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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A12, PAGES 28,351-28,360,DECEMBER 1, 1999

Structure of the magnetospheric magnetic field during magnetic storms
L. A. Dremukhina and Y. I. Felds·ein Institute of Terrestrial Magnetism, Ionosphereand Radio Wave Propagation, TYoitsk, Russia

I. I. Alexeev and V. V. Kalegaev
Institute of Nuclear Physics, Moscow State University, Moscow

M. E. Greenspan
Department of Physics, University of Maryland, College Park

Abstract. We report the results of a study of the contributions of the largescale magnetospheric currents to the observedDst variation. Ground-based magnetometer data duringfour magneticstorms(January27-30, 1985; November 23-27, 1986; January 14-16, 1988; and May 6-8, 1988) were used to calculate Dst, and the paraboloidmodelof the magnetospheric magneticfield [Alezeevet al., 1996]was usedto determinethe contribution eachmagnetospheric of current
system. Input data for our model were the solar wind plasma parameters, the

interplanetary magneticfield (IMF) Bz, DMSP F6, F7, FS, and F9 satellite
observations precipitating auroral particles, and Active MagnetosphericParticle of

TracerExplorers (AMPTE)/CCE satellitemeasurements the total energyof the of ring currentionswith energy charge per between and 300 keVq We found 1.5 -·.
good agreementbetween observedand modeled magnetic fields during the main phaseof the magnetic storms. Using the paraboloidmodel, we have determined the contributionsto Dst of different magnetospheric current systemsincluding the magnetopause current BCF, the symmetric ring current BR, and the geotail current BT. Such separation showsthat values of BT and BCF are comparable with the value of BR during the main phase of the storms. During the recoveryphase the
effect of BR predominates.

magnetic disturbance shows characteristics more of a magnetospherictail sheet current than a ring current", The Dst variation of the geomagneticfield commonly is ascribed to the developmentof the ring current in but the triangulated hypothetical current method used the inner magnetosphere. Howeverthe ring current de- by Campbell is too rough for numerical estimations. velopment alone cannot explain certain observations. To calculate the magnetic perturbation caused by the First, the magnetopause's earthwardshift during a mag- tail current system, it is necessary to know the curnetic storm cannot be causedby the ring current en- rent distribution in the plasma sheet and on the magnetopause exactly. Such information is unavailable at hancement. Second, thereare rapid (~ 1-3 hours)Dst present. However, it is possibleto estimate the tail curchangesduring the recovery phase, while the typical rent system·seffect on the Dst index by using the value timescalefor change-exchange of ring current ions loss

1. Introduction

of the tail lobe magneticflux [Maltsev, 1991; Alexeev (the mainmechanism losses) ~ 10 hours[Kozyra, of is et al., 1992]. In this work we present the results of 1989;Kistler et al., 1989;Fok et al., 1993].
a detailed study of four geomagneticstorms based on the paraboloid model of the magnetosphericmagnetic

In recent years some authors have developed the idea that the tail current system significantly influencesthe

1978;Alexeevet al., 1996]. The paraboDst index [Alexeev al., 1992,1996;Arykovand Malt- field [Alexeev, et loid model allows us to separate relative contributions sev,1993;Maltsev et al., 1994]. Indeed, the conclusion to the Dst index caused by various sources: the ring madeby Campbell [1973,p. 171] was that "the large
Copyright 1999 by the American Geophysical Union.
Paper number 1999JA900261.

0! 48-0227 99/ ! 999JA90026! $09.00 /

current, the current on the magnetopause,and the current system of the magnetospherictail. It is important that the paraboloid model describesthe time variations of these magnetosphericcurrent systems,becauseeach of them develops on its own timetable. Therefore the paraboloid model has been selectedto analyze the four

28,351


28,352

DREMUKHINA

ET AL.- STRUCTURE OF MAGNETIC FIELD DURING STORMS

events that occurred on November 23-27, 1986; January

27-30,1985;January14-16,1988;and May 6-8, 1988.
2. Calculations

ponent at low latitudes. The fields of these currents must be taken into accountif the longitudinalasymmetry of the magneticfield at low latitudes during magnetic storms is to be analyzed. However, model calculations of the disturbance fields caused by the Re-

Accordingto the paraboloid model, the total mag- gion 1 and 2 field-alignedcurrents show that averaged netic disturbance inside the magnetosphere any point overmagnetic at local time (MLT), the H component of
is presented as

