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Pi 2 pulsations: High latitude results
Planet. Space Sci., Vol. 30, No. 12, pp. 1239-1247, 1982 Printed in Great Britain.
0032-0633/82/121239-09$03.00/0 0 1982 Pergamon Press Ltd.
Pi 2 PULSATIONS:
HIGH LATITUDE
RESULTS
J. C. SAMSON
Institute of Earth & Planetary Physics, Department of Physics, University Alberta, Canada T6G 2Jl (Received
of Alberta, Edmonton,
7 April 1982)
Abstract-A review of recent experimental results from studies of high latitude Pi 2 pulsations indicates that these pulsations are fundamentally related to the initiation of the aurora1 breakup and substorm. At high latitudes, the Pi 2’s show their peak intensities in the region where the breakup begins and appear to remain in this region after the breakup has spread poleward. In addition, the Pi 2’s occur simultaneously with, or before all other ionospheric phenomena associated with the breakup. The field aligned and ionospheric currents associated with the Pi 2 resemble those of a typical substorm, but the ionospheric currents are phase shifted compared to the field aligned current. The periodic oscillations of the Pi 2’s are probably caused by a reflection of the initial field aligned current pulse from the aurora1 ionosphere. This pulse is trapped on dipolar field lines leading to multiple reflections from North and South aurora1 ionospheres.
INTRODUCTION
onset or aurora1 breakup (Akasofu, 1968) is accompanied by marked increases in the power in ULF (l-20 mHz) magnetic fields. Much of this power appears random and chaotic, particularly near the region of the breakup. The substorm onset is, however, almost always accompanied by distinct, quasi sinusoidal magnetic field oscillations with frequencies in the range 620 mHz. These oscillations, or Pi 2 pulsations, are most clearly seen on the ground at mid latitude (50-60” geomagnetic latitude) stations, but they also exist at high latitudes near the region of the breakup. This review will consider, for the most part, Pi 2’s which occur at high latitudes. I shall define high latitude to be any region which is influenced by magnetic fields from the ionospheric substormelectrojet. This region typically is between 55 and 80” geomagnetic latitude. The classification of Pi 2’s at high latitudes often leads to some ambiguity. At high latitudes, the Pi 2-band (6-20 mHz) shows marked increases in energy, but the spectra are often broad or chaotic. Conversely, at low latitudes, the spectra are often sharply peaked, indicating the presence of quasi sinusoidal pulsations. In fact, high latitude magnetometers record broad band electrojet noise as The substorm
An invited review paper presented at the 4th IAGA Scientific Assembly, Edinburgh, 14 August 1981.
well as the Pi 2 pulsations (Olson and Rostoker, 1975). It is extremely important to distinguish between these two types of oscillations in the Pi 2-band since they probably come from different sources. Consequently, I shall call only the damped quasi sinusoidal oscillations, which are clearly seen at mid latitudes, by the name Pi 2. The broad band electrojet noise, I shall call Pi 2-band noise. Any high-latitude Pi 2 which we identify should have a corresponding Pi 2 at mid latitudes. Pi 2’s have a number of features which allow us to separate them from Pi 2-band, electrojet noise. Some of these features are: (a) The Pi 2 begins at exactly the time of the substorm onset (Rostoker and Samson, 1981; Fig. la, 2a). (b) All stations that see the Pi 2 show minimal delay for Pi 2 onset ( < 20 s for nightside stations). (c) The Pi 2 oscillations seldom last for more than 10-15 minutes after the substorm onset. (d) The Pi 2 spectrum is narrow band and the oscillations are damped and quasi sinusoidal. (e) The Pi 2’s have spectra that are typical of pure states, or polarized waves (Rostoker and Samson, 1981). All of the above criteria must apply to data from high latitude stations, as well as mid latitude stations, if we are to be sure that a Pi 2 is present. The Pi 2-band electrojet noise seldom shows any of the above features. This energy is almost always broad band and unpolarized, can last for 1239
1240
J. C. SAMSON
the duration of the substorm-enhanced electrojet and is detected only when the electrojet passes over a station (Olson and Rostoker, 1975), often after delays of minutes from the substorm onset. CORRELATIONS WITH OTHER SUBSTORM PHENOMENA
Since Pi 2’s are directly connected with substorm onsets, it is not surprising that they are closely correlated with other breakup phenomena, including brightening of aurora1 arcs, X-ray intensifications, riometer absorption events and ionospheric electrojet intensifications. One of the classic signatures of the beginning of an aurora1 substorm or breakup is the sudden brightening of a quiet arc (Akasofu, 1968, and references therein). Pi 2 pulsations occur in conjunction with the brightening and there have been numerous attempts to correlate Pi 2 frequencies with the latitude of the brightened arc (Afanasyeva et al., 1970; Kuwashima, 1978). An evaluation of the data given by Samson and Rostoker (1972) and by Kuwashima and Afanasyeva et al. shows different latitudinal trends and consequently these latitudinal features are far from conclusive. Precipitating, high energy electrons (energies 20 kev and greater) which lead to brightening of aurora1 arcs, also lead to bursts of bremsstrahlung, X-rays (Akasofu, 1968). Pytte and Trefall (1972) have shown that bremsstrahlung events associated with breakups start and end with the Pi 2’s accompanying the breakup. The high energy electrons also lead to increased absorption of cosmic, radio noise. Consequently the Pi 2 is often associated with strong absorption spikes in riometer data (Pytte and Trefall, 1972; Pytte et al., 1977, and references therein). These riometer spike events can show very dynamic and rapid latitudinal motions (Hargreaves et al., 1979), which may be intimately related to the mechanism for the formation of Pi 2’s and aurora1 surges. One important point which must be emphasized before further discussion, is that almost all correlative studies have shown that the Pi 2 begins at or before most other breakup phenomena. The implication here is that the Pi 2 must be closely associated with the enhanced field aligned currents which ultimately lead to the breakup. The enhanced ionospheric electrojets associated with substorms have been studied extensively, and are now reasonably well defined (Rostoker, 1980). Thus it is important to compare the patterns of the Pi 2 polarizations and intensities with those of the substorm electrojet. Olson and Rostoker (1975) used low latitude
signals to identify the spectral region of the Pi 2 at high latitudes and their results showed that the peak in Pi 2 energy was at the center of the substorm-enhanced electrojet. Wilhelm et al. (1977) concluded that Pi 2-band energy was due to fluctuations in electrojet and field aligned current intensities. Unfortunately they did not discriminate between Pi 2-band noise and Pi 2 pulsations. A study by Rostoker and Samson (1981) was designed to extract only the polarized component of the high latitude recordings. In order to ensure that the polarized pulsations were in fact Pi 2’s, they were also compared with pulsations occurring at lower latitude stations. We found that the maximum intensity of Pi 2’s is at the southern borders of the substorm enhanced westward and eastward electrojets (Fig. 1), not at the center as indicated by the results of Olson and Rostoker. MORPHOLOGICAL STUDIES
The most common parameters of Pi 2’s which are analysed in morphological studies are the spectra (power vs frequency, latitude, longitude (L.T.)), and the polarization ellipses (usually in the horizontal plane). In order to obtain an ensemble average of the morphology, these data are often ordered by latitude, and L.T., and the averages are plotted in these coordinates. While these methods have yielded useful results in the past, particularly at mid latitudes, high latitude detail is often blurred or lost because of the large range of coordinates over which breakups can occur. In order to make further progress it will be extremely important to correlate each example of the Pi 2 morphology with the center of the breakup, before averaging. The characteristic feature of the spectra of Pi 2’s is a transition from sharply peaked spectra at Poleword border after onset
Fe;
PI
PI2
2
Poleword border before onset
FIG. 1. THE REGIONS OF THE INTENSITY MAXIMA OF Pi 2’s. The poleward border after onset corresponds to the border of the substorm enhanced electrojet (after Rostoker and Samson, 1981).
