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JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 101, NO. A9, PAGES 19,921-19,936, SEPTEMBER 1, 1996

Electromagnetic characteristics of the high-latitude ionosphere during the various phases of magnetic
substorms

Y. I. Feldstein, L. I. Gromova, and A. E. Levitin
IZMIRAN, Troitsk, Moscow Region, Russia

L. G. Blomberg,G. T. Marklund, and P.-A. Lindqvist
Alfv·n Laboratory, Royal Institute of Technology,Stockholm, Sweden

Abstract. Model calculations the electrodynamics the high-latitude ionosphere of of are comparedto measurements made by the Viking satellite during July-August 1986. The model calculationsare basedon the IZMEM procedure,where the electric field and currentsin the ionosphere given as functionsof the interplanetary are magnetic field. The events chosencorrespondto the growth, the expansion,and the recoveryphasesof substorms. During the growth and expansionphasesthe correlationbetween the model results and the satellite data is rather good. During recoveryphase the correlationis not as good. The correlationbetween modeled and observedquantities suggestthat during growth and expansionphase the magnetosphere mainly directly driven by the solar wind, whereasduring recovery is phaseit is mainly driven by internal processes, loading-unloading.Best fit is i.e., obtained when averagingthe measuredquantitiesover a few minutes, which means adjustingthe spatialresolutionof the measurements the resolutionof the model. to Different time delaysbetweenthe interplanetarymagneticfield observations and thoseof Viking were examined. Best agreementwas obtained,not surprisingly, for time delayscorresponding the estimatedinformation transit time from the solar to wind spacecraftto the ionosphere.

Introduction

netism Electrodynamical Model (IZMEM)is another
model for calculatingthe sameparameters,but the in-

Observationsof various phenomenain the polar caps are relatively sparse, and therefore our knowledgeof the global electrodynamics theseregionsis generally in based on inferences drawn from statistical analysis of large data sets or from event studies.

put data are the interplanetary magneticfield (IMF) magnitude and direction[Levitir·et al., 1984;Feldsteir· arid Levitire,1986; Papitashviliet al., 1994]. The influence of the IMF on the upper atmosphereelectrodynamics is crucial also in the models of Friis-Christer·ser·

In the geospace environment modeling (GEM) pro- et al. [1985] and Mishir·[1990].The latter two models
gram, two modelsare considered synthesis sparse for of high-latitudedata [Lotko,1993]. Both modelsallow the computation of nearly instantaneous snapshotsof electric field and potential distribution in the entire auroral region. The assimilative mapping of ionospheric
are used by their respective authors only, whereas the IZMEM model is publicly available through the World

Data CenterA for Rockets Satellites and [Bilitza,1990].
The IZMEM model is here used to determine the

global convection pattern and its temporal evolution

of high-latitude electric fields and currents from sets over the northern high-latitude region. The model elecalong of localized observational data [Richmond Kamide, tric field is comparedto the satelliteobservations and the trajectory, and the global convectionpattern and 1988;Richmor·d al., 1988;Richmor·d, et 1992]. A similar modelhasalsobeenused Marklundet al. [1988]. its temporal evolutionis estimatedin the entire highby of SeealsoMarklund aridBlomberg [1991] and Blomberg latitude region. The sensitivityto averaging the cor-

of satelliteViking electrodynamics (AMIE) modelis usedfor calculation duringa numberof passes the Swedish

aridMarklur·d [1993]. The Inetituteof Terrestrial Mag-

relation

between

the modeled

and measured

values is

discussed. The averaginginterval selected influences the sensitivity of the correlation to changesin the model,
and it is of interest to determine to what extent the cor-

Copyright 1996 t)y the American GeophysicalUnion.
Paper number 96JA00514.

0148-022796/ 96JA-00514509.00 /

relation coefficientis dependent on the correct timing of the changes betweengross featuresof the convection system during the period studied.
19,921


19,922

FELDSTEIN ET AL.: HIGH-LATITUDE

IONOSPHERE

There is today a consensus that there are two processesresponsiblefor the solar wind energy input to

height-integratedHall and Pedersen conductivities.Z· and Zp are composedof the conductivities resulting themagnetosphere sUbstorms. during These direct from both solar UV radiation and particle precipitaare driving and loading-unloading processes. Somefraction tion. The contribution of particle precipitation is based of the energy input is directly dissipatedin the iono- on the statistical conductivity model of Wallis and sphere by Joule heating and particle precipitation re- Budzinsk·t [1981], and the contributionof solar UV lated to enhanced convection and enhanced ionospheric conductivity is based on the model of Robinson and currents. This power dissipationis directly correlated Vondrak [1984]. with the solar wind parameters and is thus a driven The model current is unambiguously related to the process [Akasofu,1981]. The remainingpart of the var. iation of the horizontalmagneticfield at the Earth's powertransfetedinto the magnetosphere storedtem- surfaceand was estimatedfrom the spario-temporaldisis porarily in the Earth's magnetosphere subsequently tribution of geomagneticfield variations measuredby and releasedat substorm breakup. The latter is known as the world-wide ground-basedmagnetometernetwork.

loading-unloading process [Bakeret al., 1984,1993].
The delay time between a change in the IMF and the related effects in the ionosphereis different for the two processes. For the directly driven process is 10it 20 min due to the inductance of the magnetosphereionospheresystem. For the loading-unloading process it is typically 40-60 min. Which one of these.processes
dominates remains an open question.

