Dawn Meridian Research Articles

Ionospheric disturbances over East Asia during intense December magnetic storms of 2006 and 2015: similarities and differences

Using data from ionosondes, located in East Asia, and total electron content maps, we have made a comparative analysis of ionospheric disturbances associated with the intense geomagnetic storms of December 14–16, 2006 and December 19–22, 2015. These storms had almost equal peak intensities (Dstmin=–162 and –155 nT), but different durations of the main phases (2.5 and 19 hr). At the beginning of both the storms, the region under study was located in the vicinity of the midnight meridian. Ionospheric responses to magnetic storms differed in: i) an increase in the F2-layer critical frequency at subauroral latitudes, caused by an increase in auroral precipitation, during the initial phase of the former storm and the absence of this effect in the latter; (ii) a sharp drop in the critical frequency in the evening hours of the main phase of the latter storm, caused by a shift of the main ionospheric trough to lower latitudes, and the absence of this effect during the former storm; (iii) generation of a short-term positive disturbance observed at subauroral latitudes only in the early recovery phase of the former storm after the negative ionospheric disturbance. During both the storms at middle latitudes there were positive disturbances and wave-like fluctuations of the critical frequency which increased in the vicinity of the dawn meridian. The main causes of the differences between the ionospheric storms are shown to be the differences between the initial conditions of the magnetosphere–ionosphere system and durations of the main phases of magnetic storms.

Ионосферные возмущения над Восточной Азией во время сильных декабрьских магнитных бурь 2006 и 2015 гг.: сходствo и различиe

Using data from ionosondes, located in East Asia, and total electron content maps, we have made a comparative analysis of ionospheric disturbances as-sociated with the intense geomagnetic storms of De-cember 14–16, 2006 and December 19–22, 2015. These storms had almost equal peak intensities (Dstmin=–162 and –155 nT), but different durations of the main phases (2.5 and 19 hr). At the beginning of both the storms, the region under study was located in the vicinity of the midnight meridian. Ionospheric re-sponses to magnetic storms differed in: i) an increase in the F2-layer critical frequency at subauroral latitudes, caused by an increase in auroral precipitation, during the initial phase of the former storm and the absence of this effect in the latter; (ii) a sharp drop in the critical frequency in the evening hours of the main phase of the latter storm, caused by a shift of the main ionospheric trough to lower latitudes, and the absence of this effect during the former storm; (iii) generation of a short-term positive disturbance observed at subauroral latitudes only in the early recovery phase of the former storm after the negative ionospheric disturbance. During both the storms at middle latitudes there were positive dis-turbances and wave-like fluctuations of the critical fre-quency which increased in the vicinity of the dawn me-ridian. The main causes of the differences between the ionospheric storms are shown to be the differences be-tween the initial conditions of the magnetosphere-ionosphere system and durations of the main phases of magnetic storms.

Globally averaged exospheric temperatures derived from CHAMP and GRACE accelerometer measurements

Neutral densities (ρ) inferred from accelerometer measurements on the polar‐orbiting Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites are used to compile exospheric temperatures (T∞) during extended periods in 2003 and 2004 when their orbital planes were nearly parallel and at quadrature, respectively. Exospheric temperatures were first estimated using ρ‐h‐T∞ relationships implicit within the Jacchia models, then averaged over individual orbits. We found good agreement between the orbital‐averaged T∞ obtained from CHAMP and GRACE accelerometer data as well as with globally averaged exospheric temperatures derived from drag measurements from the constellation of satellites used in the High‐Accuracy Satellite Drag Model. Our analysis corrects a critical conjecture by Burke (2008) that globally averaged T∞ is essentially the same as orbit‐averaged values obtained by polar‐orbiting satellites, independent of the local time of their orbital planes. Unlike the symmetric 0200–1400 LT distribution of T∞ minima and maxima found in early Jacchia models, presented data indicate that the minima are located closer to the dawn meridian. We also demonstrate that the averaging technique used to estimate T∞ affects the outcomes. Statistical analyses provide an empirical basis for improving estimates of the thermosphere's total energy budget.

