Download Magnetic cloud field intensities and solar wind velocities

GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 7, PAGES 963-966, APRIL 1, 1998
Magnetic cloudfield intensitiesand solar wind
velocities
W. D. Gonzalez, A. L. (",Idade Go•zalez, A. Da.1La,go
Institute Nacional de PesquisasEspaciais,She Jos6dos Campos, SP, Brasil
B. T. Tsurutani, .1. K. Arballo, G. K. La,klfina,B. Butt, C. M.
SpacePla.smaPhysics,Jet PropulsionLaboratory,C,MiforniaInstitute of Technology,Pasadena,
S.-T.
Wu
(',enterfor SpacePlasmaand AeronomicResearch,and Departmentof Mechanicaland Aerospace
Engineering,The 11niversity
of Alabamain Huntsville,Huntsville
Abstract.
For the sets of magnetic clo•ds studied in this work
[Dungcy,1961]. In the aboveexpression,
v is the so-
we have shown the existenceof a relationship between
lar wind velocity and Bs is the southward component
their peak magneticfield strengthand peak velocity of the interplanetarymagneticfield (IMF). Gonzalez
values,with a clear tendencythat, cloudswhich move and Tsurutani [1987]haveestablishedempiricallythat
at,higherspeedsalsopossess
highercoremagneticfield the interplanetary electric field must be greater than 5
strengths.This resultsuggests
a possibleintrinsicprop- mV/m for longerthan 3 hoursto createa Dst _• --100
erty of magneticcloudsand alsoimpliesa geophysical nT magnetic storm. This correspondsto a southward
consequence.
The relativelylow field strengthsat low field componentlarger than 12.5 nT for a solar wind
velocitiesis pres•mablythe causeof the lackof intense speedof • 400 km/s.
Although the positive correlation })el,weenfast (',MEs
stormsduringlow speede.jecta. There is alsoan indication that, this type of behavior is peculiar for mag- and magnetic storms have been stressedand is reasonnetic clouds, whereas other types of non cloud-driver ably well understood, little attention has been paid to
gaseventsdo not,seemto showa similarrelationship, the opposite question, why don't, slow CMEs lead to
at least,for the data studied in this paper. We suggest rnagnetic storms? Since this question involvesthe role
that, a field/speedrelationshipfor magneticclouds,as of the magnetic field of the CME, it, leads to a more
that obtained in our presentstudy, could be associated generalquestion,namely how are the CME's speedand
with the cloud release and acceleration mechanism a.t magnetic field related to each other? These are the
the sun.
questionswe wish l,o addressin this letter.
Sincefor magneticcloudsthe total field tyically has a
If the speed of the solar ejecta is less than the upsubstantialsouthwardcomponent,B•, our resultsin,ply stream slow solar wind plus the magnetosonicwave's
that the interplanetary dawn-duskelectric field, given phasevelocity, a shockwill not forin at, the leading (anby v x Bs (wherev is the cloud'svelocity),is enhanced tisunward) edge of the e;jecta, and there will not be
by both factors. Therefore, the consequentmagneto- compressed
sheathfields. ttowever,one might ask "why
sphericenergization(that is governedby this electric can't the ejecta t,hemselveshave fields intense enough
field) becomesmore efficientfor the occurrenceof mag- to create magnetic storms with intensitiesDst < -100
netic storms.
nT7". A•nong the studied relationshipof e.jecta with
magneticstorms,practically all intensest,orms were asIntroduction
It has been well establishedthat major (D.,t _( -100
nT) magnetic storms are associatedwith fast,Coronal
Mass Ejections((•MEs) comingfrom the sun from both
socia.
ted with magneticclouds[ e.g. Burlagactal.,
1987; Tsurutani et al., 1988] that followedshocks,although Tsurutani ctal. [1988]claimedthat the intense
storm of September18 1979, wasassociatedwith a magnetic cloud without a shock (with an average speed
around370 kin/s). This latter type of associationapduringsolarmaximum[Tsurulaniel al., 1988, Gosling pears to be uncommon.
et al. 1991, Gonzalezet al., 1994] and the descending
phaseof the solar cycle[Tsur•tani ctal., 1995]. The
energytransfer mechanisnifroni the solar wind to the Method of Data Analysis.
magnetosphere
appearsto be magneticreconnectionbeTo address this issue first we use magnetic cloud
tween the interplanetary magneticfield and the earth's
magneticfield. The interplanetary dawn-duskelectric events.Magneticclouds[e.g. Burla•a, 1995]are a type
field, which is given by v x B•, governs this process of ejecta that have a mesoscaledimensionof about 0.25
AU at 1 AU and crossesthe spacecrafttypically in about
24 hours. Magnetic cloudshave a large and smooth roCopyright
1998bytheAmerican
Geophysical
Union.
tation in the field's direction, an enhancedfield strength
and low proton temperature and/• values. It is thought
Papernumber98GL00703.
[Farragiae! al., 1997andreferences
therein]that clouds
0094-8534/98/98GL-00703505.00
963
964
GONZALEZ
ET AL.'
MAGNETIC
CLOUD
are giant flux ropes formed by field aligned currents.
