Geotail Spacecraft Research Articles

Geotail observations of plasma sheet ion composition over 16 years: On variations of average plasma ion mass and O+ triggering substorm model

We examined long‐term variations of ion composition in the plasma sheet, using energetic (9.4–212.1 keV/e) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. EPIC/STICS observations are available from 17 October 1992 for more than 16 years, covering the declining phase of solar cycle 22, all of solar cycle 23, and the early phase of solar cycle 24. This unprecedented long‐term data set revealed that (1) the He+/H+ and O+/H+ flux ratios in the plasma sheet were dependent on the F10.7 index; (2) the F10.7 index dependence is stronger for O+/H+ than He+/H+; (3) the O+/H+ flux ratio is also weakly correlated with the ΣKp index; and (4) the He2+/H+ flux ratio in the plasma sheet appeared to show no long‐term trend. From these results, we derived empirical equations related to plasma sheet ion composition and the F10.7 index and estimated that the average plasma ion mass changes from ∼1.1 amu during solar minimum to ∼2.8 amu during solar maximum. In such a case, the Alfvén velocity during solar maximum decreases to ∼60% of the solar minimum value. Thus, physical processes in the plasma sheet are considered to be much different between solar minimum and solar maximum. We also compared long‐term variation of the plasma sheet ion composition with that of the substorm occurrence rate, which is evaluated by the number of Pi2 pulsations. No correlation or negative correlation was found between them. This result contradicts the O+ triggering substorm model, in which heavy ions in the plasma sheet increase the growth rate of the linear ion tearing mode and play an important role in localization and initiation of substorms. In contrast, O+ ions in the plasma sheet may prevent occurrence of substorms.

The role of magnetic field fluctuations in nonadiabatic acceleration of ions during dipolarization

We statistically examine changes in the composition of two different ion species, proton and oxygen ions, in the near‐Earth plasma sheet (X = −16 RE ∼ −6 RE) during substorm‐associated dipolarization. We use 10 years of energetic (9–212 keV/e) ion data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particles and ion composition (EPIC) instrument on board the Geotail spacecraft. The results are as follows: (1) Although the percentage increase in the energy density of O+ ions before and after a dipolarization exceeds that of H+ ions in the low‐energy range (9–36 keV/e), this property is not evident in the high‐energy range (56–212 keV/e); (2) the energy spectrum of H+ and that of O+ become harder after dipolarization in almost all events; and (3) in some events the energy spectrum of O+ becomes harder than that of H+ as reported by previous studies, and, importantly, in other events, the spectrum of H+ becomes harder than that of O+. In order to investigate what mechanism causes these observational results, we focus on magnetic field fluctuations during dipolarization. It is found that the increase of the spectrum slope is positively correlated with the power of waves whose frequencies are close to the gyrofrequency of H+ or O+, respectively (the correlation coefficient is 0.48 for H+ and 0.68 for O+). In conclusion, ions are nonadiabatically accelerated by the electric field induced by the magnetic field fluctuations whose frequencies are close to their gyrofrequencies.

Escape of high-energy oxygen ions through magnetopause reconnection under northward IMF

Abstract. During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed high-energy (>180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. We focused on the time period from 11:20 UT to 13:00 UT, when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walén analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP, which is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, we conclude that these oxygen ions are of ring current origin. Our results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause.

Evaluation of the Asymmetry in Photoelectron Distribution Around the GEOTAIL Spacecraft

