Satellite Observations of Pi 2 Activity at Synchronous Orbit...UCLA fluxgate magnetometer on ATS 6. Events that occurred in September 1974 have been studied by using digital power

JOURNAL OF GEOPkrYSICAL RESEARCM, VOL. 88, NO. A9, PAGES 7015-7027, SEPTEMBER 1, 1983

SATELLITE OBSERVATIONS OF Pi 2 ACTIVITY AT SYNCHRONOUS ORBIT

T. Sakurai

Department of Engineering, Aeronautics and Space Sciences, Tokai University

R. L. McPherron

Department of Earth and Space Sciences, Institute of Geophysics and Planetary Science University of California, Los Angeles

Abstract. Pi 2 magnetic pulsations are frequently observed at synchronous orbit by the UCLA fluxgate magnetometer on ATS 6. Events that occurred in September 1974 have been studied by using digital power spectra and coherency analysis to determine wave characteristics. From examination of wave form and application of spectral analysis, these Pi 2 events can be divided into three types. The first is a superposition of a 100-s oscillation and a large- amplitude, higher-frequency Pi 1 activity. The second is a 100-s wave unaccompanied by Pi 1. Both types have a significant compressional component. The third is a pure transverse wave in the azimuthal component. The pure transverse waves are quite rare, while the compressional waves occur during almost every substorm. In general, if the satellite is in the local time sector 1900-0300 LT, a Pi 2 burst accompanies every onset. In addition, onsets at the satellite are associated with ground Pi 2 bursts. The peak occurrence time of the satellite Pi 2 is 2100 LT. An examination of Pi 2 polarization at the satellite suggests that a polarization reversal occurs around midnight during quiet magnetic conditions (Kp • 3+), left-handed premidnight and right-handed postmidnight. This result is similar to that obtained from ground- based studies at stations equatorward of the auroral electrojet. The initial perturbation in the azimuthal component of a Pi 2 event is in the same sense as the perturbations caused by substorm-associated, field-aligned currents, positive (eastward) premidnight, negative (westward) postmidnight. This observation indicates that there may be a very close association between their causative mechanisms.

Introduction

During the last two decades, numerous studies of Pi 2 magnetic pulsations have been carried out by using magnetic data obtained from ground-based stations. One of the important results of this work is recognition of the intimate relation between these waves and the onset of the

expansive phase of magnetospheric substorms. Recent studies of Pi 2 magnetic pulsations have therefore concentrated on understanding their generation mechanism and its relation to substorm onset [McPherron, 1979]. Saito [1961] was the first to note that Pi 2 pulsations accompany the onset of magnetic bay disturbances. Since this

Copyright 1983 by the American Geophysical Union.

Paper number 3A0847. 0148-0227/83/003A-0847505.00

work, the relation between substorm onsets and Pi 2 has been studied by using ground-based magnetograms [Rostoker, 1967a,b, 1968; Saito, 1969; Saito and Sakurai, 1970; Sakurai, 1970]. These studies have also revealed a close

correlation between sudden brightening of aurora and the onset of high-latitude and low-latitude Pi 2 [Rostoker, 1971; Sakurai, 1975; Sakurai and Saito, 1976; Saito et al., 1976a,b]. In the auroral zone, Pi 2 magnetic pulsations appear as irregular bursts of magnetic pulsations accompanying a sudden brightening of the aurora and an intensification of the westward electrojet [Olson and Rostoker, 1977, 1978; Rostoker and Olson, 1978]. They are seen simultaneously at midlatitude [Sakurai et al., 1978], usually in the form of a damped sinusoid accompanying the east-west field perturbations caused by field- aligned currents [Clauer and McPherron, 1978; McPherron, 1978, 1979].

From these studies it has been concluded that

Pi 2 magnetic pulsations provide an accurate indicator of substorm expansive onset; however, it is not known what physical mechanism generates them. One suggestion is that Pi 2 pulsations result from a disruption of the cross-tail current as a neutral line forms in the near-earth

plasma sheet. To date there have been few reports of Pi 2 in space and little opportunity to determine the validity of this idea. Only a few cases of Pi 2 events observed simultaneously at both the satellite and the ground have been reported. Of these, only one occurred exactly at the time of a major substorm onset [Arthur and McPherron, 1980]. In another report, Lin and Cahill [1975] described characteristics of Pi 2 observed simultaneously in the magnetosphere at L = 5 by Explorer 45 and at the middle latitude stations, Tucson and Fredericksburg. Recently, the longitudinal extension of Pi 2 waves in space has been found to be more restricted than it is

on the ground [G.J. LaQuadra and W.J. Hughes, personal communication, 1982].

As a first step in determining the generation mechanism of Pi 2 waves, we have examined a large number of Pi 2 events recorded by the UCLA fluxgate magnetometer on the synchronous satellite, ATS 6. In this report we describe initial results of an attempt to characterize the properties of these pulsations as observed at synchronous orbit.

