Geotail Satellite Research Articles

Correction to “Polar cap bifurcation during steady‐state northward interplanetary magnetic field with ∣BY∣ ∼ BZ”

[1] In the paper “Polar cap bifurcation during steady-state northward interplanetary magnetic field with ∣BY∣ ∼ BZ” by M. Watanabe et al. (Journal of Geophysical Research, 109, A01215, doi:10.1029/2003JA009944), Figure 6f was described as the magnetosheath magnetic field observed by the Geotail satellite (paragraph 25). However, the satellite was actually in the upstream solar wind during the period shown, and Figure 6f represents the interplanetary magnetic field (IMF). The Geotail data were originally intended to reduce the ambiguity of the arrival time of the IMF disturbances observed by the ACE satellite (Figures 6b–6e). With the reexamination, we note that in this event the near-Earth IMF observed by Geotail is not exactly the same in detail as the IMF observed by ACE. However, the discrepancies are not significant enough to affect the content of the paper.

Similarity and difference of the durations of the small-scale solar wind structure boundaries by multipoint observations

This report presents observations of very fast (from a few seconds to a hundred seconds) and large (from 50% up to several hundred percents) changes in the solar wind ion flux. These changes are connected with the sharp boundaries of solar wind small-scale and middle-scale structures that may be as thin as tens of proton gyroradii. These observations of solar wind plasma and magnetic field changes were performed aboard the Interball-1 satellite with very high time resolution (1 s or 60 ms) and “simultaneously” by the WIND spacecraft with a time resolution of 3 s and in some cases also by the Geotail satellite with resolution of 12 s. We found some events in which boundaries of the very fast changes of the solar wind ion flux and magnetic field have similar thicknesses and amplitudes over the hour it takes the solar wind to propagate from WIND to Interball-1 and Geotail. In many cases, we find that event boundaries which have durations of 20–100 s at WIND 50–200 R E upstream of Earth are steeper, with durations of 3–10 s, at Interball-1 and Geotail near Earth. These differences result from local irregularities along the disturbance fronts or from the steepening of these boundaries as they move together with the solar wind.

Experimental evidence of phase coherence of magnetohydrodynamic turbulence in the solar wind: GEOTAIL satellite data

Magnetohydrodynamic (MHD) turbulence is commonly observed in the solar wind. Nonlinear interactions among MHD waves are likely to produce finite correlation of the wave phases. For discussions of various transport processes of energetic particles, it is fundamentally important to determine whether the wave phases are randomly distributed (as assumed in the quasi-linear theory) or have a finite coherence. Using a method based on the surrogate data technique, we analysed the GEOTAIL magnetic field data to evaluate the phase coherence in MHD turbulence in the Earth's foreshock region. The results demonstrate the existence of finite phase correlation, indicating that nonlinear wave-wave interactions are in progress.

Observed X‐rays associated with kilometric continuum

Kilometric continuum (KC) is a recently identified nonthermal magnetospheric radio/plasma wave phenomenon that consists of many narrowband emission lines of nearly constant frequency. The temporal structures of the KC wave intensities measured from the Geotail satellite in near equatorial positions at geocentric distances of 9–30 Earth radii were, on several occasions, similar to the temporal structures of X‐rays simultaneously emitted from the atmosphere and mapped in position from the Polar satellite at lower altitudes and at high latitudes. The similarity in behavior of KC waves and X‐rays might result from the fact that bremsstrahlung X‐rays can be generated in the atmosphere by precipitating electrons produced at high altitudes by incident KC waves. KC waves at low latitudes appear correlated with X‐rays at high latitudes. Our study supports the premise that KC waves interacting with high‐altitude energetic electrons can lead to the production of X‐rays at low altitudes from the resultant precipitation of the electrons. Thus the Polar and Geotail satellites may be used to map the spatial spread of KC, a feat that cannot be currently accomplished with waves observed from a single satellite. In this investigation, it is found that the X‐ray emissions extend over a wide range of latitudes and may span a broad longitudinal interval. The energy spectra of X‐rays emitted from the Earth at the times of KC events have been examined and found to display a wide variety of intensities and spectral shapes as do the spectra of X‐rays associated with auroral kilometric radiation (AKR) events. For most of the high‐latitude satellite passes when KC but no AKR was present, the X‐ray energy spectra were harder than at times associated with AKR.

