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Pi2 pulsation polarization patterns on the U.K. sub-auroral magnetometer network (SAMNET)
Pi2 PULSATION POLARIZATION PATTERNS ON THE U.K. SUB-AURORAL MAGNETOMETER NETWORK (SAMNET)
Department
T. K. YEOMAN.* 1). K. MILLING and D. ORR of Physics, University of York. Heslington. York YO1 5DD, U.K.
array of seven fluxgale magnctometcrs has been deployed covering mid-latitudes in western Europe. The array is configured to allow simultaneous observations ofmid-latitude pulsation characteristics in both latitude and longitude. Here the two-dimensional variations in the horizontal polarization of Pi2 pulsations arc related to the substorm current wedge position. deduced from the mid-latitude bay behaviour, and an estimated plasmapause position. Features of the polarization pattern which arc not in agreement with the substorm current wedge model arc found. These include unexpected polarization ellipse azimuths and longitudinal phase propagations. and latitudinal variations in polarization ellipse azimuths not associated with ellipticily reversals. An extension of previous travclling w’ave models to include low latitude ellkts is considered. which may explain at least some of these discrepancies. Abstract-An
I. INTRODUCTION
The polarization patterns of mid-latitude Pi2 pulsations have been the subject of a considerable amount of previous work. most notably on the Air Force Geophysics Laboratory (AFGL) magnetometer chain (e.g. Lester et d., 1983). It is now known that anticlockwise (ACW) polarized signals arc predominant at mid-latitudes but that clockwise (CW) polarization becomes more frequent at aurora1 latitudes. It has also been commonly observed that the mid-latitude pulsations have a mostly westward apparent phase propagation and that the polarization pattern is ordered by the substorm current wedge. It is helpful to first consider the higher latitude characteristics and polarization patterns which occur near the primary aurora1 amplitude maximum of the damped Pi2 wavcpackct. Typically. ITI-values of around lO&30 are seen in this region (e.g. Lester er (/I., 1985 ; Samson and Harrold. 19X5). Coiljugate results (e.g. Kuwashima and Saito, 1981) have shown that the M component is in phase in the Northern and Southern Hcmisphcres. whereas the /I) component is out of phase. This is characteristic of an odd mode oscillation guided along a field lint (Sugiura and Wilson, 1964). Pashin et d. (1952) and Lcstcr c’r (I/. (1985) both showed that in this region pulsation cquivalcnt currents in the ionosphcrc had ;I Vortex structure which appcarcd CO bc coincident with the ionosphei-ic intersection of the substorm current
* No\\ at : Department of Physics. (Jnivcrsity of Leicester. llnivcrsity
Road.
Leicestcr
LEI 7RH. U.K. 589
wedge field-aligned currents. Sakurai (at al. (1988) showed that the H component phase coherence was greater than that of the D component both conjugatcly and between three Icelandic stations. These observations may be taken as evidence that the am-oral Pi2 signature is characterized by a transient, Alfvtnic field line resonance more or less co-located with the field-aligned current system. Samson and Harrold (1983) measured statistical Pi2 polarization characteristics at high latitudes. Their pattern has not been reproduced by any model, most notably in the lower latitude region of their observations. It dots. however. show many of the features predicted by the oscillating field-aligned current model of Samson (1982). Whilst the high latitude Pi2 signal is predominantly CW polarized. it undergoes a polarization reversal to becomc ACW at lower latitudes. The position of thih reversal has often been shown to be close to the plasmapausc position (Fukunishi, 1975 ; Lester and Orr. I YXI. 1983). Studies of the wave polarization on meridional magnetometer chains have shown that an additional switch in the wave azimuth quadrant sometimes occurs in latitude equatorward of the cllipticity reversal. This has been interpreted in terms of field line resonance theories (E‘ukunishi, 1975 ; Stuart et C/l.. 1979). In the mid-latitude rcyion m-values of around 3 predominate (c.g. Lester et rrl.. 1983). In this region Lcstcr CI nl. ( 1983) used the AFGL chain to study the magnetic bay variations which arc caused by the fieldaligned currents of the substorm current wedge. These were then related to the longitudinal variations of the
590
T. K. YEOMAN(‘I cl/.
Centreof
of the substorm currcnl wedge a switch to CW polarization was observed at this latitude. The azimuthal phase propagation and horizontal polariration of these low-latitude waves is not consistent with a field line resonance. Gclpi Q/ rrl. (198%) showed that the ordering of the mid-latitude pattern rclatcd to geostationary orbit substorm observations as well as ground observations of the substorm ccntrc. Singer ct LI/. (1985) went Midlatitude 1 on to confirm the value of ground observations in DC Magnetic determining substorm onset times and longitudes in order to interpret satellite signatures. Satcllitc data have since been interprctcd using the mid-latitude Pi2 polarization pattern by, for cxamplc, Smits ct al. (19X6). Gelpi cut(I/. (1985b) studied the mid-latitude polarization on the AFGL array further and showed that the apparent longitudinal phase propagation of the H I Midlatitude ’ T and D components behaved differently. They also Pi2 Polarization noted that ACW polariration usually implied wcstI I I ward propagation whereas CW polarization corFIG. I. SC,,I:MAT,(’,
F*=
current Wedge
I
ado
Downward FAC
I
nc&Y2
Pi2 pulsation polarization
591
patternson SAMNET EAST
FIG. 2. SCHTMA.rl(‘
KT.I’RtSI:NTATION
OF
‘I Ht
LC)NGITIJI~IUAI
I’OLAKILATION
tLLIPX:S
MAGNETOMETEK
OF
THKtt
WAWS
OBSFKVT,~
ON
h
CHAIN.
The top row is an ACW circularly polarized uestward travelling wave. The second is a CW circularly polarized wave of half the amplitude of the first. The third row is the superposition of the previous two. It is elliptically polarized with aestward phase motion and a longitudinally varying polarization ellipse azimuth (Southwood and Hughcs. 1985).
how the latitudinal observations of Fukunishi (l975), Stuart c’r rrl. (I 979) and Lester and Orr ( I98 I, 1983) fit in with the longitudinal behaviour observed on the AFGL array. Most of the current understanding of the longitudinal bchaviour is based on the results obtained from the AFGL longitudinal chain. This consisted 01 tivc magnetometers at a geomagnetic latitude of 55 with tWo stations at the lower latitude of 40 _ thus the field lines linking each AFGL station with the magnetosphere invariably permeate plasmaspheric plasma. The new SAMNET array has two longitudinal chains. one is at a geomagnetic latitude of 56 and therefore usually linked with the plasmasphere. The other is at 61 and is often associated with higha latitude plasmatrough plasma. Thus experiments are now possible in which both regions may bc studied concurrently. revealing new features in the polarization pattern.
The U.K. Sub-Aurora1 Magnetometer Network (SAMNET). is an array of magnetometers deployed by the University ofYork. The array consists of seven magnetometers. These arc arranged in two longitudinal chains of three stations covering 28 in gcomagnetic longitude. scparatcd by 6.5 in geomagnetic latitude. The seventh station forms a connecting mcridional chain at the western edge covering IO of geo-
magnetic latitude. Figure 3 shows the locations of the SAMNET stations. Their geographic coordinates, geomagnetic coordinates and L-shells arc given in Table I. The fluxgate electronics and cartridge logging systcm used in the array were designed and built by the Geomagnetism Research Group of the British Geological Survey (Riddick, 1987). The fluxgate heads were built at York. The output from the three component fluxgtc sensor is sampled every 5 s and is recorded in H. /I, Z coordinates on an industry standard (ECMA 46) quarter inch magnetic cartridge. Magnetic variations over a range of k 5 I2 nT are measured with a resolution of 0.25 nT. SAMNET started recording on I October 1987 and has operated continuously since then. The array provides two-dimensional monitoring of hydromagnetic wave fields and substorm current systems and can address questions which were not possible with previous magnetometer arrays.
3. SAMNET LklJ~
OBSEK\‘ATIONS
302. 19x7 This event occurred on day 302 1987 at 23: I I U.T. Magnctograms of the event, bandpass filtered between 200 and 20 s, arc shown in Fig. 4. A considerable signal is observed in both the H and /I components right across the array. with a typical Pi2 damped wavcpackct structure clearly visible. A some-
592
T. K. YEOMAN ct ul.
Ftc;. 3. LOCATIONSOF THE SEVEN STATIONSOF THE U.K.
SUB-AURORAL MAGNETOMETERNETWORK
(SAMNET).
what longer period seems to dominate the H component at FAR. Spectral analysis shows most stations to be dominated by a period of 48 s, with the If component at FAR being dominated by a period 01 130 s. A polarization map for the event is shown in Fig. 5a and a key for this map and similar maps presented
in this paper is given in Fig. 5b. The map displays polarization characteristics and apparent longitudinal phase propagations where it is possible to calculate them. along with their position in geomagnetic latitude. longitude and magnetic local time. The station positions are marked with a solid square. Also displayed are the estimated plasmapausc position and
TABLE I. GEOGRAPHICAND W>MAC;NETICC‘OOKDINATFS or THYSAMNET STATIONS*
station
Code name
Faroes Glenmore York
FAR GML YOR NOR
Geographic N. lat.
coords. E. long.
Geomagnetic cords. N. lat. E. long.