Proposed model for Saturn's auroral response to the solar wind: Centrifugal instability model

We present a model of Saturn's global auroral response to the solar wind as observed by simultaneous Hubble Space Telescope (HST) auroral images and Cassini upstream measurements of the solar wind taken during the month of January 2004. These observations show a direct correlation between solar wind dynamic pressure and (1) auroral brightening toward dawn local time, (2) an increase of rotational movement of auroral features to as much as 75% of the corotation speed, (3) the movement of the auroral oval to higher latitudes, and (4) an increase in the intensity of Saturn Kilometric Radiation (SKR). Our model, referred to as the centrifugal instability model, provides an alternative to the reconnection model of Cowley et al. (2004a, 2004b, 2005); we suggest the above observations result from Saturn's magnetosphere being a fast rotator. Since the torques on Saturn's outer magnetosphere are relatively low, its outer magnetosphere will tend to conserve angular momentum. When compressed on the dayside, the outer magnetosphere spins up to higher angular velocities, and when it expands, the outer magnetosphere spins down to lower angular velocities. This response occurs since Saturn's ionosphere is unable to enforce corotation. The outer boundary of the plasma sheet at L ∼ 15 is identified as the primary source location for the auroral precipitating particles. Enhanced wave activity, which can precipitate the auroral producing particles, may be present at this boundary. If radial transport is dominated by centrifugally driven flux tube interchange motions, when the magnetosphere spins up, outward transport will increase, and the precipitating particles will move radially outward (since the radial gradient in electron energy flux is negative). This mechanism will cause the auroral oval to move to higher latitudes as observed. The Kelvin‐Helmholtz instability may contribute to the enhanced emission along the dawn meridian, as observed by HST, via enhanced wave activity and corresponding charged particle precipitation.

Supercooled ion temperatures observed in the topside ionosphere at dawn meridian during storm periods

A few rare events of supercooled ion temperature near the magnetic equator at dawn meridian have been observed by the first satellite of the Republic of China, ROCSAT‐1, at 600 km altitude. The ion cooling event occurs within a narrow longitude region where a large density reduction together with a large downward, eastward, and field‐aligned flow is observed. Although ion temperature troughs around the magnetic equator have been observed by many satellites before, and the interhemispheric plasma transport of adiabatic cooling effect for ions near the equator has been modeled to explain the past observations, the current observation enunciates some new facts of the ion cooling effect that have not been reported. First, ion cooling events are observed by ROCSAT at an unprecedented low altitude of 600 km during the solar maximum years of 2000 and 2001. Second, the ion temperature trough is observed, together with a large density reduction and a large downward and eastward drift that were absent in the past observations. Third, the present events are only observed near the dawn meridian during the disturbed periods. Comparing the observational facts with the existing models, we conclude that the model of interhemispheric plasma transport [Bailey et al., 1973; Heelis et al., 1978; Bailey and Heelis, 1980] seems to fit best with the current observations. However, instead of resorting to the quiet time neutral wind to induce the quasi steady state plasma transport as modeled before, a storm time transient dynamical process could have occurred within a limited longitude region where ions at high latitudes are forced to move along the field lines to the dip equator to cause ion cooling observed at 600 km lower than predicted in the model. This storm time ionospheric process could be related to the creation of a westward electric field in the inner magnetosphere near the dawn meridian from a sudden enhancement of the convection field during a disturbed period modeled in the comprehensive ring current model [Fok et al., 2003].

VLF chorus emissions observed by Polar during the January 10, 1997, magnetic cloud

VLF chorus emissions occur near the dawn meridian in fast response (<60 s) to magnetospheric perturbations caused by sudden fluctuations in solar wind dynamic pressure and southward turnings of the interplanetary magnetic field (IMF) at the arrival of the Jan. 10, 1997 magnetic cloud. Raytracing analysis indicates the likely chorus source region to be near the magnetic equator with rays launched at oblique wave normal angles, for which likely resonant electron energies are in the range E∼14–30 keV, consistent with observed particle data.

Sondrestrom and EISCAT radar observations of poleward-moving auroral forms

During many magnetospheric substorms, the auroral oval near midnight is observed to expand poleward in association with strong negative perturbations measured by local ground magnetometers. We show Sondrestrom and EISCAT incoherent scatter radar measurements during three such events. In each of the events, enhanced ionization produced by the precipitation moved northward by several degrees of latitude within 10–20 min. The electric fields measured during the three events were significantly different. In one event the electric field was southward everywhere within the precipitation region. In the other two events a reversal in the meridional component of the field was observed. In one case the reversal occurred within the precipitation region, while in the other case the reversal was at the poleward boundary of the precipitation. The westward electrojet that produces the negative H-perturbation in the ground magnetic field has Hall and Pedersen components to varying degrees. In one case the Hall component was eastward and the Pedersen component was westward, but the net magnetic H-deflection on the ground was negative. Simultaneous EISCAT measurements made near the dawn meridian during one of the events show that the polar cap boundary moved northward at the same time as the aurora expanded northward at Sondrestrom. Most of the differences in the electrodynamic configuration in the three events can be accounted for in terms of the location at which the measurements were made relative to the center of the auroral bulge.