We examine photoelectron distributions detected by the low-energy-particle (LEP) instrument onboard the GEOTAIL spacecraft by means of both data analysis and numerical simulations. Statistical data analysis shows asymmetries in the photoelectron distributions. For photoelectrons incident normal to the spacecraft spin axis, a higher flux is observed in the dawnward than in the duskward sector of the LEP. The distribution significantly depends on the ratio of the photoelectron energy to the spacecraft potential. Our numerical simulations reveal that the asymmetry is caused by the electrostatic potential around the thin antenna located at +18deg anticlockwise (viewed from the top) relative to the LEP. Photoelectrons in the dawnward sector are preferentially carried from the sunlit surface by this potential. For upward/downward incident photoelectrons, a higher flux of upward photoelectrons is observed in the antisunward than in the sunward sector, whereas downward photoelectrons show a weak asymmetry. Our numerical simulations demonstrate that the greater flux of upward photoelectrons is caused by the electrons emitted from the sunlit surface; they are attracted to the antisunward sector. Based on these results, the asymmetries in the photoelectron distribution measured around GEOTAIL are found to be caused by the asymmetric positioning of the thin antennas relative to the LEP.

Topology of magnetic flux ropes in the magnetospheric plasma sheet as measured by the Geotail spacecraft

Topology of magnetic flux ropes in the magnetospheric plasma sheet as measured by the Geotail spacecraft

Evidence for subauroral proton flashes on the dayside as the result of the ion cyclotron interaction

A series of proton aurora flashes produced by the precipitation of energetic protons equatorward of the main auroral oval was observed between 0700 and 1300 UT on 31 May 2005 on the dayside with the Imager for Magnetopause‐to‐Auroral Global Exploration (IMAGE) Far Ultraviolet (FUV) imager. At the same time multiple bursts of geomagnetic pulsations within the Pc1 range known as “hydromagnetic emission bursts” were observed on several ground stations located in the conjugate MLT sector. The pulsation bursts are well correlated with the appearance of the proton aurora conjugated with the ground stations. The frequency width of the pulsation burst correlates and the upper frequency anticorrelates, respectively, with the latitudinal width and lowest latitude of the proton aurora at the meridian of the ground station. Observations at the meridional network of pulsation magnetometers show that the maximum pulsation spectral density is detected at stations conjugated with proton flashes. The close relationship between pulsations and proton precipitation (aurora) indicates their common source. We conclude that the source is the cyclotron instability of the ring current ions that is stimulated by impulsive magnetospheric compressions. The magnetosphere compressions are, indeed, confirmed by plasma data from the Geotail spacecraft in the duskside magnetosheath. They show a series of plasma pressure pulses during the time interval of interest. Also standard midlatitude magnetograms indicate large‐scale magnetospheric compressions.

Statistical study of electrostatic solitary waves associated with reconnection: Geotail observations

The role of waves in the dynamics of the magnetotail has long been a topic of interest in magnetospheric physics. The characteristics of Electrostatic Solitary Waves (ESWs) associated with reconnection have been studied statistically in the magnetotail by surveying the large amounts data obtained from Waveform Capture (WFC) which is an important component of Plasma Wave Instrument (PWI) on the Geotail spacecraft. About 150 reconnection events with WFC data available are selected, and approximately 10 thousands of ESW waveforms are picked up by hands for statistical study. The ESWs are observed near diffusion region and near the plasma sheet boundary layer (PSBL). Two kinds of waveforms of ESWs are observed: bi-polar and tri-polar pulses. It is found that the pulse width of the ESWs is in the order of 1–5 ms and the peak-to-peak amplitude is in the order of 0.1–5 mV/m. The amplitudes of ESWs are larger in the near-earth tail region than that in deep tail region. ESWs have been observed with or without guide magnetic field 〈By〉. The characteristics of ESWs in different reconnection region and under different strength of guild magnetic field, their possible generation mechanism will be discussed.

