VOL. 78, NO. 16 3OURNAL OF GEOPHYSICAL RESEARCH 3UNE 1, 1973

SatelliteStudies of MagnetosphericSubstorms on August15, 1968 1. Stateof the

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 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 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 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 . 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 ofthe north lobe of the tail, nearly along the ofthese magnetograms 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 ( 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

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-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- coveryusually 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. at the midlatitude station, Honolulu, at the In this modified picture the onset of a mag- same local time as ATS 1, tracks the H com- nerosphericsubstorm refers to the beginning ponent at the satellite. of the growth phase, and the onset of the This tracking effect can be used to identify expansionphase refers to the beginningof the the onset of magnetosphericsubstorms at mid- auroral substorm.Since the onset of the ex- latitudes [McPherron et al., 1970; Homing pansion phase is generally sudden and quite et al., 1971]. These authors find that the onset dramatic, we believe it is the appropriateref- of an increasein H at the midlatitude station erence point to use in timing substormphe- closestto midnight is either coincidentwith nomena. or precedesthe onset of the expansionphase Unfortunately, the interpretation of auroral as determined from auroral zone magneto- zone magnetogramsis frequently ambiguous, grams.Furthermore, midlatitude changesin the and all-sky camera data are not readily avail- H componentare generallysimpler than those able. Therefore it is necessaryto use addi- in the auroral zone, and these stationsare less tional information. Such information is pro- sensitiveto localizedevents than auroral zone vided by midlatitude magnetograms.The as- observatories.Finally, there existsa better dis- sociation of changes in these magnetograms tribution of midlatitude stations. MCPHERRON:SUBSTORM STUDIES, 1 3047 Because of these facts, we use midlatitude graphicand geomagneticcoordinates are listed magnetogramsaugmented by thosein the in Table 1. The observatories are divided in auroral zone to identify the onset of the ex- two groups,auroral zoneand midlatitude,each pansion phase of magnetosphericsubstorms. ordered by increasingwest longitude. The lo- We point out, however,that it is not necessary cations of these observatorieson a geographic to accept the precedingargument to justify north polar projection are shown in Figure 2 such use. Any characteristicand easily identi- for the universal times of the two substorms fied signature of a magnetosphericsubstorm chosenfor detailed study. It should be noted can be usedas a referencepoint for establishing that the two substorms occurred at times such the sequenceof events that occursthroughout that the distribution of observatories around the magnetosphere. midnightwas the best possible. Auroral zone. The horizontal component Substorm Expansio•.• at 0•30 and 071• UT,' traced from magnetogramsof some of the August 15, 1968 auroral zone stations are shown in Figure 3. Magnetogramsfrom a number of magnetic The stations are ordered by progressivewest observatorieswere available for this study. The longitude, corrected geomagnetic midnight names of these observatoriesand their geo- [Montbriand, 1970] shown by a black dot at

TABLE 1. MagneticObservatories and Their Geographicand GeomagneticCoordinates

Geomagnetic Geographic Coordinates Station Coordinates (lat., long.)

Auroral Zone Sodankyla (SO) 67ø22•N, 26ø38•E 63 76 ø, 119 99 ø Leirvogur (LE) 64ø11•N, 21 ø42'W 70 22 ø 71 04 ø Great Whale River (GW) 55ø16•N, 77ø4TW 66 57 ø, 347 36 ø Fort Churchill (FC) 58ø48'N, 94ø6'W 68 70 ø, 322 77 ø Baker Lake (BL) 64ø20•N, 96ø2•W 73 77 ø, 315 22 ø Meanook (ME) 54ø37'N, 113ø20'W 61 82 ø, 301 02 ø Tungsten (TN) 61ø58•N, 128ø12•W 65 91ø, 278 43 o Sitka (SI) 57 ø4'N, 135ø20'W 59 99 ø, 275 34 ø College (CO) 64ø52•N, 147ø50•W 64 63 ø, 256 52 ø Barrow (BA) 71ø18•N, 156ø45•W 68 54ø, 241 15 ø Tixie Bay (TI) 71ø35•N, 129ø0•E 60 44ø, 191 41 ø Cape Chelyuskin (CC) 77ø43•N, 104ø17•E 66 26 ø, 176 46 ø Dixon Island (DI) 73 ø33•N, 80ø34•E 63 02 ø, 161 57 ø Heiss Island (HI) 80ø3TN, 58ø3'E 71 30 ø, 156 05 ø

