Office Geophysik

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Office Geophysik

Japan
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Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Koga D.,National Institute for Space Research
Journal of Geophysical Research: Space Physics | Year: 2012

Polarizations of Pi2 pulsations in the magnetosphere and on the ground in the auroral zone are inconsistent when field line motions in the meridian and transverse planes are both in the fundamental harmonic. To resolve these inconsistencies, we propose a third harmonic mode in the meridian planes. The excitation of the third harmonic is explained by dusk-to-dawn currents at the equatorial plane, which are driven by diamagnetic currents during substorm injections. We propose diamagnetic currents in the equatorial plane and compressional input at the outer boundary as the source of Pi2 pulsations in the magnetosphere. © 2012. American Geophysical Union. All Rights Reserved.


Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Koga D.,National Institute for Space Research
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2012

Using magnetometer data acquired by geosynchronous satellite and all-sky images from a ground optical station in the conjugate area, we show propagation of aurora surge repeating in a thin and narrow aurora arc at the poleward boundary of the aurora zone. The surge repetition correlated with the Pi2s at the geosynchronous altitudes. One-to-one conjunctions of the surge and Pi2 pulse were not always observed. The westward or eastward propagation directions of the surge corresponded to the clockwise or counterclockwise wave polarizations in the equatorial plane. Propagation velocities of the poleward surge were in the range of 0.2-0.5°/s or 12-30. km/s at 100. km altitudes. We discuss the propagating surge at poleward boundary in conjunction with bi-directional flows in the first 10. min intervals of Pi2 onset that were inferred by Saka et al. (2010) from the Pi2 epoch analyses of magnetometer data at geosynchronous altitudes. © 2012 Elsevier Ltd.


Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Thomsen M.,Los Alamos National Laboratory
Annales Geophysicae | Year: 2014

Substorm auroras observed on 17 January 1994 were localized within the field of view of an all-sky imager installed at Dawson City (DWS, 65.7° ILAT). In association with the enhancement of the anti-sunward convection in the polar cap and the ion flux enhancement in 1-6 keV at geosynchronous altitudes, a wave-like structure propagating equatorward to the onset latitudes with a high wave number in azimuth (m ~ 76, T ~ 120 s) was observed 30 min prior to the activation in the equatorward latitudes. The activation of the auroras in the equatorward latitudes and the subsequent poleward expansion lasted for approximately 6 min until a diffuse aurora formed. The auroras in the last 6 min were isolated and localized within the field of view of DWS, from 400 km west to 400 km east, and accompanied the magnetic pulse at the optical station. The magnetic pulse is interpreted by the propagating ionospheric current loop with a size comparable to the isolated auroras (~ 1000 km). We conclude that the wave-like structures in the pre-onset interval relate to the intrusion of the plasma-sheet plasmas from the tail by the convection. The plasmas from the tail eventually developed the ionospheric loop currents at the onset latitudes, in association with the triggering of the bead-like rippling of auroras and subsequent breaking out from the onset latitudes. © Author(s) 2014.


Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Thomsen M.,Los Alamos National Laboratory
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2010

We examined the temporal variations of the geomagnetic field and energetic ions at geosynchronous altitudes associated with substorms during the nighttime using a superposed epoch analysis timed by Pi2 onset. We focused on the first 10. min intervals of Pi2 onset and on subsequent intervals to study the substorm expansion. We conclude that the first 10. min interval of Pi2 onset is a transitional state of the substorm dominated by MHD processes associated with earthward flow and its bifurcation. Intervals of field line variations following the first 10. min were well organized by dipolarization (substorm current wedge) due to the reduced cross-tail current. We also show that energetic ion regions localized in the local time sector from 2000 to 0000. LT in the first 10. min intervals of Pi2 onset expanded to the post-midnight sector, reaching 0400. LT within 20. min after Pi2 onset. We conclude that the expansion of the energetic plasma regions can be attributed to the inflation of the inner magnetosphere during dipolarization. © 2010 Elsevier Ltd.


Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Thomsen M.F.,Los Alamos National Laboratory
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2016

An all-sky imager installed at the midnight sector in Dawson City (66.0° in geomagnetic latitude) recorded the equatorward evolution of auroras from the auroral poleward boundary. The auroras evolved as shear layers expanding southeastward with velocities of 1-4 km/s, referred to as N-S auroras, and occurred during the transient intensification of the convection electric fields in the nighttime magnetosphere, as inferred from an electron spectrogram at geosynchronous altitudes. A continuous increase in the inclination angle of the field lines and magnetic field perturbations associated with propagating ionospheric loop currents were observed in the auroral zone during the N-S auroras. Simultaneously, Pc4 pulsations were observed at low latitudes from night to day sectors. We conclude the following: (1) the N-S auroras are an auroral manifestation of the earthward drift of plasma sheet electrons in the equatorial plane associated with transient and localized convection electric fields; (2) the Pc4 pulsations are produced in the magnetosphere by plasma sheet ions in the plasmasphere. The localized convection fields produce a vortical motion of plasmas in the equatorial plane, which may initiate the N-S auroras and ionospheric loop currents in the auroral zone. © 2016 Elsevier Ltd.


Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Klimushkin D.Y.,Russian Academy of Sciences | Mager P.N.,Russian Academy of Sciences
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2014

Geomagnetic Pc5 pulsations were observed in the dawn sector of the auroral zone on 17 January 1994 in association with increased energetic ion fluxes at geosynchronous orbit 10. min after the Pi2 onset. The characteristic properties of auroras associated with these pulsations were studied using movies taken by an all-sky imager. It was found that a pulsating aurora (PA) can be an optical manifestation of the Pc5 waves by a strong poloidal component observed with ground-based magnetometers. Goes7 observations showed compressional pulsations with the same period which can be attributed to the influence of the finite pressure of plasma and field line curvature on the poloidally polarized Alfvén waves. These poloidal pulsations may be generated by the ion injection observed with the LANL 1989-046 satellite. Two auroral arcs were observed north of the PA with optical features characteristic for the toroidal field line resonances: strong localization across L-shells, 180° phase change across the resonance, poleward phase propagation. Thus the Pc5 oscillations split into the toroidal and poloidal mode and oscillated coherently at latitudes from 62°N to 70°N. This study provides observational evidence of polarization splitting of the Alfven oscillation spectrum. Such a polarization splitting would occur in association with the reappearance of the energetic particles at geosynchronous orbit. © 2014 Elsevier Ltd.


Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Leonovich A.S.,Russian Academy of Sciences
Journal of Geophysical Research A: Space Physics | Year: 2015

The modes of Pi2 oscillations in space and on the ground, particularly in the auroral zone, have been discussed individually. The former is associated with the cavity oscillations either ideally trapped or partially trapped in the magnetosphere, and the latter is associated with the transient oscillations of the currents in the ionosphere. In this report, we show that ground polarization patterns (ellipticity and major axis orientation) of Pi2 pulsations in the nighttime sector from auroral zone to midlatitudes can be explained by the wave polarizations in the magnetosphere transmitted by the fundamental and the third harmonic deformations of the geomagnetic field lines. These ground polarization patterns are, however, understood alternatively as propagating large-scale ionospheric loop currents in auroral zone expanding ~1000km longitudinally. One pattern propagates eastward in the dawn sector and the other westward in the dusk sector. The propagating loop currents for the generation of the ground polarizations were consistent with the vortical motion of auroras observed by all-sky imager in association with the Pi2 pulsations. Propagation of the ionospheric loop currents and vortical motion of auroras are a consequence of the Pi2-associated magnetosphere and ionosphere coupling through the Alfvén waves. ©2015. American Geophysical Union. All Rights Reserved.


Saka O.,Office Geophysik | Hayashi K.,University of Tokyo | Leonovich A.S.,Russian Academy of Sciences
Journal of Geophysical Research A: Space Physics | Year: 2015

A substorm aurora was observed at 04:52 UT on 27 January 1986 by an all-sky imager installed at Shamattawa (66.3°N, 336.0° in corrected geomagnetic coordinates) and a magnetometer on board a conjugate satellite (GOES 6) at geosynchronous altitudes. In the preonset intervals lasting for approximately 50min prior to the expansion onset, an equatorward drift of the auroras beginning from 71°N to 64°N was observed. Meanwhile, GOES 6 observed ULF oscillations in 25-200s periods at geosynchronous altitudes after about 10min following the start of the equatorward drift. During the equatorward drift of the arc, the flow reversal occurred, where the auroral arc propagating eastward was replaced by a westward propagation. Simultaneously, the major axes of the ULF oscillations rotated clockwise by ~90° in the equatorial plane. We conclude that the ULF oscillations are azimuthally small-scale Alfvén waves excited in the magnetosphere by field-aligned currents associated with ionospheric loop currents. Key Points Equatorward drift of presubstorm auroras Ionospheric currents associated with equatorward drift of auroras Ionospheric currents generate high-m ULF waves in the magnetosphere ©2015. American Geophysical Union. All Rights Reserved.

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