L-shell
62.05 57.16 53.95
352.98 356.32 358.95
60.77 54.94 50.99
78.12 77.99 7x.57
4.26 3.08 2.57
KVI
64.37 59.50
13.36 17.63
61.28 55.83
95.28 95.95
4.40 3.22
OUlU Nurmijarvi
OUL NUR
65.10 60.51
25.85 24.66
61.30 56.59
105.56 102.17
4.41 3.35
*Geomagnetic 120 km.
coordinates
Nordli Kvistaberg
and L-shells are calculated
using the IGRF
for 1988.0 at
Pi2 pulsation
pohktion
patterns
7
I
593
on SAMNET I
I
1
I
I
I
6.0
KVI
KVI
D COMPONENT
H COMPONENT L 23.0
I 23.10
23.20
UT
1 23.30
1 23.0
I 23.10
I 23.20
J
UT
23.30
FIG. 4. H ANL, D C’OMPONLNTMAtiNETOCiKAMS FOK THE tVENT OK DAY 302. 1987. The mapnetograms have been bandpass filtered between 200 and 20 s.
the substorm current wedge position. The method of calculation of the parameters shown in Fig. 5 and other similar figures is given below. The polarizaGon characteristics and phase propagation nz-values are calculated by a complex demodulation technique applied over a frequency band chosen to match the natural bandwidth of the Pi2 signal (e.g. Beamish c’t ol., 1979). Events are selected such that this bandwidth includes the dominant frequency of all the stations in the array. Pulsation n+valucs have been examined previously by e.g. Hughes c’t (I/. (1978), Olsen and Rostokcr (1978) and Mier-Jcdrzejowicz and Southwood (1979). Here a negative IF?-value implies westward propagation and a positive one implies eastward propagation. The /Tz-values displayed in the polarization maps are calculated using only the D component of the pulsation. This is to avoid incorrect apparent phase propagation measurements which can sometimes arise in the H component due to its variability with latitude. The instantaneous values of the demodulated time series parameters at the demodulated time scrics amplitude maximum are used. It is felt that this gives the best results for highly non-stationary Pi2 pulsations. rather than averaging over the full pulsation length. In selecting events for analysis here, we restricted ourselves to events with very clear. unambiguous polarization and propa-
gation bchaviour, in order to facilitate comparison with previous work. In the AFGL array studies of e.g. Lester pt trl. (l983), wave parameters were calculated using a cross-spectral FFT technique in addition to examining bandpass filtered hodograms. As an additional precaution to cnsurc comparability of results all polarization parameters calculated here using complex demodulation techniques were also checked with bandpass filtered hodograms to ensure consistency. The plasmapause position is calculated using a method similar to that ofOrr and Webb (1975). In this technique an empirical measure of the plasmapause shape in local time (Chappell et nl., 197 I ) is scaled by the observed dependence on K,, of the plasmapause position at 02:OO L.T. (Chappell et LI/.. 1970). The value of K,, chosen is that of an average K,] measured in the midnight -dawn sector. The midnight&dawn scctor is generally considered to be the formative sector of the plasmaspherc (c.g. Chappell et Al., 1971). Thus the average prevailing K,>is used for events occurring in the midnight&dawn sector, but the average K,, prevailing I8 h previously is used for events occurring in the dusk-midnight sector. This method is found to give the best agrcemcnt with a study of 41 crossings of the plasmapause observed on GEOS-2 cold plasma data. A correction for the rotation of the dusk bulge
T. K.
594
YFOMAN
Magnetic 23.2 1
6
0.00
et
01.
local time
0.40
1.20
2.00
2.40
d7o-o
- 65.0
5-4-
-_ -
--n FAR
OUL I, .NOR - I__ mz-1.6 - -
/ F--m: -2.6
-
-
-
-
60.0
---_
z
--__
3-
0
GML
n
m=-3.0
Q. YOR 70.0
I
80.0
I
90.0
50.0 120.0
I
100.0
Geomagnetic
-____-____-
$ zz z! .0 z 6
110.0
longitude
Expected range of plasmapauseposition Anticlockwise Clockwise Westward Eastward Upward
polarisation
polarisation apparent apparent
ellipse
ellipse
phase propagation phase propagation
Held aligned current
Substorm centre Downward
FIG. 5. Pulsation magnetic
(a) POLAKl7hTl0N
field aligned current
MAP FOR THE EVENT ON IIAY
302.
1987
shown over the SAMNET array. These are shown m relation to local time. the substorm current wedge position and the plasmapause position (SW text for details). (b) Key for the various symbols used in the polarization maps. polarization
characteristics
are
position with magnetic activity (Higel and Lci. 1984) is also included here. This allows for the observation that at times of low magnetic activity the dusk bulge moves towards midnight, whereas it moves towards noon at times of high magnetic activity. In a comparison of the calculated plasmapause position with 41 GEOS-2 crossings of the plasmapausc an agrecment to within 0.5 R, (Earth radii) occurred in 20% of casts. to within I R, in 33% of cases and to within I .5 R, in 72% of cases. These figures rose to 3 I, 38 and Xl %. respcctivcly, if only nightside crossings wcrc considered. The longitudinal position of the substorm current wedge is calculated from the magnetic bay structure across the array at low latitudes. In this region the contamination of the signal by aurora] zone ionospheric currents is smallest. This method has been
used before (c.g. Lester at (11.. 1983) and is based on the magnetic disturbance expected from upward and downward field-aligned currents. These produce fields such that inside (outside) the substorm current system the H component bay is positive (ncgativc) and that to the west (cast) of the system the D component bay is positive (negative). The ccntre of the wedge is defined by the longitude at which the D component bay changes sign. Nolc that in the symbolic rcprescntation of the SCW in Fig. 5 no attempt is made to fix the longitudinal position of the wedge exactly, but we just position the upward and downward currents and wedge ccntre relative to the SAMNET stations, according to the bay signatures above. No attempt is made to calculate the bay latitude. Figure 5 shows the substorm to occur to the west of and over the westward edge of the array. This
Pi? pulsation polarization patterns on SAMNET wedge position is predicted by the bay bchaviour on both the longitudinal chains. The associated pulsation is observed at magnetic local times of between 23:30 and 02:OO. A characteristic cllipticity reversal is seen across the estimated plasmapause position, which is at quite a low latitude as K,,has been ?I+ for some time bcforc substorm onset. The ellipse azitnuth also switches quadrant across the estimated plasmapausc position. Westward phase propagation is observed between three of the four station pairs, with KVIL NUR being too closely spaced to rcsolvc any phase diffcrencc for this event. This propagation is observed bctwcen station pairs which lit east of the eastward edge of the substorm current system. The magnitude of the observed HI-value is variable in both latitude and longitude. suggesting a signature due not to a simple w’ave travelling uniformly in longitude, but to 21combination of more complex ctt‘ects. The event also shows the expected CW rotation of the polarization cllipsc azimuth the further west it is measured. In this event the longitudinal azimuth change is less obvious than the latitudinal change. Neither the size of the change nor the orientations of the ellipse azimuths at FAR, KVI and NUR agree with the substorm current wcdgc model. k/J, 303. I YX7 A Pi2 pulsation was observed on the following day, starting at 21:52 U.T. H and .!I component magI
1
I
I
1
‘
H COMPONENT
L
21.30
21.50
595
netograms for this event, bandpass filtered between 200 and 20 s are shown in Fig. 6. For this cvcnt there arc no data from NUR as interference from a nearby radar cxperimcnt had contaminated the signal. A similar period pulsation of -72 s is seen in all stations of the array. The longer period seen in this event corresponds to a magnetically quieter day (the prevailing K,>is 3-). This is reflected in the larger plasmasphcre seen in the polarization map in Fig. 7. The estimated plasmapause position lies just north of the array and all the stations apart from OUL arc ACW polarired. For this cvcnt the substorm ccntre lies between the FAR and NOR meridians and the whole array is inside the substorm current system. Again westward propagation with a variable r?z-value is seen. Here. however, the ellipse Gmuth appears not to vary with latitude and the longitudinal variation agrees with the pattern cxpectcd from the substorm current wedge model for observations within the substortn current system.
The third case study considered here occurred on day 27. 1988, starting at 20:2X U.T. Magnetograms, bandpass filtered between 400 and 20 s are shown in Fig. 8. Again a pulsation is seen right across the array in the If and /I components. The signal here is very small at YOR and stems to have enhanced long period I
I
I
1
I
I
1
I
t
D COMPONENT
22.10
1
UT
22.30
1
2 1.30
I
21.50
I
22.10
FIG. 6. H AND0 (‘OMI’OhFNT ~fAGNETOGKA,MS tWKTHEtWNT ON DAY303. 1987 The magnetograms have been bandpass liltered between 200 and 20 s.
I
UT
1
22.30
T. K. YEOMAN ct
596
Magnetic 21.58 CJ
local time
23.17
22.37 I
cd, 0.37
23.57
1.17 70.0
8
”
6-
3-
0
n
/ & 70.0
t_ m= -6.0
GML
. YOR I 80.0
0.
E : (3
KW
I 90.0 Geomagnetic
- 55.0
I 110.0
1 100.0
50.0 120.0
longitude
FIG. 7. PULAKIZATION MAPJ;OR THLEVEKT ON DAY 303, 1987. Format as for Fig. 5.
DC
H COMPONENT
I
6.0n
6.0nT I
GML
KVI
L I
20.0
I
I
20.20
’
20140
iT
FIG. 8. ff AND D COMPONENT
The magnctograms
2j.o MAGNETOGKAMS
have been bandpass
20.0
1
1
FOK THC EVENTON
I
1
20.20
20.40
DAY 27. 1988.
filtered between 400 and 20 s.
UT
4
21.0
597
Pi2 pulsation polarization patternson SAMNET Magnetic 20.36
21.15
local time
2 1.55
22.35
3.5 5 li
10.0
I I65.0
uFAR
-
60.0
o 0
m GML
)
0
In=-3.0
17lwf
i
80.0
FIG. 9.
e F s (II
mNUR 55.0
-m= -4.3
I
I
I
go.0
100.0
110.0
POLAKI7.ATlON
MAP FOR THE CVLNT ON UAY
activity in the Hcomponent at OUL. Spectral analysis shows a dominant period of 96 s is seen at all stations cxccpt in the H component at OUL. which shows peaks at periods of 96 and 217 s. The prevailing K,, was 3 - and the polarization map (Fig. 9) shows that all stations lay south of the estimated pl~~smapause position and that they all had ACW polarization. In this case the substorm current wedge lies to the East and over the eastern part of the array. Again a variable tl?-value, westward prop~~g~~ting signal is seen. This event, like the day 302 event, has a more obvious polarization ellipse rotation with latitude than with longitude and only FAR, KVI and NUR have polari7ation cllipsc azimuths in agreement with the substorm current wedge model. For this event the polarization ellipse rotation occurs when all the stations are lying within the estimated plasmasphcrc and arc not subject to a latitudiiial ellipticity change between the two longitudinal chains.
OF Pi2 POl,ARI%ATION
P<%TTERNS
Twenty-eight Pi2 pulsations have been examined from the SAMNET data-set so far. Many more have been observed but for this preliminary study we chose to analyse only those with a clearly defined central frequency in both the H and D components and stable polarization characteristics over a number of
I!50.0 120.0
longitude
Format as for Fig.
