We have assembled a data set of 1821 magnetopause crossings. Separate fits to subsets of this data set determine the magnetopause location as a function of solar wind dynamic pressure and interplanetary magnetic field orientation. Solar wind dynamic pressure variations produce self‐similar magnetopause motion on time scales of one hour or longer. We verify the pressure balance relationship between the solar wind dynamic pressure and the location of the subsolar magnetopause. We quantify the relationship between the IMF Bz, region l Birkeland current strength, the position of the subsolar magnetopause, and the shape of the dayside magnetosphere. Cross sections of the dayside magnetopause in planes perpendicular to the Earth‐Sun line are oblate.

We present observations made in space and on the ground during the growth phase and the onset of a substorm on August 31, 1986. About 20 min after the ε parameter at the magnetopause had exceeded 1011 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 RE close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere. However, it did not lead to a full‐scale substorm expansion, as determined by several ground‐based instruments, nor did it produce large particle injections at geostationary orbit. Only after another 20 min of continued growth phase the entire magnetosphere‐ionosphere system could apparently allow the onset of a regular substorm expansion. The initial activation is interpreted in the present paper as a “pseudobreakup.” We examine the physical conditions in the near‐Earth plasma sheet using spacecraft observations and analyze the development in the ionosphere using ground‐based magnetometers and electric field observations from the STARE radar. We find that the main observable differences between pseudobreakups and ordinary breakups are the strength and consequences. Furthermore, it is shown that ionospheric activity at the time of a pseudobreakup is not necessarily as localized in longitude as generally believed.

We elaborate upon the leakage model for the escape of energetic magnetospheric particles into the magnetosheath. Unlike the merging model, no interconnection (or merging) of magnetospheric and magnetosheath magnetic field lines is required. Because outer magnetospheric energetic particle drift paths intersect the magnetopause, the leakage model requires the continual escape of ions at postnoon local times and electrons at prenoon local times, regardless of solar wind conditions. It also predicts a division between dawnward and duskward streaming ions at the point where most magnetosheath magnetic field lines make their closest approach to the magnetopause, typically near 1500 LT. Like the merging model, the leakage model predicts equatorward streaming just inside the magnetopause. We study the motion of an escaping energetic ion at a planar magnetopause to show that, without scattering, ions must move dawnward and northward in a duskward magnetosheath magnetic field and dawnward and southward in a dawnward magnetosheath magnetic field. Scattering permits some ions to move duskward. We present new observations of streaming ions outside the dayside magnetopause made by the Charge Composition Explorer satellite, a part of the Active Magnetospheric Particle Tracer Explorers program. To place these observations in context, we have performed a statistical study of previous particle observations both inside and outside the dayside magnetopause. The ensemble of observations indicates that energetic magnetospheric ions of all species continually escape from the dayside magnetosphere and stream along magnetosheath magnetic field lines, even when no merging is expected. The magnetosheath magnetic field controls the direction in which the ions stream: they move away from the magnetosphere. The results of this work indicate that energetic particle observations at the dayside magnetopause need not be taken as evidence for merging of magnetosheath and magnetospheric magnetic field lines. 12,097 12,098 SIBECK ET AL.: ENERGETIC MAGNETOSPHERIC IONS AT THE DAYSIDE MAGNETOPAUSE

This paper reports the multisatellite and ground observations of two pseudo‐substorm onset events that occurred successively at 0747 UT and 0811 UT, May 30, 1985, with more attention to the 0747 UT onset. The distinguishing features of the 0747 UT event are as follows. (1) The substorm‐associated tail reconfiguration started in a very localized region in the near‐Earth magnetotail. (2) The magnitude of the current disruption decreased markedly as the disruption region expanded tailward. (3) On the ground the onset of a very small negative bay (∼ 40 nT) was observed simultaneously with the onset of the current disruption, but over a much wider local time sector than the near‐Earth tail reconfiguration. Positive bay onsets at mid‐latitudes also had a longitudinally wide distribution. From these features we infer that in the present event the current disruption took place filamentarily near AMPTE/CCE at ∼8.8 RE. It is also inferred that pseudo‐substorm onsets are distinguished from standard substorm onsets by the absence of a global expansion of the current disruption, and that the spatial scale of the onset region in the magnetosphere is not a major difference between the two. The present study suggests that the spatial distribution of the magnetic distortion before onsets is an important factor to determine the expansion scale of the current disruption. It is also suggested that the current disruption is basically an internal process of the magnetosphere.

