The magnetic and optical signature of a Pg pulsation

0032-0633/90 s3.00+0.00 Pergamon Press plc

P/me-c. Sprrce Sci., Vol. 38, No. 11, pp. 1443-1456, 1990 Printed in Great Britain.

THE MAGNETIC AND OPTICAL SIGNATURE PULSATION

OF

G. CHISHAM PI@ D. ORR Department of Physics, University of York, Heslington, York YOl SDD, U.K.

M. J. TAYLOR Department of Physics, The University, Sou~ampton,

Hants 509 SNH, U.K.

H. LUHR lnstitut fur Geophysik und Meteorologic, Technische Universitiit Braunschweig, F.R.G.

(Received ~~nal~~r~

23 Ju!y 1990)

Abstract-A giant pulsation (Pg), observed on two magnetometer arrays in Northern Europe, has been analysed. The integrated video records from an all-sky TV camera that had been recording pulsating aurora concurrently, had an identical periodicity to the Pg. Complex demodulation analysis of the event has yielded the amplitude, phase and polarization characteristics of the wave packet. The dominant East-West (D) component wave signature in amplitude and phase suggests that this Pg has guided poloidal mode characteristics, On the basis of equatorial mass density calculations it is shown that the Pg is most likely to be a second harmonic oscillation. The bounce resonance instability with 17 keV protons is discussed as being a possible candidate for the generation of the Pg. The mechanism of Coroniti and Kennel (1970, J. geophys. Res. 75, 1279) is discussed as being the most likely mechanism for the generation of the pulsating aurora seen on this night.

1. INTRODUCTION

Giant pulsations (Pg) have intrigued observers since the beginning of this century. They are detected at the Earth’s surface as variations in the magnetic field with a very sinusoidal appearance and with periods in the range 45-150 s. No universally accepted generation mechanism has been proposed. Previous studies of Pgs have been carried out using arrays of magnetometers (Green, 1979,1985 ; Rostoker et al., 1979 ; Glassmeier, 1980) spacecraft (Hillebrand ef al., 1982) and aurora1 radar (Poulter et al., 1983). From this work, much has been discovered about Pgs. All the studies (except Green, 1985) observe Pgs within the aurora1 zone, occurring preferentially in the early morning hours, around the equinoxes in years of minimum solar activity (Brekke et al., 1987). It has been shown that Pgs Can be localized in both latitude and longitude (Glassmeier, 1980), with large azimuthal wavenumbers (Rostoker et al., 1979). The period of Pgs has a tendency to change slowly with time (Green, 1979), and there is also a tendency for Pgs to recur on successive days, 24 h apart (Rostoker et at., 1979). It has also been noticed that the majority of Pgs occur during the recovery phase of a substorm, and have resonant characteristics like other Pc4 pulsations, i.e. an opposite sense of polarization North and South of

a demarcation line where the pulsations are linearly polarized and where maximum amplitudes occur (Glassmeier, 1980). A phenomenon previously unrelated to that of Pgs is pulsating aurora. Pulsating aurora have been reviewed by Johnstone (1978) and Southwood and Wughes (1983). The temporal fluctuations of pulsating aurora1 luminosity have also been reviewed by Yamamoto (1988). Pasting aurora occur predominantly in the midnight to morning hours during the recovery phase of a substorm. Yamamoto (1988) defines pulsating aurora as aurora which show no localized shear-rotational deformations, but have a clear patch structure where the luminosity fluctuates in a quasi-periodic fashion with frequency normally between 0.05 and 2 Hz. The brightness of pulsating aurora is normally a few kilorayleighs in the aurora1 green line at 5577 A, and rocket observations suggest that pulsating aurora are produced by energetic electrons precipitating into the atmosphere. Recently, Tayior et al. (1989) presented details of a Pg pulsation observed on the night of 29/30 October 1987, by the EISCAT magnetometer cross, accompanied by pulsating aurora observed by an all-sky camera and a photometer. It was shown that the optical data, while having periods of 5-15 s associated with the puisating aurora1 patches, also had a period

1443

G. CHISHAM et al.

20°

3$

Longltudo

(E)

40U

FIG. 1. MAP SHOWINGTHE LOCATIONOF THE EISCAT MAGNETOMETER CROSS SITES,TWO SAMNET MAGNETOMETER SITESAND THEOPTICALSITEAT PITTIOVAARA.

The two circles indicate the field of view (zenith angle of 80”), and the integration region used (zenith angle of 30”) for the ail-sky camera. Both assume an aurora1 emission height of 100 km.

of approximately 77 s, associated with the Pg, and that the oscillation was phase coherent with the Pg. The present paper extends the work of Taylor et al. (1989) and analyses both the Pg and the temporal nature of the integrated video records in detail. Earth current data from the same night are also presented giving a clearer picture of the event.

