Tohru Sakurai

Magnetic pulsation Pi2 and substorm onset

Coincidence between the onset of sudden brightening of the auroral arc in the auroral oval and the onset of Pi2 magnetic pulsation in low latitudes is examined based on the auroral data obtained at a chain of stations in Alaska and the Pi2 data obtained at the low-latitude station Onagawa. The result shows that the low-latitude Pi2 occurs almost simultaneously with the sudden brightening of the auroral arc, i.e. the onset of an auroral substorm (T = 0). It is concluded that the onset of substorms can be identified quite well with the onset of the low-latitude Pi2.

Local time asymmetry in the characteristics of Pc5 magnetic pulsations

Abstract The wave characteristics of Pc5 magnetic pulsations are analyzed with data of OGO-5, ISEE-1 and -2 satellites. The toroidal modes (δBD >δBH) of Pc5 pulsations are observed at a higher magnetic latitude in the dawnside outer magnetosphere. The compressional and poloidal modes (δBz.dfnc; ∼ δBH >δBD) of Pc5 pulsations are mostly observed near the magnetic equator in the duskside outer magnetosphere. This L.T. asymmetry in the occurrence of dominant modes of Pc5s in space can be explained by the velocity shear instability (Yumoto and Saito, 1980) in the magnetospheric boundary layer, where Alfvenic signals in the IMF medium are assumed to penetrate into the magnetospheric boundary layer along the Archimedean spiral. The asymmetrical behaviour of Pc5 pulsation activity on the ground across the noon meridian can be also explained by the ionospheric screening effect on the compressional Pc5 magnetic pulsations. The compressional modes with a large horizontal wave number in the duskside magnetosphere are expected to be suppressed on the ground throughout the ionosphere and atmosphere.

The earth's palaeomagnetosphere as the third type of planetary magnetosphere

From the viewpoint of dynamical topology, planetary magnetospheres are classified into three: Types 1, 2 and 3. When the rotation vector and dipole moment of a planet and the velocity vector of the solar wind are denoted as Ω, M, and V, respectively, the planetary magnetosphere with Ω∥M⊥V is called Type 1. The magnetospheres of the present Earth, Jupiter, and Uranus at its equinoctial points belong to this type. The magnetosphere with Ω∥M∥V is called Type 2, which includes the Uranian magnetosphere at its solstitial points. The magnetosphere with Ω⊥M and Ω⊥V is called Type 3. The Earths palaeomagnetosphere is considered to have experienced Type 3 during excursions and transition stages of palaeomagnetic polarity reversals. In the Type 3 magnetosphere, drastic diurnal variations are expected in configurations of the dayside cusps, tail axis, neutral sheet, polar caps, and so on. A possible relation between the Type 3 palaeomagnetosphere and palaeoclimate of the Earth during polarity reversals and geomagnetic excursions is suggested. It is also suggested that the heliomagnetosphere during polarity reversals of the general field of the Sun exhibits a drastic configuration change similar to the Type 3 palaeomagnetosphere of the Earth. A relation between the perpendicular condition Ω⊥M and magnetic variable stars and pulsars is briefly discussed.

Dayside magnetopause Pc 3 and Pc 5 ULF waves observed by the GEOTAIL Satellite

This study is intended to reveal the oscillation and propagation characteristics of ULF waves observed near the dayside magnetopause. A comparative study of such ULF wave phenomena is made by using the data obtained from the successive skimmings of the dayside magnetopause by the GEOTAIL Satellite performed on three successive days of 9, 18, and 27 December 1994. The instrumentations of GEOTAIL are good for such a task, since the satellite simultaneously measured the magnetic and electric fields, and low energy plasma data. Based the observed data, we studied the dynamic spectral characteristics of both magnetic and electric fields. The results revealed that the dominant ULF waves appearing in the dynamic spectra were Pc 3 and Pc 5 oscillations. The Pc 3 oscillations were observed during a limited local time around noon without an extension to the dawn and dusk flanks, and dominantly appeared at a frequency of ~25 mHz in the azimuthal component of the magnetic field. This frequency component showed clear resonant oscillation characteristics. In addition to this, a fast mode earthward propagation characteristic was also recognized. The Poynting flux of the Pc 3 signals showed that the energy flux was about 10 nJ/m2s on average, the strongest being along the magnetic field line. Pc 5 oscillations showed another dominant spectral power which appeared in the radial component of the electric field, suggesting that the resonance oscillations were well established. However, the Pc 5 oscillations in the subsolar region did not show any earthward propagation characteristics.

Examination of the resonance theory on Pcs by means of an analysis of magnetic fluctuations in the magnetosheath and the magnetosphere

Abstract Magnetic fluctuations observed in the magnetosheath and the outer magnetosphere with Ogo-5 during 6 months from November 1968, are analyzed to examine the resonance theory that monochromatic waves excited outside the magnetosphere are transmitted in the compressional mode into the magnetosphere, being transmitted further along the closed field lines in the torsional mode and are finally observed as long-period pcs on the Earths surface. Ten observed results on the wave characteristics of the fluctuations including variance, spectrum, relation to the plasma stream, integrated power, longitudinal dependence are obtained and summarized. The fluctuations in the magnetosheath are found to be dominantly Alfvenic. Several pieces of evidence to support the resonance theory are found.