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Pi 2 pulsations: high latitude results mid latitudes (Stuart and Booth, 1974), to broad band spectra at high latitudes. Olson and Rostoker (1975), indicated that the Pi 2-band of the spectrum is filled in around the Pi 2 peak as the center of the electrojet is approached. The frequency of the Pi 2 spectral peak is, however, unchanged with latitude (see also the example to be given later in this discussion). Studies of the polarizations of Pi 2’s and Pi 2-band pulsations by Bjornsson et al. (1971) indicate that the mean direction of polarization (Fig. 2) is compatible with a source comprising field aligned currents and closure electrojets. Field aligned current is predominantly downward (upward) from 0 to 03.00 L.T., and upward (downward) from 20.00 to 23.00 L.T. The electrojet can be seen in the strong negative H direction polarization at 70” over the interval 22.00-04.00 L.T. The change in the orientation of the vectors at 55”, from North-East to North-West, is due to net, oscillating field aligned currents. These features will be explained more clearly in the model to be given later. Studies of the sense of polarization of Pi 2’s occurring at high latitude, indicated that there are four quadrants of polarization (Kuwashima, 1978) with the approximate boundaries given by the latitude of the breakup, and - 21.00-22.00 L.T. (Fig. 3). Note that this L.T. is also close to the change in the orientation of the direction of polarization (Fig. 2) and the region for the maximum occurrence frequency of Pi 2’s. The morphology of the polarization is, however, not quite so simple as Fig. 3 suggests. At high latitudes, in the Northern Hemisphere, Rostoker and Samson (1981) found that most Pi 2’s occurring in the morning sector had counterclockwise
FIG. 2.
THE
DIRECTION OF POLARIZATION OF Pi 2 EVENTS (AFTER BJ~RNSSON et al., 1971).
22
23
Co
01
FIG. 3. MAGNETICL.T.DEPENDENCES OF THEPOLARIZATION OFPi2’S INTHEHORIZONTALPLANE. The symbol R.H. indicates counterclockwise polarization when viewed downward (Southern Hemisphere). AE indicates the aurora1 electrojet (after Kuwashima, 1978).
polarization equatorward and clockwise polarization poleward of the Pi 2 maximum. Kuwashima’s results suggest that these polarizations should be clockwise and counterclockwise respectively, especially when the 180” phase shift in the conjugate D’s are considered (Kuwashima, 1978, 1981). In addition, Rostoker (1967) deduced that the polarizations at - 56”N geomagnetic latitude (equatorward of the Pi 2 maximum) were predominantly counterclockwise. I shall discuss these possibly conflicting results in a polarization model to be presented below. AN EXAMPLE
Probably the best way to finish this brief review is to illustrate the features of high latitude Pi 2’s by giving an example of a substorm onset and its associated Pi 2’s. To further clarify the discussion, I have chosen an isolated, and particularly simple substorm event which is accompanied by mid latitude Pi 2’s similar to the d Pi’s discussed by Stuart and Booth (1974). High latitude magnetograms from the University of Alberta array are shown in Fig. 4. The substorm onset is very abrupt, and a strong westward electrojet can be seen beginning at -8.08 U.T., indicating the breakup is centered slightly to the West of the stations. The Z-components indicate that the electrojet is initially centered South of FTCH, then rapidly moves northward to be centered near URAN (Note the sharp Z-spike caused by the northward motion). At 08.08.30 U.T. the electrojet is centered at -6S’N, at 08.10.30 at - 67”N and by 08.12.00 the electrojet is at - 69”N.
1242
J.C. SAMSON Magnetometer
I
200
Day
59
1979.28
Feb
NT
FTCH
MCMU
URAN
SMIT
HAYR
PROV
7
8
9
FIG. 4. FLUXGATEMAGNETOMETERRECORDINGS OFA SLJBSTORMOCCURRINGON~~ THEUNIVERSITYOF ALBERTAARRAY.
FEBRUARY~~~~,OVER
The components are: H, magnetic North; D, magnetic East; and Z, downward. Coordinates for these stations can be found in Rostoker and Samson (1981). Latitudes are given in Fig. 6.
A comparison of the magnetograms at PROV, FTCH and URAN indicate that the electrojet is tilted by - 10” from the E-W direction and PROV is much further South of the electrojet than URAN. Data from riometers at three of the Alberta stations indicate that absorption peaks first near MCMU at -8.10 U.T. and then at -8.12 U.T. near SMIT and PROV. This rapid poleward motion (-2 km s-l) of the absorption (precipitation) is a common feature of isolated substorm onsets (Hargreaves et al., 1979). The rapid northward motion of the precipitation and the westward electrojet commonly occurs to the East of developing surges (Akasofu, 1968, and references therein; Pytte et al., 1977) and is in-
ti.mately related to the formation of Pi 2’s and the configuration of their polarization. The dynamic motions in the electrojet indicate that the polarizations of Pi 2’s will be quite complicated near the region of the breakup. The filtered Pi 2’s (pure states) are shown in Fig. 5. The Pi 2 putsations are clearest at LEDU (see the box), and these Pi 2’s correspond to the Pi 2’s (d Pi’s) seen at mid latitudes. The spectra of the pure states (polarized waves) are shown in Fig. 6. The sharpest spectral peak is at LEDU, with a frequency near 9 mHz. OnIy this peak corresponds to the mid latitude Pi 2’s. The Pi 2 intensity (9mHz) drops at MCMU, rises again at FTCH, and again drops to very low values at URAN. As I indicated previously the electrojet is tilted at 10”
1243
Pi 2 pulsations: high latitude results MRGNETOMETEF DRY
59
20
1975,
FEB.