due to the field-alignedand ionospheric closure currents of the partial ring currentbehavesimilarly [Takahashi the Dst ;,ndex the symmetric is Here BM(t) is the total magneticdisturbance, B(t) is et al., 1991]. Because the total magnetic field intensity inside the magneto- part of the magnetic field disturbance,the contribution and field-alignedcurrentsto Dst does sphereas a function of the time t, Bz> is the dipole field, of the ionospheric BR is the field of the ring current's symmetric compo- not exceed a few nanotesla. Therefore the contribution nent, B·, is the field of the magnetospherictail current of those currents to Dst was not considered. system,and BcF -- BsR + Bsl9 is the field of the magTo calculatethe magneticfield disturbance using netopausecurrents screeningthe ring current field and the paraboloid model, it is necessaryto set five timethe dipole field. A screeningterm for the tail current dependentinput parameters: the geomagnetic dipole is included in the calculated tail current system's field. tilt angle q·;the geocentricdistance from the Earth to point, Rx; the geocentric distance from the In the .paraboloid model, Chapman-Ferraro (CF) cur- the subsolar tail rents are calculatedusingthe conditionthat the normal Earth to the earthwardedgeof the magnetospheric component the total magneticfield (the sum of the currentsheet,R2; the geotail lobe magneticflux ·; of dipole magneticfield and the magneticfield createdby and the intensityof the ring current perturbationfield the CF currents)on the magnetopause equalto zero. at the Earth's center, Bao. is The geomagneticdipole tilt angle is a known funcThe magnetic perturbations due to the tail currentsare determined by the tail lobe magnetic flux and magne- tion of the UT [see, e.g., Alexeev al., 1996].In order et tosphericgeometric parameters. The tail currents close to obtain the other four model parametersfor the cases on the dayside magnetopauseand the tail lobes. The studied, empirical data were used: when available, solar magneticfield due to the closurecurrentsis calculated wind densityand velocity (N and V) and the northfrom the condition that the component normal to the southinterplanetary magnetic field (IMF) component magnetospheric boundary of the summedmagneticfield Bz, resultsof a calculationof the total energycarriedby of the tail currents and their closure currents on the ring current ions basedon Active Magnetospheric Par(AMPTE)/CCE satellitedata in magnetopause equal to zero. In other words,the mag- ticleTracerExplorers netic perturbation due to the tail currents is calculated the inner magnetosphere,and DMSP FY-F9 satellite obgiving the locationsof differentprecipitating by solving Laplace's equation for a potential with the servations boundaryconditionB-·0, similarly for the CF cur- particle populationsin the high-latitude auroral region. The Dst variation in the course of four selected storms rents. There is a spatial overlap between the dayside has been calculated using 1 min data from the seven closure of the tail current and the CF currents. The observed magnetic field disturbance Bin(t) also low-latitude magnetometer stations listed in Table 1. includes contributionsof other currentsystems the such Deviations of the magnetic field horizontal components as the Region 1 and 2 field-alignedcurrents and the AH and AD from the quiet level during each minute partial ring current. The disturbancesdue to the field- were calculatedusingmagnetograms from the sevenobaligned and ionosphericcurrents make noticeable con- servatories.We used magnetic field valuestaken on one tributions to the asymmetry of the magnetic H corn- of the quietestdays before eachstorm as the quiet level.

-

-

(t)

(t)

(t)

(t).

the disturbance field is close zero [Sun et al., 1984; to Dremukhinaet al., 1990]. The magneticdisturbances

Table

1. The List of Observatories

Used for Calculations

of Ds't

Geographic Coordinates, deg

Geomagnetic Coordinates, deg

Observatory
Sun Juan Tenetire Tbilisi

Latitude
18.1 28.5 44.7

Longitude
293.8 343.7 47.9

Latitude
29.9 19.8 36.8

Longitude
8.2 61.4 116.6

Lunping
Kaldoka Honolulu Dell Rio

25.0
36.2 21.3 29.3

121.2
140.2 202.0 259.2

17.6
28.3 21.8 39.0

192.0
210.8 268.7 324.1


DREMUKHINA

ET AL.: STRUCTURE OF MAGNETIC

FIELD DURING STORMS

28,353

The quiet days were January 25, 1985; November 22, 1986; January 9, 1988; and May 1, 1988. The resulting 1-min values of AH and AD were transformed to geomagneticcomponents AX and AY by rotation of the coordinate systems. We calculated values of AX at the Earth's equator from the data from each observatoryby

dividing by cosA,where A is the geomagnetic latitude
of the observatory. The equatorial values of AX for all observatories were averaged to give the 1-min Dst. Then we found the 1-hour Dst by averaging over 60
1-min Dst.

flux · is an important parameter of the paraboloid model becauseit determines the intensity of the magnetospheric tail current system. Another parameter of the paraboloid model is the geocentricdistance, R2, to the earthward edge of the dawn-dusk directed plasma sheet current. This parameter together with the parameter R1 completelydefines the scaleof the magnetospheric current systemclostail ing over the magnetopause. Like ·o·, the parameter R2
was derived from DMSP satellite data. As described

