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Flapping motions of the tail plasma sheet induced by the interplanetary magnetic field variations
~,ane,~~cesci..vo~.24,pp.147to 159. ~ei-mmmPresa.1976.~A~tedisNort&m~~~~
FLAPPING MOTIONS OF THE TAIL PLASMA SHEET INDUCED BY THE INTERPLANETARY MAGNETIC FIELD VARIATIONS TSUTOMU
TOICHI
Institute of Energy Economics, NO. 10 MORI BLDG. 28 Nishikubo Sakuragawa-cho Minato-ku, Tokyo, 105, Japan TERUKI
MIYAZAKI
Geophysical Institute, University of Tokyo, Bunkyo-ku Tokyo, 113, Japan (Received injnalform
8 August 1975)
Ah&act-Flapping motions of the magnetotail with an amplitude of several earth radii are studied by analysing the observations made in the near (X = -25 N -30 RlBfand the distant (X ‘IJ -60 Rx) tail regions. It is found that the flapping motions result from fluctuations in the interplanetary magnetic field, especially Alfvenic fluctuations, when the magnitude of the interplanetary magnetic field is larger than ~10 y, and they propagate behind the Earth with the solar wind flow. Flappings tend to be observed in early phases of the magnetospheric substorm, and they have two fundamental modes with periods of -200 and -500 sec. In some limited cases a good correspondence with the long period micropulsations (Pc5) in the polar cap region is observed. Thw observational results are explained by the model in which the Alfvenic fluctuations in the solar wind penetrate into the netosphere along the connected ~te~~e~-magnetosph~c field lines. The characteristics m% of e flapping reveal that the geomagnetic tail is a good resonator for the hydromagnetic disturbances in the solar wind.
Since the discovery of the geomagnetic tail, the llapping motions of the tail plasma sheet have been studied extensiveIy. In the distant tail region at --6o Ru the multiple crossings of the neutral sheet were observed by Explorer 33 ma~etometer (Mihalov et al., 1970) and it was shown that most oscillations of the neutral sheet have periods of from & to 10 min with a peak occurring at 2 min intervals. From the magnetotail pIasma sheet observations at 18 RE by a pair of Vela satelhtes, Hones et al. (1971) found a phenomenon that is interpreted as a longperiod (4 hr) large amplitude (several Rx) flapping motion of the plasma sheet and this was observed during a magneticalIy very quiet period. Another example of a multiple crossing of the plasma sheet boundary with period co. 2 mm has been observed at 12.5 RB: behind the Earth by the OGO-5 spacecraft and Russell et al. (19721 have interpreted these crossings as a consequence of the plasma sheet expansion due to a substorm. Many theoretical studies of the flapping motions of the geomagnetic tail have aIso been carried out. Walters (1964) has argued that an oblique angle between the interplanetary field and solar wind flow introduces an appreciable tilt of the tail axis away from the solar wind direction toward the
inte~lanetary field direction, and quotations of the tail orientation over a short period may occur in concert with the variations of these inte~l~~ conditions. Obayashi (1967) has proposed the possibility of the tail fluctuation due to the effect of wave vortices produced behind the magnetosphere and predicted the fluctuation periods of 30-60 min. McKenzie (1971), by taking into account the existence of the plasma sheet, has shown that Iongwavelength perturbations on the taii surface can be coupled with waves in the plasma sheet and that these waves can be driven into an unstable condition by the KeIvin-Hehnholtz mechanism. Recently Ershkovich and Nuisov (1972) have suggested that the tail o&Rations with periods of some tens up to some hundred of minutes can be excited due to variations in the solar wind velocity. The relation between the flapping motions of the tail plasma sheet and the interplanetary conditions has been much discussed, but no experimental studies has been performed and the origin of the flapping motions and their relation to the long period micropulsations in the polar cap region have not become very clear. In this paper these problems are investigated by analysing the observations made in the interplanetary space, the tail magnetosphere and the ground. In 147
I48
T.
TOICHI
and T. MIYAZA~~
Section 2 the typical examples of flapping motions of the tail plasma sheet observed by Explorer 34 (x= -25Rx-3012,) and Explorer 35 (x = -60 IQ are presented with the interplanetary conditions monitored sirn~t~~~ly by Explorer 35 and the AE index obtained from the magnetic data by 13 magnetic observatories in the aurora1 zone. The cause of flapping motions of the plasma sheet is discussed in connection with the interplanetary magnetic field conditions and the magnetospheric substorms. Section 3 is devoted to the statistical comparisons between the tail plasma sheet motions and the ~t~~l~et~ conditions and the maoroscopic interplanetary structure is also discussed. Using the multi-satellite observational data by Explorer 33, 34 and 35, flapping motions of the plasma sheet observed simultaneously by two satellites are examined in Section 4. In Section 5 characteristics of the tail plasma sheet oscillations are studied by means of the power spectral analysis of the oscillations observed by Explorer 34 in the near tail region (X = -25 - -30 Rx) and its relation with the long period micropulsation in the polar cap region is also discussed. It is concluded in Section 6 that the ~v~tigation of the flapping motions of the tail plasma sheet is very important in clarifying the interaction rn~h~i~ between the solar wind and the magnetosphere. The magnetic field and electron experiments on board Explorer 35 are described in Behannon (1970) and in Nishida et al. (1972), respectively and the solar wind proton experiment on board Explorer 33 is described by Lyon et al. (1967). The solarma~etospheric coordinate system (Ness, 1961) is used throughout this paper. 2. OBSEIWATION
THE
OF
TAIL
MOTIONS SHEET
PLAPPING
PLASMA
2.1 Ub~e~atj~~l results in the mar (x = -25 - -3ORx)
OF
tail region
In Figs. 1 and 2 two typical records of the flapping motion of the tail plasma sheet obtained by Explorer 34 are shown together with the corresponding interplanetary and ground data. In the top panel ~~ is the square root of the dynamic pressure of the solar wind plasma, F is the strength of the interplanetary magnetic field and B,, By and 3, are components of the ~~~l~et~ magnetic field, all of which are obtained by the Explorer 33 satellite. The interplanetary magnetic field data are means over 82 set and the solar wind data are observations at every 5.46 min (Fig. 1) or one hour average (Fig. 2). In the middle panel of the figures, B, is the x-component
1. &LE3 OF THE OSCILLATORYMOTIONSOF l?KE TAIL PJ..AWA SEEET OBSERVEDIN THE NEAR TAIL -GION (x = -25 w -30 Rd. FIG.
In the upper panel ZTP is the mean square of the solar wind dynamic pressure, F is the interplanetary field strength, B,, & and & are each components of the interplanetary magnetic fieId. In the middle panel B, is the component of B along the Sun-Ear& line. In the lower panel the geoma~etic AE index is &own and the triangle is SC events observed on the ground magnetograms. of the tail magnetic field observed by the Explorer 34 magnetometer and the mean values for 3 min are used. In the bottom panel the AE index obtained from the magnetic data of the 13 magnetic observatories in the aurora1 zone is shown. (The longitudes of those observatories are summarized in Table 1.) The triangles indicate sudden commencement (SC) events observed on the ground (after Solar-Geophysical Data). When we compare the tail observations with the ~t~l~et~ data, it is necessary to take account of the time delay between the two satellites Explorer 33 and 34. In the top and middle panels the approximate positions of these satellites are given in the solar-magnetospheric coordinate system. The time delay effects are estimated, assuming that the solar wind velocity is 400 kmlsec and they are indicated by the oblique lines between the top and middle panels. In Figs. 1 and 2 oscillations in the tail I& demonstrate the oscillatory motions of the tail plasma sheet. In the middle panel of Fig. 1 series of oscillatory motions are observed during --09:2OlO:OOUT, -12:40-13:4OUT and -16:20-17:00 UT on 15 February 1968, respectively. Until
149
Motions of magnetotail
In Fig. 2 examples of the plasma sheet oscillations on 6 April 1968 are shown. In the tail field record of the middle panel the Be component is about 20 y and almost constant until 46:4OUT, indicating that the satellite lies outside the plasma sheet. At --06:4OUT the B, component begins to oscillate with a period of approx. 10 min and the oscillation continues until ^08 : 10 UT. The dynamic pressure of the solar wind is almost constant and the magnitude of the interplanetary magnetic field stays at a nearly constant level of about 10 y, but the components of the magnetic field oscillate appreciably, revealing the presence of fluctuations of the incompressible mode. The dominant direction of B, is southward during the interval, when the plasma sheet oscillations are observed in the tail. As shown in the bottom panel a magnetospheric substorm starts to develop in the auroral zone at -06:40 UT almost simultaneously as the plasma sheet oscillations are observed in the tail. The same features are recognized in another event which is observed at -ll:OO-12:OOUT.
6
FIG.
April
2. EXAMPLEOF THE
1968
OSCILLATORY TAIL PLASMA SHEET.
MOTIONS
OF THE
For further details, see the legend of Fig. 1. ^09:30 UT the satellite is located well above the neutral sheet but at --09 : 30the B, component suddenly decreases and begins to oscillate around zero with a period of approximately 10min. Since the satellite velocity is less than 2 km/set and the average half thickness of the plasma sheet at 30 Rx is approx. 4 RE (Fairfield and Ness, 1971), it is reasonable to consider that the oscillating plasma sheet has repeatedly crossed the satellite, rather than that the satellite has moved across the motionless plasma sheet. In the corresponding period the dynamic pressure of the solar wind is almost constant as shown in the upper panel. The strength F of the interplanetary field is about 10 Y and is also kept almost constant, but the components of the magnetic field, especially By and B,, fluctuate appreciably. The direction of B, is negative (southward) on average. On the other hand, as shown in the bottom panel, the magnetospheric substorm starts to intensify in the aurora1 zone at --09 :20, -13 :00 and 16:50UT. The oscillatory motions of the plasma sheet are observed in the early phases of the magnetospheric substorms.
2.2 Observational results in the distant tail region (x = -60 R,& Figures 3-5 display examples of the plasma sheet flapping motions observed by the Explorer 35 satellite fixed to the Moon; the corresponding interplanetary and ground data is in the same format as used in Figs. 1 and 2. In the middle panel a,, is the low-energy (0.1-3 keV) electron flux in the plasma sheet obtained at every 5.46 mm. The details of electron experiments are described in TABLE1. GeoMAGNEnc STATION Station
T
Geomagnetic
Lat.
Long.
Narsarssuaq
71.4
37.1
Heiss
71.0
156.3
Reykjavik
70.3
71.6
Churchill
68.6
322.5
Point Barrow
68.4
240.7
Great Whale
66.8
C. Chelyuskin
66.1
347.2 176.5
Abisko
65.9
115.4
College
64.6
256.1 161.7
Dixson Is.
