K. Tsuruda

Magnetotail flow bursts: Association to global magnetospheric circulation, relationship to ionospheric activity and direct evidence for localization

A series of bursty bulk flow events (BBFs) were observed by GEOTAIL and WIND in the geomagnetotail. IMP8 at the solar wind showed significant energy coupling into the magnetosphere, while the UVI instrument on POLAR evidenced significant energy transfer to the ionosphere during two substorms. There was good correlation between BBFs and ionospheric activity observed by UVI even when ground magnetic signatures were absent, suggesting that low ionospheric conductivity at the active sector may be responsible for this observation. During the second substorm no significant flux transport was evidenced past WIND in stark contrast to GEOTAIL and despite the small intersatellite separation ((3.54, 2.88, −0.06) RE). Throughout the intervals studied there were significant differences in the individual flow bursts at the two satellites, even during longitudinally extended ionospheric activations. We conclude that the half-scale-size of transport-bearing flow bursts is less than 3 RE.

Multisatellite observations of rapid subauroral ion drifts (SAID)

We present the first conjugate observations of subauroral ion drifts (SAID) in the magnetosphere (∼9000 km altitude) and ionosphere and coincident measurements by four ionospheric satellites. The parameters measured include ion drifts, energetic precipitating electrons and ions, and the magnetic field perturbations associated with field-aligned currents. Observations indicate that SAID are very coherent features that occur simultaneously over a large magnetic local time (MLT) range, from at least 1600 to 2400 MLT. The equatorward extent of SAID, the ion precipitation, and the region 2 field-aligned currents (FAC) flowing into the ionosphere are all shown to be coincident at all MLT locations where SAID are observed. They also appear to be closely related to the conductivity distribution in the subauroral ionosphere and the midlatitude trough. This is interpreted as an indication that their latitudinal distribution is a consequence of the subauroral conductivity structure and the movement of the plasma sheet ion and electron boundaries. Conjugate measurements at diverse altitudes when mapped along field lines are nearly identical, indicating the absence of significant field-aligned potential drops. Temporally separated SAID measurements in similar MLT regions show a reduction with time in the field-aligned current densities with little reduction in the potential drop across the SAID. We interpret the results as an indication that the magnetosphere acts as a current generator in which large FAC are initially required to support the electric field gradient in a SAID event. Subsequent evolution in the E and F regions produces large conductivity gradients that are in the right sense to remove the intense FAC requirement but maintain the large subauroral electric fields. The reported potential drops in the subauroral region can be a significant fraction of the total, up to 60 kV or more, and must be taken into account when deriving any magnetospheric convection pattern.

Cross polar cap diameter and voltage as a function of PC index and interplanetary quantities

Measurements of precipitating particles and the electric field on board EXOS D spacecraft for the period January–June 1990 have been used for estimation of the diameter of the polar cap along dawn-dusk meridian and the polar cap voltage. Identification of the polar cap boundaries has been made on the basis of specific features of precipitating ions. The data on the polar cap boundary location obtained for different geophysical conditions have been used to derive the statistical relationship between the polar cap diameter and PC index. The analysis has shown an approximately linear relationship between the polar cap diameter and the PC index for values PC < 3, the diameter tending to be asymptote when the PC index reaches large positive values. Cross polar cap voltage derived from EXOS D data is in good correlation with interplanetary quantities including the interplanetary magnetic field (IMF) southward component. The best correlation is obtained for the merging electric field υBT sin2 θ/2, with a coefficient of correlation higher than 0.82. Almost the same correlation is observed between polar cap voltage and PC index. The effect of “saturation” is not traced in the voltage dependencies on the PC index and interplanetary quantities up to values BT ≤ 10 nT.

Low‐frequency electromagnetic turbulence observed near the substorm onset site

On the basis of wave and plasma observations of the Geotail satellite, the instability mode of low-frequency (1-10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined. Quantitative estimation has also been made for the anomalous heating and resistivity resulting from the electromagnetic turbulence. Four possible candidates of substorm onset sites, characterized by the near-Earth neutral line, are found in the data sets obtained at substorm onset times. In these events, wave spectra obtained by the search-coil magnetometer and the spherical double-probe instrument clearly show the existence of electromagnetic wave activity in the lower hybrid frequency range at and near the neutral sheet. The linear and quasi-linear calculations of the lower hybrid drift instability well explain the observed electromagnetic turbulence quantitatively. The calculated characteristic electron heating time is comparable to the timescale of the expansion onset, while that of ion heating time is much longer. The estimated anomalous resistivity fails to supply enough dissipation for the resistive tearing mode instability.

A statistical study of variations in the near and middistant magnetotail associated with substorm onsets: GEOTAIL observations

We have studied the three-dimensional structure of substorm-associated variations in the magnetotail with GEOTAIL data. For this study we selected 342 substorm events from the Pi2 pulsation and applied the method of superposed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion β. It was found that the fast tailward flows start to develop in the premidnight plasma sheet around X ∼ −28 RE (GSM) about 0 ∼ 2 min before onset, associated with the plasmoid formation. Immediately after onset, the fast tailward flows develop further, and the magnetic field substantially increases southward. Simultaneously, the northward magnetic field increases around X ∼ −10 RE, corresponding to the dipolarization. In the lobe, the perpendicular plasma flow toward the plasma sheet, as well as the dawn-dusk electric field, first starts to be enhanced around (X, Y) ∼ (−20,7) RE about O ∼ 2 min before onset and then in the surrounding regions successively. The total pressure decrease first occurs around (X, Y) ∼ (−18,7) RE about 0 ∼ 2 min before onset, and then propagates to the surrounding regions successively. The dawn-dusk electric fields both calculated with E = −V × B and measured directly by the double probe, simultaneously develop around X ∼ −10 RE and X ∼ −28 RE immediately after onset, while those in the plasma sheet around.(X, Y) ∼ (−20,5) RE do not develop much even after onset. These observational results strongly suggest that an efficient magnetic reconnection takes place at least about 0 ∼ 2 min earlier than the Pi2 onset, that the substantial plasmoid evolution and the dipolarization occur simultaneously immediately after onset, and that, on average, the center of the energy release, where the near-Earth neutral line (i.e., the diffusion region) is possibly created, is initially located around (X, Y) ∼ (−19,6) RE. These features are consistent with a thin-current reconnection model.

