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Collision between ions and neutral particles is an essential characteristic of Earth's ionosphere. This ion-neutral collision is usually caused by the polarization of neutral particles. This collision can also be caused by charge exchange, if the particle pair is parental, such as atomic oxygen and its ion. The total collision frequency is not the sum of the polarization and charge-exchange components, but is essentially equal to the dominant component. The total is enhanced only around the classic transition temperature, which is near the ionospheric temperature range (typically 200–2000 K). However, the magnitude of this enhancement has differed among previous studies; the maximum enhancement has been reported as 41% and 11% without physical explanation. In the present study, the contribution of the polarization force to the charge-exchange collision is expressed as a simple curved particle trajectory effect. As a result, the maximum enhancement is found to be 22%. It is discussed that the enhancement has been neglected in classic ionospheric studies partly due to confusion with the glancing particle contribution, which adds 10.5% to the polarization component. The enhancement has been neglected presumably also because there has been no functional form to express it. Such an expression is derived in this study.

The collision between atomic oxygen and its first positive ion plays a major role in Earth's F region ionosphere. An accurate corresponding collision frequency model is necessary to quantitatively understand the ionosphere. However, the widely used classic Banks theoretical model typically provides a collision frequency that is 30% lower than the expectation from ionospheric observations. Accordingly, the classic collision frequency is often adjusted by multiplying it by a constant known as the Burnside factor. This correction-factor model adopted the classic model as its basis due to a misunderstanding that the classic model was based on a laboratory experiment; that is, the correction factor was originally meant to compensate for laboratory contamination. In this study, a collision frequency model is constructed based on the laboratory experiment, and the resultant laboratory-based model is found to be consistent with ionospheric observations. In this construction, the impact of laboratory contamination is determined to be small (7%) and is mostly canceled by a misinterpretation regarding the conventional definitions of energy. Thus, the 30% difference is mainly caused by a theoretical error in the classic model itself. This error is energy-dependent and corrected by the later wide-energy theoretical model. Thus, the classic model cannot be corrected by a temperature-independent constant and should be replaced by the later model.

Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora scenario of substorm triggering. In this scenario, a substorm is triggered by a fast earthward flow generated at the distant neutral line which corresponds to a preonset auroral streamer or arc in the ionosphere propagating from the auroral poleward boundary to the initial auroral brightening site, i.e., “preonset aurora”. In the present paper, we revisited three substorm events reported as being triggered by such a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data. Unlike previous studies, we examined the arrival timing of the preonset aurora relative to the three steps of auroral onset arc development (initial brightening, enhancement of the wave-like structure, and poleward expansion) to make the role of the preonset aurora in the auroral steps clearer. Our detailed timing analysis found that preonset auroral streamers reached the auroral onset arc but away from the initial brightening site after initial brightening for two events, while no preonset aurora reaching the initial brightening site could be identified for the other event. This result suggests that the processes associated with auroral streamers are unlikely to affect at least initial brightening, even if we consider not only the presence and arrival timing and location of the auroral streamers but also the scale of the corresponding flow and flow vortices. We list a series of open questions for testing the preonset aurora scenario further in future studies. Keywords. Magnetospheric physics (storms and substorms; auroral phenomena; magnetotail)

In the original publication of this article [1], some reference lines are missing in Fig. 5a and Fig. 6b(6). This correction shows the correct figures. The publisher apologizes to the readers and authors for the inconvenience.