The instrument, the magnetic data, and the analysis procedure used in this study are described briefly in section 2. In section 3 the wave characteristics, including ellipticity, polarization, and power spectrum, are studied

_

individually and statistically. Local time occurrence characteristics of Pi 2 bursts at the

7015

7016 Sakurai and McPherron: Satellite Observations of Pi 2 Activity

Umversol Time Substorm P•2 Onset

at Sotell/te

0600 0610 061•

OOOl O0O5 0010 0015 Substorm Pi2 Onset

on the Ground

75*West Locol Time

Fig. l. A typical Pi 2 event of type I as seen at synchronous orbit on September 8, 1974, 0604 UT. The burst is a very irregular oscillation in all components of the field. A simultaneous Pi 2 burst is seen at the midlatitude station, Tucson.

satellite and on the ground are studied in section 4 by using the ground magnetograms from two stations closest to the satellite meridian.

The geomagnetic coordinates of these stations are Fredericksburg (8 = 51.5 ø N, k = 352.8 ø E) and Tucson (8 = 39.9 ø N, k = 311.4 ø E). These stations are situated one hour east and west of

the satellite meridian, respectively. Simultaneity of Pi 2 events at the ground and the satellite is examined in section 5. A close

relation between an initial deflection of the

azimuthal component of Pi 2 wave and dc shift of the magnetic field is demonstrated in section 6 in connection with the development of the field- aligned current associated with substorm onset. The final section summarizes several important characteristics of Pi 2 waves observed at the

satellite, with a brief discussion of possible generation mechanisms.

Description of Data and Analysis Procedure

Ins t rument at ion

On May 31, 1974, the ATS 6 spacecraft was placed in synchronous orbit at 96 ø west longitude. In May 1975 it was moved to a new location at 35 ø east longitude. In both locations the spacecraft orbit was close to the geographic equatorial plane, but -initially was at 10 ø magnetic latitude and later at 3 ø south. Magnetic field changes at the spacecraft were measured by the UCLA fluxgate magnetometer. A detailed description of the instrument and its prototypes has been given by Barry and Snare [1966], McPherron [1974], and McPherron et al. [1975]. The noise spectrum of this instrument is sufficiently low that it is possible to carry out extensive studies of ULF waves at synchronous orbit [McPherron, 1976]. The magnetometer

o

Spectral Analysis of Pi 2 Burst 0600-0620 UT September 8, 1974

10 3 X,

10 3

10 3

Autospectra Eigenvalues 10 4

10 3

10 2

101

10 0

I i i ii1111, i i i i1111 i i i iiiii i i i illll 10- 2 10 -4 10 -3 10 -2 10 '• 10 0

Frequency (Hz) Fig. 2. Spectral analysis of a Pi 2 burst at 0600-0620 UT, September 8, 1974. Left panel shows auto spectra of field components. Right panel shows eigenvalues of spectral matrix. Following figure shows eigenvalue spectra of first difference time series for same event.

Sakurai and McPherron: Satellite Observations of Pñ 2 Activity 7017

sampling period is 0.125 s. The noise spectrum is roughly I/f, with arms noise of 65 nT over two decades of bandwidth.

Data Presentation

To study wave forms of Pi 2 bursts at the satellite, we have used 5-s averaged data plotted with a common format having a vertical scale of 10 nT per inch and a horizontal scale of 1 min per inch. Original data were transformed from spacecraft to dipole coordinates and plotted with tl, D, and V components and magnitude B from top to bottom. The H axis is parallel to the dipole axis and positive north, the D axis is in a direction perpendicular to H and R (east), where R is the radius vector from the center of the

earth to the spacecraft and V is outward, parallel to the magnetic equatorial plane.

Analys is Procedure

In this study we have used data of 5-s time resolution to obtain the power spectral matrix

Period (sec) 100 50 25 17 13 10

$EP. 8, Ig74 0600- 0620 UT

N

v .01

O

ß .OOl

ß •- 40

o

13. o 1

• o

-1 I i I I I I i I I I J

0 2 4 6 8 10

Frequency (x10 '2 Hz) Fig. 3. The top panel shows the power spectrum of the Pi 2 event of Figure 1o Several spectral peaks appear, with the longest period being 100 (shaded portions). The middle panel shows percent polarization. High polarization at all frequencies is primarily a consequence of having only 6 degrees of freedom in spectral estimates.

Unlversol Time Substorm Pe2 Onset

of Sate#de

03V50 0•5 0400

Substorm P•2 Onset on the Ground

75"West Local T•rne

Fig. 4. A typical Pi 2 event of type 1 as seen at synchronous orbit on September 11, 1974, 0350 UT. The format is the same as described in

Figure 1.

and polarization parameters for 57 Pi 2 events. Each event was defined as a 20-min interval

approximately centered on a Pi 2 burst. The data were first differenced to "pre-whiten" the signal and edited for bad data. A fast Fourier

transform was used to calculate the cross-

spectral matrix. To obtain sufficient frequency resolution, only six degrees of freedom were used in each spectral estimate.

