VOL. 78, NO. 16 3OURNAL OF GEOPHYSICAL RESEARCH 3UNE 1, 1973 SatelliteStudies of MagnetosphericSubstorms on August15, 1968 1. Stateof the Magnetosphere R. L. M CPHERRON Department o] Planetary and SpaceScience and Institute o] Geophysicsand Planetary Physics University o] California, Los Angeles,California 90024 The sequenceof eventsoccurring throughou.t the magnetosphereduring a substormhas not been precisely determined. This paper introduces a collection.of papers that attempts to establish this sequencefor two substormson August 15, 1968. Data from a wide variety of sourcesare used, the major emphasisbeing changesin the magnetic field. In this paper we use ground magnetograms to determine the onset times of two substorms that occurred while the Ogo 5 satellite was inbound on the midnight meridian through the cusp region of the geomagnetictail (the region of rapid changefrom taillike to dipolar field). We concludethat at least two worldwide substormexpansions were precededby growth phases.Probable begin- nings of these phaseswere at 0330 and 0640 UT. However, the onset of the former growth phase was partially obscuredby the effects of a preceding expansionphase around 0220 and a possible localized event in the auroral zone near 0320 UT. The onsets of the cor- respondingexpansion phases were 0430 and 0714 UT. Further support for these determina- tions is provided by data discussedin the subsequentnotes. The precise sequenceof events that occurs Ogo 5 in the near tail, and ATS I at syn- during a magnetosphericsubstorm has not been chronousorbit. Solar wind plasma parameters established.Among the reasons for this are were measured by Vela 4A. Magnetospheric lack of consistencyin the definition of sub- convection is inferred from a combination of storm onset and the wide variability of suc- plasmapauseobservations on Ogo 4 and 5 in cessivesubstorms. A recentattempt to establish conjunctionWith groundwhistler observations this sequencefor particle phenomenais the at Byrd station. The behaviorof energeticelec- work of Hones et al. [1971]. Since it is known trons is examined by using measurementson that magnetosphericsubstorms involve large Ogo 5 supported by observationsof electron changesin the configurationof the geomagnetic precipitation at the ATS I conjugate point. tail, it seemsparticularly important to estab- Interpretation of the electron observationsis lish this sequencefor magnetic phenomena. aided by simultaneousmeasurements on Ogo 5 Although it is po•ible to construct such a of magnetic ELF and VLF and electrostatic sequenceout of existing reports, problems of ELF waves. timing make this sequenceuncertain. Conse- The main res{fit of our work is the relative quently,in this collectionof paperswe attempt timing of various substorm-associatedphe- to definethis sequenceby combiningdata from nomena. We find that this sequenceis con- a wide variety of sourcesin a detailedstudy sistent with theoretical speculationbased on of two substormson August 15, 1968. Similar more limited observations. To motivate our work with more limited data has beenreported presentationsof the data, we briefly outline by Aubry and McPherron [1970]. this speculativemodel. Recent reviews include Substormonsets are definedby a combina- Atkinson [1966], Axford [1969], and F. V. tion of midlatitude and auroral zone magneto- Coroniti and C. F. Kennel (unpublishedmanu- grams. Satellite observationsof the magnetic script, 1971). field include Explorer 35 in the solar wind, Basically, this model indicates that, in a growth phase, magnetic flux is eroded from Copyright¸ 1973 by the American GeophysicalUnion. the front of the magnetosphereand convected 3044 I•CPHERRON.'SUBSTORM STUDIES, 1 3045 to the tail. This convectionappears to be a con- magnetosphere,magnetospheric convection, the sequenceof a southwardinterplanetary mag- magneticfield of the near tail, energeticelec- netic field. The flux is not immediatelyreturned trons and protonsin the near tail, and plasma but instead builds up on the nightside. The waves in the near tail. In the final note we plasmasheet responds by thinningand moving gather the diversedata in the form of a phe- toward the earth. A substormexpansion phase nomenologicalmodel of a magnetosphericsub- then commencesat the inner edgeof the plasma storm. sheet near the midnight meridian. This phase STATE OF THE MAGNETOSPHERE returns flux to the nightsideof the inner mag- netosphere,decreasing the tail field and ex- MagneticIndices pandingthe plasma sheet. In a finalrecovery Substormactivity on August 15, 1968, oc- phasethe returned flux flows around the earth curredduring a 6-day interval of considerable to thefront side. magneticdisturbance. The initial phase of a Theleast understood process in this model gradual commencement