The

definition

of the model

current

in the IZMEM

model is similar to that in the model by Karaide et

al. [1981].
The dependenceof the magnetic variations at high latitudes on the magnetic field and plasma parameters of the solar wind may be obtained by regression meth-

ods. According Levitin eta!. [1982], the horizontal to

and vertical components of the field variation at the Additional light may be shedon this problemby com- Earth's surfacein the 0 _<300 colatitude regionmay be paring the IZMEM model electricfieldswith thoseob- expressedas servedby Viking during different phases substorms. of The IZMEM model assumes linear dependence the a of

ionospheric electric fieldon the IMF (i.e., a directdriving). Thereforethe correlation obtainedbetweenthe whereBy, Bz, and Bz are the IMF components v and
measured and the modeled electric fields may be used as an indicator of which processis dominating. The second section contains brief descriptionsof the model calculationsand of the Viking electric field mea-

and n are the solar wind velocity and density. This seriesis assumed be sufficientfor describing relation to the betweenquasistationaryvariationsin the Earth's magnetosphere and variationsin the interplanetarymedium. surements.Resultsof the model calculationsprojected The addition of new terms doesnot improvethe agreeon the Viking trajectory are compared with observa- mentbetween observed the modelvalues the and of tional data in section 3. The last sections contain a The coefficients are found by least squaresanalysis, Ki discussionof the results and conclusionsof this study. as described Levitin et al. [1982]. Eachof the terms by in the seriesrepresentsthe geomagnetic response a to certain parameter of the interplanetary medium, while Model and Experiment Description
. ú

The large-scaledistribution of convectionabove the ionosphereis conveniently representedby the electrostatic potential ·I,. Here the IZMEM model is used where ·I, is obtainedby numericalintegrationof a secondorder partial differentialequationwith a knowncurrent

the residualterm H· represents extrapolation the the of midlatitudeSq currentsystem highlatitudes. to

The coefficientsare Kf · much smaller Kf yand than
K?, while terms the depending n andv maybe on combined Hoi asingle with to constant - K·nv2-{ Ho K?·vq- which 2 H·, characterizes theaverage conditions
in the solarwind(n = 4 cm andv = 500km/s [cf. -3 oeevitin al., 1982]). Thus for average et solarwind plasma parameters, may be simplified (3) to

function· [Faerrnark, 1977].
For the case of a nonuniform ionosphericconductivity, this equation is

-

sin -·8·n

q sin 0A ·n · 8

H· - K? By K? Bz+ Ho. -1Spatial distributions coefficients ofthe

(4)

Ü
-·
with the boundary condition

(z,, )]
sin0· +sinOOX 2
·(·S4 - 0 Ü)

Equation(3) mustbe used extreme for values n and of
and H0 are an integral part of the model of Levitin

(1)
(·)
are the

al. [1984]. This modelalsocontains values the poof

tentials ·,

·,

and·

obtained solving with by (1)

the boundarycondition(2), wherethe modelcurrent

is determined the corresponding of coefficients bY sets

Kf ·, K?, and H0.
The potential·I,,· at the point (IL, MLT) is represented by

Here 0 and A are the colatitudeand longitude,respectively, in spherical coordinates. Uu and ·


FELDSTEIN ET AL.: HIGH-LATITUDE IONOSPHERE

19,923

lite's velocityvector(the component E2). The latter,
which is used in this study, is typically the dominant electric field componentperpendicular to B. The angle between axis 2 and the satellite velocity where andBz arethevalues theIMF forthetime vector varied in the range from ~ 1600 to ~ 175Ü. InBy of when ·,, is determined. For Bz ·_ 0 and B· ( 0 condiformation about the third electric field component is tionsthe modelof Wallis and Budzinski [1981] is used unfortunatelymissingsinceone of the probeswasdam-

at Kp ( 3 andKp ) 3, respectively. ·I,·(IL, MLT), agedearly in the mission.For moreinformationon the
·7 (IL, MLT) and +· (IL, MLT) are basis functions electricfield instrument,seeBlocket al. [1987].

for ·he electrostatic potential(anclhence·he convection) in the high-latitude ionosphere. a result,one As canobtainthedistribution ·'"(IL, MLT, By,Bz) for of
any orientation of the IMF.

Convection

and

Electric

Field

Variations

The representation theionospheric of convection pat: Over the Ionosphere During the

ternby the sumof elementary contributions ·7, ·y,
and ·
ment with the three typesof magnetospheric convection cellssuggested Burchet al. [1985] and by Reiff and by

m

according Levitin et al. [1984] is in agree- Substorms to

Different Phases of Magnetospheric

Burch[1985]. Their merging cells equivalent · are to
for Bz < 0, the viscouscells are equivalentto ·

Bz - By - 0, andthelobecells correspond · and to ·m for B· > 0. Since - -V· (if V x E - 0, which E is
assumed here), V is a linear differential operator, and

Five Viking passes wereselected electricfield modfor eling. UV imagesfrom Viking of the globalauroral disfor tribution are availablefor all events[Cogget and Mur-

By andBz have spatial no dependence ionospheric (at altitude),an expression exactly analogous (5) exists to
for the modeled electric field E m = -X7(I )m .

VIKINO

Opb I t 847

25 JULY

1986
mV/m

The electric field experiment on Viking consisted of six sphericalprobesextendedon wire boomsfrom the

E2
·o

satellite[e.g.,Blocket al., 1987]. Fourof these booms
were 40 m long and located in the plane perpendicular to the spin axis. These probes provide information about the two spin plane componentsof the electric
.... ! ........... !

UT

20

21

field, one often being nearly parallel to the magnetic field (the component and the otherbeingperpenEl) dicular to B and directed nearly oppositeto the satel-

24-26
, 842 843,

JULY
847

!986
, 848,
, ,

RU
500
5OO

UT '

O

'

$

12

18

'

0

'

ú, ,

,

.....