Wave analysis of the spectral matrix was performed by using the technique of principal axis transformation, followed by coherency analysis, as discussed by McPherron et al. [1972], Arthur et al. [1976], and Arthur [1979]. The real part of the spectral matrix was diagonalized at each frequency, and the eigenvectors were used to define a similarity transformation that is applied to both the real and the imaginary parts of the spectral matrix. Subsequently, the upper left 2 x 2 submatrix corresponding to the principal plane of polarization was separated into perfectly polarized and unpolarized parts. Then the ellipticity and sense of rotation were calculated for the polarized part. Results were displayed as a set of three graphs showing as a function of frequency, the eigenvalues, the ellipticity and percent polarization, and the polar and azimuthal angles of the x and z eigenvectors. The x eigenvector is interpreted as the direction of

7018 Sakurai and McPherron: Satellite Observations of Pñ 2 Activity

Period (sec) 100 50 25 17 13 10

1II SEP. 11,1974

.001

% E 80 O

.--

N ß •- 40

O

0 1

I I I I i i i i I 1

0 2 4 6 8 10

Frequency (x10 -2 Hz) Fig. 5. Power spectrum, percent polarization, and ellipticity of Pi 2 event on September 11, 1974, 0350 UT. The format is the same as in Figure 2.

the major axis of polarization of the wave and the z eigenvector as its direction of propagation.

Characteristics of Pi 2 Waves

Examples of Pi 2 Oscillations a__t Synchronous Orbit

In the set of 57 Pi 2 bursts we identified 29

Pi 2 events having a well-defined, Pi 2 wave form at synchronous orbit. These Pi 2 bursts can be classified into three major types, which are illustrated in the figures of this section. Several characteristics of the waves, including their spectrum, ellipticity, and percent polarization, are discussed as well.

Event of 0640 UT September 8, 1974. Figure 1 shows a typical Pi 2 event seen at synchronous orbit. The vertical scale is 10 nT/division and the horizontal scale is 1-min/division. This example illustrates the main characteristics of the most common types of Pi 2 events seen at the satellite. This type of Pi 2 burst consists of two frequency components, a broad peak near 10 mHz (100s), and a higher frequency noise band. The long-period component of the oscillation becomes apparent near the end of the event (see D

component). Its period corresponds roughly to that observed simultaneously on the ground at Tucson, one hour west of the satellite meridian. The Pi 2 burst at the satellite is a very irregular oscillation in all components of the field and contains a substantial compressional component, as is evident from the variations in the B total trace. Usually, the burst is superimposed on a dc shift in the azimuthal component caused by the field-aligned currents above the satellite. In this case, the dc shift due to the field-aligned currents indicates a westward perturbation. Assuming currents above a satellite at north latitude, they must flow from the magnetosphere to the ionosphere, which is a typical flow direction for field-aligned currents in the postmidnight quadrant.

The auto power spectra of the Pi 2 burst before pre-whitening and wave analysis are presented in panel 1 of Figure 2. The low frequency component of the burst is very obvious in the radial component (V), and evident in the azimuthal component (D) as well. High frequency noise is largest in the dipole component (H). The same spectrum after eigenanalysis is displayed in panel 2 of the figure. The dominant characteristic of this burst is the broad peak centered at 10 mHz. However, two other peaks originating from the D component are present at 20 and 34 mHz.

Umversol T•rne

Subsform Pl2 Onset of Sofelhfe

September 6, 1974

AT$-6 Locol T•rne

2240 J 2•45 2•50 Subsform P•2 Onset

on the Ground

75øWest Locol T•me

Fig. 6. A typical Pi 2 event of type 2 on Sep- tember 6, 1974, 0343 UT. The format is the same as in Figure 1.

Sakurai and McPherron: Satellite Observations of Pi 2 Activity 7019

For the purposes of this study, the time series data were detrended and pre-whitened by first differencing the time series records. Applying this procedure to the data of Figure 1, we obtained the spectrum shown in Figure 3. In this case, pre-whitening almost completely flattens the spectrum. The spectral peaks mentioned above are still obvious, but two other peaks at 58 and 78 mHz also appear to be significant. For this event, 100 s was taken as the dominant wave period. The superposition of the 20, 34, 58, and 78 mHz components is what produces the noisy appearance of the waveforms plotted in Figure 1.

In most Pi 2 events, the spectrum is considerably steeper than for this case, and it is not easy to identify spectral peaks in the spectrum of the original data. It is for this reason that the data were pre-whitened. Examples of steeper spectra are shown below in Figures 4, 6, and 8.

Event of 0350 UT September 11, 1974. Figure 4 shows another example of a Pi 2 event of the first type. This Pi 2 burst, which occurred on September 11, 1974, at 0350 UT, is also an irregular oscillation in all components of the field and contains a compressional component, as

Period (sec)

100 50 25 17 13 10 i i i i i i I i --- i

SEP. 6, 1974 0340 - 0400 UT

I_ I I I _L_i J J L_

I

.Ol

.OOl

a• 40 o

o 1

0 2 4 6 8 10

Frequency (x10 -2 Hz) Fig. 7. Spectrum, percent polarization, and ellipticity of the September 6, 1974, 0343 UT event. The format is the same as in Figure 2. A dominant spectral peak appears around the 80-s period.

