Feb 1, 1988 - The sign of the latter correlation depends on the Kp index. For Kp -- 0 the pulsation power decreases with increasing solar wind kinetic energy ...
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 93, NO. A2, PAGES 877-883, FEBRUARY
1, 1988
Dayside Long-Period Magnetospheric Pulsations' Solar Wind Dependence HANS
JUNGINGER
lnstitut f•r Angewandte Geod&sie,SatellitenbeobachtunõsstationWettzell K•tztinõ, Federal Republic of Germany WOLFGANG
BAUMJOHANN
Maz-Planck-lnstitut f•r Physik und Astrophysik,lnstitut f•r eztraterrestrischePhysik Garchinõ, Federal Republic of Germany
The spectralpowerand occurrence rate of long-periodmagnetospheric pulsations(predominantly fundamentalmode toroidal PcS) observedby the electronbeam experimenton board GEOS 2 are comparedwith IMF and solarwind parameters.No clearinfluencesof IMF orientationand magnitude on pulsation amplitudes and occurrencerate are found. Significant correlations,however, do exist betweenthe spectralpower of pulsationsand the solar wind bulk velocity,and betweenthe spectral powerand the solarwind kineticenergyflux. The signof the latter correlationdependson the Kp index. For Kp -- 0 the pulsationpowerdecreaseswith increasingsolar wind kinetic energyflux, whereasit increasesfor Kp >_1. Our results are consistentwith the Kelvin-Helmholtz instability at the inner side
of the low-latitude boundarylayer being the dominant mechanismfor the generationof fundamental modetoroidalPc5 magnetospheric pulsations.Flux transfereventsplay only an inferiorrole as energy sourcesfor these pulsations. The couplingefficiencyof surfacewavesat the boundary layer to shear A!fv•n wavesnear geostationaryorbit seemsto changesignificantlyif the geostationaryorbit is inside the plasmasphere.
1.
INTRODUCTION
to surface waves propagating into the direction of U if
Electric field fluctuations with periods between 150 and
[Sen,1963;Fejer, 1964]
600 s and amplitudes between0.1 and 1 mV/m are everyday phenomenain the daysidemagnetosphereat geostationary orbit. This has been revealed by the electron beamexperiment[Melzneret al., 1978]on boardthe
n•-n2 •1 _•_' •-2U2 •- •i
($12 cos2 •1q-B22 cos2 •2)
(1)
geostationary satellite GEOS2. A systematic studyof where nl andn•.aremass densities oftheplasmas, B•and theelectric andmagnetic fieldsignatures measured on B2aremagnetic fields, and/iland/ia areangles between
board GEOS 2 during numerouslong-period magneto- magnetic field lines and the relative velocity U. spheric pulsation events showed a consistent behaviour The critical Kelvin-Helmholtz shear velocity Ucr at for the majority of events (see the paper by Jungin- which the boundary becomesunstable is determined ac-
ger et al. [1984,1985]henceforthreferredto as papers 1 cordingto (1) by the orientationand magnitudeof the and 2). This led to their interpretationsas fundamen- magnetic fields inside the plasmas. Ucr becomessmaller tal mode toroidal eigenoscillations of magnetosphericflux for weakermagneticfieldsand when/i• and/i•. comecloser tubes driven by the solar wind. to •r/2. Shear layers betweenplasmaswith parallel mag-
Twodifferent mechanisms arecurrently discussed inthe netic fields areunstable forallshear velocities perpendic-
literaturethat cancouple kineticsolarwindenergyinto ularto B.