BEHrZVIOl’R
5 .$
. YOR
Geomagnetic
4. ST,tTISTICAI,
g
27. 1988
5.
demodulates in the region of the maxitnum amplitude. The behaviour of the three case studies discussed above illustrates the great variety of results which arc typically seen, even in this restricted data-set. Average polarization ellipse azimuth rotations of 74 for the northern iong~tudinal chain and 69 for the southern longitudinal chain are observed with a standard dcviation of 36 in both casts. Considering the three meridional station pairs FAR-GML, NOR-KVI and OUL-NUR we find that there is a 56 average rotation in polarization azitnuth between the two chains, with this azimuth switching quadrants (from -90 ---t0 to 0 + 90 or vice versa) in about half of all casts. Thirty-seven per cent of these switches occurred inside the wedge and 59% occurred outside it. These latitudinal changes in polarization ellipse azimuth. which can occur both with and without a corresponding ellipticity reversal, are an impo~-tant featurp of the mid-latitude Pi2 polarization pattern. Such azimuth switches lead to deviations from the SCW pattern as latitude changes, both inside and outside the wedge, although changes outside the wedge are more frequent, Negative m-values implying westward propagation have been found in 939/o of the cases so far studied. Only six instances of eastward phase prooagation have been observed, lwo of which wcrc CW polarized. The mcasurcd polarization azimuths of individual stations agreed with the SCW pattern in 48 and 5 1%
T. K. YEOMAY 1’1rd.
598
of cases for the northern and southern chain stations, respectively. The polarization pattern for each chain as a whole agreed with the model predictions in only 17% of events for the upper chain and 23% for the lower. If a weaker criterion for the substorm current pattern is adopted. requiring a north pointing azimuth near the substorm centre and an ACW rotation of the polarization cllipsc the farther east it is measured, then there is a 63 and 67% agrecmcnt on the upper and lower chains, rcspcctively. The poor lcvcl of agreement with the SCW pattern may in part be explained by recent modclling of the substorm current system by Lester ct cd. (1989). They showed that the further away from the ionospheric intcrscction of the fieldaligned current system the observations arc made, the greater the longitudinal extent of the wcdgc appears to bc. This can introduce errors in the determination of the field-aligned current longitudes. This cffcct is exacerbated for wedges of small angular width. Pi2 observations in that paper showed that pulsation polarization azimuth quadrants at 55 and 40 gcomagnetic latitude were the same inside the wedge but were frcqucntly different outside it. Our results, howcvcr. show that latitudinal variations of azimuth arc not confined to observations made outside the wedge. Clearly the polarization ellipse azimuth pattern is far more complex and varied in its spatial structure than is predicted by a straightforward SCW model. We may conclude. however. that a SCW model type pattern. or part of enc. can almost always be seen but that its longitudinal position with respect to the wedge may bc shifted and that this shift may vary with latitude. The ACW rotation of the polarization ellipse the farther east it is measure is confirmed to bc a common feature of the mid-latitude Pi2 signature, but latitudinal variations are frequent and the orientation of ellipse azimuths is only a weak indicator of the substorm current wedge position. 5. ,\2\! INVESTIGATION
OF TRAVELLING WAVE
POLARIZATION
PATTERNS
In the previous sections we have seen a poor agreement with the substorm current wedge model polarization pattern in spite of the use of clearly defined pulsation data. Latitudinal switches of azimuth and westward phase propagation east of the eastern edge of the substorm current system have also been observed. These observations suggest that the substorm current wedge model as put forward by Lester et cd. (I 983) and the travclling wave mechanism put forward by Southwood and Hughes (1985) are inadcquate to explain all of the polarization features seen in mid-latitude Pi2 pulsations. Rcccntly Yeoman and
Orr (1989) suggested that a plasmasphcric cavity resonance is the most likely mechanism for the midlatitude amplitude enhancement and polarization rcvcrsal seen in Pi2 pulsations, rather than driven field lint resonances or surface waves. Such a mechanism has also recently been supported by the work of Sutcliffc and Yumoto (1989) and Kitamura et (I/. (1988). This mechanism provides a low latitude wave source spatially distinct from the wave source in the aurora1 zone ionosphcrc. but triggered by the same substorm activity. Consideration of the Pi2 wave as a twosource phenomenon may hold an explanation for the deviations from the classical substorm current wcdgc model polarization pattern in the region where both aurora1 zone activity and lower latitude activity are important constituents of the Pi2 signal, without rccoursc to low latitude field line resonances. Here the travelling wave ideas of Southwood and Hughes (1985) are extended to include a second source at a different longitude and a different latitude to the primary wave source. The basic model consists of two sinusoidal wave sources with circular polarization. A high latitude CW polarized wave with phase propagation in longitude represents the aurora1 zone signal and a low latitude ACW polarized wave with phase propagation in longitude represents the plasmaspheric cavity resonance signal. This arrangement is shown schematically in Fig. IOa. Polarization ellipses are calculated. in the same way as for the experimental data. from the superpositon of the waves. The spatial variation of the polarization ellipse is displayed on a 6 x 9 grid (Fig. lob). CW and ACW polarization ellipscs arc represented in the same way as in the experimental result polarization maps. The D component m-value is displayed as a lint between stations with an arrow head indicating the wave propagation direction. The model has eight variable parameters: the aurora1 zone signal amplitude and decrement, the auroral and lower latitude signal nl-values. the aurora1 and lower latitude signal source meridians from which there is phase propagation and the aurora1 and lower latitude pulsation periods. The CW aurora1 wave amplitude is kept at a constant value of IO within a “highly conducting aurora1 zone” marked in Fig. IOb as a box. Outside this arca the amplitude falls off cxponcntially with distance. For simplicity the ACW low latitude signal has an amplitude of I cvcrywhere. The aurora1 signal also has a phase delay with latitude to mimic the propagation delay introduced by the fast mode coupling of the transverse mode aurora1 signal to lower latitudes (Cough and Orr, 1984). In Fig. IOb the aurora1 zone signal has an nr-value of IO. The aurora1 source is located at the westward edge of the aurora1 zone at coordinates (2,2) counting
Pi2 pulsation polarization
I
1
-30
I
-20
-10
599
patterns on SAMNET
I
I
I
I
0
10
20
30
I
I
40
50
I
Longitude (b) FIG. IO. (a) SCHL~L~TK
tmc;uM
01 THE
I~IIOGKLSSIOU
01: WAVEPKONTS
THE MOL~ItII’I~ TRAv~LLIN(;
WAVI:
I:KOS~ THE TWO
S~UKCXS
us~u
II\
MOIIEL.
The amoral LWC source (A) propagates out radially from its amplitude maximum in the westward travelling curgc. whilst at loucr latitudes W;IVCScharacteristic of a plasmaspheric cavity resonance with a small IWv~duc exist (6). The Pi? polarrration is the result of the superposition of the two wave sources. (b) An example of results from the travelling wave model. The aurora1 zone source is in the box at grid position (2,2) measured from the top left hand corner. and has an/17-value of IO. The plasmaspheric cavity resonance propagates from the 6th grid meridian from the left with an ,n-value of 3. Tlrc wave superposition gives a substorm current wedge model type polarization pattern beneath the amoral zone box. The polarix~tion pattern also shows nr-values variable in longitude and latitude along with latitude dependent polarization cllipsc a,knuths.
from the top Mt of the grid. In this example the auroraI zone signal amplitude has an cxponcntial decrement or 1.5 between adjacent ellipses. The low latitude source is located on grid meridian 6 and has an m-value of 3. Figure 1Ob shows a substorm current wedge model type polarization pattern below the SLIToral zone box. The III-values vary with both latitude and longitude and propagation is westward west of the aurora1 source meridian and eastward cast of the
low latitude source meridian. Between the sources the direction of propagation depends on the dominant wave, changing from westward to eastward with increasing latitude. Some anomalous longitudinal propagation directions occur just to the east ot” the low latitude source in this example. These are caused by the reversal in ellipticity where the source dominance switches. Few details of the high latitude source are included in the model, so the polarization ellipses
600
T. K. YEOMANCI til.
in the upper part of the tigure have little physical meaning. In Fig. IOb the orientation of the polarization cllipsc in grid meridian 2 switches quadrant as the latitude changes. This quadrant switch is caused by the propagation delay introduced into the amoral signal. which gives the aurora1 signal phase a latitudinal dependency in addition to its longitudinal phase propagation bchaviour. Although in this example the quadrant change is accompanied by an ellipticity switch, this is due to the relative amplitudes of the wave sources. The ellipse azimuth quadrant switch is not depcndcnt on this and can occur without such a switch. In this modelling the degree of rotation of the polarization ellipse and the longitudinal position of the substorm current wedge model type polarization pattern which emerges from the wave superposition will depend on the values used for the source meridians, n?-values and wave periods. Varying these parameters will lead to quite different polarization patterns. A reduced azimuth rotation will result if nz-values are reduced or if the source meridians are brought closer together. whereas increased azimuth rotation will result from increasing tither of thcsc paramctcrs. The superposition of waves of different periods will give the same degree of azimuth rotation, but will lcad to a different section of the characteristic wave superposition azimuth orientation pattern lying below the substorm current wedge.
6. DISCUSSION An examination of the polarization of 28 Pi2 ~LIIsations observed on SAMNET has shown a characteristic ellipse azimuth rotation with longitude. The SAMNET longitudinal chains have allowed this behaviour to be examined at two latitudes and it has been found that azimuth switches between the chains are common and may occur with or without an ellipticity reversal. The case study on day 302. 1987, shows a polarization ellipse quadrant swaitch accompanying an ellipticity reversal across the estimated plasmapause position and longitudinal phase propagation inconsistent with high latitude Pi2 mod&; the pulsation appears to be propagating towards its high latitude source. Although Lester PI al. (1984) pointed out that castward propagation was the most frequent east of the substorm current wedge. observations such as that on day 302 are not uncommon. The day 303, 1987. event is in agreement with the model of Southwood and Hughes (1985). showing westward phase propagation and a polarization ellipse azimuth orientation in agreement with the substorm current wedge model.
The day 27, 1988, event shows phase propagation consistent with the Southwood and Hughes (1985) model but shows a change of polarization ellipse quadrant with latitude without any ellipticity reversal and apparently not associated with the plasmapause. An example of the results of a very simple travelling wave model of the Pi2 disturbance in the region where both the high latitude aurora1 disturbance and a lower latitude plasmasphcric cavity resonance signature are important have been presented. This model seems able to explain some of the outstanding problems of the mid-latitude Pi2 signature and also some of the new features observed on SAMNET. In its simplest form the model reproduces the substorm current wedge model polarization pattern in longitude and is thus able to accommodate results such as those seen in the day 303. 1987, event. In addition the model predicts a greater likelihood of eastward propagation for CW polarized waves. This agrees with the observations of Gelpi et al. (1985b). The possibility of longitudinal phase propagation towards the aurora1 zone source is also allowed for in regions where the low latitude source is dominant and on a different meridian to the aurora1 zone source. The model also predicts variable ellipticities across the array. Such ellipticity variations have been obscrvcd in two of the case studies presented here and were also noted by Lester et rd. (I 984). Other factors such as the different dominant periods seen in some components at some stations. also noted by Lester cf al. (1984) may bc accounted for by the model, as variations in dominant period could occur due to the two dif’fcrcnt wave sources being dominant in different regions. The model can also explain polarization ellipse azimuth quadrant switches in latitude without an cllipticity reversal, such as was observed in the day 27. 1988, cvcnt. This is a consequence of a phase delay due to the propagation of the high latitude signal. SAMNET results show that such azimuth orientation quadrant switches with constant cllipticity occur in 20% of observed events and constitute 63% of the observed polarization ellipse azimuth switches. The remaining 37”/0 of azimuth switches are associated with ellipticity reversals at the plasmapause. The extent of the rotation is probably somewhat exaggerated in the model (a phase delay of IO” or typically about I .5 s per degree of latitude is assumed to make the rotation clear), but this mechanism could explain sonic latitudinal variations. Thus a low latitude cavity resonance is able to explain the observed features of the Pi2 polarization pattern without considering a field line resonance just within the plasmasphcre, as suggested by Fukunishi (I 975).