28
Nl
i
FTCH
LEDU
URRN
SMIT
PROV
7
a
97 UNIVERSRL
i
8 TIME
a
IHRSI
FIG.~. MAGNETOMETERDATAFILTEREDTOEXTRACTPURESTATES(POLARIZEDWAVES)INTHE (SEE ROSTOKERAND SAMSON (198l), OR SAMSON AND OLSON (1981).
and so apparently is the center of the intensity of the Pi 2, since PROV shows much more 9mHz power than does URAN. Note also that three spectral peaks, at 4, 9 and 16 mHz, predominate over all the stations, especially at PROV. These three spectral peaks have also been reported by Olson and Rostoker (1977). Latitude profiles of the substorm electrojet and the data in Fig. 6 indicate that the Pi 2 is localized near the southern border of the westward electrojet, in agreement with the results of Rostoker and Samson (1981) (Fig. 1). Figure 7 gives the latitude-dependent polarizations and powers of the Pi 2 event. The minimum in the H-component near MCMU, is probably due to an antiphase, oscillating electrojet (ambient eastward electrojet) to the South of the main eiecelectrojet). The trojet (ambient westward
Pi2 BAND.
polarizations to the South of the maximum are counterclockwise (positive) and clockwise to the North, with a demarcation near MCMU. These results are also consistent with those given by Rostoker and Samson (1981) but appear to contradict those of Kuwashima (1978). A MODEL FOR HIGH LATITUDE Pi 2%
The task now is to construct a relatively simple model for the polarization states of high latitude Pi 2’s which will be compatible with the observations I have aheady discussed. The Pi 2 model should also be compatible with the characteristics of the substorm current systems. It is clear that breakup phenomena begin with the brightening of a quiet arc, and substantial precipitation of electrons. These features suggest large field aligned currents initially flowing upward
J. C. SAMSON 1.0
FREWENCY
FIG. 6.
(mHz
.%irgoo
1
GEOMAGNETIC
STATE SPECTRA OF THE Pi 2 EVENT FROM 08.08 to 08.18 U.T., 28 FEBRUARY. If x(t) = [H(t), D(t), Z(t)] at one station, the pure state spectra are given by P(j) = (1 - (Az/A,)“~)‘(lr,tzCf))’ PURE
where z(w) =
A, and LI, are respectively
H
FIG. 7.
72
LATITUDE
POLARIZATION CHARACTERISTICS OF THE 9 2 OCCURRINGONFEBRUARY 28.'
mHz Pi
The ellipticity (solid line) is the ratio of the minor to major axis of the polarization ellipse. A positive ellipticity indicates counterclockwise in the horizontal plane, viewed from above. The polarization angle, theta, is measured clockwise from magnetic North ( - 90” zz theta I 90”).
c x(t)e’“‘, the largest eigenvalue
and
corresponding eigenvector of ‘?’ z(o)zt(w), and A?is the U=lmAlnext largest eigenvalue. The symbol t denotes time and t is Hermitian adjoint. The plots show the function fP(f).
on field lines intersecting the arc. The high energy electrons are possibly accelerated by fields which are caused by topside ionospheric current instabilities (Coroniti and Kennel, 1972) or double layers (Block, 1972), both of which can originate from large field aligned currents. These polarization electric fields map to the ionosphere as horizontal electric fields pointing toward the center of the arc (initially) (see Fig. 8). The net electric field will be a combination of this field and the ambient convection field (usually pointing equatorward to the East of the Harang discontinuity, and poleward to the West). However, for simplicity I shall ignore these ambient fields. In the region of the precipitation near the arc, the Hall, u”, and Pederson, up, conductivities in the E region are significantly increased, with Us = 3u, (Brekke et al., 1974). Because of the gradients in u” and ap, the initial current pulse must close
FIG. 8. A MODEL
FOR THE INITIAL PULSE OF A HIGH LATITUDE Pi2.
After this initial pulse, all currents oscillate at frequency o but different phases. Since Pi 2’s propagate azimuthally, this picture shows only the fust half cycle in the azimuthal direction. The symbol E denotes ionospheric electric fields, ji denotes ionospheric electrojet currents, and jr denotes field aligned currents.
Pi 2 pulsations: high latitude results
through the ionosphere, until the non conducting edge is reached, and then flow down field lines (Fig. 8). Since uH + op the ionospheric current is initially westward in the poleward morning sector and eastward in the equatorward evening sector. Thus the field aligned return current flows are separated longitudinally from the precipitation region. The configuration in Fig. 8 most likely gives the initial pulse of the Pi 2, but remember this pulse originates from field aligned and electrojet currents. This pulse might then be reflected from the conducting ionosphere near the aurora1 arc (Maltsev et al., 1974; Mallinckrodt and Carlson, 1978). If the field lines where precipitation initially occurred become dipolar, the reflected wave can bounce from hemisphere to hemisphere as a shear Alfven wave, leading to the typical Pi 2 oscillation. The difficulty with the above model is that it predicts linear polarization for Pi 2’s on the ground, since field aligned and electrojet currents are in phase. We have found experimentally, however, that the Pi 2 fields originating from the field aligned currents are phase shifted with respect to the magnetic field associated with electrojet oscillation. In the morning sector, the return current system (downward in Fig. 8), leads the electrojet oscillation by 90”. In fact, this phase shift is expected if the Pi 2 propagates azimuthally. To see this, consider the very simplified picture shown in Fig. 9. The region of enhanced conductivity and precipitation extends from yI to y2. Because of the sharp gradients in the conductivity, field-aligned sheetcurrents must flow at the edge of the conducting
FIG.
9.
1245
strip and these sheet currents are connected by Hall and Pedersen currents in the ionosphere. The field-aligned sheet-current at y, is jfl = Aeicormkx) and at y, is jf2 = - Aei’ot~k’x~dZHTp~“‘. The net fieldaligned current at any longitude x, is = AeiOr (,_,PX,_ e~ik(x,-dP”Zp-l)). The net Hall current at longitude x1 is I”(X,) = cos 0
YZ jfi(xI - y&J,-‘)dy I Yl = cos 8
VI dx I X,~dXllYp-Ijfl(x)
where 0 = tan-’ (Z&-‘). Thus, &(x,) = ik-’
jf(xl),
and consequently the net Hall current at any longitude is 90” phase shifted with respect to the net field-aligned current at that longitude. This leads to relative phase shifts in the magnetic fields associated with these current systems. If a 90” phase shift is introduced into the electrojet currents, the Pi 2’s on the ground become 10 shows the elliptically polarized. Figure polarization characteristics calculated numerically for the model in Figs. 8, 9. The breakup for this model is similar to the substorm in Fig. 4. A comparison of this model with Figs. 2, 3 and the results of Rostoker and Samson (1981) indicate
FIELD-ALIGNED, SHEET-CURRENTS, AND IONOSPHERIC CURRENTS AZIMUTHALPROPAGATION.
ASSOCIATED WITH A
Pi 2
WITH
The sheet-current at latitude y, is jr, = Aei(“‘-IU’ and at y2 the sheet-current is jf2= - Aei(w’~k(X~d~HTp~‘)). Height-integrated Hall and Pedersen conductivities are CH and Cp respectively. The electric field is approximately parallel or antiparallel to the y direction.
1246
J.C. SAMSON
FIG. 10. THE POLARIZATION OF THE Pi 2 IN THE HORIZONTAL PLANE, CALCULATED FOR THE MODEL IN FIG. 8.