Midlatitude M'Bour (MB) 14ø24•N, 16ø58•W 21 25ø, 55.02 ø San Juan (SJ) 18ø7'N, 66ø9•W 29 60 ø, 3.12 ø Fredericksburg (FR) 38ø12•N, 77ø22'W 49 55ø, 349.85 ø Dallas (DA) 32ø59•N, 96ø45'W 42 96 ø, 327.75 ø Tucson (TU) 32ø15'N, 110ø50•W 40 43 ø, 312.18 ø Honolulu (HO) 21ø19•N, 158ø0•W 21 07ø, 266.53 ø Guam (GU) 13ø55•N, 144ø52'E 4 30 ø, 212.85 ø Kakioka (KA) 36ø14'N, 140ø11•E 26 03ø, 205.96 ø Irkutsk (IR) 52ø10•N, 104ø2TE 40 73 ø, 174.70 ø Tashkent (TA) 41ø20•N, 69ø37•E 32 33 ø, 144.02 ø Tbilisi (TB) 42ø5•N, 44 ø42•E 36 68 ø, 122.08 ø Gnangara (GN) 31ø4TS, 115ø57•E - 43 23 ø, 185.78 ø Hermanus (HE) 34ø25•S, 19ø14•E -33 28ø, 80.53 ø Dipole Axis (DP) 78ø30'N, 69 øO'W 90.00 ø, 0.0 ø Geographic Pole (GP) 90ø0'N, OøO'W 78.50 ø, 180.00 ø

Stations are ordered by progressivewest longitude. 3048 McPHERRON'SUBSTORM STUDIES, 1 04:30:00.0 07:00:00.0

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s•d •he off-ce•te• c•c]e •e •he s•sfis•ic• au•o•] so•e. the appropriateuniversal time on eachtrace. currentbeginning to move northwardprior The scale of each trace is indicated at the to 0451. right of the figureby verticalarrows corre- Argumentssimilar to thoseabove apply to spondingto 4007. Finalresults of the analysis the 0714 UT substorm.The beginningof a of both auroral zone and midlatitudemag- trigger bay in H [Rostoker,1968] occurred nerogramsas describedbelow are shownby at 0714at FC, followedby the beginningof verticaldashed lines at the approximateonset the main bay at 0723 UT. Againthe Z corn- of eachsubstorm expansion. ponentat BL (Figure 4) indicatesthat a Figure3 indicatesthat manysubstorms oc- northwardexpansion occurred prior to 0740. curred on August 15, 1968. Some of these Throughoutthe day of August 15, 1968, substormshad reasonablyclear signaturesat there werea numberof additionalsubstorms. severalobservatories; for example,consider The approximatetimes of the onsetsof their the baysat moststations after 0430and 0714 expansionphases are shownby verticaldashed UT. Becauseeffects were seen throughout the lines in Figure 3. During the selectedtime nightsector, we tentativelyidentify these bays intervalthere was one substormfor which as worldwidepolar substorms.For the 0430 the auroralzone evidenceis not clear.The UT substormGreat Whale (GW) and Fort suddendrop at FC at 0248 may be a delayed Churchill(FC)were just beforemagnetic mid- effectof a weak event seenat GW at 0205 night (blackdots on GW and FC tracesand and0242 UT. The X componentat BL (Figure alsoleft panelof Figure2). The very sharp 4) supportsthe FC data.,however, showing drop at FC at exactly0430 UT (measured a sharponset of a negativebay at 0250UT. on originalmagnetograms) is the characteristicThis suggeststhat the 0220UT expansionwas signatureof thewestward-traveling surge. Since nottypical and may have been a multiple-onset FC was so closeto midnight, the actual onset event. of the expansionphase cannot have been much On the ba•s of theseauroral zone data alone, earlierthan 0430 UT. The suggestionthat a we concludethat two well-developedpolar significantexpansion occurred after this time magneticsubstorms occurred between 0000 and is supportedby data from BakerLake (BL), 0900 UT on August15, 1968.Possible onset 5øN of FC (Figure4). The Z componentex- timesfor their expansionphases are 0430and hibited the characteristicsignature of a line 0723 UT if we assumethat the triggerbay MCPHE•ON: SUSSTO•MSTUDIES, 1 3049 precedesthe onsetof the auroralsubstorm, as From evidencepresented below and also in wassuggested by Rostoker[1968]. successivenotes we believeit is morelikely There are many baylike disturbancesat that this bay is a precursor. individual stationsthat are not easily in- A very similarfeature appears in the GW terpretedin termsof substormexpansion. We trace at 0631 UT. Again,on the basisof evi- discusstwo of thesedisturbances here because dence discussedlater, we believe that this they are relevantto our later interpretationof 'featureis a precursorof the 0714 substorm. ATS I and Ogo5 magnetometerdata in terms Midlatitudes.Magnetic effectsin the H of substormgrowth phases. componentfor August15, 1968,at midlatitudes One anomalousfeature in Figure 3 is the are shownin Figure 5. Baselinesfor each sta- onsetof a negativebay at LE at 0320 UT tion representthe averageof the Q and QQ with correspondingonsets of positivebays at (quiet) daysfor the monthof August.Each Sitka (SI), College (CO), and Point Barrow stati'ontrace is the deviation of the observa- (BA). The questionarises of whetherthis tionsat that stationfrom the quietday. Local bay is a precursorof the 0430UT substorm midnightis shownby blackdots, and, as in or an aftereffectof the 0220 UT substorm. Figure3, the stationsare orderedby progres-