Pi2 pulsation polarization patterns on SAMNET
Variations in m-values, source meridians and period can also produce substorm current wedge model-type polarization patterns which are incomplete or displaced with respect to the wedge. Lester rt ~1. (1983) noted that 33% of patterns on the AFGL array were shifted in such a manner. They ascribed this to the presence of pre-existing substorm current systems. Lester (‘2~1. (1984) examined Pi2s only from isolated substorms to avoid this effect, but found a very similar level of agreement of 70%. The travelling wave model may provide an explanation for at least some of these displacement effects. Observations on SAMNET have shown that a simple application of the SCW polarization pattern is frequently inadequate to explain the observed mid-latitude polarization. More sophisticated modelling of the high and low latitude wave sources will be required before full understanding of the mid-latitude region can be achieved Clearly the travclling wave model has quite a numbcr or variable parameters. This allows it to successfully reproduce the very varied mid-latitude Pi2 signature in a large number of cases. It is also a drawback of the model as it makes it rather difficult to establish its validity. Continued observations of Pi2 characteristics on a large number of events are anticipated during the current campaign. More observations should enable the processes which combine to make up the mid-latitude signature of Pi2 pulsations to be established In addition the consideration of the lower latitude SAMNET Pi2 data may enable the nightside Pi2 to be used as a test of global cavity resonance theory, a subject on which considerable theoretical and modelling analysis has been concentrated recently (e.g. Allan rt rd., 1986: Kivelson and Southwood, 1986) mostly in a daysidc context. but of which there has been little observational confirmation. inclusion of such a magnetospheric cavity rcsonancc model in modelling of Pi2 pulsations might explain some puzzling features of the lower latitude polarization patterns. Kivelson and Southwood (1986) noted that the node and antinodc structure of a cavity mode allow the possibility of polarization rcvcrsal without field line rcsonanccs. This may be able to account for the low latitude reversal seen by Lester cut trl. (IHY).
~lc.X,ro~~~k~c~ymro~t.~~Wethank the SERC for funding the SAMNET project and for a studentship for T.K.Y. The invaluable contributions of our Litc operators and Mews John Riddick and John McDonald and.Dr Bill Stuart of the British Geological Survey are also acknowledged. We also thank a referee for constructive criticism,
601 REFERENCES
Allan, W.. Poulter, E. M. and White, S. P. (1986) Hydromagnetic wave coupling in the tnagnetospherc~plasmapause effects on impulse excited resonances. Plwwr. Spuw Sci. 34. I 189. Baranskiy. L. N.. Troitskaya. V. A., Sterlikova. I. V., Gokhberg, M. B., Ivanov, N. A., Khartchcnko. I. P.. Munch, J. W. and Wilhelm, K. (I 980) The analysis of simultaneous observations of nighttime Pi pulsations on an cast west protilc. J. Groph_vx. 48. I. Beamish, D., Hanson, H. W. and Webb, D. C. (1979) Conplex demodulation applied to Pi2 geomagnetic pulsations. C;wpl~w. J. R. usir. Sec. 58. 471. Chappell. C. R., Harris. K. K. and Sharp, G. W. (1970) A study of the influence of magnetic activity on the location of the plasmapause as measured by OGO 5. J. ,qeopl~>,.s. Rc.v. IS. 50. Chappell, C. R., Harris. K. K. and Sharp. G. W. (1971) The daysidc of the plasmasphere. J. g-leoph~x. Res. 76. 7632. Fukunishi. H. (1975) Polarization changes of geomagnetic Pi2 pulsations associated with the plasmapause. J. ,qwpl~>a.r.Res. 80, 98. Gelpi. C.. Hughes, W. J., Singer, H. J. and Lester, M. (1985a) Mid-latitude Pi2 polarisation pattern and synchronous orbit magnetic activity. J. ,qeop/~!~.s.Ret. 90, 6451, Gelpl, C., Hughes, W. J. and Singer, H. J. (lYX5b) Longitudinal phase and polarisation characteristics in mid-latitude Pi2 pulsations. J. ,ycqj/l~.r. Re.v. 90, 9905. Gough, H. and Orr. D. (1984) The cffcct of damping on geomagnetic pulsation amplitude and phase at ground observatories. P/ww/. Spuc’c &i. 32, 619. Higel, B. and Lci, W. (1984) Electron density and plasmapause characteristics at 6.6 Rr: a statistical study of the GEOS 2 relaxation sounder data. .I. ,ywph,rs. RKS. 89.
1583. Hughes, W. J., McPherron, R. L. and Barfield, J. N. (1978) Geomagnetic pulsations observed simultaneously on three geostationary satellites. .I. geop/z~~s. Res. 83. I 109. Kitamura. T.-I., Saks. 0.. Shimoizumi. M., Tachihara. H., Oguti, T.. Araki. T., Sate, N.. Ishitsuka. M.. Veliz. 0. and Nyobc. J. B. (198X) Global mode of Pi2 waves in the equatorial region-difference of Pi2 mode between high and equatorial latitudes. J. Gcormy~~. Gcwe/c~c~/~.40. 621. Kivelson, M. G. and Southwood. D. J. (19X6) Coupling ol’ global magnetosphcric MHD eigenmodes to field line resonances. J. ycwp/~~.r. Rcs. 91. 4345. Kuwashima, M. and Saito. T. (19X I) Spectral characteristics ofmagnctic Pi2 pulsations in the aurora1 region and lower latitudes. J. ,qco~~~/~~~s. Rc.c. 86, 4686. Lester, M.. Glal3m&cr. K.-H. and Behrcns. J. (1985) Pi2 pulsations and the castual-d electrojct: a cast study. Plrrw/. s/Jr/w %i. 33, 35 I Lester, M., Hughes. W. J. and Singer. H. J. (1983) Polarisation patterns of Pi2 magnetic pulsations and the substorm current wdge. J. ,qrq+?~~.c. Xc.\. 88. 795X. Lester. M.. Hughcs. W. J. and Singer. I-1. J. (1984) Longitudinal structure m Pi2 pulsations and the aubstorm current wedge. J. ,qcwp/~~~.s.RCA. 89, 5489. Lester. M. and Orr. D. (1981) The spatio-temporal charactcristics of Pi2’s. J. afnmc. Icw. P/I~.~. 43. 947. Later, M. and Orr. D. (1983) Correlation between ground observations of Pi2 geomagnetic pulsations and satellite plasma density observations. Pltrwt. S’/xrw Sci. 31. 143. Lester, M.. Singer. H. J., Smits, D. P. and Hughes, W. J.
602 (1989)
T. K.
YFOMAN
Pi2 pulsations and the substorm current wedgelow latitude polarisation. J. qqeoph)x. Res. 94, 17, 133. Mier-Jedrzejowicz, W. A. C. and Southwood, D. J. (1979) The East West structure of mid-latitude gcomagnelic pulsations in the 8 25 mHz band. P/uric/. Spcm Si. 27. 617. Olsen. J. V. and Rostokcr, G. (1978) Longitudinal phase variations of micropulsations. J. gc~o~~/~~xmRc.~.83, 24X I. Orr. D. and Webb, D. C. (1975) Statistical studies of geomagnetic pulsations with periods between IO and 70 s and their relationship to the plasmapause region. P/u?zc/. .‘jp~ce SC,/. 23, I 169. Pashin, A. B.. GlaBmeier. K.-H.. Baumjohann, W.. Raspopov. 0. M.. Yahnin, A. G.. Opgenorth, H. J. and Pellincn, R. J. (1982) Pi2 magnetic pulsations. aurora1 break-ups and the substorm current wedge : a case study. J. Gcc&~.~. 51. 223. Riddick, J. C. (1987) BGS Geomaynctism Research Group Report. Number X7:23. 1987. Sakural. T.. Tonesawa. Y. and &to. N. (1988) Oscillation characteristics ocfPi2 in aurora1 zone : Syowa Iceland conjugatc study. Proc. NIPR S.mp. tipp’per A/ma.\. P/I~v. I, I IO. Samson. J. C. (1982) Pi2 pulsations : high latitude rcsuhs. P/urw/. Spm SC,;. 30. 1239. Samson. J. C. and Harrold. B. G. (1983) Maps of the polarisation of high latitude Pi2’s. J. q~)p/?w. Rcs. 88. 5736.
et
ul.
Samson. J. C. and Harold. B. G. (1985) Characteristic time constants and velocities of high latitude PiZ’s. J. ,qeop/~~s. Rrs. 90. 12.173. Singer, H. J.. Hughes. W. J.. Gelpi. C. and Ledley, B. G. (1985) Magnetic disturbances in the vicinity of synchronous orbit and the substorm current wedge: a case study. Smits. D. P.. Hughes, W. J.. Cattcll, C. A. and Russell. C. T. (19X6) Observations of field aligned currents, waves and electric fields at substorm onset. J. ,qycoph~~.~.Rex. 91, 121. Southwood. D. J. and Hughes, W. J. (19X5) Concerning the structure of Pi2 pulsation. J. ,qcwph~~.~. RCT. 90. 3X6. Stuart. W. F., Brett, P. M. and Harris, T. J. (1979) Midlatitude secondary resonance in Pi2‘s. J. tr/mo.r. ICW. P/J.I~.s. 41. 65. Sugiura. M. and Wilson. C. R. (1964) Oscillation of the gcomagnctic lield lines and associated magnetic perturbations at conjugate points. J. yeo[?l~\:s. Rcs. 69. I21 I. Sutcliffc. P. R. and Yumoto. K. (1989) Dayside Pi2 pusations at low latitudes. Geop/~~.\. Re.c. Left. 16. 887. Yeoman. T. K. and Orr, D. (1989) Phase and spectral power of mid-latitude Pi2 pulsations: evidence for a plasmaspheric cavity resonance. P/trwf. Spuw Sci. 37. 1367.