resonant fieldlineeigenoscillations inside themagneto-Theideathatthemagnetopause could besubject to sphere: theKelvin-Helmholtz instability of themagne-a Kelvin-Helmholtz instability of thetypedescribed in
topause and(most recently)flux transfer events. Both(1) came upin the1950s [Dunyey, .1955] andstimumechanisms aresupposed to produce surface wave-like lateda considerable amount ofdataanalysis [e.g., Boiler
or impulsive disturbances ontheouterboundaries of andStolov, 1970; Aubry etal.,1971; LedIcy, 1971; Fairthemagnetosphere which feed energy intoresonant shearfield,1976; Wolfe andKaufmann, 1975; Southwood, 1979;
Alfvdn waves deep inside themagnetosphere through pro- Wolfe, 1980] andtheoretical work [e.g., Fejer, 1964; Sen,
cesses describedby Southwood [1974],Chenand Hasegawa 1965; Southwood,1968; McKenzie, 1970; Ong and Rod[1974a,b], and Allan et al. [1986]. erick, 1972; Yumoto and Saito, 1980; Walker, 1981; Lee
A plane magneticshearlayer betweentwo plasmasmov- et al., 1981; Miura and Pritchett, 1982; Pu and Kivelson, ing with the velocity U relative to each other is unstable 1983]whichprovidedevidencefor the existenceof surface
Paper number 6A8587.
waves of the Kelvin-Helmholtz type on the outer boundaries of the magnetosphere. As our knowledge of magnetopause boundary layer structure has increased, so too has the evidence for the
0148-0227/88/006A-8587505.00
Kelvin-Helmholtz instability acting in these regions. Re-
Copyright 1988 by the American GeophysicalUnion.
877
878
JUNGINGER AND BAUMJOHANN: SOLAR WIND DEPgNDgNCg OF PC 5
cent papers favor the instability occurring at the inner edge of the low-latitude boundary layer, where the stabilizing effectof the magneticfield is least [e.g., Walker, 1981; Lee et al., 1981]. Surfacewaveswith wavelengthsof the order of 10 Rls and periods of a few hundred seconds, i.e., suitable to drive long-period pulsations, seem not to be uncommonaccordingto these studies. Flux transfer events are magnetic signaturesfirst observedjust outside the magnetosphere[Russelland EIphic, 1979]. They are interpretedas small-scale,patchy reconnectionphenomena[e.g., Scholeret al., 1982]. One end of the flux tube connectsto the magnetosheathfield with the other end remaining anchoredin the magnetosphere. Flux transfer eventshave been recordedpredominantly in the northern dawn sectorand the southern dusk sector,a bias reflectingthe ISEE spacecraftorbital configuration [Berchernand Russell,1984;Rijnbeeket al., 1984]. They have an averagediameter of drTE • 1.5 Rls [Saunders et al., 1984]. The magnetic tensionforce and the magnetosheathflow combineto causethe two open flux tubes to contract poleward along the magnetopauseat a typical velocityvrr• • 100km/s [e.g.,Dalaiand Keppler, 1983]. Reconnectedflux tubes being pulled along the magnetopause by the magnetosheath plasma are expected to deform the latter locally with a time scale rrr• • drr•/vrrg • 100s which matches typical periods of
long-periodmagnetosphericpulsations.Furthermore,flux transfer events are repetitive, occurring on average every 7 to 8 min [Rijnbeeket al., 1984]. For thesereasons, flux transfer events are also consideredto causepulsations
tr threshold one would be unable to distinguishbetween
signaland noise;seepaper 1). As describedin papers1 and 2, an overwhelmingmajority of the pulsationeventsfound this way turned out to be-
long to the classof so-calledfundamentalmodePc 5 pulsationswith predominantlytoroidal (azimuthal) plasma drift and magnetic field polarization. Furthermore, over 90 % of the pulsation events were pure shear-like variations without any distinguishablecompressionalmagnetic field variation.
The interplanetarymagneticfield(IMF) components in geocentric-solar-magnetospheric (GSM) coordinatesand the other solar wind parameters were taken from King
[1983]. They were previouslycomparedwith GEOS 2 electrongun measurementsof the 1-hour averagedc electric field [Baumjohannand Haerendel,1985] 2.2.
Pc 5 Power
and IMF
Orientation
Figure 1 shows the relationship between normalized IMF components (indicatingIMF orientation)and pulsation amplitudesfor the 5303 eventsmeasuredby the electron beam experimenton board GEOS 2 betweenAugust 1978 and April 1979. The spectralpowerP of most events
rangesbetween0.01 and 1 mV2/m2 (paper 1). The IMF observationsduring the pulsationeventsclusterin the pa-
rameterregionsIB/BI > 0.5, IB/BI > 0.5, IB/BI < 0.5, typical for the IMF. The correlation coefficientsfor
log(P) versusB•/B, By/B, and Bz/B are -0.10, 0.17, and-0.10, respectively,thus indicating that there is no correlation between the IMF orientation and the spectral power of the electric field pulsations.
