Two basic modes of interaction between the ... - Wiley Online Library

VOL. 76, NO. 10

:JOURNAL OF GEOPHYSICAL

RESEARCH

APRIL 1, 1971

Two Basic Modes of Interaction between the

SolarWindandtheMagnetosphere ATSUHIRO I•TISHIDA1 AND KIYOSHI MAEZAWA

Institute oi Space and Aeronautical Science University oi Tokyo, Komaba, Tokyo, Japan

By usingExplorer33 and 35 interplanetaryplasmaand magneticfield data, it is confirmed that there are at least two basic modes in the interaction mechanism between the solar

wind and the magnetosphere. The first is the changein the magnetospheric dimensionthat

resultsfrom changes in the solarwind dynamicpressareexertedat the magnetopause, and the second is the fluctuation in the magnetospheric DP 2 electric field that is related to

fluctuationsin the north-south componentof the interplanetary magneticfield. Whereasthe

first can be interpretedessentiallyby the classicalChapman-Ferraro theory, the second suggests that the interplanetaryelectricfield penetrates deepinto the magnetosphere.

The comparisonof the ground magnetic nerosphere.The existenceof this mode was recordswith the plasmaand field observations predicted by Chapman andFerraro[1931]. in the interplanetaryspaceyieldsusefulinforTheDP 2 fluctuations, ontheotherhand,show

mation

on the mechanism of the interaction

peak-to-peakcorrespondence to the north-south

betweenthe solar wind and the magnetosphere. component of the interplanetary magneticfield

Althoughthe magneticvariationsobservedon the groundare the mixture of the fieldsgeneratedby numerousmagnetospheric and ionosphericprocesses, they can be separatedinto several distinct components when their localtime and latitude dependences are examined.

[Nishida,1968b].This demonstrates the existence of anothermode of the interactionbetweenthe

solarwindand the magnetosphere. The DP 2 electricfield in the magnetosphere is directed principallyin the morning-evening direction, and a possiblecauseof the observedcorrelation

Amongthesecomponents, correlations with the is the penetration of the interplanetary electric interplanetaryconditionshave been detected fieldE -- --u x B intothemagnetosphere. The for suddenimpulses(includingssc's), polar possibility of sucha process wassuggested by substorms(namely, DP 1 fields), and DP 2

Alfv•n[1963]and Dungey[1963]priorto the

fluctuations.

finding of the DP 2.

Suddenimpulseshave been found to occur The activityof the polarsubstorm (DP 1) following sudden changesin the momentum is alsoinfluenced by the electromagnetic state

flux P -- Nmv •' of the solar wind [Gosling

of the interplanetary space.This activityis

et al., 1967]. The primary feature of sudden stronglyreflected in indices suchasKp andAE. impulsesis the worldwideincreaseor decrease It hasbeenfoundthat threeparameters of the in the field strength,which resultsfrom the interplanetary magnetic fieldshowappreciable contractionor expansionof the entire magneto-

correlation with these indices. These are the

sphere.The variationin the magnetosphericmagnitude,the north-southcomponent,and dimensionin response to the changein P repre- the variabilityof the transverse component. It sents the first of the basic modes of the interis noted,however, that variations in anysingle action between the solar wind and the mag- one of theseparameterscannotfully explain the observedactivity [Hirshbergand Colburn, 1969]. It may be that some interplanetary x Also at the Center for Space Research, Massachusetts Institute of Technology, Cambridge

Copyright ¸

parameter other than those listed above is in

fact responsible for polar substorms,or that a combinationof morethan oneparameterdetermines their activity. The nature of this inter-

02139.

1971 by the American GeophysicalUnion.

,

2254

2255

SOLARWIND AND MAGNETOSPHERE

cio. tween these two modes by using adata set netic field isreported, the solar wind plasma DP 2are fiuctuatiøns and the interplanetary mag• "'• data notexamined. This was because solar•ø paperwe concentrate on the first two basic

modesof the interactionmentionedpreviously.