The B•, componentof the cloud has typically an ampIf
tude that represents a substantial [raction of the total
IMF intensity. For example, for the intenseand super-
intensestormsstudiedby Tsurutaniel, al. [1988]and
Tsurutani et al. [1992],the magneticcloudsresponsible for about half of the storm events had their B.,
fields with intensities of 70% or more than that of the
correspondingtotal IMF intensities. This association
can explain why magnetic cloudswith strong magnetic
fields have typically strong B, componentsand there-
FIELDS
AND
VELOCITIES
Second Set, of Clouds (1979)
i
ß
!
!
!
ß
25
20
m 15-
10
fore cause intense storms.
We have chosento examinefirst previouslypublished
magneticcloudeventsbecausetheir identificationexists
õ
in the literatureand the readerhas the opportunityto
300
350
400
450
500
550
600
examinethe eventsin detail. We use(1) the five cloud
Peak v (kin/s)
eventspublishedand illustrated by Klein and Burlaga
[1982];(2) the two cloudeventspublishedand illusfor the
trated by Burlagaet al. [1987],•sing data.[rotl• t,he Figure 2. Scatterplot ForBr,•. vers,sv•,•,a•set of 13 •agnetic clo;•(!s,as observedby the
IMP8/ISEE-3 satellites;(3) six cloudeventsreported second
ISEE-3 satellite in 1979. These evc•l,s i•volved clouds
by Tsurutaniet al. [1988]and Tsurutaniel al. [1992],
that causedintenseand •o(!erate magneticstorn•s.
usingISEE-3 data;(4) threecloudeventsdiscussed
by
Farrugiaet al. [1997]whoreferto previousstudiesthat
useddata from the IMP8/ISEE-3 satellites;and (5) the
oneevent,reportedby Burlagaet al. [1996],usingdata
from the WIND
served at •
satellite.
All these 17 events were ob-
1 AU and the criteria to select the events,
that are cormnon in all these cases, are: a large rotation in the ejecta field's direction, an enhancedfield
strength (typically > 10 nT), a low proton te•nperature
and a duration of about, 24 hours for the ejecta to cross
the spacecraft.
Because most of these events were studied
in asso-
satellite, and identified the driver gas events using a
combined criteria from those discussedby Zwickl et al.
[98.q]
Ts,vttani
at. [988]. The combined
cri-
teria involved' a smooth ]nagneticfield (lack o[ Alfvdn
wavesand discontinuities),high magneticfield strength,
and a low proton temperature. For this study we have
used5 to 15 minute averagesof the ISEE-3 data to identify the eventsand higher t,ime resolutionto checkthe
smoothness
of the field.
These
events were dividcd
in
two subsets,one including only clear magnetic clouds,
eventsreportedby Klein and Burlaga[1982],who se- using the selection criteria mentioned in the previous
lected clouds without any association with magnetic paragraph (with field rotation anglesthat for this data,
storms, we have looked for another (and independent) set lie in the domain of abo]•t 200 to 320 degrees),and
set of magnetic cloudsthat would not have an (a pri- the other subset was formed with the remaining driver
ori) associationwith magneticstorms. We obtained gas-noncloud events. For the first s•bsot we obtained
this secondset of clotadsin the followingway. We took 13 events and for the second one 24 events.
As for the first, set, of clouds, we also performed the
the whole year of 1979, for which a fifil set,of plaslna
and magnetic field data was recordedby the ISEE-3 same study for the peak values of the solar wind speed
and the magnetic field within the intervalsof thesetwo
ciation with intense magnetic storms, except,the five
subsets
First
40
'
i
'
Set, of Clouds
i
ß
i
i
35
30
to the fact that the year 1979 was near solar maxi]num,
but. it, has been shown that, the solar cycle distribution
of storms does not necessarilyfollow the sunspot solar
25
cycledistribution[e.g. Gonzalezctal., 1994].
On the other hand, it is also possiblethat magnetic
clouds(followingshocks)are ]nest of t.!•cl,i•es asso-
a0
ciated with
15
10
3OO
of events.
It is interesting to point, out that the secondset,
of cloud events, even though they were selectedwithout looking to their relationship to magnetic storms,
they were later seen to still be associatedwith magnetic storms of at. least,moderate intensities (-100 nT
_<Dst <_-50 nT). One may think that this could be due
,
I
400
I
,500
•
I
600
,
I
700
,
800
Peak v (kin/s)
intense storms or at least with st.orn•s of
moderate intensity, becausethey tend to involvefairly
largevaluesof B.• [e.g. Bnrlagact al., 1987' Tsurutani
c! al., 1988].
Results.
Figure 1. Scatter[)lot,k)r Bl,eat:versus•.'•,•a•:
for the
first set of 17 magnetic clouds,taken from severalreferences(see text), involving cloudsthat ca,•se(t
intense and very intense •nagnet,icst,orn•s.