The arrows indicate theta and the contours are ellipticity. Shaded regions indicate negative ellipticity. The initial precipitation region extends from 290 to 295”, longitude is tilted at 10” to E-W, and has its approximate center at 68-69”N. The currents associated with the westward propagating Pi 2 are one third of those associated with the eastward propagating Pi 2, and the azimuthal wavelength is 60”. To _simulate induction effects a perfect conductor is placed at a depth of 200 km. Geomagnetic, local midnight is at 300” E geomagnetic longitude.
that it predicts many of the observed polarization features of high latitude Pi 2’s. The possible conflicts in the data given by Rostoker (1967), Kuwashima (1978) (Fig. 3), and Rostoker and Samson (1981) can probably be explained by the features of this model. Since Rostoker averaged all events before local midnight, and all events after local midnight, the small region of clockwise polarization near local midnight might not be apparent. Equatorward of the breakup, Kuwashima’s data showed a transition from clockwise before 21.00-22.20 L.T. to counterclockwise after (counterclockwise to clockwise in the Northern Hemisphere). This transition is at 287” longitude in Fig. 9. In the morning at higher latitudes ( > 62” the transition is from counterclockwise (equatorward) to clockwise (poleward) of the intensity maximum, in agreement with the results of Rostoker and Samson (1981)). SUMMARY
Experimental studies of high latitude Pi 2’s presently suggest the following picture for the development of Pi 2 pulsations. (a) The Pi 2 perturbation begins with the initial upward field aligned current pulse which is responsible for precipitation, and arc brightening. This precipitation typically occurs near the equatorward border of the Harang discontinuity (ROStoker and Samson, 1981). (b) The field aligned currents associated with the Pi 2 close largely as Hall currents in the
E-region of the ionosphere. The net ionospheric currents are 90” phase shifted with respect to the net field aligned current at one longitude. (c) Within the first cycle of the Pi 2, the field lines on which the original current pulse started collapse to dipolar. The electrojet and precipitation then move to higher latitudes as an aurora1 surge forms. (d) The original current pulse is reflected on what are now dipolar field lines and propagates along field lines as a shear AlfvCn wave. Multiple reflections from the North and South aurora1 ionosphere lead to the typical Pi 2 wavetrain (Kuwashima, 1981). (e) Since the Pi 2 is trapped on dipolar field lines, the Pi 2 maximum tends to be just equatorward of the ambient, substorm electrojet (Fig. l), which has expanded poleward following the surge. (f) Since much of the Pi 2 energy is propagating along the dipolar field lines, a good part of the mid latitude Pi 2 energy will be due to the field aligned currents associated with the shear Alfven mode. In order to use Pi 2’s to detect the plasmapause (Fukunishi, 1975), the field aligned and Pi 2 electrojet fields must be removed from the mid latitude data. In a sense, the Pi 2 pulsations might give an “oscillating picture” of the initial configuration of the breakup. What is clear, however, is that Pi 2’s are fundamentally related to the initiation of the breakup and that an understanding of the mechanisms involved in the generation of Pi 2’s
Pi 2 pulsations: hig;h latitude results will lead to an understanding involved in aurora1 breakups, in general.
of the mechanisms and substorm onsets
Acknowledgements--I wish to acknowledge many valuable discussions with G. Rostoker, J. V. Olson, H. Singer and W. J. Hughes, on the topic of Pi 2’s. Numerical values for the model in Figs. 8 and 9 were computed by M. Mareschal. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).
REFERENCES Afanasjieva, I.,. T., Raspopov, 0. M., Schepetnov, R. V., Koshelevskiy, V. K. and Hazarov, M. 13. (1970) Relationship between geomagnetic pulsations of the Pi 2 type and the parameters of the aurora1 zone. Geomagn. Aeron. 10,600. Akasofu, S. I. (1968) Polar and Mapnetospheric Substorms. Springer, New York. .
Biiirnsson. A.. Hillebrand. 0. and Voelker. H. (19711 First obser;ational rest&s of geomagnetic ‘Pi 2 &d PC 5 pulsations on a North-South profile through Europe. Z. Geophysik 37, 1031. Block, L. P. (1972) Potential double layers in the ionosphere. Cosmic Electrodynamics 3, j45. Brekke, A., Douonik. J. R. and Banks. P. M. (19741 Incoherent sc^atter’ measurements of’ the E iegioi conductivities and currents in the aurora1 zone. J; geophys. Res. 79, 3773. Coroniti, F. V. and Kennel, C. F. (1972) Polarization of the aurora1 electrojet. J. geophys. Res. 77, 2835. Fukunishi, H. (1975) Polarization chances of eeomaenetic Pi 2 pulsatidns associated with the plasmapause. J. geophys. Res. 80, 98. Hargreaves, J. K., Chivers, H. J. A. and Nielsen, E. (1979) Properties of spike events in auroral radio absorption. .I. geophys. Res. 84,424s. Kuwashima, M. (1978) Wave characteristics of magnetic Pi 2 pulsations in fhe aurora1 region-spectral and polarization studies. Mem. natn. Inst. Polar Res. Japan, Series A 15, 79. Kuwashima, M. K. (1981) A model of magnetic Pi 2 pulsations based on a concurrent ULF observation from high to middle latitudes on the ground. Reprint
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from poster session presented to 4th IAGA Assembly, 14 August, 1981. Mallinckrodt, A. J. and Carlson, C. W. (1978) Relations between transverse electric fields and field-aligned current. J. geophys. Res. 83, 1426. Maltsev, Yu. P., Leontyev, S. V. and Lyatsky, W. B. (1974) Pi 2 pulsations as a result of evolution of an Alfvtn impulse originating in the ionosphere during a brightening of aurora. Planet. Space Sci. 22, 1519. Olson, J. V. and Rostoker, G. (197.5) Pi 2 pulsations and the aurora1 electrojet. Planet. Space Sci. 23, 1129. Olson, J. V. and Rostoker, G. (1977) Latitude variation of the spectral components of aurora1 zone Pi2. Planet. Space Sci. 25,663.
Pytte, T. and Trefall, H. (1972) Aurora]-zone electron precipitation event observed before and at the onset of negative magnetic bays. J. atmos. terr. Phys. 34, 315.
Pytte, T., McPherron, R. L. and Kokubun, S. (1976) The ground signatures of the expansion phase during multiple onset substorms. Planet. Space Sci. 24, 1115. Rostoker, G. (1967) The polarization characteristics of Pi 2 micropulsations and their relation to the determination of possible source mechanisms for the production of nighttime impulsive micropulsation activity. Can. J. Phys. 45, 1319. Rostoker, G. (1980) The aurora1 electrojets,in Dynamics of the Magnetosphere (Edited by Akasofu, S.-I.), p. 201. D. Reidel, Dordrecht. Rostoker, G. and Samson, J. C. (1981f Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: location of source regions with respect to the aurora1 electrojets. Planet. Space Sci. 29, 225.
Samson, J. C. and Olson, J. V. (1981) Data adaptive polarization filters for multichannel geophysical data. Geophys. 46, 1423. Samson, J. C. and Rostoker, G. (1972) Latitude-dependent characteristics of high-latitude PC 4 and PC 5 micropulsations. J. neo&vs. Res. 77,6133. Stuart, vs. F. and Booth,-DI C. (1974) A study of the power spectra of d Pi’s and Pi 2’s. .7. atmos. terr. Phys. 36, 835.
Wilhelm, K., Miinch, J. W. and Kremser, G. (1977) Fluctuations of the auroral zone current system and geomagnetic pulsations, f. geophys. Res. 82,270s.