[e.g., Glafirneieret al., 1984; Gillis et al., 1987]. An attempt will be made in this paper to put constraints on the relative importance of the two mechanisms describedabove with respect to the generation of dayside toroidal fundamental mode magnetosphericpulsations. This will be done by a comparisonof solar wind parameters with electric field fluctuation amplitudes measured at geostationary orbit during events which mostly show a behavior as predicted for solar wind driven pulsations. 2.
DATA
2.3.
Pc 5 Power and Solar [Find •locit!l
In Figure 2 the power of pulsation events is plotted versus the plasma bulk velocity of the solar wind. The minimum solar wind velocity was 270 km/s in the time intervalsconsidered.The maximumvelocitywas 740km/s. The spectral power of the pulsationsis clearly related to the solar wind bulk velocity. It increaseswith increasing velocity. The slopeof the increaseis steeperfor velocities between250 and 500 km/s than for velocitiesbetween500
and 750km/s. The correlationcoefficient betweenlog(P) and vs• was found to be 0.43.
Within this section we shall first briefly describe the Pc 5 Power and Solar Wind Kinetic Energy Fluz characteristicsof the data set used in this study and then 2.4. presentthe resultsof the regressionanalysesbetweenthe In Figure 3 the dependenceof the spectral power of spectral power of long-period magnetosphericpulsations the magnetosphericpulsationson the solar wind kinetic and interplanetary magneticfield and plasmaparameters. energy flux is shown. The correlation coefficientbetween 2.1.
Data
Set
The data set usedin this study comprises5303 pulsation events measured by the electron beam experiment on board the GEOS 2 satellite betweenAugust 1978 and April 1979. These events were discussedwith respect to their polarization behaviorand their Poynting vectorsin
log(P)andlog(•nswmpvsw 3 ) is 0.50. This is the most pronounced correlation found between pulsation power and any of the solar wind parameters. It is interesting to consider the latter relationship in
mpvsw moredetail.In Figure4, log(P)versus 1og(•nsw a )
is shownfor six different Kp levelsand two different local time sectorsof occurrence. The 1080 prenoon eventsconpapers 1 and 2. They represent electric field fluctuations sidered were observedbetween 0600 and 1000 LT, and the found in a 186-day sampleof data by calculating1-hour- 2243 afternooneventsbetween1400 and 1800LT. (Note long power spectra throughout the whole interval, each that the predominanceof afternoon pulsation eventsis an spectrum shifted 10 min with respect to the earlier one apparent one only and stems from more favorable condiand taking those spectra with a significanceof greater tions for operation of the electron gun in this local time than five standard deviations (for anything below the 5- sector; in fact, the normalizedoccurrencerate given by
JUNGINGER AND BAUMJOHANN' SOLAR WIND DEPENDENCE OF Pc5
102
879
102 [[:-0
:043 5303
10
N : 5303 !
• •
.
lO0 ....•-.•.•.
.
...,.,
:•.S,:'4 2 ,' 'l.-'
, .
100
.:':-..•.
lO4
>
lO-2 ß ,,. ,. •..,..
.,
,.;,:
10-•
10-•
10-3
1ø-] I ß .[:...:'. "4.r'' ] ß
i
i
-1
o
1
250
500
Bx
102
i.
,-., 100, •
Vsw, km/sec
'
101
CC: 0.17[
ß N:$303 t
i .i" ß
Fig. 2. Spectral power P of long-periodmagnetospheric pulsation versussolar wind bulk velocity. The pulsationpowerincreases with increasingsolar wind velocity betweenabout Vsw : 250 and 500 km/s.
:' i.,.iß *'''• ,, •:. ,'•l•
.'.
