The interplanetary magnetic field Bz-component influence on the geomagnetic field variations and on the auroral dynamics

PImet. m

Ski., Vol. 27. PP. -716.

FWcamm

PmnsLtd..1979.RintdhNc1rtkm~

THE INTERPLANETARY MAGNETIC FIELD &-COMPONENT INFLUENCE ON THE GEOMAGNETIC FIELD VARIATIONS AND ON THE AURORAL DYNAMICS R. V. REZHEBIOV ad V. 6. VORORJEV Polar Geophysical Institute, Apatity, U.S.S.R. Y.LFEUSTEW

Izmiran, p/o Akademgorodok, (Received

Moscow Region, U.S.S.R.

14 June 1978)

Aha&aet- The influence of the interplanetary magnetic field (IMP) B,-component on the geomagnetic field variations and on the auroral dynamic are examined. The following results are obtained: (1) The negative (positive) pulse in the B, -component of the IMP caused the current from the nightaide to the dayside (from the dayside to the nightside) in the polar cap and the eastward (westward) current at the Equator. (2) In the interval OS-14 L.T. the fluctuations of the B,-component always provoke those of the magnetic field’s H-component at the magnetic Equator, the relationship between them being expressed in the form: AH = -4AB,. In the interval 14-17 L.T. this relationship could not always be revealed. (3) There is a tendency to an increase (decrease) of the 3 h mean intensity of the equatorial electrojet with an increase (decrease) of the B,-component average values. (4) The variations of the equatorial boundary of aurorae in the interval OS-20M.L.T. with a duration of 30-6Omin are synchronous with the occurrence and the increase of the DP-2 current system. The dependence of the equatorial amoral boundary position in the noon sector, on the horizontal disturbance vector value in the polar cap is expressed by the relation:

A~‘(krn)= (4.0* l.O)AT(v). (5) The region in which the direction of the auroral discrete forms meridional motion changes, consists of a system of 1ongiUinal overlapping arcs of the nocn and the evening types. The short-lived noon rayed arcs always occur in this region at lower latitudes than the homogenecns evening 81%~. During the period of DP-2 increase the short-lived noon arcs and the evening arcs move towards each other.

tion the existence of DP-2 as a peculiar type of geomagnetic field variation. The main difficulties arising when investigating the relationship between B,-component fluctuations and the DP-2 type disturbances of the geomagnetic field are: firstly, the choice of the reference level for determining the value and the sign of the geomagnetic field variation and, secondly, the fact that this variation is a self dependent phenomenon and not the result of a superposition of variations of the other type (Matsushita and Balsley, 1972; Kawasaki and Akasofu, 1972; Akasofu et al., 1973; Troshichev et al., 1974; Feldstein, 1969; Afonina et al., 1975). In particular, the necessity of a new mode in solar wiudmagnetosphere interaction is disputed by Kawasaki and Akasofu (1972) and Akasofu et al. (1973). In their opinion DP-2 is either a manifestation of a magnetospheric substorm or the effect of the intensification and stretching of the Sqp current system

IM’RODUCIlON

The disturbance of the DP-2 type first described by Nishida (l%Sa, 1968b) is one of the manifestations of the direct influence of the interplanetary magnetic field (Ih@) and, in particular, of its B,component on the Earth’s magnetosphere. Further investigations have shown that DP-2 had other morphology than the disturbances of DP-1 type or DCF (Nishida, 1971a; Nishida and Maezawa, 1971). The equivalent current system of DP-2 consists of the morning and the evening vortices stretched from high-latitudes and up to the Equator. The functional agreement between the variations of the North-8outh component of the IMF and the DP-2 was assumed to be caused by the penetration of the interplanetary electric field E = VXB in the magnetosphere. However, some methodical di8kulties in the above-mentioned papers gave some authors the occasion to call in ques699

700

B. V. RWHFWI!, V. G.

VORORJRV

towards the Equator. Matsushita and Balsley (1972) assume that the reference level for determining the sign and value of the disturbance should not be the same as that chosen by Nishida (1968a). If it were chosen according to Matsushita and Balsley (1972), the variation of the DP-2 would have the reverse sign. If the distur&nce is reckoned from the most quiet day’s level flroshichev et al., 1974) then the current system DP-2 co~ected with the fluctuations of the B, IMF component breaks down into two independent parts. The first one, lying within high-latitudes (4 > 500) is analogous to a well-known current system S,‘. The other part is represented by a system of zonal currents caused by extra ionospheric sources. Feldstein (1969) and Afonina et al. (1975) think that the contribution of substorm fields and of asymmetrical ring currents into the field variations generally attributed to DP2, is important in a number of cases. The ambiguity in choosing the reference level may be eliminated by analyzing the geomagnetic field variations in the period of isolated IMF fluctuations, for then the instants of the beginning and of the end of a fluctuation and the corresponding geomagnetic field changes at the Earth% surface are known exactly. To lind the physical cause of the correlation between the fluctuation in B, of the IMF and the geomagnetic field, it would be of interest to determine whether the fluctuation in B, is always accompanied by a fluctuation in the geomagnetic field and whether the long-period B, variations influence the geomagnetic field values at the equatorial latitudes in particular. According to Vorobjev et al. (1975) and Vorobjev ef al. (1976), an equatorward shift of the day and evening sector of the oval occurs 30-6Omin before the onset of the expansive phase of the substorm. T&is shift is connected with the turn of the B,-component of the IMF towards the South. Such a shift during the period of the southward turn of B, was observed by Horwits and Akasofu (1977) and was associated with the corresponding shift of the dayside cusp towards the Equator (Kamide et al., 1976). The shift begins with a l&15 min delay with respect to the beginning of the B, decrease. In this coMection it would be of interest to carry out the investigation of a possible relationship between the location of aurorae and the disturbance of the DP-2 type geomagnetic field. The daytime aurorae whose location and motion are closely connected with the m components variations are determined directly by plasma injection from the magnetosheath. Therefore, the study of their dynamics and the correlation with evening

and Y. I. m

aurorae permits us to follow the dynamica of different parts of the magnetosphere, in particular to investigate how the injections from the plasma sheet leading to evening aurorae are conjugated with the injections from the magnetosheath responsible for the daytime aurorae. Therefore we consider: (1) the sign and the value of the geomagnetic field disturbance following the isolated pulse in the B,-component of the IMF; (2) whether the fluctuation of the B,-component of the IMF is always followed by the geomagnetic field fluctuation at the Equator; (3) the influence of long-term B, variations on the geomagnetic field at the Equator; (4) the relationship between the variations of the auroral equatorial boundary and the DP-2 type geomagnetic disturbances; (5) the reciprocal position of evening and noon aurorae, their dynamics and relationship with different regions of the magnetosphere. GEOMAGNFXlCPIELDRFSPONSETOTHEISOLATED PUUEINlWEIMF&-COhWONETW