Auroral stepwise poleward expansions were clarified by investigating a multiple-onset substorm that occurred on 27 February 2009. Five successive auroral brightenings were identified in all-sky images, occurring at approximately 10 min intervals. The first brightening was a faint precursor. The second brightening had a wide longitude; thus, it represented the Akasofu substorm onset. Other brightenings expanded poleward; thus, they were interpreted to be auroral breakups. These breakups occurred stepwise; that is, later breakups were initiated at higher latitudes. Corresponding reconnection signatures were studied using Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observations between 8 and 24 RE down the magnetotail. The Akasofu substorm onset was not accompanied by a clear reconnection signature in the tail. In contrast, the three subsequent auroral breakups occurred simultaneously (within a few minutes) with three successive fast flows at 24 RE; thus, these were interpreted to be associated with impulsive reconnection episodes. These three fast flows consisted of a tailward flow and two subsequent earthward flows. The flow reversal at the second breakup indicated that a tailward retreat of the near-Earth reconnection site occurred during the substorm expansion phase. In addition, the earthward flow at the third breakup was consistent with the classic tailward retreat near the end of the expansion phase; therefore, the tailward retreat is likely to have occurred in a stepwise manner. We interpreted the stepwise characteristics of the tailward retreat and poleward expansion to be potentially associated by a stepwise magnetic flux pileup.

Molecular oxygen collides with its first positive ion in the Earth's ionosphere. The collision frequency of this particle pair is used to calculate the electric conductivity of the ionosphere. However, for this parental pair there are two collision types, resonant and nonresonant, and the selection of the collision type has differed among previous studies in calculation of conductivity. In the present study, we clarify that the nonresonant collision is physically essential for this pair because the relevant temperatures are low. That is, the peak of the ionospheric conductivity occurs at altitudes between 100 and 130 km, where the temperatures of ions and neutral particles are usually lower than 600 K, and for these temperatures nonresonant collisions are dominant. The collision frequency would be underestimated by 30% if the resonant collision was assumed at an altitude of 110 km (where the temperature is 240 K). The impact of this difference on the conductivity is estimated to be small (3%), primarily because molecular nitrogen is much more abundant than molecular oxygen. Although we have confirmed that the nonresonant collision is essential, we also include the resonant type, primarily in case of possible elevated temperature events. A set of ion‐neutral collision frequency coefficients for calculating the conductivity is summarized, including other particle pairs, in the Appendices. Small corrections to the traditional coefficients are made., ファイル公開：2020-08-01

Substorm onset has originally been defined as a longitudinally extended sudden auroral brightening (Akasofu initial brightening: AIB) followed a few minutes later by an auroral poleward expansion in ground-based all-sky images (ASIs). In contrast, such clearly marked two-stage development has not been evident in satellite-based global images (GIs). Instead, substorm onsets have been identified as localized sudden brightenings that expand immediately poleward. To resolve these differences, optical substorm onset signatures in GIs and ASIs are compared in this study for a substorm that occurred on December 7, 1999. For this substorm, the Polar satellite ultraviolet global imager was operated with a fixed-filter (170nm) mode, enabling a higher time resolution (37 s) than usual to resolve the possible two-stage development. These data were compared with 20-s resolution green-line (557.7nm) ASIs at Muonio in Finland. The ASIs revealed the AIB at 2124:50UT and the subsequent poleward expansion at 2127:50UT, whereas the GIs revealed only an onset brightening that started at 2127:49UT. Thus, the onset in the GIs was delayed relative to the AIB and in fact agreed with the poleward expansion in the ASIs. The fact that the AIB was not evident in the GIs may be attributed to the limited spatial resolution of GIs for thin auroral arc brightenings. The implications of these results for the definition of substorm onset are discussed herein.

Understanding of underlying mechanisms of drastic variations of the near-Earth space (geospace) is one of the current focuses of the magnetospheric physics. The science target of the geospace research project Exploration of energization and Radiation in Geospace (ERG) is to understand the geospace variations with a focus on the relativistic electron acceleration and loss processes. In order to achieve the goal, the ERG project consists of the three parts: the Arase (ERG) satellite, ground-based observations, and theory/modeling/integrated studies. The role of theory/modeling/integrated studies part is to promote relevant theoretical and simulation studies as well as integrated data analysis to combine different kinds of observations and modeling. Here we provide technical reports on simulation and empirical models related to the ERG project together with their roles in the integrated studies of dynamic geospace variations. The simulation and empirical models covered include the radial diffusion model of the radiation belt electrons, GEMSIS-RB and RBW models, CIMI model with global MHD simulation REPPU, GEMSIS-RC model, plasmasphere thermosphere model, self-consistent wave–particle interaction simulations (electron hybrid code and ion hybrid code), the ionospheric electric potential (GEMSIS-POT) model, and SuperDARN electric field models with data assimilation. ERG (Arase) science center tools to support integrated studies with various kinds of data are also briefly introduced.

