Dependence of Pc5 micropulsation power on conductivity variations in the morning sector

b’lanet. Space Sci., Vol. 27, pp. 631-642.

F’ersamor~Rar

DEPENDENCE CONDUCTIVITY

Ltd., 1979. Rioted io NorIhem Imhd

OF Pc5 MICROPULSATION POWER ON VARIATIONS IN THE MORNING SECTOR PETERRS-

Magnetic Observatory of the CSIR, P.O. Box 32, Hermanus 7200, South Africa

GQRDON

ROSTOKW

Institute of Earth and Planetary Physics and Department of Physics, University of Alberta, Edmonton, Canada T6G

251

(Received 9 October 1978) Ah&ad-For many years it has been known that the most intense and continuous Pc5 micropulsation activity occurs in the local time quadrant between dawn and noon. Recently, Lam and Rostoker (1978) have shown that Pc5 pulsations occur in the latitudinal regime occupied by the westward auroral electrojet and have suugested that part of the oscillating current system responsible for the pulsations involves upward field-aligned current at the boundary between the sunlit and dark ionosphere at local dawn. In this paper, we show that power in the Pc5 micropulsation range is markedly enhanced as one moves across the dawn terminator at 100 km from the nightside to the dayside. It is further shown that there is a significant increase in pulsation strength at -0730 L.T. The increase in Pc5 pulsation strength across the dawn terminator favors the concept that Pc5 micropulsations can be viewed as oscillations of a three-dimensional current loop involving downward current in the pre-noon sector diverging to flow in the ionosphere as part of the westward amoral electrojet and returning to the magnetosphere along field lines penetrating the ionosphere across the region separating the dark and sunlit ionosphere. We further suggest that the region of enhanced high energy electron precipitation shown by Hartz and Brice (1967) to maximize in the pre-noon quadrant is associated with the marked enhancement of Pc5 activity near 0730 L.T.

INTRODUCI’ION

The ultra-low frequency oscillations of the Earth’s magnetic field in the frequency range 1.5-6.5 mHz are classified as Pc5 pulsations (Jacobs et al., 1964). These pulsations attain maximum amplitude and frequency of occurrence in the aurora1 zones (Jacobs and Sinno, 1960; Sugiura, 1961; Hirasawa, 1970). Samson (1972) showed that, except for a short time around local noon, the PCS intensity peak overlaps the aurora1 zone. In the auroral zone the diurnal variation of the Pc5 activity has a clearly defined peak in the morning with a second small peak in the afternoon. In a study of 11 yr of data from Fort Churchill, Rao and Gupta (1978) confirmed the occurrence of a dominant peak at 08 f 1 h L.T. and a less clearly seen peak at 16* 2 h L.T. In sub-aurora1 regions the occurrence frequency is double peaked with peaks in the morning and afternoon (01, 1963; Saito, 1964). 01 (1963) demonstrated that the ratio of the intensity of the morning maximum to the intensity of the afternoon maximum decreases with decreasing latitude. In a study of Pc5 pulsations in the morning sector, Lam and Rostoker (1978) found that the behaviour of Pc5 pulsations is intrinsically related 631

to the character of the spatial and intensity variations of the westward aurora1 electrojet. Specitically, they found that the dominant spectral bands in the Pc5 range peak within the latitudinal conlines of the westward electrojet, and that the Pc5 activity is enhanced in conjunction with rapid reconligurations of the electrojet. Furthermore, the character of the polarization parameters can be explained if the pulsations are regarded as the magnetic signature of a three-dimensional current system involving the westward electrojet and its associated fieldaligned currents in the morning sector. Numerous theories have been proposed to explain the mechanism of generation of Pc5 pulsations. To date, most of these theories have made use of hydromagnetic wave theory in terms of which the pulsations are considered to be the signature of steady state oscillations of resonant field lines in a localized L-shell regime. Such theories have been able to explain many of the characteristics, such as the latitudinal dependence of amplitude and certain aspects of the polarization of PcSs (Hughes and Southwood, 1976). The most commonly suggested energy source for the resonant oscillations of field lines is the interaction of the solar wind with the