2. INSTRUMENTATION

This study has made use of two ground magnetometer arrays, and the locations of the stations used in this analysis are shown in Fig. 1. These are SAMNET, the U.K. sub-aurora1 magnetometer network (Yeoman et al., 1990), and the EISCAT magnetometer cross (Luhr et al., 1984). The SAMNET magnetometers are three component fluxgate magnetometers with a sampling rate of 5 s. They can measure magnetic field variations over a range of + 512 nT with a resolution of 0.25 nT. The EISCAT magnetometer cross instruments are also three component fluxgate magnetometers, but with a sampling rate of 20 s. They can measure magnetic field variations over a range of f2000 nT with a resolution of 1 nT. The EISCAT magnetometer cross data had to be rotated

from geographic to geomagnetic coordinates (X, Y,Z to H,D,Z) for this analysis. Also, for direct comparison of the two data sets using digital analysis techniques it was necessary to reduce the sampling interval of the SAMNET data to 20 s. This was achieved by low-pass filtering at 20 s, while discarding three out of four data points. Magnetometer data were also obtained by the Geophysical Observatory at Sodankyla ; however they were of insufficient temporal resolution for this study. The optical observations were made from Pittiovaara, near Sodankyla, Finland, during the new Moon period in October 1987. The measurements were made as part of the MAC-EPSILON campaign and were primarily directed towards monitoring the low elevation sky to the West of Sodankyla in the direction of the sounding rocket sites at Andoya, Norway, and Kiruna, Sweden. As part of these measurements an all-sky aurora1 TV system with a temporal resolution of 0.3 s was operated alongside a zenith pointing wide angle photometer (- 30”) (Sodankyla Geophysical Institute) to monitor local sky conditions (see Taylor el al., 1989). For simplicity the TV system (which had an S25 photocathode response), and the photometer were operated without filters to provide

i445

The magnetic and optical signature of a Pg pulsation TABLE 1.TABLE SHOWING THE COORDINATES OF ALL THE STATIONS USED IN THIS ANALYSIS AND THEIR

RESPECTIVE

kWELLS

coords Long. E.

Geomag. Lat. N

coords Long. E.

22.22 22.96 23.05 23.53 24.08 20.70 27.01

67.01 66.28 65.43 64.39 63.23 65.61 65.95

106.91 106.83 106.17 105.73 105.34 104.41 109.82

6.66

SAMNET (U.K. sub-aurora1 magnetometer network) Oulu OUL 65.10 25.85

61.30

105.56

4.41

Optical observations Pittiovaara

PIT

67.42

26.39

63.91

107.56

5.13

Earth currents Sodankyla

SOD

67.37

26.63

63.57

107.66

5.13

Station

Code name

EISCAT magnetometer cross Soroya SOR Alta ALT Kautokeino KAU Muonio MU0 Pello PEL Kilpisjarvi KIL Kevo KEV

Geograph. Lat. N 70.54 69.86 69.02 68.01 66.90 69.05 69.76

general information on aurora1 activity. Intensity plots were derived from the video records by integrating the video signal within a 30” rectangular area centred on the zenith (for comparison with the photometer trace). The inner circle in Fig. 1 shows this sample area, which assumes an aurora1 emission altitude of 100 km. The intensity plots were digitized with 1 s sampling to make them available for FFT analysis. This study also makes use of Earth currents recorded at Sodankyla. Table 1 shows the stations used in this analysis and their locations in geographic and corrected geomagnetic coordinates for the end of 1987, and their L-value. The main analysis technique used in this study is the technique of Complex Demodulation (Beamish et al., 1979). This is a method for analysing particular frequency components of a waveform in terms of amplitude, phase and polarization characteristics and is based upon the Fast Fourier Transform (FFT). The results of Complex Demodulation are shown in the form of contour diagrams. The contour diagram form of presentation necessarily assumes that all the stations lie on the same geomagnetic meridian. Since all the stations on the latitudinal chain lie within 1.5” of geomagnetic longitude we can say that this is approximately so.

L-Shell

6.28 5.88 5.44 5.01 5.96 6.12

I

I

6.00

10.00

14.00

18.00

22.00

1

I

2.00

6.00

UNIVERSAL TIME

FIG. 2. A

PLOT

OF

24 h

OF MAGNETOMETER

DATA

AT

MU0

(NO FILTER).

Plot starts at 0600 U.T., 29 October 1987, and ends 0600 U.T., 30 October 1987. The start and finish times of the Pg event are indicated by the vertical dotted lines.

