Browsing by Author "Nishimura, Yukitoshi"

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    Dayside Magnetospheric and Ionospheric Responses to a Foreshock Transient on 25 June 2008: 1. FLR Observed by Satellite and Ground-Based Magnetometers
    Shen, Xiao-Chen; Shi, Quanqi; Wang, Boyi; Zhang, Hui; Hudson, Mary K.; Nishimura, Yukitoshi; Hartinger, Michael D.; Tian, Anmin; Zong, Qiu-Gang; Rae, I. J.; Degeling, Alexander W. (2018-08)
    As one type of driver of magnetospheric Alfven waves, foreshock transients have received less attention than, for example, the Kelvin-Helmholtz instability, discrete and broadband frequency solar wind dynamic pressure oscillations, and interplanetary shocks. Previous works show that foreshock transients can induce both Alfven mode and compressional mode Pc 3-5 ULF waves inside the magnetosphere. However, to our knowledge, none of these reported Pc 3-5 waves, induced by foreshock transients, are proved to be localized in the magnetosphere. In this paper, using in situ and ground-based observations, we report the generation of localized magnetospheric compressional waves and field line resonances (FLRs) by a foreshock transient. Both the foreshock transient and Pc 5 ULF waves were found on the duskside; while on the morning side of the magnetosphere, no clear wave signatures were captured. Our results demonstrate that in addition to the global effects of foreshock transients on the magnetosphere reported earlier, foreshock transients can also generate localized magnetospheric responses in the Pc 5 range with clear dawn-dusk asymmetry. A suite of eight dayside spacecraft plus ground magnetometer measurements make possible the determination of the foreshock transient driver and dawn-dusk asymmetry of the magnetospheric response not previously reported with such a complete data set.
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    First Simultaneous Lidar Observations of Thermosphere-Ionosphere Fe and Na (TIFe and TINa) Layers at McMurdo (77.84 degrees S, 166.67 degrees E), Antarctica With Concurrent Measurements of Aurora Activity, Enhanced Ionization Layers, and Converging Electric Field
    Chu, Xinzhao; Nishimura, Yukitoshi; Xu, Zhonghua; Yu, Zhibin; Plane, John M. C.; Gardner, Chester S.; Ogawa, Yasunobu (2020-10-28)
    We report the first simultaneous, common-volume lidar observations of thermosphere-ionosphere Fe (TIFe) and Na (TINa) layers in Antarctica. We also report the observational discovery of nearly one-to-one correspondence between TIFe and aurora activity, enhanced ionization layers, and converging electric fields. Distinctive TIFe layers have a peak density of similar to 384 cm(-3) and the TIFe mixing ratio peaks around 123 km, similar to 5 times the mesospheric layer maximum. All evidence shows that Fe+ ion-neutralization is the major formation mechanism of TIFe layers. The TINa mixing ratio often exhibits a broad peak at TIFe altitudes, providing evidence for in situ production via Na+ neutralization. However, the tenuous TINa layers persist long beyond TIFe disappearance and reveal gravity wave perturbations, suggesting a dynamic background of neutral Na, but not Fe, above 110 km. The striking differences between distinct TIFe and diffuse TINa suggest differential transport between Fe and Na, possibly due to mass separation.
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    Local time extent of magnetopause reconnection using space-ground coordination
    Zou, Ying; Walsh, Brian M.; Nishimura, Yukitoshi; Angelopoulos, Vassilis; Ruohoniemi, J. Michael; McWilliams, Kathryn A.; Nishitani, Nozomu (European Geosciences Union, 2019-04-10)
    Magnetic reconnection can vary considerably in spatial extent. At the Earth's magnetopause, the extent generally corresponds to the extent in local time. The extent has been probed by multiple spacecraft crossing the magnetopause, but the estimates have large uncertainties because of the assumption of spatially continuous reconnection activity between spacecraft and the lack of information beyond areas of spacecraft coverage. The limitations can be overcome by using radars examining ionospheric flows moving anti-sunward across the open-closed field line boundary. We therefore infer the extents of reconnection using coordinated observations of multiple spacecraft and radars for three conjunction events. We find that when reconnection jets occur at only one spacecraft, only the ionosphere conjugate to this spacecraft shows a channel of fast anti-sunward flow. When reconnection jets occur at two spacecraft and the spacecraft are separated by < 1 Re, the ionosphere conjugate to both spacecraft shows a channel of fast anti-sunward flow. The consistency allows us to determine the reconnection jet extent by measuring the ionospheric flows. The full-width-at-half-maximum flow extent is 200, 432, and 1320 km, corresponding to a reconnection jet extent of 2, 4, and 11 Re. Considering that reconnection jets emanate from reconnections with a high reconnection rate, the result indicates that both spatially patchy (a few Re) and spatially continuous and extended reconnections (> 10 Re) are possible forms of active reconnection at the magnetopause. Interestingly, the extended reconnection develops from a localized patch via spreading across local time. Potential effects of IMF B-x and B-y on the reconnection extent are discussed.
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    Understanding Strong Neutral Vertical Winds and Ionospheric Responses to the 2015 St. Patrick's Day Storm Using TIEGCM Driven by Data-Assimilated Aurora and Electric Fields
    Lu, Xian; Wu, Haonan; Kaeppler, Stephen; Meriwether, John; Nishimura, Yukitoshi; Wang, Wenbin; Li, Jintai; Shi, Xueling (American Geophysical Union, 2023-02)
    As one of the strongest geomagnetic storms in Solar Cycle 24, the 2015 St. Patrick's Day storm has attracted significant attention. We revisit this event by taking advantage of simultaneous observations of high-latitude forcings (aurora and electric fields) and ionosphere-thermosphere (I-T) responses. The forcing terms are assimilated to drive the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) using a newly adopted Lattice Kriging method (Wu & Lu, 2022, https://doi. org/10.1029/2021SW002880; Wu et al., 2022, https://doi.org/10.1029/2022SW003146). Compared to the default run, the TIEGCM simulation with assimilation captures: (a) secondary E-region electron density peak due to aurora intensification; (b) strongly elevated ion temperatures (up to similar to 3000 K) accompanied by a strong northward electric field (similar to 80 mV/m) and associated ion frictional heating; (c) elevation of electron temperatures; and (d) substantially enhanced neutral vertical winds (order of 50 m/s). Root-mean-square errors decrease by 30%-50%. The strong neutral upwelling is caused by large Joule heating down to similar to 120 km resulting from enhanced aurora and electric field. Data assimilation increases the height-integrated Joule heating at Poker Flat to a level of 50-100 mW/m2 while globally, its maximum value is comparable with the default run: the location of energy deposition becomes guided by data. Traveling atmospheric disturbances in the assimilation run show stronger magnitudes and larger extension leading to an increase of vertical wind variability by a factor of similar to 1.5-3. Our work demonstrates that data assimilation of model drivers helps produce realistic storm-time I-T responses, which show richer dynamic range, scales, and variability than what has been simulated before.