above, the angular radius of a circle for the boundary

discreteand diffuseprecipitation,O(b3a),and AMPTE/CCE data were used to calculate the ring between current ion energy W. We used measuredfluxes of hy- the angular displacementof this circle from the geomagmeridian,d(b3a), drogen,helium, nitrogen,and oxygenionswith energies netic polealongthe midday-midnight per charge from 1.5 to 300 keVq to compute local were calculated using DMSP observations. According -1 a ring current energy density. We then multiplied that to Newell et al. [1996], b3a is the boundarybetween energy density by the appropriate dipole L shell vol- discrete and diffuse auroral plasma precipitation, correume, and we summed the resulting energies over the spondingto the stable trapping boundary of electrons range L- 2-7 to get W. Longitudinal symmetry of the with energyE > 35 keV. This boundary marks a change ring current was assumed.The time requiredfor CCE in the magnetic field line character on the nightside to crossthe range L = 2-7 was --, 3 hours. During in- of the magnetospherefrom quasi-dipolar to extended tervals when the satellite was outside t. ring current, magnetotail. The change results from the presenceof he W was calculated by linear interpolation. The intervals the magnetospherictail current sheet, and the boundary b3a identifies the location of the tail current sheet's were · 1 hour at perigeeand · 9 hours at apogee. To calculate the other input parameters of the para- earthward edge. The midnight colatitude of the boundboloid model, we have utilized DMSP F6, F7, FS, and ary of discrete precipitation is equal to F9 electronand ion spectra taken every second,covering t·. = t·(b3a)+ d(b3a). (3) particle energiesfrom 30 to 20 eV. These spectra have been used to identify the precipitating plasma boundaries in high-latitude regions and to determine the co- In order to obtain the geocentricdistanceR2, the boundlatitude of the polar cap boundaryt·p·. Only Northern ary t·, was projected to the magnetosphericequatorial Hemisphereobservationswere used. During · 15 min plane assumingthat the magnetic field lines are quasidipolaf. During magnetic storms the inner boundary of the DMSP satellite orbit intersects the auroral boundthe tail current frequently is located at R2 = 4-5 RE ary twice in two different MLT sectors. Newellet al. [1996]and Feldstein and Galperin[1996] near midnight. Direct comparisonbetween model valdescribe the physical principles for selection of appropriate boundaries. We assume that the border marked by the equatorward boundary of the cusp or open low-

ues R2 and measuredAMPTE/CCE locationsof the
plasma sheet's inner boundary in the equatorial plane gives discrepancies from 0.1 to 0.6 Rz. Thus

to mark the equatorward boundary of the auroral oval. That boundary maps along the magnetic field lines to the inner edge of the geotail plasma current sheet near midnight. Then, supposingthat the polar cap is a circle, the angular displacement of the polar cap center from the geomagneticpole along the midday-midnight

latitudeboundary layer(LLBL) (onthe dayside) by and R2= 1/sin 8,. 2 (4) the poleward boundaryof the subvisual drizzle (on the nightside) the polarcapboundary[Newellet al., 1996; is The distance from the Earth to the subsolarpoint on Feldsteinand Galperin, 1996]. The boundarybetween the magnetopause is the input parameter which deR· discrete and diffuse auroral precipitations is assumed terminesthe scaleof the magnetosphere. Mead [1964]
made the first significant computation of the depen-

denceof the magnetopause position(Rx) on the solar
wind pressure.We have calculated R· using solar wind plasma and IMF data when available. Initially, we calculated R· using both the formula given by Roelof and

Sibeck [1993] and that givenby Shueet al. [1997] the for meridian, dpc, andthe angular polarcapradius·pcwere dependenceof R1 on the dynamic pressureof the solar calculated usingthe measured by coordinates (corrected wind Psw and the IMF Bz. Both functional forms give geomagnetic latitude A and MLT) of two pointscorre- similar results for small values of the IMF Bz. Howspondingto every intersectionof the auroral boundary ever, the formula of Roelof and Sibeckis valid only when and the satellite orbit. After that it became possible Psw < 8 nPa and Bz < 7 nT. For this reason, we have to calculate the paraboloid model parameter tail lobe used the functional form of Shue et al., which is valid magnetic flux ·oo as over a wider range of Psw and Bz IMF values:

ú· -- 2·'BER·sin t·pc, 2

(2)

/1·1/6'6 R· - (11ú 0.013x Bz)/- sw 4+

Bz < O;

where Bz is the dipole field at the Earth's equator and Rz is the Earth's radius. The tail lobe magnetic

/1 sw R· -- (ll.4 + 0. 140x Bz)/· )1/6'6

> O.

(5)


28,354

DREMUKHINA

ET AL.' STRUCTURE

OF MAGNETIC

FIELD DURING STORMS

During the storms studied, there were many time intervals when the solar wind data were missing. During such intervals we calculated R· by the following method: the empirical dependence of R· on (I,· was obtained for the periods during the four storms when
solar wind data were available. The data were divided

phase of a magnetic storm, when the total particle energy distribution strongly dependson MLT.