3. RESULTS

A substorm occurred on the night of 29/30 October 1987. Figure 2 shows a 24 h plot of magnetometer data from MU0 on the EISCAT magnetometer cross. The plots from other stations across the array are very similar to that at MUO, so only the one is shown. The

plot starts at 06:OO U.T. on 29 October and finishes at 06:OO U.T. on 30 October. It can be seen from this figure that following a sudden impulse (SI) at approximately 13:30 U.T. a series of substorms develop. The recovery phase after the substorm begins

G. CHISHAMet al.

PIT VDEO RECORDS

L

0.22

I

0.27

I

,

0.32

,

0.37

0.42

1

0.47

t

0.52

I

0.57

1.02

UNIVERSAL TME FIG.~.D COMPONENTOFTHE

Pg EVENTON

ANDOU~FROM The

THELATITuDINALCHAINOFTHE SAMNET, FWEREDBETWEEN~0AND

EISCATMAGNETOMETER CROSS 120 S.

top trace is of the unfiltered video records of the all-sky TV camera data integrated over a 30” area centred in the zenith.

at approximately 23:30 U.T. and continues on into the early morning of 30 October. It is within this recovery phase at approximately 00:30 U,T. that the Pg occurs. Figure 3 shows magnetometer data from the EISCAT magnetometer cross and the SAMNET station OUL, together with the integrated video records, which are calculated by integrating the video signal within a 30” rectangular area eentred on the zenith, and which have been digitized. Only the a component ‘of the magnetometer data is shown since it is the dominant component with respect to Pgs. A data gap occurs in the video records between 00~36 and 00:43 U.T. Looking at the video records, it is noticeable that a periodic oscillation with similar period to the magnetic oscillation begins at 00:30 U.T. and continues until approximately 0052 U.T. There also seems to be good phase coherence between the optical and magnetic signatures. The magnetometer data show sinusoidal variations

with the largest amplitude at a similar latitude to the optical pulsating aurora1 patches. From Fig. 3 we can see that the pulsation amplitude has decreased to about a quarter of its maximum value at the northernmost (SOR) and southernmost (OUL) stations. It is not visible at NUR (South of Oulu, on the SAMNET array). This demonstrates that the pulsation is localized in latitude. Data from KIL and KEV {to the West and the East of KAU, respectively), on a similar geomagnetic latitude to KAU, indicate the pulsation amplitude to have decreased by half at these points, and the pulsation is not visible at NOR (West of OUL. on a similar geomagnetic longitude, on the SAMNET array), showing that it is probably also localized in longitude. Spectral analysis of the pulsation illustrates its narrow spectral bandwidth (see Fig. 4) with a central period of approximately 77 s. The pulsation occurs in the early morning hours and has a large azimuthal wavenumber (calculated from phase estimates at KIL and KEV) with westward phase propa-

The magnetic and optical signature of a Pg pulsation

0.0

12.5 FREQUENCY

25.0 fmHz)

FIG. 4. FFT POWER SPECTRA FOR THE U.T., FORBOTHTHEVIDEO RECORDSAND

PERIOD 00:22-01:02 THEDCOMPONENT OFTHEMAGNETOMETERTRACES,

The scale denotes increasing powers of 10.

gation. All these properties are characteristic of Pg pulsations. Figure 4 shows a stacked power plot of the video records (top panel) and the D component of the six stations in the latitudinal chain. The power spectra have been calculated over the interval from 00:22 to 01:02 U.T. the same as is shown in Fig. 3. The spectral estimates are calculated with four degrees of freedom, and the data are high pass filtered at 400 s. It can be seen that all the magnetometer stations and the video records have the same dominant peak at approximately 13 mflz (77 s). The peak is less prominent in the video records. This is probably due to the creation of broadband noise due to the data gap in the video records.