62.8
Meanook
61.9
300.7
Cape Wellen
61.7
236.8
Sitka
60.0
275.0
1
20 0 -%I aD 15 10 5 0 o5
2”;
5b
0e ~-400 34 Mirth FKS*
3* ~~
OF QE
1966
oscE~ToRY
~~*N~
OF
components are very large. Qn the ground a ma~eto~he~c substo~ begins to deveiop at ~x4:OO UT. In Fig. 4 a similar example of the plasma sheet osc~~a~on obse~ed on 13 zebras 1968 is displayed* In the mid&e paue$ the 23, ~orn~~e~t begins to oscillate at If : 57 UT with a period of ca. S min, and when the satellite crosses the neutral sheet the large flux of the plasma sheet electron is observed. It is seen in the upper panel that the dynamic pressure of the solar wind is almost constant and the field ma~itude is about 10 Y. The B* ~rnpo~~t is sout&w~d during the interval when the plasma sheet oseilfations are observed in the tail. The magnetospheric substorm starts to intensify at ~12:OO UT as shown in the iower panel* Thus the exampks of the plasma sheet o~~~at~onsobtained at the lunar distance (Figs. 3 and 4) show the same features as observed by Explorer 34 in the near taif region (Figs. 1 aud 2). Another type of 3app~g motion of the t&Z uT plasma sheet is also observed in the geomagnetic tail region. Figm+e5 displays the examples of the sudden crossing motions of the plasmasheet observed nEtr by explorer 3J at the iunar distance (X CT-6Q R&
TAIL 7&MbSA SIigBT OElSBXtV33D IN THE DISTANT TAB. REXXON
(Xc% --60-R&
In the rn~d~epanel @a is the low-energy (Cl-3.0 kevf eketron flux. For further details, see the legend of Fig+I*
3..
3 8 d‘g
29 i+3
data are g Nishida d ak (3972). The 0 20 )aobeyed by explorer 33. It is shown in the middle panel of Fig_ 3 that the g $ satellite lies near the neutral sheet until 4s: 10 UT p 2 and at AS: 1OUT the 3, component suddenly 2 ; begins to oscillate with a period of approx. 15 min. 22 3 In the ~te~~~et~ space the rna~~t~de of the X G f&Id and the v~~ab~ity of its ornaments are at first smaJ1, bnt at -07:4OUT the Geld intensity increases from ca. 7 to 14 y and the notations of $ B, and B8 cornpo~e~~ become large. On the gromd the SC events are observed at 07 : 37 UT and g ? 08 : 39 UT, and the ma~etospheric substo~ starts I- ,$ to develop at No8 :OOUT in the aurora1 zone. 8 Another example of the oscillatory motions of the @’ plasma sheet is observed in the time interval of ~~4~~~4~~UT. In this case the satellite Iies +1 - 2_. outside the plasma sheet until A4:OOUI’ as indicated by the small values of @,, but it suddenly crosses the neutml sheet and the large Bux of the In the inter~ow~ner~ electron is observed. pl~et~‘~~ the dynamic pressure of the solar wind is &ost instant and the BeId no. 4. ~~ is slightly huger than 10 y, and flu~t~tions of its
13irebruary OP TER
1968
~~TORY
YXn PLW
MOTION.¶
suzur.
OF
‘IX&%
Motions of magnetotail
14 FIG.
5.
EXAMPLE
OF THE
THE TAIL
February SUDDEN PLASMA
1968 CROSSING
MGTIONS
OF
SIiEST.
The time variations
of the 3, component in the middle panel of Fig. 5 show that the satellite is at first embedded in the plasma sheet but it moves out to the southern lobe of the tail at -21:55 UT. Then at -22:40 UT B, changes very rapidly from -17 to f17 y within 164sec and the increased low-energy electron flux is observed when B, becomes almost zero y. This implies that the satellite was traversed by the moving plasma sheet and the plasma sheet velocity relative to the satellite can be estimated to he about 16Okm/sec and 80 km/set if the plasma sheet thickness is assumed to be 4 and 2 RX, respectively. The same phenomena were also observed at ~23 :35 and at -24:OO UT. In the top panel the inte~l~et~y magnetic field and the solar wind plasma data are shown, The sudden change of BB observed on the tail at +2240 UT probably corresponds to the change in the direction of the interplanetary magnetic field observed at -22:OO UT, where the solar wind dynamic pressure stays almost constant. Another change in the tail B, at -24:OO UT can also be related to the interplanetary magnetic field variations of the ~comp~ssible mode at ~23 : 20 UT.
151
to the interplanetary magnetic field conditions. In the present section this result is examined statistically by collecting more examples of the plasma sheet motions. In Fig. 6 the quiet event of the tail plasma sheet is illustrated with the corresponding interplanetary conditions. In the upper panel Fis smaller than 5 Y and the fluctuations of the components are small. In the lower panel the magnitude B, is smaller and the low-energy electron flux intheplasma sheet is observed until 42:35UT, which means that the satellite lies in the plasma sheet until --02:35 UT and then moves out to the tail lobe, but no flapping motion is observed for more than one hour. In the foilowing analysis we define the “quiet” condition of the plasma sheet as follows; the satellite lies near the plasma sheet boundary, but no large magaitude change of the B, component is observed for more than one hour. As shown in Fig. 6, the low-energy electron flux data in the plasma sheet is available in addition to the magnetic data, so we can check whether the satellite lies near the plasma sheet aids or not. 13 events of the flapping motions and of the quiet states of the plasma sheet have been collected from Explorer 34 data during the period of February-April 1968, respectively. And 17events of the flappingmotions and of the quiet stat& of the plasma sheet have been obtained from Explorer 35 data during the interval of August 1967-June 1968, respectively. The
z
,
-X=-32& IO-Y =-6SRe
-c
I
I
I,
t
,
(
I,
(
,
-
3, ~RR~ATION BETWRRN THE FLAPPING MOTIONS OF THE TAIL SHEET AND THE INTERPLANETARY CONDITIONS
3.1 Statistical relation between tail plasma motions and interplanetary conditions
sheet
It has been shown in Section 2 that flapping motions of the tail plasma sheet are closely related
12 May FIG.
6. &WF%E
PLASMA
SHEFJT
OF
1968
THJZ
OBSERVED
QUIET IN
THE
STATS DISTANT
OF
THE
TAIL
TAIL
REGION
T. TOICHIand T. MIYAZAKI
152
corresponding interplanetary magnetic field and the plasma data have been obtained by Explorer 33. In Fig. 7 the occurrence frequencies of the flapping motions (right, shaded) and of the quiet states of the plasma sheet (left) are shown versus the one hour mean value of the solar wind bulk velocity. There is apparently DO tendency that the faster the solar wind velocity, the more frequently the flapping motions are observed in the tail. Thus the flapping motions of the tail plasma sheet are not likely to be the result of the Kelvin-Helmholtz mechanism as proposed by several authors (e.g. McKenzie, 1971). Figure 8(a) illustrates the occurrence frequencies of the flapping motions of the plasma sheet (shaded part) and of the quiet states (unshaded part) versus the mean value of the interplanetary magnetic field magnitude F. Mean values F are calculated as follows. First the time delay due to the separation between the satellites, (i.e. Explorer 33 and Explorer 34 or 35) is taken into consideration assuming the solar wind velocity 400 km/set. For cases of oscillatory motions, F is the mean value of the interplanetary field magnitude for the interval when the plasma sheet oscillations are observed in the tail; for the cases of sudden crossing motions, F is the mean value over a 22 min interval extending equally before and after the time of the sudden crossing motion in the tail. For the quiet states, F is the mean value for one hour. In Fig. 8(a) the overall averages of F are also indicated for each category. It is seen from the figure that the larger the magnitude of the interplanetary magnetic field, the more frequent is the detection of flapping motions in the tail plasma sheet. In Fig. 8(b) the occurrence frequencies of the plasma sheet motions (shaded part) and of the quiet
F(y) (4 20
0
8
16
J-mm,.
24
(y)
0) CB FREQUENCY FIG. 8. (a) O~~~JRREN
OF THE FLAPPING MOTIONS (SHADED PARTS) AND THE QUIET STATES OF THE PLASMA SHBET VS THE INTERPLANETARY FIELD MAGNITUDE. (b) OcoURWNcE FREQUENCY OF THE FLAPPING MOTIONS (SHADED PARTS) AND THE QUJJZT STATES OF THE PLASMA SHEET VS THE MAXMUM VARIATIONS OF THE INTERPLANETARY By AND B. COMPONENTS.
Occurence
FfG. 7.
&XXJRRENCE FREQUJ3NCY OF PLASMA SHEET CONDlTIONS VS ONE HOUR AVERAGE VALIJFS OF THE SOLAR UlND FLOW VELOCITY.
Right shaded panel is the case for the flapping motions of the plasma sheet and left panel is the case for the quiet states of the plasma sheet.
state (unshaded part) are shown versus the maximum values of the interplanetary magnetic field variations {2/(ABF1)2 + (AB,)2},,,. Here AB, and ABS are ranges of variations in By and B, components in each 1 l-minute period, and the highest value during the interval when the mean value F is calculated is taken as the maximum. The figure shows that the large variations in the non-radial components of the interplanetary magnetic field tend to be associated
153
Motions of magnetotail
with the plasma sheet flapping motions in the tail.
From the Solar Terrestrial Activity Chart compiled by Obayashi (1971) the relationship between the plasma sheet flapping motions and the macroscopic structure of the ~te~l~et~ space can be seen to be as follows. In Fig. 9 hourly average values of the solar wind velocity V, nnmber density N, total field intensity B, solar ecbptic latitude 6 and solar ecliptic longitude (p observed by Explorer 33 and 35 dnring the period of l-8 April 1968 are ~ustmt~. In this figure the arrows indicate the times of the flapping motions of the plasma sheet observed by Explorer 34. It is shown that the plasma sheet motions are observed when the ~te~l~et~ field intensity and the number density are large and the solar wind velocity increases with time. The same feature is also observed in other examples (Toichi, 1973). 4. MIJLTI-SA’iT?XZ,~ OBSERVATIONS WLAPPING MOTIONS
OF
In one instance the flapping motion of the plasma sheet is observed s~~t~~ously by a pair of satellites which are ca, 30 RE apart. In Fig. 10(a) Rot. Et,‘l
.,”..
E
<
2
1842 March-April 3
4
5
.:
2or
I
IO
o
20r
1968 6
I<
____-
.k --+
..”