Wecompare modelmagnetic the fieldBM (t) obtained from (1) with the Dst variation,whichis calculated relative to the quiet magnetic field. Therefore we must

subtractfrom BM(t) the magneticfieldsproduced by
quiet time currents. Thi.s means that the correction,

into three groups corresponding three different levto els of geomagneticdis[urbance, which were specifiedby R2' strong disturbed conditionsif R2 < 4.5 RE, moderately disturbedcond!tions 4.5 RE _ 7.0 RE. Assuminga linear relation betweenRx and (I'o·, the followingdependence
was found:

(B·D B·n+ B· + B·), containingcontributions + the
of all sourcesduring the quiet periods is required. Let us evaluatethis correctionusing the paraboloidmodel. It is well known that during quiet intervals the auroral oval equatorward boundary is located at A · 700

Rx - 8.30

P·2 < 4.5 RE,

Rx -- -0.49(I'oo + 9.45

4.5 .RE _
Rx- 10.0 R2 > 7.0 RE, Here Rx is in RE, and ·oo is in 10s Wb.

(6)

near midnightand at A · 800 near midday [Feldstein andStarkov,1967].This givesvalues the parameters of /·2 · 8 RE and 'I'oo· 0.5 x 10Ü Wb. ParametersRx and Bn0 are assumed be · 10 RE and-15 nT (owto ing to the ring currentenergyW beforea storm). Then
the paraboloidmodel givesthe followingvaluesof mag-

neticdisturbances the Earth'ssurface: on B}D · 33

For periods when the solar wind data are available, nT, B·n m 0.4 nT, B} m -15 nT, andB· · -22 nT. values of parameter Rx obtained on the basis of rela- The total correction is equal to-3.6 nT. It was omitted

BM(t) with Dst because i.ssmall. it tions (5) or (6) wererecalculated requiring by balance whencomparing
between the solar wind dynamic pressureand the pa-

raboloidmodelmagneticpressure. Thesevalues R· of

were used as input parametersfor subsequent calculations of the model magnetic variations on the Earth's We have shown that it is possibleto calculate the time surface.Figureslb, 3b, 5b, and 7b showR· and Rx as dependence Rx, R2, ·c·, and BR0 during magnetic of well as other input parameters of the paraboloid model storms then use them to calculate the time dependence (R· is plotted as a solid line, and Rx is plotted as a of the model field at the Earth's equator, and compare dashedline). As one can seefrom figureslb, 3b,5b, it with the IDstl variation. In our study we analyzed and 7b, the valuesof Rx and R· do not differfrom each four moderate magnetic storms with Dst · 120- 150 othersignificantly (differences O(0.1 RE)). During nT during the main phase. are

3.

Results

periods with southward IMF (Bz << 0), R· > R·, and,
3.1. Magnetic storm of November 23-27, 1986 in contrast, R· < Rx during periodswith Bz · O. The last parameter that is necessary input to the for Figure 1 showsthe solar wind data available to evalparaboloid model is the ring current intensity. We cal- uate the input parameters of the model. This storm culatedthis parameterfrom the total ring current ion wasstudiedby Kalegaev al. [1998].Figure la shows et

energy measured the AMPTE/CCE satelliteduring by the intervalsstudied. The total ring current particle kinetic energy W is related to the magneticfield perturbation at the Earth's center by the Dessler-ParkerSckopkerelation:

the solarwind data (IMF Bz, densityN, and velocity V) and the calculateddynamicpressure Psw in the interval November 23-27, 1986. Figure lb represents the paraboloidmodel parameters: Rx, obtainedfrom

(5) (dashed line) and R·, obtained from the balance between dynamicand magnetic pressures (solidline). - - (2/ 3) w/w . (7) Two other parameters plottedin Figure lb, R2(RE) Here BR0 is the magneticfield caused the ring cur- and(I, by oo/10s(Wb), been have calculated (2) and from rent at the Earth's center, BE is the dipolaf field at the (4) usingDMSP satellitedata. Duringintervals when Earth's equator,WM -- (1/3)BEM is the dipolaf field the solar wind data are absent, valuesof Rx are reconenergyoutside the Earth (M is the dipolafmagnetic structedfrom (6). Colatitudesof the polar cap midof moment), and W is the total energyof the ring cur- nightboundary = ·p· + dp·obtained ·o fromempirical rent particles. Becauseof the high conductivityof the data are shownin the bottom plot of Figure lb. As Earth's mantle and core, the contributionof the sym- one can seefrom Figure 1, variations of the paraboloid metricring currentto Dst is equalto Bj· - (3/2)Bj·o. model input parameters reflect the effect of solar wind Since -- 3.2 x 104nT and WM -- 8 x 1024 BE ergs, for parameter changes. The solar wind velocity did not ionswith total energyW -- 103xkeV onefindsB·=- changesignificantlyduring the considered interval. Its 64 nT. Calculationsof W are based,as a rule, on the values were· 400-500 kms-x. However, density the N measurements made inside a limited local time sector and the IMF Bz changed very significantly: changed N by only one satellite. Therefore it is supposed that the from 5 to 30 cm -3, and Bz changed from-10 to +15 ring current did not depend on longitude. However, nT. There are three intervalsof increase ,I)·, corof thissupposition clearlyis not accurate duringthe main responding increasingly to southward IMF. During Bz


DREMUKHINA ET AL.: STRUCTURE OF MAGNETIC FIELD DURING STORMS
23.11
lO

28,355

24.11

25.11

26.11

27.11.86
IMF Bz, nT

23.11

24.11

25.11

26.11

27.11.86

i

.