Variations of magnetospheric convection electric fields during substorms as inferred from pc1 hydromagnetic waves

Abstract The convection electric field in the vicinity of the plasmapause in the midnight sector during magnetospheric substorms has been obtained on the basis of spectral analysis of Pc1 hydromagnetic (HM) waves observed at the low latitude station, Onagawa (Φ = 28.°3, Λ = 206.°8). Variations of the field are consistent for four independent substorm events studied. The calculation implies that the convection electric field increases westwards up to ~1.0 mV/m during the expansion phase of the substorms, changes polarity near the end of the expansion phase, and then points eastwards during the recovery phase.

Alfvénic plasma velocity variations observed at the inner edge of the low‐latitude boundary layer induced by the magnetosheath mirror mode waves: A THEMIS observation

[1] With unique simultaneous observations both in the magnetosheath and magnetosphere by the THEMIS probes, Alfvenic variations in the plasma velocity are observed at the inner edge of low-latitude boundary layer (LLBL) and are induced by the mirror mode waves in the magnetosheath near the subsolar magnetopause on 31 July 2007. These Alfvenic variations appeared as the wavy perturbations in the V x and V y components observed by THEMIS C, D, and E, which had the same periodicity as associated magnetic field variations. Simultaneously, THEMIS B observed the mirror mode waves in the magnetosheath. The periodicities of the magnetic and plasma pressure variations of mirror modes in the magnetosheath were consistent with those of the Alfvenic wavy variations in the LLBL. Therefore, the mirror mode waves can induce the magnetopause undulations, launchingAlfven waves, and resultant Alfvenic variations are observed in the LLBL. Also, in the succeeding magnetosheath interval by THEMIS B, we examined whether the mirror mode waves occurred and associated Alfvenic variations were observed in the LLBL. However, no clear evidence for an existence of the mirror mode waves was obtained, and THEMIS C, D, and E do not also observe associated magnetic field and plasma Alfvenic responses in the LLBL. These results suggest that the Alfvenic variations in the LLBL are strongly related to the mirror mode waves in the magnetosheath. On the basis of these results, we emphasize that the magnetosheath energy is transmitted and transported into the magnetosphere via magnetopause surface waves.

Poynting vectors of Pc 5 pulsations observed by the GEOTAIL satellite in the dayside outer magnetosphere

Poynting vectors of Pc 5 pulsations observed in the dayside outer magnetosphere are examined with the magnetic and electric field data simultaneously observed by the GEOTAIL satellite. Poynting energy can be estimated as about 10−8–10−6 W/m2 in both directions across and along the magnetic field-line. However, the energy propagating across the magnetic field-line is larger than the energy propagating along the magnetic field-line. From this result we can estimate the Pc 5 wave energy flowing into the inner magnetosphere during an hour as 1010–1013 J, which is one or two orders less than the substorm energy (about 1014–1015 J). However, by taking into account of the continuous activation of Pc 5 pulsations in the outer magnetosphere, Pc 5 waves should play an important role for energetics in the magnetosphere.

Double-frequency oscillations of low energy plasma associated with transverse Pc 5 pulsations: GEOTAIL satellite observations

The GEOTAIL satellite observed an interesting oscillation phenomenon of low energy plasma (LEP) in the dawnside outer magnetosphere. The oscillation was taking place with a frequency double that of the transverse oscillation of Pc 5 pulsations. The double-frequency oscillation appeared in the plasma density and temperature, clearly showing an out-of-phase relationship between them. However, this phenomenon is revealed to be an instrumental effect of the LEP detector, which has a low energy threshold of measuring an ion population at 32 eV/Q. The imbedded ion population is found to be composed of cold ions with an energy of less than the threshold. They are convected past the LEP detector by the Pc 5 wave and enter the detector energy window twice per wave period. Plasma bulk parameters calculated using the detected ions produce an oscillation that has a frequency exactly double that of the Pc 5 wave. However, it should be noted that this phenomenon is observed with a large amplitude electric field oscillation only in intervals when the satellite passes through the dawnside outer magnetosphere under very quiet magnetic conditions, i.e., periods of the northward interplanetary magnetic field.

Geomagnetic effects of high-density plasma with southward magnetic field in the interplanetary coronal mass ejection observed on May 2-3, 1998

This paper aims to clarify the effect of high-density plasma in interplanetary coronal mass ejection (ICME) observed during the May 2–3, 1998 geomagnetic storm. The examination is performed based on the estimation of Dst index, which is calculated with the observed solar wind parameters of the ICME. The estimated Dst index variation is compared with Dst index variation provided by the World Data Center for Geomagnetism, Kyoto (WDC, Kyoto). From this examination, we find that the trend of the estimated Dst is in good agreement with that of the provided Dst when the thresholds are taken into account for both the solar wind plasma density and the dawn-to-dusk solar wind electric field, as 30 #/cc and 0.49 mV/m, respectively. From the result, we can conclude that the effect of high-density plasma is important on the enhancement of geomagnetic storm as well as the effect of the other solar wind parameters, such as the interplanetary magnetic field (IMF) Bz and solar wind velocity. On the other hand, the solar source of the magnetic field of this ICME is examined. The magnetic field structure of the ICME is examined by fitting the flux rope model to the observed magnetic field and solar wind speed. The results are compared with the magnetic structure of the bases of coronal helmet streamers. From this comparison we can find that the magnetic structure of the interplanetary flux rope is in good agreement with that of the neutral line of the base of coronal helmet streamers. The result suggests that if we look for the causes of geomagnetic storm we should take into account both the plasma structure and the magnetic structure of the base of coronal helmet streamers.