magnetic storm began isthe triggering mechanism responsible forthe earlyon August 13and ended late in the day. onsetof theexpansion phase. Without precise The mainphase of thisstorm developed timingit is impossibleto determine which of throughoutAugust 14, 15, and 16, reaching themany substorm associated phenomena it a minimumof -41 ¾at 1500UT onAugust maydepend on. Since it is thoughtthat this 17.The storm ended near the middle ofAugust mechanismoccurs in theregion of transition19. FigureI showsthe hourlyDst index fromdipolar to taillikefield (cusp) near the andthe three hourly Kp indexfor thefirst inneredge of the electron plasma sheet [Vasy- 4 daysof thisstorm. As is indicatedin this liunas,1968] around local midnight, studies in figure,the Kp was approximately 4 throughout thisregion are particularly important. Con- theentire day of August 15. The Dst averaged sequently,inthis work we concentrate onthe -10 ¾in the first half of the day and -20 ¾in neartail (5-20 R• behindthe earth)and the the lasthalf. midnightmeridian. The relativelyconstant value of the Kp To date,the satellite with the best instru- index and the prolonged development of the menrationandmost appropriate orbitto study main phase are suggestive of an intervalof thenear tail is the Ogo 5 spacecraft.During sustained substorm activity. Examination of the monthsof August1968 and 1969 the groundmagnetograms during this time con- inboundportion of its orbitwas down, out firmsthis interpretation. A detailed analysis of thenorth lobe of the tail, nearly along the of thesemagnetograms allowsus to determine midnightmeridian. (Here we define the lobe theapproximate onset times of thevarious as the regionsof highstable field oriented substorms. parallel to the •arth-sun line.) On August 15, 1968, Ogo 5 passedthrough the cuspbetween SubstormTiming with Ground Magnetograms 0000and 0900 UT. In thisinterval there were Theuse of groundmagnetograms to define severalmagnetospheric substorms. Two of these the 'onset' of substormsis a subject of con- substorms had very clear signatures at the siderablecontroversy. Originally, onset was de- satellite as well as worldwide effects on the fined in terms of the auroral substorm as the ground.These two substormshave been chosen fordetailed study. timeat whichthe most equatorward auroral Forconvenience inorganizing thedata avail- arc brightened andbegan to expandnorth- ablefor thisevent we havechosen to divide ward[Akaso[u, 1964]. Later a relationwas thisstudy into a collectionof short notes. established between details of associatedmag- Thisnote describes the state of themagneto- netic disturbances (polar magnetic substorm) sphereon August15, 1968, as definedby mag- and the auroral substorm[Akaso[u et al., 1965; neticindices and ground magnetograms. Herein Akaso•uet al.,1966; Akaso•u and Meng, 1969]. we definethe onsettimes of variousphases of The mostobvious magnetic signature of the the two magnetosphericsubstorms. Successive onset of an auroralsubstorm is a suddende- notes describe the solar wind and the inner crease in the horizontal componentas a west- 3046 MCPHERRON;SUBSTORM STUDIES, 1 6 i i i I 20 o I I I I -4c , , I , I ' , I AUGUSTI• AUGUST14 AUGUST15 AUGUST16 Fig. 1. The Kp and Dst indices for a 4-day interval including August 15, 1968. Note the gradual commencementstorm beginning on August 13. The main phase minimum occurred on August 17. ward auroral surgepasses over an auroral zone with auroral magnetogramscould be established station located just before magnetic midnight. directly but was noted first in a study of sub- McPherron [1970] has used this signature to storm effectsin the magneticfield at the syn- show that isolatedsubstorms have a period of chronoussatellite ATS 1. weak magnetic disturbanceprior to the onset Cummings and Coleman [1968] and Cum- of the auroral substorm. Since many mag- mings et al. [1968] reported that changesin netosphericphenomena are associatedwith this the magnetic field at ATS I were correlated disturbance, McPherron concluded that a with the onset of the expansionphase of a growth phaseprecedes the onsetof the auroral substorm. Their observationsshowed that, substorm. during the growth phase, when the satellite Similar conclusionshave been reached by was just before local midnight, the magnetic a number of other workers on the basis of field gradually becomesdepressed below the independent work, e.g., Morozumi [1967], quiet day value. At the onsetof the expansion Iijima [1968], and Pudovkin et al. [1968]. phase the field magnitudebegins a rapid re- More recentlyS. Kokubun (unpublishedmanu- covery usually returningto its quiet day value. script, 1971), Oguti [1969], and Nishida [1971] The authorsalso noted that the H component have also studied the substormgrowth phase.