0032-0633/82/121239-09$03.00/0 0 1982 Pergamon Press Ltd.
Pi 2 PULSATIONS:
HIGH LATITUDE
RESULTS
J. C. SAMSON
Institute of Earth & Planetary Physics, Department of Physics, University Alberta, Canada T6G 2Jl (Received
of Alberta, Edmonton,
7 April 1982)
Abstract-A review of recent experimental results from studies of high latitude Pi 2 pulsations indicates that these pulsations are fundamentally related to the initiation of the aurora1 breakup and substorm. At high latitudes, the Pi 2’s show their peak intensities in the region where the breakup begins and appear to remain in this region after the breakup has spread poleward. In addition, the Pi 2’s occur simultaneously with, or before all other ionospheric phenomena associated with the breakup. The field aligned and ionospheric currents associated with the Pi 2 resemble those of a typical substorm, but the ionospheric currents are phase shifted compared to the field aligned current. The periodic oscillations of the Pi 2’s are probably caused by a reflection of the initial field aligned current pulse from the aurora1 ionosphere. This pulse is trapped on dipolar field lines leading to multiple reflections from North and South aurora1 ionospheres.
INTRODUCTION
onset or aurora1 breakup (Akasofu, 1968) is accompanied by marked increases in the power in ULF (l-20 mHz) magnetic fields. Much of this power appears random and chaotic, particularly near the region of the breakup. The substorm onset is, however, almost always accompanied by distinct, quasi sinusoidal magnetic field oscillations with frequencies in the range 620 mHz. These oscillations, or Pi 2 pulsations, are most clearly seen on the ground at mid latitude (50-60” geomagnetic latitude) stations, but they also exist at high latitudes near the region of the breakup. This review will consider, for the most part, Pi 2’s which occur at high latitudes. I shall define high latitude to be any region which is influenced by magnetic fields from the ionospheric substormelectrojet. This region typically is between 55 and 80” geomagnetic latitude. The classification of Pi 2’s at high latitudes often leads to some ambiguity. At high latitudes, the Pi 2-band (6-20 mHz) shows marked increases in energy, but the spectra are often broad or chaotic. Conversely, at low latitudes, the spectra are often sharply peaked, indicating the presence of quasi sinusoidal pulsations. In fact, high latitude magnetometers record broad band electrojet noise as The substorm
An invited review paper presented at the 4th IAGA Scientific Assembly, Edinburgh, 14 August 1981.
well as the Pi 2 pulsations (Olson and Rostoker, 1975). It is extremely important to distinguish between these two types of oscillations in the Pi 2-band since they probably come from different sources. Consequently, I shall call only the damped quasi sinusoidal oscillations, which are clearly seen at mid latitudes, by the name Pi 2. The broad band electrojet noise, I shall call Pi 2-band noise. Any high-latitude Pi 2 which we identify should have a corresponding Pi 2 at mid latitudes. Pi 2’s have a number of features which allow us to separate them from Pi 2-band, electrojet noise. Some of these features are: (a) The Pi 2 begins at exactly the time of the substorm onset (Rostoker and Samson, 1981; Fig. la, 2a). (b) All stations that see the Pi 2 show minimal delay for Pi 2 onset ( < 20 s for nightside stations). (c) The Pi 2 oscillations seldom last for more than 10-15 minutes after the substorm onset. (d) The Pi 2 spectrum is narrow band and the oscillations are damped and quasi sinusoidal. (e) The Pi 2’s have spectra that are typical of pure states, or polarized waves (Rostoker and Samson, 1981). All of the above criteria must apply to data from high latitude stations, as well as mid latitude stations, if we are to be sure that a Pi 2 is present. The Pi 2-band electrojet noise seldom shows any of the above features. This energy is almost always broad band and unpolarized, can last for 1239
1240
J. C. SAMSON
the duration of the substorm-enhanced electrojet and is detected only when the electrojet passes over a station (Olson and Rostoker, 1975), often after delays of minutes from the substorm onset. CORRELATIONS WITH OTHER SUBSTORM PHENOMENA
Since Pi 2’s are directly connected with substorm onsets, it is not surprising that they are closely correlated with other breakup phenomena, including brightening of aurora1 arcs, X-ray intensifications, riometer absorption events and ionospheric electrojet intensifications. One of the classic signatures of the beginning of an aurora1 substorm or breakup is the sudden brightening of a quiet arc (Akasofu, 1968, and references therein). Pi 2 pulsations occur in conjunction with the brightening and there have been numerous attempts to correlate Pi 2 frequencies with the latitude of the brightened arc (Afanasyeva et al., 1970; Kuwashima, 1978). An evaluation of the data given by Samson and Rostoker (1972) and by Kuwashima and Afanasyeva et al. shows different latitudinal trends and consequently these latitudinal features are far from conclusive. Precipitating, high energy electrons (energies 20 kev and greater) which lead to brightening of aurora1 arcs, also lead to bursts of bremsstrahlung, X-rays (Akasofu, 1968). Pytte and Trefall (1972) have shown that bremsstrahlung events associated with breakups start and end with the Pi 2’s accompanying the breakup. The high energy electrons also lead to increased absorption of cosmic, radio noise. Consequently the Pi 2 is often associated with strong absorption spikes in riometer data (Pytte and Trefall, 1972; Pytte et al., 1977, and references therein). These riometer spike events can show very dynamic and rapid latitudinal motions (Hargreaves et al., 1979), which may be intimately related to the mechanism for the formation of Pi 2’s and aurora1 surges. One important point which must be emphasized before further discussion, is that almost all correlative studies have shown that the Pi 2 begins at or before most other breakup phenomena. The implication here is that the Pi 2 must be closely associated with the enhanced field aligned currents which ultimately lead to the breakup. The enhanced ionospheric electrojets associated with substorms have been studied extensively, and are now reasonably well defined (Rostoker, 1980). Thus it is important to compare the patterns of the Pi 2 polarizations and intensities with those of the substorm electrojet. Olson and Rostoker (1975) used low latitude
signals to identify the spectral region of the Pi 2 at high latitudes and their results showed that the peak in Pi 2 energy was at the center of the substorm-enhanced electrojet. Wilhelm et al. (1977) concluded that Pi 2-band energy was due to fluctuations in electrojet and field aligned current intensities. Unfortunately they did not discriminate between Pi 2-band noise and Pi 2 pulsations. A study by Rostoker and Samson (1981) was designed to extract only the polarized component of the high latitude recordings. In order to ensure that the polarized pulsations were in fact Pi 2’s, they were also compared with pulsations occurring at lower latitude stations. We found that the maximum intensity of Pi 2’s is at the southern borders of the substorm enhanced westward and eastward electrojets (Fig. 1), not at the center as indicated by the results of Olson and Rostoker. MORPHOLOGICAL STUDIES
The most common parameters of Pi 2’s which are analysed in morphological studies are the spectra (power vs frequency, latitude, longitude (L.T.)), and the polarization ellipses (usually in the horizontal plane). In order to obtain an ensemble average of the morphology, these data are often ordered by latitude, and L.T., and the averages are plotted in these coordinates. While these methods have yielded useful results in the past, particularly at mid latitudes, high latitude detail is often blurred or lost because of the large range of coordinates over which breakups can occur. In order to make further progress it will be extremely important to correlate each example of the Pi 2 morphology with the center of the breakup, before averaging. The characteristic feature of the spectra of Pi 2’s is a transition from sharply peaked spectra at Poleword border after onset
Fe;
PI
PI2
2
Poleword border before onset
FIG. 1. THE REGIONS OF THE INTENSITY MAXIMA OF Pi 2’s. The poleward border after onset corresponds to the border of the substorm enhanced electrojet (after Rostoker and Samson, 1981).