144-l

The Pg has been analysed using the technique of complex demodulation (Beamish ef al., 1979). Using this technique, amplitude, phase and polarization characteristics of the pulsation have been determined using a frequency band centred on 13 mHz (77 s). Figures 5a-d show contour plots of the variation of the amplitude and phase of the H and D components with latitude and time, and Fig. Se displays the ellipticity variation during the event. Looking first at the H amplitude, the maximum amplitude of the event is about 2.5 nT and is rather constant over the four stations PEL, MUO, KAU and ALT. This maximum occurs at approximately O&42 U.T. The H phase changes by more than 180” over the region of maximum amplitude. The 1) amplitude maximum occurs at MU0 at approximately 00:44 U.T., and has a value between 5 and 6 nT. The D phase, in contrast to the large variation in the H phase, remains approximately constant with time across the array. The plot of ellipticity shows an ellipticity reversal in the region of the amplitude maximum, a characteristic associated with the field line resonance theory (Southwood, 1974). The polarization characteristics of the event are presented in more detail in the form of polarization maps in Figs 6 and 7. Figure 6 shows a plot of the pola~zation ellipses calculated at each station in the latitudinal chain, in the universal time interval which covers the major part of the event. The first noticeable thing is that the ellipses at the lower latitude stations have anti-clockwise polarization, and those at the higher latitude stations have clockwise polarization. At the intermediate station MUO, the polarization is extremely linear and alternates between clockwise and anti-clockwise. However, the polarization ellipses are approximately constant with universal time, and due to their consistency, they can be averaged over the event to give a reliable estimate of the average polarization of the event. Figure 7 shows the polarization across the whole EXSCAT magnetometer cross array and OUL. In this figure the polarization eilipses have been averaged across the centre of the event (00:3& 0050 U.T.). Noticeably, the polarization is linear at MUO, where the maximum D amplitude occurs, and the polarization is clockwise poleward of MUO, and anti-clockwise equatorward of MUO. Such polarization changes have been detected in Pg studies by Green (1979) and Hillebrand et al. (1982). Also shown in Fig. 7 is the azimuthal wavenumber calculated between KIL and KEV. These stations give reliable m values which are consistent with other measurements such as STARE. KAU is not used in these calculations since it is somewhat contaminated by ground anomalies. This value has a large error due to the small

1448

-90.00

(4

-135.00

0.49

PEL

MU0

KAU

ALT

SOR

0

0.29

62 0.34 0.39 0.44 Universal Time

0.49

-1.00

-0.75

W

Anliclockwise

UNIVERSALTIME

PEL

MU0

564 E 8 a 63

KAU

3 65 .o zi

ALT

SOR

g 66 3 C

67

Ellipticity

FIG. ~.CONTOURDIAGRAMSSHOWINGTHEVARIATIONOFAMPLITUDE,PHASEANDPOLARIZATION CHARACTERISTICS OFTHEEVENTWITH THEARRAY,ASCALCULATED USINGCOMPLEXDEMODULATION. (a) H amplitude; (b) D amplitude; (c) H phase; (d) D phase; (e)ellipticity.

-,35.00-180.00~

&g

0.00 -45.00

-45.00 -90.00 -

m

90.00 45.00

45.W~ o.oo-

:

135.00 180.00

135.w. 90.00 -

0.34 0.39 0.44 Universal Time

:

0.29

62

67

D Phase

ACROSS

G. CHISHAM et al.

PEL ,77’T”““isqq~csc?44c?

Out

9 0.29

Q

e

uWvW?mG!44?

0.33

Y

0.37

0.42

0.46

0

UNIVERSAL TIME 0, 0,

Antlctockwise poierisation Clockwise polarisetion

FIG. 6. DETAILED PLOT OF m

ellipse

ellipse

VARIATION OF m ~L~IZATION ELLIPSESAT ALL M LATITUDINAL CHAIN, WITH UNIVERSALTIME.

station separation, the low sampling frequency and to the two stations being at slightly different geomagnetic latitudes. It has been calculated using the D component of the pulsation only. This is to avoid incorrect apparent phase propagation measurements that can sometimes arise in the H component due to its variability with latitude. The calculated m value is large, and the phase propagation is westward. The large azimuthal wavenumber observed and the localization of the pulsations suggest that Pgs are oscillations dominantly in the guided poloidal mode (Orr and Matthew, 1971). The plasmapause position has been estimated using the method of Yeoman (1988) which is based on that used by Orr and Webb (1975). This method is found to give the best agreement with a study of 41 crossings of the plasmapause observed on GEOS-2 cold plasma data. The position was estimated to be just below L = 4, out of the range of the polarization map shown in Fig. 7. This confirms the event as being located within the plasmatrough on this occasion, and not being related to the plasmapause as was suggested by Rostoker ef al. (1979). The video records from the TV camera show the occurrence of pulsating aurora from around midnight U.T. These aurora are seen for a couple of hours. They consist mainly of large-scale, irregularly-shaped luminous patches occurring equatorward of a series