We report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) of 17–25 February 2012 at subauroral latitudes at Athabasca (L=4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising-tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling-tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty-patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions.

To investigate the physical mechanism responsible for substorm triggering, we performed a superposed-epoch analysis using plasma and magnetic-field data from THEMIS probes. Substorm onset timing was determined based on auroral breakups detected by all-sky imagers at the THEMIS ground-based observatories. We found earthward flows associated with north–south auroral streamers during the substorm growth phase. At around X = −12 Earth radii (RE), the northward magnetic field and its elevation angle decreased markedly approximately 4 min before substorm onset. Moreover, a northward magnetic-field increase associated with pre-onset earthward flows was found at around X = −17 RE. This variation indicates that local dipolarization occurs. Interestingly, in the region earthwards of X = −18 RE, earthward flows in the central plasma sheet (CPS) reduced significantly approximately 3 min before substorm onset, which was followed by a weakening of dawn-/duskward plasma-sheet boundary-layer flows (subject to a 1 min time lag). Subsequently, approximately 1 min before substorm onset, earthward flows in the CPS were enhanced again and at the onset, tailward flows started at around X = −20 RE. Following substorm onset, an increase in the northward magnetic field caused by dipolarization was found in the near-Earth region. Synthesizing these results, we confirm our previous results based on GEOTAIL data, which implied that significant variations start earlier than both current disruption and magnetic reconnection, at approximately 4 min before substorm onset roughly halfway between the two regions of interest; i.e. in the catapult current sheet.

[1] We devise an approach to calculate the polarization electric field in the ionosphere, when the ionospheric conductances, the primary (modeled) or the total (measured) electric field, and the Cowling efficiency are given. In contrast to previous studies, our approach is a general solution which is not limited to specific geometrical setups, and all parameters may have any kind of spatial dependence. The solution technique is based on spherical elementary current (vector) systems (SECS). This way, we avoid the need to specify explicit boundary conditions for the searched polarization electric field of its potential which would be required if the problem was solved in a differential equation approach. Instead, we solve an algebraic matrix equation, and the implicit boundary condition that the divergence of the polarization electric field vanishes outside our analysis area is sufficient. In order to illustrate our theory, we then apply it to two simple models of auroral electrodynamic situations, the first being a mesoscale strong conductance enhancement in the early morning sector within a relatively weak southward primary electric field, and a morning sector auroral arc with only a weak conductance enhancement, but a large southward primary electric field at the poleward flank of the arc. While the significance of the polarization electric field for maximum Cowling efficiency is large for the first case, it is rather minor for the second one. Both models show that the polarization electric field effect may not only change the magnitude of the current systems but also their overall geometry. Furthermore, the polarization electric field may extend into regions where the primary electric field is small, thus even dominating the total electric field in these regions. For the first model case, the total Joule heating integrated over the analysis area decreases by a factor of about 4 for maximum Cowling efficiency as compared to the case of vanishing Cowling efficiency. Furthermore, for this case the resulting total electric field structurally shows a strong resemblance to that frequently observed during auroral omega band events.

The solar zenith angle (SZA) dependence of the conductance is studied and a simple theoretical form for the Hall-to-Pedersen conductance ratio is developed, using the peak plasma production height. The European Incoherent Scatter (EISCAT) radar observations at Tromso (67 MLAT) on 30 March 2012 were used to calculate the conductance. The daytime electric conductance is associated with plasma created by solar extreme ultraviolet radiation incident on the neutral atmosphere of the Earth. However, it has been uncertain whether previous conductance models are consistent with the ideal Chapman theory for such plasma productions. We found that the SZA dependence of the conductance is consistent with the Chapman theory after simple modifications. The Pedersen conductance can be understood by approximating the plasma density height profile as being flat in the topside E region and by taking into account the upward gradient of atmospheric temperature. An additional consideration is necessary for the Hall conductance, which decreases with increasing SZA more rapidly than the Pedersen conductance. This rapid decrease is presumably caused by a thinning of the Hall conductivity layer from noon toward nighttime. We expressed this thinning in terms of the peak production height in the Chapman theory.