,

, I0
-50C -50C

,

UT ,

0

6

12

18

'

O'

'

500

,,
UT 0 6 12 18 0

500

o

20

21

Figure 1. The perpendicular component the electric of field measuredby Viking, E2, the interplanetary mag- Figure 2. One-minute averagesof, E·, IMF Bz and

neticfield(IMF) z and y components, AU, AL, By, and AU, AL, and AE for Viking pass847 over and
and AE, respectively, the four eventsoccurring for July
24-26, 1986.

the high-latitude region on July 25, 1986. The IMP 8

spacecraft position (6.1, 32.5,-5.1) Rr. is


19,924
(a)

FELDSTEINET AL.' HIGH-LATITUDE IONOSPHERE

POTENTIFIL OF ELECTRIC FIELD
VIKING
UT 19.50

1

8z:,

O. 10

By: -4.95

orb84?

75

MLT

POTENTIFIL OF ELECTRIC FIELD
VIKING
UT 20.30

1·2

Bz: -8.15

By: 0.00

orb 847

55
',, I

MLT

Figure 3. The convection patterns (a) 1950UT and (b) 2030UT on July 25, 1986for the at IMF component valuesof B, - 0.1 nT and By - -4.95 nT; B, - -8.15 nT and By - 0.0 nT,
respectively.


FELDSTEIN
V IK !NO Orb I t 867

ET AL.' HIGH-LATITUDE

IONOSPHERE

19,925

25 JULY 1986
·V/·

uesof E2 are spin-averaged (20 s) which data havebeen
further averagedover the sametime as E·. When correlatingE·' and E2, E2 wasmappedfrom the satellite altitude down to 100 km altitude, assumingno parallel potential drop in between. This altitude corresponds to the ionospheric region where the main current flows. The correlation coefficientand the dispersionbetween

E· and E2 werecalculated eachof the fiveexamples for
presented below.
Growth Phase

20

21

mV/m

L

Ob,.rv,d·
25

Viking pass 847 on 25 July 1986 1940-2145 UT occurredduring substormgrowth phaseand the beginning of expansionphase. Figure 2 showsthe transverse electric field observedby Viking, the IMF componentsBz

and By, and the magnetic activityindices AU, AL and

'·--::"-'·D.:i'm
I ]MF ft - 20mln eh '·1111111··1·1·1:::::::
20

man 3ml -

DIS= 1:!

ú

- -75
50 25
0

-25
-50
-25
-50

AE. Magnetic activity in the auroral zone began 15 min after the southwardturning of Bz because the siof
multaneous increase of both the westward and eastward

21

mV/m

/

/

I

Oat,, mean J·- eh ft -

I

I

I

I

t·mln 20mln

I

t

I

I

I

RR ú 0.85 OISP ú 13.23

I

I

I

I

I

I

I
21

I

I

I

I

I

I

I

-75
mV/m

20

.......

MOde

50
25 0

-25

Data mean 20

Stain

RR

= 0.33
21

-50 -75

electrojets. As the IMF evolveswith time so doesthe large-scale ionosphericconvectionpattern. Figure 3 presentsthe modeled electrostatic potential at 1950 and 2030 UT. At 1950 UT the dawn convectioncell is dominating, whereasat 2030 UT the dawn and dusk cellsare roughly equal. A comparison was made between the measured transverseelectric field mapped to 100 km altitude and the corresponding componentof the modeledfield along the satellite trajectory. The result is shown in Figure 4 where the solid curves are the observedfield, and the dashed ones are the model field. The different panels representdifferent time averagingintervals ranging

from 1 min (top) to 5 min (bottom). The IMF time delay (AT) is 20 min in all cases.Maximumcorrelation Figure 4. Comparisonof the horizontal electric field coefficient RR = 0.85, and mean square componentsmeasured by Viking and mapped along (correlation deviation (dispersion) DISP = 13mV/m) between obdipolemagnetic fieldlinesto 100km altitude(E2 solid servations and modelingresultsis obtainedfor an averline) and calculated (E·' dottedline) with various averagingtime step for the IMF and E2 valuesbut with a aging interval of 4 minutes. Thus, for this event there constant IMF time shift AT -- 20 min. From top to the is a rather good correlation between the modeled elecbottom time-averaging valuesof 1, 2, 3, 4, and 5 min are tric field and the observations (alongthe satellitetrapresented. The correlation coefficientand mean square jectory). This implies that directdrivingdominates as deviationsvaluesare also presented. the energy transfer mechanismbetweenthe solar wind and the magnetosphere during the growth and expansion phases of substorms. We have also checkeddifferent delay times of the IMF data. Best fit was obtained

phree,1990]. Figure1 shows perpendicular the electric
field measuredby Viking and mapped to 100 km altitUde, the IMF z and 3tcomponents, the electrojet and indices which indicate that Viking pass 842 on July 24 occurredduring the expansionphase, passes843 and 848 on July 25 and 20 occurred during the recovery phase and pass 847 occurredduring the growth phase and the subsequenttransition to the expansionphase

for AT-

20 minutes.