The magneticcloudintensity versusspeedfor e.ach of
the 17 eventsof the first set,of cloudsis shownin Figure
1, iu a scatter-plot forn•at (Prcli•]•inary results related
GONZALEZ ET AL.- MAGNETIC CLOUD FIELDS AND VELOCITIES
Combined
4o
ß
i
ß
i
Set of Clouds
ß
i
ß
i
Driver
ß
i
Gas -
ß
i
965
Non Clouds
ß
I
ß
!
ß
i
2o
3o
m 20
lo
ß
0
ß
300
I
400
•
I
500
•
I
600
•
I
700
•
i
800
300
Peak v (kin/s)
•
I
400
•
I
•
1500
I
600
•
I
700
Peak v (kin/s)
Figure 3. Scatter plot for the coml,ination of tlw two Figure 4. Driver gas eventsthat apparentlydid not
independentsetsof cloudsshownin Figures1 and 2.
involvecloudsin 1979(seetext), as measured
by the
ISEE-3
to thisfigurewerebrieflymentioned
by Tsurutanictal.
satellite.
velocity is presumably the causeof the lack of intense
[1998]).A linearregression
fit isaddedto theplot,.The stormsd•ring low speedejecta.
correlation
coefficient
is 0.71and the linearregression There is also an indicationthat this type of behav-
line gives,
ior is peculiarfor magneticclouds,whereasother types
of non cloud-drivergas eventsdo not seemto show a
Bp,•.(nT) = 0.047,,•,•(km/s) - 1.l
similar relationship.
Due to the B-v relationshipfor magneticcloudsthat
Figure 2 shows a similar plot, for the secondset
we presentin this paper,a fastercloudimpliesa larger
of clouds,namelyfor the 1979drivergas-clearcloud amplitude of B (which typically has a substantialB•
events,and Figure 3 refers to the combineddata set,of component). Thus, since B• and v are related to one
thoseplottedin FiguresI and 2, involving30 clouds. another, the magnetosphericenergytransfer, which is
The correlation coefficients obtained from the linear fits
in Figures 2 and 3 are, respectively,0.73 and 0.7.5.
From Figure 3, we note that, at cloud velocitiesof
v m 400 km/s, the field magnitudescan be 1,5to 20 nT,
which could have a B., componentco•npara.ble
to the
governedby v x Bs [e.g. Gonzalezet al., 1994],is enhanced by bothfactors and therefore becomesmore effi-
cient for the occurrenceof magneticstorms.This rela:
tionshipexcludesthe possibilitythat a magneticcloud
with a high speed and a negligibleor absentB., field
valueexpectedfromthe Gonzalezand Tsurutani[1987] could lead to the developmentof an intensestorm. To
criteria, discussedin [he Introduction. It, is also inter-
our knowledgeno one has observedsuch a case. This
esfing[o note t,ha[ the largely documentedmagnetic lackof observationalsoreinforcesthe B-v relationship
cloudevent of Jan 10, 1997, with peak valuesof about, reported in our paper.
1,5nT and 480 km/s, fits fairly well the trend shownin
At.this time, the physicalcausesof the reportedrelaFigure 3.
tionship betweenthe magneticfield and plasmaspeed
Figure4 refersto the ISEE-3 subsetof drivergas-non of the magnetic cloudsare uncertain. Compressionof
cloudevents.One canseethat this plot is largelyscat- the cloud is certainly occurring. Thus, it is possible
tered, without any cleartrendfor a relationshipbetween that in some cases the field increases can be accounted
the peak valuesof the magneticfield and the solarwind for by such a.neffect.
speed. One can speculatethat this set of eventscould
Another possibilityis that this relationshipmay be
still include cloudsthat were crossedby the satellite far
from their center, for which a rotation in the field was
much smaller.
In such a case much lower B fields are
associated with the CME release and acceleration mech-
anismsat the sun. For example,a recentself-consistent
numericalMHD simulationstudy [Wu e! al., 1997]
expectedfor a similar v value of the cloud. It is pos- showsthat the radial velocity of the center of the flux
sible that other driver gas structurescould be involved
rope (i.e. the centerof the magneticcloud),erupting
in Figure 4, and deservea detailedinvestigation.
from a helmetstreamer,is proportionalto the strength
of the azimuthal componentof the magneticfield, in
Discussion
and Conclusions.
a way consistentwith the magnetic cloud B- v re]ationshipreported in this paper. A detailedparametric
For the sets of magnetic cloudsstudied in this work study of this kind of simulationalwork is currentlyunwe have shownthe existenceof a relationshipbetween der way.
their peak magneticfield strength and peak velocity
values,that may suggesta possibleintrinsicproperty
Acknowledgments.
Portions of this work were supof magneticcloudsand also imply a geophysicalcon- ported by the Fundo N acional de Desenvolvimento Cient[fico
sequence.There is a clear tendency that cloudswhich e Tecno16gicoof Brazil and by the National Aeronauticsand
moveat higherspeedsalsopossess
highercoremagnetic SpaceAdministration/ Jet PropulsionLaboratory,Califorfieldstrengths.The relativelylowfieldstrengthat a low nia Institute of Technology.
966
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(ReceivedNovember10, 1997; revisedDecember26, 1997;
Tsurutani, B. T, W. D. Gonzalez, W. D., F. Tang, and Y.
accepted February 10, 1998.)