STATIONS IN m

of discrete aurora1 arcs. The majority of the patches are spatially stable with lifetimes of 5-l 5 s, although the position of the patches overhead varies during the magnetic cycle as explained by Taylor et al. (1989). A number of the patches, however, exhibit streaming and split-streaming characteristics, travelling at velocities of the order of tens of kilometres per second. A singular ring-like structure is exhibited by some of the observed patches and is as yet unexplained. These unique patches are shown in detail in Taylor et ai. (1989) (see their Fig. 4). Figure 8 shows a more detailed plot of the integrated TV camera data. This figure is separated into two parts, the oscillations before the data gap, and those after. Before the start of the definite 77 s modulation (00:22-00:30 U.T.), the trace is varying in a quasi-sinusoidal manner with a period in the 5-15 s range. Just after 00:30 U.T. the character of the trace changes, and these 5-l 5 s oscillations are modulated by a 77 s period. This is seen clearly on the video records as all pulsating aurora1 phenomena cease every 77 s. This modulation can be seen very clearly between 00:30 and 00:36 U.T., and between 00:44 and 0054 U.T. Towards the end of the plot, this modulation dies away, and the trace reverts back to its original character: Figure 9 shows power spectra calculated for four intervals of the video records. The

1451

The magnetic and optical signature of a Pg pulsation 2.3 I

CENTRAL MAGNETIC LOCAL TIME 2.69 3.07 3.13 3.23

I

r----l

have eight degrees of freedom and have been high pass filtered at 200 s. Trace (a) is calculated from data between 00:22 and 00:32 U.T. where the dominant period of the oscillations is shown to be between 7.65 and 16.25 s. Trace (b) is calculated from data between 00:29 and 00:37 U.T., and in this case the dominant spectral estimate is that corresponding to 77 s. This is also seen in trace (c) which is calculated from data between 00:42 and 00:54 U.T. Trace (d), showing data between 00:52 and 01:02 U.T. shows only broadband features. Figure 10 shows earth currents recorded at Sodankyla on the same night between 00:30 and 00:43 U.T. This shows a dominant 77 s period with oscillations of the order 5-20 s superimposed. This indicates that magnetic variations in this period range were occurring as well as the 77 s variation although they were not visible with the EISCAT magnetometer cross due to the 20 s sampling rate of the magnetometers. spectra

3.31

A SOR

33.O

0.s -

0

. ALT

0 A KEV

0.0 -

-.

KIL

3

!i :

-

so.0

x

-

s3.0

b

z

m--23

3

). KAU

Y

3

\

6.S -

s

A MU0

0 -s&o

0

LO-

l

g

PEL - 33B

- *2.a 4.6

-

4. DISCUSSION

Q + OUL 102.0

104.0

106.0

GEOMAGNETIC

103.0

110.0

4.1. Interpretation of giant pulsations as oscillations predominantly in the guided poloidal mode

112.0

LONGITUDE

m._r.oWentward

appar.nt

ph...

propagation

z

aw.r.nt

phas.

prop.~.tlon

Eastward

Q+

a*

*“tlslockrl*. ClOCLIIS.

pol.rls.tlon

Polwl84tlon

FIG. 7. PLAITOFTHEPOLARIZATION

The experimental data presented in Figs 5-7 allow us to speculate on the mode of oscillation of the hydromagnetic wave reported in this paper. From these figures the following observations can be made about the characteristics of the H and D components at 00:44 U.T. (i.e. at the time of the strongest signals detected on the ground) :

.IllPS.

*IlIP** ELLIPSES AVERAGED

THE CENTRE OF THE EVENT, ON THE MAGNETOMETER

ACROSS ARRAY.

The calculated m value is also shown.

I

I

0.22

0.24

0.44

0.46

I

0.26

I

0.28

I

0.30

(i) The D amplitude peaks quite strongly and is greater than 5 nT at MUO. At SOR and OUL,

I

0.32

1

0.34

I

0.36

I

0.38

UNIVERSAL

TIME

I

,

0.48

FIG. 8. DETAILEDPU)T OFTHE

I

0.50 INTEGRATED

I

0.52

I

I

0.54

0.56

1

0.58 1.0 UNIVERSAL

c

1.2 TIME

VIDEORECORDS ON THEMORNINGOF 30 OCTOBER1987.

1452

G. CHISHAMet al.

(iii) The H amplitude has a broad peak centred on MUO. (iv) The H phase changes systematically with latitude, the lower latitude stations leading the higher latitude ones. (v) The polarization is almost linear at MU0 and has a clockwise sense of rotation to the North and an anti-clockwise sense to the South of MUO.

lOOr

ts

Id)

I

0.0

I

I

100.0

200.0 FREQUENCY

FIG.

9. FFT

POWER

INTERVALS

SPECTRA

OF TH!? FOLLOWING

OF THE INTEGRATED

VIDEO

mHz FOUR

TIME

RECORDS.

(i) 00:22-00:32 U.T. ; (ii) 00:29-00:37 U.T. ; (iii) 00:42-00:54 U.T. ; (iv) 00:52-01:02 U.T.

approximately 3” poleward and equatorward of MUO, respectively, the D amplitude has dropped to about 1 nT. (ii) The D phase is remarkably constant over the array.