Chorus and hiss-like emissions in the VLF band (several hundred Hz to 10 kHz) are the most common and intense whistler mode waves in the inner magnetosphere. Though frequency spectra of chorus waves having rising tone frequencies are different from those of hiss-like emissions, their wave amplitude and propagation characteristics are similar [1]. A recent simulation study shows that geomagnetic field gradient along a field line near the equatorial plane plays an important role in the generation of hiss-like emissions and chorus emissions [2]. When the geomagnetic field gradient varies to more gradual one, a threshold amplitude for triggering chorus emissions becomes smaller in the nonlinear wave growth theory. Then, multiple chorus emissions are frequently generated, and hiss-like emissions are produced by merging these chorus emissions in frequency spectra. We observed a high correlation between luminosity of a pulsating aurora (PA) and intensities of grouped chorus and hiss-like emissions on 7 February 2013 at Athabasca (L=4.3) in Canada. In this event, the periodic PA luminosity disappeared when the chorus emissions changed to hiss-like emissions. For this event, variations of the geomagnetic field gradient near the equator were estimated by using the Tsyganenko model [3]. The estimated geomagnetic field gradient became more gradual when the hiss-like emissions were observed. It is consistent with the nonlinear wave growth theory of the generation process of hiss-like emissions. Also, these observation results suggest a close relationship between periods of PA and hiss-like emissions. In this presentation, we will discuss the importance of the effect of geomagnetic field gradient on periods of PA and hiss-like emissions.

Accepted: 2000-04-26, 資料番号: SA1002541000

Accepted: 1998-12-23, 資料番号: SA1002490000

[1] A physical process for the latitudinal motion of an auroral arc based on the four-side bound Cowling channel model is proposed. Assuming that an upward field-aligned current (FAC) is associated with the auroral arc that forms a Cowling channel with finite lengths not only latitudinally but also longitudinally and that the upward FAC region is primarily embedded in a purely northward electric field, the primary Hall current driven by the northward electric field accumulates positive excess charges at the eastern edge of the channel and negative charges at the western edge for a perfect or partial Cowling channel with a nonzero Cowling efficiency. The charges produce a westward secondary electric field, indicating that a westward electric field can thus be produced by a purely northward primary electric field. This secondary electric field moves the arc with its magnetospheric source drifting together with the magnetospheric plasmas equatorward and simultaneously produces the electric field outside the channel that moves the downward FAC equatorward of the upward FAC region equatorward together with the upward FAC. Thus, the whole 3-D current system is expected to move equatorward as often observed in the afternoon auroral zone.

[1] Toward the understanding of the effect of the magnetosphere originated disturbances on the global ionospheric electric field and current system, we developed a two-dimensional ionospheric potential solver based on the so-called “thin shell model.” The important extension from the previous studies is that our model covers the pole-to-pole ionosphere without placing any boundary at the equator. By using this solver, we investigate how the ionospheric electric field changes from undershielding condition to overshielding condition as the field aligned current (FAC) distribution changes. Calculations are performed by changing IR2/IR1 (the ratio of current intensities of region 2 (R2) and region 1 (R1) FACs) and by moving R2-FAC relative to the fixed R1-FAC. The results are summarized as follows: (1) The turning point, at which the ionosphere turns from undershielding to overshielding is IR2/IR1 = 0.7 ∼ 0.8. (2) With increasing the local time deference between the R1 and R2-FAC peaks, the efficiency of the shielding by R2-FAC increases but the associated potential skews to the nightside. (3) At the same time the shielding effect is weakened around noon, where the R1-potential intrudes to the low latitude region instead, but the R2-potential remains dominant at other local times. The result suggests that the overshielding or undershielding should be identified by observations not only in a limited local time sector but also in the overall ionosphere as much as possible. In order to accurately describe the ionospheric condition, we suggest new classification terms, “complete-overshielding” and “incomplete-overshielding.”