-.___
14
7
the records obtained by two satellites on 14 February 1968 are reproduced. In the second panel of the left side of the figure, I& observed by Explorer 34 in the near tail region at x = -31 Rx (3 min Avery) is displayed. A highresolution plot of the same data using 20 see average values is shown in the right panel. The letters A, B, C and D in both panels indicate the same point of the flapping motions. In the third panel, B, and fs, observed by Explorer 35 in the distant tail region at x z -57 RE axe shown. (This record is already described in Fig. 5.) In Fig. 10(b) the positions of three satellites I?xplorer 33,34 and 35 are plotted versus time for each case of the flapping motion. The apparent transit velocities corr~pond~g to the lines ~nn~ting the ~te~~~eta~ and the m~etot~l oblations are given for eaclr case, The solar wind velocity in the interplanetary space observed by Explorer 35 is N4OOkmlsec during this period, but in the magnetosbeath the flow velocity decreases to -200 km/ set (G~~tadt et al., 1967). Therefore the apparent transit velocity of ~265 km/see obtained from Fig. to(b) lies in the reasonable range. This means that the flapping motions of the plasma sheet induced by the interplanetary magnetic field fluctuations propagate behind the Earth with the solar wind flow. S. C~~C~RiST~CS PLASMA
SHEET
OF THE TAIL OSC~ATIONS
Powerzpectrai ana@& The record of the plasma sheet oscillations
5.1 8
1
1
1 3
observed by Explorer 34 on 5 April 1968 is shown in Fig. 11(a), using 20 see average values of &, For this event the power spectral densities of the oscillations are calculated by the fast Fourier analysis method and the results are shown in Fig. 11(b). The numbers I-IV in both figures correspond to each other. The frequency range treated in the present analysis is several tens seconds to several tens minutes. It is clearly shown that there are two dominant peaks at -2 and ~5 mIIz or ~500 and ~200 sec. The large amplitude Sapping motions in Fig. 11(a) correspond to 2 mHz oscillations and the small amplitude ones superposed on them are 5 mHz oscillations
5.2 ~tati~ti~~l s&&y of the o~ciI~utio~period The power spectral densities similar to the above are calculated for 16 examples of the oscillations and h3.9. HOURLYAVSRAOB VALUES OF TKS SOLAR WIND the statistical ~v~tigation has been made. Each VELOCll’Y v, NUMBSR RENSIlY N, TOTAL FIELD INTENSITY power spectral density is normalized by the powerB, SOLAR ECLIPTIC LATITUDE e AND THR SOLAR BCL~C LONGITUDB ye DURING THB PERIOD OF l-8 APR& 1968. density at the lowest frequency and the normalized The times of the plasma she-etflapping motions observed power densities for 16 cases are superposed, The result is illustrated in Fig. 12, which suggests that by Explorer 34 are illustrated by arrows.
T. TOICHI and T,
154
h&YAZAKI
Ground
FIG.
10.
(a)
h
THB RIQHT
IN
THE
Wl’T
FLAPPING
MOTIONS
HAND
PANEL
PANEL
3 tin
OPTHE
20 SeC
PLASMA
SHEET OBSERVED SIMULTANBOUSLY
MEAN VALWJ33 OF
AVERAGE
VALUES
EXPUXWR 33,34 AND35 vs TIME,WHBN
Ba OBSERVED
ARE ILLUSTRATED. THE FLAPPING
there are two dominant periods in the plasma sheet oscillation. In Fig. 13 the occurrence frequency of the powerdensity peak frequency (and period) is shown, where peaks smaller than 10 r”lmHz are not mcluded. Here again it is seen that there are two domiuant peaks of the oscillation period, i.e. .-5OO and -200s~. It can be concluded that there are two fundamental modes of the plasma sheet flapping motions on the geomagnetic tail in the frequency range of several tens seconds to several tens minutes. 5.3 The tail plasma sheet osciikztion and the Iong period mi~ropulsation (PC 5) in the polar cap regibn It has been suggested by MeClay and Radoski (1967) and Pate1 (1968) that long period (periods of
BY EXPLORER
(b)
MOTIONS
BY TWO
34 ARE
POSITIONS
SATJJLLITES.
SHOWN,
OF THREE
THOUGH
SATELLITES
ARE OBSERVED IN THE TAIL.
the order of 25 mm) magnetic field fluctuations observed in the geomagnetic tail and at the surface of the Earth (Patel, 1966; Herron, 1967; Ness, 1969) are caused by natural resonances of the geomagnetic tail. Therefore it is very interesting to study the mutual relation between the plasma sheet oscillation and the long period micropulsation in the polar cap region. In the lower panel of Fig. 14(a) the observational data obtained by Explorer 35 in the distant tail region (X -N 60 RE) are reproduced, The components of the interplanetary magnetic field observed by Explorer 33 on 14 March 1968 are shown in the upper panel. It is seen that the plasma sheet begins to oscillate at -08:lO UT and the oscillation continues until -09: 10 UT in asssociation with the interplanetary field variations as already described in Fig. 3. In Fig. 14(b) the normal
I
,
moo
1
t
t
16xX3
5 April
1968 (a)
5 April
1968 14~43 -16:06
rnHz
mHr
%xl200 100500 200 IOO~ see S@G
mflz
mWz 500 300 200 150
5%
100
S@C @I
FQG.
If.
(a) I?&MPLE OF THE PLASM SHEET ~LAPPIM Mwrso~s OBSBRVED ON 5 APRU PO~R~~LD~S~VSP~Q~~~PE~~DOFTWBSAMBEVENT.
rna~eto~~ data at four magnetic obse~ato~es in the polar cap region, Thule, Resolute Ray, Godhavn and South Pole, are shown and the ~rn~etic latitudes are given for each station. From 48 : 10 to 49: 20 WT magnetic field pub sties with a period of -15miu are observed ah?rost coherently at all these poIar cap stations, It is noticeable that both the micropulsatio~ md the tail plasma sheet oscillation events start to develop almost s~~t~eausly at 48: 10 UT. In order to clarify the mutual relation among the int~l~et~ magnetic field ~uct~tions, the tail plasma sheet oscillations and the long-period micropul~tions in the polar cap region, the power spectral analysis has been performed by means of the fast IWrier t~~i~~. In Fig. 15(a) the power spectml density of each component of the inte~I~et~ magnetic field is ~l~trated. This figure shows that the B, component has a fairly broad peak at -1000 set with the range of 400 to -15OOsee and the B, component has a small peak at -7Wsec, but
196%
{b)
the Be,component has no peak. The power spectral density of the corresponding tail plasma sheet oscillation observed at the lunar distance is shown in Fig. 15(b), which shows that there is a very clear peak at *sec. Figure 15(c) shows the corr* sending power spectral densities of the ma~etic field variations observed at four magnetic observatories in the polar cap. A fairly broad peak at -loo0 see is observed at Thule, and -800 set peak at Resolute Bay, -1500 and -700 see peaks at Godhavn and -1500 set peak at South Pole are observed, respectively. Thus periods of the tail plasma sheet oscillation and mi~opulsatio~s in the polar oap region are in fair a~rn~t with the &retuation periods of the interplanetary magnetic field. These features appear to suggest that the tail plasma sheet oscillation and the long period micropulsations in the polar cap region observed on 14 March 1968 are caused by the interplanetary magnetic field variations, especially J3, oomponent ~uctuatio~. The very sharp peak of the power
T. TOICHI and T. h’flYAZAKl
156
14 March
1968
(6)
mHz
tH
1 low FIG.
12.
FLAPPING
500
NORMALIZED MOTIONS
250 set
POWER ARE
Thule (8B2O)
62
SPECTRAL
VS FREQUBNCY
AMPLES
126
AND
DENSITY
OF THE
PERIOD.
116 EX-
to
SUPERPOSED.
Resolute
spectral density of the tail plasma sheet oscillation means that the geomagnetic tail behaves as a good resonator to the hydromagnetic disturbances in the solar wind. The above example is a very clear case, but in other cases it is difhcult to find a clear relation between the plasma sheet oscillations and the micropulsations in the polar cap region, because the field disturbances in the polar cap region are very large, due to the substorm, and in other cases
Bay
(83@)
tH
Godhovn (8OV)
to
South-Pole C-78.3’)
ID
tz 07
08
09
IO UT
0314-1968
FIG. THB
14. (a) LUNAR
SPONDING PANEL). OBSERVED
PLASMA
INTERPLANETARY IN THE POLAR
GEOMAGNBTIC
mHz
t 100
200 set
Fb.
13. NUMBEROF THB FLAPPING
POWER
MOTIONS
SPECTRAL
DBNSITY
VS FREQUBNCY
AND
PEAKS PERIOD.
OF
(LOWER
OSCILLATION PANEL)
CAP
REGION
ARE OBSERVED
LATITUDES
OF
EACH
OBSERVED
AND
MAGNETIC
(b) EXAMPLES OF LONG-PERIOD
SHEET OSCILLATIONS
I 1 1000500
SHEET
DISTANCE
TIiB
AT
CORRB-
FIBLD
(UPPER
MICROPULSATIONS WHEN
THE
PLASMA
IN THE DISTANT STATION
ARB
TAIL. GIVEN.
there is no micropulsation event, even if the plasma sheet oscillations are observed in the tail. Therefore it can be said that when the plasma sheet oscillations are observed in the tail, in some cases the corresponding micropulsations are observed in the polar cap region, but in other cases are not observed.
Motions of magnetotail 6. CONCLUSIONS AND DISCUSSIONS
Interplanetary 14 March
1968
mHz
07:45-09120
mHz
In this paper flapping motions of the tail plasma sheet have been investigated by means of the tail magnetic field and plasma data obtained by the satellites Explorer 34 and 35. Large amplitude flapping motions of the tail plasma sheet have been observed both in the near tail region (x = -25 to -30 R&and in the distant tail region (x N -60 R,). The data of these flapping motions are compared with the interplanetary conditions monitored simultaneously by Explorer 33 and the magnetospheric substorm conditions represented by the AE index. It has become clear that flapping motions of the plasma sheet are induced by the interplanetary magnetic field variations, especially incompressible Alfvenic fluctuations, and that the oscillatory motions are observed in the early phases of the magnetospheric substorm. When the flapping motions are observed in the tail, the interplanetary magnetic field strength is usually larger than 10 7 and tends to have a southward component. Simultaneous observations of flapping motions by two satellites have revealed that the flapping motions propagate behind the Earth with the solar wind flow. It has also been demonstrated from the power spectral analysis of the plasma sheet oscillations that there are two fundamental modes in the plasma sheet flapping motions with periods of -200 and WWO sec. In some cases the geomagnetic micropulsations have been observed in the polar cap region when the plasma sheet oscillations are observed in the distant tail, but a clear correspondence has not been observed in other cases. The physical processes of the flapping motions of the plasma sheet described above would be explained by the following model. When the flapping motions are observed in the tail the dominant direction of the interplanetary field B, is southward and the magnetospheric substorm is developing. It has been suggested that in the early phase of the substorm the geomagnetic field lines are connected to the interplanetary field lines (Aubry et al., 1970) and the magnetopause is not the tangential discontinuity surface. Therefore it is likely that the field line reconnection occurs between the geomagnetic and interplanetary field lines, and the normal component of the magnetic field at the magnetopause is finite when the flappong motions of the plasma sheet are
mHr
(4 Tail
I
I
I
14 March
I
I
I
1966
08:OO - 09220
0
2
I I mHz 1000500
4
6
I
200
Ground 14 March
i
1968
07,30-09:30
no.