1241
Pi 2 pulsations: high latitude results mid latitudes (Stuart and Booth, 1974), to broad band spectra at high latitudes. Olson and Rostoker (1975), indicated that the Pi 2-band of the spectrum is filled in around the Pi 2 peak as the center of the electrojet is approached. The frequency of the Pi 2 spectral peak is, however, unchanged with latitude (see also the example to be given later in this discussion). Studies of the polarizations of Pi 2’s and Pi 2-band pulsations by Bjornsson et al. (1971) indicate that the mean direction of polarization (Fig. 2) is compatible with a source comprising field aligned currents and closure electrojets. Field aligned current is predominantly downward (upward) from 0 to 03.00 L.T., and upward (downward) from 20.00 to 23.00 L.T. The electrojet can be seen in the strong negative H direction polarization at 70” over the interval 22.00-04.00 L.T. The change in the orientation of the vectors at 55”, from North-East to North-West, is due to net, oscillating field aligned currents. These features will be explained more clearly in the model to be given later. Studies of the sense of polarization of Pi 2’s occurring at high latitude, indicated that there are four quadrants of polarization (Kuwashima, 1978) with the approximate boundaries given by the latitude of the breakup, and - 21.00-22.00 L.T. (Fig. 3). Note that this L.T. is also close to the change in the orientation of the direction of polarization (Fig. 2) and the region for the maximum occurrence frequency of Pi 2’s. The morphology of the polarization is, however, not quite so simple as Fig. 3 suggests. At high latitudes, in the Northern Hemisphere, Rostoker and Samson (1981) found that most Pi 2’s occurring in the morning sector had counterclockwise
FIG. 2.
THE
DIRECTION OF POLARIZATION OF Pi 2 EVENTS (AFTER BJ~RNSSON et al., 1971).
22
23
Co
01
FIG. 3. MAGNETICL.T.DEPENDENCES OF THEPOLARIZATION OFPi2’S INTHEHORIZONTALPLANE. The symbol R.H. indicates counterclockwise polarization when viewed downward (Southern Hemisphere). AE indicates the aurora1 electrojet (after Kuwashima, 1978).
polarization equatorward and clockwise polarization poleward of the Pi 2 maximum. Kuwashima’s results suggest that these polarizations should be clockwise and counterclockwise respectively, especially when the 180” phase shift in the conjugate D’s are considered (Kuwashima, 1978, 1981). In addition, Rostoker (1967) deduced that the polarizations at - 56”N geomagnetic latitude (equatorward of the Pi 2 maximum) were predominantly counterclockwise. I shall discuss these possibly conflicting results in a polarization model to be presented below. AN EXAMPLE
Probably the best way to finish this brief review is to illustrate the features of high latitude Pi 2’s by giving an example of a substorm onset and its associated Pi 2’s. To further clarify the discussion, I have chosen an isolated, and particularly simple substorm event which is accompanied by mid latitude Pi 2’s similar to the d Pi’s discussed by Stuart and Booth (1974). High latitude magnetograms from the University of Alberta array are shown in Fig. 4. The substorm onset is very abrupt, and a strong westward electrojet can be seen beginning at -8.08 U.T., indicating the breakup is centered slightly to the West of the stations. The Z-components indicate that the electrojet is initially centered South of FTCH, then rapidly moves northward to be centered near URAN (Note the sharp Z-spike caused by the northward motion). At 08.08.30 U.T. the electrojet is centered at -6S’N, at 08.10.30 at - 67”N and by 08.12.00 the electrojet is at - 69”N.
1242
J.C. SAMSON Magnetometer
I
200
Day
59
1979.28
Feb
NT
FTCH
MCMU
URAN
SMIT
HAYR
PROV
7
8
9
FIG. 4. FLUXGATEMAGNETOMETERRECORDINGS OFA SLJBSTORMOCCURRINGON~~ THEUNIVERSITYOF ALBERTAARRAY.
FEBRUARY~~~~,OVER
The components are: H, magnetic North; D, magnetic East; and Z, downward. Coordinates for these stations can be found in Rostoker and Samson (1981). Latitudes are given in Fig. 6.
A comparison of the magnetograms at PROV, FTCH and URAN indicate that the electrojet is tilted by - 10” from the E-W direction and PROV is much further South of the electrojet than URAN. Data from riometers at three of the Alberta stations indicate that absorption peaks first near MCMU at -8.10 U.T. and then at -8.12 U.T. near SMIT and PROV. This rapid poleward motion (-2 km s-l) of the absorption (precipitation) is a common feature of isolated substorm onsets (Hargreaves et al., 1979). The rapid northward motion of the precipitation and the westward electrojet commonly occurs to the East of developing surges (Akasofu, 1968, and references therein; Pytte et al., 1977) and is in-
ti.mately related to the formation of Pi 2’s and the configuration of their polarization. The dynamic motions in the electrojet indicate that the polarizations of Pi 2’s will be quite complicated near the region of the breakup. The filtered Pi 2’s (pure states) are shown in Fig. 5. The Pi 2 putsations are clearest at LEDU (see the box), and these Pi 2’s correspond to the Pi 2’s (d Pi’s) seen at mid latitudes. The spectra of the pure states (polarized waves) are shown in Fig. 6. The sharpest spectral peak is at LEDU, with a frequency near 9 mHz. OnIy this peak corresponds to the mid latitude Pi 2’s. The Pi 2 intensity (9mHz) drops at MCMU, rises again at FTCH, and again drops to very low values at URAN. As I indicated previously the electrojet is tilted at 10”
1243
Pi 2 pulsations: high latitude results MRGNETOMETEF DRY
59
20
1975,
FEB.