The estimated

time needed for

the IMF to propagatefrom IMP 8 to the magnetopause is 14 min assumingthat the "IMF wavefront" is parallel to the direction of propagation,and 3 min assuming that the IMF propagatesradially away from the Sun. Hence intramagnetospheric processes introduce a time delay of between 6 and 17 min.
Expansion Phase

(vertical linesmarktime intervals each of pass).
The values of E·' were calculatedby averagingthe IMF valuesover between 1 and 15 min. The delay time
relative to the IMF measurements was varied from 10 to

Viking pass 842, July 24, 1986, 2215-2350 UT occurred during the expansionphaseof a substorm. Figure 5 shows the transverse electric field observed by

25 min. The valuesof E·' thus obtainedwerecompared Viking, the IMF components and By, and the magBz to measured values of the horizontal component E2 of netic activity indices AU, AL, and AE (the format is the electric field along the Viking trajectory. The val- equivalent that of Figure 2). IMF B· is strongly to


19,926

FELDSTEIN ET AL.: HIGH-LATITUDE

IONOSPHERE

southward, almost20 nT, until 2310UT whenit changes abruptly to beingstronglynorthward. Duringthe course of the passthe modeledionospheric convection pattern changes from a two-cellpattern with the flowlinesconvergingat the centerof the polar cap at 2225 UT, to a two-cellsystemwith the flow lines converging the at dawn side at 2305 UT. The asymmetry is introduced because a substantial of change IMF By. Later, at of 2320 UT the convection pattern is practicallya singlecell systemcenteredaround the magneticpole. Using the IMF data as are givesa modeledpolar cap potential drop exceeding 300 kV. This unrealistically high value is a result of the linear dependence the model in of the potential on the IMF. The solarwind- maÓnetosphere interaction knownto saturatewhenthe southis ward IMF Bz exceeds nT [Reiffand oeukmann, 10 1986;
usedthe value 10 nT as input to the model wheneverBz exceeds nT. FiÓure 6a showsthe convection 10 pattern modeled at 2305 UT takinÓ this saturation effect into

account. The polar cap voltage is 181 kV, still high. The model electric field vectors along the Viking trajectory are shownin Figure 6b. The field magnitudeis high on the daysidebut decreases the satellite moves as toward the nightside. This is not only a result of the spatial variation of the convectionpattern but it also reflects the northward turning of the IMF at 2310 UT, about half ways through the Viking pass. The agreementbetweenmodeledand measuredelectric field depends critically on the IMF delay time as well as on the averaginginterval chosen.Figure 7 shows the transverse electric field measured by Viking and mapped to 100 km altitude together with the modeled field along the trajectory. The IMF delay is 25 rain in all panels, but the averaginginterval changes from 1 rain

(top) to 5 rain (bottom). The correlation practically is
5 min. The overall agreementis good. The only significant discrepancyoccurstoward the end of the pass when Viking enters the morning sectorauroral oval. The effect of different IMF delay times on the modeling resultsis shownin Figure 8. The delay increases

Dremukina ai., 1990] To account this,wehave the same(r = 0.9) for averaging et for intervals 3, 4, and of

from15rain (top) to 30 rain (bottom)insteps 5 min. of
The averaginginterval is 5 rain in all panels. Best fit is obtained for AT -- 25 rain, for which the correla24 JULY 1986
mV/m

tion maximizes - 0.9) and the dispersion (r minimizes (d = 20.8 mV/m). In addition,it is clearlyseenthat
the peaks and reversalsof the electric field are well colocated for this particular choiceof AT. The time delay of IMF propagation from IMP 8 to the magnetopause estimated to be 20 rain, allowing is for the IMF direction, and 7 rain assumingradial expansion. Thus the intramagnetospheric delay was between
5 and 18 rain for this event.

E2
lO o -1o

10
0

-·0

Recovery Phase

II I I I I I I I I I I I I I I I I I I
BY
I I I I I I t I I I I I I I I I I I I I

·0 o

Viking pass 843, July 25, 1986, 0225-0415 UT occuffed during substormrecovery.In Figure 9 are shown the transverseelectric field observedby Viking, the IMF

RU

-·0 components and By, and the magnetic Bz activity indices AU', AL, andAE (the formatis equivalent that to .· of Figure2). The IMF is significantly weaker than in the previouscases,the electrojetshave faded, but the
s00

electric field remains rather strong. All this is characteristic for the recoveryphase. The modeled high-latitude

0
-50C

I

I

I

I

I

I

I

I

I

[

I

I

I

I

I

I

I

I

I

I

convection pattern (Figure 10a)is basically a singleof cell type with anticlockwise plasmacirculationin the cell. The flow lines are somewhatsqueezed togetheron the dayside. Figure 10b showsthe model electricfield vectorsalongthe Viking trajectory for a AT of 20 min. Regardless the averaginginterval, the correlation of is low (r < 0.58), i.e., significantly lowerthan for the
This implies that the directly driven processdoes not dominate during recoveryphasein contrast to growth

s00 growth expansion events and phase discussed above.
and expansion phase. Rather, during recovery phase the magnetosphere driven, at least to a significant is part, by a loading-unloading process. The maximum is Figure 5. Similar to Figure 2, but for Viking pass842. (i.e., least bad) correlation obtainedfor AT -- 20 min. IMF propagationtime from IMP 8 to the magneThe IMP 8 locationwas(18.6, 27.5,-10.9) RE.
UT 22,

-'

i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

o


FELDSTEIN ET AL.: HIGH-LATITUDE

IONOSPHERE

19,927

(a)

POTENT I RL OF ELECTR I C FIELD
VIKING
UT 23.05

1

Bz=- 10. O0

By=

2.96

orb842

C

MLT

(b)

ELECTRIC VECTORS RLONO FIELD .1.2 TRFIJECTORY
V I K I NG orb 842 D I FF ERENT UT

18

O6

Figure 6. (a) The northern hemisphere high-latitude convection patternat 2305UT on July 24, 1986, allowing the IMF Bz component for intensity restriction saturation (the effect).Extrema of
the potential valuesare indicatedin the loci. Solidlinescorrespond anticlockwise to convection, and dotted lines to clockwiseconvection. The polygonalline is the satellite track, the dots

correspond the satellite to location every5 rain between 2215and 2350 UT. (b) Electricfield
vectorsE "· alongthe satellitetrack for each5-rain interval usingIMF shift AT = 25 min.