A survey of flux transfer events recorded by the UKS spacecraft magnetometer

The UKS spacecraft operated from August 1984 through to January 1985. During that time, it made multiple crossings of the magnetopause in local time sectors extending from mid-afternoon to just behind the dawn meridian. We have surveyed the magnetometer records from these magnetopause encounters and have compiled a catalogue of flux transfer events (FTEs using criteria identical to those employed by Rijnbeek et al. (1984, J. Geophys. Res. 89, 786) in their survey of ISEE spacecraft magnetometer data. Using the catalogue, we find that FTE occurrence determined from the UKS data set is substantially less than that detected using data from the early ISEE 1 2 spacecraft orbits. The UKS data set shows a correlation between FTE occurrence and southward external magnetic field, but there are several instances of passes in which no FTEs are detected but for which the external field was unam- biguousluy southward. The passes with the largest number of events are those for which the field outside the magnetopause has a large B M component. We conclude that the lower latitude of the UKS encounters is responsible for the discrepancy with the ISEE occurrence. The most likely source region appears to be near the subsolar region.

ISEE 1 and 2 observations of Birkeland currents in the Earth's inner magnetosphere

Data from the ISEE 1 and 2 magnetometers were examined for evidence of Birkeland currents in the earth's inner magnetosphere, from 2.4‐ to 7‐RE radial distance. This survey reports on the two spacecraft measurements of the properties of the Birkeland currents. Both line and sheetlike currents were observed. Large‐scale Birkeland current signatures were observed throughout the 18 hours of local time in which the data coverage was sufficient. In local time this coverage was from local noon through dusk to local midnight to the dawn meridian. In approximately 27% of the spacecraft's outbound passes, large‐scale current structures were identifiable as parts of the region 1 or region 2 current systems. In approximately 19% of the outbound passes, no distinguishable region 1 or region 2 Birkeland currents were observed. These cases occurred during periods of low magnetic activity. In the remaining 54%, both multiple current structures and ambiguous magnetic signatures were observed. The multiple structures were generally of smaller scale than the structures identified as the region 1 and 2 current structures, and the ambiguous cases had no clear sources or causes. For these cases the spatial and temporal variations were not easily separated. Two passes through the dayside magnetosphere showed Birkeland current layers at the interface between the outer plasma sheet convection boundary and a plasma mantlelike boundary region, located inside the postnoon magnetopause. These two cases provided clear examples of region 1 Birkeland current flow in the dayside magnetosphere.

Wave characteristics of Ps 6 magnetic variations and their implications for convective flow in the magnetotail

Ps 6 magnetic variations are fluctuations with periods of 10–30 min and amplitudes of tens to hundreds of nT observed in the dawn sector auroral oval during substorm activity. The causative current system appears to be associated with auroral omega bands which follow the substorm breakup, and drift eastward from the midnight sector with typical velocities of 0.5–2.0 km s −1 . Quadrature techniques are applied to Ps 6 waveforms to separate their group and phase velocities, and to obtain their wave characteristics. The Ps 6 phenomenon seems to be confined to the 03:00–09:00 L.T. sector, and over the range of longitudes involved, its wave properties are relatively constant in the ionosphere. The group velocity has an average value of 1.0 km s −1 and, on the average, marginally exceeds the phase velocity by about 15% ; both these velocities are directed eastward. When the Ps 6 wave parameters observed in the ionosphere are mapped to the equatorial magnetospheric plane, the wavelength of the disturbance shows, on an average, values ranging from ∼47 Re for the 03:00–04:00 L.T. sector to ∼12 Re for the 08:00–09:00 L.T. sector; the phase velocity changes from ∼ 300 km s −1 in the deep magnetotail to ∼ 20 km s −1 near the dawn meridian. Assuming the disturbance to originate through a Kelvin-Helmholtz instability, at the interface between the low-latitude boundary layer (LLBL) and the central plasma sheet (cps), the width of the momentum shear zone narrows from ∼ 4.5 R E at 03:00–04:00 L.T. to ∼ 1.2 R E at 08:00–09:00 L.T. The inferred large sunward convection velocity of ∼ 300 km s −1 for plasma in the 03:00 L.T. region of the LLBL/cps interface is interpreted as confirmation of magnetic field line reconnection at a neutral line antisunward of the region of formation of Ps 6 in the magnetotail.

