Local time asymmetry in the characteristics of Pc5 magnetic pulsations

Local time asymmetry in the characteristics of Pc5 magnetic pulsations

Planet. Printed Space Sci., Vol. 31, No. 4, pp. 459471, in Great Bntain CK132-0633/83/040459-13$03.00/O Pergamon Press Ltd. 1983 LOCAL TIME ASYMME...

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Planet. Printed

Space Sci., Vol. 31, No. 4, pp. 459471, in Great Bntain

CK132-0633/83/040459-13$03.00/O Pergamon Press Ltd.

1983

LOCAL TIME ASYMMETRY IN THE CHARACTERISTICS OF Pc5 MAGNETIC PULSATIONS KIYOHUMI

Onagawa

Magnetic

Observatory

YUMOTO

and Geophysical

and TAKAO SAITO Institute, Tohoku University,

Sendai 980, Japan

and TOHRU

Laboratory

SAKURAI

of Space Science, Tokai University,

Sibuya-Ku,

Tokyo

153, Japan

(Received infinalform 6 October 1982) Abstract-The wavecharacteristicsofPc5 magnetic pulsations are analyzed withdataofOGO-5, BEE-1 and -2 satellites. The toroidal modes (6B, > 6B,) of PCS pulsations are observed at a higher magnetic latitude in the dawnside outer magnetosphere. The compressional and poloidal modes (6B,, _ 6B, > 6B,) of Pc5 pulsations are mostly observed near the magnetic equator in the duskside outer magnetosphere. This L.T. asymmetry in the occurrence of dominant modes of PCS’S in space can be explained by the velocity shear instability (Yumoto and Saito, 1980) in the magnetospheric boundary layer, where Alfvtnic signals in the IMF medium are assumed to penetrate into the magnetospheric boundary layer along the Archimedean spiral. The asymmetrical behaviour of Pc5 pulsation activity on the ground across the noon meridian can be also explained by the ionospheric screening effect on the compressional Pc5 magnetic pulsations. The compressional modes with a large horizontal wave number in the duskside magnetosphere are expected to be suppressed on the ground throughout the ionosphere and atmosphere.

1. INTRODUCTION

The study of the wave characteristics of PCS magnetic pulsations has advanced substantially in the last decade by the new techniques of measurement such as the availability of extensive magnetometer chains (Samson et al., 1971; Samson and Rostoker, 1972) multisatellite observations (Hughes et al., 1977, 1978; Hughes, 1980) and aurora1 raders (Unwin and Knox, 1971; McDiarmid and McNamara, 1973 ; Walker et al., 1978, 1979). The coordinated works also led to strong theoretical advances (Inoue, 1973; Southwood, 1974; Chen and Hasegawa, 1974; Hughes, 1974; Hughes and Southwood, 1976a and b ; Tamao, 1978 ; Southwood and Hughes, 1978). Long-period magnetic pulsations in the daytime have been generally believed to be a resonance oscillation of a local magnetic field line based on the idea of a resonance coupling between a monochromatic surface wave excited at the magnetopause due to Kelvin-Helmholtz instability (and/ or other mechanism) and a shear Alfvtn wave in the magnetosphere. However, new questions are recently pointed out by several workers (Gupta, 1976; Yumoto and Sakurai, 1977 ; Lam and Rostoker, 1978). There are strong asymmetries in the level of Pc5 activity on the ground and the variance direction of Pc5 magnetic pulsations in the outer magnetosphere (see Figs. 5 and 6). There are no a priori reasons for an instability to be asymmetrically excited on the magnetopause, thus, 459

extended research is required to explain the asymmetric behaviors of Pc5 pulsations across local noon. The aim of the present paper is to clarify the asymmetric behaviour of the wave characteristics of PCS magnetic pulsations in the outer magnetosphere and attempt an interpretation ofthat with respect to the analysis of Pc5 pulsations from in situ observations. First we will describe the selected data and analysis procedure in Section 2. Wave characteristics of Pc5 magnetic pulsations in space will be analyzed to examine the dawn-dusk asymmetry ofdominant mode of PCS in Section 3. The observational results of PCS pulsations will be summarized and theoretically discussed in the final Section. 2. DATA AND ANALYSIS PROCEDURE

The data used in this study are from the University of California, Los Angeles, triaxial fluxgate magnetometers on OGO-5, ISEEand -2 satellites. Analog data of OGO-5 in the solarmagnetospheric coordinates had been provided by the National Space Science Data Center through the World Data Center-A. Analog data of BEE-1 and -2, which were represented by 4 s average data in the solarrecliptic coordinates, are the courtesy of Prof. C. T. Russell, UCLA. In order to obtain clear Pc5 wave characteristics observationally, we had to put the following four criteria on the data sampling.

460

K.

YLJM~T~ et al.

(1) The data were chosen where the OGO-5, ISEE-I and -2 satellites were in the dayside magnetosphere and in the outer flanksides (L 2 10) between -0400 and -2000 L.T. The data in the midnight sector were excluded in the present study. (2) In order to avoid contamination by Ps6 type Pi3 events (Saito, 1978; Saito and Yumoto, 198 1) having the irregular, burst-like appearance of Pi2 magnetic pulsations as seen on the ground were also excluded from consideration. (3) Pc5 events whose amplitudes are larger than - 3nT were selected to avoid artificial noise caused by data processing. (4) The discussions in this paper are restricted within the Pc5 pulsations during intervals of magnetic quiet days (0, d K, d 3_ and AE = 20 - 300~). Being restricted by the above-mentioned four severe conditions, we could select 40 events of OGO-5 during 21 months from March 1968 to November 1969 and 20 events of ISEE- and -2 during 4 months from November to December 1977 and from July to August 1978 as shown in Tables 1 and 2, respectively. The three-component analog magnetograms of OGO-5 were expanded and digitized with a semiautomatic digitizer, Graf/Pen, with a sampling period of 30 s. The digitized magnetograms in the SM coordinate system were then transferred into a magnetic field line coordinate system as illustrated in Fig. 1. In the magnetic field line coordinates (B,, D,H),the &-axis is taken parallel to the local geomagnetic field, the D-axis azimuthally westward perpendicular to the &-axis and the Z,,-axis, and the third H-axis, defined by eH = e, x e,,, nearly radial outward from the center of the Earth through the satellite near the magnetic equator. The transferred data in the (B,, D, H)-coordinates system were subjected to power spectral and polarization analyses. Figure 2 indicates an example of time-amplitude records of the selected Pc5 event in the SM coordinates obtained at OGO-5, whose orbit is shown to be outbound in the dawn sector for the 2 h from 0030 to