19,928

FELDSTE1N

ET AL.: HIGH-LATITUDE
VIKING

IONOSPHERE
Orbit 842 24 JULY
.......

topauseis estimatedat 17 min accounting the IMF for directionand 5.5 min without accounting it. Hence for the intramagnetospheric delay was between7 and 14.5
min in this event.

1986
mV/m

ú...-'.....,,.. ..o o ..,, ",, .",
IMFshift - ISmIn
I I I i t I I I

Model

Observed 1 O0
50

Another example of recoveryphaseis Viking pass 848, July 26, 1986, 0020-0210UT. Figure 11 presen, ts the measured electric field, the IMF data, and the electrojetindices. The electricfield is rather strong throughoutthe passwhile the IMF and electrojetsare rather weak, again typical of recoveryphase. The modeled large-scale convection pattern is of a two-celltype with antisolarplasmaflow across centralpart of the the
polar cap.
The measured and the modeled electric fields were.
I
t

DiS?ú 32.98
/ I I

I

I

i

i

(

i

i

i

i

-10C
mV/m

,'' .o-.,- ·", ·..o .-''' .... ... -...
.·

.......

Observe 100
Model
50 -50

IMF shift - 20mln
I I i I I I I I

DISP = 32.91
I I I I I I ,i i i I I I

-10C
mV/m

2·

..........o

oo,

Observed 100

comparedalong the Viking trajectory using different

...o,'"'
i Oste meen- 5mln IMF shift - 25mln

·. ..... ''ÜÜ·'',,· ' 50 .......
-50

RR = 0.87 OIS· : 30.91
23

-10C
mV/m

VIKING

Orbit

842

24 JULY 1986
mV/m

., ,·,,· .....

' ·,,,;,., ,-,-'"·, ,,-, ",

......

- ......

Ob d ..... Mo·.·

lOO

I

.........

.·

Observed lO0

50
0 -50

[
-10C
,

Oete mean 5mln IMFshift - 30mln
, , i , ,

i

,

i

RE - 0,75 DIS?ú 33.79
m , ,

.... 0
o

-50

,,

,

,

,

,

,

)

,

,

-10C

UT

23

Figure 8. Comparison E2 andE·' forvarious of values
of the IMF time shift AT relativeto E2 usinga constant time averaginginterval of 5 min.
23

O
Observed
,...... Model

mV/m
100

ú.,,,· 50 .-'·.. '·
0

IMF delay(or lead) timesand alsodifferent averaging intervals.Bestcorrelation foundfor a 5-minaveragwas ing, consistent with the eventsdiscussed above. Figure
12 compares the measured and modeled fields for dif-

-50

I

s

IMF shift
s i

- 25mln
i i

i

s

i

DISP = 30.09
m · i

-10C

·

I

I

I

·

I

·

t

23
, · ',' '' -·
....... Mode[

0

mV/m

Observed 100
50

.,.....,,·
UT 23

0
-50

0

mV/m

Observed + 100
Model m

ferent AT. In the top panel AT -- -5 min, i.e., the ionospheric convection leads the IMF variationsby 5 min. In the lower three panelsAT equals,0, 5, and 10 min, respectively. The best correlation is obtained for a zero or slightly negativedelay time. In all cases there is an overallagreementof the directions the measured of and modelelfields,but the magnitudeof the modelfield is consistentlysmaller than the measuredone.
The fact that the best correlation was obtained for a

IHF shift

-

25mln

DISP = 30.91

I
UT

I

I,

I

I

I

I

I

t

[
23

I

t

t

"1-50
+ -]0c

· 0[m

nonpositive delay time demonstrates things. First, two that it is crucialin genera]that the delaytime be chosen properly. Second,that in this particular case,substorm recoveryphase, the directly driven assumptionis not viable but rather a loading-unloading process likely. is

line) and calculated allowing the saturation for effect by et (E· dottedline) with various averaging intervals havebeendiscussed Feldste{r· al. [1995]. At the time
for IMF and E2 but with a constant IMF time shift AT

Figure 7. Comparisonof the horizontal electric field Growth Phase FollowinÓ a Transpolar Arc componentsmeasured by Viking and mapped along Parameters the interplanetarymediumand magof dipolemagnetic field linesto 100 km altitude (E2 solid netic activity during the substormon August 3, 1986, beginningof Viking pass896, 1710-1920UT, both B·

the UT, both compoof 1, 2, 3, 4, and 5 min are presented. The correlation crossing zero level at 1750-1802 coefficient and mean square deviations values are also nents were negative at the end of the pass. A polar presented. arc existedin the polar cap of the southernhemisphere

(Bz ~ 9 nT, Bv ~ 8 nT). After = 25 min. Fromtop to bottom time averaging intervals and Bv werepositive


FELDSTEIN
¾!K !NO Orb I t 863

ET AL.' HIGH-LATITUDE

IONOSPHERE

19,929

25 JULY 19815
mV/m

E2
1o o -1o

phase at 2015 UT is accompaniedby an increasein the AL index, stemmingfrom the corresponding decrease in the horizontal componentat IL~ 67Ü. Thus the Viking passbegan during relatively quiet magnetospheric conditions and finished during the substorm growth phase. During the initial part of the pass the whole polar cap is coveredby a vortex with clockwiseplasma con-

BZ
1o

vection(Figure13). Sucha one-cell convection system
is similar to that described earlier by Feldstein et al.
, 0

[1984],Burchet al. [1985], and Friis-Christensen al. et

[1985] for the sameIMF By polarity.Startingat the
I ( I I I I I I I I I I I I I I I I I I
I i
i

substorm growth phase, a two-cell convectionsystem
1o o
-1o

is established, which means that the model convection distribution and hence the electric fields substantially change during the satellite pass. This means that in

I
UT

I

I

I

I

I

I
3

I

I

I

I

I

I

I

t

I

I

t

I

I

I

I

I

the model calculation of ionosphericparametersalong the satellite track temporal variations in the IMF must
be taken into account.