Universol T•me Substorm Pi2 Onset

ot Sotellite

0145 • 0150 0155 .__ :_. I I I I I I I I I =• -

.• ø September II, 1974 -

• -

o • 20n -

_

• -

_

_

0•45 Ol.• 0•55 ATS-6 Locol Time

• • •• •5.0n• 0245 0250 0255

Substorm Pi2 Onset on the Ground

75øWest Loco/ T/me

Fig. 8. A typical Pi 2 event of type 3 on Sep- tember 11, 1974, 0746 UT. Oscillations occur primarily in the azimuthal component. The format is the same as in Figure 1.

evident in the trace of the total field. The

burst is also superimposed on a dc shift in the azimuthal component, owing to field-aligned currents above the satellite. In contrast to the

event previously described, this burst is superimposed on a positive dc shift, which would be caused by a field-aligned current from the ionosphere toward the magnetosphere. This event is typical of those occurring in the premidnight sector. A Pi 2 is seen simultaneously at Tucson at midlatitudes, with a •typical damped sinusoidal wave form of about 130 s.

The power spectrum plotted in Figure 5 shows three spectral peaks at about 130, 50, and 26 s. The shaded portion corresponds to the dominant 130-s period seen on the ground. In this case, the wave is almost linear at the period of the wave seen on the ground.

Event of 0343 UT September 6, 1974. An example of a second type of Pi 2 is shown in Figure 6 In this case, the high frequencies evident in the waveform and spectrum of the previous events are not seen. The dominant period in the waveform is ~80 s, although a 30-s wave is also present. The correlation with an east-west oscillation at the

ground is particularly clear. Note also that the wave at the satellite is superimposed on a large dc offset of the azimuthal component. This ev'•nt occurs in the premidnight sector, as was the case

702Q Sakurai and McPherron: Satellite Observations of Pi 2 Activity

N .1

(D

.O1

O

._>

• .OO1

.OOOl

c: %

._N

O

13. O 1

Period (sec) 100 50 25 17 13 10

! I I I I i I I I

SEP. 11,1974 0745 - 0805 UT

i i i i I i i i I i I

0 2 4 6 8 10

Frequency (x10 -2 Hz) Fig. 9. Spectrum, percent polarization, and ellipticity for the event of September 11, 1974, 0746 UT. A dominant spectral peak appears at around 100 s in the period.

ß ß ß g occu BII lIB = ß ß 4FREQ

ß BII •1• ß ß BIIm 2 m ß BII lB Bill Bill 4681

l, 4' ß ß /lB --•••--- &••11111

( +•--II

T--• --Till

ß II

l

200

I00 80

2O

I0 - Io+ -2•+ -•+ -4o+ -5o+

MAGNETIC ACTIVITY Kp Fig. 10. Magnetic activity dependence of the dominant period found in the spectra of Pi 2 bursts observed at synchronous orbit for 31 events during September 1974. The dominant and the subsidiary spectral peaks for each event are shown with large and small solid circles, respectively. Because of pre-whitening the dominant peak may be in the Pi 1 band.

LOCAL TIME DEPENDENCE OF ELLIPTICITY OF Pl 2 BURST

AT SYNCHRONOUS ORB IT

1.0 I | i [ I [

o 4_

Sakurai and McPherron: Satellite Observations of ?i 2 Activity 7021

ONE TO ONE CORRESPONDENCE oF Pi 2 AT A TS-6 AND ON THE GROUND

•= • 40

•- o • -' 20 22 O0 02 20 22' O0 02 ,o • ATS-6 Local Time ATS-6 Local Time

• 0

• •o

• • 40 0 02 04 • 06 08 UT(h) 02 04 06 • 08 IO

MN Fred. MN Tucson

ATS-6 vs Fredericksburg A•S-6 vs T•son (96 • •) ( •5 • •) (96 ß W) (110 • W) Sepl

Fig. 13. Probability of simultaneous ground-satellite Pi 2 occurrence as a function of local time.

120 s. A Pi 2 oscillation with a similar period can be seen at the ground. In this case, the wave event is highly polarized and almost linear. However, a slight right-handed polarization is emphasized by the shaded portion of the ellipticity. The orientation of the major axis indicates that the wave was almost transverse in

the east-west direction. During this event the ambient magnetic field was directed almost radially inwards with an elevation angle 23 ø from the V-D plane, which is a very tail-like field configuration.

As illustrated by the typical events presented in this section, the characteristics of Pi 2 bursts at synchronous orbit can be summarized as

• •q • o/ • o5 oz

•o • • o• 04

AT&-G LOCAL TIME

LOCAL TIME

Fig. 14. Distribution of occurrence of time lag of Pi 2 at the satellite and on the ground. (top) ATS 6 and Fredericksburg. (bottom) ATS 6 and Tucson. Almost all of the Pi 2 events ob-

served at the synchronous satellite occur simultaneously with a ground Pi 2 event.

follows• Three types of Pi 2 •Pents are observed at the satellite.

