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2. Solar Wind and Outer Magnetosphere VOL. 78, NO. 16 •[OURNAL OF GEOPHYSICAL RESEARCH •[UNE 1, 1973 SatelliteStudies of MagnetosphericSubstorms on August15, 1968 2. SolarWind and Outer Magnetosphere R. L. MCPHE'RRON,• G. K. P'ARIiS,•' D. S. COLBURN,3 AND M.D. MONTGOMERY4 We continue the study of the sequer•e of events o.ccurringin the magnetosphereduring several substormson August 15, 1968. We show that the onsets of expansion phas'esidentified in the preceding paper at 0220, 0430, and 0714 UT were each preceded by almost an hour of southward solar wind magnetic field. During the entire interval containing these sub- storms there was no significant change in the solar wind velocity. Intermittent observations of the solar wind particle density and temperature suggestthere were no large changesin the dynamic and static pressure. The fact that the solar wind field remained southward after the onsets of two substorm expansions is interpreted as evidence that the expansion is a process internal to the magnetosphere. However, coincidence of an expansion onset with a large fluctuation of the solar wind field makes it impossible to rule out the possibility that the expansion can be triggered externally. Magnetic field observations in the premidnight sector of the synchronousequatorial orbit show that the earth's field here begins to decrease in responseto the beginning of the southwardsolar wind field. Throughout the interval prior to the onset of the expansion (growth phase) the field continues to decrease.During one of the substorms, energetic electron precipitation was observed during this growth phase. In the expansion phase the field at synchronousorbit recovers. In the first note of this series [McPherro•, expansionswere precededby intervals of south- 1973] we discussedthe importanceof establish- ward solar wind magnetic field and nearly con- ing the sequenceof magnetic changesthat stant solar wind plasma pressure.During both occurswithin the magnetosphereduring a sub- preliminary intervals the magnetic field mag- storm. We noted that it was possibleto estab- nitude at synchronous orbit in the evening lish this sequenceon the basis of existing sector became severely depressed.During one studies, but such a sequencewas subject to substorm for which balloon observations were considerableerror, owing to inconsistenciesin available this depressionwas accompaniedby timing. Consequently,in this set of coordinated energeticelectron precipitation at the conjugate notes we are attempting to establish the sub- point of the synchronoussatellite. storm sequence for two worldwide substorms The general associationbetween southward on August 15, 1968. solar wind field and geomagnetic activity is This second note examines changesin the well established(see review by Hirshberg and solar wind and the outer magnetosphere(syn- Colburn [1969]). Recently, Arnoldy [1971] has chronous orbit, 6.6 Rs) associatedwith these examined this relation statistically using the substorms. Our results show that both substorm hourly auroral electrojet index (AE) and the integral of southwardsolar wind magneticfield. He findsthe parameterbest correlatedwith AE • Department of Planetaw and Space Science is the integral of the GSM southwardZ compo- and Institute of Geophysicsand Planetaw Phys- nent duringthe hour precedingthe maximumof ics, University of California, Los Angeles, Cali- fornia 90024. AE. A similar result has been obtained by Fos- •Geophysics Program, University of Washing- ter et al. [1971], using superposedepoch ton, Seattle, Washington 91805. analysis.Their work showsthat the beginning 8Space SciencesDivision, Ames Research Cen- of a rapid increasein AE is precededby more ter, Moffett Field, California 94035. than an hour of southwardsolar wind magnetic 4Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico. 87544. field. Aubry and McPherron [1971] have examined Copyright ¸ 1973 by the American Geophysical Union. the magnetotail responseto both the solar 3054 MCPHERRONET AL.: SUBSTORMSTUDIES, 2 3055 wind magnetic field and magnetosphericsub- events in the solar wind and outer magneto- storms. Using individual substorms,they find spherehave been made by severalinvestigators that the onsetof southwardcomponent of the using different data. They do, however, sug- solar wind magneticfield is followedby an in- gest a sequenceof events which should be creasein the tail field magnitude, a thinning observableduring a single substorm.Our ob- of the plasma sheet, a depressedfield at syn- servations during the two major substorms chronousorbit, and a taillike field on the dawn on August 15, 1968,are in agreementwith these meridian. The start times of these various reports.and show that such a sequenceexists. phenomenawere consistentwith the convection These observationsare discussedbelow. of the southward component past the earth inthe solar wind. Hourly averages andstandard SOLARWIND deviations of various plasma parameters in Observationsof the solar wind from 0000 to the solar wind showedno significantvariations 0900 UT, August 15, 1968, are plotted in Figure correlatedwith the substorms. 