Magnetospheric plasma drifts during a sudden impulse

An interplanetary shock front (monitored by the ISEE 1/2 satellite pair) hit the earth's magnetosphere on October 17, 1978, around 0430 UT. The changes in magnetospheric plasma drift and magnetic field associated with this sudden impulse (SI) were measured by GEOS 2 in the equatorial plane near the dawn meridian. After the SI the plasma drift first increased (in an inward direction), followed by an exponential decay. The magnetic field changes were purely compressional with an amplitude increase which also exponentially decayed. These features can be explained by adiabatic (quasi‐static) compression of the magnetopause due to the increased solar wind pressure behind the shock front. In addition, both data sets exhibit also the excitation of a highly damped compressional hydromagnetic oscillation by the SI, thus hinting of a second, wavelike component in the compression of the magnetosphere.

Corotation anisotropies in Saturn's magnetosphere

Ion data from the Voyager 1 and 2 low‐energy charged particle (LECP) experiment are fit to a second‐order harmonic expansion to determine anisotropies within the Saturnian magnetosphere. Anisotropies from the low‐energy channels (28–130 keV) are often consistent with those expected from corotation (generally with a small radial component on the dayside) but are at times distinctly different from corotation. On the dayside, near the noon meridian, first‐order anisotropies often depart significantly from the corotation direction. On the nightside, amplitudes are consistent with full corotation but begin to drop below corotation values at a dipole L of ∼27 Rs (corresponding to a velocity of ∼265 km/s). Second‐order anisotropies are significant, even dominant at times, on the dayside where a relatively broad range of pitch angles is sampled by the instrument. In all cases there is a departure of ∼180° from the first‐order anisotropies expected due to corotation beginning outside the Rhea L shell and continuing through at least the orbit of Dione. This is the region where the cold and hot plasma tori have been observed. We have computed the anisotropies expected from rigid corotation of the observed flux distributions and conclude that parts of the Saturnian magnetosphere are not rigidly corotating. Calculations based on the combined effects of corotation plus solar wind generated convective electric fields appear to provide insights into the anisotropies along the dawn meridian. However, on the dayside near noon, turbulence and/or time variations seem necessary to explain the departure from simple convection models.

Ionospheric hot spot at high latitudes

A hot spot (or spots) can occur in the high‐latitude ionsophere depending on the plasma convection pattern. The hot spot corresponds to a small magnetic local time‐magnetic latitude region of elevated ion temperatures located near the dusk and/or dawn meridians. For asymmetric convection electric field patterns, with enhanced flow in either the dusk or dawn sector of the polar cap, a single hot spot should occur in association with the strong convection cell. However, on geomagnetically disturbed days, two strong convection cells can occur, and hence, two hot spots should exist. The hot spot should be detectable when the electric field in the strong convection cell exceeds about 40 mV m−1. For electric fields of the order of 100 mV m−1 in the convection cell, the ion temperature in the hot spot is greatest at low altitudes, reaching 4000 °K at 160 km, and decreases with altitude in the F‐region. An ionospheric hot spot (or spots) can be expected at all seasons and for a wide range of solar cycle conditions.