1. A 100-s irregular oscillation in all components of the field with superimposed high-frequency oscillations.

2. A 100-s irregular oscillation unaccompanied by high-frequency oscillations.

3. A much rarer, quasi-sinusoidal transverse oscillation only in the azimuthal component of the field.

In regard to the latter two types of Pi 2 there

PREMIDNIGHT POS TMIDNIGHT ATS-6 Local Time

2145 2150 2155 2200 0145 0150 0155 02.00

0• O• O• 0• ON• 0• 0•

2245 2250 2255 2,.TO0 0245 0250 0255 O,TO0 &-- Substorm

Pi 2 Onset

75'W Local Time

Fig. 15. Initial deflection in the east-west component due to the wave is in the same sense as de shift, positive (eastward) in premidnight, negative (westward) in postmidnight.

7022 Sakurai and McPherron: Satellite Observations of Pi 2 Activity

Pi2 INITIAL DEFLECT•0N __ OC SHIFT DUE TO FIELD- IN THE EAST-WEST COMPONENT ALIGNED CURRENTS

PRE MI ONIGHT POSTM IDNIGHT

• PI 2- SUBS TORM ONSET ß %,. P• 2 - SUBSTORM ONSET

• -IOMIN fi I"---I0 MIN

z

Fig. 16. Schematic representation of initial deflection of Pi 2 wave and dc shift (upper panel) is interpreted as an effect of field- aligned currents (bottom panel).

appears to be a tendency for the east-west oscillation at the satellite to be rotated toward

the north-south direction at the ground. The first type of Pi 2 event has a large compressional component, while the third type has an almost pure transverse oscillation at the satellite.

Statistical Studies of Characteristics of Pi 2

Wave

Dominant period of Pi 2 at the synchronous orbit. We have used spectra such as those discussed above to examine the frequency of occurrence of the two largest peaks in the spectrum of Pi 2 events. For the 57 Pi 2 events identified by waveform data in the month of September 1974, it was possible to identify obvious peaks in 31 cases. We have plotted the location of these peaks as a function of period and the level of magnetic activity in Figure 10. Large dots are used to denote the dominant peak in each spectrum while smaller dots denote the secondary peaks. In a number of cases the dominant peak falls in the Pi 1 band rather than the Pi 2. The primary, reason for this is the pre-whitening of the data carried out to reduce the slope of the spectrum.

It is apparent from the figure that there is no simple dependence of the period of either the dominant or secondary peak on the level of magnetic activity. For the lowest level of activity (Kp ~ 2), five of the six dominant peaks are located at 100 s or longer. Two of the three longest period events (T = 200 s) occurred during extremely quiet conditions. As magnetic activity increases, the spectrum tends toward greater complexity and becomes flatter with more spectral power in higher frequencies. Within the Pi 2 band, the most probable location of a spectral peak is 80-100 s.

Ellipticit¾. The local time dependence of ellipticity of Pi 2 waves is shown in Figure 11.

Solid and open circles represent, respectively, the ellipticity of Pi 2 occurring during relatively quiet (Kp < 3+) and disturbed (4- < Kp < 5+) conditions. In quiet times, the ellipticity changes from left-handed to right- handed polarization around midnight, but at higher levels of magnetic activity, this tendency cannot be discerned. Another important pointe is that the ellipticity is distributed symmetrically about zero, implying that highly elliptical polarization is the most common circumstance.

Local Time Dependence of Pi 2 Bursts at Synchronous Orbit

We have examined the local time occurrence

pattern of Pi 2 bursts at the satellite and on the ground and summarize the results in Figure 12. Normalized occurrence probability of Pi 2 bursts is shown for all events observed during September 1974. All events were observed while the satellite was in the night sector within approximately six hours of local midnight. The most probable occurrence time is 2100 hours, with more events seen premidnight than postmidnight. To compare Pi 2 waves at the satellite with ground observations we also examined rapid-run magnetograms obtained at the two ground-based stations, Fredericksburg and Tucson. These stations are situated one hour east and west, respectively, of the satellite meridian. Very similar results are obtained for these two

stations. The most probable occurrence time for Pi 2 at these stations is ~2200 hours at

Fredericksburg and ~2000 hours a.t Tucson, one hour later and one hour earlier than that at the

satellite.

The results indicate that most of the Pi 2

RELATION BETWEEN SPECTRUM PEAKS OF P i BURSTS AND MAGNETIC FIELD CHANGES

200 100

_ c.., 80

20

ß •! • OCCUR. II lB I--., .,

I II I IIIllllll/111 I-II 22 4 (' 81

II I ,. , II/ -- -- '-• H•'I'--'--'"4 V'--- -- I I I!

-- -- '•' '-e-----• '-'-, ------III

,dm•d .,. i.

•* ß '4P'4! ' III

-2 ø 0 2 4 6 8 10 12 14 ø

POLAR ANGLE DEVIATION

(Z•O= OpRE.--OpOST..DEGREE ) ASSOCIATED WITH P i BURST I'HE MOST DOMINANT SPECTRUM PEAK

Fig. 17. The relation between the dominant period of Pi bursts and changes in field configuration associated with substorm Pi 2 onset. Longer period Pi 2's are associated with smaller changes of the polar angle of the magnetic field.