Nineteen isolated pulses in the B,-component of the Ih@ have been considered during the period from 1 September to 30 November 1965. The pulses of different sign preceded by a quiet period of -1-5 h have been considered. When there was a series of pulses only the first was considered. A comparison of fluctuations in the &-component with the field variation in the polar cap and at the Equator was carried out, for it is in just these regions that the characteristic variations of the DP2 type are most distinctly pronounced. The beginning and the end of fluctuations in the geomagnetic field and in the B,-component were connected by a straight line which was taken as a reference level. The IMF measurements of Explorer 28 were used in the present paper. A relatively small quantity of the selected data is due to the method of analysis. The cases of a pulse in the B,-component, following the quiet interval, are very rare. Figure la shows the variations of the B,component and the magnetograms of the polar cap stations Thule (Th), equatorial stations Addis Ababa (Ad), Annamalainagar (An), low-latitude station Alibag (Al) and m-index for 2 November 1965. B, >O in the interval from 0540 U.T. to - 0700 U.T. and made on the average - 1.5~ with variations of f 0.5~. The southward fluctuation of B, set on about 0700 U.T. and the amplitude of the fluctuation was -6~. A distinct fluctuation of the geomagnetic field occurs at the magnetograms of

701

Interplanetary magnetic geld and amoral dynamics

(b)

(a)

s, 6

& 8

IO U.-r IOY

Th

0 H

f=Y

f

19oy

Ad

f 481

1 391 1 44Y t 44Y 1 36~

ICQY

FIG. 1. a. GEOMAGNETIC

FIFLDS RESFQNSE TO THE NEGATIVE PUUE ON 2 NOVEMBER 1965.

IN THE

B, -cor+noumrr OF THE IMF

From the top: B,-component of the IMF in the solar-ecliptic system; H- and D-components of the geomagnetic field at Thule (Th) observatory; H-component at the equatorial stations Addis Ababa (Ad), Annamalainagar (An), low-latitude station Alibag (Al) and the mean hour M-index. The horizontal lines arc the reference level for B, and AE; they are plotted for other field variations for convenience; they indicate the time scale. b. GEOMAGNKTIC FIELD’S RESFONSE TO IHE FOSITMSPUISEMTHEB,-COMP~NENTOFTI~E~ON~ 0CIDBER 1965. The notations are analogous to those in Fig. la. The equatorial station Huancayo (Hu) is added. nearpole and equatorial stations with a delay of -15 min. The fluctuation sign is determined unambiguously everywhere. As it follows from the magnetogram of Thule, the equivalent current in the polar cap is sunward. The equatorial stations record the eastward current. The maximal value of the TABLE1. ILSr OF N 1 2 3 4 5 6

STATIONS,

Station

Wieze C. Chelyuskin Thule Alert

THEZR CORRFXXID

Symbol I% HIS WI1 CCH Th ALE

disturbance is observed at Addis Ababa (the amplitude -15-y). It follows from the comparison between the field fluctuation values at the observatories Annamalainagar and Alibag that a considerable part of the field fluctuation is caused by the current having ionospherical origin. It also

GFiOMAGNEITC

COORDINATES

AND

CORRFKTI ON FROM

U.T. TOL.T.

Correc. gccm. latitude

Ccrrec. geom. longitude

L.T. = U.T. + (h)

74.9 74.7 73.3 70.8 86.0 87.5

114.7 144.4 155.6 174.2 60.6 136.5

+1.2 +3.8 +5.1 +7.0 -4.6 -4.1

B. V. E&ZHEINOV, V. G. VOR~BJEVand Y. I. Fkuxmmv

702 TABLET. Ltrr

OFsTATIoNS,THEIRGEoGRApHlCcoORDINATEs,INcLINAnoNANDcORREcIl

N

station

Symbol

1 2 3 4 5 6

m%J TliWUldrUm BaflgUi Addis Ababa Fuquene HUanCayO

AL Tr Bll Ad Fu Hu

Geographic coordinates latituck longitude 18.6” 08.5” 04.4” 09.0” 05.5” 12.0”

72.9” 77.0” 18.5” 38.8” 286.3” 284.7”

follows from Fig. la that, in the period observed, there is no disturbance connected with the sllbstorms (AE -30-y). Figure lb shows the B,-component and the magnetograms of ground based observatories for 3 October 1965. During -1.5 h the B,-component was changing insignificantly and made on the average - - 2~. A positive pulse in the B, -component which began at 08lOU.T. was followed by the development, with a 15 min delay, of a current in the polar cap flowing from the noonside to the nightside. Meanwhile the equatorial stations were recording a westward current with maximum intensity on the dayside, its major part being the ionos-

ON FROM

U.T. TDL.T.

Inclination

L.T. = U.T. + (h)

24.6” 00.6” 13.9” 1.0” 33.7” 1.0”

+4.8 +5.0 +1.2 +2.5 -4.9 -5.0

pherkal origin. The magnetic activity in the auroral zone is moderate with a maximum of -150~ at the end of fluctuation. Figure 2a represents the disturbance vectors in the geomagnetic field for 12 cases when the pulse in the B,-component was negative. The small circle designates the geomagnetic latitude of the station Alert (4~= 86”), the external circle-the geomagnetic Equator. The same figures near the vectors correspond to the same fluctuation. A horizontal disturbance vector T, = [(AX)‘+ (AI’)‘]“’ has been used for the station Alert whereas the amplitude of the H-component has been used for the Equator. For the negative pulse in B,, the current always AH,

y

s,_ L

FIG.~. a.THE

+

D-~TI~NOFGE~~~AG~C~ELDDISNRB TRRE~IUATORFOR~E Bz-co~~

The s8me figuresat the bottom of the vectors correspond

4+

ANcRVlXTORSpTTHEPOLARCAPANDAT NRGATWEPLJLSEL

to the same case. The time is local.

b.‘IImsAMetinv

FIG.~SFORTHE~~PULSEOF~B,CO~~C.~RELA~ONSHIP BE~~EN~GE~~G~CFIELD~~~~L~~~AL~EATTHE~UA~R(~~~)~T~~~~AL~E~F THE B,-comm d.Tmx R~LA~~N~HI~OF?~EGE~MA~NET~C~ELDVAL~EAT~WLAR~A~ ~ITI-I THAT OF 'IHE B,-COMPONEIW.