Our purposeis to clarifythe distinction be-

o

that iS morecomplete than the oneemployed

earlier.In the earlieranalysis[Nishida,1968b], where the coherencebetweenthe geoma•o•netic

wind plasma data with sufficient time and space • resolution werenot available for the Imp 1

satellite, whose magnetic fieldrecords were analyzed at thattime.Theabsence of the

cases presented there dynamic pressure P plasma data invited athe speculation that inthe and the north-southfieldB, of the solarwind

were varying coherently, sothatthegeomagnetic fluctuations that appearedto be correlatedto B, were in fact causedby variations in P

17

•uGusr 29 •967

UT

MARc. •2 •6•

Fig. 1. Examplesof suddenimpulse (left) and

through theinteraction ofthesimode[Heppner, DP 2 fluctuations (right).Intervalbetween time 1969]. Thus a further examination of the dis- marksis 1 hour. tinction between the two basic modes of the interaction will be made.

(In the figures where ground magnetograms

In the followinganalysiswe use the data are reproduced, H or X component recordis obtainedin the interplanetaryspaceby the usedin middleand low latitudes,but nearthe

Explorer33 (AIMP-D) and Explorer35 geomagnetic polewepresent the component in (AIMP-E) satellites. Both plasmaand field whichthephase of theDP 2 fluctuations relaobservations are successfully obtainedby these tive to their equatorialobservationdoesnot

satellites; they providethe solarwindproton vary throughout the intervalincluded in the

spectrum every328seeandtheinterplanetary figure. Aswould bededuced fromtheequivalent magnetic fieldvectorevery6 sec.The mag- currentsystem of DP 2 [Nishida, 1968a],the netic field data usedhere are averagesof the

abovephaseis the sameor oppositedepending

originaldata overonesequence (duration81 onthe localtime.) see)for thosesequences in whichthe solar A sudden impulse (Figure1, left) starts•ll

windprotondataarenormally available. Ex- overthe worldwithina fewminutes, but its amples are selected from14 days(August 3 formdiffers considerably by theposition of the through 9 and24 through 30, 1967)of the observation, as summarized schematically in Explorer35 observation and from 15 days Figure2b. Whenseenin the horizontal corn(January 12 through 15,February 11through ponent magnetogram, a positive sudden impulse 15,andMarch11 through 16, 1968)of the typically shows anincrease likea stepfunction Explorer 33observation. in theevening, night,andmorning partsof the ,

low-latituderegion(whichincludes Tbilisi), but

SUDDEN IMPULSES ANDDP 2 FLUCTUATIONS in the afternoon side(whichincludes I-IuanBeforecomparing the interplanetary with cayo)thisincrease ispreceded by a smallnega-

theground data,themorphological difference rivekick(Preceding reverse impulse). In the

betweensi andDP 2 is explained in FigureI

late morninghoursaboveabout35ø geomag-

by a setof examples. Themagnetograms pre- neticlatitude(e.g.,Victoria) the increase is

sented in thisfigure areobtained at theobserv- followed immediately by a deeperdecrease atorieswhose positions (in geomagnetic lati- (following reverse impulse), andtheobserved tudeandlocaltime)areshown in •'igure2a. formof thevariation is different froman in-

2256

NISI-IIDA AND MAEZAWA o

o

o

TBLL,ISI•

'"'/'Z 60ø ß• ß FUQUENE

AT18UT

/

•HUAN•AYO 12

(•)

6

12

(b)

18 DP 2

.5Fnin

12

(c)

3OFnin

1•i•.•. (•) Lo½•fJon• of tNeob•e•¾Atofie• •No•e•e½o•d• A•esNo•nin l•igu•e1. (b) ])i•fxi-

butionof the fo•m of si Asseenin

oœ D? 2.