,

.

i

.

30

N, cm '3
20

10
0 '

500

V, kms 'l

400
ú

,

.

i

.

i

·

i

,

i

,

i

,

i

i

i

lO

Psw,nPa

5

o

ú

i

.

i

.

i

.

i

.

i

.

i

.

,

.

,

.

12

24

12

24

12

24

12

24

12

24

12

24

12

24

12

24

12

24

12

UT

b

Figure 1. The magnetic stormof November 23-27,1986. (a) The solarwinddataand (b) the paraboloid modelinput parameters.The solidline shows and the dashed shows R·, line
the first increase of (I'oo the solar wind and IMF data creases when Bz tu:ns northward. After 1500 UT on are absent, but it is plausible that Bz < 0. Increases November when 24, Bz remains 0 for a longtime,R2 < of Psw cause Rz to decrease. Variations of R2 do not decreases 4-5 RE and remains at that level for about to showany relation to the solar wind parameters, out R2 --, 2 days. decreaseswhen the IMF Bz turns southward and inFigure 2a shows the contributions to Dst of the

geotailcurrentsystemBT, Chapman-Ferraro currents BcF, and the ring currentfieldBR, including field the
23.11
100
50

24.11

25.11

26.11

27.11.86

of the induced current inside the Earth. There is a full set of solar wind and DMSP satellite data to determine

the polar cap radiusand midnightauroralovalboundary duringthis storm. During the periodNovember 23-

27, 1986, AMPTE/CCEsatellite the crossed ring the
current between --,2130 and 0230 LT inbound and be-

-100

50
-50

tween--,1330 and 1830LT outbound.We useda stepwise linear interpolationbetweenvaluesof BR calculatedfromAMPTE/CCE satellite measurements. Fluctuationsof BR are caused changes the total ion by in energy wellasby the MLT asymmetry theringcuras of

rentions.We cannot distinguish MLT dependence the
of the ring current ions flux and the time variation of

-100

-150

.................

12

24

12

24

12

24

12

24

12

2· UT

the total ion energyby usinga singlesatellite. As one can see, valuesof Bc·,, BR, and BT have comparable magnitudes duringthe main phaseof the storm.The timedependence Bc· andBT is similar, of but they haveopposite signs.Three BT extremadur-

Figure 2. The maÓneticstorm ooe November23-27,

ing the stormcorrespond increases the polarcap to of
area, which causemagnifications (·ooand reductions of of R2. Simultaneous increases Bc·, are connected of

neropause screeninÓ current (trianÓles),and the rinÓ
A comparison between Ds½(solidline) and modeled total magnetic fieldBM (dashed line).

to decreases R· and earthward of displacement the of
magnetopause.

Figure2b represents comparison a between model the


28,356

DREMUKHINA

ET AL.: STRUCTURE

OF MAGNETIC

FIELD DURING

STORMS

15.01.88

16.01.88

15.01.88

16.01.88

lO

IMF Bz, nT
12

,

i

i

i

,

i

8 ·
4

N, cm 4
i i I i i
!

o 55o

o

V, kms 4

12

5OO

R·, R·, R·
450 3
2
, i i i i

Psw,nPa

2o

.

lO
0 i , i ,
, i i

Tet0o '
O0 12 24
b

00

12

24

12

24

12

24

UT

paraboloid model inputparameters. solid shows andthedashed shows The line R·, line

Figure 3. Themagnetic storm January of 14-16, 1988.(a) Thesolar winddataand(b) the

magneticfield BM and the Dsi index. In general,the and of the tail current BT to Dst. As Figure 4 shows, phaseof the storm the ring current comparison showsgood agreement betweenour model duringthe recovery and ground-based measurements. However,sometimes producesthe largest effect on Dst. However, at the of phasethe magnetic field prothe differencesreach · 25 nT, specifically,from 0000 beginning the recovery to 1200 UT on November 25, 1986, and on November ducedby the tail current systemreachessometens of 27, 1986, during the recoveryphase. Such differences may be caused the valuesof Rz obtainedfrom (6) by 15.01.88 16.01.88 when the solar wind data are missing. The correlation
coefficientr between Dst and BM is 0.82, with standard
deviation
interval.
lOO

rr=16.1

nT as calculated

for the whole storm

&&

3.2. Magnetic Storm of January 15-16, 1988
The DMSP auroral particle data needed to evaluate the paraboloidmodel input parametersare availablefor only 2 days during the storm of January 14-16, 1988. The solar wind plasma data are missingafter 1200 UT, January 15, 1988, as well, so Rz has been calculated

0
-50 .... ·, .· · I
i i i i i i

o

using(6). The AMPTE/CCE satellite intersected the
ring currentinboundnear midnightand outboundnear 1900MLT. Figures3a and 3b showavailablesolarwind
data and calculated parameters R2, (·m, Rx, and 00. The auroral particle data usedto calculatethe tail lobe -150 magneticflux (·ooand the distanceR2 from the Earth O0 to the tail current sheet'sinner edgehave been obtained from only one satellite, DMSP F7. Thereforethesepa- Figure 4. rameters are calculated with a-· 1.5-hour time step. We estimated intermediate valuesof (·ooand R2 by linú

-100 .:'·
.......

b
12 24 12 24 UT

The magnetic s·o·m of 5&·u&W [&-[6,

ear interpolation.