Magnetic Field Studies by Voyager 2: Preliminary Results at Saturn

Further studies of the Saturnian magnetosphere and planetary magnetic field by Voyager 2 have substantiated the earlier results derived from Voyager 1 observations in 1980. The magnetic field is primarily that of a centered dipole (moment = 0.21 gauss-RS(3); where one Saturn radius, RS, is 60,330 kilometers) tilted approximately 0.8 degrees from the rotation axis. Near closest approach to Saturn, Voyager 2 traversed a kronographic longitude and latitude range that was complementary to that of Voyager 1. Somewhat surprisingly, no evidence was found in the data or the analysis for any large-scale magnetic anomaly in the northern hemisphere which could be associated with the periodic modulation of Saturnian kilometric radiation radio emissions. Voyager 2 crossed the magnetopause of a relatively compressed Saturnian magnetosphere at 18.5 RS while inbound near the noon meridian. Outbound, near the dawn meridian, the magnetosphere had expanded considerably and the magnetopause boundary was not observed until the spacecraft reached 48.4 to 50.9 RS and possibly beyond. Throughout the outbound magnetosphere passage, a period of 46 hours (4.5 Saturn rotations), the field was relatively steady and smooth showing no evidence for any azimuthal asymmetry or magnetic anomaly in the planetary field. We are thus left with a rather enigmatic situation to understand the basic source of Saturnian kilometric radiation modulation, other than the small dipole tilt.

Equatorial disc and dawn‐dusk currents in the frontside magnetosphere of Jupiter: Pioneer 10 and 11

Observations by Pioneer 10 and 11 show that the strongest azimuthal fields are observed near the dawn meridian (Pioneer 10) while the weakest occur near the noon meridian (Pioneer 11), suggesting a strong local time dependence for the corresponding radial current system. Modeling studies of the radial component of the field observed by both spacecraft suggest that the corresponding azimuthal current system must also be a strong function of local time. Both the azimuthal and the radial field component signatures exhibit sharp dips and reversals, requiring thin radial and azimuthal current systems. There is also a suggestion that these two current systems either are interacting or are due, at least in part, to the same current. We suggest that a plausible current model consists of the superposition of a thin, local time independent, azimuthal current system plus the equatorial portion of a taillike current system that extends into the dayside magnetosphere.

Intensity variations in plasma flow at the dawn magnetopause

Voyager 1, exiting the earth's magnetosphere along the dawn meridian at a velocity of ~11 km/sec, measured strong tailward flows of ions ( E≳30 keV) immediately outside the magnetopause. These flows are found to originate sunward of the dawn meridian and to exhibit significant variabilities on the time scale of 400 msec. The variations are not related to changes in the magnetosheath magnetic fields and are likely produced up-stream by the leakage of magnetosphere protons or by a magnetopause particle energization process. The intensities of the dawn meridian ion flows are greater in the magnetosheath than in the magnetosphere. The flows appear to penetrate inside the dawn magnetosphere to a depth ≲0.1 R> E , less than an ion gyroradius.

Energetic protons in the Jovian magnetosphere

The time histories, angular distributions and energy spectra of energetic protons have been measured over an energy range extending from 0.2 to 20 MeV for the four passes of Pioneers 10 and 11 through the Jovian magnetosphere. The energetic particle data from these four passes are remarkably different. Azimuthal asymmetries appear to dominate with time variations also contributing to the very complex topology. On the inbound P‐10 pass the expected corotation anisotropy was not observed in the outer magnetosphere. The simplest explanation is that the particle reference frame (i.e., the magnetospheric plasma in this region) is moving nearly radially, suggesting the existence of a planetary wind at that time. Near the dawn meridian, particle streaming away from the planet begins at ∼15 RJ. On both the P‐10 inbound and P‐11 outbound passes, there are regions where only partial corotation is achieved. In the mid‐magnetosphere, field‐aligned streaming away from the near‐equatorial current sheet region is the most prominent and puzzling feature. At mid‐latitudes in the subsolar regime, the streaming pattern is more chaotic and its magnitude is smaller. Qualitative discussions are presented for a number of possible mechanisms which could produce this streaming. In the context of our present understanding of the Jovian magnetosphere, each of three mechanisms show promise: perpendicular electric fields, a strong azimuthal intensity gradient at the equator, and strong diffusion processes. In the Jovian wind regions, proton energy spectra are generally of the form E−γ, where E is the kinetic energy of the particle with γ generally ranging between values of 3.0 and 4.2. For the remaining times, the spectra are most frequently of the form exp −P/P0, where P is the proton momentum with P0 varying mainly between 8.0 and 12 MV.