(AT)

Intup~

5ows through the polar cap from midnight towards noon while the current is directed eastwards at the Equatol. Seven cases are shown in Fig. 2b when the pulse in the B,-component of IMF was positive. The notations are the same as in Fig. 2a. The current 5ows from noon to midnight in the polar cap and it is directed westwards at the Equator. The above method of calculating the 5uctuation field is free of arbitrariness in choosing the reference level in the case of repeated 5uctuations, since only the isolated pulses are considered. It has advantages in comparison with the field scaling from the most quiet day, which may cause the following di5iculties. Firstly, it would be a changing reference level since the field values would be ditferent even for the most quiet days. Secondly, and more importantly, all more long-period field variations and their variability from day to day, would be taken into account and their contribution to the calculated field variations would be the main one. Figure 2c shows the dependence of the field 5uctuation intesity at the Equator (vertical axis) on that of the &-component (horixontal axis) for the same cases as in Figs. 2a, b. The cases of the B, decrease are shown to the left of the horizontal axis, those of the increase are shown to the right. The positive values AH correspond to the eastward current, the negative to the westward one. The correlation coefficient r and the regression equation are calculated by the least squares method, assmning the existence of a linear dependence between AH andAB,.Thehighvalue r = -0.91*0.03showsa close connection between 5uctuations in B, and in the geomagnetic field at the Equator, valid both for AR, <0 and for AB, > 0. The regression equation has the form: AH, = -26AI3,. Figure 2d represents the dependence of AT, in the polar cap on the fluctuation intensity of the IMF B, -component. AT,> 0 corresponds to a current from the night to the dayside and AT, < 0 to a current in the opposite direction. The high value r = -0.95 f 0.1 is also indicative of a close relationship between field 5uctuations in the polar cap with AB. fluctuations, both positive and negative. The regression equation has the form: AT,= -8.3AB.. The linear dependence of the magnetic field variation’s intensity in the polar cap on the Bzcomponent intensity has also been discovered by Maexawa (1976) and Kuxnetsov and Troshichev (1977). In this way, the analysis performed shows that 10

703

magnetic !icld and amoral dynamics

the sign and the value of the IMF B,-component are controlling the sign and the value of field fluctuations in the polar cap and at the Equator. In this case the relationship between AB, and AI-I-, AB, and AT, tends to be a linear one.

THElNFL~cEoFTBEIMF&FLucl7JATKDNs

ON lm3

EQu.4mluAL

AT DlPFEtWW QVIICRVAIS

B LT.

There is nowadays no doubt about a close relationship between the a-component variations and those of the geomagnetic 5eld in the nearpole region. However, such a relationship for the equatorial region is not accepted unanimously. In order to know whether the pulse in the B,component is always causing the 5uctuation in the geomagnetic field at the Equator, the variations of the B,-component and of the magnetic field have been analyxed for the equatorial observatory Huancayo in the interval from 1300 to 2200U.T. when the observatory was on the sunlit side of the Earth. To eliminate the infhrence of the 5eld from the extra-ionospheric sources (DCF and DR), the difference has been analysed between the Hcomponents of Huancayo and Fuquene observatories, both situated along one meridian (Gouin and Mayaud, 1967). The field variations from extra-ionospheric sources proceed analogously and have the same values at these observatories, therefore the di5erence AHuu+u is the field of a source having purely ionospheric origin and it represents the field 5uctuations of the equatorial electrojet. AH,, values in the interval 0000-0300 U.T. are taken as a reference level of the electrojet intensity since it is known that its 5eld does not exceed several gammas at near-midnight hours. The data for 1 September 1%5,31 July 1965,lS October 1965 and 19 November 1965 are shown in Figs. 3a, b, c, d, respectively. The mean values for 15 min AH,,, the field value in the polar cap at Thule (Fig. 3b) or Alert (Figs. 3a, c, d) and the B,component values are shown above. The hour values of the I3,-component of IMF and AH-indices are shown below. A certain relationship may be observed between the 5uctuations in the B,-compoaent of the equatorial electrojet field and field 5uctuations in the polar cap on 1 September 1965 (Fig. 3a). Synchronous changes of the equatorial electrojet field and polar cap field correspond to each 5~tuation of the B,-component. 31 July 1965 (Fig. 3b) is an “anomalous” case of the IMR and geomagnetic 5eld variations. Up to

704

B. V. aov,

V. G.

VOROBJEV

and Y. I. W (b)

31.pIL.65

h-HFU

t 207 l637 1767

15.X.65

FIG. 3. a.

&WAl-ORIAL

ELECTROJJ?T INTENSITY

THEPoL.ARcAPcoNNEcmJJ

WlTH

(d)

lS.H.65

Hnu -+u

VARIATIONS m

HHU-“FU

AND

B,-COMf’ONENT

THOSE

OF THE GROMAGNETK!

ON 1 sEF’TEM6 RR

FIELD

IN

1965.

From the top: the difference between H-components of the geomagnetic field at stations Huancayo and Fuquene; X- and Y-field components at Alert; B,-component of the IMF; mean hour values of By-component of the IMF; mean hour values of AE mdex. Horizontal lines show the reference level except the line corresponding to X and Y-components at Alert station. b. ‘IIn? SAMEAS IN FIG. 3a

FOR

31 JULY 1965.

The magnetic field in the polar cap is given from Thule data. c. W SAMEAS IN FIG. 3a FOR 15 Ocroe~~ d. THE SAMEAS IN FIG. 3a FOR 19 NOW=

-1900 U.T. the fluctuations of the equatorial electrojet field are synchronous with those in the B,component and in the polar cap field. But between 1900 and 2000U.T. the fluctuation in B, hardly causes an appropriate change in the equatorial electrojet, whereas the field in the polar cap shows a normal response to the fluctuation in B,. The interval 1900-2000U.T. corresponds to 14001700L.T. at the Huancayo observatory. Figure 3c shows an exclusively magneto-quiet day of 15 September 1965. In the interval 14001700 U.T. when B,does not change, no noticeable field fluctuations are observed at ahe Equator and in the polar cap. Figure 3d shows another “anomalous” case of 19 November 1965. A noticeable field decrease is observed at the Equator from 1100 U.T. to 2000U.T. with the maximal value of -70~ with

1965. 1965.

respect to the midnight value, which may be interpreted as a rise of a westward electrojet. In spite of the anomalous character of the equatorial electrojet, a usual relationship between the B, -component fluctuations and those of the geomagnetic field at the Equator remains. All simultaneous IMF recordings of Explorer 28 and the magnetograms from Huancayo observatory over the period July-November 1965 were considered with the following conclusions: (1) in the interval 0800-1400L.T. a distinct relationship between the B, -component fluctuation and the equatorial electrojet field, independent of the intensity and direction of the latter, is always observed; (2) the relationship between the B,-component of IMF and the equatorial electrojet field is not always observed in the interval 1400-1700 U.T.

Interplanetary magnetic field and auroral dynamics

ABZ.

705

Y

FIG. 4. a. THE DEPENDENCE PULSE

BETWEECN THEFIELDPULSE VALUE AT THESTATION HUANCAYO ANDTHE B,-COMPONENT OF THEIMP FORTHEINTEFWAL 0800-1400 L.T. b. THE SAME As IN FIG. &I FORTHEINI-ERVAL 1400-1700L.T.

