wan, W.; Yumoto, K.; Watari, S.; Alex, S.A prompt penetration electric field (PPEF) usually manifests itself in the form of an equatorial ionospheric electric field being in correlation with a solar wind electric field. Due to the strong Cowling conductivity, a PPEF on the dayside can be inferred from Delta H (ΔH), which is the difference in the magnitudes of the horizontal (H) component between a magnetometer at the magnetic equator and one off the equator. This paper aims to investigate the performance of ΔH in response to a PPEF on the nightside, where the Cowling conductivity is not significant. We first examine the strongest geomagnetically active time during the 20 November 2003 superstorm when the Dst drops to −473 nT and show that the nightside ΔH can indeed manifest a PPEF but with local time dependence and longitude dependence. We then examine a moderately active time by taking advantage of the multiple‐penetration event during 11–16 November 2003 when the Dst remains greater than −60 nT. During this event, a series of PPEF pulses recorded in Peru, Japan, and India form a database, allowing us to examine PPEF effects at different local times and longitudes. The results show that (1) the nightside ΔH was caused by attenuation of the effects of the polar electric field with decreasing latitude; (2) the nightside ΔH can manifest a PPEF at least in the midnight‐dawn sector (0000–0500 LT), but not always; and (3) the magnitude of the nightside ΔH in the midnight‐dawn sector in Peru is on average only 1/18 of that of the dayside ΔH in response to a given PPEF.Item Restricted Lunar-dependent equatorial ionospheric electrodynamic effects during sudden stratospheric warmings(American Geophysical Union, 2010) Fejer, B. G.; Olson, M. E.; Chau Chong Shing, Jorge Luis; Stolle, C.; Lühr, H.; Goncharenko, L. P.; Yumoto, K.; Nagatsuma, T.We have used plasma drift and magnetic field measurements during the 2001–2009 December solstices to study, for the first time, the longitudinal dependence of equatorial ionospheric electrodynamic perturbations during sudden stratospheric warmings. Jicamarca radar measurements during these events show large dayside downward drift (westward electric field) perturbations followed by large morning upward and afternoon downward drifts that systematically shift to later local times. Ground-based magnetometer measurements in the American, Indian, and Pacific equatorial regions show strongly enhanced electrojet currents in the morning sector and large reversed currents (i.e., counterelectrojets) in the afternoon sector with onsets near new and full moons during northern winter warming periods. CHAMP satellite and ground-based magnetic field observations indicate that the onset of these equatorial afternoon counterelectrojets is longitude dependent. Our results indicate that these large electrodynamic perturbations during stratospheric warming periods are due to strongly enhanced semidiurnal lunar wave effects. The results of our study can be used for forecasting the occurrence and evolution of these electrodynamic perturbations during arctic winter warmings.Item Restricted Prompt effects of solar wind variations on the inner magnetosphere and midlatitude ionosphere(Elsevier, 2005) Huang, Chao-Shong; Foster, J. C.; Yumoto, K.; Chau Chong Shing, Jorge Luis; Veliz, OscarIt is well known that the solar wind can significantly affect high-latitude ionospheric dynamics. However, the effects of the solar wind on the middle- and low-latitude ionosphere are much less studied. In this paper, we report observations that large perturbations in the middle- and low-latitude ionosphere are well correlated with solar wind variations. In one event, a significant (20–30%) decrease of the midlatitude ionospheric electron density over a large latitudinal range was related to a sudden drop in the solar wind pressure and a northward turning of the interplanetary magnetic field, and the density decrease became larger at lower latitudes. In another event, periodic perturbations in the dayside equatorial ionospheric E × B drift and electrojet were closely associated with variations in the interplanetary electric field. Since the solar wind is always changing with time, it can be a very important and common source of ionospheric perturbations at middle- and low-latitudes. The relationship between solar wind variations and significant ionospheric perturbations has important applications in space weather.Item Open Access Variations of low-latitude geomagnetic fields and Dst index caused by magnetospheric substorms(American Geophysical Union, 2004-05-25) Huang, Chao-Song; Foster, J. C.; Goncharenko, L. P.; Reeves, G. D.; Chau Chong Shing, Jorge Luis; Yumoto, K.; Kitamura, K.We present observations of periodic magnetospheric substorms and corresponding ionospheric disturbances. Since the periodic substorms occur during a stable interplanetary magnetic field, we are able to identify which ionospheric signatures are caused solely by substorms. We find that the low-latitude ionospheric electric field perturbation after substorm onsets is eastward on the dayside and westward on the nightside and that the ground magnetometer northward (H) deviations at middle and low latitudes show an increase (a positive bay) after each substorm onset, no matter whether the magnetometers are located on the dayside or on the nightside. The nightside magnetometer H deviations are closely correlated with the inner magnetospheric magnetic field Bz component during the dipolarization process. The Dst index shows a significant increase of 20–40 nT after each substorm onset. We propose that the increase in the magnetometer H field and Dst index in response to substorm onsets is related to the field dipolarization. In this scenario the nightside magnetosphere earthward of the near-Earth neutral line is highly compressed during the dipolarization, and the magnetic flux density within the inner magnetosphere is greatly enhanced, resulting in an increase in the ground magnetometer H component and in Dst.