Characteristics of electrostatic solitary waves in the Earth's foreshock region: Geotail observations

We observed electrostatic solitary waves (ESW) in the Earth's foreshock region by the Geotail spacecraft. The foreshock region is classified into electron foreshock and ion foreshock regions, which are dominated by energetic electrons and superthermal ions, respectively. The Geotail waveform observations show the existence of ESW in both electron and ion foreshock regions. To understand the wave features of ESW, we examine the waveform and perform statistical analyses on the spatial distribution of the ESW. The results show that the occurrence and amplitude of ESW decrease as the distance from the bow shock transition increases. In the electron foreshock region, we find that ESW and electron beams are observed simultaneously. The plasma conditions are similar to those in the plasma sheet boundary layer (PSBL), in which ESW were first discovered by the Geotail spacecraft. In the ion foreshock region, ESW are simultaneously observed with superthermal ions reflected by the bow shock. We find two types of ESW in the ion foreshock region based on the orientation of their bipolar waveforms. We examine the angle between the magnetic field and the shock normal dependence of the occurrences of ESW in the ion foreshock region. The results show that ESW observed in the quasi‐parallel shock have characteristics that differ from those observed in the quasi‐perpendicular shock. From waveform and statistical analyses, the most plausible generation mechanism of the first type of ESW is the Buneman instability based on the superthermal ions and background electrons. A possible mechanism of the second type of ESW is the positive potential ESW propagating from the upstream region to the bow shock due to the reflection by the negative potentials or the negative potential ESW generated by the reflected superthermal ions.

Statistical characteristics of the storm time plasma sheet

The present study addresses the convection and configuration of the storm time plasma sheet by examining the occurrence ratios of fundamental plasma quantities. Measurements made by the Geotail spacecraft in the midnight sector are used. The results are summarized as follows: (1) the occurrence ratio of the plasma flow velocity is not noticeably different between storm and nonstorm time or between the main and recovery phases; (2) the convection electric field is more intense during the main phase; (3) the equatorial magnetic field is stronger during the main phase; (4) the ion pressure is higher, therefore the tail current is more intense, during the main phase. It is suggested that the stronger equatorial magnetic field (result 3) counteracts the enhancement of the convection electric field (result 2) so that the occurrence ratio of the flow velocity does not depends on geomagnetic activity (result 1). The storm time increase in the equatorial magnetic field can be attributed to the X‐dependent intensification of the dawn‐to‐dusk electric current (the ring current and tail current); if the current is more intensified on the earthward side than on the tailward side, the net change in BZ is positive. It is also found that the reconnection takes place, if infrequently, at X > −15 RE during storm periods. However, result 1 suggests that despite the intense tail current (result 4), the X distribution of the near‐Earth reconnection site does not statistically depend on storm activity. This can also be attributed to the stronger equatorial magnetic field as well as to the storm time enhancement of the near‐Earth plasma number density. It is therefore suggested that the ring current affects the plasma sheet dynamics through its contribution to the tail magnetic configuration.

Geotail observations of two-component protons in the midnight plasma sheet

Abstract. Through the effort to obtain clues toward understanding of transport of cold plasma in the near-Earth magnetotail under northward IMF, we find that two-component protons are observed in the midnight plasma sheet (−10>XGSM>−30 RE, |YGSM| <10 RE) under northward IMF by the Geotail spacecraft. Since the two-component protons are frequently observed on the duskside during northward IMF intervals but hardly on the dawnside, those found in the midnight plasma sheet are thought to come from the dusk flank. The cold proton component in the midnight region occasionally has a parallel anisotropy, which resembles that in the tail flank on the duskside. The flows in the plasma sheet with two-component protons were quite stagnant or slightly going dawnward, which supports the idea that the observed two-component protons in the midnight region are of duskside origin. Because the two-component protons in the midnight plasma sheet emerge under strongly northward IMF with the latitudinal angle larger than 45 degrees, and because the lag from the strongly northward IMF to the emergence can be as short as a few hours, we suggest that prompt plasma transport from the dusk to midnight region occurs under strongly northward IMF. We propose that the dawnward flows result from viscous interaction between the high-latitude portion of the plasma sheet and the lobe cell. Another candidate for plasma transport process from the dusk to the midnight region is turbulent flow due to vortical structures of the Kelvin-Helmholtz instability that developed around the dusk low-latitude boundary under strongly northward IMF. In addition, we also suggest that gradual cooling of hot protons under northward IMF is a global phenomenon in the near-Earth magnetotail.