Department
T. K. YEOMAN.* 1). K. MILLING and D. ORR of Physics, University of York. Heslington. York YO1 5DD, U.K.
array of seven fluxgale magnctometcrs has been deployed covering mid-latitudes in western Europe. The array is configured to allow simultaneous observations ofmid-latitude pulsation characteristics in both latitude and longitude. Here the two-dimensional variations in the horizontal polarization of Pi2 pulsations arc related to the substorm current wedge position. deduced from the mid-latitude bay behaviour, and an estimated plasmapause position. Features of the polarization pattern which arc not in agreement with the substorm current wedge model arc found. These include unexpected polarization ellipse azimuths and longitudinal phase propagations. and latitudinal variations in polarization ellipse azimuths not associated with ellipticily reversals. An extension of previous travclling w’ave models to include low latitude ellkts is considered. which may explain at least some of these discrepancies. Abstract-An
I. INTRODUCTION
The polarization patterns of mid-latitude Pi2 pulsations have been the subject of a considerable amount of previous work. most notably on the Air Force Geophysics Laboratory (AFGL) magnetometer chain (e.g. Lester et d., 1983). It is now known that anticlockwise (ACW) polarized signals arc predominant at mid-latitudes but that clockwise (CW) polarization becomes more frequent at aurora1 latitudes. It has also been commonly observed that the mid-latitude pulsations have a mostly westward apparent phase propagation and that the polarization pattern is ordered by the substorm current wedge. It is helpful to first consider the higher latitude characteristics and polarization patterns which occur near the primary aurora1 amplitude maximum of the damped Pi2 wavcpackct. Typically. ITI-values of around lO&30 are seen in this region (e.g. Lester er (/I., 1985 ; Samson and Harrold. 19X5). Coiljugate results (e.g. Kuwashima and Saito, 1981) have shown that the M component is in phase in the Northern and Southern Hcmisphcres. whereas the /I) component is out of phase. This is characteristic of an odd mode oscillation guided along a field lint (Sugiura and Wilson, 1964). Pashin et d. (1952) and Lcstcr c’r (I/. (1985) both showed that in this region pulsation cquivalcnt currents in the ionosphcrc had ;I Vortex structure which appcarcd CO bc coincident with the ionosphei-ic intersection of the substorm current
* No\\ at : Department of Physics. (Jnivcrsity of Leicester. llnivcrsity
Road.
Leicestcr
LEI 7RH. U.K. 589
wedge field-aligned currents. Sakurai (at al. (1988) showed that the H component phase coherence was greater than that of the D component both conjugatcly and between three Icelandic stations. These observations may be taken as evidence that the am-oral Pi2 signature is characterized by a transient, Alfvtnic field line resonance more or less co-located with the field-aligned current system. Samson and Harrold (1983) measured statistical Pi2 polarization characteristics at high latitudes. Their pattern has not been reproduced by any model, most notably in the lower latitude region of their observations. It dots. however. show many of the features predicted by the oscillating field-aligned current model of Samson (1982). Whilst the high latitude Pi2 signal is predominantly CW polarized. it undergoes a polarization reversal to becomc ACW at lower latitudes. The position of thih reversal has often been shown to be close to the plasmapausc position (Fukunishi, 1975 ; Lester and Orr. I YXI. 1983). Studies of the wave polarization on meridional magnetometer chains have shown that an additional switch in the wave azimuth quadrant sometimes occurs in latitude equatorward of the cllipticity reversal. This has been interpreted in terms of field line resonance theories (E‘ukunishi, 1975 ; Stuart et C/l.. 1979). In the mid-latitude rcyion m-values of around 3 predominate (c.g. Lester et rrl.. 1983). In this region Lcstcr CI nl. ( 1983) used the AFGL chain to study the magnetic bay variations which arc caused by the fieldaligned currents of the substorm current wedge. These were then related to the longitudinal variations of the
590
T. K. YEOMAN(‘I cl/.
Centreof
of the substorm currcnl wedge a switch to CW polarization was observed at this latitude. The azimuthal phase propagation and horizontal polariration of these low-latitude waves is not consistent with a field line resonance. Gclpi Q/ rrl. (198%) showed that the ordering of the mid-latitude pattern rclatcd to geostationary orbit substorm observations as well as ground observations of the substorm ccntrc. Singer ct LI/. (1985) went Midlatitude 1 on to confirm the value of ground observations in DC Magnetic determining substorm onset times and longitudes in order to interpret satellite signatures. Satcllitc data have since been interprctcd using the mid-latitude Pi2 polarization pattern by, for cxamplc, Smits ct al. (19X6). Gelpi cut(I/. (1985b) studied the mid-latitude polarization on the AFGL array further and showed that the apparent longitudinal phase propagation of the H I Midlatitude ’ T and D components behaved differently. They also Pi2 Polarization noted that ACW polariration usually implied wcstI I I ward propagation whereas CW polarization corFIG. I. SC,,I:MAT,(’,
F*=
current Wedge
I
ado
Downward FAC
I
nc&Y2
Pi2 pulsation polarization
591
patternson SAMNET EAST
FIG. 2. SCHTMA.rl(‘
KT.I’RtSI:NTATION
OF
‘I Ht
LC)NGITIJI~IUAI
I’OLAKILATION
tLLIPX:S
MAGNETOMETEK
OF
THKtt
WAWS
OBSFKVT,~
ON
h
CHAIN.
The top row is an ACW circularly polarized uestward travelling wave. The second is a CW circularly polarized wave of half the amplitude of the first. The third row is the superposition of the previous two. It is elliptically polarized with aestward phase motion and a longitudinally varying polarization ellipse azimuth (Southwood and Hughcs. 1985).
how the latitudinal observations of Fukunishi (l975), Stuart c’r rrl. (I 979) and Lester and Orr ( I98 I, 1983) fit in with the longitudinal behaviour observed on the AFGL array. Most of the current understanding of the longitudinal bchaviour is based on the results obtained from the AFGL longitudinal chain. This consisted 01 tivc magnetometers at a geomagnetic latitude of 55 with tWo stations at the lower latitude of 40 _ thus the field lines linking each AFGL station with the magnetosphere invariably permeate plasmaspheric plasma. The new SAMNET array has two longitudinal chains. one is at a geomagnetic latitude of 56 and therefore usually linked with the plasmasphere. The other is at 61 and is often associated with higha latitude plasmatrough plasma. Thus experiments are now possible in which both regions may bc studied concurrently. revealing new features in the polarization pattern.
The U.K. Sub-Aurora1 Magnetometer Network (SAMNET). is an array of magnetometers deployed by the University ofYork. The array consists of seven magnetometers. These arc arranged in two longitudinal chains of three stations covering 28 in gcomagnetic longitude. scparatcd by 6.5 in geomagnetic latitude. The seventh station forms a connecting mcridional chain at the western edge covering IO of geo-
magnetic latitude. Figure 3 shows the locations of the SAMNET stations. Their geographic coordinates, geomagnetic coordinates and L-shells arc given in Table I. The fluxgate electronics and cartridge logging systcm used in the array were designed and built by the Geomagnetism Research Group of the British Geological Survey (Riddick, 1987). The fluxgate heads were built at York. The output from the three component fluxgtc sensor is sampled every 5 s and is recorded in H. /I, Z coordinates on an industry standard (ECMA 46) quarter inch magnetic cartridge. Magnetic variations over a range of k 5 I2 nT are measured with a resolution of 0.25 nT. SAMNET started recording on I October 1987 and has operated continuously since then. The array provides two-dimensional monitoring of hydromagnetic wave fields and substorm current systems and can address questions which were not possible with previous magnetometer arrays.
3. SAMNET LklJ~
OBSEK\‘ATIONS
302. 19x7 This event occurred on day 302 1987 at 23: I I U.T. Magnctograms of the event, bandpass filtered between 200 and 20 s, arc shown in Fig. 4. A considerable signal is observed in both the H and /I components right across the array. with a typical Pi2 damped wavcpackct structure clearly visible. A some-
592
T. K. YEOMAN ct ul.
Ftc;. 3. LOCATIONSOF THE SEVEN STATIONSOF THE U.K.
SUB-AURORAL MAGNETOMETERNETWORK
(SAMNET).
what longer period seems to dominate the H component at FAR. Spectral analysis shows most stations to be dominated by a period of 48 s, with the If component at FAR being dominated by a period 01 130 s. A polarization map for the event is shown in Fig. 5a and a key for this map and similar maps presented
in this paper is given in Fig. 5b. The map displays polarization characteristics and apparent longitudinal phase propagations where it is possible to calculate them. along with their position in geomagnetic latitude. longitude and magnetic local time. The station positions are marked with a solid square. Also displayed are the estimated plasmapausc position and
TABLE I. GEOGRAPHICAND W>MAC;NETICC‘OOKDINATFS or THYSAMNET STATIONS*
station
Code name
Faroes Glenmore York
FAR GML YOR NOR
Geographic N. lat.
coords. E. long.
Geomagnetic cords. N. lat. E. long.