632

P. R. S-

magnetosphere which, by way of the KelvinHehnholtz instability, sets up surface waves on the magnetopause. Recently, Chen and Hasagawa (1974) and Southwood (1974) have indicated how energy can be transferred from surface waves on the magnetopause to the resonant field lines. On the basis of Lam and Rostoker’s (1978) findings, and since no hydromagnetic wave theory at present correctly predicts all aspects of long period pulsations as observed at the Earth’s surface, Rostoker and Lam (1978) suggested that Pc5 pulsations can be regarded as LC-oscillations of a three-dimensional current loop. This current loop involves downward field-aligned current flow near noon, which diverges in part to form the ionospheric westward electrojet and returns back along magnetic field lines into the magnetosphere in the vicinity of the ionosphere conductivity discontinuity at the dawn meridian. The energy for driving the current system is extracted from the magnetospheric plasma drifting sunward past the flanks of the magnetosphere in a manner discussed by Rostoker and Bostrom (1976). Since the theory put forward by Rostoker and Lam (1978) suggests that the ionospheric current which causes the Pc5 pulsations, diverges up along magnetic field lines in the vicinity of the ionospheric conductivity discontinuity at the dawn terminator (see their Fig. 4), one might expect to observe a significant enhancement of Pc5 amplitude at that meridian. That is, since much of the pulsation amplitude is due to oscillations of the westward electrojet on the sunward side of the dawn terminator, one would expect a sharp rise in micropulsation power as one crossed the dawn terminator at 100 km altitude moving from the nightside to the dayside. This increase in power would not be related to any other local time feature (e.g. local dawn, local magnetic dawn) but specifically to sunrise at 1OOkm. The major objective of this study, then, was to determine, on a statistical basis, whether a sunrise effect can be detected in PCS activity. We used the same statistical method to investigate a local time dependence in Pc5 activity which also became apparent during the wurse of this study. DATA

AND

METHOD

OF AN~Y!W

The data for this study were recorded on two of the magnetometers in the East-West line operated by the University of Alberta during the IMS, namely those at Uranium City (IRAN) and Fort Providence (PROV). The geomagnetic coordinates of URAN are (67.4“N, 304.3”E) and for PROV

and G. Rocimmx~ they are (67.5”N, 292.O“E), so that the two stations are nearly one time zone apart. Three wmponents (H, D and 2) of the geomagnetic field were recorded digitally on magnetic tape with a sampling interval of 2.56 s per component. Technical details of the magnetometers have been described by Kisabeth (1972). Data for 29 days recorded at URAN, and 44 days recorded at PROV, on which Pc5 activity was observed during the morning hours were selected for this study. Each data set (which was either 9 or 10 h in length) was low-pass filtered to prevent aliasing (cut-off frequency =O.O24Hz) and decimated to give a sampling interval of 10.24 s. A high-pass tilter was applied to remove low frequency variations from each data set, after which each set was normalized to unit standard deviation. The latter normalization was carried out in order to give each day equal weight in the subsequent analysis, irrespective of pulsation amplitude. In order to assess the importance of the change in ionospheric conductivity from dayside to nightside on the strength of Pc5 activity, it was decided to use the superposed epoch analysis technique @PEA). In this technique one attempts to extract a specific signal from a set of data with a high noise level. The use of the technique requires one to specify the time at which it is expected that the signal takes place. One then uses the time series of the observed parameter (signal plus noise) and sums the magnitudes of the parameter for discrete intervals on either side of the reference time (at which the signal onset is assumed to take place). If the assessment of the reference time is correct, superposition of the time series of the parameter results in the signal being amplified (through addition of identical signals) and the noise being reduced through cancellation. Since the assumption in this study is that the conductivity discontinuity across dawn is important, the reference time chosen for our SPEA was sunrise at 100 km altitude (where horizontal ionospheric current flow tends to maximize). In the secondary analysis to investigate the local time dependence of Pc5 activity, wmmon local time was selected as the reference time. The parameters used for the SPEA study were the power in the H- and D-wmponents obtained for 1 h lengths of the micropulsation time series. The “power” using the series of data points normalized to unit standard deviation was computed using the Fast Fourier Transform Technique, each raw spectrum being smoothed to produce a spectrum with 24 degrees of freedom and an equivalent