[1] A high correlation between a pulsating auroral patch and grouped chorus waves was observed on 17 April 2006 at Syowa Station in Antarctica. The spatial distribution of aurora–chorus correlation coefficients is evaluated in order to determine the source region. A pulsating patch at the highest-correlation pixel shows a one-to-one correspondence with the intensity variation of the grouped chorus waves, consisting of successive rising-tone elements with a duration and spacing of 2–3 s and 20–30 s, respectively. The generation region of the chorus waves is estimated from the latitude and longitude dependence of the equatorial electron gyrofrequencies using the IGRF geomagnetic field model. The extent of the estimated latitude and longitude is consistent with the spatial distribution of the high-correlation aurora–chorus region. The time difference between the chorus waves and the scattered electrons is also evaluated to discuss the validity of the source region. It shows that electrons reached the ionosphere sooner than the associated chorus waves by ∼1 s, consistent with the theoretical value for conjugate pulsating aurora generated at the equator. These results support the hypothesis that pulsating aurora is caused by pitch angle scattering of high-energy electrons by whistler mode chorus waves, via a cyclotron resonance at the equator. These results are the first ground-based observations of high correlations between a spatially extended aurora and chorus waves.

[1] We have studied successive substorm expansions that occurred in spite of a period of prolonged northward interplanetary magnetic field (IMF). After the northward IMF persisted for almost ∼19 h, a series of 11 very weak to moderate substorm expansions occurred during the following 6-h interval of northward IMF on 19 January 1998, separated from each other by a few tens of minutes. Most of these substorm expansions did not accompany significant changes of the solar wind and the IMF. As for typical substorms that occur under southward IMF conditions, spacecraft and ground-based instruments observed auroral breakups and enhancements of the westward auroral electrojet and large-scale convection in the ionosphere, together with plasmoids and dipolarizations in the magnetotail, in association with the substorm expansion onsets. These signatures suggest that the mechanism of substorm expansion onsets is the same as for typical substorms, even under prolonged northward IMF conditions. We suggest that the large IMF ∣By∣ affects large-scale convection and energy accumulation in the magnetotail and that the enhanced convection due to the large IMF ∣By∣ effect and substorm expansions can be an important factor for the promotion of succeeding substorm expansion onsets.

We have statistically studied substorm-associated evolution of the near-Earth magnetotail to understand the substorm triggering mechanism. In the present chapter we focus on changes in the total and plasma pressures. We find that energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization, i.e., at –12 > X > –18 R e than in the surrounding regions. Unlike previously reported results, the plasma pressure increases in association with the initial dipolarization at X > –12 R e, the increase largely contributed by high-energy particles. This result suggests that the rarefaction wave scenario proposed in the current disruption model is questionable.

[1] We have studied plasma (ion) pressure changes that occurred in association with the dipolarization in the near-Earth plasma sheet around substorm onsets. Using Geotail data, we have performed a superposed epoch analysis in addition to detailed examinations of two individual cases with special emphasis on the contribution of high-energy particles to the plasma pressure. It is found that, unlike previously reported results, the plasma pressure does increase in association with the initial dipolarization at X > ∼−12 RE and −2 < Y < 6 RE, with the increase largely due to high-energy particles. Outside the initial dipolarization region, particularly tailward and duskward of this region, the plasma pressure begins to decrease owing to the magnetic reconnection before onset or before the dipolarization region reaches there. At later times, the plasma pressure tends to increase there, related to the expanding dipolarization region, but the contribution of high-energy particles is not very large. These observations suggest the following. The rarefaction wave scenario proposed in the current disruption model is questionable. The radial and azimuthal pressure gradients may strengthen between the initial dipolarization and outside regions, possibly resulting in stronger braking of fast earthward flows and changes in field-aligned currents. The characteristics of the dipolarization may differ between the initial dipolarization and tailward regions, which would be possibly reflected in the auroral features. Furthermore, we have examined the specific entropy and the ion β. The specific entropy increases in the plasma sheet in the dipolarization region as well as in the midtail region in conjunction with substorm onsets, suggesting from the ideal MHD point of view that the substorm processes are nonadiabatic. The ion β is found to peak at the magnetic equator in the initial dipolarization region around dipolarization onsets.