L-shell
62.05 57.16 53.95
352.98 356.32 358.95
60.77 54.94 50.99
78.12 77.99 7x.57
4.26 3.08 2.57
KVI
64.37 59.50
13.36 17.63
61.28 55.83
95.28 95.95
4.40 3.22
OUlU Nurmijarvi
OUL NUR
65.10 60.51
25.85 24.66
61.30 56.59
105.56 102.17
4.41 3.35
*Geomagnetic 120 km.
coordinates
Nordli Kvistaberg
and L-shells are calculated
using the IGRF
for 1988.0 at
Pi2 pulsation
pohktion
patterns
7
I
593
on SAMNET I
I
1
I
I
I
6.0
KVI
KVI
D COMPONENT
H COMPONENT L 23.0
I 23.10
23.20
UT
1 23.30
1 23.0
I 23.10
I 23.20
J
UT
23.30
FIG. 4. H ANL, D C’OMPONLNTMAtiNETOCiKAMS FOK THE tVENT OK DAY 302. 1987. The mapnetograms have been bandpass filtered between 200 and 20 s.
the substorm current wedge position. The method of calculation of the parameters shown in Fig. 5 and other similar figures is given below. The polarizaGon characteristics and phase propagation nz-values are calculated by a complex demodulation technique applied over a frequency band chosen to match the natural bandwidth of the Pi2 signal (e.g. Beamish c’t ol., 1979). Events are selected such that this bandwidth includes the dominant frequency of all the stations in the array. Pulsation n+valucs have been examined previously by e.g. Hughes c’t (I/. (1978), Olsen and Rostokcr (1978) and Mier-Jcdrzejowicz and Southwood (1979). Here a negative IF?-value implies westward propagation and a positive one implies eastward propagation. The /Tz-values displayed in the polarization maps are calculated using only the D component of the pulsation. This is to avoid incorrect apparent phase propagation measurements which can sometimes arise in the H component due to its variability with latitude. The instantaneous values of the demodulated time series parameters at the demodulated time scrics amplitude maximum are used. It is felt that this gives the best results for highly non-stationary Pi2 pulsations. rather than averaging over the full pulsation length. In selecting events for analysis here, we restricted ourselves to events with very clear. unambiguous polarization and propa-
gation bchaviour, in order to facilitate comparison with previous work. In the AFGL array studies of e.g. Lester pt trl. (l983), wave parameters were calculated using a cross-spectral FFT technique in addition to examining bandpass filtered hodograms. As an additional precaution to cnsurc comparability of results all polarization parameters calculated here using complex demodulation techniques were also checked with bandpass filtered hodograms to ensure consistency. The plasmapause position is calculated using a method similar to that ofOrr and Webb (1975). In this technique an empirical measure of the plasmapause shape in local time (Chappell et nl., 197 I ) is scaled by the observed dependence on K,, of the plasmapause position at 02:OO L.T. (Chappell et LI/.. 1970). The value of K,, chosen is that of an average K,] measured in the midnight -dawn sector. The midnight&dawn scctor is generally considered to be the formative sector of the plasmaspherc (c.g. Chappell et Al., 1971). Thus the average prevailing K,>is used for events occurring in the midnight&dawn sector, but the average K,, prevailing I8 h previously is used for events occurring in the dusk-midnight sector. This method is found to give the best agrcemcnt with a study of 41 crossings of the plasmapause observed on GEOS-2 cold plasma data. A correction for the rotation of the dusk bulge
T. K.
594
YFOMAN
Magnetic 23.2 1
6
0.00
et
01.
local time
0.40
1.20
2.00
2.40
d7o-o
- 65.0
5-4-
-_ -
--n FAR
OUL I, .NOR - I__ mz-1.6 - -
/ F--m: -2.6
-
-
-
-
60.0
---_
z
--__
3-
0
GML
n
m=-3.0
Q. YOR 70.0
I
80.0
I
90.0
50.0 120.0
I
100.0
Geomagnetic
-____-____-
$ zz z! .0 z 6
110.0
longitude
Expected range of plasmapauseposition Anticlockwise Clockwise Westward Eastward Upward
polarisation
polarisation apparent apparent
ellipse
ellipse
phase propagation phase propagation
Held aligned current
Substorm centre Downward
FIG. 5. Pulsation magnetic
(a) POLAKl7hTl0N
field aligned current
MAP FOR THE EVENT ON IIAY
302.
1987
shown over the SAMNET array. These are shown m relation to local time. the substorm current wedge position and the plasmapause position (SW text for details). (b) Key for the various symbols used in the polarization maps. polarization
characteristics
are
position with magnetic activity (Higel and Lci. 1984) is also included here. This allows for the observation that at times of low magnetic activity the dusk bulge moves towards midnight, whereas it moves towards noon at times of high magnetic activity. In a comparison of the calculated plasmapause position with 41 GEOS-2 crossings of the plasmapausc an agrecment to within 0.5 R, (Earth radii) occurred in 20% of casts. to within I R, in 33% of cases and to within I .5 R, in 72% of cases. These figures rose to 3 I, 38 and Xl %. respcctivcly, if only nightside crossings wcrc considered. The longitudinal position of the substorm current wedge is calculated from the magnetic bay structure across the array at low latitudes. In this region the contamination of the signal by aurora] zone ionospheric currents is smallest. This method has been
used before (c.g. Lester at (11.. 1983) and is based on the magnetic disturbance expected from upward and downward field-aligned currents. These produce fields such that inside (outside) the substorm current system the H component bay is positive (ncgativc) and that to the west (cast) of the system the D component bay is positive (negative). The ccntre of the wedge is defined by the longitude at which the D component bay changes sign. Nolc that in the symbolic rcprescntation of the SCW in Fig. 5 no attempt is made to fix the longitudinal position of the wedge exactly, but we just position the upward and downward currents and wedge ccntre relative to the SAMNET stations, according to the bay signatures above. No attempt is made to calculate the bay latitude. Figure 5 shows the substorm to occur to the west of and over the westward edge of the array. This
Pi? pulsation polarization patterns on SAMNET wedge position is predicted by the bay bchaviour on both the longitudinal chains. The associated pulsation is observed at magnetic local times of between 23:30 and 02:OO. A characteristic cllipticity reversal is seen across the estimated plasmapause position, which is at quite a low latitude as K,,has been ?I+ for some time bcforc substorm onset. The ellipse azitnuth also switches quadrant across the estimated plasmapausc position. Westward phase propagation is observed between three of the four station pairs, with KVIL NUR being too closely spaced to rcsolvc any phase diffcrencc for this event. This propagation is observed bctwcen station pairs which lit east of the eastward edge of the substorm current system. The magnitude of the observed HI-value is variable in both latitude and longitude. suggesting a signature due not to a simple w’ave travelling uniformly in longitude, but to 21combination of more complex ctt‘ects. The event also shows the expected CW rotation of the polarization cllipsc azimuth the further west it is measured. In this event the longitudinal azimuth change is less obvious than the latitudinal change. Neither the size of the change nor the orientations of the ellipse azimuths at FAR, KVI and NUR agree with the substorm current wcdgc model. k/J, 303. I YX7 A Pi2 pulsation was observed on the following day, starting at 21:52 U.T. H and .!I component magI
1
I
I
1
‘
H COMPONENT
L
21.30
21.50
595
netograms for this event, bandpass filtered between 200 and 20 s are shown in Fig. 6. For this cvcnt there arc no data from NUR as interference from a nearby radar cxperimcnt had contaminated the signal. A similar period pulsation of -72 s is seen in all stations of the array. The longer period seen in this event corresponds to a magnetically quieter day (the prevailing K,>is 3-). This is reflected in the larger plasmasphcre seen in the polarization map in Fig. 7. The estimated plasmapause position lies just north of the array and all the stations apart from OUL arc ACW polarired. For this cvcnt the substorm ccntre lies between the FAR and NOR meridians and the whole array is inside the substorm current system. Again westward propagation with a variable r?z-value is seen. Here. however, the ellipse Gmuth appears not to vary with latitude and the longitudinal variation agrees with the pattern cxpectcd from the substorm current wedge model for observations within the substortn current system.
The third case study considered here occurred on day 27. 1988, starting at 20:2X U.T. Magnetograms, bandpass filtered between 400 and 20 s are shown in Fig. 8. Again a pulsation is seen right across the array in the If and /I components. The signal here is very small at YOR and stems to have enhanced long period I
I
I
1
I
I
1
I
t
D COMPONENT
22.10
1
UT
22.30
1
2 1.30
I
21.50
I
22.10
FIG. 6. H AND0 (‘OMI’OhFNT ~fAGNETOGKA,MS tWKTHEtWNT ON DAY303. 1987 The magnetograms have been bandpass liltered between 200 and 20 s.
I
UT
1
22.30
T. K. YEOMAN ct
596
Magnetic 21.58 CJ
local time
23.17
22.37 I
cd, 0.37
23.57
1.17 70.0
8
”
6-
3-
0
n
/ & 70.0
t_ m= -6.0
GML
. YOR I 80.0
0.
E : (3
KW
I 90.0 Geomagnetic
- 55.0
I 110.0
1 100.0
50.0 120.0
longitude
FIG. 7. PULAKIZATION MAPJ;OR THLEVEKT ON DAY 303, 1987. Format as for Fig. 5.
DC
H COMPONENT
I
6.0n
6.0nT I
GML
KVI
L I
20.0
I
I
20.20
’
20140
iT
FIG. 8. ff AND D COMPONENT
The magnctograms
2j.o MAGNETOGKAMS
have been bandpass
20.0
1
1
FOK THC EVENTON
I
1
20.20
20.40
DAY 27. 1988.
filtered between 400 and 20 s.
UT
4
21.0
597
Pi2 pulsation polarization patternson SAMNET Magnetic 20.36
21.15
local time
2 1.55
22.35
3.5 5 li
10.0
I I65.0
uFAR
-
60.0
o 0
m GML
)
0
In=-3.0
17lwf
i
80.0
FIG. 9.
e F s (II
mNUR 55.0
-m= -4.3
I
I
I
go.0
100.0
110.0
POLAKI7.ATlON
MAP FOR THE CVLNT ON UAY
activity in the Hcomponent at OUL. Spectral analysis shows a dominant period of 96 s is seen at all stations cxccpt in the H component at OUL. which shows peaks at periods of 96 and 217 s. The prevailing K,, was 3 - and the polarization map (Fig. 9) shows that all stations lay south of the estimated pl~~smapause position and that they all had ACW polarization. In this case the substorm current wedge lies to the East and over the eastern part of the array. Again a variable tl?-value, westward prop~~g~~ting signal is seen. This event, like the day 302 event, has a more obvious polarization ellipse rotation with latitude than with longitude and only FAR, KVI and NUR have polari7ation cllipsc azimuths in agreement with the substorm current wedge model. For this event the polarization ellipse rotation occurs when all the stations are lying within the estimated plasmasphcrc and arc not subject to a latitudiiial ellipticity change between the two longitudinal chains.
OF Pi2 POl,ARI%ATION
P<%TTERNS
Twenty-eight Pi2 pulsations have been examined from the SAMNET data-set so far. Many more have been observed but for this preliminary study we chose to analyse only those with a clearly defined central frequency in both the H and D components and stable polarization characteristics over a number of
I!50.0 120.0
longitude
Format as for Fig.