633

Dependence of Pc5 mIcropulsationpower on conductivityvariations resolution of -3 mH2. Spectra were calculated at half-hour intervals on either side of the reference time, and the superposed spectra were constructed for each frequency component (0-12.6mHz at intervals of 0.28mHz). During the course of this investigation, we found it necessary to group the data according to season in order to separate and clarify the various factors influencing Pc5 activity. While studying the magnetograms in order to select days on which reasonably clear PCS activity occurred during the morning hours, we noted a deficiency of suitable days during the summer months. The data presented here, therefore, are for days during winter and equinoctial months.

RE%JLlS

OF THE STATISTICAL ANALYSIS

Figure l(a) shows the spectra obtained from the superposition of the H-component spectra for 14 days of data recorded at URAN during winter months. The individual spectra for the 14 days were superposed using the time of local sunrise at 100 km height (henceforth referred to as SR) as the reference time. The plotted spectra are for intervals 1 h in length but with half-hour separation and have been recoloured by the response of the highpass filter. Note that the spectrum at SR(marked by a horizontal arrowhead in Fig. 1) is the superposition of 14 spectra (from 14 different days) each computed at the time of SR on the specific day. The remaining

ll-

02.

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0

4

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12

FREQUENCY

FIG. DATA

1. %BClRA RIXGRDED

(I&

COMPWTEDBYTHESUFBRFOS ITIONOF THE H-cotmom DURING

WINTBR

MONTHS.

AND

(b) 16

MONTHS

DAYS

OF DATA

4

8

12

mxm4 RECORDED

FOR DURING

(a) 14

DAYS

OF

EKXJINOCTJAL

AT um.

In each case the pointer indicates the position of SR which was used as the reference time for the superposedepoch analysis.

634

P. R. SLVZLEFEand G. Rosroxna

spectra in the plot are the superposition of individual spectra computed for intervals with halfhour separation either side of SR. Table 1 indicates the range of dates defined as winter and equinoctial months; each seasonal group consists of two subgroups, and the dates of the shortest-, median- and longest-day in each subgroup are tabulated together with the mean times of SR on these dates. The annotated local times in the figure are measured from the time of SR (to the nearest half-hour) on the median-day of the subgroups. From the table it is apparent that the times of the individual SR’s utilized in computing Fig. l(a) cover a range of 1; h in L.T., implying that the spectrum for any specific local time in Fig. l(a) is in effect “smeared out” over a period of 1; h. From Fig. l(a) we see that there is a significant enhancement of power in the Pc5 spectral band at the time of SR which, for the median-day in the winter group, occurs at 0656L.T. The Pc5 activity appears to start developing an hour or so before the tune of SR, and then starts to decrease about 3 h after SR.

I

I

03

FIG. DATA

2.

.%NOGRAMS

RECORDED

06 LOCAL

COMPUTRD

DURING

WINTER

FROM

THE

MONTHS,

TABLE 1. DATES LQNGELW @ACH

CONSISITNG OF LOCAL

SHORT AND

OF TWO GROUPS SUNRISE

AT

MEDIAN

EQUINocrL‘u

OF DAYS)

100 km

AND SRASONS

AND THE MEAN

AT u%iN

ON THESE

DATES

SR at UFXAN Dates Group 1 Group 2 Mean L.T.

Season Day

Shortest

Winter

Median Longest Shortest

Median Longest

Equinox

21 21 22 22

Dee Nov act Feb

22 21 21 22

Dee Jan Feb act

0723 0656 0552 0552

21 March 21 Scpt 21 April 21 Aug

0436 0258

The spectra plotted in Fig. l(b) were computed in the same manner as those in Fig. l(a), except that the data were recorded on 15 days during the equinoctial months. We see some evidence of activity half an hour before SR, and clear evidence of an increase in Pc5 activity half an hour after SR, which now occurs at 0436 L.T. This increase is not as large as that observed at SR during the winter months.