Coupling of perturbations in the solar wind density to global Pi3 pulsations: A case study

A typical Pi3 pulsation is examined by magnetic field measurements from multiple satellites and ground stations. Low‐latitude ground observations with a wide longitudinal span indicate that the amplitude of the Pi3 pulsation peaks on the dayside and is gradually decreasing toward the nightside. This effect and the fact that the wave phase on the dayside leads that on the nightside, imply that the source of the Pi3 lies on the dayside. Variations in solar wind dynamic pressure observed by the GEOTAIL satellite (just outside of the magnetosphere) are highly correlated with these ground magnetic field variations. We argue in this case that the global Pi3 pulsation is directly driven by impulsive variations in the solar wind dynamic pressure. The Pi3 pulsation observed along the latitudinal magnetometer chain at 0930LT shows significant equatorial enhancement and additional observations along a latitudinal chain at 1630LT show that the phase of the Pi3 pulsation at high latitudes lags behind that at low latitudes. The low‐altitude polar orbiting satellite Oersted also observed this pulsation in the dayside inner magnetosphere. The B∥ (northward) component at Oersted is strictly out of phase with the X component observed at the dip equator below the spacecraft path, which indicates that the Pi3 pulsation at the dip equator is caused by oscillation of an ionospheric current. We propose that the Pi3 pulsations at different latitudes are generated by different mechanisms.

Alfven Waves in a Plasma Sheet Boundary Layer Associated withNear-Tail Magnetic Reconnection

We report observations from Geotail satellite showing that largePoynting fluxes associated with Alfven waves in the plasma sheetboundary layer (PSBL) occur in the vicinity of the near-tailreconnection region on 10 December 1996. During the period of largePoynting fluxes, Geotail also observed strong tailward plasma flows.These observations demonstrate the importance of near-tail reconnectionprocess as the energy source of Alfven waves in the PSBL. Strongtailward (Earthward) plasma flows ought to be an important candidate ingenerating Alfven waves. Furthermore, the strong perturbations not onlyof the magnetic field but also of the electric field observed in thePSBL indicate that the PSBL plays an important role in the generationand propagation of the energy flux associated with Alfven waves.

Micro–Type III Radio Bursts

We present a detailed description of the features of solar radio bursts, which are elements of the so-called type III storms, using long-term observations made by the Geotail and Akebono satellites. Micro-type III bursts are characterized by short-lived, continuous, and weak emission. Their average power is estimated to be well below that of the largest type III bursts, by 6 orders of magnitude. When they occur, these bursts have a distribution of emitted power flux that is different from that of ordinary type III bursts, indicating that they are not just weaker versions of the ordinary bursts. Micro-type III burst activity is not accompanied by significant solar soft X-ray activity. We identify the active regions responsible for micro-type III bursts by examining the concurrence of their development and decay with the bursts. It is found that both micro and ordinary type III bursts can emanate from the same active region without interference, indicating the coexistence of independent electron acceleration processes. It is suggested that the active regions responsible for micro-type III bursts generally border on coronal holes.

Gamma-ray detection efficiency of the microchannel plate installed as an ion detector in the low energy particle instrument onboard the GEOTAIL satellite

A microchannel plate (MCP) assembly has been used as an ion detector in the low energy particle (LEP) instrument onboard the magnetospheric satellite GEOTAIL. Recently the MCP assembly has detected gamma rays emitted from an astronomical object and has been shown to provide unique information of gamma rays if they are intense enough. However, the detection efficiency for gamma rays was not measured before launch, and therefore we could not analyze the LEP data quantitatively. In this article, we report the gamma-ray detection efficiency of the MCP assembly. The measured efficiencies are 1.29%+/-0.71% and 0.21%+/-0.14% for normal incidence 60 and 662 keV gamma rays, respectively. The incident angle dependence is also presented. Our calibration is crucial to study high energy astrophysical phenomena by using the LEP.

Plasma sheet expansion: Statistical characteristics

The present study addresses the cause of plasma sheet expansion by statistically comparing the characteristics of lobe‐to‐plasma sheet (LB‐to‐PS) and PS‐to‐LB crossings observed by the Geotail satellite. Whereas the flapping motion of the magnetotail causes both types of crossing, the PS expansion (thinning) can be associated only with the LB‐to‐PS (PS‐to‐LB) crossing. Thus any systematic difference between the two types of crossing should reflect the difference between the PS expansion and thinning. Geotail observed more LB‐to‐PS crossings (744 events) than PS‐to‐LB crossings (640 events), and the preferred occurrence of the LB‐to‐PS crossing is more manifest closer to the Earth. It is found that at the PS‐to‐LB crossing, the plasma moves in the same direction as the boundary motion. At the LB‐to‐PS crossing, in contrast, the plasma often moves in the opposite direction to the boundary motion, indicating that there is a finite electric field in the frame of the boundary motion associated with the PS expansion. The PS expansion is therefore considered to be a manifestation of magnetic reconnection. That is, the PS expands because new PS flux tubes are added onto the preexisting PS. In the course of the PS expansion, the total pressure decreases, which may be interpreted in terms of the replacement of the preexisting PS plasma with new low‐pressure plasma originating from the tail lobe. The PS expansion is also characterized by relaxation (dipolarization) of the local magnetic field, which is inferred to be a direct consequence of reconnection. On the basis of recent reports of the lack of a one‐to‐one correspondence between reconnection and substorm onset, it is suggested that the PS expansion cannot be uniquely associated with a specific substorm phase.