BEHrZVIOl’R
5 .$
. YOR
Geomagnetic
4. ST,tTISTICAI,
g
27. 1988
5.
demodulates in the region of the maxitnum amplitude. The behaviour of the three case studies discussed above illustrates the great variety of results which arc typically seen, even in this restricted data-set. Average polarization ellipse azimuth rotations of 74 for the northern iong~tudinal chain and 69 for the southern longitudinal chain are observed with a standard dcviation of 36 in both casts. Considering the three meridional station pairs FAR-GML, NOR-KVI and OUL-NUR we find that there is a 56 average rotation in polarization azitnuth between the two chains, with this azimuth switching quadrants (from -90 ---t0 to 0 + 90 or vice versa) in about half of all casts. Thirty-seven per cent of these switches occurred inside the wedge and 59% occurred outside it. These latitudinal changes in polarization ellipse azimuth. which can occur both with and without a corresponding ellipticity reversal, are an impo~-tant featurp of the mid-latitude Pi2 polarization pattern. Such azimuth switches lead to deviations from the SCW pattern as latitude changes, both inside and outside the wedge, although changes outside the wedge are more frequent, Negative m-values implying westward propagation have been found in 939/o of the cases so far studied. Only six instances of eastward phase prooagation have been observed, lwo of which wcrc CW polarized. The mcasurcd polarization azimuths of individual stations agreed with the SCW pattern in 48 and 5 1%
T. K. YEOMAY 1’1rd.
598
of cases for the northern and southern chain stations, respectively. The polarization pattern for each chain as a whole agreed with the model predictions in only 17% of events for the upper chain and 23% for the lower. If a weaker criterion for the substorm current pattern is adopted. requiring a north pointing azimuth near the substorm centre and an ACW rotation of the polarization cllipsc the farther east it is measured, then there is a 63 and 67% agrecmcnt on the upper and lower chains, rcspcctively. The poor lcvcl of agreement with the SCW pattern may in part be explained by recent modclling of the substorm current system by Lester ct cd. (1989). They showed that the further away from the ionospheric intcrscction of the fieldaligned current system the observations arc made, the greater the longitudinal extent of the wcdgc appears to bc. This can introduce errors in the determination of the field-aligned current longitudes. This cffcct is exacerbated for wedges of small angular width. Pi2 observations in that paper showed that pulsation polarization azimuth quadrants at 55 and 40 gcomagnetic latitude were the same inside the wedge but were frcqucntly different outside it. Our results, howcvcr. show that latitudinal variations of azimuth arc not confined to observations made outside the wedge. Clearly the polarization ellipse azimuth pattern is far more complex and varied in its spatial structure than is predicted by a straightforward SCW model. We may conclude. however. that a SCW model type pattern. or part of enc. can almost always be seen but that its longitudinal position with respect to the wedge may bc shifted and that this shift may vary with latitude. The ACW rotation of the polarization ellipse the farther east it is measure is confirmed to bc a common feature of the mid-latitude Pi2 signature, but latitudinal variations are frequent and the orientation of ellipse azimuths is only a weak indicator of the substorm current wedge position. 5. ,\2\! INVESTIGATION
OF TRAVELLING WAVE
POLARIZATION
PATTERNS
In the previous sections we have seen a poor agreement with the substorm current wedge model polarization pattern in spite of the use of clearly defined pulsation data. Latitudinal switches of azimuth and westward phase propagation east of the eastern edge of the substorm current system have also been observed. These observations suggest that the substorm current wedge model as put forward by Lester et cd. (I 983) and the travclling wave mechanism put forward by Southwood and Hughes (1985) are inadcquate to explain all of the polarization features seen in mid-latitude Pi2 pulsations. Rcccntly Yeoman and
Orr (1989) suggested that a plasmasphcric cavity resonance is the most likely mechanism for the midlatitude amplitude enhancement and polarization rcvcrsal seen in Pi2 pulsations, rather than driven field lint resonances or surface waves. Such a mechanism has also recently been supported by the work of Sutcliffc and Yumoto (1989) and Kitamura et (I/. (1988). This mechanism provides a low latitude wave source spatially distinct from the wave source in the aurora1 zone ionosphcrc. but triggered by the same substorm activity. Consideration of the Pi2 wave as a twosource phenomenon may hold an explanation for the deviations from the classical substorm current wcdgc model polarization pattern in the region where both aurora1 zone activity and lower latitude activity are important constituents of the Pi2 signal, without rccoursc to low latitude field line resonances. Here the travelling wave ideas of Southwood and Hughes (1985) are extended to include a second source at a different longitude and a different latitude to the primary wave source. The basic model consists of two sinusoidal wave sources with circular polarization. A high latitude CW polarized wave with phase propagation in longitude represents the aurora1 zone signal and a low latitude ACW polarized wave with phase propagation in longitude represents the plasmaspheric cavity resonance signal. This arrangement is shown schematically in Fig. IOa. Polarization ellipses are calculated. in the same way as for the experimental data. from the superpositon of the waves. The spatial variation of the polarization ellipse is displayed on a 6 x 9 grid (Fig. lob). CW and ACW polarization ellipscs arc represented in the same way as in the experimental result polarization maps. The D component m-value is displayed as a lint between stations with an arrow head indicating the wave propagation direction. The model has eight variable parameters: the aurora1 zone signal amplitude and decrement, the auroral and lower latitude signal nl-values. the aurora1 and lower latitude signal source meridians from which there is phase propagation and the aurora1 and lower latitude pulsation periods. The CW aurora1 wave amplitude is kept at a constant value of IO within a “highly conducting aurora1 zone” marked in Fig. IOb as a box. Outside this arca the amplitude falls off cxponcntially with distance. For simplicity the ACW low latitude signal has an amplitude of I cvcrywhere. The aurora1 signal also has a phase delay with latitude to mimic the propagation delay introduced by the fast mode coupling of the transverse mode aurora1 signal to lower latitudes (Cough and Orr, 1984). In Fig. IOb the aurora1 zone signal has an nr-value of IO. The aurora1 source is located at the westward edge of the aurora1 zone at coordinates (2,2) counting
Pi2 pulsation polarization
I
1
-30
I
-20
-10
599
patterns on SAMNET
I
I
I
I
0
10
20
30
I
I
40
50
I
Longitude (b) FIG. IO. (a) SCHL~L~TK
tmc;uM
01 THE
I~IIOGKLSSIOU
01: WAVEPKONTS
THE MOL~ItII’I~ TRAv~LLIN(;
WAVI:
I:KOS~ THE TWO
S~UKCXS
us~u
II\
MOIIEL.
The amoral LWC source (A) propagates out radially from its amplitude maximum in the westward travelling curgc. whilst at loucr latitudes W;IVCScharacteristic of a plasmaspheric cavity resonance with a small IWv~duc exist (6). The Pi? polarrration is the result of the superposition of the two wave sources. (b) An example of results from the travelling wave model. The aurora1 zone source is in the box at grid position (2,2) measured from the top left hand corner. and has an/17-value of IO. The plasmaspheric cavity resonance propagates from the 6th grid meridian from the left with an ,n-value of 3. Tlrc wave superposition gives a substorm current wedge model type polarization pattern beneath the amoral zone box. The polarix~tion pattern also shows nr-values variable in longitude and latitude along with latitude dependent polarization cllipsc a,knuths.
from the top Mt of the grid. In this example the auroraI zone signal amplitude has an cxponcntial decrement or 1.5 between adjacent ellipses. The low latitude source is located on grid meridian 6 and has an m-value of 3. Figure 1Ob shows a substorm current wedge model type polarization pattern below the SLIToral zone box. The III-values vary with both latitude and longitude and propagation is westward west of the aurora1 source meridian and eastward cast of the
low latitude source meridian. Between the sources the direction of propagation depends on the dominant wave, changing from westward to eastward with increasing latitude. Some anomalous longitudinal propagation directions occur just to the east ot” the low latitude source in this example. These are caused by the reversal in ellipticity where the source dominance switches. Few details of the high latitude source are included in the model, so the polarization ellipses
600
T. K. YEOMANCI til.
in the upper part of the tigure have little physical meaning. In Fig. IOb the orientation of the polarization cllipsc in grid meridian 2 switches quadrant as the latitude changes. This quadrant switch is caused by the propagation delay introduced into the amoral signal. which gives the aurora1 signal phase a latitudinal dependency in addition to its longitudinal phase propagation bchaviour. Although in this example the quadrant change is accompanied by an ellipticity switch, this is due to the relative amplitudes of the wave sources. The ellipse azimuth quadrant switch is not depcndcnt on this and can occur without such a switch. In this modelling the degree of rotation of the polarization ellipse and the longitudinal position of the substorm current wedge model type polarization pattern which emerges from the wave superposition will depend on the values used for the source meridians, n?-values and wave periods. Varying these parameters will lead to quite different polarization patterns. A reduced azimuth rotation will result if nz-values are reduced or if the source meridians are brought closer together. whereas increased azimuth rotation will result from increasing tither of thcsc paramctcrs. The superposition of waves of different periods will give the same degree of azimuth rotation, but will lcad to a different section of the characteristic wave superposition azimuth orientation pattern lying below the substorm current wedge.
6. DISCUSSION An examination of the polarization of 28 Pi2 ~LIIsations observed on SAMNET has shown a characteristic ellipse azimuth rotation with longitude. The SAMNET longitudinal chains have allowed this behaviour to be examined at two latitudes and it has been found that azimuth switches between the chains are common and may occur with or without an ellipticity reversal. The case study on day 302. 1987, shows a polarization ellipse quadrant swaitch accompanying an ellipticity reversal across the estimated plasmapause position and longitudinal phase propagation inconsistent with high latitude Pi2 mod&; the pulsation appears to be propagating towards its high latitude source. Although Lester PI al. (1984) pointed out that castward propagation was the most frequent east of the substorm current wedge. observations such as that on day 302 are not uncommon. The day 303, 1987. event is in agreement with the model of Southwood and Hughes (1985). showing westward phase propagation and a polarization ellipse azimuth orientation in agreement with the substorm current wedge model.
The day 27, 1988, event shows phase propagation consistent with the Southwood and Hughes (1985) model but shows a change of polarization ellipse quadrant with latitude without any ellipticity reversal and apparently not associated with the plasmapause. An example of the results of a very simple travelling wave model of the Pi2 disturbance in the region where both the high latitude aurora1 disturbance and a lower latitude plasmasphcric cavity resonance signature are important have been presented. This model seems able to explain some of the outstanding problems of the mid-latitude Pi2 signature and also some of the new features observed on SAMNET. In its simplest form the model reproduces the substorm current wedge model polarization pattern in longitude and is thus able to accommodate results such as those seen in the day 303. 1987, event. In addition the model predicts a greater likelihood of eastward propagation for CW polarized waves. This agrees with the observations of Gelpi et al. (1985b). The possibility of longitudinal phase propagation towards the aurora1 zone source is also allowed for in regions where the low latitude source is dominant and on a different meridian to the aurora1 zone source. The model also predicts variable ellipticities across the array. Such ellipticity variations have been obscrvcd in two of the case studies presented here and were also noted by Lester et rd. (I 984). Other factors such as the different dominant periods seen in some components at some stations. also noted by Lester cf al. (1984) may bc accounted for by the model, as variations in dominant period could occur due to the two dif’fcrcnt wave sources being dominant in different regions. The model can also explain polarization ellipse azimuth quadrant switches in latitude without an cllipticity reversal, such as was observed in the day 27. 1988, cvcnt. This is a consequence of a phase delay due to the propagation of the high latitude signal. SAMNET results show that such azimuth orientation quadrant switches with constant cllipticity occur in 20% of observed events and constitute 63% of the observed polarization ellipse azimuth switches. The remaining 37”/0 of azimuth switches are associated with ellipticity reversals at the plasmapause. The extent of the rotation is probably somewhat exaggerated in the model (a phase delay of IO” or typically about I .5 s per degree of latitude is assumed to make the rotation clear), but this mechanism could explain sonic latitudinal variations. Thus a low latitude cavity resonance is able to explain the observed features of the Pi2 polarization pattern without considering a field line resonance just within the plasmasphcre, as suggested by Fukunishi (I 975).