INTENSITY

IN THE

Figure 4a represents the dependence of field fluctuations at the Equator and in the B,component for the interval 080O-1400L.T. 196 isolated fluctuations have been considered altogether for AJ3 c 200-y. The crosses correspond to the mean values of AH and the points to concrete cases since the mean was calculated for n 2 5 only. Vertical and horizontal segments designate the mean square error value. The dependence between AB, and AH in the linear approximation in the mentioned interval L.T. may be described by the equation AH, = -4AB,. A relationship AH, = -2.6AB, was given above in order to show the dependence between AH, and AB,. The di8erence is due to the fact that the lirst relation was obtained not only from Huancayo observatory data but also from other stations which are further away from the equatorial electrojet than Huancayo is and are situated in the other longitudinal sector. The dependence of field fluctuation values at the Equator and of the B,-component for the interval 140O-1700L.T. is shown in Fig. 4b. It is clearly seen that the dependence of AH on AB, is significantly dzerent from that shown in Fig. 4a. Apparently, the whole set of data may be classified into two groups. The first one includes the cases for which the relationship between AH, and AB, wrresponds to the equation AH, = -4AB,. The second one (it may be conventionally bounded with

dashes) embraces the cases which do not show such a relationship. In this way, in the interval 080O-1400L.T. B, fluctuations always give rise to field fluctuations at the Equator. The relationship between AH, and AB, approaches the linear one; the current responsible for equatorial field fluctuations has an ionospheric origin. The asymmetry of the influence of the B,component fluctuation on the intensity of the equatorial electrojet may probably explain the results of the paper of Troshichev et al. (1974) in which a good correlation has been found between the field variations in the polar cap and at the Equator for -1600 U.T. (forenoon hours at Huancayo) and a bad correlation for later hours. Such an asymmetry may be connected with fieldaligned current peculiarities responsible for the DP-2 disturbance. In the paper of Leontjev et al. (1974) it is shown that the shape of the equivalent current system, DP-2, depends considerably on the position of downward and upward field-aligned currents with respect to the terminator, as well as on the relationship between the wnductivities of sunlit and dark sides of the polar cap. Probably the asymmetry is, to a certain degree, due to the development of westward electrojets in the equatorial ionosphere which occur must often at afternoon hours (Woodman et al., 1977).

B. V.

706

TEElNFLUENCEOFIMF& VALUE)

R~mmov.

V. G.

VORORJRV

-COMPONENT AVERAGED

ON ‘IHE EQUA’IQRIAL

GlWb¶AGNEllC

FIELD

The existence of a relationship between the short-period fluctuations of the B,-component and the geomagnetic field at equatorial latitudes leaves no place for doubts. As to the influence of mean values of the B,-component on the geomagnetic field intensity at low-latitudes, this problem remains open to discussion. According to the theoretical concepts of Lyatsky and Maltsev (1975), the field variations at low and middle latitudes may be caused by a stationary magnetospheric electric field and a threedimensional current system associated with it; this system being composed of a downward fieldaligned current at the morning boundary of the polar cap and an upward field-aligned current at its evening boundary. This system of field-aligned currents produces a current system in the ionosphere which resembles the S, current system characterized by the eastward current in low-latitudes. The southward (northward) turning of B, produced the growth (attenuation) of field-aligned currents and consequently, the growth (attenuation) of the eastward current at low and equatorial latitudes. Several attempts have been made to fmd a direct relationship between the intensity of the S, variation and the value of the B,-component of the IMF. Pate1 and Rastogi (1974), Rastogi and Pate1 (1975) and Rastogi (1977) have cited several cases when the increase of B, > 0 was accompanied by a sharp decrease of the equatorial electrojet intensity at the magnetic Equator. Mishin et al. (1975) concluded that the full current value in the S, vortex has a tendency to increase with an increase of B. positive values both in the northern and in the southern hemispheres. This conclusion is valid for mean hour current values as well as for the mean diurnal ones. Kane (1975) has investigated the dependence of the H-component maximal value at the Equator on the mean diurnal value of the B,-component. The contribution of extra-ionospheric sources has been eliminated. The results obtained showed that for negative values of B, the intensity of the equatorial electrojet tends to decrease. This is valid both for the quiet and disturbed days. The decrease of S,variations intensity at the geomagnetic Equator on moderately disturbed days, has also been noted by Onwumechili et al. (1973). The contribution of DP-2 disturbances was not always eliminated in the above-mentioned papers. To do so it is necessary to average the equatorial

ad

Y.

I.

F~uxmm

field for an interval much greater than the characteristic time of a DP-2 duration. Such field averaging is done below for a 3 h interval. The IMF measurements from Explorer 28 have been used, as well as the data for the geomagnetic field from the observatories Trivandrum (inclination - 1”) and Alibag (inclination 25“) for the periods from July to November 1965 and from July to November 1966. To eliminate the influence of extra-ionospheric sources (DCF and DR), the difference between the H-components, at these observatories has been considered. The mean daerence I&__+,_ was compared with the k?* mean value in a 3 h interval. Taking into account that the electric field connected with the IMF B,-component variation penetrates most effectively to the Equator during the forenoon and near-noon hours, we have chosen the interval 0508 U.T. which corresponds to 1000-1300 L.T. at the Trivandrum and Alibag observatories. Figure 5a shows the dependence of Em_= on E for the days when AE < 200~. 115 such cases have been considered. The vertical lines show the mean sm errors. The points correspond to isolated AHm_= in a 3 h interval since the mean value was calculated for n 2 5. The big values of- the mean square error show a big distribution of AHm-values. However, one may notice a tendency to an increase of the equatorial electrojet mean intensity with an increase of the B,-component mean value. When Ez <-2-y, the above tendency is sharply broken.

z&.& .

FIG. VALUE

Y

80 t

5.

a. THE DEPEND OF THE

ia)

WCROFTHR3hMEAN~

EQUATORIAL

ELEc’mo~oNTHE3h

B,-

1000-1300L.T. FROM JULY TD NowtqRR 1965 AND1966 (FOR AE < 200.~). h. Ike SAMEAs IN hG. 5a FOR THE BmRvm wrm Al3 2s 2007.

comm

VALUE

FOR

707

Interplanctarymagneticfkldandauroraldynamk

the same time the decrease of B, leads to the rise of a disturbance of the DP-2 type. So, there should probably be a direct relationship between the auroral oval dynamics and the disturbances of the DP-2 type. To investigate this relationship, the all-sky camera data have been used, as well as the magnetograms of polar cap and equatorial stations for the winter months of the IGY period. Fourteen 6 h intervals have been considered altogether. The comparison has been carried out between the variations of equatorial boundaries of aurorae and of geomagnetic field fluctuations in the polar cap and at the Equator in the interval ONlO-2000L.T. When dete rmining the latitude of the equatorial boundary of the oval, the height of aurorae was assumed to be 15Okm in the day sector and 120km in the night sector (Starkov, 1968; Khorosheva and Emelyanenko, 1969). The variations of the equatorial boundary of aurorae at Pyramida (080&1300L.T.) and the fluctuations of the magnetic field at the polar cap station Thule on 19 December 1958 are shown in Fig. 6a. The corrected geomagnetic latitude is plotted along the ordinate, the horizontal arrow shows the position of the zenith of the station. Two