I

12

OS

TIME

(b) 15

MONTHS

DEFINING

OF THE WINTER

TIMES

SUPRRPQSRD AND

DAYS

H-COMPONENT DAYS

OF DATA

SPJXTRA RECORDED

FOR

(a)

DURING

14

DAYS

OF

EQUINOCI’JAL

AT uRb&i.

In each case the pointer indicates the position of SR which was used as the reference time for the superposed epoch analysis.

Depmdence of PC5micropulsationpower on conductivityvariations However, it now appears as if there is a second slightly larger enhancement in activity; in this case the second enhancement occurs 3 h after SR or at the “smeared out” 0730 L.T. Only after this second enhancement does the normalized power reach the same level of magnitude as that reached soon after SR during the winter months. Since the method of data presentation used in Fig. 1 is rather clumsy, the data of Fig. l(a) and (b) are repeated in Fig. 2(a) and (b) respectively in the form of contour plots. This format, where the vertical axis represents increasing frequency and the horizontal axis increasing time is commonly known as a sonogram. An arrow indicates the position of SR. Figure 2(a) dramatically shows the massive enhancement of power in the Pc.5 band around SR, while Fig. 2(b) clearly shows an initial enhancement of power following SR, with a second increase shortly after 0700 L.T. Figures 3(a) and (b) are similar to Fig. 2(a) and (b), except that they were computed using the D-component data for the days from which the H-component data of Fig. 2 were determined. The conclusions to be drawn from the D-component sonograms are essentially the same as those for the H-component.

635

The sonograms presented in Fig. 4(a) and (b) were again computed in a manner similar to that used for computing Fig. 2(a) and (b), but this time using data for 19 and 25 days recorded during the winter and equinoctial months at PROV respectively. We again see that the PCS power increases in the vicinity of SR during both the winter and equinoctial periods. Inspection of the original magnetograms showed that the small peaks in the Pc5 spectral band well before SR, such as the peak at 0500 L.T. in Fig. 4(a) were due to substorm related activity rather than Pc5 pulsations. From Fig. 4(b), it appears as if the ,activity builds up following SR, then tends to die down slightly, and then develops again between 0700 and 0800 L.T. Note that once again this 0730L.T. enhancement is “smeared out”, being made up of 25 individual spectra which wmmence at local times which range over nearly 3 h with a mean time close to 0730 L.T. In order to further study the apparent occurrence of two enhancements in PCS activity during the morning hours, we superposed spectra covering 4 shorter periods as indicated in Table 2. This finer division of the data confirmed that the first enhancement is related to SR, while the second appears to be related to local time.

A I

lu-

._ -iOCAL FIG.

TIME

3. SAMEASFIG. 2, BUTFORTHE

-D-COMPO~.

636

P. R. SWU.IFFE and G. Rcmom

““LOCAL FE. 4. SAMEAs FIG. 2,

BUT FOR

DAYS OF DATA

(a) 19

DAYS

RECORDED

0~ DATA

DATES

D EiFlNtNG THE

LONGEST DAYS OF THE? ING OF

2

1 2 3 4

SHOR’IIXST, MEDIAN,

SEASONAL

AND

PERIODS (EACH CONSIST-

GROUPS OF DAYS) AND THE MEAN TIMES OF LOCAL

SUNRISE AT 100

Seasonal Period

4

km HEIGHT AT

Day Shortest Median Longest Shortest Median Longest Shortest Median Longest Shortest Median Longest

URAN

ON THFZSE DATES

SR at URAN Dates Group 1 Group 2 Mean L.T. 22 Dee 7Jan 21 Jan 22 Jan 7 Eeb 21 Feb 22 Feb 7 March 21 March 22 March 7 April 21 April

21 7 22 21 7 22 21 7 22 22 7 22

Dee Dee Nov Nov Nov act act Ott Sept Sept Sept Aug

--

RECORDED

DURING

wINTER

DURING E?.OUINOClTAL MONTHS AT

Figures 5(a)-(d) show the sonograms for seasonal periods l-4, which were respectively computed using data for 9, 5, 6 and 9 days recorded at URAN. Figure 5(a) for the group 1 data covering the mid-winter period closely resembles Fig. 2(a). We see a large, clearly defined enhancement in PCS TABLE 2.