15. (a)
PLANETARY VS TAIL
mHz
mHz
mHz (a)
157
mHz
(C)
POWER
MAGNETIC
FREQUENCY. MAGNBTIC POWBR AT
4
(b) FIELD
SPECRUL MAGNBTIC
SPECTRAL FIELD POWER &
DENSITIES
COMPONENTS SPECTRAL VS
DENSITIES
0BSFiRVATORIE-S
THB
DENSITY
FREQUENCY OF PC
OF
B,, By AND
5 EVENTS VS
INTBRAND OF
B. THE
PERIOD. OBSERVED
FRBQUBNCY.
158
T. TOICHIand T.
observed in the tail. When the magnetopause is a tangential discontinuity surface, the Alfven waves in the solar wind can not transport energy into the magnetosphere (McKenzie, 1970). On the other hand when the normal component of the magnetic field across the discontinuous surface is thrite, the Alfven waves on one side can penetrate into the other in the form of Alfven or magnetosonic waves (Simon, 1958). This suggests that the tail plasma sheet motions are caused by the penetration of the Alfvenic fluctuations in the solar wind into the geomagnetic tail region when the geomagnetic field lines are connected to the interplanetary field lines. The model is illustrated in Fig. 16. The eigenoscillations of the geomagnetic tail have been studied by several authors. Assuming the cyclindrical tail geometry, McClay and Radoski (1967) predicted that the tail has natural resonant modes with periods of 5-3 min. Siscoe (1969) discussed the resonant compressional waves in the geomagnetic tail by use of a single-layered, twodimensional model and obtained fundamental periods of approx. 11 min for the antisymmetric mode and approx. 2 rnin for the symmetric mode. McKenzie (1970) criticized Siscoe’s model in that the magnetopause was treated as a solid wall and the important effects of the solar wind on the eigenmodes were ignored. He derived the dispersion equation for hydromagnetic oscillation of the tail by taking account of the solar wind flow effect and showed that the characteristic period of the tail oscillation is about 13 min for disturbances with wavelength of the order of the diameter. The plasma sheet oscillation
\ Fro. 16. SCHEMATIC OF
THE
FIGURE ILLUSTRATING
i%fAGNETOSPHERB
WHEN
THE
THE CONDITION
FL.APPING
MOTIONS
OF THE PLASMA SHEST ARE OBSERVED lN THE TAIL.
MWAZAKI
was also studied and the periods of 6 min for the symmetric mode were obtained for wavelengths of the order of the thickness of the plasma sheet. In all of these studies the magnetopause has been assumed to be a tangential discontinuity surface and the characteristic periodsof oscillationswith several mmutes to several tens of minutes have been obtained. These are in fairly good agreement with the periods of the plasma sheet oscillations observed in the present study. In order to estimate the oscillation neriods of the tail it would not matter much whether the magnetopause is a tangential discontinuity surface or not. However, the nature of the magnetopause would be essential for the understanding of the interaction mechanism of the hydromagnetic disturbances in the solar wind with the geomagnetic tail. No attempt has been made to calculate the reflection and transmission rates of the hydromagnetic waves in the solar wind at the magnetopause with the tiite normal components of the magnetic field, and this remains as a future problem. It was proposed (McClay and Radoski, 1967; Patel, 1968; Ershkovich and Nuisov, 1972) that the long-period (periods of order of 25 min) magneticfield fluctuations observed at the surface of the Earth (Patel, 1966; Herron, 1967) are caused by the natural resonances of the goemagnetic tail, but no observational evidence has been given. We have investigated the mutual relation between the tail plasma sheet oscillation and the long-period micropulsation in the polar cap region, and our results indicate that in some cases a good correspondence can be obtained but in other cases it is not. A possible explanation would be that a good correspondence depends upon the nature of the interplanetary field fluctuations, i.e. whether it is incompressible or compressible. But it may also be considered that as the tail plasma sheet oscillations propagate behind the Earth with the solar wind flow rather then toward the ground, a good correspondence is not always to be expected. Belcher and Davis (1971) have shown that the largest amplitude Alfvenic fluctuations are found in the compression regions at the leading edges of high-velocity streams where the velocity increases rapidly with time. According to their model, in the compression regions the field intensity and the number density are large and the large amplitude Alfvenic fluctuations are dominant because the hydromagnetic waves, except the Alfven wave, are strongly damped in the solar wind plasma (Barnes, 1966< These are consistent with our results in Section4.2 that the plasmasheetmotionsareobserved when the interplanetary field intensity is large and
Motions of magnetotail the field fluctuations, especially incompressible Alfvenic modes, are dominant. Therefore it can be concluded that the flapping motions of the tail plasma sheet are induced by the incompressible Alfvenic fluctuations of the interplanetary magnetic field in the compression regions where the fast- and slow-stream interactions occur. Ack~wie~ement~We are grateful to Dr. A. Nishida of the Institute of Space and Aeronautical Science, University of Tokyo, for his continuous encouragement and valuable comments and discussions. One of us (T. T,) wishes to express his thanks to Dr. T. Tamao of University of Tokyo for his valuable suggestions and his continuousencouragement, and to Professor T. Obayashi of the Institute of Space Aeronautical Science, University of Tokyo, for his interest and valuable suazestions. Thanks-are also due to Professor T. Nagata, %ofessor N. Fukushiia. Professor T. Ormti. Dr. T. Tohmatsu. Dr. S. Kokub&, Dr. E. Kanedz, Dr. T. Hirasawa, Drl T. Iijima, Dr. T. Ogawa Dr. Y. Kamide, the staff of Geophysical Research Laboratory, University of Tokyo, for their encouragement and useful discussions and comments. We wish also to express our gratitude to the Center for Space Research, M.I.T. which has providedthesolarwind and the tail electron data. The magnetic dam used in this paper are distributed by the World Data Center A, Rocket‘s and Satellites, at NASA Goddard Space Flight Center. We would like to express our gratitude for the supply of geomagnetic data for the present study from all the principal magnetic observatories in the world, through the IGY World Data Center C2 for Geomagnetism. REFERENCES
Aubry? M. P., Russel, C. T. and Kivelson, M. G. (1970). On mward motion of the magnetopause preceding a substorm. J.geophys. Res. 75,701s. Barnes, A. (1966). Collisionless damping of hydromagnetic waves. Phys. Fit&is 9,1483. Behannon, K. W. (1970). Geometry of the geomagnetic tail. J.geopkys. Res. 75,743. Belcher, J. W. and Davis, L. Jr., (1971). Large-amplitude Alfven waves in the interplanetary medium. J.geophys. Res. 76,3534. Ershkovich, A. I. and Nuisov, A. A. (1972). Geomagnetic tail oscillations. Cosm. Electrodyn. 2,471. Fairtield, D. H. and Ness, N. F. (1970). Configuration of the geomagnetic tail during substorms. J. geophys. Res. 75,7032.
159
Greenstadt, E. W., Inoue, G. T., Green, I. M. and Judge, D. L. (1967). Vela 3 magnetograms at 18 Ra structure and pulsations in the magnetosheath. J. geophys. Res. 72, 3855. Herron, T. J. (1967). An average geomagnetic power spectrum for the period range 4.5 to 12,900 seconds. J. geophys. Res. 72, 759. Hones, 3. W., Asbridge, 3. R. and Bame, S. J. (1971). Time variations of the magnetotail plasma sheet at 18 RIP determined from concurrent observations by a nair of Vela satellites. J. peonhvs. Res. 76.4402. Lion. E., Egidi, A., Pizzella: G., Bridge, H.,.Binsack, J., Baker, R. and Butler, R. (1967). Plasma measurement on Explorer 33, 1. Interplanetary. Space Res. 8, 100. McClay, J. F. and Radoski, H. R. (1967). Hydromagnetic propagation in a theta-model geomagnetic tail. J.geophys. Res. 72,452s.
McKenzie, J. F. (1970). Hydromagnetic oscillations of the geomagnetic tail and plasma sheet. J.geophys. Res. 75,533l.
McKenzie, J. F. (1971). Hypothetic wave coupling between the solar wind and plasma sheet. J. geophys. Res. 76,295s. Miialov, J. D., Sonett, C. P. and Colbum, D. S. (1970). Reconnection and noise in the geomagnetic tail. Costn. Electrodyn. 1, 178. Ness, N. F. (1965). The Earth’s magnetic tail. J.geophys. Res. 70,2989. N”,“s:y. F. (1969). The geomagnetic tail. Rev. Geophys. Nishida, A. and Lyon, E. F. (1972). Plasma sheet at lunar distance: Structure and solar-wind dependence. J. geophys. Ref. 77,486.
Obayashi, T. (1967). Flapping motions of the magnetospheric tail. Rep. Iotzosph. Space Res. Japan 20, 36. Patel, V. L. (1966). Long-period hydromagnetic waves. Space Res. 6,758. Patel, V. L. (1968). Origin of long period micropulsations. Nature, Lond. 218,857. Russel, C. T., McPherron, R. L. and Coleman, P. J. Jr. (1971). OGO-5 observations of magnetic noise in the geomagnetic tail. EOS Trans. Am. Geophys. Un. 52, ??LI
33L.
Simon, R. (1958). On reflection and refraction of hy~o~gnetic waves at the boundary of two compressible gaseous media. Ap. J. l23,392. Siscoe, 6. L. (1969). Resonant comoressional waves in the geomagnetic tail. J. geophys. Ifes. 74,6482. Toichi. T. (1973). Structure and dvnamics of the magnetospheric~ tail. Ph.D. thesis: University of Tokyo, Tokyo. Walters, G. K. (1964). Effect of oblique interplanetary magnetic field on shape and behaviour of the magnetosphere. J.geophys, Res. 69,1769.