0230 U.T. on 16 January 1969. It is clearly recognized to be difficult to identify the dominant mode of the Pc5 magnetic variations in the SM coordinates, which can easily be examined by comparing the variance powers in the (B,, D,H)-coordinates. After the digitized magnetograms in the SM coordinates are transferred into the (B,, D,H)-coordinates, the trends of the transferred data are removed in the input time series by least mean squares fitting ofa second order polynomial as shown in Fig. 3A. Amplitudes of the Pc5 event are larger in D and H than in B,. The variance direction of the Pc5 event is nearly confined to the plane normal to the local magnetic field. Magnetic pulsation of this type can be categorized as a transverse mode. The polarization analysis was given by drawing disturbance hodographs in the H-D plane. The hodograph of the Pc5 event indicates the left-handed polarization. Power spectra of the Pc5 pulsation observed by OGO-5 were also obtained to examine the power ratios of (Pl/P1 +P,,) and (PHIPI), and the dominant period as illustrated in Fig. 3B, where P, = P, + P, and P,,, P,, and P, are power spectra in the B,-,D-,and Hdirections, respectively. Trajectories where the selected Pc5 pulsations were observed at OGO-5, ISEE- and -2 are drawn in L.T.L-value and magnetic latitude (@)-radial distance projections, respectively (Fig. 4). Trajectories where transverse modes (PI>>Pii) dominated are expressed by solid lines, and those with compressional modes (PI1 >>PJ by dotted lines. It is noteworthy that the occurrences of the transverse and the compressional Pc5 pulsations tend to have a maximum around 0700 L.T. and @ 2 20” and a peak around 1900 L.T. and There is no dawn-dusk @ 6 20”, respectively. asymmetry in the OGO-5 trajectory, which had been confirmed with the OGO-5 Ephemeris provided by the World Data Center-A. OGO-5 also made many trajectories at higher latitude in the dusk-side and near the magnetic equator in the dawn-side magnetosphere. Therefore, the dawn-dusk asymmetry of the Pc5 occurrences must be due to an excitation and/or a propagation mechanism of Pc5 pulsation in the magnetosphere. 3. WAVE CHARACTERISTICS

OF PCS IN THE

MAGNETOSPHERE

FIG. 1. A MAGNETIC FIELD COORDINATES SYSTEM (B,, D, w). &axis is parallel to the ambient magnetic field, D-axis azimuthally westward normal to the B. -ZSM plane, and Haxis defined by eH = e, x e,,, i.e., radial outward from the center of the Earth through the satellite position near the magnetic equator.

In the last decade, in situ observations have revealed the existence of various modes of PC pulsations in the magnetosphere (Barfield et al., 1972; Bossen et al., 1976a and b; Yumoto and Sakurai, 1977; Kokubun et al., 1977; Hughes et al., 1978 ; Kokubun, 1980). Statistical studies have shown that most of Pc3%5 pulsations can be classified into the two types, i.e.,

Local time asymmetry in the characteristics of Pc5 magnetic pulsations

UT

event

:1968)

0820-0940

MAY 8

0950-lYO0

MAY 18 MAY 21 UY 31 OCT.19 OCT.21

OCT.27 NOV. 1 NOV,'Il

NOV.29

1950-2020 1050-1740 1920-7950 1950-2030 0200-0300 0300-0400 1230-1305 1350-1420 1445-1530 1640-1715 1715-1800 1820-1950 0030-0230 0510-0530 05‘30-0610 1470-7510 1410-1510 2150-2220 2220-2250

0020-0100 0100-0120 otzo-0210 131o-1 340 FEZ.18 l350-1420 E'EB.26 0930-1020 2310-0000 ?&AR.3 0010-0050 xAR.4 APR.

1

-3ITK-ic --iiEE!?-MAY

23

MAY 25 NOV. 2

-+$wi400-0600

0404-0440 0450-0530 0125-020~ 0205-0245 0245-0325 0400-0430 0445-0515 0525-0600 1635-1715 1775-1740 1735-1805 TABLE

1. DETAILS PC5

OF SELECTED

PCS

IN MAGNETOSPHERE

PULSATIONS

(000-5,

OHSERVED BY OGO-5.

1968-1969)

461

462

K.

TABLE 2. SELECTED Pc5 MAGNETIC PULSATIONSOBSERVED ISEE- AND-2 SATELLITES

JAN. 16,1969

YUMOTO et al.