TIME

0723 0716 0656 0656 0627 05.52 0552 0516 0436 0436 0349 0258

MONTHS AND

(b) 25

PROV.

activity at SR, although it appears as if the activity actually started to develop about an hour prior to sunrise. Such behaviour would be expected on the basis of the existence of edge effects for the causative three-dimensional current system. SR for this period occurs at 0716 L.T., close to the time at which we previously noted a second enhancement in activity. We contend that this exceptionally clear enhancement results from the SR effect and 0730L.T. enhancement occurring almost sirnultaneously. This contention is supported by the results presented in Fig. 5(b)-(d). In going from group 1 to 4, SR occurs progressively earlier; thus, if there are two distinct enhancements in the Pc5 frequency band, one of which occurs at SR and the other at 0730 L.T., then these should be observed to become increasingly separated in time. From Fig. 5(b)-(d) we see that this is indeed the case. Figure 5(b) shows an increase in activity developing half an hour before SR at 0600L.T. and a second enhancement following 1; h later at 0730 L.T. In Fig. 5(c), it appears as if the activity starts to develop half an hour before SR, although this is probably due to contamination by some substonn activity. (Substorm activity was present on 2 of the

Dependence of Pc5 micropulsation power on amductivity variations

637

b

03

OS &CAL

12

TIME

H-COMPONENT SPECIRAMIR DATA RECORDED AT URAN. The sonograms (a)-(d) are for the seasonal periods l-4 respectively as detiued in Table 2. In each case the pointer indicates the position of SR which was used as the reference time for the superposed epoch aualysis.

FIG.

5.

fhNOGRAM.9

COMPUTED

FROM

SUPERPOSED

6 days in this sample prior to sunrise.) There is certainly power in the PCS band half an hour after SR, followed by a second marked enhancement at 0800L.T. From Fig. 5(d) we note a signiticant increase in PCS power at 0430L.T., half an hour after SR, and a second enhancement which develops between 0700 and 0800 L.T. e The results for the PROV data (not shown) distributed over the same four seasonal periods were similar to those of Fig. 5, and confhmed our findings of an initial development of PCS power around SR, followed by a second marked enhancement in the vicinity of 0730 L.T. We thus see that the time separation between the two enhancements of PCS activity conforms with that between SR and 0730 L.T. for the four seasonal groups, providing convincing evidence that the enhancements are in 6

fact related to SR and 0730 L.T. Since it appears as if 0730 L.T. is a key time for the generation mechanism of Pc5 pulsations, we repeated the process of superposing the spectra from individual days; but this time using local time rather than SR as the reference time. The results for the data recorded on the days during the winter and equinoctial periods (Table 1) at URAN are presented in Figs. 6(a) and (b), respectively. Note that in this figure the spectrum used to compute the sonogram at a specific local time is made up by superposing spectra for that local time. Further, the position marked SR in this figure is not the position of the superposed SRs as was the case in previous figures, but’ is now marked at the local time of SR on the median-day of the seasonal group. Thus, the spectrum representing SR is “smeared out”, since it

P. R. SUI‘CLIFFE and G. Rosroxnn

638

is

made up of spectra commencing at times which range by as much as 3 h around the actual SR times. If the enhancements in Pc5 activity do, in fact, occur at SR and 0730 L.T. on all days, one might expect the SR enhancements in Fig. 6 to appear “smeared out” when compared with the previous figures, with the 0730L.T. enhancements being sharper. It is difllcult to say whether the SR effect is smeared or not when comparing Fig. 6 with Fig. 2; neither is the second enhancement markedly sharper. The implication of this is that, although the enhancements are observed at SR and 0730L.T. when determined statistically, there is a sign&ant day-to-day variability in their times of occurrence. One might expect this to be so, since the SR and 0730L.T. effects were determined on the basis of probability of occurrence. An increase in Pc5 activity will be observed on a particular day when crossing the sunrise terminator, provided the conditions for Pc5 generation pre-exist; that is,