FLAPPING MOTIONS OF THE TAIL PLASMA SHEET INDUCED BY THE INTERPLANETARY MAGNETIC FIELD VARIATIONS TSUTOMU
TOICHI
Institute of Energy Economics, NO. 10 MORI BLDG. 28 Nishikubo Sakuragawa-cho Minato-ku, Tokyo, 105, Japan TERUKI
MIYAZAKI
Geophysical Institute, University of Tokyo, Bunkyo-ku Tokyo, 113, Japan (Received injnalform
8 August 1975)
Ah&act-Flapping motions of the magnetotail with an amplitude of several earth radii are studied by analysing the observations made in the near (X = -25 N -30 RlBfand the distant (X ‘IJ -60 Rx) tail regions. It is found that the flapping motions result from fluctuations in the interplanetary magnetic field, especially Alfvenic fluctuations, when the magnitude of the interplanetary magnetic field is larger than ~10 y, and they propagate behind the Earth with the solar wind flow. Flappings tend to be observed in early phases of the magnetospheric substorm, and they have two fundamental modes with periods of -200 and -500 sec. In some limited cases a good correspondence with the long period micropulsations (Pc5) in the polar cap region is observed. Thw observational results are explained by the model in which the Alfvenic fluctuations in the solar wind penetrate into the netosphere along the connected ~te~~e~-magnetosph~c field lines. The characteristics m% of e flapping reveal that the geomagnetic tail is a good resonator for the hydromagnetic disturbances in the solar wind.
Since the discovery of the geomagnetic tail, the llapping motions of the tail plasma sheet have been studied extensiveIy. In the distant tail region at --6o Ru the multiple crossings of the neutral sheet were observed by Explorer 33 ma~etometer (Mihalov et al., 1970) and it was shown that most oscillations of the neutral sheet have periods of from & to 10 min with a peak occurring at 2 min intervals. From the magnetotail pIasma sheet observations at 18 RE by a pair of Vela satelhtes, Hones et al. (1971) found a phenomenon that is interpreted as a longperiod (4 hr) large amplitude (several Rx) flapping motion of the plasma sheet and this was observed during a magneticalIy very quiet period. Another example of a multiple crossing of the plasma sheet boundary with period co. 2 mm has been observed at 12.5 RB: behind the Earth by the OGO-5 spacecraft and Russell et al. (19721 have interpreted these crossings as a consequence of the plasma sheet expansion due to a substorm. Many theoretical studies of the flapping motions of the geomagnetic tail have aIso been carried out. Walters (1964) has argued that an oblique angle between the interplanetary field and solar wind flow introduces an appreciable tilt of the tail axis away from the solar wind direction toward the
inte~lanetary field direction, and quotations of the tail orientation over a short period may occur in concert with the variations of these inte~l~~ conditions. Obayashi (1967) has proposed the possibility of the tail fluctuation due to the effect of wave vortices produced behind the magnetosphere and predicted the fluctuation periods of 30-60 min. McKenzie (1971), by taking into account the existence of the plasma sheet, has shown that Iongwavelength perturbations on the taii surface can be coupled with waves in the plasma sheet and that these waves can be driven into an unstable condition by the KeIvin-Hehnholtz mechanism. Recently Ershkovich and Nuisov (1972) have suggested that the tail o&Rations with periods of some tens up to some hundred of minutes can be excited due to variations in the solar wind velocity. The relation between the flapping motions of the tail plasma sheet and the interplanetary conditions has been much discussed, but no experimental studies has been performed and the origin of the flapping motions and their relation to the long period micropulsations in the polar cap region have not become very clear. In this paper these problems are investigated by analysing the observations made in the interplanetary space, the tail magnetosphere and the ground. In 147
I48
T.
TOICHI
and T. MIYAZA~~
Section 2 the typical examples of flapping motions of the tail plasma sheet observed by Explorer 34 (x= -25Rx-3012,) and Explorer 35 (x = -60 IQ are presented with the interplanetary conditions monitored sirn~t~~~ly by Explorer 35 and the AE index obtained from the magnetic data by 13 magnetic observatories in the aurora1 zone. The cause of flapping motions of the plasma sheet is discussed in connection with the interplanetary magnetic field conditions and the magnetospheric substorms. Section 3 is devoted to the statistical comparisons between the tail plasma sheet motions and the ~t~~l~et~ conditions and the maoroscopic interplanetary structure is also discussed. Using the multi-satellite observational data by Explorer 33, 34 and 35, flapping motions of the plasma sheet observed simultaneously by two satellites are examined in Section 4. In Section 5 characteristics of the tail plasma sheet oscillations are studied by means of the power spectral analysis of the oscillations observed by Explorer 34 in the near tail region (X = -25 - -30 Rx) and its relation with the long period micropulsation in the polar cap region is also discussed. It is concluded in Section 6 that the ~v~tigation of the flapping motions of the tail plasma sheet is very important in clarifying the interaction rn~h~i~ between the solar wind and the magnetosphere. The magnetic field and electron experiments on board Explorer 35 are described in Behannon (1970) and in Nishida et al. (1972), respectively and the solar wind proton experiment on board Explorer 33 is described by Lyon et al. (1967). The solarma~etospheric coordinate system (Ness, 1961) is used throughout this paper. 2. OBSEIWATION
THE
OF
TAIL
MOTIONS SHEET
PLAPPING
PLASMA
2.1 Ub~e~atj~~l results in the mar (x = -25 - -3ORx)
OF
tail region
In Figs. 1 and 2 two typical records of the flapping motion of the tail plasma sheet obtained by Explorer 34 are shown together with the corresponding interplanetary and ground data. In the top panel ~~ is the square root of the dynamic pressure of the solar wind plasma, F is the strength of the interplanetary magnetic field and B,, By and 3, are components of the ~~~l~et~ magnetic field, all of which are obtained by the Explorer 33 satellite. The interplanetary magnetic field data are means over 82 set and the solar wind data are observations at every 5.46 min (Fig. 1) or one hour average (Fig. 2). In the middle panel of the figures, B, is the x-component
1. &LE3 OF THE OSCILLATORYMOTIONSOF l?KE TAIL PJ..AWA SEEET OBSERVEDIN THE NEAR TAIL -GION (x = -25 w -30 Rd. FIG.
In the upper panel ZTP is the mean square of the solar wind dynamic pressure, F is the interplanetary field strength, B,, & and & are each components of the interplanetary magnetic fieId. In the middle panel B, is the component of B along the Sun-Ear& line. In the lower panel the geoma~etic AE index is &own and the triangle is SC events observed on the ground magnetograms. of the tail magnetic field observed by the Explorer 34 magnetometer and the mean values for 3 min are used. In the bottom panel the AE index obtained from the magnetic data of the 13 magnetic observatories in the aurora1 zone is shown. (The longitudes of those observatories are summarized in Table 1.) The triangles indicate sudden commencement (SC) events observed on the ground (after Solar-Geophysical Data). When we compare the tail observations with the ~t~l~et~ data, it is necessary to take account of the time delay between the two satellites Explorer 33 and 34. In the top and middle panels the approximate positions of these satellites are given in the solar-magnetospheric coordinate system. The time delay effects are estimated, assuming that the solar wind velocity is 400 kmlsec and they are indicated by the oblique lines between the top and middle panels. In Figs. 1 and 2 oscillations in the tail I& demonstrate the oscillatory motions of the tail plasma sheet. In the middle panel of Fig. 1 series of oscillatory motions are observed during --09:2OlO:OOUT, -12:40-13:4OUT and -16:20-17:00 UT on 15 February 1968, respectively. Until
149
Motions of magnetotail
In Fig. 2 examples of the plasma sheet oscillations on 6 April 1968 are shown. In the tail field record of the middle panel the Be component is about 20 y and almost constant until 46:4OUT, indicating that the satellite lies outside the plasma sheet. At --06:4OUT the B, component begins to oscillate with a period of approx. 10 min and the oscillation continues until ^08 : 10 UT. The dynamic pressure of the solar wind is almost constant and the magnitude of the interplanetary magnetic field stays at a nearly constant level of about 10 y, but the components of the magnetic field oscillate appreciably, revealing the presence of fluctuations of the incompressible mode. The dominant direction of B, is southward during the interval, when the plasma sheet oscillations are observed in the tail. As shown in the bottom panel a magnetospheric substorm starts to develop in the auroral zone at -06:40 UT almost simultaneously as the plasma sheet oscillations are observed in the tail. The same features are recognized in another event which is observed at -ll:OO-12:OOUT.
6
FIG.
April
2. EXAMPLEOF THE
1968
OSCILLATORY TAIL PLASMA SHEET.
MOTIONS
OF THE
For further details, see the legend of Fig. 1. ^09:30 UT the satellite is located well above the neutral sheet but at --09 : 30the B, component suddenly decreases and begins to oscillate around zero with a period of approximately 10min. Since the satellite velocity is less than 2 km/set and the average half thickness of the plasma sheet at 30 Rx is approx. 4 RE (Fairfield and Ness, 1971), it is reasonable to consider that the oscillating plasma sheet has repeatedly crossed the satellite, rather than that the satellite has moved across the motionless plasma sheet. In the corresponding period the dynamic pressure of the solar wind is almost constant as shown in the upper panel. The strength F of the interplanetary field is about 10 Y and is also kept almost constant, but the components of the magnetic field, especially By and B,, fluctuate appreciably. The direction of B, is negative (southward) on average. On the other hand, as shown in the bottom panel, the magnetospheric substorm starts to intensify in the aurora1 zone at --09 :20, -13 :00 and 16:50UT. The oscillatory motions of the plasma sheet are observed in the early phases of the magnetospheric substorms.
2.2 Observational results in the distant tail region (x = -60 R,& Figures 3-5 display examples of the plasma sheet flapping motions observed by the Explorer 35 satellite fixed to the Moon; the corresponding interplanetary and ground data is in the same format as used in Figs. 1 and 2. In the middle panel a,, is the low-energy (0.1-3 keV) electron flux in the plasma sheet obtained at every 5.46 mm. The details of electron experiments are described in TABLE1. GeoMAGNEnc STATION Station
T
Geomagnetic
Lat.
Long.
Narsarssuaq
71.4
37.1
Heiss
71.0
156.3
Reykjavik
70.3
71.6
Churchill
68.6
322.5
Point Barrow
68.4
240.7
Great Whale
66.8
C. Chelyuskin
66.1
347.2 176.5
Abisko
65.9
115.4
College
64.6
256.1 161.7
Dixson Is.