10-1
correlated,with correlationcoef•cientsof-0.48 and -0.25, respectively.For Kp _• I pulsation power and solar wind kinetic energy flux correlate with correlation coef•cients
10-2
between 0.14 and 0.57. The correlation coef•cients for the ,::.•i.•-,'• -'
10-3
750
-:
afternooneventsincreasesteadily(from-0.25 at Kp = O)
.-_:.,:. :... :. -"" ::
with increasingKp values until they reach a maximum
ß
at 0.52 for Kp = 3 and 4. The correlation coei•cients I
o
1
By
for the prenoonevents,however,seemto switchfromsignificantlynegativevalues(-0.48) to significantlypositive values(+0.48) betweenKp = 0 and 1. 3.
DISCUSSION
102
In this sectionwe will first discussthe implicationsof our regressionanalyseswith respect to the excitation of pulsations via flux transfer events and Kelvin-Helmholtz instability and then try to interpret the influenceof the
101
100
•
Kp index on the excitation.
10-1 10-2
102
10-]
101
lO0 Bz
Fig. 1. SpectralpowerP of long-periodmagnetospheric pulsations versusnormalized IMF componentsin GSM coordinates. The corre-
lation coefficients (CC) for the N - 5303eventsare -0.10, 0.17, and -0.10 for B•/B, By/B, and Bz/B, respectively.
10-1 10-2
10-3
Junginger et al. [1984]evenshowsslightlymoremorning-
] ' wlm2 T1 nswmp vsw sideevents.) The correlationcoef•cientsof the pre-noonand afterFig. 3. SpectralpowerP of long-periodmagnetospheric pulsations noon eventsas a function of Kp behavein an approxi- versussolarwind kineticenergyflux. It representsthe most promately similar way. For Kp -- 0, logarithmicpulsation nouncedcorrelationfound betweenpulsationpowerand any of the power and logarithmicsolar wind energyflux are anti- solar wind parameters. , / /
880
•UN•IN•gR
102 • i G-1OUT
F
XND BXUMJOHXNN'
SOLXR WIND
DZPZ•Z•½Z
o•
102F
-
Pc •
T
1
14-18LT KP=O-/+
101 -
101•
100 -
lOo
[[ =051I N :2243
..
,:..
:i
,:r.'
ß'.: ' ...,::.•.ii • , .•. -.:.!' . -.. ß o: o
ß a*
10-1-
>
10-1 -
10-2 _
10-2
10-] _
10-3
.
: !i'
'
'
! ß
I
i
I
10 -3
102
102 6 - 10 LT KP:O
101
101
_
I
EE = -0 25 N = 171
_
_
10o
10-1
>
10-1
10-2
10-2
10-3
10-] I
i
10-•
102
I
I0 -]
102 101
10o
100
•
[[: 0.14 N = 597
_
10-1
10-2
10-2
10-]
10-] I
i
14- 18LT KP = 1
101
10-•
10-]
i
0.48 20?
KP=I
I
10-•
I
6 - 10 LT
•
I
124
_
10-]
14 - 18LT KP = 0
-0.48
10o
•
i
10-•
I
10-•'
10-•
1 rlswmpVs•w z '•, W/m Fig. 4. SpectralpowerP of long-periodmagnetospheric pulsationsversussolarwind kineticenergy flux for two differentlocal time sectorsand six differentKp levelsof occurrence.For Kp - 0, the spectralpoweris anticorrelatedwith the kineticenergyflux in the prenoon,as well as in the afternoon sector. For Kp >_O, P correlateswith the solar wind kineticenergyflux.
3.1.
Fluz Transfer
fer events as a dominant
mechanism
for the conversion of
solar wind kinetic energyinto pulsationenergy,sinceflux The power and occurrenceof the fundamental mode transfer eventsare strictly controlledby the IMF orientatoroidal Pc 5 pulsationsdo not significantlydependon any tion: they occur when the IMF direction is southward or of the IMF components.This finding rulesout flux trans- nearly horizontal. Only very few flux transfer events were
JUNGINGER AND BAUMJOHANN' SOLAR WIND
1021
102
i
6 - 10 LT
[[:033
KP-2
N - 258
•
1/+-18LT i KP = 2
[[ =040 N 666
_
101 [ 10o
ß
10-2 ß
10-] i
i
102
10-•, ,
102
i
6 - 10LT KP: 3
1/, - 18 LT KP: 3
C[ = 0.21 N = 428
101
10-] , CC= 0.52 N = 643
101
.