BY

transverse and compressional pulsations. Arthur et al. (1977) Arthur (1978) and Kokubun (1980) had also classified the Pc3-5 pulsations into the two types, azimuthally and radially polarized pulsations, according to the orientation of the wave ellipse. These azimuthal and radial Pc5 pulsations are correspondent to the transverse and compressional pulsations, respectively, which will be demonstrated in Fig. 7. The magnetic variance ratios (P,/P,+P,,) of Pc5 pulsations observed by ISEEand OGO-5 are indicated in Fig. 5 against L.T. and magnetic latitude, where P, and Pi, stand for variance powers normal and parallel to the local magnetic field, respectively. The transverse modes are represented by (PJP, + PI,) - 1.0, and the compressional modes by (Pi/Pi+P,,) - 0.0.A diamond and a circle express Pc5 pulsations observed by ISEE- and OGO-5, and solid and open ones stand for Pc5 pulsations observed at higher magnetic latitude (@ 2 20”) and near magnetic equator (@ < 20”), respectively. It is found that the transverse modes in the Pc5 frequency range dominate in the dawn sector, and the compressional modes are mostly observed in the dusk sector. It should also be worth mentioning that latitudinal structure ofhydromagnetic waves in space must be considered in interpretation of statistical characteristics such as the L.T. dependence of occurrence. The transverse modes are observed at

OGO-5

IL-1

LT & L

FIG. 2. TIME-AMPLITUDERECORDSOFPCS INTHESOLAR-MAGNETOSPHERICCOORDINATESWHILEOGO-~ORB~T WASOUTBOUNDINTHEDAWNSECTOR.

Local time asymmetry in the &aracteristics of Pc5 magnelic pufsations

FlG. 3. (A)

TfiE

TRANSFERRED The

(3)

@JWXX

PC5

EVENT FROM

Fir;. 2

IS REPRESENTED

IN THE MAGNETIC

FIELD i’OORDINATE.5.

homographin the H-D plane is also shown in the lower panel.

SPECTRA

OF THE &-

D-,

AND ff-COMPONENTS

higher magnetic iatitude,and the compression& modes near magnetic equator. If Pc5 pulsations are the fundamental mode of standing oscillations of the local field line, the transverse magnetic component should not be observed on the equatorial plane. Such a feature was first noted in the survey of PCS pulsations by Kokubun et 01. (1976 and 1977). They found that transverse PCS pulsations were not detected near the magnetic equator (@ 5 loo). Singer and Kivelson (1979) had further examined the latitudinal structure of Pc5 wave by using simultaneous magnetic field and ion ffux data. Hughes et al. (1978) demonsttated that the azimuthal wavenumber of the transverse wave (i.e., azimuthal mode)in thedawn sector issmaller than that of the compressional wave(i.e., radial mode) in the dusk sector, as shown by multi-satellite observations. Theory of eigen-oscillation predicts that hong-period pulsations with small azimuthal wave number correspond to the standing resonance wave, i.e., (u2 = V,?&,, and long-period pulsations with large azimuthal wavenumber to acompressional evanescent wave, e.g., u$ = (k: -Ck$ - ifi and kf = k&,i -k&,, cz 0 or cck$ (Yumoto and Saito, 1982). Local time dependence of the variance direction of Pc5 pulsations in the Jf-4 plane is illustrated as a function of the power ratio [I?p,i:P,) in Fig. 6. We find a strong L.T. asyn~metry in the variance direction of Pc5 in themagnetosphere, i.e., theazimuthal PC5 pulsations dominate in the pre-noon sector and the radial pulsations are mostly observed in the post-noon sector. The relation between the major axis of the perturbation

ARE OBTAINED

BY hffi%f

METHoI~

fields in the N-D plane and the power ratio of the normal component to the total variance power is represented in Fig. 7 against (P,/P,) vs (FL/P, + P,,). The transverse Pc5 pulsations in the pre-noon magnetosphere have a significant azimuthal component, and the compressional Pc5 pulsations in the post-noon magnetosphere have a large radial component as compared with the transverse puisations. The transverse and azimuthal PCS pulsations in the dawnside magnetosphere and the compressional and radial Pc_S in the duskside correspond to the toroidal and poloidal oscillations of a local field line (Dungey, 1954 ; Cummings et al., 1969; Yumoto et al., 1983), respectively. Polarization characteristics of Pc.5 pulsations observed in the magnetosphere are presented in Fig. 8, which are associated with the generation and propagation mechanism in space. Thelinear resonance theory, i.e., the coupling resonance between an evanescent compressional wave and a shear AlfvCn wave in the magnetosphere (Southwood, 1974; Chen and Hasegawa, 1974), predicts that the polarization reversal occurs in accordance both with the switch in propagation direction and the side of a maximum in amplitude. The left-handed and the right-handed polarizations of Pc5 events dominate in the dawnside and in the duskside magnetosphere, respe~tive~~r,which is agreement with the polarization reversal on the ground across the noon meridian (Kate and Utsumi, 1964 ; Samson, 1972). The polarization reversal at local noon and the largeinput ofenergy at Pc5 frequencies on

464

K.YuMOToeetat.

PROJECTION ,

OF OGO-5

(MARCH

1966

ORBITS

-

FOR

NOVEMBER

PC 5

45*

90”

PROJECTION

OF ISEEt NW-OEC.tsn

ORBITS

EVENTS

1969)

FOR

Pc5

EVENTS

IuL-huG.I978)

I

-45

-90' g

vs

R

FIG.4.TRAJECTORIESOFOGO-~ANU BEE-I, WHILEPC~PU~SATIO~SWERE~BSERVING,INL.T.-L-VALUEANDIN MAGNETIC LA~~oE-RADIAL DISTANCE PROJECTIONS. Solid and dotted fines express the transverse and the compressional PCS pulsations, respectively.

the dayside aurora1 oval (Lam and Rostoker, 1978) suggest that the energy ofdaytime PCS events originates from a Kelvin-Helmholtz type instability at the magnetopause (Chen and Hasegawa, 1974). However, the dawn-dusk asymmetry of the dominant modes, i.e., the toroidal Pc5 in the dawnside magnetosphere and the poloidal Pc5 in the duskside (cf. Figs. 5 and 6) cannot be explained by the previously published theories. The local time asymmetries in the variance direction and the sense of the Pc5 polarizations in the magnetosphere will be theoretically discussed later,

which may be strongly associated with the generation mechanism for long-period pulsations at the magnetospheric boundary. On the other hand, we could not find the clear tendency of the polarization reversal of the Pc5 pulsation on each side of a maximum in amplitude in the outer magnetosphere as shown in Fig. 3. This inconsistency with the classic prediction of the polarization near the resonance point may give a clue to the ground-satellite correlation of the long-period magnetic pulsations. The further study is needed to

Local time asymmetry

in the characteristics VARIANCE OF Pc5 IN THE

of Pc5 magnetic

465

pulsations

DIRECTION MAGNETOSPHERE

0

0,

,

OOh

,

,

,

l,

,

06

03

I

,

L*CAL

.:l!m>20’

0,0:lpI20” FIG.