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.... AAAA 0500 0600 0900 UNIVE•/•_ TIME Fig. 4. Tracingof a portionof the Baker Lake magnetogramfor August15, 1968.Vertical dashedlines are the probablebeginning of expansionphases. Note the large increasein the Z componentduring each expansion. sive west longitude.The uniformscale of 40 midlatitudestation was coincidentwith or y/divisionis indicatedat the right of the prior to the onsetof the polarmagnetic sub- figure.Dashed vertical lines show the results stormin the auroralzone. From the data of of our analysis. Figure5 we notethat this is the casefor both The effectsof the weak magneticstorm in selectedsubstorms. For the 0430 UT substorm, progresson August15, 1968,are evidentin SJ is at midnightand FR just before.Re- all traces.All stationsare depressedrelative spectiveonsets of the increasein H were at to the quiet day throughoutmost of the day. 0428 and 0420 UT (partly obscuredby black The index of storm time activity, Dst, could dots; timing determinedfrom originaldata). be calculatedat any universaltime by averag- It shouldbe noted,however, that the increases ing the observeddeviations over all stations at DS and TU beginningat 0430 UT were (longitudes).At the timesof the two selected larger, possiblyindicating that the substorm substorms(0430 and 0714 UT) it is apparent was centered somewhat premidnight. From that the field depressionis larger in the after- these observationswe chose0430 UT as the noonsector, Honolulu (HO) and Guam (GU), most probableonset time for the expansion than in the midnightsector, San Juan (SJ), phase.This time is coincidentwith what was Fredericksburg(FR), Dallas (DS), and Tucson chosenfrom the auroralzone data. (TU). This is the characteristiceffect of a Similararguments apply to the0714 UT sub- partial [Akasoiuand Chapman, storm.In this casethe midnightstations have 1964]. very sharp onsetsat 0714 UT and the largest The midlatitude magnetic effects of sub- increases.It shouldbe pointed out that this storms describedin detail by .A_kasoiuand time correspondsto the onsetof a trigger bay Meng [1969] are apparentover a wide range at the midnightauroral station (FC in Figure of longitudes.For example,after 0714 UT 3) and precedesthe onsetof the main bay. there is a largeincrease in H aroundmidnight From the midlatitudedata we thus choose (SJ, FR, DS, and TU) and a corresponding0714 UT as the most probableonset of the decreasearound dusk. A similarsituation occurs expansionphase. after the 0430 UT substorm,although it is Sinceit is relevantto our later discussionof much le• clear. the growthphase of the 0430 UT substorm,we In a previoussection we mentionedthat the commenthere on the midlatitudeeffects of beginningof the increasein H at the midnight the 0220 UT substorm.As is apparentfrom McPI-IERRON'SUBSTORM STUDIES, 1 3051