Figure 4a displaysthe contributions the ring cur- be·wee·Ds· (sotMt[·e) &·d ·he mode] of rent BR, of the magnetopausescreeningcurrent BcF,


DREMUKHINA ET AL.: STRUCTURE OF MAGNETIC FIELD DURING STORMS
27.01
5

28,357

28.01

29.01

30.01.85

27.01

28.01

29.01

30.01.85

-lO

·/· ··/
, i , i ,

i

,

i

,MF Bz,·n; l
, i

,

I

,

i

15

2o
10 lO

o 500

,

i

i

i

i

N, cm '3
i ,
, i , i , i , i , i , i

10 400

V, kms 4
300 8
i , i ,

'-'· ';
I , I , I
i I

R·*, R·, R E
, i

2O

10
· i , i ,
i · i

Psw,nPa
i i ,

·

i

,

i

,

i

i

I

·

I

,

Teto 'o
i ,

12

24

12

24

12

24

12

24

12

24

12

24
b

12

24

12

24 UT

Figure 5. The magnetic stormof January27-30, 1985. (a) The available solarwind data and (b) the paraboloid modelparameters. The solidline shows parameter and the dashed the R·,
line shows

nanotesla.Figure 4b demonstrates comparison Dst a of with the model field BM during storm recovery, showing that there is goodagreement betweenthe two. The correlationcoefficient betweenBM and Dst is 0.94, r
and ·r=10.8 nT.

We propose that becauseof local time asymmetry of the ring current, deternfiningthe total energy of the ring current ionsduring the main phaseof the storm by usingthe 0900 MLT data givesvaluesof W and thus of [Bn[ that are too small. For this reason modelfield the /·M was net calculated during the interval 1300-1600
UT.

3.3. Magnetic Storm of January 27-30, 1985
Figures 5 and 6 show input data and the model resuits for the magnetic storm of January 27-30, 1985. There were few solar wind measurements during the storm, and during intervalswith no measurements, val-

27.01
1 oo

28.01

29.01

30.01.85

5o

uesof Rz werecalculated using(6). Continuous DMSP F6 and F7 data were available,yielding precipitating plasmaboundaries and thusallowingthe determination
of R2 and ·. There are two maxima of ·. The first

-5o -lOO
-150 o

of them, near 2200 UT on January 28, is larger, and it occurs near minimum Dst. This locationsuggests that the increase [Dst[ may be explained geotail in by currentgrowth, because ring current energydeterthe minedfrom AMPTE/CCE data did not increase this at
time.

-5o -lOO

A calculation the magneticperturbationdue to the of -150 ring currentbasedon CCE data taken on January28, -200 1985, 1300-1600UT, gives Bn - -11 nT, while ear12 24 12 24 12 24 12 24 UT lier, from 0100-0400 UT, CCE data give Bn - -74 nT, althoughDst shows that the stormgrewlargerbe- Figure 6. (a) Contributions Dst of the tail curto tween 0100 and 1400 UT. This indicates that the torent system (asterisks), the screening of currenton the tal ring currention energyobtainedby assuming az- magnetopause (triangles), of theringcurrent and (solid imuthalsymmetryof the ring currentis incorrect.Dur- line); (b) A comparison between the modelmagnetic ing this storm the AMPTE/CCE satellitecrossed L=5 fieldB· (dashed line) and the Dst index(solidline)
i i i i , i , i , i , i ,

at ·

0900 MLT

inbound

and ·

0300 MLT

outbound.

during January 27-30, 1985.


28,358
06.05
lO

DREMUKHINA ET AL.: STRUCTURE OF MAGNETIC FIELD DURING STORMS
07.05 08.05.88 06.05 07.05 08.05.88

-lO

IMF Bz, nT
i I , I , I i I , I

3

,

i

i

i

·

i

i

i

i

i

i

20

N,cm '·

lO

,

i

i

I

i

I

i

I

i

I

I

i

6OO

5OO
, i , i , i , .i ! i

10

Psw,nPa

20

lO
, i , i , i , i , i ,

O0

12

24

12

24

12

24

O0

12

24

12
b

24

12

24

UT

'Figure 7. The magnetic storm May 6-8, 1988.(a) The solar of winddataand (b) the paraboloid
input parameters.ParameterR· is shownby a solidline, and Rx is shownby a dashed line.

the model field BM and Dst coincide with accuracy equal to 10-15 nT. Note that the Dst minima at 2200 UT, November 28, and 2200 UT, November 29, coincide ;·ith Ba- maxima which are the results of abrupt increases the polar cap area and thus of (I,·. The corof relation coefficient between Dst and BM for this storm is equal to 0.79, with rr = 15.5 nT.