[1] Based on observations by a high-resolution narrow field-of-view CCD camera, we found small-scale (5–25 km) finger-like structures at the western boundary of auroral patches in images obtained at Gillam (geomagnetic latitude: 65.5°N), Canada, in January 2008. Since shear motion was not observed along the boundary of the patches, we suggest that these structures are formed by macroscopic Rayleigh-Taylor type plasma instability arising in the magnetospheric equatorial plane from the force balancing of the (eastward) magnetic tension force and the (westward) pressure gradient force.

Accepted: 2009-09-28, 資料番号: SA1000794000

[1] The dawn–dusk locations of reconnection in the near-earth magnetotail at the time of isolated auroral breakup are studied to clarify whether breakup is always accompanied by reconnection. The near-earth reconnection is identified by tailward plasma flows faster than 200 km/s with southward magnetic field. We first identified 66 breakups in the Polar ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail in situ observations of the plasma sheet between 25 and 31 RE down the tail. It was found that the dawn–dusk (Y) locations of relatively fast (≥400 km/s) tailward flows were associated with breakup magnetic local time (MLT) by a regression line of YAGSM = −5.7 × (MLT + 0.6) RE with a correlation coefficient of 0.8. Most tailward flows were observed within 5 RE of the modeled Y locations, where tailward flows occurred in 88% of the 26 cases of breakups between 22 and 0 MLT. It is thus inferred that in most cases, breakup is accompanied by tailward flow near the breakup MLT with its dawn–dusk dimension ∼10 RE. There were only two events without tailward flows in the region where flows have been expected. These two events were an earthward flow event and a traveling compression region event, which are not inconsistent with the initiation of the near-earth reconnection. Auroral breakup is thus likely to always be accompanied by near-earth reconnection near breakup MLT. It is also inferred that reconnection and breakup occur simultaneously within a few minutes, assuming a time delay between reconnection onset and the arrival of tailward flows at satellite locations.

[1] In this study we first identify earthward plasma sheet fast flows from Geotail plasma and magnetic fields with a criterion of V⊥x > 300 km/s, where V⊥x is the X component of the plasma flow perpendicular to the ambient magnetic field. We then estimate rates of change of the nightside auroral power over the courses of the fast flows using Polar Ultraviolet Imager auroral images. It is found that 68 earthward fast flows observed at ∣Y∣ 2 mV/m for an identification of fast flows, where Vx is the X component of the plasma flow and Bz is the Z component of the geomagnetic field. It is found that V⊥x is a better parameter than VxBz to differentiate the two classes of earthward fast flows in the plasma sheet.

[1] Two coronal mass ejections associated with the X17 and X10 solar flares reached the Earth's environment at very high speeds on 29 and 30 October 2003, respectively, causing very intense geomagnetic storms (Dst ∼ −400 nT). The present study focused on the main phase of the 30 October storm during which the Geotail spacecraft was within the near-Earth magnetotail at X ∼ −8 RE. A number of extremely intense substorms occurred during this period. In one of them, the intensity of the westward auroral electrojet exceeded 3000 nT, which was one of the largest magnitudes ever observed. The energetic particle observations from the low-altitude, polar-orbiting NOAA satellites indicate that the auroral oval shifted equatorward to magnetic latitudes much lower than usual, as low as 50°. Throughout the interval, the magnetic field in the near-Earth magnetotail, and possibly the plasma density, was much larger than usual, indicating a considerable degree of energy accumulation in the lobe region and compression of the plasma sheet and very intense cross-tail currents. The dense plasma may be responsible for the intense auroral electrojet and the intense ring current. Very large, rapid dipolarizations occurred in relation to the intense substorms. High-energy particle fluxes were an order of magnitude higher than usual, and their increases took place immediately after the dipolarizations. Fast tailward flows with large southward magnetic fields as well as fluxes of energetic heavy ions (oxygen) were also observed, suggesting that the magnetic reconnection took place in the near-Earth magnetotail, associated with the very intense substorms. This location is much closer to the Earth than usual, probably as close to the Earth as ever reported. These magnetotail and auroral observations as well as other results reported previously suggest that the entire magnetosphere was considerably distorted during the storm.