RU
500

...,·/· I I·1 ·-·,. I I I I I I I I· I

I t I'
-50C

I

I'

i

I

I

I

I

I

I

t

I

I

I

I

I

I

I

I

I

I

I

Figure 14 shows E· and E2 for differentvaluesof the IMF delay time AT, with an averaginginterval of 5 min. It is worth mentioning the improvement in the correlation up to RR=0.90 as AT increasesfrom 10 to 20 min. Even the sharp decreaseof E2 at 1805 UT is reflected in the model output for AT -- 20 min. RR decreases and DISP increasesrapidly as AT increasesbeyond 20 min. It appearsthat for this Viking passon August 3,

1986, whichoccurs during the substorm growthphase,
500

the model reasonably well describesthe distribution of and variations in the observedelectric field. This agree-

ment betweenE· and E2 suggests that the modelcan be used to describethe temporal evolution of the electrostaticpotentialdistribution (convection pattern)and the electricfield overthe entire high-latituderegion. Figure 9. Similar to Figure 2 but for Viking pass843 The high correlation between the observedelectric over the high-latitude regionon July 25, 1986. The IMP fields and those predicted by the model indicates that 8 spacecraft at (17.3, 28.3,-10.3) RE. is the magnetosphere closelycontrolledby the IMF duris ing the substorm growthphase.Apparently, magnethe tosphereis at this stageof the substormdirectly driven by the solar wind. Processes within the magnetosphere connected with the unloadingof magneticenergystored prior to the Viking pass[Vorobjev al., 1995], which et tail role. was seen after 1800 UT simultaneouslyin both hemi- in the magnetospheric have only a secondary Small-scale variations of the electric field in the vicinspheres [Cravenet al., 1OOl]. After Bz turns southo

ward, the polar are rapidly movesacrossthe polar cap sphere. Thus the event under considerationis a rare

ity of a 19aurora lies beyond the possibilities of the

towards dawn(dusk)in the northern(southern) hemi- model. In this event, this is seen in the time interval 1837-1906UT, marked with crosses Figure 13b. in In this interval plasma measurementson the satellite caseof a persistent19 aurora during southwardIMF. that it intersected magneticfield linesconnected From 1500to 1800UT the magnitudeof magneticdis- showed to the polar arc. The 20-s resolution electric field also turbancesin the auroral zoneis small and decreasing for
northward IMF. At 1800 UT, the AU and AL indices shows characteristic variations near the arc, which are

start to increase,reflecting a smooth increaseof both the eastward and westward electrojets. A study of the

horizontal (northward) magnetic fieldcomponent along
the meridional chain of magnetic observatories traversing the auroral zone in the midnight sectorshowsthat the substorm expansionphase started at 2015 UT. Before this time the decreaseof the X component is maximum at 70Ü-72 invariant latitude, with essentially Ü no disturbances IL ~ 67Ü. These characteristics typat are ical for the growth phaseof a magnetospheric substorm

[McPherron, 1970;Feldstein, 1974]. The expansion

smoothedout when averagingover severalminutes. Marklund et al. [1991] found that the relationship between the polar arc and the total convectionpattern can change alongthe arc length depending the intenon sity of the convectiondirectly associatedwith the arc. If a polar arc has low intensity,it can be crossed conby vectionlines. The resultingtotal convection the sum is of the quasistationary convection,which exists continuously due to the interaction betwen the solar wind and the Earth's magneticfield, and additional convection in the vicinity of the polar arc. The resulting convection


19,930
(a)

FELDSTE1N AL.' HIGH-LATITUDE IONOSPHERE ET

POTENTIRL-OF

ELECTRIC

FIELD

1

Bz=

2.35

By= -5.78

VIKING
UT 03.40

Ürb84$

C

MLT

ELECTRIC

FIELD

VECTORS

!

RLONG TRRJECTORY
IN DIFFERENT UT

VIKING

orb 843

liLT

125mv/m

Figure (a)The 10. convection at 0340 on·July 1986, IMFcomponents pattern UT 25, for wJues JSz 2.35 and - -5.78 Thedots nT By nT. along satellite correspond satellite the track tothe location every rainbetween and0415 (b) Theelectric vectorsm along 4 0225 UT. field E the
satellite trackat 4-rainintervals using IMF timeshiftAT - 20 min. an


FELDSTEIN
VIKING Orbit 848

ET AL.: HIGH-LATITUDE

IONOSPHERE
26 JULY 1986

19,931

26 JULY 1986
mV/m

VIK I NG Orb I t 840

mV/m

Oboorvod
E2
lO o -lO

....... t50 MoOeL
Oate SmlnRR0.73 me· ú
,
IMF ahl((

,

I

,

- -S mln

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.

t

I

OISP - 18.73

'
I I I ,
.

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,

,

I

,

I

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,

2

:

,

:
,

ú

mV/m

Observed
BZ
lO o
-10

Date - 5mln mean
BY
10 0

.....
I
.

.......t50 Mo·©L
I I I I I I I t
, ,

I

IHF ahlft - 0 mln

RR = 0.73
DISP = lB, JÓ

I

I

I

I

I

I

I

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I

:

,

:

,

Obaerved l

-10

....... t50 Model
0

AU
500

Data - $mln mean
I
IMF ahlft

I

·

I

- 5 mln

I

I

I

[

I

.