62.8
Meanook
61.9
300.7
Cape Wellen
61.7
236.8
Sitka
60.0
275.0
1
20 0 -%I aD 15 10 5 0 o5
2”;
5b
0e ~-400 34 Mirth FKS*
3* ~~
OF QE
1966
oscE~ToRY
~~*N~
OF
components are very large. Qn the ground a ma~eto~he~c substo~ begins to deveiop at ~x4:OO UT. In Fig. 4 a similar example of the plasma sheet osc~~a~on obse~ed on 13 zebras 1968 is displayed* In the mid&e paue$ the 23, ~orn~~e~t begins to oscillate at If : 57 UT with a period of ca. S min, and when the satellite crosses the neutral sheet the large flux of the plasma sheet electron is observed. It is seen in the upper panel that the dynamic pressure of the solar wind is almost constant and the field ma~itude is about 10 Y. The B* ~rnpo~~t is sout&w~d during the interval when the plasma sheet oseilfations are observed in the tail. The magnetospheric substorm starts to intensify at ~12:OO UT as shown in the iower panel* Thus the exampks of the plasma sheet o~~~at~onsobtained at the lunar distance (Figs. 3 and 4) show the same features as observed by Explorer 34 in the near taif region (Figs. 1 aud 2). Another type of 3app~g motion of the t&Z uT plasma sheet is also observed in the geomagnetic tail region. Figm+e5 displays the examples of the sudden crossing motions of the plasmasheet observed nEtr by explorer 3J at the iunar distance (X CT-6Q R&
TAIL 7&MbSA SIigBT OElSBXtV33D IN THE DISTANT TAB. REXXON
(Xc% --60-R&
In the rn~d~epanel @a is the low-energy (Cl-3.0 kevf eketron flux. For further details, see the legend of Fig+I*
3..
3 8 d‘g
29 i+3
data are g Nishida d ak (3972). The 0 20 )aobeyed by explorer 33. It is shown in the middle panel of Fig_ 3 that the g $ satellite lies near the neutral sheet until 4s: 10 UT p 2 and at AS: 1OUT the 3, component suddenly 2 ; begins to oscillate with a period of approx. 15 min. 22 3 In the ~te~~~et~ space the rna~~t~de of the X G f&Id and the v~~ab~ity of its ornaments are at first smaJ1, bnt at -07:4OUT the Geld intensity increases from ca. 7 to 14 y and the notations of $ B, and B8 cornpo~e~~ become large. On the gromd the SC events are observed at 07 : 37 UT and g ? 08 : 39 UT, and the ma~etospheric substo~ starts I- ,$ to develop at No8 :OOUT in the aurora1 zone. 8 Another example of the oscillatory motions of the @’ plasma sheet is observed in the time interval of ~~4~~~4~~UT. In this case the satellite Iies +1 - 2_. outside the plasma sheet until A4:OOUI’ as indicated by the small values of @,, but it suddenly crosses the neutml sheet and the large Bux of the In the inter~ow~ner~ electron is observed. pl~et~‘~~ the dynamic pressure of the solar wind is &ost instant and the BeId no. 4. ~~ is slightly huger than 10 y, and flu~t~tions of its
13irebruary OP TER
1968
~~TORY
YXn PLW
MOTION.¶
suzur.
OF
‘IX&%
Motions of magnetotail
14 FIG.
5.
EXAMPLE
OF THE
THE TAIL
February SUDDEN PLASMA
1968 CROSSING
MGTIONS
OF
SIiEST.
The time variations
of the 3, component in the middle panel of Fig. 5 show that the satellite is at first embedded in the plasma sheet but it moves out to the southern lobe of the tail at -21:55 UT. Then at -22:40 UT B, changes very rapidly from -17 to f17 y within 164sec and the increased low-energy electron flux is observed when B, becomes almost zero y. This implies that the satellite was traversed by the moving plasma sheet and the plasma sheet velocity relative to the satellite can be estimated to he about 16Okm/sec and 80 km/set if the plasma sheet thickness is assumed to be 4 and 2 RX, respectively. The same phenomena were also observed at ~23 :35 and at -24:OO UT. In the top panel the inte~l~et~y magnetic field and the solar wind plasma data are shown, The sudden change of BB observed on the tail at +2240 UT probably corresponds to the change in the direction of the interplanetary magnetic field observed at -22:OO UT, where the solar wind dynamic pressure stays almost constant. Another change in the tail B, at -24:OO UT can also be related to the interplanetary magnetic field variations of the ~comp~ssible mode at ~23 : 20 UT.
151
to the interplanetary magnetic field conditions. In the present section this result is examined statistically by collecting more examples of the plasma sheet motions. In Fig. 6 the quiet event of the tail plasma sheet is illustrated with the corresponding interplanetary conditions. In the upper panel Fis smaller than 5 Y and the fluctuations of the components are small. In the lower panel the magnitude B, is smaller and the low-energy electron flux intheplasma sheet is observed until 42:35UT, which means that the satellite lies in the plasma sheet until --02:35 UT and then moves out to the tail lobe, but no flapping motion is observed for more than one hour. In the foilowing analysis we define the “quiet” condition of the plasma sheet as follows; the satellite lies near the plasma sheet boundary, but no large magaitude change of the B, component is observed for more than one hour. As shown in Fig. 6, the low-energy electron flux data in the plasma sheet is available in addition to the magnetic data, so we can check whether the satellite lies near the plasma sheet aids or not. 13 events of the flapping motions and of the quiet states of the plasma sheet have been collected from Explorer 34 data during the period of February-April 1968, respectively. And 17events of the flappingmotions and of the quiet stat& of the plasma sheet have been obtained from Explorer 35 data during the interval of August 1967-June 1968, respectively. The
z
,
-X=-32& IO-Y =-6SRe
-c
I
I
I,
t
,
(
I,
(
,
-
3, ~RR~ATION BETWRRN THE FLAPPING MOTIONS OF THE TAIL SHEET AND THE INTERPLANETARY CONDITIONS
3.1 Statistical relation between tail plasma motions and interplanetary conditions
sheet
It has been shown in Section 2 that flapping motions of the tail plasma sheet are closely related
12 May FIG.
6. &WF%E
PLASMA
SHEFJT
OF
1968
THJZ
OBSERVED
QUIET IN
THE
STATS DISTANT
OF
THE
TAIL
TAIL
REGION
T. TOICHIand T. MIYAZAKI
152
corresponding interplanetary magnetic field and the plasma data have been obtained by Explorer 33. In Fig. 7 the occurrence frequencies of the flapping motions (right, shaded) and of the quiet states of the plasma sheet (left) are shown versus the one hour mean value of the solar wind bulk velocity. There is apparently DO tendency that the faster the solar wind velocity, the more frequently the flapping motions are observed in the tail. Thus the flapping motions of the tail plasma sheet are not likely to be the result of the Kelvin-Helmholtz mechanism as proposed by several authors (e.g. McKenzie, 1971). Figure 8(a) illustrates the occurrence frequencies of the flapping motions of the plasma sheet (shaded part) and of the quiet states (unshaded part) versus the mean value of the interplanetary magnetic field magnitude F. Mean values F are calculated as follows. First the time delay due to the separation between the satellites, (i.e. Explorer 33 and Explorer 34 or 35) is taken into consideration assuming the solar wind velocity 400 km/set. For cases of oscillatory motions, F is the mean value of the interplanetary field magnitude for the interval when the plasma sheet oscillations are observed in the tail; for the cases of sudden crossing motions, F is the mean value over a 22 min interval extending equally before and after the time of the sudden crossing motion in the tail. For the quiet states, F is the mean value for one hour. In Fig. 8(a) the overall averages of F are also indicated for each category. It is seen from the figure that the larger the magnitude of the interplanetary magnetic field, the more frequent is the detection of flapping motions in the tail plasma sheet. In Fig. 8(b) the occurrence frequencies of the plasma sheet motions (shaded part) and of the quiet
F(y) (4 20
0
8
16
J-mm,.
24
(y)
0) CB FREQUENCY FIG. 8. (a) O~~~JRREN
OF THE FLAPPING MOTIONS (SHADED PARTS) AND THE QUIET STATES OF THE PLASMA SHBET VS THE INTERPLANETARY FIELD MAGNITUDE. (b) OcoURWNcE FREQUENCY OF THE FLAPPING MOTIONS (SHADED PARTS) AND THE QUJJZT STATES OF THE PLASMA SHEET VS THE MAXMUM VARIATIONS OF THE INTERPLANETARY By AND B. COMPONENTS.
Occurence
FfG. 7.
&XXJRRENCE FREQUJ3NCY OF PLASMA SHEET CONDlTIONS VS ONE HOUR AVERAGE VALIJFS OF THE SOLAR UlND FLOW VELOCITY.
Right shaded panel is the case for the flapping motions of the plasma sheet and left panel is the case for the quiet states of the plasma sheet.
state (unshaded part) are shown versus the maximum values of the interplanetary magnetic field variations {2/(ABF1)2 + (AB,)2},,,. Here AB, and ABS are ranges of variations in By and B, components in each 1 l-minute period, and the highest value during the interval when the mean value F is calculated is taken as the maximum. The figure shows that the large variations in the non-radial components of the interplanetary magnetic field tend to be associated
153
Motions of magnetotail
with the plasma sheet flapping motions in the tail.
From the Solar Terrestrial Activity Chart compiled by Obayashi (1971) the relationship between the plasma sheet flapping motions and the macroscopic structure of the ~te~l~et~ space can be seen to be as follows. In Fig. 9 hourly average values of the solar wind velocity V, nnmber density N, total field intensity B, solar ecbptic latitude 6 and solar ecliptic longitude (p observed by Explorer 33 and 35 dnring the period of l-8 April 1968 are ~ustmt~. In this figure the arrows indicate the times of the flapping motions of the plasma sheet observed by Explorer 34. It is shown that the plasma sheet motions are observed when the ~te~l~et~ field intensity and the number density are large and the solar wind velocity increases with time. The same feature is also observed in other examples (Toichi, 1973). 4. MIJLTI-SA’iT?XZ,~ OBSERVATIONS WLAPPING MOTIONS
OF
In one instance the flapping motion of the plasma sheet is observed s~~t~~ously by a pair of satellites which are ca, 30 RE apart. In Fig. 10(a) Rot. Et,‘l
.,”..
E
<
2
1842 March-April 3
4
5
.:
2or
I
IO
o
20r
1968 6
I<
____-
.k --+
..”