5. LOCAL

TIME DEPENDENCE

OF THE VARIANCE

1:

ISEEOGO-5:

DIRECTION

T

, ,1;,

: +

0

.

0

(f’,/P,

+ P,,)

OF PC5

PULSATION

IN SPACE.

A diamond and a circle express PCS’S observed by [SEE-l and by OGO-5, respectively. Solid and open ones stand for Pc5 pulsations observed at higher magnetic latitude and near magnetic equator.

clarify the simultaneous polarization distribution by means of appropriately separated multi-satellite and to explain the observational result. Singer and Kivelson (1979) indicated that Pc5 periods in the dawnside of the plasma trough are generally consistent with the fundamental resonance period. The variations of Pc5 periods with L-shell in both the dawnside and the duskside magnetosphere are summarized in Fig. 9. It is clearly found that the rate of change of Pc5 period with L-values in the dawnside magnetosphere is approximately the same both with the observational results on the ground obtained by Samson and Rostoker (1972) and with the theoretical predictions of toroidal eigen-oscillations (Warner and Orr, 1979; Yumoto et al., 1983). Where the results of Samson and Rostoker (1972) discussed were obtained using data from 1969 to 1970, on the other hand, the in situ observations by OGO-5 were taken in 196%1969.

Although Kato and Saito (1964) had shown that the relative sunspot number is an important parameter in determing Pc5 period, all 3 years have very similar sunspot numbers and so a comparison is quite straightforward. It is interesting to note that the Pc5 pulsations in the duskside magnetosphere have larger compressional component (cf. Fig. 5) and a poor correlation between the observed periods and the Lvalues. The powers of Pc5 variations are shown in Fig. 10 against the L-values where Pc5 pulsations were obtained at OGO-5. The variance powers of Pc5 in the duskside are comparable to those in the dawnside. It is also interesting to note no relation between the variance power of PCS and the L-value in the dawnside and a positive correlation in the duskside. These results presented here thus suggest that energy sources such as the solar wind, or from instabilities at the magnetopause, are coupled into a toroidal eigen-mode

FIG. 6. LOCAL TIME DEPENDENCE OF VARIANCE DIRECTION OF PC5 IN THE H-D PLANE. Solid and open circles stand for Pc5 events which were observed by OGO-5 at the higher magneticlatitude near the magnetic equator, respectively.

and

K.

466

YUMOTO et al.

magnetic equator in the duskside outer magnetosphere. in the sense of Pc5 (2) The L.T. asymmetry polarization is also confirmed in the satellite data. (3) The ratio of change of Pc5 periods with L-values in the dawnside magnetosphere is consistent both with that on the ground (Samson and Rostoker, 1972) and with the theoretical prediction of toroidal eigen-oscillations (Warner and Orr, 1979). Pc5 pulsations which have a large compressional component in the duskside magnetosphere indicate a poor correlation between the observed periods and the L-values.

.

l . p;

.

Jo.

PO

P,o.5_.

. . l

;/ _ 0

.

.

l

r IO.70 .

.

-@

.

.

. I,I.Ir,II

0

1.0

es

Pd Pl+P,, FIG. 7. RELATION BETWEENTHE VARIANCE DIRECTION IN THE H-D PLANE AND THE TOTAL VARIANCE POWER.

From these results ofthe observational analyses, we can summarize that Pc5 pulsations sources from instabilities at the magnetopause or in the solar wind are coupled into a toroidal eigen-oscillation at local field line within the dawnside magnetosphere, and into a poloidal and compressional oscillation in the duskside magnetosphere. Recently, several workers (Gupta, 1976; Lam and Rostoker, 1978; Kokubun, 1980) pointed out that there is a strong local time asymmetry in the level of PC&5 activity with a preponderance of activity observed on the ground in the prenoon sector and the activity level being clearly suppressed in the postnoon sector. However, there is no a priori reason for a instability driven by the solar wind on the magnetopause to be preferentially excited on the prenoon flank as opposed to the postnoon flank, since L.T. asymmetry in the level of Pc5 activity observed by OGO-5 cannot be confirmed in the outer magnetosphere as shown in Fig. 10. Magnetic pulsations in the

configuration within the dawnside magnetosphere, and into a poloidal and compressional mode configuration within the duskside magnetosphere.

4. SUMMARY

AND DISCUSSIONS

In this paper satellite data are analyzed to clarify the wave characteristics of Pc5 pulsations, which give clues to the generation and propagation mechanisms in the magnetosphere. We can summarize the major results of these analyses as follows :

(1) The toroidal modes (SS, >> 6B,) ofPc5 pulsations are observed at a higher magnetic latitude in the dawnside outer (L 2 10) magnetosphere. The compressional and poloidal modes (6B,, - 6B, > 6B,) of Pc5 pulsations are mostly observed near

POLARIZATION DlSTRl8UTlON OF PCS 1N THE MAGNETOSPHERE

t -(,GO-5; . 6

ooh

02

04

06

L80 C’1

12

L FIG. 8. POLARIZATION A diamond double

and a circle are indicate

0

,,,,(r,,,,,,,,,,,,,,,I,

Pc5 pulsations

the left-handed.

14

10

18

20

22

TIME

DISTR~BIJTIONS OF Pc5 PULSATIONS IN SPACE. obtained

by ISEE-

the right-handed,

and OGO-5,

and the confused

respectively. polarizations,

A white,

black

respectively.

and

DUSK

DAWN 600-

Lu

467

in the characteristics of Pc5 magnetic pulsations

Local time asymmetry

, ,' ,'

. 0

500-

. .