A generation mechanism for Pc 5 micropulsations in the morning sector

In the companion paper (Lam and Rostoker, 1978) we have shown that Pc 5 micropulsations are intimately related to the behaviour and character of the westward auroral electrojet in the morning sector. In this paper we show that Pc 5 micropulsations can be regarded as LC-oscillations of a three-dimensional current loop involving downward field-aligned current flow near noon, which diverges in part to form the ionospheric westward electrojet and returns back along magnetic field lines into the magnetosphere in the vicinity of the ionosphere conductivity discontinuity at the dawn meridian. The current system is driven through the extraction of energy from the magnetospheric plasma drifting sunwards past the flanks of the magnetosphere in a manner discussed by Rostoker and Boström (1976). The polarization characteristics of the pulsations on the ground can be understood in terms of the effects of displacement currents of significant intensity which flow near the F-region peak in the ionosphere and induced currents which flow in the earth. These currents significantly influence the magnetic perturbation pattern at the Earth's surface. Model current system calculations show that the relative phase of the pulsations along a constant meridian can be explained by the composite effect of oscillations of the borders of the electrojet and variations in the intensity of current flow in the electrojet.

Measurements of the Poynting vector of standing hydromagnetic waves at geosynchronous orbit

Particle and magnetic field data from the ATS 6 spacecraft have been combined to study a Pc 4 pulsation event. The event occurred as the satellite passed the dawn meridian during the recovery phase of a moderate magnetic storm, Plasma drifts associated with the wave are determined from the measured modulation of low‐energy protons. Using the observed ambient and perturbation magnetic fields in conjunction with the plasma drift velocity, we calculate the Poynting vector of the wave, assuming that the electric field is given by the frozen field approximation. The Poynting vector gives convincing evidence that the wave is a standing hydromagnetic wave along the magnetic field. In addition, it shows that the wave is propagating predominantly azimuthally from noon toward the dawn meridian. The phase relationships between the magnetic and plasma drift velocity oscillations are consistent with the occurrence of an odd harmonic of the standing wave. Comparison of observed particle fluxes with model predictions demonstrates that the fundamental mode is the most reasonable interpretation of the data. Ambient ion number densities, predicted by the wave data on the basis of a simple model of standing hydromagnetic waves, are found to be somewhat higher than observed values, on the basis of the assumption that the ambient plasma consists of protons and electrons only. Predicted and observed values of the ion number densities can be reconciled if it is assumed that a significant fraction of the ambient plasma consisted of He+ and/or O+. By using the fact that the wave is a standing wave the particle flow velocity is predicted from the observed magnetic variation and a model‐dependent theory. These predictions are found to agree very well with the flow values determined from the data.

Complete plasma dropouts at vela satellites during thinning of the plasma sheet

Five satellite years of Vela data are examined for plasma sheet thinnings. Complete proton disappearances (plasma dropouts) are the main subject here. During such times, the Vela satellite is temporarily in the high-latitude tail lobe. The distribution of such plasma dropouts within the magnetotail suggests that the semithickness of the plasma sheet near midnight seldom reaches less than 1 RE during substorms and that the dawn and dusk portions of the plasma sheet remain thicker than the midnight portion. But it is also shown that the plasma sheet occasionally becomes very thin near the dusk magnetopause. This seems to occur when the magnetopause near the dawn and dusk meridians lies further away from the earth than is normal. Extension of the magnetopause to these greater distances does not seem to be related to magnetically quiet periods nor to the Z component of the interplanetary magnetic field (IMF). No such severe thinnings of the plasma sheet are found near the dawn magnetopause. Plasma dropouts can occur regardless of the sign of the Z component of the IMF, but their frequency of occurrence seems to be greater when the Z component is negative. The decreased frequency of occurrence with northward IMF may reflect weaker thinning of the plasma sheet at ∼ 18 RE during the weaker substorms that occur then. Weaker thinning of the plasma sheet may also imply a greater than normal distance from the earth to the magnetic neutral line thought to form in the magnetotail at substorm onset. Three plasma dropouts which occurred in the midnight sector at unusually large distances from the estimated position of the neutral sheet were observed during geomagnetic storms. It is likely that the midnight sector of the plasma sheet can become very thick (∼18 RE) at certain times during the main phase of storms. Detailed measurements in the plasma sheet were obtained near the beginning of a geomagnetic storm whose sc triggered a substorm. A compression of the plasma sheet at XSM ≈ −15 RE occurred about 10 min after the sc onset at the earth and about 5 min after the start of plasma sheet thinning associated with the sc-related substorm. If compression-thinning of the plasma sheet initiated this substorm, the triggering action must have occurred earthward of XSM ≈ −15 RE.