SEPT. II, 1974

2hUTL i... r,., 5 6 7 8 9 -70 nt V - -80 2 -90

I0 D

u> 0

80 nT

o• H 70 w 60

- BT

[ ! • i [ ] i , [ [ ! [ ! [ i i i i i i ! i ! i i '• (.:3 ' N • H -.--.•-----•- .• • IOnT o3 S

•O ..... • EiOn T

• 22 h 23 O0 01 02 03 75 • West locoI Time

Pi 2- SUBSTORM ONSET •

Fig. 18. An example of the relation between the occurrence of Pi 2, substorm onset and the development of field-aligned current at synchronous orbit. The onset of Pi 2 (solid triangle) is closely associated with the dc shift (shaded portion) in the D component of the magnetic field, eastward in the premidnight and westward in the postmidnight. The Pi 2's are clearly recognized in the ground magnetogram at Fred- ericksburg.

events observed at the satellite can be detected

simultaneously at these midlatitude stations. To qualify this observation, we also examined the simultaneous occurrence probability of Pi 2 bursts at the satellite and at the ground. The results are summarized in Figure 13. When the satellite is near 2200 LT, virtually every event is seen simultaneously at both locations. It is only when one of the ground stations or the satellite is past dawn that this correlation breaks down.

Onset Time of Pi 2 Bursts at the Satellite

and Ground

It is well known that an understanding of the physical mechanisms causing magnetospheric substorms requires accurate timing. Recent papers show that most expansion onsets are accompanied by a Pi 2 burst [Olson and Rostoker, 1978; Sakurai and Saito, 1976; Saito et al., 1976a; Pytte et al., 1976a,b; McPherron, 1978; Rostoker and Olson, 1978; Sakurai et al., 1978]. In this section we present results of an attempt to determine onset time of Pi 2 events at the

satellite and on the ground. To do this, we examined the onset time of Pi 2 bursts at each

location and then compared the time delay between them. The result shows that 60% of the Pi 2

events are simultaneous at the two locations

within the accuracy of the measurements. Two major problems with ground magnetograms limit the accuracy. The first is reading the films on which the rapid-run magnetograms are recorded.

The second is the low sensitivity of the ground magnetograms. For Fredericksburg this is only 1.6 nT/mm and for Tucson it is 2.5 nT/mm. In our study, Pi 2 with amplitude exceeding 0.8 nT at Fredericksburg and 1.25 nT at Tucson were identified as ground Pi 2. Results are presented in Figure 14. Virtually all events are simultaneous within 1 min. There were, however, a small number of cases with greater delays. The time delays of Pi 2 onset at each station are indicated along the ordinate and plotted with

time lag AT = TAT S - TG; in which TAT S and T G mean onset time of Pi 2 burst at the satellite

and on the ground, respectively. The curves plotted at the left-hand side of each panel show the total occurrence probability for all local times. The probability of simultaneous ground- satellite observation within a time comparable to the measurement error exceeds 60%.

Initial Deflection of Pi 2 Bursts

at Synchronous Orbit

An important property of Pi 2 bursts observed at synchronous orbit is that the initial deflection in the east-west component caused by the wave is in the same direction as the dc

shift. Examples of this are shown in Figure 15. These deflections show a remarkable change with local time, as illustrated in the left and right panels. In the premidnight sector they are positive, in the postmidnight, negative. The dc shift of the field begins approximately at the onset of substorm expansive phase and continues

7024 Sak•arai and McPherron: Satellite Observations of Pi 2 Activity

SEPT 6, 1974

2 hUT 3 4 5 6 7 8 9 I0

0 u /

h :E 0 U U U

0

mN 80nT

90

-I00

-ll0

20nT

I0

0

-I0

?OnT

60

50

40

120nT

I10

I00

N

2 2 h 23, O0 03, h 04 05 75 e West LT P i 2 -- SUBSTORM ONSET IE •#T

A E254 NT

Fig. 19. Pi 2 bursts occur in close relation to the development of field-aligned currents at synchronous orbit and also with changes in the field configuration from tail-like to more dipolar. The upper and lower panels show the magnetograms at synchronous orbit and on the ground (Dallas), respectively.

for 10-20 min. Its sign and reversal at midnight indicate that it is caused by field-aligned currents above the satellite. Similar results

have been reported from ground observations [Rostoker, 1967b]. These field changes, including the initial perturbation of the wave and dc shift, are schematically shown in the top panel of Figure 16. Both effects can be explained by a diversion of the cross-tail current down field lines to the westward

electro jet, as shown in the bottom panel. Assuming that the perturbations are caused by currents flowing on field lines above the satellite, an eastward perturbation of the magnetic field should be seen in the premidnight sector and a westward one in the postmidnight sector.

Discussion

Polarization Characteristics of Pi 2 at

Synchronous Orbit

The polarization characteristics of Pi 2 observed at auroral zone stations have been

statistically studied by Sakurai [1970]. The study demonstrated that the rotational sense of Pi 2 in the auroral zone changes from left to right around midnight at the magnetic latitude of College (6 = 64.8 ø N, I = 259.7 ø E), but at the higher- latitude station, Point Barrow (6 = 69.6 ø N, I = 246.6 ø E), no clear dependence on local time can be observed.