creaselike a step functionin the polar cap (e.g.,Mould Bay) as well. The abovespatial variation of the form of positivesuddenimpulseshasbeenwidelyrecognized [Matsushita, 1962], and what hasbeenshownas the equivalentcurrentsystemfor the DS part of sudden impulsescorresponds to the maximumstage of the following reverseimpulse [Obayashi and Jacobs,1957]. In the easeof negative suddenimpulsesthe signof the changeis com-

distinguishes suddenimpulses andDP 2 fluctuationsis the spatialdependence of the form of

the variation, andthe comparison of the geomagneticpolar with the daytime equatorial magnetograms is a convenientway to separate si and DP 2. This methodworksunlesspolar substorms become so active that their influence dominates the observed variation even

at the pole and the equator.It is pertinentto notethat eventhe typicalsi as shownin Figpletely oppositeto the easeof positivesudden ure i (left) does not literally look like an impulses[Nishidaand Jacobs,1962]. 'impulse,'but like a stepfunction,in the region DP 2 fluctuations(Figure 1, right) are also wherereverseimpulsesare absent.Thus we have

observed all over the world within several min-

chosento definesi by the spatial distribution

utes,but in contrastto si their shapedoesnot involve localizedfeatureslike precedingand followingimpulsesof si, soit is possibleto draw the equivalentcurrentsystemfor the amplitude of DP 2 fluctuationsas scaledby arrows in Figure 2c [Nishida, 1968a, b]. This method of sealinghas beenmostusefulin eliminatingthe contribution of other types of variations on which DP 2 fluctuationsare superposed,al-

of its form, without interpretingthe term 'suddenimpulse'rigorously. Suddenimpulses in this senseoftenhavebeenseento appearas a step function or a successionof increasesand

decreases plus appropriatereverseimpulses [Nishidaand Jacobs,1962]. There is • difference between si and DP 2

also in the degreeof the daytime equatorial enhancement.In Figure i the ratio of the

though it has the disadvantagethat the direction of the current must be assumed.The sign magnitudesof disturbancesobservedat Huanof the changein the horizontal componentis cayo and Fuqueneis approximately2.3 for si oppositein the morningside of the midlatitude and 4.5 for DP 2. This difference seems to be region (which includesVictoria in this ease)to due to the difference in contributions of the that observedelsewhere,so the equivalent cur- extra-ionosphericcurrent flow on the observed rent system in middle and high latitudes can disturbance field: for si the enhancement is be representedby twin vortices. (The equiva- lesssinceits primary sourceis locatedon the lent current system of DP 2 was examined in magnetopause,whereas for DP 2 the enhancedetail by Nishida [1968a].) ment is larger sincethe ionosphericcontribution is dominant. Thus the principal morphologicalfeature that

SOLARWINV AND MAGNETOSPhERE COMPARISONWITH INTERPLANETARY DATA

2257

The same comparisonfor the caseof sudden impulse is made in Figure 4. The satellite (Explorer 35) positionis (18, --56, 23) in the solar magnetosphericcoordinate.The sudden impulseclearly corresponds to a sharp rise in •/-•. Almost simultaneouslyB, increased slightly, and this may have causeda slight

The interplanetary data correspondingto Figure1 are presentedin Figures3 and 4. The interplanetaryparametersshownare the componentB, (perpendicular to the solarmagnetosphericequatorialplane) of the interplanetary magneticfieldandthe squarerootof thedynamic reductionin the height of the pulse. pressure P = Nm½. The grounddata shownare are made for three more tracingsof the recordof the Huancayoobserva- Similar comparisons sets in Figures 5, 7, and 8. Suddenimpulse,DP 2, tory at the geomagnetic equator. and the interplanetary featuresthat correspond In Figure 3 the comparison is made for the to theseare emphasizedby shadings.These sets are taken from different rangesof UT, and the 33) positionin the solarmagnetospheric coordicaseof DP 2 fluctuations.The satellite (Explorer

grounddata are represented by the tracing of recordsfrom a polar stationAlert (geomagnetic latitude 85.9ø, geomagneticlongitude 168.2ø) arepoorlycorrelated with•/•. Thisimpressionand an equatorialstation (Bangui, 5.0ø, 88.6ø; or Guam,4.0ø,212.9ø) is confirmedby the cross-correlation analysis; Huancayo,--0.6ø,353.8ø;