Simultaneous observations of fluctuating cusp aurora and low‐latitude magnetopause reconnection

The location and variability of magnetic reconnection are investigated using simultaneous in situ observations of boundary layer flows at the dayside magnetopause and remote sensing of proton cusp aurora in the ionosphere. Two events when the Geotail spacecraft was at the magnetopause and the Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft was observing cusp proton precipitation are used in this investigation. The directions of high‐speed flows observed in the boundary layer by the Geotail spacecraft are compared to predicted directions from the antiparallel reconnection model and from two component reconnection models. The IMAGE cusp proton aurora observations provide additional information on the type of reconnection and on variability in the reconnection rate. For the first event, antiparallel reconnection may be occurring at the magnetopause and there is a long‐duration (10 s of minutes) decrease in the proton aurora intensity. For the second event, component reconnection is occurring and variability in the cusp emissions on a timescale of several minutes appears to indicate variability in the reconnection rate.

Equator‐S observations of drift mirror mode waves in the dawnside magnetosphere

The mirror mode is a plasma instability that is typically excited in high‐beta plasmas where there is significant pressure anisotropy and is most commonly observed in the magnetosheath. However, it is possible under sufficiently anisotropic conditions to generate the mirror instability inside the magnetosphere, though as yet there are few examples. We present an extended interval of ∼7 hours of mirror mode activity on 19 March 1998 when the Equator‐S spacecraft was traversing the dawnside magnetosphere above the equatorial plane at radial distances of up to L ∼ 11 and encountered quasi‐monochromatic ∼2‐min oscillations in magnetic field and ion number density, temperature, and velocity. The magnetic field strength and number densities were in antiphase, and the plasma and magnetic field pressure perturbations were also in antiphase while the observed total pressure remained constant. This is consistent with excitation via a mirror instability. We are able to discern that the mirror activity must be confined to approximately within ±20° of the equatorial plane through a conjunction with the Geotail spacecraft. We find that the condition for mirror mode waves to grow is strongly met throughout the interval (Hasegawa, 1969). We believe that this is an excellent high temporal resolution example of the mirror instability exciting ULF waves inside the Earth's magnetosphere. Given the coexistence of toroidal oscillations at almost the same frequency, we suggest coupling between the mirror mode and local standing Alfvén waves. Our observations hence add to the understanding of how energy can be transferred from hot plasma into ULF waves in the magnetosphere before being dissipated in the ionosphere.

Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

The Geotail spacecraft made an inbound passage perpendicular to the dusk equatorial magnetopause on 1 August 1998 when the interplanetary magnetic field had been very northward for more than 10 hours. As the spacecraft moved through the low‐latitude boundary layer, it detected waves with ∼3 min period that caused transitions between cool dense magnetosheath plasma and mixed magnetosheath and magnetosphere plasmas. A region of highly fluctuating magnetic fields with variations up to 50 nT/s was observed on leaving the magnetosheath plasma but not on its return to this region. This observation suggests that the boundary region is not laminar but more likely consists of Kelvin‐Helmholtz vortices. Such fluctuations are probably caused by twisting of the field by the vortices, and they could be important in transporting plasma to the magnetosphere where densities of 5/cc were observed. A global MHD simulation of the event reproduced the observed low‐frequency waves, their counterclockwise polarization, and a region of high‐velocity magnetosheath flow outside the magnetopause. This flow was associated with a decreased density and increased field strength which are characteristics of a plasma depletion layer. When Geotail moved further inward into the magnetosphere, it detected waves that are probably the magnetotail vortices which had been detected earlier by the ISEE spacecraft.

Comparative Study of the Initial Spikes of Soft Gamma-Ray Repeater Giant Flares in 1998 and 2004 Observed withGeotail: Do Magnetospheric Instabilities Trigger Large-Scale Fracturing of a Magnetar's Crust?