I

I

I

1

SODANKYLA

I

0.30

I

I

0.32

I

I

I

I

The hydromagnetic oscillation that is occurring in the magnetosphere is obviously a complex coupled system. We can, however, imagine a simplified situation and consider the structure of the Hand D components separately. The dominant yet localized D component, which is in phase over the ground magnetometer array, and which has a very regular period, is suggestive of a localized standing hydromagnetic wave driven by energetic charged particles. As is inferred later in this discussion, the wave appears to match well with a second harmonic guided poloidal mode centred on a latitude close to MU0 (L = 5.44). A candidate for the energetic particles sustaining the oscillations are westward-drifting protons. The protons give up energy to the wave and amplify it in the bounce resonance mechanism as described by Dungey (1965) and Southwood er al. (1969). The bounce resonance mechanism is considered again later in this discussion. The H component of this Pg is relatively localized in space and can be described as an asymmetric toroidal mode driven by the second harmonic guided poloidal

I

I

I

I

I

I

I

1

1

1

EARTHCURRENTS

1

I

0.34

I

I

0.36

I

I

0.38

I

0.40 UNIVERSAL

FIG. 10. EARTHCLJRRENTRECORDSFROMSODANKYLAON

30 OCTOBER

0.42 TIME 1987.

The magnetic and optical signature of a Pg pulsation

mode. This mode obtains energy from the radially inward diffusion of energetic positive ions and is analogous to the driven standing waves discussed by Orr and Hanson (1981) and Gough and Orr (1984). Here, the transverse mode component in the magnetosphere (6$ is the response to the compressional driving term b, sin (wrt), and 6, on adjacent L-shells are solutions of the differential equation describing forced damped simple harmonic motion :

a2b,

x

+ 2~ 2

+W,?b+ = o,fb,c sin (art),

where w, is the natural eigenfrequency of the geomagnetic flux tubes, c is a coupling constant and y relates to the damping in the system. The compressional driving term b,, in the guided poloidal mode, will be localized over a small range of L-shells, but over this range the disturbance will be in phase in the magnetosphere. The signal detected by a magnetometer on the ground will have the wave vector rotated by 90” due to the influence of the ionosphere as described by Hughes and Southwood (1976). Consequently, these features will appear in the D component on the ground with constant phase over a small latitudinal range. The magnetospheric asymmetric toroidal mode (b,) response to the guided poloidal driving term will result in a maximum amplitude where the guided poloidal driving frequency matches the toroidal eigenfrequency. This will appear in the H component on the ground and for the second harmonic of both modes it will occur at approximately the same latitude. At adjacent latitudes the natural toroidal eigenfrequencies will vary, typically for plasmatrough latitudes the eigenfrequency decreases with increasing latitude. The magnetospheric toroidal response will decrease away from the resonance position and the phase will change by 180” through the resonance. In the above case the H phase on the ground on the lower latitude side of resonance will lead that at the higher latitude side of resonance. This description of giant pulsations has parallels with the field line resonance theory developed by Southwood (1974) and Chen and Hasegawa (1974). They considered fast mode waves coupling to guided toroidal mode waves ; here we have discussed compressive guided poloidal mode waves coupled to guided toroidal mode waves through the non-uniformity of the ambient geomagnetic field. 4.2. Giant pulsations: mechanism

harmonic mode and generation

Whether Pgs are odd or even mode oscillations has been a matter of continuing debate. Annexstad and