This study has been extended recently by Kuwashima [1978]. This study focuses on the polarization characteristics with respect to location of the auroral electrojet. The results show that a polarization reversal is often observed across the auroral breakup region and the maximum occurrence time of the Pi 2's. At

the equatorward side of the auroral electrojet the polarization of the Pi 2 is left-handed in the premidnight and right-handed in the postmidnight sector. According to Kuwashima [1978], these polarization characteristics are consistent with the theoretical interpretations proposed by Chen and Hasegawa [1974] and by Southwood [ 1974 ].

The results obtained from this ATS 6 satellite

study show that during magnetically quiet conditions the polarization of Pi 2's is left- handed in the premidnight and right-handed in the postmidnight sector, similar to the results obtained on the equatorward side of the auroral electro jet. By taking into account the observed dc shifts at the satellite, it is apparent that the satellite is situated equatorward of the field-aligned currents. Therefore, it seems reasonable to assume that the rotational sense of

Pi 2 waves observed at the satellite is the same

as that on the equatorward side of the auroral electrojet. From the ground observations, Pi 2 maximizes in magnitude just below the auroral electrojet [Olson and Rostoker, 1977; Saito et al., 1976a]. Thus, when the satellite is closer to the field-aligned current, the Pi 2 waves


Midlatitude Pi :) Bursts equatorial density for the ambient plasma of 1-2 September 6. 1974 particles/cm 3 These calculations yield periods ß

0833 0011 ranging from 90 to 140 s, depending on the

I I density of the cold plasma and the intensity of 03•8•i the magnetic field. These periods are roughly 2•k•r consistent with the observed values which indicates that the period of Pi 2 observed at the

200¾

satellite may probably be attributed to the fundamental mode of the oscillation of the

magnetic field line passing through the satellite. However, in situ measurements of the ambient cold plasma at the time of wave events and a better field model are required to verify this conclusion. Recent observations on the GEOS 2 satellite show that the ambient cold

• plasma density decreases in the midnight

• O I I [•••l quadrant, reaching values of 10 cm 3 during I prolonged quiet periods ([Kp = 4 - 6) [Decreau et I I I I al., 1982]. However, during periods of higher 0t 04 05 08 0o 10 magnetic activity (Y-K = 10 - 20) the density is

P Un,versal hme much lower, in the range 0.1-1.0 cm 3 Taking ß Fig. 20. An expanded view of the Fredericksburg into account these observational facts, our 3 and Dallas magnetograms showing Pi 2 bursts as- assumption of a plasma density of 1 or 2 cm sociated with effect of field-aligned currents during moderately disturbed times seems illustrated in Figure 19. reasonableß

should have larger magnitude than those dealt with in this study. The typical magnitude of the Pi 2 waves observed at synchronous orbit is 10 to 20 nT, even during magnetically quiet conditions.

Relation of Frequency Content of Pi Bursts to Field Configuration

As discussed in previous sections, the peaks in the spectra of Pi 2's observed at ATS 6 are found near 100 s, with longer-period peaks of 200 s or more being rarely observed. The characteristics of these Pi 2 wave events were used to define

three types. The first is a superposition of a 100-s period oscillation and a large-amplitude, higher-frequency Pi 1 activity. The second is a 100-s wave unaccompanied by Pi 1. The third is a pure transverse wave in the azimuthal component.

We examine the spectra of Pi 2's more carefully in this section to determine whether there is a

connection between the type of Pi 2 observed at substorm onset and the magnitude of changes in the main field. To do this, we have plotted in Figure 17 the periods of all Pi 2 spectral peaks used in this study as a function of the change in

•._

the polar angle of the main field. This change is defined as the difference between the polar angle of the field just before the burst and this angle just after the rapid decrease caused by the substorm expansion. Thus, negative and positive changes correspond to growth of a tail-like field and return to a more dipole-like configuration, respectively. Large circles correspond to the dominant spectral peaks in each wave event and small circles to subsidiary peaks. It is evident that the dominant period in the spectrum becomes shorter as the change in the polar angle increases. Since small changes in angle are characteristic of weak substorms, this result suggests that long period Pi 2 are more frequent in time of low magnetic activity.

To interpret the period of Pi 2 component seen at the satellite location, we have calculated the period of the fundamental mode of torsional oscillation using a dipole field model and an

The Relation Between Pi 2 Bursts and Field-

Aligned Currents

As discussed in section 6, there exists a close relation between the onset of Pi 2 bursts and the

development of field-aligned currents. This relation is particularly obvious at synchronous orbit, where rapid fluctuations associated with the Pi 2 are superimposed on dc shifts caused by the currents in the azimuthal component of the field. An example of this relation is shown in Figure 18, where solid triangles denote Pi 2 onsets and shading denotes dc shifts in the east- west component of the synchronous field. Simultaneous ground data demonstrate that these events are typical Pi 2 bursts.