28
Nl
i
FTCH
LEDU
URRN
SMIT
PROV
7
a
97 UNIVERSRL
i
8 TIME
a
IHRSI
FIG.~. MAGNETOMETERDATAFILTEREDTOEXTRACTPURESTATES(POLARIZEDWAVES)INTHE (SEE ROSTOKERAND SAMSON (198l), OR SAMSON AND OLSON (1981).
and so apparently is the center of the intensity of the Pi 2, since PROV shows much more 9mHz power than does URAN. Note also that three spectral peaks, at 4, 9 and 16 mHz, predominate over all the stations, especially at PROV. These three spectral peaks have also been reported by Olson and Rostoker (1977). Latitude profiles of the substorm electrojet and the data in Fig. 6 indicate that the Pi 2 is localized near the southern border of the westward electrojet, in agreement with the results of Rostoker and Samson (1981) (Fig. 1). Figure 7 gives the latitude-dependent polarizations and powers of the Pi 2 event. The minimum in the H-component near MCMU, is probably due to an antiphase, oscillating electrojet (ambient eastward electrojet) to the South of the main eiecelectrojet). The trojet (ambient westward
Pi2 BAND.
polarizations to the South of the maximum are counterclockwise (positive) and clockwise to the North, with a demarcation near MCMU. These results are also consistent with those given by Rostoker and Samson (1981) but appear to contradict those of Kuwashima (1978). A MODEL FOR HIGH LATITUDE Pi 2%
The task now is to construct a relatively simple model for the polarization states of high latitude Pi 2’s which will be compatible with the observations I have aheady discussed. The Pi 2 model should also be compatible with the characteristics of the substorm current systems. It is clear that breakup phenomena begin with the brightening of a quiet arc, and substantial precipitation of electrons. These features suggest large field aligned currents initially flowing upward
J. C. SAMSON 1.0
FREWENCY
FIG. 6.
(mHz
.%irgoo
1
GEOMAGNETIC
STATE SPECTRA OF THE Pi 2 EVENT FROM 08.08 to 08.18 U.T., 28 FEBRUARY. If x(t) = [H(t), D(t), Z(t)] at one station, the pure state spectra are given by P(j) = (1 - (Az/A,)“~)‘(lr,tzCf))’ PURE
where z(w) =
A, and LI, are respectively
H
FIG. 7.
72
LATITUDE
POLARIZATION CHARACTERISTICS OF THE 9 2 OCCURRINGONFEBRUARY 28.'
mHz Pi
The ellipticity (solid line) is the ratio of the minor to major axis of the polarization ellipse. A positive ellipticity indicates counterclockwise in the horizontal plane, viewed from above. The polarization angle, theta, is measured clockwise from magnetic North ( - 90” zz theta I 90”).
c x(t)e’“‘, the largest eigenvalue
and
corresponding eigenvector of ‘?’ z(o)zt(w), and A?is the U=lmAlnext largest eigenvalue. The symbol t denotes time and t is Hermitian adjoint. The plots show the function fP(f).
on field lines intersecting the arc. The high energy electrons are possibly accelerated by fields which are caused by topside ionospheric current instabilities (Coroniti and Kennel, 1972) or double layers (Block, 1972), both of which can originate from large field aligned currents. These polarization electric fields map to the ionosphere as horizontal electric fields pointing toward the center of the arc (initially) (see Fig. 8). The net electric field will be a combination of this field and the ambient convection field (usually pointing equatorward to the East of the Harang discontinuity, and poleward to the West). However, for simplicity I shall ignore these ambient fields. In the region of the precipitation near the arc, the Hall, u”, and Pederson, up, conductivities in the E region are significantly increased, with Us = 3u, (Brekke et al., 1974). Because of the gradients in u” and ap, the initial current pulse must close
FIG. 8. A MODEL
FOR THE INITIAL PULSE OF A HIGH LATITUDE Pi2.
After this initial pulse, all currents oscillate at frequency o but different phases. Since Pi 2’s propagate azimuthally, this picture shows only the fust half cycle in the azimuthal direction. The symbol E denotes ionospheric electric fields, ji denotes ionospheric electrojet currents, and jr denotes field aligned currents.
Pi 2 pulsations: high latitude results
through the ionosphere, until the non conducting edge is reached, and then flow down field lines (Fig. 8). Since uH + op the ionospheric current is initially westward in the poleward morning sector and eastward in the equatorward evening sector. Thus the field aligned return current flows are separated longitudinally from the precipitation region. The configuration in Fig. 8 most likely gives the initial pulse of the Pi 2, but remember this pulse originates from field aligned and electrojet currents. This pulse might then be reflected from the conducting ionosphere near the aurora1 arc (Maltsev et al., 1974; Mallinckrodt and Carlson, 1978). If the field lines where precipitation initially occurred become dipolar, the reflected wave can bounce from hemisphere to hemisphere as a shear Alfven wave, leading to the typical Pi 2 oscillation. The difficulty with the above model is that it predicts linear polarization for Pi 2’s on the ground, since field aligned and electrojet currents are in phase. We have found experimentally, however, that the Pi 2 fields originating from the field aligned currents are phase shifted with respect to the magnetic field associated with electrojet oscillation. In the morning sector, the return current system (downward in Fig. 8), leads the electrojet oscillation by 90”. In fact, this phase shift is expected if the Pi 2 propagates azimuthally. To see this, consider the very simplified picture shown in Fig. 9. The region of enhanced conductivity and precipitation extends from yI to y2. Because of the sharp gradients in the conductivity, field-aligned sheetcurrents must flow at the edge of the conducting
FIG.
9.
1245
strip and these sheet currents are connected by Hall and Pedersen currents in the ionosphere. The field-aligned sheet-current at y, is jfl = Aeicormkx) and at y, is jf2 = - Aei’ot~k’x~dZHTp~“‘. The net fieldaligned current at any longitude x, is = AeiOr (,_,PX,_ e~ik(x,-dP”Zp-l)). The net Hall current at longitude x1 is I”(X,) = cos 0
YZ jfi(xI - y&J,-‘)dy I Yl = cos 8
VI dx I X,~dXllYp-Ijfl(x)
where 0 = tan-’ (Z&-‘). Thus, &(x,) = ik-’
jf(xl),
and consequently the net Hall current at any longitude is 90” phase shifted with respect to the net field-aligned current at that longitude. This leads to relative phase shifts in the magnetic fields associated with these current systems. If a 90” phase shift is introduced into the electrojet currents, the Pi 2’s on the ground become 10 shows the elliptically polarized. Figure polarization characteristics calculated numerically for the model in Figs. 8, 9. The breakup for this model is similar to the substorm in Fig. 4. A comparison of this model with Figs. 2, 3 and the results of Rostoker and Samson (1981) indicate
FIELD-ALIGNED, SHEET-CURRENTS, AND IONOSPHERIC CURRENTS AZIMUTHALPROPAGATION.
ASSOCIATED WITH A
Pi 2
WITH
The sheet-current at latitude y, is jr, = Aei(“‘-IU’ and at y2 the sheet-current is jf2= - Aei(w’~k(X~d~HTp~‘)). Height-integrated Hall and Pedersen conductivities are CH and Cp respectively. The electric field is approximately parallel or antiparallel to the y direction.
1246
J.C. SAMSON
FIG. 10. THE POLARIZATION OF THE Pi 2 IN THE HORIZONTAL PLANE, CALCULATED FOR THE MODEL IN FIG. 8.