Dayside and nightside reconnection rates inferred from IMAGE FUV and Super Dual Auroral Radar Network data

The spectrographic imager at 121.8 nm (SI12) of the far ultraviolet (FUV) experiment onboard the IMAGE spacecraft produces global images of the Doppler‐shifted Lyman α emission of the proton aurora. This emission is solely due to proton precipitation and is not contaminated by dayglow, allowing us to monitor the auroral oval on the dayside as well as on the nightside. Remote sensing of the polar aurora can be advantageously supplemented by use of ground‐based data from the Super Dual Auroral Radar Network (SuperDARN) that monitors the ionospheric convective flow pattern in the polar region. In the present study, the SI12 images are used to determine the location of the open/closed field line boundary and to monitor its movement. The SuperDARN data are then used to compute the ionospheric electric field at the location of the open/closed boundary. The total electric field is then computed along the boundary accounting for its movement via Faraday's law so that the dayside and nightside reconnection voltages can be derived. This procedure is applied to several substorm intervals observed simultaneously with IMAGE FUV and SuperDARN. The dayside reconnection voltage feeds the magnetosphere with open flux, which is later closed by nightside reconnection. The calculated dayside reconnection rate is consistent with the solar wind properties measured by the Geotail, Wind, and ACE satellites. We identify the presence of nightside reconnection due to pseudobreakups taking place during the growth phase. In several cases, we establish that the nightside reconnection rate is maximum at the time of the substorm expansion phase onset or shortly after, reaching ∼120 kV, and then slowly returns to undisturbed values of ∼30 kV. The flux closure rate can also start intensifying prior to expansion phase onset, producing pseudobreakups.

Effects of the fast plasma sheet flow on the geosynchronous magnetic configuration: Geotail and GOES coordinated study

The present study statistically examines how (or if) the geosynchronous (GOES) magnetic field responds to fast earthward flow observed by the Geotail satellite in the plasma sheet. The change of the GOES H (north‐south) component within 15 min of the detection of fast flows, ΔH, is used as a primary measure of the geosynchronous response. It is found that following the detection of fast flows, the geosynchronous magnetic field rarely dipolarizes, but it often becomes more stretched, which is manifested by negative ΔH. This H decrease is not accompanied by any correlated variation of the D (azimuthal) component, suggesting that the associated stretching is not an edge effect of the substorm current wedge formation, but it can be attributed to the intensification of the local tail current. No systematic dependence of ΔH on the satellite separation can be found. On the other hand, the geosynchronous magnetic field tends to dipolarize if it is already stretched significantly, although the associated changes in the H and V (radial) components are not much larger than those in events that are not preconditioned. The flow intensity does not seem to be a controlling factor, either. However, caution needs to be exercised because the present study is not able to address the azimuthal structure of the fast flow. It is concluded that in most events the fast plasma flow does not reach geosynchronous orbit and that the generation of the fast plasma flow in the plasma sheet is not sufficient for causing geosynchronous dipolarization.

Decrease in Bz prior to the dipolarization in the near‐Earth plasma sheet

We examine in detail the rapid decrease in Bz just before dipolarizations observed by the Geotail satellite in the near‐Earth plasma sheet at (XGSM, YGSM) = (−8.3 RE, −5.1 RE). The observations were made using high‐time‐resolution data from a fluxgate magnetometer (16‐Hz sample), a search‐coil magnetometer (128 Hz), and an electric field antenna (64 Hz). Two dipolarizations were observed during a short time interval of 2 min. The magnetic Bz component suddenly decreased 2–4 s prior to the dipolarization. Characteristic waves with frequencies of 5–20 Hz and amplitudes of 1–3 mV m−1 and 5–15 nT s−1 were observed in the electric and magnetic field data at the time of the sudden decreases in Bz. We discuss two possible causes of the sudden decreases in Bz prior to the dipolarizations: (1) passage of a field‐aligned line current associated with the substorm current wedge and (2) explosive growth phase and subsequent disruption of the tail current caused by the observed characteristic field oscillations.