[1] To understand magnetotail dynamics, it is essential to determine where magnetic reconnection takes place in the near-Earth magnetotail during substorms. The Geotail spacecraft thoroughly surveyed the near-Earth plasma sheet at radial distances of 10–31 RE during the years 1995–2003. Thirty-four clear reconnection events were identified using the criterion of strong electron acceleration. Various solar wind parameters prior to each reconnection event were examined in order to find the factor controlling the location of the magnetic reconnection site in the magnetotail. The same analyses were carried out for fast tailward flow events. The most important factor was determined to be the solar wind energy input, which can be expressed by −Vx × Bs, where Vx is the x component of the solar wind velocity and Bs is the southward component of the interplanetary magnetic field. It is likely that higher efficiency of energy input, rather than the total amount of energy input, primarily controls the location of magnetic reconnection; magnetic reconnection takes place closer to the Earth when efficiency of energy input is higher. The effect of solar wind dynamic pressure is minor. The present result suggests that the tail magnetic reconnection location during substorms is controlled by solar cycle variations in the solar wind.

[1] Examination of observations taken by radially aligned International Solar Terrestrial Physics spacecraft in the nightside magnetosphere on 9 July 1997 has revealed close temporal correlations between earthward flow bursts in the plasma sheet and the ejection of plasmoids. A one-dimensional model of plasma sheet flow is applied to these observations to determine the time and location for the initiation of lobe flux tube reconnection. For the single clear flow burst-plasmoid pair observed during the first substorm and the three pairs produced by the second substorm, lobe flux reconnection was inferred to have started at X ∼ −15 to −18 RE, respectively, about 2–5 min prior to the observations of substorm expansion phase onset. These time delays and propagation speeds are shown to be consistent with the measured plasma sheet bulk flow speeds. Substorm expansion phase onset was essentially coincident with the arrival of the flow bursts at Geotail, which was located near the inner edge of the plasma sheet at X ∼ −9 RE. The dipolarization of the magnetic field at geosynchronous orbit, auroral kilometric radiation (AKR) emissions, Pi2 pulsations, high-latitude negative magnetic bays, and auroral breakup marking substorm expansion onset are all coincident within the ±1 min resolution of the measurements. Accordingly, it appears that earthward of the inner edge of the plasma sheet, where Geotail was located, substorm effects propagated at speeds comparable to the Alfven speed characteristic of the high-latitude inner magnetosphere, ∼103 km s−1. In summary, the results of our investigation strongly support the modern near-Earth neutral line (NENL) model of substorms in which the onset of lobe flux tube reconnection in the near tail is followed ∼2–5 min later by the braking of earthward flow bursts as they encounter the inner magnetosphere and within ∼1 min, by Pi2s generations, current wedge development, and AKR and auroral expansion, and finally, ∼10–20 min later, by the tailward retreat of the neutral line and either the development of a new NENL or the initiation of the recovery phase.

[1] We analyze Geotail observations of magnetic field and flow oscillations in the dayside magnetosphere that occurred in direct association with an event of traveling convection vortices observed in the northern high-latitude ionosphere. Geotail was positioned on a magnetic flux tube that mapped very close to the centers of the driving field-aligned currents in the ionosphere. These are the first in situ observations from the source region of such events. The flow signature is seen clearly in the cold, dense plasma population, which the satellite encounters during the event, in addition to a hot magnetospheric (plasma sheet-like) population. This report will present the observations and briefly discuss their implications for our understanding of the traveling convection vortex (TCV) signature. We interpret the event in terms of a boundary wave at the magnetopause. Our analysis suggests that strong gradients in the magnetic field strength or plasma density in the inner magnetosphere should play an important role in this process.

Accepted: 1997-10-29, 資料番号: SA1002533000