-.___
14
7
the records obtained by two satellites on 14 February 1968 are reproduced. In the second panel of the left side of the figure, I& observed by Explorer 34 in the near tail region at x = -31 Rx (3 min Avery) is displayed. A highresolution plot of the same data using 20 see average values is shown in the right panel. The letters A, B, C and D in both panels indicate the same point of the flapping motions. In the third panel, B, and fs, observed by Explorer 35 in the distant tail region at x z -57 RE axe shown. (This record is already described in Fig. 5.) In Fig. 10(b) the positions of three satellites I?xplorer 33,34 and 35 are plotted versus time for each case of the flapping motion. The apparent transit velocities corr~pond~g to the lines ~nn~ting the ~te~~~eta~ and the m~etot~l oblations are given for eaclr case, The solar wind velocity in the interplanetary space observed by Explorer 35 is N4OOkmlsec during this period, but in the magnetosbeath the flow velocity decreases to -200 km/ set (G~~tadt et al., 1967). Therefore the apparent transit velocity of ~265 km/see obtained from Fig. to(b) lies in the reasonable range. This means that the flapping motions of the plasma sheet induced by the interplanetary magnetic field fluctuations propagate behind the Earth with the solar wind flow. S. C~~C~RiST~CS PLASMA
SHEET
OF THE TAIL OSC~ATIONS
Powerzpectrai ana@& The record of the plasma sheet oscillations
5.1 8
1
1
1 3
observed by Explorer 34 on 5 April 1968 is shown in Fig. 11(a), using 20 see average values of &, For this event the power spectral densities of the oscillations are calculated by the fast Fourier analysis method and the results are shown in Fig. 11(b). The numbers I-IV in both figures correspond to each other. The frequency range treated in the present analysis is several tens seconds to several tens minutes. It is clearly shown that there are two dominant peaks at -2 and ~5 mIIz or ~500 and ~200 sec. The large amplitude Sapping motions in Fig. 11(a) correspond to 2 mHz oscillations and the small amplitude ones superposed on them are 5 mHz oscillations
5.2 ~tati~ti~~l s&&y of the o~ciI~utio~period The power spectral densities similar to the above are calculated for 16 examples of the oscillations and h3.9. HOURLYAVSRAOB VALUES OF TKS SOLAR WIND the statistical ~v~tigation has been made. Each VELOCll’Y v, NUMBSR RENSIlY N, TOTAL FIELD INTENSITY power spectral density is normalized by the powerB, SOLAR ECLIPTIC LATITUDE e AND THR SOLAR BCL~C LONGITUDB ye DURING THB PERIOD OF l-8 APR& 1968. density at the lowest frequency and the normalized The times of the plasma she-etflapping motions observed power densities for 16 cases are superposed, The result is illustrated in Fig. 12, which suggests that by Explorer 34 are illustrated by arrows.
T. TOICHI and T,
154
h&YAZAKI
Ground
FIG.
10.
(a)
h
THB RIQHT
IN
THE
Wl’T
FLAPPING
MOTIONS
HAND
PANEL
PANEL
3 tin
OPTHE
20 SeC
PLASMA
SHEET OBSERVED SIMULTANBOUSLY
MEAN VALWJ33 OF
AVERAGE
VALUES
EXPUXWR 33,34 AND35 vs TIME,WHBN
Ba OBSERVED
ARE ILLUSTRATED. THE FLAPPING
there are two dominant periods in the plasma sheet oscillation. In Fig. 13 the occurrence frequency of the powerdensity peak frequency (and period) is shown, where peaks smaller than 10 r”lmHz are not mcluded. Here again it is seen that there are two domiuant peaks of the oscillation period, i.e. .-5OO and -200s~. It can be concluded that there are two fundamental modes of the plasma sheet flapping motions on the geomagnetic tail in the frequency range of several tens seconds to several tens minutes. 5.3 The tail plasma sheet osciikztion and the Iong period mi~ropulsation (PC 5) in the polar cap regibn It has been suggested by MeClay and Radoski (1967) and Pate1 (1968) that long period (periods of
BY EXPLORER
(b)
MOTIONS
BY TWO
34 ARE
POSITIONS
SATJJLLITES.
SHOWN,
OF THREE
THOUGH
SATELLITES
ARE OBSERVED IN THE TAIL.
the order of 25 mm) magnetic field fluctuations observed in the geomagnetic tail and at the surface of the Earth (Patel, 1966; Herron, 1967; Ness, 1969) are caused by natural resonances of the geomagnetic tail. Therefore it is very interesting to study the mutual relation between the plasma sheet oscillation and the long period micropulsation in the polar cap region. In the lower panel of Fig. 14(a) the observational data obtained by Explorer 35 in the distant tail region (X -N 60 RE) are reproduced, The components of the interplanetary magnetic field observed by Explorer 33 on 14 March 1968 are shown in the upper panel. It is seen that the plasma sheet begins to oscillate at -08:lO UT and the oscillation continues until -09: 10 UT in asssociation with the interplanetary field variations as already described in Fig. 3. In Fig. 14(b) the normal
I
,
moo
1
t
t
16xX3
5 April
1968 (a)
5 April
1968 14~43 -16:06
rnHz
mHr
%xl200 100500 200 IOO~ see S@G
mflz
mWz 500 300 200 150
5%
100
S@C @I
FQG.
If.
(a) I?&MPLE OF THE PLASM SHEET ~LAPPIM Mwrso~s OBSBRVED ON 5 APRU PO~R~~LD~S~VSP~Q~~~PE~~DOFTWBSAMBEVENT.
rna~eto~~ data at four magnetic obse~ato~es in the polar cap region, Thule, Resolute Ray, Godhavn and South Pole, are shown and the ~rn~etic latitudes are given for each station. From 48 : 10 to 49: 20 WT magnetic field pub sties with a period of -15miu are observed ah?rost coherently at all these poIar cap stations, It is noticeable that both the micropulsatio~ md the tail plasma sheet oscillation events start to develop almost s~~t~eausly at 48: 10 UT. In order to clarify the mutual relation among the int~l~et~ magnetic field ~uct~tions, the tail plasma sheet oscillations and the long-period micropul~tions in the polar cap region, the power spectral analysis has been performed by means of the fast IWrier t~~i~~. In Fig. 15(a) the power spectml density of each component of the inte~I~et~ magnetic field is ~l~trated. This figure shows that the B, component has a fairly broad peak at -1000 set with the range of 400 to -15OOsee and the B, component has a small peak at -7Wsec, but
196%
{b)
the Be,component has no peak. The power spectral density of the corresponding tail plasma sheet oscillation observed at the lunar distance is shown in Fig. 15(b), which shows that there is a very clear peak at *sec. Figure 15(c) shows the corr* sending power spectral densities of the ma~etic field variations observed at four magnetic observatories in the polar cap. A fairly broad peak at -loo0 see is observed at Thule, and -800 set peak at Resolute Bay, -1500 and -700 see peaks at Godhavn and -1500 set peak at South Pole are observed, respectively. Thus periods of the tail plasma sheet oscillation and mi~opulsatio~s in the polar oap region are in fair a~rn~t with the &retuation periods of the interplanetary magnetic field. These features appear to suggest that the tail plasma sheet oscillation and the long period micropulsations in the polar cap region observed on 14 March 1968 are caused by the interplanetary magnetic field variations, especially J3, oomponent ~uctuatio~. The very sharp peak of the power
T. TOICHI and T. h’flYAZAKl
156
14 March
1968
(6)
mHz
tH
1 low FIG.
12.
FLAPPING
500
NORMALIZED MOTIONS
250 set
POWER ARE
Thule (8B2O)
62
SPECTRAL
VS FREQUBNCY
AMPLES
126
AND
DENSITY
OF THE
PERIOD.
116 EX-
to
SUPERPOSED.
Resolute
spectral density of the tail plasma sheet oscillation means that the geomagnetic tail behaves as a good resonator to the hydromagnetic disturbances in the solar wind. The above example is a very clear case, but in other cases it is difhcult to find a clear relation between the plasma sheet oscillations and the micropulsations in the polar cap region, because the field disturbances in the polar cap region are very large, due to the substorm, and in other cases
Bay
(83@)
tH
Godhovn (8OV)
to
South-Pole C-78.3’)
ID
tz 07
08
09
IO UT
0314-1968
FIG. THB
14. (a) LUNAR
SPONDING PANEL). OBSERVED
PLASMA
INTERPLANETARY IN THE POLAR
GEOMAGNBTIC
mHz
t 100
200 set
Fb.
13. NUMBEROF THB FLAPPING
POWER
MOTIONS
SPECTRAL
DBNSITY
VS FREQUBNCY
AND
PEAKS PERIOD.
OF
(LOWER
OSCILLATION PANEL)
CAP
REGION
ARE OBSERVED
LATITUDES
OF
EACH
OBSERVED
AND
MAGNETIC
(b) EXAMPLES OF LONG-PERIOD
SHEET OSCILLATIONS
I 1 1000500
SHEET
DISTANCE
TIiB
AT
CORRB-
FIBLD
(UPPER
MICROPULSATIONS WHEN
THE
PLASMA
IN THE DISTANT STATION
ARB
TAIL. GIVEN.
there is no micropulsation event, even if the plasma sheet oscillations are observed in the tail. Therefore it can be said that when the plasma sheet oscillations are observed in the tail, in some cases the corresponding micropulsations are observed in the polar cap region, but in other cases are not observed.
Motions of magnetotail 6. CONCLUSIONS AND DISCUSSIONS
Interplanetary 14 March
1968
mHz
07:45-09120
mHz
In this paper flapping motions of the tail plasma sheet have been investigated by means of the tail magnetic field and plasma data obtained by the satellites Explorer 34 and 35. Large amplitude flapping motions of the tail plasma sheet have been observed both in the near tail region (x = -25 to -30 R&and in the distant tail region (x N -60 R,). The data of these flapping motions are compared with the interplanetary conditions monitored simultaneously by Explorer 33 and the magnetospheric substorm conditions represented by the AE index. It has become clear that flapping motions of the plasma sheet are induced by the interplanetary magnetic field variations, especially incompressible Alfvenic fluctuations, and that the oscillatory motions are observed in the early phases of the magnetospheric substorm. When the flapping motions are observed in the tail, the interplanetary magnetic field strength is usually larger than 10 7 and tends to have a southward component. Simultaneous observations of flapping motions by two satellites have revealed that the flapping motions propagate behind the Earth with the solar wind flow. It has also been demonstrated from the power spectral analysis of the plasma sheet oscillations that there are two fundamental modes in the plasma sheet flapping motions with periods of -200 and WWO sec. In some cases the geomagnetic micropulsations have been observed in the polar cap region when the plasma sheet oscillations are observed in the distant tail, but a clear correspondence has not been observed in other cases. The physical processes of the flapping motions of the plasma sheet described above would be explained by the following model. When the flapping motions are observed in the tail the dominant direction of the interplanetary field B, is southward and the magnetospheric substorm is developing. It has been suggested that in the early phase of the substorm the geomagnetic field lines are connected to the interplanetary field lines (Aubry et al., 1970) and the magnetopause is not the tangential discontinuity surface. Therefore it is likely that the field line reconnection occurs between the geomagnetic and interplanetary field lines, and the normal component of the magnetic field at the magnetopause is finite when the flappong motions of the plasma sheet are
mHr
(4 Tail
I
I
I
14 March
I
I
I
1966
08:OO - 09220
0
2
I I mHz 1000500
4
6
I
200
Ground 14 March
i
1968
07,30-09:30
no.