nate is approximately(--12,--35,--20) in the unit RB (earth radius). It is noted that DP 2 fluctuationscorrespond peak-to-peakto B,, but

the ttuancayomagnetogramtraces show the correlationof r •,• 0.8 with B, observedabout

that is closest to the local noon. The inter-

planetarydataare •/• andB,. In the correla-

10 min earlierby Explorer33 but are essentially tional analysis the ground magnetogramsare

independent of thevariations in %/• observeddigitizedat the intervalof 328 sec.To avoidthe

by the samesatellite.(Beforeestimatingthe influenceof slowlyvarying componentssuchas type cross-correlation, we screenedthe data by the S•, the ultralowpassfilter of the Chebyshev with the main lobe of half-width 0.33 hour -• is

low-cut filter describedbelow.)

r

h 90 o

• eo •

•o

T

0.5

_

20 min

5

17

19 (UT)

MARCH 12,

1968

Fig. 3. Left' comparison of DP 2 fluctuations withvariations in B, andx/P in the interplanetaryspace.Right' cross-correlation betweenHuancayoH and interplanetaryparameters.Time lag is negativewhenthe interplanetaryobservationprecedes the groundobservation.

2258

Nis•iDA

AND MArZ•WA

filtration process is illustratedin Figure5a (right) •

120-

o

90

o

60

•

50

and 5b. The DP

!

o .

_

2 fluctuations

with the time

scaleof-1 to 2 hours (Figure 5a, right) clearly standsout after the slowlyvarying background is eliminated (Figure 5b). Figure 5a showsexamplesof si and DP 2 in the range600 to 1200 UT, and the corresponding cross-correlation curvesare shownin Figure 6. In the caseof the si (Figure5a, left, andFigure6, above)for whichequatorialand polarrecordsdo not look similar and poorly correlated,peak-topeak correspondence can be noted between the

2

6

equatorial geomagnetic field and the inter-

planetary x/P, yielding the correlationcoefficient of about 0.7. The equatorial field is little •

l.o

•

¸ø9

correlatedwith B•, but the polar field showsa highercorrelation with B• than with x/-•. Thus it appearsthat at the equator the influenceof

• 0.8 I

x/• dominates overthe influence of B,, but at

16

18

2O

(UT)

the pole the influenceof B• becomesmore ap-

parent, sincevariationsin x/'• are reflected

AUGUST 29, 1967

there only as small transientspikeswhoseforms

are differentfrom the form of x/•. In the case

i

of DP 2 fluctuations(Figure5a, right, and Figure applied, and the resultingslowly varying part is 6, below), on the other hand, a high correlation subtracted from the original data (both inter- is noted between the equatorial and the polar planetary and geomagnetic)before correlation records.A closesimilarity is seenalso between coefficients are obtained. The effect of this the groundfield and the interplanetaryB,, and

150 •'

•-2.0

o. or 2.0

,,. ]

I

•

2.o

0.0 2.0

1.2

6

1.2

,......8,

7

,

B

,

10

,

11

SI (August 4, 1967)

,

I

12 (UT)

',7

,

8

,

B

,

10

,

11

,

12

,

13 (UT.)

DP 2 ( August29, 1967)

Fig. 5a. Comparisonof the ground and the interplanetary data for a case of si (left) and DP 2 (right).