-
ß
100
r. :
I-
i
100
:.
./.. i .-'.Li.'i: .-
..,,•-
10 -1h L
t...'•i'•':' :' ;', ; '.',,'.'., '
10-2
i :' ' '•• :
.
.
ß
J
E
;
1
-
.
'*'
10-•,
10-3
i
102
i
6- 10LT
[[:
KP=4
N :63
101
100
10o
10-1
• ......--
10-3
I
0.57
101
10-2
..o
10-]
_ I
•
:
o!
10-2
10-]
102
ß
: .' ::.;; .'I,'a ß .. ß
"
10-•,
ß
I
14-18LT
CC: 052
KP: 4
N
166
_
--
_
ß
• '
10-1
:
ß . ;'•t
:i: ;..
.'r:'
'•.
10-2
10-3
10-3
i
i
i
1
10-•
10-3
10-• )
Vs ]w, •1 nswmp
2
•-nswmpVsw, W/m
i
10-]
W/m 2
Fig. 4. (continued)
reported for slightly northward orientation of the IMF
damental mode toroidal Pc 5 pulsations:First, flux trans-
[Berchemand Russell,1984;Rijnbeeket al., 1984]. Fun- fer events produce impulsive disturbancesof the magdamental mode magnetosphericpulsations,however,were observedirrespectiveof the signof the IMF z component. In fact, the fraction of pulsationsfound with Bz > 0 was greater than the one found with Bz < O. There might be two physicalreasonsfor the inferior role flux transfer eventsare playing in the generationof fun-
netosphericboundary. Hence, the responsenear geostationaryorbit shouldbe transient [Allan et al., 1986]. Transient events are not very likely to be identified as
long-period magnetosphericpulsations [Baumjohann et al., 1984; paper 1]. Second,flux transfer eventsare found predominantly outside the equatorial plane. For this rea-
882
JUNOlNO!gR
AND BAUMJOHANN'
SOLAR WIND
son they are probably more efficient in generating asym-
metric (with respectto the equator) modesof field line eigenoscillations,to which an electric field experiment near the equator is not particularly sensitive[Junginger and Baumjohann,1984].
DZPZI•I)ZI•½Z
o1• Pc: 5
orbit is in most cases inside the plasmasphere for very low Kp indices, whereas it is in the plasma trough for
high valuesof Kp [Hi9el and Wu, 1984]. It is natural to assume that the coupling properties of surface waves to shear Alfv•n waves inside and outside the plasmasphere are different.
3.2.
Kelvin-Helmholtz Instabilit•t 4.
The simple-minded picture of the Kelvin-Helmholtz in-
SUMMARY
AND CONCLUSIONS
complicatedthan is assumedin (1).
The influenceon long-period magnetosphericpulsation power of the IMF orientation and magnitude and of the solar wind velocity and kinetic energy flux has been studied. Pulsation power and occurrenceswere not found to be sensitive to the IMF Bz component. Therefore flux
Sincewe are comparingpulsation amplitudeswith IMF and solar wind parameters, an additional complication arises when the role of the Kelvin-Helmholtz instability
the generationof fundamental mode toroida1Pc 5 pulsations observedby the electron beam experiment on board
is discussed.
G EOS 2.