. -.

. .

l .

.

. . .

.

a .

_

K

.

IOO--- ;Samsm&Rostoker T:-58+(3,d?37)L

:THISSTUDY T=80+25AL

oi,.,,,,,,

,,,,.

6

6

10

12

14

I

16

I,

I 10

18

I

I

I

12

I

I

14

I

I

16

I

16

LF1c.9. L-VALUE DEPENDENCES OF Pc5 PERIODS OBSERVED BY OGO-5 IN THE DAWNSIDE MAGNETOSPHERE,RESPECTIVELY.

postnoon magnetosphere have not yet been identified on the ground (Barfield and McPherron, 1972a and b ; Hedgecock, 1976). Hughes et al. (1978) demonstrated that an azimuthal wave number of transverse wave, i.e., azimuthal mode, in the dawn sector is smaller than that of the compressional wave, i.e., radial mode, in the dusk sector, as shown by multisatellite observations. The dawn-dusk asymmetry of Pc5 activity on the ground must be associated with the dawn-dusk asymmetry of the dominant-mode occurrence in the outer magnetosphere, i.e., the toroidal PCS in the dawnside and the

poloidal and compressional Pc5 in the duskside as shown in Figs. 5 and 6. On the other hand, theoretical considerations of the screening of magnetospheric signals by the atmosphere and ionosphere have indicated that the ground observations are limited to detecting magnetospheric signals of a horizontal scale length less than - 120 km at the ionosphere level (Hughes and Southwood, 1976a and b). The activity level of the compressional Pc5 pulsations with large horizontal wave number is expected to be suppressed on the ground. Although the horizontal wave numbers

SPECTRUM

PEAKS

IN THE DAWN-AND DAWN 2 c

-

I

I

B fz

-

le

AND THE DUSKSIDE

OF

DUSK-SIDE

Pc5

MAGNETOSPHERE DUSK

l

l

a

000.

8

l

-

102-

6

I 6

I

I I I, 10 12

I

14

I,

I

I,

16

16

6

I,

I,

I

6

IO

12

I,,

,

14

16

,

I 16

L -VALUE FIG. 10. L-VALUEDEPENDENCEOFTHEVARIANCEPOWEROF Pc5 PULSATIONSOBSERVEDBY OGO-5. Left- and right-panels indicate PCS’S in the dawnside and in the duskside magnetosphere, respectively.

468

K. YUMOTO et al.

of magnetic pulsations were not analyzed in this paper, the compressional Pc5 pulsations observed by OGO-5, ISEEand -2 satellites in the duskside outer magnetosphere are thought to have larger horizontal wave numbers and to be suppressed on the ground throughout the ionosphere and atmosphere. Candiates of PC&5 pulsations observed at the inner magnetosphere (L 6 8) are drift waves, which are associated with hot ring-current proton during intervals of high magnetic activity (Hasegawa, 1975; Tamao, 1978; McPherron, 1981; Southwood, 1981). In this paper, however, the discussions are restricted within the selected Pc5 pulsation which were observed at the outer magnetosphere (L ,> 8) and during intervals of magnetic quiet days (0, 6 K, 6 3_ and AE = 20 - 300nT). The selected Pc5 pulsations must be little due to the drift waves driven by the internal sources. Daytime ULF pulsations in the PC&5 ranges are believed to be driven by surface waves on the magnetopause where the bulk flow of solar wind has quite a strong velocity shear. Many workers (Aubry et al., 1971; Ledley, 1971; Wolfe and Kaufmen, 1975; Fairfield, 1979; Mauk et al., 1980; Hones et al., 1981) presented evidence that surface waves of KelvinHelmholtz type instabilities exist on the magnetopause. These observational works led to theories of the instability driven by the velocity shear on the magnetospheric boundary 1955; (Dungey, Southwood, 1968 ; McKenzie, 1970; Ong and Roderick, 1972; Southwood, 1979; Mekki, 1980; Lee et al., 1981; Walker, 1981 ; Melander and Parks, 1981). However, there is no explanation of the strong local time asymmetry in the variance direction of Pc5 pulsations in the magnetosphere, i.e., the azimuthal Pc5 events dominate in the pre-noon sector and the radial and compressional pulsations are mostly observed in the post-noon sector (see Figs. 5 and 6). A more theoretical consideration is therefore required to explain the L.T. asymmetry in the occurrence ofdominant modes of Pc5 pulsations in the magnetosphere. Yumoto and Saito (1980) recently presented expressions for the polarization ofthe HM-wave driven by the velocity shear in the magnetospheric boundary layer, where the complexity of the real physical system of the layer was simplifed in the paper and analytical study was restricted to the shear flow i.e.,

aInV, alnp,, ---->>-----ax

aX

c3lnB, ax’

The ambient magnetic field B,(x) is parallel to the z-axis and the bulk flow V,,(x) of the solar wind in the boundary layer is in the Y-axis. Thus, the x-axis and the y-z plane are nearly normal and parallel to the magnetopause, respectively. The perturbation field 6A

in the shear plasmas is modified by the velocity shear term (a&,/ax) and expressed as follows : 6A = 6A, +f (k, aV,/ax) 6A’,

(I)

where 6A, is the perturbation field in uniform plasmas. For the perturbed magnetic field in first order, the polarization in the x-y plane is given by (by/b,) =

(IJ~/v,)

+

i(a Vo,/ax)/Q s Re”,

(2)

where (tly/vX)=

i~(n*-~3(~2--~~~)(al~n/a~)k~i -(nz-c$)(v;+vf)k,(ahx~ax)l +(sr2-Wy)(!iY-W!?),

with

R = w-k, = PI(1 -t/W’

Voy, co; = V;k,Z, wf

*o&wL;- = f(1 + P,)V;(k;

+ k,2)

4k% ‘-(k;+k;)(l+/?$



PI = V,“/V% Vf = (YPo/P~ and Vii = B%wo,). The ratio of the transverse perturbed magnetic field is

component

to the total

(3)

lbi12/lb,o,ai12 = (I + lb,12/lb,12)- i> where lb,12/lbL(2 = li(dlnu,/dx)-k,(v,/v,)12k;2(1

+R2)-‘.