Pi2 pulsation polarization patterns on SAMNET
Variations in m-values, source meridians and period can also produce substorm current wedge model-type polarization patterns which are incomplete or displaced with respect to the wedge. Lester rt ~1. (1983) noted that 33% of patterns on the AFGL array were shifted in such a manner. They ascribed this to the presence of pre-existing substorm current systems. Lester (‘2~1. (1984) examined Pi2s only from isolated substorms to avoid this effect, but found a very similar level of agreement of 70%. The travelling wave model may provide an explanation for at least some of these displacement effects. Observations on SAMNET have shown that a simple application of the SCW polarization pattern is frequently inadequate to explain the observed mid-latitude polarization. More sophisticated modelling of the high and low latitude wave sources will be required before full understanding of the mid-latitude region can be achieved Clearly the travclling wave model has quite a numbcr or variable parameters. This allows it to successfully reproduce the very varied mid-latitude Pi2 signature in a large number of cases. It is also a drawback of the model as it makes it rather difficult to establish its validity. Continued observations of Pi2 characteristics on a large number of events are anticipated during the current campaign. More observations should enable the processes which combine to make up the mid-latitude signature of Pi2 pulsations to be established In addition the consideration of the lower latitude SAMNET Pi2 data may enable the nightside Pi2 to be used as a test of global cavity resonance theory, a subject on which considerable theoretical and modelling analysis has been concentrated recently (e.g. Allan rt rd., 1986: Kivelson and Southwood, 1986) mostly in a daysidc context. but of which there has been little observational confirmation. inclusion of such a magnetospheric cavity rcsonancc model in modelling of Pi2 pulsations might explain some puzzling features of the lower latitude polarization patterns. Kivelson and Southwood (1986) noted that the node and antinodc structure of a cavity mode allow the possibility of polarization rcvcrsal without field line rcsonanccs. This may be able to account for the low latitude reversal seen by Lester cut trl. (IHY).
~lc.X,ro~~~k~c~ymro~t.~~Wethank the SERC for funding the SAMNET project and for a studentship for T.K.Y. The invaluable contributions of our Litc operators and Mews John Riddick and John McDonald and.Dr Bill Stuart of the British Geological Survey are also acknowledged. We also thank a referee for constructive criticism,
601 REFERENCES
Allan, W.. Poulter, E. M. and White, S. P. (1986) Hydromagnetic wave coupling in the tnagnetospherc~plasmapause effects on impulse excited resonances. Plwwr. Spuw Sci. 34. I 189. Baranskiy. L. N.. Troitskaya. V. A., Sterlikova. I. V., Gokhberg, M. B., Ivanov, N. A., Khartchcnko. I. P.. Munch, J. W. and Wilhelm, K. (I 980) The analysis of simultaneous observations of nighttime Pi pulsations on an cast west protilc. J. Groph_vx. 48. I. Beamish, D., Hanson, H. W. and Webb, D. C. (1979) Conplex demodulation applied to Pi2 geomagnetic pulsations. C;wpl~w. J. R. usir. Sec. 58. 471. Chappell. C. R., Harris. K. K. and Sharp, G. W. (1970) A study of the influence of magnetic activity on the location of the plasmapause as measured by OGO 5. J. ,qeopl~>,.s. Rc.v. IS. 50. Chappell, C. R., Harris. K. K. and Sharp. G. W. (1971) The daysidc of the plasmasphere. J. g-leoph~x. Res. 76. 7632. Fukunishi. H. (1975) Polarization changes of geomagnetic Pi2 pulsations associated with the plasmapause. J. ,qwpl~>a.r.Res. 80, 98. Gelpi. C.. Hughes, W. J., Singer, H. J. and Lester, M. (1985a) Mid-latitude Pi2 polarisation pattern and synchronous orbit magnetic activity. J. ,qeop/~!~.s.Ret. 90, 6451, Gelpl, C., Hughes, W. J. and Singer, H. J. (lYX5b) Longitudinal phase and polarisation characteristics in mid-latitude Pi2 pulsations. J. ,ycqj/l~.r. Re.v. 90, 9905. Gough, H. and Orr. D. (1984) The cffcct of damping on geomagnetic pulsation amplitude and phase at ground observatories. P/ww/. Spuc’c &i. 32, 619. Higel, B. and Lci, W. (1984) Electron density and plasmapause characteristics at 6.6 Rr: a statistical study of the GEOS 2 relaxation sounder data. .I. ,ywph,rs. RKS. 89.
1583. Hughes, W. J., McPherron, R. L. and Barfield, J. N. (1978) Geomagnetic pulsations observed simultaneously on three geostationary satellites. .I. geop/z~~s. Res. 83. I 109. Kitamura. T.-I., Saks. 0.. Shimoizumi. M., Tachihara. H., Oguti, T.. Araki. T., Sate, N.. Ishitsuka. M.. Veliz. 0. and Nyobc. J. B. (198X) Global mode of Pi2 waves in the equatorial region-difference of Pi2 mode between high and equatorial latitudes. J. Gcormy~~. Gcwe/c~c~/~.40. 621. Kivelson, M. G. and Southwood. D. J. (19X6) Coupling ol’ global magnetosphcric MHD eigenmodes to field line resonances. J. ycwp/~~.r. Rcs. 91. 4345. Kuwashima, M. and Saito. T. (19X I) Spectral characteristics ofmagnctic Pi2 pulsations in the aurora1 region and lower latitudes. J. ,qco~~~/~~~s. Rc.c. 86, 4686. Lester, M.. Glal3m&cr. K.-H. and Behrcns. J. (1985) Pi2 pulsations and the castual-d electrojct: a cast study. Plrrw/. s/Jr/w %i. 33, 35 I Lester, M., Hughes. W. J. and Singer. H. J. (1983) Polarisation patterns of Pi2 magnetic pulsations and the substorm current wdge. J. ,qrq+?~~.c. Xc.\. 88. 795X. Lester. M.. Hughcs. W. J. and Singer. I-1. J. (1984) Longitudinal structure m Pi2 pulsations and the aubstorm current wedge. J. ,qcwp/~~~.s.RCA. 89, 5489. Lester. M. and Orr. D. (1981) The spatio-temporal charactcristics of Pi2’s. J. afnmc. Icw. P/I~.~. 43. 947. Later, M. and Orr. D. (1983) Correlation between ground observations of Pi2 geomagnetic pulsations and satellite plasma density observations. Pltrwt. S’/xrw Sci. 31. 143. Lester, M.. Singer. H. J., Smits, D. P. and Hughes, W. J.
602 (1989)
T. K.
YFOMAN
Pi2 pulsations and the substorm current wedgelow latitude polarisation. J. qqeoph)x. Res. 94, 17, 133. Mier-Jedrzejowicz, W. A. C. and Southwood, D. J. (1979) The East West structure of mid-latitude gcomagnelic pulsations in the 8 25 mHz band. P/uric/. Spcm Si. 27. 617. Olsen. J. V. and Rostokcr, G. (1978) Longitudinal phase variations of micropulsations. J. gc~o~~/~~xmRc.~.83, 24X I. Orr. D. and Webb, D. C. (1975) Statistical studies of geomagnetic pulsations with periods between IO and 70 s and their relationship to the plasmapause region. P/u?zc/. .‘jp~ce SC,/. 23, I 169. Pashin, A. B.. GlaBmeier. K.-H.. Baumjohann, W.. Raspopov. 0. M.. Yahnin, A. G.. Opgenorth, H. J. and Pellincn, R. J. (1982) Pi2 magnetic pulsations. aurora1 break-ups and the substorm current wedge : a case study. J. Gcc&~.~. 51. 223. Riddick, J. C. (1987) BGS Geomaynctism Research Group Report. Number X7:23. 1987. Sakural. T.. Tonesawa. Y. and &to. N. (1988) Oscillation characteristics ocfPi2 in aurora1 zone : Syowa Iceland conjugatc study. Proc. NIPR S.mp. tipp’per A/ma.\. P/I~v. I, I IO. Samson. J. C. (1982) Pi2 pulsations : high latitude rcsuhs. P/urw/. Spm SC,;. 30. 1239. Samson. J. C. and Harrold. B. G. (1983) Maps of the polarisation of high latitude Pi2’s. J. q~)p/?w. Rcs. 88. 5736.
et
ul.
Samson. J. C. and Harold. B. G. (1985) Characteristic time constants and velocities of high latitude PiZ’s. J. ,qeop/~~s. Rrs. 90. 12.173. Singer, H. J.. Hughes. W. J.. Gelpi. C. and Ledley, B. G. (1985) Magnetic disturbances in the vicinity of synchronous orbit and the substorm current wedge: a case study. Smits. D. P.. Hughes, W. J.. Cattcll, C. A. and Russell. C. T. (19X6) Observations of field aligned currents, waves and electric fields at substorm onset. J. ,qycoph~~.~.Rex. 91, 121. Southwood. D. J. and Hughes, W. J. (19X5) Concerning the structure of Pi2 pulsation. J. ,qcwph~~.~. RCT. 90. 3X6. Stuart. W. F., Brett, P. M. and Harris, T. J. (1979) Midlatitude secondary resonance in Pi2‘s. J. tr/mo.r. ICW. P/J.I~.s. 41. 65. Sugiura. M. and Wilson. C. R. (1964) Oscillation of the gcomagnctic lield lines and associated magnetic perturbations at conjugate points. J. yeo[?l~\:s. Rcs. 69. I21 I. Sutcliffc. P. R. and Yumoto. K. (1989) Dayside Pi2 pusations at low latitudes. Geop/~~.\. Re.c. Left. 16. 887. Yeoman. T. K. and Orr, D. (1989) Phase and spectral power of mid-latitude Pi2 pulsations: evidence for a plasmaspheric cavity resonance. P/trwf. Spuw Sci. 37. 1367.