Figure 5b represents the dependence of E,, on Bz for the days when AB > 200~. 48 such days have been considered altogether. The notations are the same as - in Fig. Sa. This sample also shows an increase of AI&_& when B, increases. However, in this case this tendency is not infringed when Bz < -27 and it is only when Bz I -6~ that three points disturb this tendency. So it can be seen that there exists a tendency (both for the quiet and disturbed period) for an increase (decrease) of the 3 h mean intensity of the equatorial electrojet with an increase (decrease) of the B,-component mean values. The result obtained probably means that the electric field tends to be smaller at the Equator when the B,component is directed southward, than when the B,-component is directed northwards. Such a dependency is quite opposite to that shown by Lyatsky and Maltsev (1975). THEBELATIONSElPBElWBEN THBvABlATloNoF THE AUROBAE BQUA'NBBIALBOUNDABYANDTEE

DISITJRBANCEOF DP-2 TYPE

The decrease of the B,-component leads to a shift of the auroral oval towards lower latitudes. At (a)

lb)

Ar FIG. 6a. ‘DIE RFLATI~N~H~P BJSWEBN THE PYRAMDA ‘~I’ULE.

AND

b. ‘l’mr RFLATIONSHU’

-A EQUATORIAL AUROEUL

ORSEEWATIONS ANJJ

&ELYRuSIGN

STATION BOUNDARY

BANGUI. SHJFT &’

THE

THE

OBSERVATIONS AND

JXXIAToRLtL

GJ5OWGNETIC

BKTWEEN c.

Y

THE

AUROFWL

FIELD

EQUATORIAL AND

THE

RELATIONSHIP

THE INTENWW CAP

AT

AUFtORAL

BEIWEDI

THE

VARIATIONS POLAR

BOUNDARY

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BOUNDARY

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substorms had taken place during the considered period; the onset of their expansive phase is shown by a vertical dashed line. It ensues from the data given in Fig. 6a that each variation of the equatorial aurora1 boundary has its corresponding variation in the geomagnetic field. The disturbance maximum in the geomagnetic field corresponds to the maximal shift of the equatorial boundary. Figure 6b represents the variations of the equatorial auroral boundary at the stations Pyramida (0800-1400L.T.) and Cape Chelyuskin (1500-2000 L.T.) as well as the If-component of the magnetic field at the equatorial station Bangui on 12 December 1957. The notations are the same as in Fig. 6a. The variations of the equatorial boundary of the aurorae in the noon and in the evening sectors occur synchronously and correspond to the fluctuations of the geomagnetic field H-component at the Equator. Besides the short-period type DP-2 variations, a long-period variation of the aurora1 boundary (0800-1100 U.T.) occurring both in the day and evening sectors, revealed in Fig. 6b. The dependence of the change of the equatorial aurora1 boundary position in the day sector A+‘(km) on the horizontal vector value of the polar AT,= disturbance in the cap [(AH)* + (AD)*]l” is shown in Fig. 6c. The straight line connecting the beginning and the end of the fluctuation was taken as a reference level for A&’ and AT. The scaling was carried out using the maximum fluctuation. The increase of A+’ corresponds to the shift of the equatorial boundary of the aurorae to lower latitudes. The periods with substorms, as well as without them, were considered. The mean A&’ values calculated for the periods without substorms are given in Fig. 6c. The vertical segments show the mean square error in A4’, the horizontal ones the error in AT. The relationship between A+’ and AT is linear and by the equation A4’(km) = is expressed (4.0* l.O)AT,(r). Substituting into it AT, = -8.3AB, as found before, we obtain the dependence between the value of the shift of the equatorial aurora1 boundary and that of the B,-component fluctuation: A+’ (km) = -(33 f lO)AB,(y). Positive values of A+’ correspond to the shift of the equatorial boundary towards the Equator. The expressions obtained are valid only in a limited interval of AT, and AB, change since A+’ cannot change within a wide range. The points in Fig. 6c indicate the cases when the variations of the equatorial boundary of aurorae occurred after the onset of the expansive phase of

and Y. 1. Runsrnm

the s&storm on the nightside. It is seen that the shift value of the equatorial aurora1 boundary during the fluctuations is independent of whether the fluctuation occurs during the quiet period or during the substorm, but depends only on the size of the change of the IMP B,-component. The independence of the occurrence of DP-2 disturbances on the occurrence of substorms has been described by Nishida (1971b). The variations of the equatorial boundary of the auroral region was used as initial data. The shift of the auroral boundary towards the Equator corresponds to positive values of A4’. This method of scaling the values of the aurora1 boundary position fluctuations seems quite reasonable since it is known (Starkov and Feldstein, 1971; Akasofu, 1972a, 1972b; Vorobjev et al., 1975; Vorobjev et al., 1976; Horwits and Akasofu, 1977) that in quiet conditions (B, > 0) the equatorial boundary of aurorae in the day sector is located at higher latitudes than when B, CO. The coincidence of variations of the equatorial auroral boundary with positive fluctuations of the geomagnetic field H-component at the Equator and with field fluctuations in the polar cap testify that the reference level for comparing B,-components fluctuations with DP-2 disturbances has been chosen correctly. The direct relationship obtained between the variations of the equatorial boundary of aurorae and DP-2 shows that the B,-component of the IMF influences both of these phenomena simultaneously. DP-2 disturbances reading the Equator prove that the electric field connected with shortperiod variations of B,, penetrates to low-latitudes. THE DYNAMICS

OF AURORAE DI!SCREl-E FORMS EVENINGPARTSOFTHE INTHENOONAND AURORAL OVAL

Vorobjev et al. (1976) have discovered that the behavior of discrete auroral forms in the noon and evening sector have ditIerent morphology. In the period of the oval’s shift towards lower latitudes the discrete auroral forms move polewards in the noon sector and equatorwards in the evening sector. These results show that, in the afternoon hours, a region in which the discrete forms of aurorae change the direction of their motion exists, i.e. the discrete forms move to the pole when they are to the West of this region and to the Equator when they are to the East of it. Since the dynamics of the discrete forms shows the changes of the magnetosphere’s structure, as well as the processes taking place in it, below we investigate in detail their motions and their relative positions.