stability sketchedin equation(1) is not directly applicable to the situation at the magnetosphericboundary. The boundary is not a plane layer, nor is it a sharp discontinuity; moreover, the magnetic field topology is far more
In terms
of the models
referred
to in sec-
tion 1 of this paper, pulsation amplitudes are the result of fully developedsurfacewavespropagatingon the mag-
netosphericboundary,whereasequation(1) (therebyrepresentingmost of the work done on the Kelvin-Helmholtz
instability hitherto) is only a criterionfor the onsetof the surfacewave instability. The lack of a well-defined model, taking all these complicationsinto account, is the main difficulty in interpreting our data. However, we found a clear relationship between the spectral power of pulsations and the solar wind velocity,
transfer eventscan play only a minor role (if any) in
The orientationand magnitudeof Bz and By seemnot to have a recognizableeffect on pulsation power and occurrence. Solar wind bulk velocity and kinetic energyflux, however, clearly do correlate with pulsation power on av-
erage. Similar resultswerereportedby Wolfeet al. [1980] and Wolfe [1980]for comparisonsof ground-baseddata
with IMF and solar wind parameters. The correlations between pulsation power and solar wind speed, or solar wind kinetic energy flux, support the assumptionthat fundamental mode toroidal magneSince tospheric pulsations are caused by the Kelvin-Helmholtz aswellasbetween log(P) andlog(•nswrnpvsw). a shear velocity is the most important critical parameter instability at the magnetosphericboundary. If we follow for the Kelvin-Helmholtz instability, our finding strongly this interpretation, the insignificanceof the IMF points points towards this instability as being responsiblefor the towards the inner boundary of the boundary layer as the excitation of fundamental mode toroidal Pc 5 pulsations. dominant regionof surfacewavegeneration, in accordance One more noteworthy point is the saturation of the pul- with theoreticalconsiderations made by Walker [1981] sation amplitude for fast solar wind flows. Amplitudes and Lee et al. [1981]. In the magnetospheric boundary of pulsationsincreasewith increasingsolar wind velocity layer the magneticfield has approximately the samedirecbetween vsw = 250 and 500km/s on average. Beyond tion and magnitude as in the magnetosphere.The plasma vsw • 500 km/s, pulsation amplitudesseemto be inde- bulk velocity, however,is related to the solar wind speed. pendent of the solar wind speed. This might indicate that Consequently,the magneticfield has no stabilizingeffect, in terms of our interpretation, surfacewaveamplitudeson as indicated by the observations. The pulsation power was found to anticorrelate with the magnetosphericboundary are limited. The saturation of surfacewaveamplitudeswouldstart at vsw • 500km/s the solar wind kinetic energy flux for Kp - O, whereas it correlatesfor Kp _• 1. This is most probably due to a accordingto this interpretation. differentcouplingof surfacewaveson the magnetospheric 3.3. Influence of the Kp Indez boundaryto shearA-lfv•nwavesneargeostationary orbit if the geostationaryorbit is inside the plasmasphere.
As shownin Figure 4, the Kp index has a striking inAcknowledgments. H.J. thanks the European Space Agency for fluenceon the relationshipbetweenthe spectral power of pulsations and the solar wind kinetic energy flux. For an internal fellowship, by which this work was supported. The work of W. B. was financially supported by the Deutsche ForschungsseKp = 0 the power decreaseswith increasingenergyflux; meinschaftthrough a Heisenbergfellowship. for Kp > 1 it increases. The Editor thanks M. Saunders and T. Tamao for their assistance There are two possibilities which could cause this ef- in evaluating this paper. fect. The various states of the magnetospheredescribed R!•FERBNCES by different Kp indicescould either have different boundaries on which surfacewave generation takes place, or they Allan, W., S. P. White, and E. M. Poulter, Impulse excited hydromagnetic cavity and field-line resonancesin the magnetosphere, could have different properties of the couplingof surface waves on the outer boundaries
to shear Alfv•n
waves near
geostationary orbit. Sinceit is hard to imagine that surfacewaveamplitudes decreasewith increasingsolar wind kinetic energy flux, we think that most likely the couplingpropertieschange between Kp - 0 and Kp •_ 1. Additional evidence for this interpretation is given by the fact that the geostationary
Planet. Space $ci., 34, $71, 198B. Aubry, M.P., M. G. Kivelson, and C. T. Russell, Motion and structure of the magnetopause,•. Ceophlls.Res., 76, 1B75, 1971. Baumjohann, W., and G. Haerendel, Magnetosphericconvectionobservedbetween0600 and 2100 LT: Solar wind and IMF dependence, J. Geoph•ls.Res., 90, 6370, 1985. Baumjohann, W., H. Junginger, G. Haerendel, and O. H. Bauer, Resonant Alfv•n wavesexcited by a sudden impulse, J. Geoph•ts. Res., 89, 2765, 1984.
JUNGINGER XND BXUMJOHXNN:
SOLXR WIND
DEPENDENCE OF Pc5
883
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(ReceivedJune 10, 1986; revised September 16, 1987;
acceptedOctober 19, 1986.)