We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak profile revealed four characteristic features: an initial steep rise, an intermediate rise to the peak, an exponential decay, and a small hump in the decay phase. From this light curve, we found that the isotropic peak luminosity was 2.3 × 1046 ergs s-1 and that the total energy was 4.3 × 1044 ergs s-1 (E 50 keV), assuming that the distance to SGR 1900+14 is 15 kpc and that the spectrum is optically thin thermal bremsstrahlung with kT = 240 keV. These values are consistent with the previously reported lower limits derived from Ulysses and Konus-Wind observations. A comparative study of the initial spikes of the SGR 1900+14 giant flare in 1998 and of the SGR 1806-20 giant flare in 2004 is also presented. The timescale of the initial steep rise shows a magnetospheric origin, while the timescale of the intermediate rise to the peak indicates that it originates from crustal fracturing. Finally, we argue that the four features and their corresponding timescales provide us with a clue to identify extragalactic soft gamma-ray repeater giant flares among short gamma-ray bursts.

Temperature anisotropies of electrons and two-component protons in the dusk plasma sheet

Abstract. To investigate the cold plasma sheet formation under northward IMF, we study the temperature anisotropies of electrons and two-component protons observed by the Geotail spacecraft. The two-component protons, which are occasionally observed in the dusk plasma sheet near the low-latitude boundary, are the result of spatial mixing of the hot protons of the magnetosphere proper and the cold protons from the solar wind. Recent research focusing on the two-component protons reported that the cold proton component at times has a strong anisotropy, and that the sense of the anisotropy depends on the observed locations. Since electrons have been known to possess a strong parallel anisotropy around the low-latitude boundary layer, we compare anisotropies of electrons and protons to find that the strengths of parallel anisotropies of electrons and the cold proton component are in good correlation in the tail flank. The parallel anisotropy of electrons is stronger than that of the cold proton component, which is attributed to selective heating of electrons. We further find that the strengths of the parallel anisotropies in the tail flank depend on the latitudinal angle of the IMF; strong parallel anisotropies occur under strongly northward IMF. We discuss that the Kelvin-Helmholtz vortices, which developed under strongly northward IMF, and the resultant magnetic reconnection therein may lead to the strong parallel anisotropies observed in the tail flank.

MHD flow visualization of magnetopause boundary region vortices observed during high‐speed streams

We present statistics for a total of 304 vortices found near the ecliptic plane on the magnetopause flanks, using simulated magnetohydrodynamics (MHD) data driven by real solar wind conditions. The study concentrates on 9 hours, from 29 March, 2000 UT to 30 March, 0500 UT, during the onset of a high‐speed stream that existed from 29 March to 5 April 2002. Magnetopause crossings seen by the Geotail spacecraft for the 9‐hour time interval were also analyzed and compared with the MHD simulation to validate our results. Vortices were classified by solar wind input provided by the Wind satellite located 60–80 RE upstream from Earth. Two hundred seventy‐three of the vortices were generated under northward Interplanetary Magnetic Field (IMF), and 31 were generated under southward IMF. The vortices generated under northward IMF were more prevalent on the dawnside than on the duskside and were substantially less ordered on the dawnside than on the duskside. Most of the vortices were large in scale, up to 10 RE, and with a rotation axis closely aligned with the ZSM direction. They rotated preferentially clockwise on the dawnside, and counterclockwise on the duskside. Those generated under southward IMF were less ordered, fewer in number, and also smaller in diameter. Significant vortex activity occurred on the nightside region of the magnetosphere for these southward cases in contrast to the northward IMF cases on which most of the activity was on the magnetopause flanks. The IMF is primarily northward for our time interval, and the development of these vortices with their rotation preference depending on their local time position suggest that a Kelvin‐Helmholtz (KH) instability is likely present.