SOLARWIND AND MAGNETOSPHERE but the correlation

--B,

HI20r

2259 of 0.6 is obtained

between

and the field at Alert. The smooth trace

in B, is drawn roughly to showthe similarity of the general trend to the ground records. The presenceof several points that do not fit this trend suggestthat the geomagneticfield does not respondto interplanetary electromagnetic variations with a time scale of 5 min or less. For several cases of DP 2 fluctuations identi-

fied in the presentdata set, dependences on the interplanetaryfield componentsother than B, are alsostudied,and correlationcoefficients are tabulated in Table 1. B: and By are x and y componentsin the solar magnetosphericcoordinate system,and B is the magnitudeof the field. As seen in Table

I the correlations with

B:, By, and B are lessthan the correlationwith Sz. '7

8

9

10

11

12

13 (UT

DP2(AUgust 29, 1967') Fig. 5b. DP 2 of Figure 5a after the low-cut filtration.

the correlation

coefficient between the Alert

data and --B, is about 0.8. Throughout this interval the interplanetaryP remainspractically constant and cannot be consideredresponsible for variations observedon the ground.

Figure 7 is anotherpair of examplesobserved in the range1300 to 2000 UT. As in the previous example,we note that, in the caseof the si, the equatorialand polarvariationsare not correlated

(]rl_• 0.4), and the equatorialfield is highly correlated withtheinterplanetary •/• (r = 0.8), but the polar field is correlated better with

B,(r = --0.7) than•/•([r[ _• 0.4). In the case

The time lag obtained from the cross-correlation analysisis due primarily to the separation between the satellite and the earth. Since

the satellites used here have large apogees (about 80 Rs for Explorer 33 and 60 Rs for

Explorer 35), the magnitudeof the lag producedby the separationcan be as large as 30 rain and sometimesthe groundobservationleads the satellite observation.The time lag due to

the separationis estimatedby a simplemodel: the solar wind parametersare assumedto be constanton a plane that is perpendicularto a vector (--u, --•r, 0) in the solar ecliptic coordinate system,where u, •, and r are solar wind velocity, angularvelocity of the solar rotation, and sun-earth distance.This plane is assumed to propagatetowardthe earth radially with the velocity u, and the time interval betweenpassages of this plane acrossthe satellite

of DP 2 fluctuations,we note that the equatorial and the center of the earth is taken as T•x,. The observedlag To•s and the expectedlag and polar variations are highly correlated T•x, are comparedin Figure 9. The accuracyof (r = 0.8), and they correspondpeak-to-peak To•s is about 5 min and that of T•x, would not to the interplanetary B,(r = --0.7), but the

interplanetary •/•

doesnot showcorrelation be

with the groundfield ([r[ _• 0.3). In the next example obtained in the range 2200 to 0500 UT, for si (Figure 8, left) we can see the low correlation between the polar and equatorial records,and the occurrenceof si in

responseto changein •/P(r = 0.8 at the equator). For the DP 2 (Figure 8, right), the polar and the equatorial recordsshow coherent variations(r = 0.8), but theseare not correlated

much better. Within

the limit

of this ac-

curacyit can be said that for suddenimpulses To•s • T•x,, but for DP 2 fluctuationsTo•sis appreciablylarger than T•x,, the difference (To•.s-- TEx,) being about 15 min in average. From the examinationof the originalsolarwind

data it can be seenthat all the suddenimpulses studied here are due to discontinuitysurfaces

in the solarwind, not to shockwaves.Henceit is reasonablethat for si's the observedtime lags

with •/•. In thiscaseB, is relativelyscattered, are closeto the estimateby the abovemodel.

NISt-IIDA AND MAEZAWA

2260

r -1.O

•.o'

-1.0

0.5

-0.5

sI

!

Omin

-0.5

-0.5

ALERT AND BANGUI

DP2

-0.5

BANGUI AND INTERPL.

11.O

1.O

0.5

-B

I

ALERT AND INTERPL.

•.5

cr•'Crl •3'

I

-O.5

-Bz I 3•O

--0.5

Fig. 6. Cross-correlation curYes •o• •Ne si (•bo•e) u•e

a•d •Ne D•

ci-30

n

+-0.5 • (below) sNo•

i• •i•-

5•.