1453

Wilson (1968) first reported Pgs as being even mode oscillations, using results from approximately conjugate stations at College, Alaska, and Macquarie Island. Green (1979), once again using conjugate stations, but at the lower latitude stations of St Anthony and Halley bay, classed the Pgs he observed as odd mode oscillations. However, for the Pgs he observed, the conjugate stations were some distance from the centre of the events. This can be deduced from Green’s polarization analysis. Hillebrand et al. (1982) concluded that the Pg event that they studied was an odd mode standing wave on the basis of the relative sizes of the transverse magnetic perturbation near the equator and on the ground. However, as was pointed out by Poulter et al. (1983) the dipole field geometry produces an enhancement factor between the equatorial plane and the ionosphere which they did not take into account. Poulter et al. (1983), on the basis of estimated equatorial mass densities, suggested that a second harmonic even mode oscillation was the most likely. Similarly, we have estimated equatorial mass densities for this event using the method of Orr and Matthew (1971) for a dipole field geometry. The results are shown in Table 2. A period of 77 s and an L-shell value of 5.44 were used, and the wave was assumed to be a guided poloidal mode wave. Calculations were made for both an Rm4 and Re3 density variation along the field line, for both a dipole and more realistic field [using a - 50% correction suggested by Singer et al. (1981)], and for both fundamental and second harmonic mode waves. Poulter et al. (1984) deduced a large number of estimates of the equatorial mass density at different L-shell values using the eigenperiods of pulsations observed on the Slope Point and STARE radars (see their Fig. 5). At an L-shell value of 5.44, Poulter et al. (1984) calculated equatorial mass densities in the range 3(rl80 a.m.u. cmm3. Our estimates for a second harmonic wave in a realistic field were 21 and 25 a.m.u. cme3 . However, the results presented by Poulter et al. (1984) were from events observed during solar maximum and they assumed N 50% 0 + ions were present, to obtain such large mass densities. From Young et al. (1982), we know that the 0+/H+ ratio decreases with decreasing geomagnetic activity and so for our event we could possibly expect - 10% O+ ions. This could explain the lower values of equatorial mass density that we have calculated. Poulter et al. (1984) also show (see their Fig. 6) that in the early morning hours, the equatorial mass density is lower than at other times of the day and therefore we would expect our calculation to be at the lower end of the Poulter equatorial mass density range. On the basis of our results and those of Poulter et al. (1984), we conclude that our event is most likely to be a second

1454

G. CHISHAM

TABLE 2. TABLE

SHOWING

et

al.

CALCULATIONS OFEQUATORIAL MASS DENSITIES FOR

FUNDAMENTAL

ANDSECONDHARMONICWAVEFORMS

Mode Fundamental 2nd harmonic

Field configuration Dipole Realistic Dipole Realistic

harmonic mode wave. From 34 Pg events observed on the EISCAT magnetometer cross, we have concluded (Chisham and Orr, 1990), using the same method, that all these Pgs are second harmonic mode waves. Because of this, we can discount generation mechanisms which require the wave to be an odd mode, e.g. the drift wave instability of the compressional Alfvtn wave, as suggested by Green (1979, 1985). A likely candidate for Pg generation, as discussed earlier, is the bounce resonance mechanism. If standing waves are set up on the field lines, resonance can take place between these standing waves and unstable particle distributions when the condition w = mw,+No, is satisfied, where m is the azimuthal wavenumber, wd is the particle’s azimuthal drift frequency, w,, is the particle’s bounce frequency and N is an integer. The N = + 1 bounce resonances have been suggested as a generation mechanism for Pgs by Glassmeier (1980) and Poulter et al. (1983). Southwood et al. (1969) showed that the resonance condition results in a quadratic in the particle resonant velocity. The roots of this quadratic give a high energy solution and a low energy solution. For this Pg event, if it is assumed to be a second harmonic wave in a dipole field geometry, the most likely solution is the low energy root for protons, which is N 17 keV. Poulter et al. (1983), using a similar method, calculated a low energy solution for their event observed on aurora1 radar of 10 keV. From 34 Pg events observed on the EISCAT magnetometer cross, we have calculated, using the same method, a range of values for the low energy root of 8-20 keV. However, Glassmeier (personal communication) has suggested, on the basis of GEOS-2 observations, that Pgs are generated by the bounce resonance instability occurring with westward-drifting proton clouds with a non-Maxwellian distribution at around 60 keV. Hillebrand et al. (1982) have observed fluctuations in proton fluxes at geostationary orbit, at the time of a Pg event, in the range 25.5-234 keV. The bounce resonance instability seems a likely candidate for Pg excitation but more coincident spacecraft particle and

Equatorial mass density (a.m.u. cm-‘) Rm4 variation R- 3variation 4.5 2.2 41 21

4.7 2.3 49 25

wave measurements are needed to determine the exact proton populations that are involved.

4.3. Pulsating aurora Pulsating aurora have been associated with long period geomagnetic pulsations before. Berger (1963) presented a case of simultaneously occurring giant pulsations and pulsating aurora recorded at a single observatory. Barcus and Rosenburg (1965) observed similar temporal behaviour between energetic electron precipitation, cosmic noise absorption and magnetic variations in the 150-300 s period range. Berkey (1974) observed pulsations recorded with a riometer, photometer and TV camera at the same time as a PCS oscillation. He suggested that hydromagnetic waves may have been the source of the optical pulsations and that a possible effect of a wave-particle interaction could be the modification of the particle pitchangle, moving particles into and out of the loss cone. Kokubun et al. (1981) observed an event that was associated with a slowly varying magnetic pulsation with an approximate period of 2 min. In the shorter period range, Campbell (1970) observed Pc3 pulsations at the same time as photometer modulations of a similar period. We suggest that the lack of observations of long period modulations in pulsating aurora may be due to the fact that longer periods are not obvious to the observer and shorter period fluctuations dominate. However, the most common geomagnetic pulsation associated with pulsating aurora is the short period Pil, or Pcl-2. Oguti et al. (1984) suggested that this magnetic pulsation is mostly due to fluctuations of the ionospheric electric currents caused by temporal and spatial changes in electric conductivity produced by the pulsating precipitation of aurora1 electrons, and that they have little or no magnetospheric origin. The sampling interval of the EISCAT cross magnetometers is 20 s and therefore no short period geomagnetic pulsations related to the Pg event can be observed. However, earth current data from Sodankyla shown in Fig. 10, show, along with the 77 s