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Figure 5, these effects are as ambiguousas GROWTHPHASES .or SELECTEDSUBSTORMS theywere in theauroral zone. Possible choices In theprevious section we used ground mag- for the midlatitude expansion onset include nerogramsto establish the onsets of the ex- 0150,0220, and 0236 UT. Thebest choice pansion phases of substormson August 15, forstations closest to midnight (SJ and FR) 1968.Using these onsets, we can now discuss is0236 UT, but the more pronounced increase the possible existence ofgrowth phases for the at stationsat earlier local times (DS and TU) twoselected substorms. In Figure 5 wenote arguesthat 0220 UT maybe a betterchoice. that at 0714UT, whenthe midnight stations Theend of the expansionwas either 0306 or beganto increasein H, the stationsin the 0330UT. The subsequentdecrease in the H dusksector did not experienceany significant component.ending at 0430UT wasconsiderably change. In fact,it is clearthat HO, GU, and larger than the increaseat most stationsand KA all beganto decreasesome time earlier. may have been due in part to growth phase A thin vertical line at 0640 UT emphasizes effectsof the 0430 UT substorm,as is discussed this fact. Examining the midnight stations in the followingsection. during the interval 0640-0714 UT, we note 3052 McP•ERRON: SUBSTORMSTUDIES, 1 that they are also decreasing.Consequently, this interference effect. The first of these we are led to the conclusionthat at mid- problemsis that of multiple-onsetevents. Both latitudes there appearsto be an initial phase the auroral zone and the midlatitude data of the substormduring which all stationsin suggest that this substorm did not begin the night sector simultaneouslyexperience a simultaneouslyat all local times. decreasein the H component. The secondproblem is that many substorms If we now examinethe 0430 UT substorm, do not appear to be centeredon the midnight we recognizea similar effect. In this casethe meridian. This appeared to be the case for initial phase probably began after 0330 UT, the 0220 UT event, in which the eventual as was discussedin the precedingsection. It expansionwas larger at the early eveningsta- shouldbe pointed out that this interpretation tions. However, we note here that 0220 UT is is not clear becauseof the effectsof recovery an inopportunetime to study substorms,in as from the substormexpansion around 0220 UT. much as the Atlantic sector is centeredat mid- In support of the precedinginterpretation night. As is clear from Figure 4, there is a gap we recall that the anomalousfeatures in the in both auroral zone and midlatitude magnetic auroral zone magnetograms(0320 and 0630 observatoriesroughly centeredabout 40øW. UT) occurredat approximatelythe sametimes The precedingproblems could be solvedby as the proposedmidlatitude effects.In both selectingfor study only those substormswith casesthe anomaly consistedof the beginning particular properties: the substorm should of a negativebay at a station in the morning be isolated,i.e., no substormin the preceding sector. This is just the effect identified by 6 hours; it shouldhave a singleclearly defined McPherron [1970] as the beginning of the onset; and it should have the most magnetic growth phasefor isolatedsubstorms. effectsat localmidnight. Although it is possible We note, however, that the 0430 UT sub- to select a reasonablenumber of such events storm is not an isolated substorm and its in the data of a year, the resultingset is clearly growth phase signatureis complicatedby re- not 'typical' in the sensethat there are many covery phase effectsof the 0220 UT substorm. more clear substorm events that do not fit In particular, the negativebay at LE after these criteria. Furthermore,with the specu- 0320 UT in Figure 3 developsmore rapidly lative model presentedin the introduction,it than the events discussedby McPherron is easy to suggestreasons why such selected [1970]. Although such behavior is suggestive eventsshould not be typical. of an expansionphase onset, the BL mag- If, in addition to the foregoingconstraints, netogramin Figure 4 and the midlatitudemag- we imposethe restrictionthat a satellitemag- netogramsin Figure 5 do not support such an nerometerbe in an appropriate region of the interpretation. geomagnetictail, we are likely to reduce our D•scuss•oN setof eventsto thenull set. Consequently, our approach in this sequenceof papers is The foregoinganalysis reveals a numberof the opposite;i.e., we take well-definedevents