15. (a)
PLANETARY VS TAIL
mHz
mHz
mHz (a)
157
mHz
(C)
POWER
MAGNETIC
FREQUENCY. MAGNBTIC POWBR AT
4
(b) FIELD
SPECRUL MAGNBTIC
SPECTRAL FIELD POWER &
DENSITIES
COMPONENTS SPECTRAL VS
DENSITIES
0BSFiRVATORIE-S
THB
DENSITY
FREQUENCY OF PC
OF
B,, By AND
5 EVENTS VS
INTBRAND OF
B. THE
PERIOD. OBSERVED
FRBQUBNCY.
158
T. TOICHIand T.
observed in the tail. When the magnetopause is a tangential discontinuity surface, the Alfven waves in the solar wind can not transport energy into the magnetosphere (McKenzie, 1970). On the other hand when the normal component of the magnetic field across the discontinuous surface is thrite, the Alfven waves on one side can penetrate into the other in the form of Alfven or magnetosonic waves (Simon, 1958). This suggests that the tail plasma sheet motions are caused by the penetration of the Alfvenic fluctuations in the solar wind into the geomagnetic tail region when the geomagnetic field lines are connected to the interplanetary field lines. The model is illustrated in Fig. 16. The eigenoscillations of the geomagnetic tail have been studied by several authors. Assuming the cyclindrical tail geometry, McClay and Radoski (1967) predicted that the tail has natural resonant modes with periods of 5-3 min. Siscoe (1969) discussed the resonant compressional waves in the geomagnetic tail by use of a single-layered, twodimensional model and obtained fundamental periods of approx. 11 min for the antisymmetric mode and approx. 2 rnin for the symmetric mode. McKenzie (1970) criticized Siscoe’s model in that the magnetopause was treated as a solid wall and the important effects of the solar wind on the eigenmodes were ignored. He derived the dispersion equation for hydromagnetic oscillation of the tail by taking account of the solar wind flow effect and showed that the characteristic period of the tail oscillation is about 13 min for disturbances with wavelength of the order of the diameter. The plasma sheet oscillation
\ Fro. 16. SCHEMATIC OF
THE
FIGURE ILLUSTRATING
i%fAGNETOSPHERB
WHEN
THE
THE CONDITION
FL.APPING
MOTIONS
OF THE PLASMA SHEST ARE OBSERVED lN THE TAIL.
MWAZAKI
was also studied and the periods of 6 min for the symmetric mode were obtained for wavelengths of the order of the thickness of the plasma sheet. In all of these studies the magnetopause has been assumed to be a tangential discontinuity surface and the characteristic periodsof oscillationswith several mmutes to several tens of minutes have been obtained. These are in fairly good agreement with the periods of the plasma sheet oscillations observed in the present study. In order to estimate the oscillation neriods of the tail it would not matter much whether the magnetopause is a tangential discontinuity surface or not. However, the nature of the magnetopause would be essential for the understanding of the interaction mechanism of the hydromagnetic disturbances in the solar wind with the geomagnetic tail. No attempt has been made to calculate the reflection and transmission rates of the hydromagnetic waves in the solar wind at the magnetopause with the tiite normal components of the magnetic field, and this remains as a future problem. It was proposed (McClay and Radoski, 1967; Patel, 1968; Ershkovich and Nuisov, 1972) that the long-period (periods of order of 25 min) magneticfield fluctuations observed at the surface of the Earth (Patel, 1966; Herron, 1967) are caused by the natural resonances of the goemagnetic tail, but no observational evidence has been given. We have investigated the mutual relation between the tail plasma sheet oscillation and the long-period micropulsation in the polar cap region, and our results indicate that in some cases a good correspondence can be obtained but in other cases it is not. A possible explanation would be that a good correspondence depends upon the nature of the interplanetary field fluctuations, i.e. whether it is incompressible or compressible. But it may also be considered that as the tail plasma sheet oscillations propagate behind the Earth with the solar wind flow rather then toward the ground, a good correspondence is not always to be expected. Belcher and Davis (1971) have shown that the largest amplitude Alfvenic fluctuations are found in the compression regions at the leading edges of high-velocity streams where the velocity increases rapidly with time. According to their model, in the compression regions the field intensity and the number density are large and the large amplitude Alfvenic fluctuations are dominant because the hydromagnetic waves, except the Alfven wave, are strongly damped in the solar wind plasma (Barnes, 1966< These are consistent with our results in Section4.2 that the plasmasheetmotionsareobserved when the interplanetary field intensity is large and
Motions of magnetotail the field fluctuations, especially incompressible Alfvenic modes, are dominant. Therefore it can be concluded that the flapping motions of the tail plasma sheet are induced by the incompressible Alfvenic fluctuations of the interplanetary magnetic field in the compression regions where the fast- and slow-stream interactions occur. Ack~wie~ement~We are grateful to Dr. A. Nishida of the Institute of Space and Aeronautical Science, University of Tokyo, for his continuous encouragement and valuable comments and discussions. One of us (T. T,) wishes to express his thanks to Dr. T. Tamao of University of Tokyo for his valuable suggestions and his continuousencouragement, and to Professor T. Obayashi of the Institute of Space Aeronautical Science, University of Tokyo, for his interest and valuable suazestions. Thanks-are also due to Professor T. Nagata, %ofessor N. Fukushiia. Professor T. Ormti. Dr. T. Tohmatsu. Dr. S. Kokub&, Dr. E. Kanedz, Dr. T. Hirasawa, Drl T. Iijima, Dr. T. Ogawa Dr. Y. Kamide, the staff of Geophysical Research Laboratory, University of Tokyo, for their encouragement and useful discussions and comments. We wish also to express our gratitude to the Center for Space Research, M.I.T. which has providedthesolarwind and the tail electron data. The magnetic dam used in this paper are distributed by the World Data Center A, Rocket‘s and Satellites, at NASA Goddard Space Flight Center. We would like to express our gratitude for the supply of geomagnetic data for the present study from all the principal magnetic observatories in the world, through the IGY World Data Center C2 for Geomagnetism. REFERENCES
Aubry? M. P., Russel, C. T. and Kivelson, M. G. (1970). On mward motion of the magnetopause preceding a substorm. J.geophys. Res. 75,701s. Barnes, A. (1966). Collisionless damping of hydromagnetic waves. Phys. Fit&is 9,1483. Behannon, K. W. (1970). Geometry of the geomagnetic tail. J.geopkys. Res. 75,743. Belcher, J. W. and Davis, L. Jr., (1971). Large-amplitude Alfven waves in the interplanetary medium. J.geophys. Res. 76,3534. Ershkovich, A. I. and Nuisov, A. A. (1972). Geomagnetic tail oscillations. Cosm. Electrodyn. 2,471. Fairtield, D. H. and Ness, N. F. (1970). Configuration of the geomagnetic tail during substorms. J. geophys. Res. 75,7032.
159
Greenstadt, E. W., Inoue, G. T., Green, I. M. and Judge, D. L. (1967). Vela 3 magnetograms at 18 Ra structure and pulsations in the magnetosheath. J. geophys. Res. 72, 3855. Herron, T. J. (1967). An average geomagnetic power spectrum for the period range 4.5 to 12,900 seconds. J. geophys. Res. 72, 759. Hones, 3. W., Asbridge, 3. R. and Bame, S. J. (1971). Time variations of the magnetotail plasma sheet at 18 RIP determined from concurrent observations by a nair of Vela satellites. J. peonhvs. Res. 76.4402. Lion. E., Egidi, A., Pizzella: G., Bridge, H.,.Binsack, J., Baker, R. and Butler, R. (1967). Plasma measurement on Explorer 33, 1. Interplanetary. Space Res. 8, 100. McClay, J. F. and Radoski, H. R. (1967). Hydromagnetic propagation in a theta-model geomagnetic tail. J.geophys. Res. 72,452s.
McKenzie, J. F. (1970). Hydromagnetic oscillations of the geomagnetic tail and plasma sheet. J.geophys. Res. 75,533l.
McKenzie, J. F. (1971). Hypothetic wave coupling between the solar wind and plasma sheet. J. geophys. Res. 76,295s. Miialov, J. D., Sonett, C. P. and Colbum, D. S. (1970). Reconnection and noise in the geomagnetic tail. Costn. Electrodyn. 1, 178. Ness, N. F. (1965). The Earth’s magnetic tail. J.geophys. Res. 70,2989. N”,“s:y. F. (1969). The geomagnetic tail. Rev. Geophys. Nishida, A. and Lyon, E. F. (1972). Plasma sheet at lunar distance: Structure and solar-wind dependence. J. geophys. Ref. 77,486.
Obayashi, T. (1967). Flapping motions of the magnetospheric tail. Rep. Iotzosph. Space Res. Japan 20, 36. Patel, V. L. (1966). Long-period hydromagnetic waves. Space Res. 6,758. Patel, V. L. (1968). Origin of long period micropulsations. Nature, Lond. 218,857. Russel, C. T., McPherron, R. L. and Coleman, P. J. Jr. (1971). OGO-5 observations of magnetic noise in the geomagnetic tail. EOS Trans. Am. Geophys. Un. 52, ??LI
33L.
Simon, R. (1958). On reflection and refraction of hy~o~gnetic waves at the boundary of two compressible gaseous media. Ap. J. l23,392. Siscoe, 6. L. (1969). Resonant comoressional waves in the geomagnetic tail. J. geophys. Ifes. 74,6482. Toichi. T. (1973). Structure and dvnamics of the magnetospheric~ tail. Ph.D. thesis: University of Tokyo, Tokyo. Walters, G. K. (1964). Effect of oblique interplanetary magnetic field on shape and behaviour of the magnetosphere. J.geophys, Res. 69,1769.