Overwhelming O+ contribution to the plasma sheet energy density during the October 2003 superstorm: Geotail/EPIC and IMAGE/LENA observations

We studied dynamics of O+ ions during the superstorm that occurred on 29–31 October 2003, using energetic (9–210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low‐energy neutral atom (LENA) imager on board the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite. Since the low‐energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near‐Earth plasma sheet of XGSM ∼ −6 RE to −8.5 RE, we found that the H+ energy density showed no distinctive differences between the superstorm and quiet intervals (1–10 keV cm−3), while the O+ energy density increased from 0.05–3 keV cm−3 during the quiet intervals to ∼100 keV cm−3 during the superstorm. The O+/H+ energy density ratio reached 10–20 near the storm maximum, which is the largest ratio in the near‐Earth plasma sheet ever observed by Geotail, indicating more than 90% of O+ in the total energy density. We argued that such extreme increase of the O+/H+ energy density ratio during the October 2003 superstorm was due to mass‐dependent acceleration of ions by storm‐time substorms as well as an additional supply of O+ ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near‐Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had the O+/H+ energy density ratio as large as 10–20 for the October 2003 superstorm.

Field‐aligned beam observations at the quasi‐perpendicular bow shock: Generation and shock angle dependence

On 19 October 1995 the Geotail satellite skimmed along the quasi‐perpendicular bow shock for more than 3 hours, and field‐aligned ion beams (FABs) were continuously observed in the foreshock region. Also observed were 11 crossings with the bow shock where these FABs are thought to be generated. The upstream condition of the bow shock was in the state of low Mach number (MA ∼ 2.9) and low beta (β ∼ 0.02). By a detailed study of the evolution of the ion distribution across one of the crossings, we have found that leakage as origin of these ions from the magnetosheath is unlikely. Comparison of the observations with simulations suggests that FAB ions are generated by multiple interaction of incoming solar wind ions with the bow shock. We have then compared the FAB flux normalized by the solar wind flux, , with the shock angle θBn and have found that falls off rapidly just above θBn > 60°, but it maintains significant level ( ∼ 0.01%) up to ∼75°.

Nonadiabatic orbit features in ion distribution functions of fast flow magnetotail configurations

One of the major problems in space physics is the limitation of measurements. We present here one way to infer global characteristics of the magnetosphere from only a single satellite position. Several consistent orbit‐tracing (COT) models of fast flows in thin plasma sheets were constructed. The resulting ion distribution functions were examined to see if there are any features which can distinguish a uniform earthward flow of plasma from flows which are confined in a narrow channel in the dawn‐dusk direction. The model distribution functions during uniform flows had the appearance of an initially isotropic f(v) that had simply been shifted by the earthward flow speed. The model f(v) exhibited lima bean–shaped features when the flow was confined to a region comparable to the diameter of a cyclotron orbit based on Bz. Distribution functions observed by the Geotail satellite were examined for comparisons, and the measured distributions were found to exhibit features similar to those obtained in the COT model. We are able to demonstrate that the distributions measured by Geotail may only be reproduced in our technique if the convective electric field is confined in the dawn‐dusk direction.

Simultaneous in-situ observations of the signatures of dayside reconnection at the high- and low-latitude magnetopause

Abstract. We present magnetic field and particle data recorded by the Cluster and Geotail satellites in the vicinity of the high- and low-latitude dayside magnetopause, respectively, on 17 February 2003. A favourable conjunction of these spacecraft culminated in the observation of a series of flux transfer events (FTEs), characterised by bipolar perturbations in the component of the magnetic field normal to the magnetopause, an enhancement in the overall magnetic field strength, and field tilting effects in the plane of the magnetopause whilst the satellites were located on the magnetosheath side of the boundary. Whilst a subset of the FTE signatures observed could be identified as being either normal or reverse polarity, the rapid succession of events observed made it difficult to classify some of the signatures unambiguously. Nevertheless, by considering the source region and motion of flux tubes opened by magnetic reconnection at low latitudes (i.e. between Cluster and Geotail), we demonstrate that the observations are consistent with the motion of northward (southward) and tailward moving flux tubes anchored in the Northern (Southern) Hemisphere passing in close proximity to the Cluster (Geotail) satellites. We are able to demonstrate that a multi-spacecraft approach, coupled with a realistic model of flux tube motion in the magnetosheath, enables us to infer the approximate position of the reconnection site, which in this case was located at near-equatorial latitudes.