The arrows indicate theta and the contours are ellipticity. Shaded regions indicate negative ellipticity. The initial precipitation region extends from 290 to 295”, longitude is tilted at 10” to E-W, and has its approximate center at 68-69”N. The currents associated with the westward propagating Pi 2 are one third of those associated with the eastward propagating Pi 2, and the azimuthal wavelength is 60”. To _simulate induction effects a perfect conductor is placed at a depth of 200 km. Geomagnetic, local midnight is at 300” E geomagnetic longitude.
that it predicts many of the observed polarization features of high latitude Pi 2’s. The possible conflicts in the data given by Rostoker (1967), Kuwashima (1978) (Fig. 3), and Rostoker and Samson (1981) can probably be explained by the features of this model. Since Rostoker averaged all events before local midnight, and all events after local midnight, the small region of clockwise polarization near local midnight might not be apparent. Equatorward of the breakup, Kuwashima’s data showed a transition from clockwise before 21.00-22.20 L.T. to counterclockwise after (counterclockwise to clockwise in the Northern Hemisphere). This transition is at 287” longitude in Fig. 9. In the morning at higher latitudes ( > 62” the transition is from counterclockwise (equatorward) to clockwise (poleward) of the intensity maximum, in agreement with the results of Rostoker and Samson (1981)). SUMMARY
Experimental studies of high latitude Pi 2’s presently suggest the following picture for the development of Pi 2 pulsations. (a) The Pi 2 perturbation begins with the initial upward field aligned current pulse which is responsible for precipitation, and arc brightening. This precipitation typically occurs near the equatorward border of the Harang discontinuity (ROStoker and Samson, 1981). (b) The field aligned currents associated with the Pi 2 close largely as Hall currents in the
E-region of the ionosphere. The net ionospheric currents are 90” phase shifted with respect to the net field aligned current at one longitude. (c) Within the first cycle of the Pi 2, the field lines on which the original current pulse started collapse to dipolar. The electrojet and precipitation then move to higher latitudes as an aurora1 surge forms. (d) The original current pulse is reflected on what are now dipolar field lines and propagates along field lines as a shear AlfvCn wave. Multiple reflections from the North and South aurora1 ionosphere lead to the typical Pi 2 wavetrain (Kuwashima, 1981). (e) Since the Pi 2 is trapped on dipolar field lines, the Pi 2 maximum tends to be just equatorward of the ambient, substorm electrojet (Fig. l), which has expanded poleward following the surge. (f) Since much of the Pi 2 energy is propagating along the dipolar field lines, a good part of the mid latitude Pi 2 energy will be due to the field aligned currents associated with the shear Alfven mode. In order to use Pi 2’s to detect the plasmapause (Fukunishi, 1975), the field aligned and Pi 2 electrojet fields must be removed from the mid latitude data. In a sense, the Pi 2 pulsations might give an “oscillating picture” of the initial configuration of the breakup. What is clear, however, is that Pi 2’s are fundamentally related to the initiation of the breakup and that an understanding of the mechanisms involved in the generation of Pi 2’s
Pi 2 pulsations: hig;h latitude results will lead to an understanding involved in aurora1 breakups, in general.
of the mechanisms and substorm onsets
Acknowledgements--I wish to acknowledge many valuable discussions with G. Rostoker, J. V. Olson, H. Singer and W. J. Hughes, on the topic of Pi 2’s. Numerical values for the model in Figs. 8 and 9 were computed by M. Mareschal. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).
REFERENCES Afanasjieva, I.,. T., Raspopov, 0. M., Schepetnov, R. V., Koshelevskiy, V. K. and Hazarov, M. 13. (1970) Relationship between geomagnetic pulsations of the Pi 2 type and the parameters of the aurora1 zone. Geomagn. Aeron. 10,600. Akasofu, S. I. (1968) Polar and Mapnetospheric Substorms. Springer, New York. .
Biiirnsson. A.. Hillebrand. 0. and Voelker. H. (19711 First obser;ational rest&s of geomagnetic ‘Pi 2 &d PC 5 pulsations on a North-South profile through Europe. Z. Geophysik 37, 1031. Block, L. P. (1972) Potential double layers in the ionosphere. Cosmic Electrodynamics 3, j45. Brekke, A., Douonik. J. R. and Banks. P. M. (19741 Incoherent sc^atter’ measurements of’ the E iegioi conductivities and currents in the aurora1 zone. J; geophys. Res. 79, 3773. Coroniti, F. V. and Kennel, C. F. (1972) Polarization of the aurora1 electrojet. J. geophys. Res. 77, 2835. Fukunishi, H. (1975) Polarization chances of eeomaenetic Pi 2 pulsatidns associated with the plasmapause. J. geophys. Res. 80, 98. Hargreaves, J. K., Chivers, H. J. A. and Nielsen, E. (1979) Properties of spike events in auroral radio absorption. .I. geophys. Res. 84,424s. Kuwashima, M. (1978) Wave characteristics of magnetic Pi 2 pulsations in fhe aurora1 region-spectral and polarization studies. Mem. natn. Inst. Polar Res. Japan, Series A 15, 79. Kuwashima, M. K. (1981) A model of magnetic Pi 2 pulsations based on a concurrent ULF observation from high to middle latitudes on the ground. Reprint
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from poster session presented to 4th IAGA Assembly, 14 August, 1981. Mallinckrodt, A. J. and Carlson, C. W. (1978) Relations between transverse electric fields and field-aligned current. J. geophys. Res. 83, 1426. Maltsev, Yu. P., Leontyev, S. V. and Lyatsky, W. B. (1974) Pi 2 pulsations as a result of evolution of an Alfvtn impulse originating in the ionosphere during a brightening of aurora. Planet. Space Sci. 22, 1519. Olson, J. V. and Rostoker, G. (197.5) Pi 2 pulsations and the aurora1 electrojet. Planet. Space Sci. 23, 1129. Olson, J. V. and Rostoker, G. (1977) Latitude variation of the spectral components of aurora1 zone Pi2. Planet. Space Sci. 25,663.
Pytte, T. and Trefall, H. (1972) Aurora]-zone electron precipitation event observed before and at the onset of negative magnetic bays. J. atmos. terr. Phys. 34, 315.
Pytte, T., McPherron, R. L. and Kokubun, S. (1976) The ground signatures of the expansion phase during multiple onset substorms. Planet. Space Sci. 24, 1115. Rostoker, G. (1967) The polarization characteristics of Pi 2 micropulsations and their relation to the determination of possible source mechanisms for the production of nighttime impulsive micropulsation activity. Can. J. Phys. 45, 1319. Rostoker, G. (1980) The aurora1 electrojets,in Dynamics of the Magnetosphere (Edited by Akasofu, S.-I.), p. 201. D. Reidel, Dordrecht. Rostoker, G. and Samson, J. C. (1981f Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: location of source regions with respect to the aurora1 electrojets. Planet. Space Sci. 29, 225.
Samson, J. C. and Olson, J. V. (1981) Data adaptive polarization filters for multichannel geophysical data. Geophys. 46, 1423. Samson, J. C. and Rostoker, G. (1972) Latitude-dependent characteristics of high-latitude PC 4 and PC 5 micropulsations. J. neo&vs. Res. 77,6133. Stuart, vs. F. and Booth,-DI C. (1974) A study of the power spectra of d Pi’s and Pi 2’s. .7. atmos. terr. Phys. 36, 835.
Wilhelm, K., Miinch, J. W. and Kremser, G. (1977) Fluctuations of the auroral zone current system and geomagnetic pulsations, f. geophys. Res. 82,270s.