Interplanetarymagnetic fieldand amoral dynamics

709

move poleward. It follows from Fig. 7 that the latitude where the short-lived arcs occur, follows the latitudinal movement of the long-lived arc. In their turn the latitude variations of the long-lived arcs determine the variation of the equatorial auroral boundary in the noon sector. It is curious that the two diiIerent types of arc, less than 200 km apart, move in opposite latitude directions (e.g. at 0950 and at 1045 U.T.). Homogeneous arcs moving equatorward (poleward) with an increase (decrease) in the intensity of DP-2 type disturbance are characteristic of the evening sector. Figure 8 (on the left hand side) shows the dynamics of discrete auroral forms at stations Pyramida, Heiss, Wieze on 16 December 1958. The corrected geomagnetic latitude is plotted along the ordinate. Horizontal arrows show the zenith of the station. Two substorms have occurred during the period in consideration; the onset of their expansive phase are shown by vertical dashed lines. Short-lived rayed arcs moving poleward are observed at Pyramida which is situated in the noon sector of the aurorai oval (1200-1400M.L.T.). Rayed arcs characteristic of the noon sector are first observed at Heiss (1500-1700M.L.T.) but after -1100 U.T. the arcs of the evening sector appear and they are easily seen at the station Wieze (1700-1900 M.L.T.). The evening sector is characterized by homogeneous arcs moving equatorward (poleward) with an increase (decrease) of DP-2 disturbance intensity. The onset of the expansive phase of the substorm at 1020 U.T. was accompanied with the appearance of a difhrse luminescense at Wieze; at 1130 U.T. the diffuse luminescence and the rays appeared at Heiss and then at Pyramida. The appearance of a diffuse luminescence at the beginning of the expansive phase of the substorm or at DP-2 maximum could often be observed in the noon sector. When consider73. ing the left hand part of Fig. 8, one may see that the place where the meridional motion of the discrete 71 . forms changes direction is clearly seen at Wieze at about 1106 U.T. For more detailed analysis of the 691 I I I I I relative positions of the arcs in the noon and evening parts of the auroral oval and of their dynamics, the 09 II 13 right hand side of Fig. 8 shows the projection of the UT arcs onto the Earth’s surface for 4 instants of time. PIG. 7. ?-HEPOSlTION OF THE LOWER AURORAL BOUNDARY At the left hand part of Fig. 8 these time instants IN THE NOON SECTOR OF THE AURORAL OVAL FROM ALLare marked by vertical arrows at the upper edge of SKY CAhlERA DATA AT bRAMlDA ON 19 DECEMBER the figure. The circles show the ranges of vision of 1957. the cameras. The coordinates are corrected One may see the presence of two types of auroras: a geomagnetic latitude and local geomagnetic time long-lived equatorial arc and a short-lived one drifting to according to Gustafsson (1974). The figures in Fig. the pole. All-sky camera data for the winter months of 1957-1959 have been used. The time interval 1000-1800 M.L.T. (1000-1400 M.L.T. of the noon and 1400-1800 M.L.T. of the evening sector of the amoral oval) has been investigated. When studying the regions in which the meridional motion of discrete auroral forms undergoes a change, a longitudinal chain of all-sky cameras with overlapping ranges of vision have been used. The radius of the ranges of vision of the camera was confined to 500 km. The aurorai height when projected onto the Earth’s surface was taken to be 150 km in the noon sector and 120 km in the evening sector. Figure 7 shows the position of noon aurorae from Pyramida data of 19 December 1957. The corrected geomagnetic latitude of the lower boundary of aurorae is plotted along the ordinate. The horizontal dash line corresponds to the zenith of the station. The aurorae of two types may be observed at the latitudes of the station. A longlived rayed arc corresponds to the fhst type. It lasted through out the whole period from 0915 to 1400 U.T. Short-lived (- 1Omin) rayed arcs correspond to the second type. Long-lived arcs are located at the equatorial boundary of the noon sector of the auroral oval. They correlate (as it has been shown by Akasofu, 1972a,b; Vorobjev et al., 1975; Vorobjev et al., 1976) with the increase of the magnetic activity and according to the above data with DP-2 disturbances. Short-lived arcs always occur polewards from the long-lived arcS and

I

710

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DYNAMICS OFDISCRET% FORMS OF

AURORAE

FROM

AIL-SKY

CAMERA6

DATA

AT hRAMlDA,

Wmzu ON 16 DWER 1958. The right hand side of the figure shows the projection of aurorsl arcs onto the Earth surface for four instants of time marked on the left upper edge of the figure by arrows. The coordinate-the corrected geomagnetic latitude-local geomagnetic time, the vertical dash lines show the begimkg of the expansive phase of the substorm. HEIS,

8 show the same arcs. The arrows show the direction of motion, the lines with vertical dashes correspond to rayed arcs, those without dashes to the homogeneous arcs. The first three cases concern the creation phase of the substorm, the fourth case the expansive phase. Three rayed arcs are seen at Pyramida at 1109 U.T. They are moving poleward and arc (3) is going to disappear in a minute whereas arc (1) has only just appeared. It is only the noon arc (2) moving poleward and the evening arc (4) that are seen at Heiss. The eastern edge of arc (4) observed at Wieze is moving equatorward while the western edge is practically stationary. Another evening arc (S)isseentotheEastofWieze.Soitcanbeseen

that the noon and the evening arcs are moving towards each other. Five minutes later, at 1114 U.T. the pattern changes considerably. The noon arcs (2,3) disappear and arc (1) has moved forward to the pole as well as to the East and occupied the place of arc (2). A new rayed arc (7) was formed instead of arc (1) at the West and to the South of the Pyramida’s zenith. Another arc (6) joined at the East, to the evening arcs which existed before and arc (4) began to move along its full length equatorward. At 1117 U.T. the noon arc (7) becomes shorter and occupies the place polewards of the evening arc (4). A newly formed rayed arc (8) is overlapped by arc (4) in the longitudinal interval -15” and they are moving in opposite

711

Interplaoctary magnetic field and auroral dynamics

ved, throughout the whole period, at station Heiss. The evening type aurorae prevailed at Chelyuskin and the aurorae of both types were observed at Wieze. In the time interval under consideration one substorm occurred, the expansive phase of which began at 1035 U.T., which is shown with a dashed line on the left hand side of the figure. The interesting peculiarity of the above case is that the aurorae disappeared at Wieze and Chelyuskin not long before the beginning of the expansive phase of the substorm and moved polewards at Heiss. It seems that the substorm set on when the &-component of the &IF was directed northwards. It has been shown above that the shifts of the equatorial boundary of aurorae in the noon sector of the auroral oval correlate well with the DP-2 disturbance and consequently, with Ih@ El,component variations. It follows from Fig. 9 that the equatorial boundary of aurorae at Heiss began moving polewards not long before 1025 U.T. and it was about that time that the B,-component of the

directions. At the East the earlier motionless arcs (5,6) start moving to the Equator. At 1130U.T., several minutes after the beginning of the expansive phase of the substorm, rayed arcs moving poleward are observed in the noon sector of the oval and those moving equatorward in the evening sector. A discontinuous region f&xl with diffuse luminescence, occurs between these two types of aurorae. Such a discontinuity occurs rather often after the beginning of the expansive phase or during the maximum intensity of the DP-2 type disturbance. On the left hand side of Fig. 9 the dynamics of discrete forms of aurorae at stations Heiss, Wieze, Chelyuskin on 18 January 1959 are shown. The notations are the same as in Fig. 8. The aurorae were faint and indistinct against the background of the bright sky at Wieze in the interval 09001000 U.T. At Chelyuskin, the all-sky camera began working after 1000 U.T. As one may see from Fig. 9, the aurorae of the noon sector type were obser-