Multi-spacecraft observation of plasma dipolarization/injection in the inner magnetosphere

Abstract. Addressing the origin of the energetic particle injections into the inner magnetosphere, we investigate the 23 February 2004 substorm using a favorable constellation of four Cluster (near perigee), LANL and Geotail spacecraft. Both an energy-dispersed and a dispersionless injection were observed by Cluster crossing the plasma sheet horn, which mapped to 9–12 RE in the equatorial plane close to the midnight meridian. Two associated narrow equatorward auroral tongues/streamers propagating from the oval poleward boundary could be discerned in the global images obtained by IMAGE/WIC. As compared to the energy-dispersed event, the dispersionless injection front has important distinctions consequently repeated at 4 spacecraft: a simultaneous increase in electron fluxes at energies ~1..300 keV, ~25 nT increase in BZ and a local increase by a factor 1.5–1.7 in plasma pressure. The injected plasma was primarily of solar wind origin. We evaluated the change in the injected flux tube configuration during the dipolarization by fitting flux increases observed by the PEACE and RAPID instruments, assuming adiabatic heating and the Liouville theorem. Mapping the locations of the injection front detected by the four spacecraft to the equatorial plane, we estimated the injection front thickness to be ~1 RE and the earthward propagation speed to be ~200–400 km/s (at 9–12 RE). Based on observed injection properties, we suggest that it is the underpopulated flux tubes (bubbles with enhanced magnetic field and sharp inner front propagating earthward), which accelerate and transport particles into the strong-field dipolar region.

Geotail observations of temperature anisotropy of the two-component protons in the dusk plasma sheet

Abstract. In search for clues towards the understanding of the cold plasma sheet formation under northward IMF, we study the temperature anisotropy of the two-component protons in the plasma sheet near the dusk low-latitude boundary observed by the Geotail spacecraft. The two-component protons result from mixing of the cold component from the solar wind and the hot component of the magnetospheric origin, and may be the most eloquent evidence for the transport process across the magnetopause. The cold component occasionally has a strong anisotropy in the dusk flank, and the sense of the anisotropy depends on the observed locations: the parallel temperature is enhanced in the tail flank while the perpendicular temperature is enhanced on the dayside. The hot component is nearly isotropic in the tail while the perpendicular temperature is enhanced on the dayside. We discuss possible mechanism that can lead to the observed temperature anisotropies.

Effects of a moving X-line in a time-dependent reconnection model

Abstract. In the frame of magnetized plasmas, reconnection appears as an essential process for the description of plasma acceleration and changing magnetic field topology. Under the variety of reconnection regions in our solar system, we focus our research onto the Earth's magnetotail. Under certain conditions a Near Earth Neutral Line (NENL) is free to evolve in the current sheet of the magnetotail. Reconnection in this region leads to the formation of Earth- and tailward propagating plasma bulges, which can be detected by the Cluster or Geotail spacecraft. Observations give rise to the assumption that the evolved reconnection line does not provide a steady state behavior, but is propagating towards the tail (e.g., Baker et al., 2002). Based on a time-dependent variant of the Petschek model of magnetic reconnection, we present a method that includes an X-line motion and discuss the effects of such a motion. We focus our main interest on the shock structure and the magnetic field behavior, both for the switch-on and the switch-off phase.

Convection electric field in the near-Earth tail during the super magnetic storm of November 20–21, 2003

[1] We examine the electric field observations made by the Geotail spacecraft in the magnetotail during the intense magnetic storm of November 20–21, 2003. During this storm, Geotail traversed the magnetotail at R ∼ 9–12 Re from dusk to dawn through the midnight sector at ∼17 UT on November 20 and observed the convection electric field in the near-Earth plasma sheet for the main phase and early recovery phase. While the electric field in the plasma sheet during the storm interval shows highly turbulent characteristics in a wide range of local time, the average intensity of the duskward electric field is not so different in magnitude from those observed during other small to moderate level storms. A comparison with those calculated using the empirical models reveals that the independent behavior of the tail convection intensity with regard to the storm driver intensity still holds for an intense storm case. These results give a further support to the hypothesis suggested by our previous study that the tail convection supplies the ring current particles rather constantly regardless of the solar wind condition.