The long time delay betweenB• and DP 2 is noted also in the earlier analysisof the Imp 1 data [Nishida, 1968b]. Since the speed of the propagationof the interplanetary agent would not be much differentfrom the solarwind speed, the excessof the observed lags indicates that the solar wind magnetosphereinteraction of this type takes longer time to manifest its effect on the ground when comparedwith the compression of the magnetosphere,if the solar wind parameter responsiblefor DP 2 fluctuations indeedhas the phaseof Be. When the activity of the polar substormis

enoughto causeappreciableDP 2 fluctuations (as notedby comparisonwith previousfigures), but the peak-to-peak correspondence between B• and the polar (Alert) record is not actually recognized.At this time an intensepolar substorm is being observedat College (geomagnetic latitude 65ø) and the Kp indicesare 4+ and 4--. Accordingto Balli/ et al. [1967], Kp is observedto increaseas the variability of the transverse component of the interplanetary magnetic field increases.Thus in the present

very high, its effectcanmaskDP 2 fluctuations over a wide area. An exampleof this situation is shownin Figure 10. The interplanetaryB, is varying with an amplitude that seemshigh

has masked DP

example it seemsthat the amplitude of the B• variation is so large that the resultant DP 1 2 over a wide area. The inter-

vals with Kp of 2 or 3 are usuallymost suitable for the detectionof the correspondence between B• and DP 2; when Kp is lower the amplitudes

SOLARWIND AND MAaNETOSr•ERE

2261

X1150x xl

-4.0

a• o.o

-4.0

2.0

h -2.o œ

o.o

.0 •

2.0

1.

1.0

L•1.4 1.2 '13

....... "':'" :•" ;" ":'"' ""/' ;:•i '": 14

15

16

17 (UT)

SI (August 9, 1967)

0

L• 0.6 14

15

16

17

18

19 (UT)

DP 2 ( January 13, 1968)

Fig. 7. Interplanetary origin of si and DP 2.

of the variations

are too small to allow a reli-

able comparison,and when Kp is higher DP 1 tends to mask DP 2.

in the solar wind, and the integration is to be made over the lateral (with respectto the sunearth line) extent L of the interplanetarymagnetic field lines that

DISCUSSION

are reconnected.

The

re-

connection at the solar wind magnetosphere Thus it is confirmed that the interaction that interface can be visualized as Figure 11. The connectsthe interplanetary magnetic field to left and right figures correspondto the cases the geomagnetic DP 2 field is a processthat is of B• being southward or northward, respecdistinct from the effect of the varying solar tively. Dotted curves show the outer limit of wind dynamic pressure.Among the magnitude the doughnut region that contains the closed and the componentsof the interplanetary mag- geomagnetic lines of force, and the internetic field, B, is correlatedbest with DP 2, but planetary and the terrestrial magnetic lines of this is not yet to say that DP 2 dependslinearly force are connected at the reconnection line on B,, sincethere could be other functionsof shown by the dashed curve. When Petschek's the interplanetarymagneticfield that give an [1964] estimate is extended to the case where the magneticfield on both sidesof the interface equal or better correlation. If the DP 2 is due to the penetrationof the makesan arbitrary angle20 but the densityand interplanetaryelectric field into the magne- the field magnitudesare symmetric,it is found tospherethroughthe reconnection of the inter- that u ec Bsin 0 [Maezawa and Nishida, 1970]. planetaryand the terrestrialmagneticlinesof In average 20 may be taken to be the angle force, the electric potential acrossthe magne- between the interplanetary field and the geotosphereis givenby ß -- j'uBdl, whereu is magneticnorth, and L is roughly proportional the velocity of the solar wind fiowi'ngtoward to sin0. Hence approximately ß cr B'sin'O, the reconneetionline, B is the magnetic field which can be expressedas

2262

Nxs•xDA a•D MAEz•wa

yi120)' ß

-4.0

•-2,o

•

-

ß

1

ß ß

o.o

ß

4.0

1.0

0.8 25 SI ( August8- 9, 1967)

I

o

1

2

3

4 (UT)

DP 2 (August26- 27, 1967)

Fig. 8. Same as Figure 6.

above parameter is also tabulated in Table 1.