1455

The magnetic and optical signature of a Pg pulsation shorter period fluctuations in the range of approximately 5-20 s. Several mechanisms have been proposed to explain pulsating aurora. Most of these assume that electrons are precipitated by VLF whistler mode turbulence causing pitch-angle diffusion of electrons into the loss cone in the vicinity of the geomagnetic equator (Kennel and Petschek, 1966). A possible mechanism that could explain the present event is that discussed by Coroniti and Kennel (1970) and Haugstad (1975). They proposed that hydromagnetic waves could modulate the whistler growth rate and hence the pitchangle diffusion and subsequently the precipitation of electrons. The Coroniti and Kennel mechanism requires that the waves be even mode waves, and as our Pg is a second harmonic it fulfils this requirement. However, it can be seen on the video records and in Fig. 3 that pulsating aurora are occurring before the Pg begins and before the 77 s modulation in the optical records has begun. This could suggest that either some other mechanism was operating before the onset of the Pg, or that the Pg was not observable on the ground due to either ionospheric shielding or inadequate magnetometer sensitivity. A mechanism that could possibly have been occurring before the Pg onset has been discussed by Royrvik (1978) in which the source of the whistler waves is actually the loss cone distribution. When the Pg is seen by the magnetometers it seems to act as a modulator or switch for precipitation, with all pulsation activity ceasing every 77 s. It could also be true that the Pg is altering the pitch-angle distribution of precipitating particles at the geomagnetic equator. However, opposition to the Coroniti-Kennel theory has come from Oguti et al, (1986) who, using data from GEOS-2, showed that in their case study, pulsation precipitation of aurora1 electrons occurred without any detection of hydromagnetic waves in the magnetospheric equatorial region. Hillebrand et al. (1982) observed oscillations of electron flux in the equatorial plane with comparable period to their Pg. The oscillation of electrons in the energy range 32-51 keV started after the onset of the Pg, making it likely that the oscillation was due to the Pg. Electrons of this energy would precipitate into the ionosphere between heights of 85 and 116 km. It is possible that Hillebrand et al. (1982) observed a similar event to the present one, and that precipitating electrons and hence pulsating aurora occurred for their event. variation,

5. SUMMARY This study has shown unique data of a Pg pulsation occurring concurrently with pulsating aurora phen-

omena on the night 29-30 October 1987. The technique of complex demodulation was used to show the localization of the Pg in latitude and to show the large azimuthal wavenumber associated with the Pg. The technique also revealed the complete polarization characteristics of the event, including the polarization reversal that occurs at the station of maximum amplitude. It was shown that the Pg was most likely to be a second harmonic oscillation predominantly in the guided poloidal mode. It was suggested that this guided poloidal mode oscillation was driving an asymmetric toroidal mode oscillation resulting in the field line resonance-like structure of the wave observed on the ground. Bounce resonance with - 17 keV westward-d~fting protons was suggested as a possible generation mechanism for the event. The mechanism of Coroniti and Kennel (1970), in which particle precipitation is modulated by hydromagnetic waves, was discussed as a mechanism for the generation of the pulsating aurora. The likelihood of this type of event being common is quite low, since pulsating aurora are extremely common and Pgs are rare. However, it is obvious from the results we have presented that some kind of coupling is occurring between the Pg and the pulsating aurora mechanism. Acknowledgements-We are greatly indebted to Dr E. Kataja (Geophysical Observatory, Sodankyla, Finland) for offering the facilities of the Pittiovaara site. The SAMNET data were supplied by Dr D. K. Milling (University of York, U.K.). We are also grateful to Dr T. K. Yeoman (University of Leicester, U.K.) for the development of some of the software used in this analysis. The EISCAT magnetometer cross is a joint enterprise of the Finnish Meterological Institute, the Sodankyla Geophysical Observatory, and the Technical University Braunschweig. The optical measurements were supported by the U.S. &r Force-Office of Scientific Research as part of the MAPSTAR programme. One of us (G.C.) is currently in receipt of a SERC studentship.

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