If the magneticstorm is considered a whole, then energy obtained from a satellite pass near 1300 MLT as achievesits minimum value, equivalent to BR -- -59 nT. Undoubtedly,the small total ring current energy is a consequence local time asymmetry of the ring of current plasma distribution. The asymmetry leads to an underestimateof the total ring current energywhen

06.05

07.05

08.05.88

3.4. Magnetic Storm of May 6-8, 1988
Figures 7a and 7b show solar wind data and the values of (I,·, Rx, R2, and 00 computed from those obser-

150
100

vations and used to calculate Bci, and Ba- during the magnetic storm of May 6-8, 1988. Solar wind data were missingduring severalshort intervals. During the main phaseof the storm the IMF Bz has a large negative value, · -10 nT, and the solar wind density N reaches

50
0

-50 -100
i I i I i I i I i I i I

· 20 cm-3, producing increase Psw, the dynamic an in
plasma pressure. The solar wind velocity V has large

0

-50

values, 600 kms duringthe wholestorminterval. · -x,
Three maxima in the tail lobe magneticflux (I,· occur. The first appears connected to the solar wind density or dynamic pressurejump, and the secondand third, seemingly, appear connectedto a reduction of the geocentric distance R2. D MSP F8 and F9 satellites' data
-100

-150
-200
, i i i , · , i i

Figure 8. The magnetic stormof May 6-8, 1988. (a) Contributions Dst dueto the tail current(asterisks), ·o the screening magnetopause's current(triangles), and side at 1300-1800 MLT does not allow us to determine ·he ringcurrent(solidline). (b) A comparison between the values of Ba correctly. At the time of Dst mini- modelmagnetic field BM (dashed line) and the Dst mum, 0900-1100UT, May 6, 1988, the total ring current index(solidline).
0,·, 0pc,and R2 duringthe wholemagnetic storm. The unsuitable locationof AMPTE/CCE on the day-

are available, and they allow us to find the parameters


DREMUKHINA

ET AL.' STRUCTURE

OF MAGNETIC

FIELD

DURING

STORMS

28,359

BR, nT
125

B nT R
125-

100

IO0

75

75

50

50

25

25

o.........
Dst, nT

o o

1(;o
Dst, nT

Figure 9. The relationof BR and Dst: (a) for the main phase Dst and (b) for the recovery of
phase of Dst. Asterisks show the nightside, and triangles show the dayside.

We have shown in section 3 that during the main phasesof magnetic storms the intensity of the geotail current system magnetic field on the Earth's surface is roughly equal to the intensity of the ring current magAs in our other cases,Figure 8a showsthe magnetic netic field. This result contradicts what is presented fields BcF, BT and B·, comprisingthe model mag- in several standard textbooks, accordingto which the netic disturbance BM. As is evident from Figure 8a, main causeof Dst is the ring current magneticfield. It is necessary note that the Dst index is not a pure to increases IB'l causedby increases ·o· during the in of ring current index [Campbell,1996a]. It shouldnot be main phase of the storm are compensatedfor by ina of creases BcF as a result of small values of Rz(· 7 considered simplerepresentation the ring current in of a storm, because other sourcescontribute to the index's RE). During the recovery phase of the storm, BM is systematicallymore negative than Dst, but differences formation. The existence of an important contribution by nightside currents to the symmetric Dst followed do not exceed --· 20 nT. The correlation coefficient r from the paper by Kertz [1964]. Campbell[1973]has for this magnetic storm has been calculated two ways: by excludingthe time interval 0800-1900 UT, May 6, described a hypothetical additional contributor to 1988, r is equal to 0.67 with rr = 15.0 nT; by includ- as a triangulated current existingin the midnight maging all data during the storm, it is equal to 0.60 with netosphereat geocentric distances2-4 rr -- 31.9 nT. However, model calculations of perturbations due to
2000 to 2300 UT, when the satellite crossed L=5 at · 1800 MLT, BR is equal to -128 and-89 nT, respectively. For this reason, in Figure 8a we show valuesfor B· after 1900 UT only.

CCE measurements are made on the dayside. During the neighboringpasses from 0400 to 0700 UT and from

4.

Conclusions

In orderto definethe relationship the measured thesecurrentsystems of usingknownvalues the curof
total energy of the ring current ions with the MLT of satellite measurements,the dependenceof Bn on Dst

rent intensities give values of the magnetic field that
they produce on the Earth's surface as · 2030 nT

and 1994]. Campbell [1996b], using hasbeenanalyzed. In Figure9 we plot Bn(Dst) for [Tsyganenko $ibeck, the dayside (0600-1200-1800 MLT) andnightside (1800- lognormal distribution functions for the hourly values 0000-0600 MLT), plotting the main phaseand the re- of the magneticfield amplitude during typical magnetic
coveryphaseseparately. Though there is a large dispersion,one can seethat BR increases with IDstI all MLT during the recoveryphase of storm. The relation is not clear during the main phase if the satellite

storms,has concludedthat although a ring current certainly existsduring a storm, there are many other contributions in the Dst index. However, the relative contributions of different not been defined. sources to the Dst variation have

is on the dayside. Figure 9 showsthat measuredtotal ring currentenergyfrom satellitepasses the dayside on is reducedbecauseof the azimuthal asymmetryof the ring current particles' distribution. An assumptionof longitudinal symmetry of the ring current particles is suitable, it seems,for the recoveryphase of a storm.