Another example of the relation between Pi 2 bursts and field-aligned currents is presented in Figure 19. In this example, multiple Pi 2 bursts plotted at the bottom of Figure 19 and in expanded form in Figure 20 are associated with more complicated changes in the east-west component of the field at the satellite. This example, as well as the one above, suggests that Pi 2 bursts are transient disturbances associated

with changes in field-aligned currents.

DIPOLE-LIKE FIELD

AIL-LIKE FIELD NEUTRAL

Fig. 21. A schematic representation of the field configuration change during the course of a Pi 2 event. The Pi 2 is assumed to be a

transient oscillation of the field-aligned current flowing along a magnetic field line during the disruption of the tail current at the onset of a substorm.

7026 Saknrai and McPherron: Satellite Observations of Pi 2 Activity

Pi 2 as a Transient Oscillation at Substorm

Expansion Onset

The intimate relation between Pi 2 bursts and

field-aligned currents illustrated above suggests that they may be caused by the same mechanism. One possibility is that the Pi 2 burst is simply a transient associated with the onset of a field-

aligned current. As pointed out by Southwood and Stuart [1980], the Alfven wave is necessarily associated with a fluctuating field-aligned current, and hence is the means by which information concerning the development of such a current in the magnetosphere is transmitted to the ionosphere. The ionosphere, however, is a good reflector of Alfven waves, which would then be expected to bounce back and forth along a field line between opposite hemispheres. This behavior would account for good Pi 2 conjugacy between both auroral zones and the equatorial plane, as is observed here.

It should be remembered, however, that the nighttime ionospheric conductivity is not high enough to support a large number of bounces between hemispheres. Consequently, it would be expected that the original transient would soon damp out, leaving only the dc current along the field line. As a typical example, we consider the Pi 2 event shown above in Figure 8. In this event, the wave period and damping time were 124 and 300 s, giving a damping rate of 0.3, respectively. Using the results of Newton et al. [1978], these values imply a nightside Pedersen

11 conductivity of 3 x 10 e.s.u., which is a reasonable value for magnetically quiet condit ions.

To account for the initiation of the field-

aligned current, we must elaborate on the above model. To do this, we recall that the Pi 2 is also associated with a dipolarization of the field line at synchronous orbit. In one widely discussed model of such dipolarization [McPherron, 1979, and references therein], a neutral line is formed in the near-earth plasma sheet, as shown schematically in Figure 21. The neutral line disrupts the cross-tail current, diverting it along field lines to the auroral ionosphere. Thus in this model the Pi 2 burst is a consequence of the sudden diversion of the tail current to the ionosphere by a near-earth neutral line.

Summary

This paper describes an initial attempt to characterize Pi 2 activity at synchronous orbit based on The magnetic field data recorded during September 1974. Our results may be summarized as follows:

1. Pi 2 bursts are seen primarily in the nighttime sector.

2, They are most probable near 2100 LT. 3. They are usually simultaneous with Pi 2

bursts. seen at midlatitude ground stations. 4. There are three main types of events at the

(3) A much rarer quasi-sinusoidal transverse oscillation of only the east-west component of the field.

5. The first and second types of Pi 2 events have a significant compressional component, while the third type is purely transverse.

6. The dominant period of Pi 2 bursts is approximately 100 s and tends to be shorter as the level of activity measured by field chang•es at the satellite increases.

?. During quiet times, there is an indication that the sense of rotation of Pi 2 bursts

changes from left-handed premidnight to right •-handed postmidnight.

8. The Pi 2 oscillations are. superimposed on dc shifts in the azimuthal component.

9. The initial Pi 2 perturbation in the azimuthal component is in the same sense as the dc shift caused by field-aligned currents.

Acknowledgements. The work described here was done while one of us (T.S.) was a Visiting Fellow at the Institute of Geophysics and Planetary Physics of the University of California, Los Angeles, and he thanks the faculty and staff at the Institute for their help and hospitality. Data on the behavior of ambient plasma at synchronous orbit as observed by GEOS 2 were provided by B. Higel and Wu Lei. Many helpful comments were made by D. J. Southwood and R. E. Gendrin. This work was partially supported by NASA grants NAS 5-11674 and NGL 05-007-004, by the Office of Naval Research N00014-75-C-0396, and by the National Science Foundation ATM 76-17O35.

The Editor thanks L. J. Cahill and G. Rostoker

for their assistance in evaluating this paper.

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Sakurai and McPherron: Satellite Observations of Pœ 2 Activity 7027

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T. Sakurai, Department of Engineering, Aeronautics, and Space Sciences, Tokai University, Hiratsuka, 259-12, Japan.

R. L. McPherron, Department of Earth and Space Sciences, Institute of Geophysics and Planetary Science, University of California, Los Angeles, CA 90024.

(Received November 30, 1981 revised April 26, 1983 accepted May 23, 1983.)

Documents

Satellite Observations of Pi 2 Activity at Synchronous Orbit...UCLA fluxgate magnetometer on ATS 6. Events that occurred in September 1974 have been studied by using digital power