A locally-shifted frequency R was introduced. The frequencies of Alfven wave, the fast and slow magnetosonic waves were represented by wA and wt, respectively. Alfven and the sound speeds were also defined as V, and V,. They demonstrated that a Alfvenlike (a,) HM-wave (T 2 40 s) has sufficient gain to be observed as a HM-wave at the Aank-side boundary layer. The sense of polarization and the variance direction of the Alfvin-like (a,) wave are functions of the angle 4 = tan- ’ (kdk,) ofthe k-vector and the sign of the velocity shear (aV,,,/dx 2 0). When an Alfvenic perturbation, i.e., SA,(GV and 6B) I B, in a uniform plasma, propagates into the velocity shear plasma as shown in Fig. 11A, the first-ordered perturbation 6A is polarized by the velocity shear (aV,,/ax 3 0). The dominant polarizations of the Alfven-like perturbations for 4 = tan - ’ (kdk,) > 0 in the shear plasma are expected to be left-handed and toroidal (6A, >> 6A,) mode in the dawn-side (aV,,/ax < 0) and right-handed and poloidal (6A, >>6A,) mode in the dusk-side (i3F&,/ax > 0) boundary layer, respectively. They indicated that the Q,-wave shows a transverse mode with left-handed polarization in the dawnside and a compressional mode with right-handed polarization in the duskside boundary layer, whenever signals of

Local

time

asymmetry

@” FIO. 11. (A). Alfvinic handed

perturbations and

(6A,

6A,, >>&A,

in the characteristics

v

k,

(toroidal)

and

PERTURBATION

B,,,)

in the

PCS magnetic

are

dawnside

FIELD 6~

polartized and

by the

right-handed

469

pulsations

tan-%cx/ky)

Q =

SCHEMATICAL I

of

IN THE SHEAR PLASMAS.

velocity 6.4,

shear

(8,/3x)

>>

(polaidal)

6A,

and

become

left-

in the duskside

boundary Iayer for C#J= tan-’ (k,/k,) s- 0. (B). THEORETICAL PREDICTIONS OF LONG-PERIOD HYDROMAGNETIC WAVES IN THE VELOCITY SHEAR PLASMAS. Polarization and variance directions are functions of the sign of velocity shear (c?V,iSx) and the angle of the k-vector [+ = tan-’ (kJk,)).

modes (Denskat and Burlaga, 1977 ; Bavassano et al., 1981) in the interplanetary medium penetrate into the magnetospheric boundary layer along the Archimedean spiral, i.e., (p = tan-’ (kJk,) > 0 as shown in Fig. 11B. For the case of Cp< 0, the polarization characteristics become reverse in both the flankside boundary layers. This theoretical assumption (4 > 0) is reasonable, since the most Pc5 pulsations observed by OGO-5 occur when the IMF direction nearly coincides with that of the Parker spiral as shown in Fig. 12. I-Iourly averages of the direction of the IMF, while PCS pulsations weremeasured by OGO-5,refer to the interplanetary medium data book (King, 1977). Hughes et al. (1978) showed phase differences between pairs of geostationary satellites (ATS-6, SMS1 and -2) for Pc3-5 magnetic pulsations. In the PC+5 ranges the sign of the azimuthal wave number changes across local noon and is consistent with phase propagation away from local noon toward the dawn and dusk flanks. By means of data from an East-West array of three ground based magnetometers separated from each other by about 5” of longitude at Alfvtnic

FIG.

12.

IMF

DIRECTION

PIJLSATIONS

Relation

(HOURLY

WERE OBSEKVED

AVERAGES)

WHILE

PCS

BY OGO-5.

between IMF direction and Pc5 polarization presented.

is also

K. YUMOTOet al.

470

geomagnetic latitude -67”N, Olson and Rostoker (1978) also demonstrated the phase propagation of Pc4.5 pulsations away from 1100 L.T. toward the flank sides. Although future observational researches in the outer magnetosphere are needed to clarify the k vector of Pc5 pulsations in the H-D plane, i.e., both azimuthal and radial wave numbers across the noon meridian, andexamine the occurrenceofthe HM-wavesdriven by the velocity shear instability (Yumoto and Saito, 1980) by means ofmulti-satelliteobservations, the theoretical prediction of the polarization characteristics for (d > 0) is in agreement with the observational results (1) and (2) in the present paper. It is concluded that the L.T. asymmetry in the occurrence ofdominant modes of Pc5 pulsations can be explained by the Alfv~n-like (0,) wave driven by the velocity shear instability (Yumoto and Saito, 1980), when signals of Alfvknic waves in the interplanetary medium (Bavassano et al., 1981) penetrate into the magnetospheric boundary along the Parker spiral direction. Acknowledgements--We would like to express our sincere thanks to Prof. T.Tamao, Dr. S. Kokubun and Prof. H. Oyafor their valuablediscussionsandcriticismson our work. We wish also to thank Prof. A. Nishida for providing the magnetic field data in space and discussions. The OGO-5 UCLA Triaxial fluxgate magnetometer data of Drs P. J. Coleman, Jr., T. A. Farley, C. T. Russell (UCLA), D. L. Judge (USC) had been provided by the National Space Science Data Center through Ihe World Data Center-A for Rockets and Satellites. ISEEand -2 magnetic field data of Prof. C. T. Russell are generated by the UCLA magnetometer group from Decom data tapes. This study was partially supported by grants-in-aid for Scientific Research Project No. 57740221 by the Ministry of Education, Science and Culture in Japan. REFERENCES Arthur, C. W., McPherron, R. L. and Hughes, W. J. (1977) A statistical study of Pc3 magnetic pulsations at synchronous orbit, ATS-6. J. geophys. Res. 82, 1149. Arthur, C. W. (1978) Pc4 magnetic pulsations at synchronous orbit, ATS-6. EOS 59, 1166. Aubry, M. P., Kivelson, M. P. and Russell, C. T. (1971) Motion and structure of the magnetopause. J. geophys. Res. 76, 1673. Barfield, J. N. and McPherron, R. L. (1972a) Investigation of interaction between Pcl and 2 and Pc5 microequatorial pulsations at the synchronous orbit. J. geophys. Rex 77, 4704. Barfield, J. N. and McPherron, R. L. (1972b) Statistical characteristics of storm associated Pc5 micropulsations observed at the synchronous equatorial orbit. J. geophys. Res. 77, 4720. Barfield, J. N., McPherron, R. L., Coleman, P. J. Jr. and Southwood, D. J. (1972) Storm-associated Pc5 micropulsation events observed at the synchronous equatorial orbit. J. geophys. Res. 77, 143. Bavassano, B., Dobrowolny, M., Mariani, F. and Ness, N. F. (1981) On the polarization state of hydromagnetic fluctuations in the solar wind. J. geophys. Res. 86, 1271. Bossen, M., McPherron, R. L. and Russell, C. T. (1976a)