RR .0.68
DISP = 18.12

I

I

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t

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I

,

2

I

,

: ',

,

.J

mV/m
50
25

0
0

-25
-50C
,,

·E
500

Figure 12. Similar to Figure 8 with a constantaveraging time interval of 5 min.

mined by the state of the interplanetary medium and may be described usingmodelsof Figure 11. Similar to Figure 2 but for Viking pass the magnetosphere, the large-scale electricfieldsand currents.Suchmodels, 848 over the high-latitude regionon July 26, 1986. The

IMP 8 spacecraft position (2.9, 32.8,-3.6) RE. is

which are based on the method of reconstruction

of the

can be directednot alongthe arc, asis usuallyassumed, but at an angle to the arc. In the case studied here, sunward convectionis observedeverywhereequatorwardof the polar arc, and it is possible that the arc is indeedassociated with a region of convectionreversal. The polar arc is located near

the focus the dawnconvection (Figure13b),i.e., of cell
in the regionof large-scale field-aligned currentflowing into the ionosphere. This corresponds the region 1 to currents Iijima and Polemra[1986], but it doesnot of excludethe possibilitythat there is a small-scale upward field-alignedcurrent immediatelyabovethe polar arc. In fact, sucha current was detectedby the Viking
satellite.

large-scale currentsystems from ground-based magnetic data [Levitinet al., 1984;Friis-Christensen al., 1985; et Mishin,1990]or satellite measurements electric of fields at ionospheric altitudes[Richand Maynard,1989] are capable of describingthe spario-temporaldistribution of the electricfieldin the high-latitude region(seeFigures4, 7, 12,and 13). The actualcapability the above of presented modelsto predict and reconstruct parameters in the near-Earth spaceis determined,first of all, by the
directness of the connection between the solar wind and

Discussion

the variationsin the magnetosphere-ionosphere system. If the processes the systemare directly driven by the in solarwind [Akasofu, 1979], the models provide can high correlation between the predicted and observednearspacecharacteristics.If the correlation is low this may imply either unadequacyof the model usedor the dominance in the system of "loading-unloading"processes over the directly driven ones. The loading-unloading processes related to the magneticenergystoragein are

The establishment of quantitative relations that describe the relation of ionosphericelectromagneticpa-

the magnetospheric (loading tail phase) with its subsequent release,i.e., an "unloading"of the magneticen-

1979;Baker et al., rameters (fieldsand currents) the conditions the ergy storedin the tail [McPherron, to of
solar wind, of which the IMF magnitude and direc- 1984]. tion are the most important ones, is one of the most The high correlationobtainedhere betweenthe model acute problemsin solar-terrestrialphysics.The electric and observedelectric fields during the growth and ex-

phases magnetospheric of substorms suggests fieldsin the high-latitudeionosphere, which are deter- pansion


19,932

FELDSTEIN

ET AL.: HIGH-LATITUDE

IONOSPHERE

(a)
POTENTIRL OF ELECTRIC FIELD
VIKING
UT 17.35
o

1

Bz:

7.9

By:

10.00

orb 896

o

MLT

(b)
POTENTIRL OF ELECTRIC FIELD
VIKING
UT 18.35
o
..

1

Bz: -1.3

By:-10. O0

orb 896

tt
it it t t
I , I

i

'

o

MLT

Figure 13. Comparison of/·2 and/·a for variousvaluesof IMF time shirtsAT relative to /·2 with a constantaveragingtime of 5 min.


FELDSTEIN

ET AL.: HIGH-LATITUDE

IONOSPHERE

19,933

that the magnetosphere thesetimes is primarily a tradictory resultsmay be explained,at least partially, at in models, but alsoto driven systemand that unloading magnetotailen- by differences the usednonlinear of to the ergycontributes a minoramount the observed someextent by the necessity discriminate disturonly to according the phases to of electric field variations. Processes within the magneto- bancesunder investigation the substorm and a more detailed consideration of time sphere moreprominent are duringthe substorm recovery phase,makingthe correlation betweenthe model shifts betweenEsw and the magneticindicesAE anti
and observedvalues substantially poorer. It is not exAL.

The optimum time shift AT- 20 min shall be comcludedthat theseprocesses connected are with energy dissipation stored the magnetospheric duringthe pared to the time it takes for IMF variationsto propin tail The aninitial phases the substorm(loading-unloading of sce- agateto the magnetosphere. solarwind needs other4.5 min to reachthe magnetopause [$preiterand nario). The contribution the intramagnetospheric of 1980]. According ClauerandBanks to [1986] processes leadsto the mitigationof direct connection Stahara, convection responds IMF variations to betweenthe magnetospheric state and the interplane- the ionospheric with ~ 14-raindelay. Greenwald tary mediumconditions.l·ecentresultson this topic on the magnetopause are contradictoryconcerning relation betweenmag- e· al. [1990] estimatethis delay to be 5-9 rain, and the eZ a netospheric disturbances and the solar wind electric $evgeev al. [1986] suggest time delayof 10 min in of to field (E,·). Goertzet al. [1993] foundcorrelation the response polar cap convection discontinuities

with the auroralzonemagnetic disturbances in(AE

in the IMF

at the bow shock. The time shifts obtained

dex)at a ~ 90 % level, Bakeret al. [1993]found in this study are consistentwith thesevalues. but

of coefficient indicatea simithe loading-unloading processes prevailing generating Highvalues a correlation in of magnetic disturbances the westward in electrojet region larity of variationand, hence,the existence a definite

(AL index)[cf. Akasofu, 1994]. Such strikingly con- relation between the correlated phenomena. Another

VIKING

Orbit

896

03 RUO

1986

," ,,,'-·
PI
1Ó 19

Ob,er-ved J_ 50
25

UT

18

19

mV/m

·
18 19

Ob..,-v,d _L so
25

quiet interval = 10nT, Bv = 6 nT) (a), andduring growth (Bz the phase a substorm = of (Bz -4 nT, Bv = -10 nT) (b)fortheViking onAugust 1986. polygonal indicates pass 3, The line the

Figure 14. Highlatitude convection systems thenorthern of hemisphere during magnetically the

ionospheric footpoint km) of thesatellite (100 track,thedots correspond thesatellite to location every minbetween and1925 5 1710 UT. Extrema thepotential of values indicated thefoci. are in The potential difference between isolines 10 kV; zeroequipotential marked the dotted is is by
line. The two crosses mark the Viking satellitepositionat 1837and 1906UT.