We have calculated quantitative contributions of different current systemsto Dst using the dynamic paraboloid model of the magnetospheric magneticfield. The calculations, carried out using ground-baseddata and AMPTE/CCE and DMSP satellitemeasurements,


28,$60

DREMUKHINA ET AL.: STRUCTURE OF MAGNETIC FIELD DURING STORMS

show that the Dst is not an index measuringthe ring Kalegaev, V. V., I. I. Alexeev, Y. I. Feldstein, L. I. Gromova, A. Grafe, and M. Greenspan, Magnetic flux in the magcurrent only. The other magnetosphericmagnetic field netotail lobes and D st dynamics during magnetic storms, sources, particularly the geotail current system and Geomagn. Acton., 38(3), 10, 1998. screeningcurrents on the magnetopause,give signifiKertz, W., Ring current variations during the IGY, Ann. cant contributions to Dst. During the main phase of Int. Geophys. Year, 35, 49, 1964. magnetic storms the contributions of Ba- BR, and Bcl· Kistler, I. M., F. M. Ipavich, D.C. Hamilton, G. Gloecker, are comparable. However, Bcl· and Ba-effect roughly B. Wilken, G. Kremser, and W. Stridemann, Energy speccompensateone another. During the recoveryphasethe tra of the major ion species in the ring current during geomagneticstorms, J. Geophys. Res., 9·, 3579, 1989. effect of BR predominates.
Acknowledgments.
for DMSP satellite data

The authors thank P. T. Newell
and A. Grafe and L. I. Groinova for

Kozyra, J. U., Sources and lossesof ring current ions: An update, Adv. Space Res., 9, 171, 1989. Maltsev, Y. P., Relation of Dst-variation to magnetospheric

the help in calculations of D s&. This work was supported by grant INTAS-RFBR-95-0932 and by the Russian Foundation of Basic Researches project 99-05-65611. Michel Blanc thanks Tuija I. Pulkkinen and Wallace H. Campbell for their assistancein evaluating this paper.

geometry,Geomagn. Acton., 31(3), 567, 1991.

Maltsev, Y. P., A. A. Arykov, E.G. Belova, B. B. Gvozdevsky, and V. V. Safm'galeev, Magnetic flux teclnfiqne for calculating storm-time depression, Paper presented at International Conference on Magnetic Storms, Sol. Terr. Environ. Lab., Nagoya Univ., Rikubetsu, Hokkaido, Japan, Oct. 6-8, 1994. References Mead, G. D. Deformation of the geomagnetic field by the solar wind, J. Geophys. Reo., 69, 1181, 1964. Alexeev, I.I., Regular magnetic field in the Earth's magneNewell, P. T., Y. I. Feldstein, Y. I. Galperin, and C.-I. Meng, tosphere,Geomagn. Acton., 18(4), 656, 1978. The morphology of nights.;deprecipitation, J. Geophys. Alexeev, I. I., V. V. Kalegaev, and Y. I. Feldstein,Modelling Res., 101, 10,737, 1996. of magnetic field in the strongly disturbed ·nagnetosphere, Roelof, E. C., and D. G. Sibeck, The magnetopauseshape Geomagn. Acton., 32(4), 8, 1992. as a bivariate function of IMF Bz and solar wind dynamic Alexeev, I. I., E. S. Belenkaya, V. V. Kalegaev, Y. I. Feldpressure, J. Geophys.Res., 98, 21,421, 1993. stein, and A. Grafe, Magnetic storms and magnetotail Shue,J.-H., J. K. Chao, H. C. Fu, C. T. Russell,P. Song, currents, J. Geophys. Res., 101, 7737, 1996. K. K. Khurana, and H. J. Singer, A new functional form Arykov, A. A., and Y. P. Maltsev, Contribution of various to study the solar wind control of the magnetopausesize sourcesto the geomagnetic storm field, Geomagn. Acton., and shape, J. Geophys.Res., 102, 9497, 1997. 33(6), 67, 1993. Campbell, W. H., The field levels near midnight at low and Sun, W., B.-H. Aim, S.-I. Akasofu, and Y. Kamide, A comparisonof the observed mid-latitude magneticdistm-bance equatorial geomagnetic stations, J. Atmos. Terr. Phys.,
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M. E. Greenspan,Department of Physics,University of
Maryland, College Park, MD 20742.

(Received October21, 1998; revised May 25, 1999;
accepted June 7, 1999.)