Relation between the ring current and the tail current during magnetic storms

Abstract. We study the dynamics of the magnetospheric large-scale current systems during storms by using three different magnetospheric magnetic field models: the paraboloid, event-oriented, and Tsyganenko T01 models. We have modelled two storm events, one moderate storm on 25-26 June 1998, when Dst reached -120nT and one intense storm on 21-23 October 1999, when Dst dropped to -250nT. We compare the observed magnetic field from GOES 8, GOES 9, and GOES 10, Polar and Geotail satellites with the magnetic field given by the three models to estimate their reliability. All models demonstrated quite good agreement with observations. Since it is difficult to measure exactly the relative contributions from different current systems to the Dst index, we compute the contributions from ring, tail and magnetopause currents given by the three magnetic field models. We discuss the dependence of the obtained contributions to the Dst index in relation to the methods used in constructing the models. All models show a significant tail current contribution to the Dst index, comparable to the ring current contribution during moderate storms. The ring current becomes the major Dst source during intense storms.

Forecasting magnetopause crossing locations by using Neural Networks

Given the highly complex and nonlinear nature of Near Earth Space processes, mathematical modeling of these processes is usually difficult or impossible. In such cases, modeling methods involving Artificial Intelligence may be employed. We demonstrate that data driven models, such as the Neural Network based approach, shows promise in its ability to forecast or predict the behavior of these processes. In this paper, modeling studies for forecasting magnetopause crossing locations are summarized and a Neural Network algorithm is presented to describe the nonlinear time-dependent response of the subsolar region of the magnetopause to varying solar wind conditions. In our approach the past history of the solar wind has, for the first time to the best knowledge of the authors, been included in forecasting the subsolar region of the magnetopause. It is proposed that the data driven approach is a valid approach to understanding and modeling the physical phenomena of Near Earth Space. The only basic requirement for the data driven approach is the availability of representative data for the phenomena. The objective of this paper is to demonstrate that by using WIND and GEOTAIL satellite data a Neural Network based model can be adapted to the modeling of the Earth’s magnetopause.

Correlations between low‐frequency and high‐frequency auroral kilometric radiation plasma wave intensity bursts and X rays in the auroral zone

Comparisons are made between X‐ray (>2 keV) emissions emanating from the Earth's Northern Hemisphere aurora observed on the Polar satellite and auroral kilometric radiation (AKR) plasma wave intensities detected on the Geotail satellite. For this comparison the plasma wave frequencies were divided into a low‐frequency (LF‐AKR) (30–100 kHz) band and a high‐frequency (HF‐AKR) (100–800 kHz) band. For the LF‐AKR band the correlation coefficients with auroral X‐ray emissions are generally high and significantly larger than the corresponding coefficients between X‐ray and HF‐AKR variations. It is found that short time‐scale variations of the LF‐AKR activity often correspond to the temporal fine structure of the intensity (5–10 min) of X‐ray auroral emissions. HF‐AKR intensity enhancements generally precede enhancements of the X‐ray emissions, while the LF‐AKR intensity enhancements generally lag the X‐ray enhancements. This sequence implies that auroral acceleration begins at lower altitudes and then moves to higher altitudes. In addition, the time profiles of the X‐ray intensities often display slower recovery times than do the LF‐AKR.

Tail dynamics during the growth phase of the 24 November 1996, substorm event: Near‐Earth reconnection confined in the plasma sheet

The present study examines a substorm event on 24 November 1996, with an emphasis on magnetic field and plasma flow measurements in the magnetotail during the growth phase. The unique and most intriguing feature of this event is a systematic change of the flow direction from earthward to tailward observed by the Geotail satellite at X ∼ −21 RE. Good correlation between VX and BX is found for the intervals of both earthward and tailward flows, which suggests that observed variations of the flow velocity were caused by satellite motion relative to the plasma sheet rather than the temporal change of the velocity itself. It is inferred that the earthward and tailward flows lasted at least 9 and 5 min, respectively. The signs of BZ and BY variations also changed systematically along with the sign of VX. These results are consistent with the idea that the observed flow reversal was caused by the earthward movement of a neutral line, which might be formed at a preceding pseudobreakup or even earlier. It is inferred that the associated reconnection did not reach the lobe magnetic field, and that the neutral line is convected toward the Earth along with the background plasma. Interestingly, during the interval of the flow reversal, other satellites observed typical growth‐phase features. The geosynchronous magnetic field continued to be stretched in the midnight sector. The Interball and IMP8 satellites observed a continuous increase of the lobe field strength at X = −20 and −34 RE, respectively, in the premidnight sector. It is therefore concluded that the tail current intensification during the growth phase is a global process, whereas the near‐Earth reconnection is a local process, which, at its early stage, can proceed independently from the overall storage of the tail magnetic energy.