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ISSHOWN. ‘Ihe projection of aurorsl arcs on to the Earth’s surface for four instants of time pointed out by the arrowsontheupperlcftedgeofthefi~isshowntotheright. HEISS,Wm,

712

B. V.

V. G.

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IMF turned northward. It was just after that moment that the aurorae disappeared at Wieze and Chelyuskin. On the right hand side of Fig. 9 the projection of auroral arcs onto the Earth% surface for four concrete time instants before the beghming of the expansive phase of the substorm are shown. At 1010 U.T. three rayed arcs are seen at Heiss, Wieze and Chelyuskin, characteristic of the noon sector of the oval and one arc of the evening type. All three rayed arcs moved poleward but at considerably different velocities. The arcs (1,2) have velocities of the order of 400 ms-’ and the rayed arc situated to the pole of them is almost stationary. At 1014 U.T. the arcs (1,2) are located polewards and considerably eastwards of the position at 1010 U.T. The arc (1) is seen at Heiss for the last minute and then it is seen only at Wieze. At this instant its western part is still moving to the pole while the eastern edge starts moving equatorward in the same direction as the newly formed evening arc (5). Five minutes later (at 1019U.T.) the arcs (1,2) disappear and arc (3) seems to take their place. Westwards and equator-wards of the station Heiss zenith a new rayed arc (4) appears. The evening arc (5) which occurred eastwards and poleward of the Chelyuskin zenith shifts equatorwards and westwards towards arc (3). At 1023 U.T.

and Y. I. F&osm~~

arcs (3) and (5) are at a minimal distance from each other but not touching, after which arc (3) and then (4) disappears. New arcs appear in the West and in the East. So it can be seen that the behavior of discrete auroral forms on 18 January 1959 in the noon and evening sectors is mainly coincident with their behavior on 16 December 1958. The noon arcs occur westward and equatorward of already existing arcs and then propagate to the East and move to the pole. The evening arcs occur eastward and poleward of those already existing and then propagate to the West and move to the Equator. On the left hand side of Fig. 10 the all-sky camera data at stations Heiss, Wieze, Chelyuskin of 20 December 1958 are shown. The notations are the same as in Fig. 8. The noon-type aurorae prevail at Heiss and those of the evening type at Chelyuskin; the aurorae typical of the noon as well as of the evening sectors of the oval are observed at the station Wieze. On this day the synchronous character of equatorial boundary variations is distinctly seen at all stations. One substorm occurred during the time interval considered. Its expansive phase began at 0845 U.T. As in Fig. 7 the short-lived rayed arcs appear poleward of the long-lived noon arc (1) and its variations coincide with those of the evening arc. On the right hand side of Fig. 10 the projections of

69 06

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Interplanetarymagnetic field and auroral dynamics auroral arcs onto the Earth’s surface are shown for three concrete cases when the long-lived noon arc (1) existed. Something like stabilisation was observed at 0706 U.T. A long-lived noon arc (1) moved poleward as well as the western part of the short-lived arc (2). The eastern part of arc (2) and the evening arc (3) were moving towards the Equator. This process lasted for 5 min. At 0711 U.T. the noon arc moved westward and became a typical arc of the noon sector of the auroral oval. The long-lived arc (1) starts moving equatorward synchronously with the evening arcs. Three minutes later (at 0714U.T.) the noon arc (2) moved still further poleward and the long-lived and the evening arcs (1,3) continued their motion to the Equator. Further behavior of discrete auroral forms on 20 December 1958 is represented in Fig. 11 in the projection onto the Earth’s surface. At 0731 U.T. the long-lived arc (1) joins the evening arc (3) and this unified arc moves eastward. A new rayed arc (4) is formed in the West and an evening arc (5) appears in the East. All arcs (besides 5) shift poleward. As one may see from the left hand side of Fig. 10, this instance corresponds to the movement of the equatorial boundaries of the aurorae to the pole, which may be associated with the B,component turning to the North. The same situation is observed at 0748 U.T. By that instant of time the arc (4) disappeared and a new noon arc (6)

/p‘?--.0731

U.T

713

arose. The joint arc (3 + 1) and the evening one shifted considerably westward. At 07.54U.T. the evening arc (5) is broken into two parts. Its eastern part moves equatorward during 4min and the western edge shifts poleward together with the other arcs (3+ 1 and 6). On the whole, the equatorial boundaries of aurorae also move to the pole at all stations. A rapid motion of the equatorial boundary to lower latitudes begins at about 0800 U.T. and continues throughout the whole creation phase of the substorm. Shortly after the begirming of the expansive phase a short increase of the diffuse luminescence is observed at Heiss. After the beginning of the expansive phase, the equatorial boundary of the aurorae starts moving quickly poleward at all stations. Such a shift of the equatorial boundary to the Equator and to the pole may be connected, as it has been mentioned above, with the B,-component turning to the South and to the North, respectively. Figure 11 shows (to the right) the projection of discrete auroral forms onto the Earth’s surface for three instants of time on 20 December 1958 when the equatorial aurora1 boundary was shifting towards the high-latitudes. At 0916 U.T. three rayed arcs (1,2,3) which are moving polewards and one evening arc moving to the Equator are seen at Heiss, Wiese, Chelyuskin. Three minutes later (at 0919 U.T.) arc (1) disappears and arc (2) occupies “?,,~

FIG. 11. THE CONTINUATION OFFIG. 10 FOR20 DECEMBER

0916

1958.

UT.

714

B. V. RBzHHNov, V. G.

VORORJEV

its place. A new arc (4) appears in the West. After another 3 min, at 0922 U.T. arc (2) propagates eastward, arc (3) vanishes and another new noon arc (5) appears in the West. In this way, these cases also show the successive eastward and poleward shift of noon arcs occurring westward and equatorward of the previous ones. After having moved to the pole by -1” the arcs vanish. At the same time the evening arc moves, as a rule, to the Equator. The top of Fig. 12 represents the projection of auroral arcs onto the equatorial plane of the magnetosphere on 16 December 1958 for two instants of time, which had been used in Fig. 8. The Fairfield and Mead magnetosphere model (1975) for I& a2 had been used for the projection. As follows from Fig. 12, the regions of particle precipitations responsible for the occurrence of the noon arcs (7 and 1) are located near the noon boundary of the magnetosphere at 1114 U.T. but at closed field lines and they are projected into the magnetospause in the near-noon hours. The regions of precipitations connected with the evening arcs (4,5,6) are situated deep in the magnetosphere and are probably oxmected with the plasma sheet of 1114 u.7:

and

Y. I. m

the tail. AT 1117 U.T. the precipitation region associated with arc (7) has moved to higher latitudes and towards the magnetosphere boundary in the equatorial plane and a new noon arc (8) is formed instead. The precipitation regions associated with the evening arcs become strongly deformed when projected onto the equatorial plane of the magnetosphere and their shift towards the noon sector of the magnetosphere can be observed. At the bottom of Fig. 12 the projections of the arcs onto the equatorial plane of the magnetosphere are represented for two time instants on 18 January 1959 (see Fig. 9). The successive shifts of the regions cormected with the noon arcs are seen there. They are moving from the noon sector of the magnetosphere towards the tail. The precipitation regions connected with the evening arcs become deformed when projected onto the equatorial plane; they move deep into the magnetosphere and towards its noon part. The noon and the evening arcs (Fig. 8 and 9) overlapped considerably along the longitude at 1117 U.T. and at 1019 U.T. As one may see from Fig. 12, the precipitation region in the equatorial