(B• zq-B, z) [I -- (B,,2 q_

The correlation is not better than the one for

B•, but it is necessary to refinethe modelby by noting that at the bow shockthe transverse droppingthe assumption of the symmetryin componentsB, and B, are enhancedmuch more

the estimate of u.

than B,. The correlation between DP 2 and the

The time lag to DP 2 is about 15 min both

TABLE 1. Time, Kp, and Dependences on the InterplanetaryMagneticParameters of the DP 2 Fluctuations

Examined

Here Correlation½

Year 1967

1968

Month Day Aug. Aug. Aug. Aug. Aug. Jan. Feb. Mar.

Hour

6 22-3 25 6-10 25 17-21 26 23-4 29 . 7-13 13 13-19 13 14-17 12 14-18

Kp 33 3 3 2 223-

Mean

0.72 0.62 0.74 0.60 0.76 0.76 0.74 0.78

0.28 0.76 0.54 0.64 0.58 0.16 0.48 0.66

0.54 0.54 0.56 0.52 0.36 0.38 0.64 0.46

0.56 0.70 0.58 0.58 0.50 0.14 0.54 0.36

0.68 0.60 0.74 0.60 0.50 0.74 0.68 0.68

0.72

0.51

0.50

0.50

0.66

-':•TheKpindex isforthecenter oftheinterval studied. a Correlstioncoefficients correspond to the maximumin the cross-correlation curveand are estimated for the polar(Alert)datain 1967andfor theequatorial(Huancayo) datain 1968.

2263

SOLARWIND AND MAGNETOSPHERE

.... 40I

'E

ß

n,-._. -•¾

20- ßo •/• o ß

ß

H1500 •,

•0

•

•

-20

Fig. •.

Xt150Y

a_

l

ß•2

Comparison of •he obse•ed •o• •h •he expectedtime

0.0

m2.0 •.0 1.4

1.2'

1.C)

for B, and above (I). Although (I) in a more refinedmodelmay showa differentvalue of the

0.86

I

I 8

I, 10

12 (UT)

FEBRUARY 15, 1968

of theinterplanetary and Fig. 10. Comparison lag, the possiblecauseof the presentresult is the ground data in the case of relatively high examined.The penetrationof the interplanetary auroral zone activity. electricfield from the tail has been suggestedas

a possiblesolution[Nishida,1968b],but this doesnot apply in a reconnection modelof Fig- wind is decelerated to about one-tenth of the ure 11. Accordingto Petschelc[1964] the solar localAlfvSnspeedin its approachto the recon-

Fig. 11. Schematic illustrationof the reconnection of the interplanetary andthe terrestrial

magnetic linesof forceat the surface of the doughnut-like regionthat contains t"•e'closed geomagneticlines of force.

2264

NISHIDA AND MAEZAWA

nectionline, but to spend15 min in transit

The Editor wishes to thank S. Matsushita and

in evaluating fromthe bowshockto magnetopause the solar anotherrefereefor their assistance wind speedin the magnetosheath hasto be of this paper. the orderof 10• cm sec -•, whichis not likely. P•EFERENCES In the reconnection model,however, the inter-

of the magnetoplanetarymagneticfield line onceconnectedto Alfv•n, H., Hydromagnetics sphere,SpaceSci.Rev., 2, 862,1963. the geomagnetic onestaysconnected and exerts

Ballif, J. R., D. E. Jones,P. J. Coleman,Jr., influence on the electricfieldin the magneto- L. Davis, Jr., and E. J. Smith, Transverse sphere for some time. Thus the electric field

fluctuations in the interplanetary magnetic field: A requisite for geomagnetic variability,J. Geo-

E • in the magnetosphere can be written as

E•(t) -- (1/T)$o•W(•)E(t -- •)d•, where

phys.Res.,72, 4357,1967.

netic field in front of the reconnectionline at

adventurein velocity space,in Geophysics, editedby DeWitt et al., Gordonand Breach,