19,934

FELDSTEIN ET AL.: HIGH-LATITUDE IONOSPHERE

important characteristic of their quantitative relation

the convectionacrossthe central part of the polar cap

is the meansquare deviation(dispersion) between E2 is antisunward. Therefore, during times of 0 aurora and E·. In the courseof the growth and expansion the convectionvia the central part of the polar cap is
phases dispersion the over the wholepassis · 15 % of frequently sunward, but in other time intervals the conthe electricfield variation amplitude, and it increases up to ~ 25 % duringthe recovery phase.A dispersion 15 of % doesnot excludethe influenceof unloadingprocesses on the substormdynamicsduring its two first phases. An important input parameter in the modelingis the ionospheric conductivity distribution. The statistical vection direction may become antisunward. This fact must be taken into accountwhen constructing models
of 0 auroras.

Another event similar to the August 3, 1986, treated here with Bz > 0 initially followedby a southwardturning of the IMF was Viking pass1188 on September25,

modelby WallisandBudzinski [1981] for Kp < 3 and 1986,analyzed detail by Mavklund al. [1991]. in et Kp > 3 whichhas beenusedherein the modelcalculations to representIMF B: ·_ 0 and IMF B: < 0 conSummary
ditions, respectively,provides, of course, only a crude approximation of the actual conditionsprevailingduring the Viking passes.In particular, during pass896 it does not take into account conductivity enhancements associated with the 0 aurora inside the polar cap. It is known, that the distribution of the electrostaticpotentiM and the electric field is critically dependent on

the ionospheric conductivity[cf. Blombevg Markand lund, 1988; Ahn et al., 1989]. However, duringthe
summer seasonthe electromagneticradiation component of the height-integratedionosphericconductivity exerts a stabilizing effect on the resultant conductivity distribution. Its existencesmooths out conductivity irregularitiesassociated with particle precipitation. Apparently, the increasein the component of the conductivity due to particle precipitation doesnot crucially influencethe large-scale featuresof the electrostaticpotentiM and electricfield distributionsduring summer. When modelingthe convectionand electricfield during the Viking passes,it was necessaryto take into account the variations in the interplanetary medium. During the time interval of about 2 hours it took the Viking satellite to crossthe high-latitude region drastic changesof the IMF orientation and intensity took place. Modeling of the whole passusingonly one representativestate of the magnetosphere thereforenot is adequate. For instance,Viking pass896 on August 3, 1986, correspondsto an interval for which a transpo-

The IZMEM model for reconstructingthe electric fieldsand horizontalcurrentsin the high-latitudeionospherewas applied to five eventsoccurringduring different phasesof magnetospheric substorms.The results werecomparedto in situ Viking electricfield data. The level of agreementbetweenthe measuredand the modeled electric field is different during different phases. This leadsus to the followingconclusions. The IZMEM model, and also other similar models relating the electrodynamics the high-latitudeionoof sphere to the interplanetary medium, are a useful tool for describing system.The high correlationbetween the model and observations growth and expansion for phase is suggeststhat at these times the magnetosphere is mainly directly driven by the solar wind. During recoveryphase processes internal to the magnetosphere, i.e., loading-unloading are more prominent, and therefore there is a poorer correlation between model and
observations.

lar luminosity band(0 aurora)existed the polarcap. in Buvchet al. [1992] useda northwardIMF merging
model containinglobe cells, mergingcells, and viscous cells to interpret the large-scaleconvectionfor such an event. It was assumed that the magnitudesof the IMF componentscan be obtained as mean valuesover a reasonably long time span to characterizethe IMF conditions during the event. If such a procedureis used for the August 3, 1986, event the averagevalue of the IMF B· component over the time interval from I hour
before the auroral observation until the end of observa-

In the modelingit is necessary accountfor the finite to transit time from the solar-windspacecraft the magto netopauseand also for the intramagnetospheric time delay. The optimum time delays found in this study are consistentwith typical delay times discussed the in literature. Anothernecessity to account changes was for in the IMF during the course ofViking's traversalof the high-latitude region.
Acknowledgments. The Viking Project wasmanaged and operated by the Swedish Space Corporation under contract from the Swedish Board for Space Activities. This work was supported by the Russian Foundation of Fun-

damentalResearches (project codes93-05-8722and 94-0516350), grant N M6P000 from the International Science
Foundation, and by the Swedish National Space Board.
The Editor thanks Roderick A. Heelis and another referee

for their assistancein evaluating this paper.

tions(1700-1926 UT) becomes - 6.6 nT. All other References B· cases considered Buvch al. [1992] werecharacterby et
ized by a northward IMF direction. Therefore one may
be led to conclude that 0 aurora events are characterized

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FELDSTEIN ET AL.' HIGH-LATITUDE

IONOSPHERE

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RAN, 142092Troitsk, MoscowRegion,Russia.

(Received June 15, 1993;revised November 1995; 10, accepted February1, 1996.)