16 I2 58

II I7 U.T.

18.01.59

‘X

‘X

FIG. 12. THE PROJEXX-ION ON To

THE

RQUATORIAL

IO 23 U.T:

OF PLANE

DISCRETE5 OF THE

FORMS

OF

AURORAE

MAGNeTospHERe

18 JANUARY 1959

FOR

ALONG

THE

GEOhfAGNETIC

FIRLD

16 DEXXMB= 1958 (ABOVE)

(BELOW).

AND

LINTS FOR

Intcrplanc~

magaetic field and auroraI dynamics

plane of the magnetosphere associated with such overlapping arcs are set apart considerably at these instants of time. Though the precipitation regions associated with the noon short-lived arcs are located in the equatorial plane at closed magnetic field lines (according to Fairfield and Meads model, 1975), their drift to the pole indicates that they are located in the polar cap, i.e. to the latitudes of open magnetic field lines projection. It seems that the averaged model we use satisfies the conditions of weak disturbance when the cusp is projected on to the geomagnetic latitude +‘- 78”. It is known that during the disturbance period the cusp shifts up to 4’-72”, which requires an essentially ditferent model, particularly for the dayside magnetosphere. As it has been shown above, two types of auroral arcs may co-exist in the noon sector of the auroral oval. When the intensity of DP-2 disturbances increases, i.e. when the convective electric field increases, the two types of arc start moving in opposite latitudinal directions and the distance between them is only several dozens of kilometers. The change of direction of the meridional motion of the discrete forms may also be observed at the “joint” point of the noon and evening arcs. This region represents a system of overlapping along the longitude, noon and evening arcs moving towards each other or in the same direction during the increase (decrease) of DP-2 intensity. Thus, the motion of the noon or the evening arcs (and their analogous noon long-lived arcs) does not seem to be associated with the convective electric fields. As the direction of short-lived arc’s motion coincides with the direction of convection obtained from satellite observations, one may assume that the motion of these arcs is just caused by the convective electric field. The variations of the equatorial boundary and consequently of the longlived arc in the noon sector and of the equatorial arc in the evening sector correlates well with DP-2 disturbances, the latter being controlled by the B.-component and consequently by the convective electric field. In spite of this, the shift of the equatorial boundary to low-latitudes with the B,component directed southwards is not caused directly by the electric field. In reality when B, is directed southward erosion of the day side of the magnetosphere (Burch, 1973) and the diminishing of the last closed field line latitude occurs. The precipitation regions associated with the discrete forms of aurorae and situated at the boundary between the regions with different characteristic

715

geomagnetic field lines, are probably “following” the location of the boundary. The longitudinal shift of the arcs does not seem to be associated with the convective motion since the velocity of such a shift is 10 km s-l and more. This motion may be caused by the disturbing factor responsible for the precipitation of the particles. CONCXIJSION Summing up the results of the present paper, one may draw the following conchrsions: (1) The negative pulse in the B,-component of the lMF causes the current from the nightside to the dayside in the polar cap and the eastward current at the Equator, which agrees with the DP-2 current system. (2) The positive pulse in the B.-component gives rise to a current from noon to night in the polar cap and the westward current at the Equator, which corresponds to the reverse DP-2 current system. (3) In the interval 0800-1400L.T. the fluctuations of the B,-component always provoke fluctuations of the magnetic field’s W-component at the magnetic Equator, the relationship between them being expressed in the form: AH = -4AB,. (4) At the Equator, in the interval 14001700 L.T., two types of geomagnetic field’s reaction to B,-component fluctuations exist. In the first type the association is also determined by the relationship AH = -4AB, and in the second type the relationship could not be revealed. (5) There is a tendency for an increase (decrease) in the 3 h mean intensity of the equatorial electrojet with an increase (decrease) of the B.component mean values. This tendency may be observed during the magneto-quiet as well as during the disturbed periods. (6) The equatorial amoral boundary’s variations in the interval 0800-2000 M.L.T. over a duration of 30-60 min are synchronous with the occurrence and the increase of the DP-2 current system. (7) The dependence of the equatorial auroral boundary position in the noon sector on the horizontal disturbance vector value in the polar cap is expressed by the relation

A&&m) = (4.0* l.O)AT(y). (8) The region in which the change of the direction of the auroral discrete form’s meridional motion occurs consists of a system of longitude-wise overlapping arcs of the noon and the evening types. The short-lived noon rayed arcs always occur in this region at lower latitudes than the homogeneous evening arcs.

716

B. V. REpIwov,

V. G.

VORORJEN

(9) During the periods of DP-2 disturbance intensity increase the short-lived noon arcs and the homogeneous evening arcs move towards each other. The noon arcs move polewards and the evening ones to the Equator. Near the DP-2 intensity maximum the noon and the evening arcs are often separated by the d&e luminescence region. (10) In the periods of DP-2 disturbance decrease the short-lived noon arcs and the homogeneous evening arcs move in the same poleward direction. (11) The long-lived noon arcs and the evening arcs move in the same direction. (12) As a rule the short-lived noon arcs shift, after their appearance, from the noon hours to the evening hours. The evening arcs move in the opposite direction from the evening hours to the noon hours. (13) When moving the opposite latitudinal direction the day and the evening arcs may merge and form one arc but most often the length of the overlapping part of the arcs diminishes and the noon arc shifts further polewards behind the evening arc. (14) Not all the arc motions in the noon and evening sector are entirely associated with the convective electric field. To all appearances, the motions of long-lived noon arcs at the equatorial boundary of the auroral oval are associated with the erosion of the magnetosphere. Acknowledgements-In conclusion we should lie to express acknowledgements to G. V. Starkov, V. B. Lyatsky, S. V. Leontiev for the useful discussion of the results. The IMF measurements on board the Explorer 28 have been carried out by F. Ness and D. Fairfield and obtained through the NSSDC.

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and Y. I. FeLDsTEIN

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