1. Plasma parameters shown at the bottom of Cummings et al. [1968] and Coleman and the figure were measuredwith an electrostatic McPherron [1970] have studied the substorm analyzer tm Vela 4A [Hundhausenet al., 1967; magnetic signature at the synchronousorbit. Montgomery et al., 1970]. The approximate The most systematic changesoccur in the H location of Vela during this interval was solar component (parallel to the earth's rotation magnetosphericlatitude --11.6 ø to --3 ø and axis). Near the midnight meridian the field longitude--11.5 ø to 27ø, at a distanceof 18.6 becomesdepressed prior to the onsetof a polar R•. This location was closeto the noseof the magnetic substorm and recoversin conjunc- magnetosphereand probablywell ahead of the tion with its expansionphase. At the dusk bow shock. meridian the behavior is more complex, de- Magnetic field observationsdisplayed under pending on the individual substorm and the top panel of Figure I were obtainedwith whether it is part of a sequenceof substorms. a fluxgate magnetometeron Explorer 35 [Col- If there is any effect,it consistsof a depression burn, 1969]. The approximate location of and recovery in H. The onset of the recovery plorer during these observationswas at the is generally delayed relative to the onset of moon, slightly behind the dawn meridian. A1- the expansionphase at midnight.The beginning though this location is well outside the bow of the depression,however, may occur at any shock,the locationis not well suited for timing time relative to the onset of the expansion the onsets of various substorm phenomenain phase. relation to the solar wind magnetic field. As Enhancedparticle precipitationprior to the was discussedby Aubry and McPherron [1971, onset of a substorm expansionhas been re- appendix 2], there can be considerableam- ported by Hones et al. [1971]. From riometer biguity in the calculatedarrival times of the data these authors have shown that, for at effectsof a solar wind changeat different points least 1 hour prior to the onset of a substorm in the magnetosphere.If we assumethat the expansion,energetic electrons (E • 10-20 key) discontinuity between northward and south- were precipitating throughout most of the ward solar wind magneticfield is perpendicular nightsideauroral oval. This weak activity was to a radius vector, this discontinuity should associatedwith a southwardcomponent of the reach the magnetopauseabout 2 min before solar wind magnetic field, a thinning of the it reachesExplorer 35. If we assumethat the plasmasheet, depressions in H at midlatitudes, discontinuity is perpendicularto the ecliptic and weak bay activity. Pytte and Tre[all and along the Parker spiral, the discontinuity [1972] have also reported electron precipita- would arrive at the magnetopause 15 min tion for more than an hour before the onset before it would arrive at the moon. of negative bays. Using balloon X ray data, Also shown in Figure 1 in the top panel is they show that these events are generally the total field observed by the Ogo 5 mag- weaker and more smoothly varying than the netometerinbound, down from the north lobe largeimpulsive fluxes associated with bay onset. of the tail, on the midnight meridian. A de- The precedingreports of substorm-associatedtailed descriptionof these data is given in 3056 MCPHERRONET AL.' SUBSTORMSTUDIES, 2 io ! , 4 6 7 8 9 UNIVERSAL TIME Fig. 1. Solar wind plasma parameters and magnetic field measurements for 0430 and 0714 UT on August 15, 1968, for Vela 66-38 located near the noon meridian and Explorer 35 near the dawn meridian. The vertical dashed lines are the onsets of substormexpansion. The shading on the solar magnetosphericZ component indicates the southward field. The Ogo 5 field magnitudeat •10 RE on midnight meridian is also shownfor comparison. paper 4 of this series.Vertical dashedlines characteristicof this graph is the nearly con- at 0220, 0430, and 0714 UT are the onsets stant solar wind velocity during the entire of substormexpansions as determinedin paper interval. A comparisonwith the onsetsof sub- 1. Vertical arrows at 0330 and 0640 UT are storm expansionsshown by vertical dashed the possible start times for two of the sub- lines reveals no correlation. storm growth phases. The static and dynamicpressures of the solar Plasmaparameters. The solar wind velocity wind for three intervals when the satellite from 0000 to 0900 UT is plotted near the was in the real time
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