Abstract. The global conditions during a moderate geomagnetic disturbance event on May 15, 1996, are examined by comparing data from several ground-based instruments and inner tail satellites with global MHD simulations of the same event.The ground-based data show two substorm intensifications about 40 rain apart, the first one being small and localized (a pseudobreakup) and the second leading to a major rearrangement of both the ionospheric auroral distribution and the magnetotail configuration. The simulation shows that during the pseudobreakup, open field lines were reconnecting in the midtail, but the flows were mainly tailward and very few effects were observable in the inner magnetosphere. The result that pseudobreakups can be associated with activity producing topological changes in the tail is an important new aspect that has not been discussed in earlier studies.Both the observations and the simulation show two distinct regions of activity' a thin current sheet in the inner tail magnetically connected with the auroral bulge and a reconnection region in the midtail associated with the most intense electrojet currents.

Abstract. A global MHD simulation of an isolated substorm that occurred on March 9, 1995 is presented. The simulation, driven with solar wind data provided by the Wind satellite, reproduced to a surprising degree the evolution of substorm activity. The onset of the expansion phase was coincident with the penetration of an electric field spike into the nearEarth region. This impulse launched a tailward propagating signal (rarefaction wave) that enhanced reconnection in the mid tail.Substorm intensification was correlated with th'e enhancement of the reconnection rate at the preexisting reconnection region located at 30 Re. The importance of the electric field spike in correlating ionospheric and magnetospheric aspects of the substorm is emphasized.

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Public r*WUM tfutbudn for ofto dalme wiommon ~ isne to evgg 1 how per reepame. Wcko*V d the* for low wait miiatim.aori xdigdeIMeG ien (March-June 1986). Event interval C comprised the period 0000-1200 UT on 3 May1986 which was a highly disturbed time near the end of a geomagnetic storm interval. A very large substorm early in the period commenced at 0111 UT and had a peak AE index value of -1500 nT. Subsequent activity was lower, but at least three other substorms occurred at 2-3 hour intervals. The substorms on 3 May were well observed by a variety of satellites, including ISEE-1, -2, and IMP-8 in the magnetotail plus SCATHA, GOES, GMS, and LANL spacecraft at or near geostationary orbit. A particularly important feature of the 0111 UT substorm was the simultaneous imaging of the southern auroral oval by DE-1 and of the northern oval by Viking. The excellent constellation of spacecraft near local midnight in the radial range 5-9 RE made it possible to study the strong cross-tail current development during the substorm growth phase and the current disruption and current wedge development during the expansion phase. We use a time-evolving magnetic field model to map observed auroral features out into the magnetospheric equatorial plane. There was both a dominant eastward and a weaker westward progression of activity following the expansion phase. A clear latitudinal separation of the initial region of auroral brightening and the region of intense westward electrojet current was identified. The combined ground, neartail, and imaging data for this event provided an unprecedented opportunity to investigate tail current development, field line mapping, and substorm onset mechanisms. We find evidence for strong current diversion within the near-tail plasma sheet during the late growth phase and strong current disruption and field-aligned current formation from deeper in the tail at substorm onset. We conclude that these results are consistent with a model of magnetic neutral line formation in the late growth phase which causes plasma sheet current diversion before the substorm onset. The expansion phase onset occurs considerably later due to reconnection of lobelike magnetic field lines and roughly concurrent cross-tail disruption in the innlrll nlzcmn thet reinn ................................................................................................................... AbstractThe ninth Coordinated Data Analysis Workshop (CDAW-9) focussed upon several intervals within the PROMIS period (March -June 1986). Event interval C comprised the period 0000-1200 UT on 3 May 1986 which was a highly disturbed time near the end of a geomagnetic storm interval. A very large substorm early in the period commenced at 0111 UT and had a peak AE index value of -1500 nT. Subsequent activity was lower, but at least three other substorms occurred at 2-3 hour intervals. The substorms on 3 May were well-observed by a variety of satellites including ISEE-1, -2, and IMP-8 in the magnetotail plus SCATHA, GOES, GMS, and LANL spacecraft...