Simultaneous Pcl observations by the synchronous satellite ATS-1 and ground stations ; Implications concerning IPDP generation mechanisms. J. atmos. terr. Phys. 38, 1157. Bossen, M., McPherron, R. L. and Russell, C. T. (1976b) A statistical study of Pcl magnetic pulsations at synchronous orbit. J. geophys. Res. 81,6083. Chen, L. and Hasegawa, A. (1974) A theory of long-period magnetic pulsations, 1. Steady state excitation of field line resonance. J. geophys. Res. 79, 1024. Cummings, W. D., O’Sullivan, R. J. and Coleman, P. J., Jr. (1969) Standing AlfvCn waves in the magnetosphere. J. geophys. Res. 74, 778. Denskat, K. U. and Burlaga, L. F. (1977) Multispacecraft observations ofmicroscalefluctuations in the Solar wind. J. geophys. Res. 82,2693. Dungey, J. W. (1954) Electrodynamics of the outer atmosphere, Ionosphere Research Lab. Pennsylvania State Univ. Sci. Rept., No. 69. Dungey, J. W. (1955) Electrodynamics of the outer atmosphere, in The Physics of the Ionosphere, pp. 229-236. Physical Society London. Fairfield, D. H. (1979) Waves in the vicinity of the magnetopause, in Magnetospheric Particles and Fields (Edited by McCormac, B. M.),pp. 67-77. Reidel, Dordrecht. Gupta, J. G. (1976) Some characteristics of large amplitude Pc5 pulsation. Aust. J. Phys. 29, 67-79. Hasegawa, A. (1975) Plasma instabilities and nonlinear effects, in Physics and Chemistry in Space (Edited by Roederer, J. G. and Wasson, J. T.), Vol. 8. Springer, Berlin, Heidelberg, New York. Hedgecock, P. C. (1976) Giant PCS pulsations in the outer magnetosphere, a survey of Heos-1 data. Planet. Space Sci. 24, 921. Hones, E. W., Jr., Birn, J., Bame, S. J., Asbridge, J. R., Paschman, G., Schopke, N. and Haerendel, G. (1981) Further determination of the characteristics of magnetospheric plasma vortices with ISEE 1 and 2. J. geophys. Res. 86, 814. Hughes, W. J. (1974) The effect of the atmosphere and ionosphere on long period magmetospheric micropulsations. Planet. Space Sci. 22, 1157. Hughes, W. J. and Southwood, D. J. (1976a) The screening of micropulsation signals by the atmosphere and ionosphere. J. geophys. Rex 81, 3234. Hughes, W. J. and Southwood, D. J. (1976b) An illustration of modification of geomagnetic pulsation structure by the ionosphere. J. geophys. Res. 81, 3241. Hughes, W. J., McPherron, R. L. and Russell, C. T. (1977) Multiple satellite observations of pulsation resonance structure in the magnetosphere. J. geophys. Res. 82,492. Hughes, W. J., McPherron, R. L. and Barfield, J. N. (1978) Geomagmeticpulsations observed simultaneously on three geostationary satellites. J. geophys. Res. 83, 1109. Hughes, W. J. (1980) Multisatellite observations of geomagnetic pulsations. J. Geomagn. Geoelect., Kyoto Supplement, pp. 41-55. Inoue, Y. (1973) Wave polarizalions of geomagnetic pulsalions observed in high latitudes on the earth’s surface. J. geophys. Res. 78,2959. Kate, Y. and Utsumi, T. (1964) Polarization ofthe long period geomagnetic pulsation Pc5. Rep. Ionosph. Space Res. Japan 18, 214. Kato, Y. and Saito, T. (1964) Secular and annual variations in the periods of Pc3 and Pc5. Rep. lonosph. Space Res. Japan 18. 183-187.