the problemsassociated with the study of in the magnetosphereand studythe associated magnetosphericsubstorms using ground mag- ground data whatever its complexity.We nerograms.The particularlyclear signature of recognizethe limitationsof this approachin the 0714 UT substorm is not really typical. as much as the conclusionsderived from these As was demonstratedby the discussionof the grounddata alonemay be ambiguous.Thus 0430 UT substorm,ground magnetograms fre- the conclusionspresented in the followingsec- quentlyinclude superimpo•d effects of succes- tion must be consideredprovisional. As will sivesubstorms. It is oftenimpossible to decide becomeapparent in later papersin this series, unambiguouslyfrom ground data alone whether these conclusionsare supportedby satellite a given magnetic perturbation is the recovery data. phase of one substorm or the growth phase of thenext. CONCLUSIONS The 0220 UT substormillustrates two other On the basisof our analysisof groundmag- problemsof substormanalysis in addition to netograms,we conclude that a number of MCPHERRON:SUBSTORM STUDIES, 1 3053 distinct substormexpansions occurred on Au- Atkinson, G., A theory of polar substorms,J. gust 15, 1968. During the interval in which Geophys. Res., 71, 5157-5164, 1966. Aubry, M.P., and R. L. McPherron, Inward mo- Ogo 5 passedinbound through the cuspregion tion of the magnetopause before a substorm, of the tail (0000-0900 UT), there were at least J. Geo,phys.Res., 75, 7018-7031, 1970. three substorms. Two of these had sudden Axford, W. I., Magnetospheric convection, Rev. sharp onsetsand worldwideeffects. The most Geophys. Space Phys., 7, 421-459, 1969. probableonset times for their expansionphases Cummings, W. D., and P. J. Coleman, Jr., Simul- taneous magnetic field variations at the earth's are 0430 and 0714 UT. For the weaker sub- surface and at synchronous,equatorial distance, storm that precededthese, 0220 UT was chosen 1, Bay-associated events, Radio Sci., 3, 758-761, 1968. as the onset of the expansionphase. • . The two stronger substorm expansionsap- Cummings, W. D., J. N. Barfield, and P. J. Cole- man, Jr., Magnetospheric substorms' observed pear to have been precededby growth phases. at the synchronousorbit, J. Geophys. Res., 73, Tentative identification of the beginning of 6687-6698, 1968. these is 0330 and 0640 UT. Respective dura- Hones, E. W., Jr., S. Singer, L. J. Lanzerotti, tions of these proposedphases are 60 and 34 J. D. Pierson, T. J. Rosenberg, The magneto- min. spheric substorm of August 25-26, 1967, J. Geo- phys. Res., 76, 2977-3009, 1971. The signaturesof the two worldwide sub- Horning, B. L., R. L. M cPherron, P. J. Coleman, storms at the auroral zone stations are similar Jr., Correlations between magnetic field changes to th'osediscussed many times.in the literature. at AT.S 1 and low-latitude ground stations (ab- At midlatitudesthe growth phase of both sub- stract), Eos Trans. AGU, 52, 324, 1971. Iijima, T., N. Fukushima, and Y. Kamide, World- storms consists of a relatively small decrease wide geomagnetic change preceding the onset of in H at nearly all stations in the night sector. auroral electrojet, Rep. Ionos. Sp.ace Res. Ja- However, the expansionphase signature de- pan, 22, 1968. McPherron, R. L., Growth phase of magneto- pends on local time. Near midnight there is spheric substorms, J. Geophys. Res., 75, 5592- a very large increasein H abovethe quiet day 5599, 1970. curve. Near dusk there is an additional decrease McPherron, R. L., P. J. Coleman, Jr., and B. L. in H. Homing, Propagation of substorm effects away from midnight as observed by the ATS I mag- Aclcno,wledgments. I thank my many co-work- netometer (abstract), Eos Trans. AGU, 51, 391, 1970. ers for their numerous helpful comments in the conduct of this work. Particular thanks are due Montbriand, L. E., A simple method for calculat- Dr. C. T. Russell for his carefill reading of the ing the local time of corrected geomagnetic manuscript.Ground magnetic data were provided midnight, J. Geophys. Res., 75, 5634-5636, 1970. in the form of microfilm and digital tape by Wil- Morozumi, It. M., Auroral-zone geophysical liam Paulishak of the World Data Center for Geo- events and their relationship to the magneto- magnetism, Greenbelt, Maryland. sphere, Proceedings of the Symposium on Pa- Partial support for the author's time was de- cific-Antarctic Sciences, Jare Sci. Rep., Spec. rived from NASA Grant NGL 05-007-004. 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