S.,andV. C. A. Ferraro,A newtheory W(•) is the weight function that satisfies Chapman, of magneticstorms,Terr. Magn. Atmos.Elec., $W(•)d• -_ 1, E(t) is the electricfield corre36, 77, 1931. or sponding to thestateof theinterplanetary mag- Dungey,J. W., The structureof the exosphere particular time t, and T is the time interval

New York,

1963.

duringwhichtheparticular interplanetary mag- Gosling,J. T., J. R. Asbridge,S. J. Bame,A. J.

netic field line significantlyaffectsthe state of

Hundhausen,and I. B. Strong,Discontinuities

in the solarwind associated with suddengeothemagnetosphere. Fromthispointof view,the magneticimpulsesand storm commencements, observedtime lag To•s can be equatedto J. Geophys.Res.,72, 3357,1967. $o•W(•)•d• in the statistical sense.Although Heppner,J.P., Magnetospheric convectionpat-

the form of W(•) is not known,it wouldbe

terns inferred from high latitude activity, in

reasonableto supposethat at least in the order

AuroraandAirglow 1968,Reinhold, NewYork,

of magnitude Tots• T. Thenthe correspond-

1969.

J., and D. S. Colburn,Interplanetary ing downstream dimension of the influenced Hirshberg, field and geomagnetic variations: A unified region is estimatedas Vsw X Tots •

60 R•.

The ideaof attributingthe geomagnetic activ-

view, Planet. SpaceSci., 17, 1183,1969. Maezawa,K., and A. Nishida, Penetrationof the

interplanetaryelectricfield into the magnetosphere,in preparation,1970. dynamicpressureand the interplanetaryelec- Matsushita, S., On geomagnetic suddencommencetric field was discussed by Rostokerand FSltments, suddenimpulses,and storm durations, hammar[1967],but they haveconsidered only J. Geophys.Res.,67, 3753,1962. DP i as the effectof the interplanetaryelectric Nishida, A., GeomagneticDP 2 fluctuationsand associated magnetospheric phenomena, J. Geofield. It seemsnow that DP 2 represents the phys. Res., 73, 1795, 1968a. more direct consequence of the magnetospheric Nishida, A., Coherenceof geomagneticDP 2 interactionwith the interplanetaryelectromag- fluctuationswith interplanetarymagneticvariations,J. Geophys.Res.,73, 5549,1968b. netic field, and for the understanding of DP 1 ity to the combinedinfluenceof the solar wind

complex magnetosphericprocessessuch as the Nishida, A., DP 2 and polar substorm,Planet. SpaceSci., in press,1971. storageand releaseof energyin the geomag- Nishida,A., and J. A. Jacobs, Worldwidechanges netic tail would have to be taken into account. in the geomagnetic field, J. Geophys.Res.,67, 525, 1962. The morphologicaldistinction between DP 2

and DP i is explainedand the relation between these two modes of the disturbance is examined

in Nishida [1971]. Acknowledgments.The solar wind plasmaand magnetic field data employedhere are provided by courtesy of Dr. J. H. Bin•ck of the Massachusetts Institute of Technology and Dr. N. F. Ness of the Goddard Space Flight Center. We wish to express our gratitude to their kindness and also to the useful comments we have received from

Rostoker,G., and C.-G. F•lthammar,Relationship between changesin the interplanetary magneticfield and variationsin the magnetic field at the earth'ssurface,J. Geophys.Res., 72, 5853, 1967.

them.

Part of the work was conductedunder the grant NGR

Obayashi,T., and J. A. Jacobs,Suddencommencements of magnetic storms and atmospheric dynamoaction,J. Geophys.Res.,62, 589, 1957. Petschek, I-I. E., Magneticfieldannihilation, AASNASA symposium on the physicsof solarflares, editedby W. N. Hess,NASA SP-50, 1964.

22-009-372.

(ReceivedAugust 10, 1970; accepted December 28, 1970.)