Local time asymmetry

in the characl .eristics of Pc5 magnetic

King, J. H. (1977) Interplanetary medium data book, Sept. 1917, NSSDC/‘WDC-A-RES 77-04. Kokubun, S., McPherron, R. L. and Russell, C. T. (1976) Ogo 5 observations of PCS waves : Ground-magnetosphere correlations. J. geophys. Res. 81, 5141. Kokubun, S., Kivelson, M. G., McPherron, R. L. and Russell, C. T. (1977) OGO 5 observations of Pc5 waves : Particle flux modulations. J. geophys. Res. 82,27?4. Kokubun, S. (1980) Observation of PC pulsations in the magnetosphere ; Satellite-ground correlation. J. Geomagn. Geoelect., Supplement, pp. 17-39. Lam, H. L. and Rostoker, G. (1978) The relationship of Pc5 micropulsation activity in the morning sector to the aurora1 westward electrojet. Planet Space Sci. 26,473. Lee, L. C., Albano, R. K. and Kan, J. R. (1981) KelvinHelmholtzinstability in the magnetopause-boundary layer region. J. geophys. Res. 86, 54. Ledley, B. G. (1971) Magnetopause attitudes during Ogo 5 crossings. J. geophys. Res. 76,6737. Mauk, B., Gurgiolo, C., Lin, C. S., Parkes, G. K., Anderson, J. A., Lin, R. P. and Reme, H. (1980) Surface waves on the Magnetopause. EOS 61, 346. (AGU 1980 Spring Meeting Abs.) McDiarmid,D. R. and McNamara, A. G.( 1973) Radio aurora, storm sudden commencements, and hydromagnetic waves. Can. J. Phys. 51, 1261. McKenzie, J. F. (1970) Hydromagnetic wave interaction with the magnetopause and the bow shock. Planet. Space Sci. 18, 1. McPherron, R. L. (1981) Substorm associated micropulsations at synchronous orbit, in ULF Pulsations in the Magnetosphere (Edited by Southwood, D. J.), pp. 57-73. Center for Academic Publications, Tokyo, Japan/D. Reidel, Dordrecht, Boston, London. Mekki, 0. M. El (1980) Hydromagnetic planetary waves in vertically sheared zonal flow and transverse magnetic field. Solar Phys. 68, 3-15. Melander, B. F. and Parks, F. K. (1981) The effects of cold plasma on the Kelvin-Helmholtz instability. J. geophys. Res. 86,4697. Olson, J. V. and Rostoker, G. (1978) Longitudinal phase variations of Pc4-5 micropulsations. J. geophys. Res. 83, 2481. Ong, R. S. B. and Roderick, N. (1972) On the KelvinHelmholtz instability of the Earth’s magnetopause. Planet. Space Sci. 20, 1. Saito,T. (1978) Long-period irregular magnetic pulsation, Pi3. Space Sci. Rev. 21,427. Saito,T. and Yumoto, K. (1981) A 15-min period geomagnetic pulsation Ps6 excited by an instability of aurora1 electrojets, in Relation between Laboratory and Space Plasmas (Edited by Kikuchi, H.), pp. 187-195. b. Reidel, Dordrecht. Samson. J. C.. Jacobs. J. A. and Rostoker. G. (1971) Latitudedependent characteristics of long-pirio> geomagnetic micropulsations. J. geophys. Res. 76, 3675. Samson, J. C. (1972) Three-dimensional polarization

pulsations

471

characteristics of high latitude Pc5 geomagnetic pulsations. J. geophys. Res. 77,6145. Samson, J. C. and Rostoker, G. (1972) Latitude-dependent characteristics of high-latitude Pc4 and Pc5 micropulsations. J. geophys. Res. 77,6133. Singer, H. J. and Kivelson, M. G. (1979) The latitudinal structure of Pc5 waves in space : Magnetic and electric field observations. J. geophys. Res. 84, 7213. Southwood, D. J. (1968) The hydromagnetic stability of the magnetospheric boundary. Planet. Space Sci. 16, 587. Southwood, D. J. (1974) Some features offield line resonances in the magnetosphere. Planet. Space Sci. 22,483. Southwood, D. J. and Hughes, W. J. (1978) Source induced vertical components in geomagnetic pulsation signals. Planet. Space Sci. 26, 715. Southwood, D. J. (1979) Magnetopause Kelvin-Helmholtz instability,inMagnetosphericBoundaryLayer,pp. 357-364. ESA SP-148, Paris. Southwood, D. J. (1981) Low frequency pulsation generation by energetic particles, in ULF Pulsations in the Magnetosphere (Edited by Southwood, D. J.), pp. 75-88. Center for Academic Publications, Tokyo, Japan/D. Reidel, Dordrecht, Boston, London. Tamao, T. (1978) Coupling modes of hydromagnetic oscillations in non-uniform, finite pressure plasmas : Twofluids model. Planet. Space Sci. 26, 1141. Unwin, R. S. and Knox, F. B. (1971) Radio aurora and electric fields. Radio Sci. 6, 1061. Walker, A. D. M., Greenwald, R. A., Stuart, W. F. and Green, C. A. (1978) Resonance region of Pc5 micropulsation examined by a dual aurora1 radar system. Nature, Land. 273, 646. Walker, A. D. M., Greenwald, R. A., Stuart, W. F. and Green, C. A. (1979) Stare aurora1 radar observations of Pc5 geomagnetic pulsations. 1. geophys. Res. 84, 3373. Walker. A. D. (19811 The Kelvin-Helmholtz instabilitv in the low-latitude boundary layer. Planet. Space Sci.-29, 1119. Warner,M.R.andOrr,D.(1979)Timeofflightcalculationsfor high latitude geomagnetic pulsations. Planet. Space Sci. 27, 679. Wolfe, A. and Kaufman, R. L. (1975) MHD wave transmission and production near the magnetopause. J. geophys. Res. 80, 1764. Yumoto, K. and Sakurai, T. (1977) Characteristics of quiet time Pc5 in the magnetosphere, in Abstracts of the 61st General meeting of Japanese Society of Geomagnetism and Geoelectricity, p, 1. Yumoto, K. and Saito, T. (1980) Hydromagnetic wave driven by velocity shear instabilityin the magnetospheric boundary layer. Planet. Space Sci. 28,789. Yumoto, K. and Saito, T. (1982) Nonlinear resonance theory of Pc3 magnetic pulsation. J. geophys. Res. 87, 5159. Yumoto, K., Eitoku, A. and Saito. T. (19831 Hvdromaenetic eigenoscillations in the dipolar field. Mem. National Inst. Polar Res.. Ser. A. No. 26. Tokyo (in press).