Geotail observations of substorm onset in the inner magnetotail

On April 26, 1995, while Geotail was in the near-equatorial magnetotail at 13 RE and 2300 LT, a substorm onset occurred that was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit. Although Geotail was initially outside a greatly thinned current sheet, plasma sheet thickening associated with the substorm dipolarization quickly caused Geotail to move into the plasma sheet where it observed field-aligned earthward moving ions with velocities of 400 km/s. During the subsequent few minutes as the magnetic field became more northward, the velocities increased with particles moving increasingly into the energy range of the energetic particle experiment. These flows culminated with 1-min worth of earthward flow of 2000 km/s that was perpendicular to the northward B field. Such flow, probably the largest ever detected at 13 RE, was confirmed by the observation of an intense dc electric field of 50 mV/m (0.3 megavolts/RE). This large field is probably inductive, caused by reconnection that occurred tailward of the spacecraft, and related to the acceleration processes associated with particle injection at geosynchronous orbit. Energy and magnetic flux conservation arguments suggest that this rapid flow has a small cross-tail dimension of the order of 1 RE. The data appear to support a simulation of Birn and Hesse [1996] which showed rapid earthward flows from a reconnection line at 23 RE that caused a tailward expansion of a region of dipolarized flux. Subsequent to the onset, Geotail observed plasma vortices with typical velocities of 50–100 km/s that occurred in a high-beta plasma sheet with a 15-nT northward magnetic field. The vortices were punctuated by occasional flow bursts with velocities up to 400 km/s, one of which was accompanied by a violently varying magnetic field where north/south field components were as large as 30 nT and as small as −8 nT.

Near-earth dipolarization : Evidence for a non-MHD process

We have investigated a near-Earth dipolarization event in the midnight sector using simultaneous observations of Polar and Geotail. We have found evidence for near-Earth dipolarization to be a non-MHD process: dipolarization occurring without significant plasma flow or with tailward flow and during dawnward electric field different from that inferred based on the frozen-in condition. These observations are inconsistent with the idea that dipolarization is an MHD process of magnetic flux pileup from braking of sunward plasma flow. Possible variances of the flow braking scenario are considered but none is satisfactory in accounting for the observed features. On the other hand, these findings are quite consistent with the expectations from the current disruption scenario.

On the relationship between bursty flows, current disruption and substorms

A current disruption (CD) event was observed by the Geotail spacecraft at (X, Y)GSM=(−10.0, 1.9) RE within 1 min from the onset of a substorm. The event was selected as one of four near-Earth current disruption events identified in a search of three years of Geotail data. Fast ion flows accompanied the magnetic field dipolarization, rendering this event a near-Earth bursty bulk flow (BBF) event. The gradient anisotropy of the 30–44 keV ions at the onset of the flow is consistent with an Earthward motion of the heated plasma and agrees with the direction of flux and energy transport. The aurora and the associated electrojet moved from low to high latitudes during substorm expansion. Our observations show that CDs and BBFs are qualitatively similar phenomena in the near-Earth tail, both associated with poleward-moving (classical) auroral substorms.

Net current density of photoelectrons emitted from the surface of the GEOTAIL spacecraft

The current density carried by photoelectrons emitted from the GEOTAIL spacecraft is estimated from the electric potential of the spacecraft measured in the single probe mode of GEOTAIL/EFD and plasma density and temperature obtained by GEOTAIL/LEP during the period from September 14, 1993 to October 31, 1998, by assuming balance of the currents carried by photoelectrons and ambient thermal electrons. Behaviour of the photoelectron current as a function of spacecraft potential is consistent with the current profile predicted by Grard (1973), and the emitted photoelectrons consist of several components with different temperatures. The saturation density of the low energy component of the photoelectron current is 85 ± 33 × 10−6 [Am−2]. Number density of the photoelectrons is estimated to be 2.9 ± 1.4 × 109 [m−3] at the surface of the spacecraft, and the average energy of the photoelectrons is 2.1 ±0.5 [eV]. These values are higher than the prediction by Grard but consistent with previous in-flight measurements from GEOS-1, ISEE-1 or Viking.

ISEE 1 and Geotail observations of low‐frequency waves at the magnetopause

Observations of waves at frequencies below ∼200 Hz obtained near the magnetopause are presented. For one case identified in the ISEE 1 data as a period when steady state reconnection was occurring, there were waves below the lower hybrid frequency with amplitudes up to ∼7 mV/m. Intense low-frequency waves with amplitudes up to ∼20 mV/m at the subsolar magnetopause have also been observed by the Geotail electric field instrument. In some cases, large spiky fields were embedded in the waves. The waves observed by ISEE 1 and Geotail were large enough to provide the dissipation required for reconnection to occur.