I

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I

tiNoGRAM.3

DATA

RECoRDED

Local

time

COMPUTED

DURING

WINI-FB

FROM

THE

MONTHS

H-COMPONENT

(b) 15

MONTHS was

used

*

I

11

TIME

sAND

I

09

67

LOCAL PIG. 6.

I

I

I

05

03

ionosphereoscillations of the provided magnetosphere electric field (which occur in conjunction with rapid reconfigurations of the westward electrojet) are in progress. If the electric field only commences oscillation some time after SR. the Pc5 pulsations will also be seen only after this time. If the electric field and, consequently, also the electrojet remain stationary, then no Pc5 pulsation activity will occur. Finally, we computed spectra by superposing all the data from URAN using local time as the reference time. The sonograms for the H- and Dcomponents are presented in Figs. 7(a) and (b), respectively. We see that the Pc5 power starts to develop at about 0500 L.T., reaches a maximum at 0830 L.T. and then dies down again. This is in excellent agreeement with determinations of diurnal variation of Pc5 activity during the morning hours by previous workers (01, 1963; Rao and Gupta, 1978), thus serving as confirmation of the validity of our superposed spectra.

DAYS

OF DATA

smcm4

FOR

(a) 14

RECOFtDFJD DURJNG

DAYS

0~

FXWINoCITAL.

AT m.

reference time for the superposed epoch analysis. the pointer indicates the position of SR on the median day of the seasonal period.

as the

Dependence of Pc5 micmpulsation power on amductivity variatious

639

b

s!.

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h)-

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~NOGRAMS

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time

was

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TIM;

(a) H-COMPONBNT AND(b) D-WNKWNX svxra~ FOR 29

DAYS OF DATA Local

I

10

LOCAL FIG. 7.

I

I

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RECORDED

AT m.

used as the reference time for the superposed epech analysis.

DISCUSSION

Two major processes are responsible for energy transfer within the magnetosphere; the first is by the streaming of energetic particles along magnetic field lines, and the second by convection normal to the magnetic lines of force. Lit the open magnetosphere model (Dungey, 1961), field lines convect across the polar cap in the anti-sunward direction and return in the sunward direction such that the feet of the field lines follow the auroral oval. Since the auroral oval represents a region with high transverse (Pedersen and Hall) conductivities, the electric field associated with convection drives electrojet currents in the ionosphere. The existence of gradients in electric field and in conductivity results in current flow diverging along the magnetic field lines and three-dimensional current systems being set up with closure currents -in the magnetosphere (Rostoker and Bostrom, 1976). Rostoker and Lam (1978) pointed out that at the dawn meridian there. is a signiIWnt conductivity discontinuity, since solar u.v. radiation causes en-

hancement of the level of ionization in the Eregion, and hence Hall and Pedersen conductivities will be markedly enhanced in moving from the dark to the sunlit ionosphere across the dawn meridian. The effect of this conductivity discontimrity is to force some of the ionospheric Hall current to diverge up the field lines at the dawn meridian. Rostoker and Lam (1978) proposed that the magnetosphere-ionosphere system can be considered as an LCR current circuit consisting of two loops (see their Fig. 4). The first loop involves a double sheet system (Bostim, 1964) where current flows into the ionosphere on the poleward side of the electrojet, then equatorward in the ionosphere as Pedersen current, and out of the ionosphere in the equatorward portion of the electrojet. Closure of the loop is made by current flowing adjacent to the magnetospheric equatorial plane and neutral sheet on the day and nightsides, respectively. In the second loop, current flows down field lines near noon, westward in the ionosphere as Hall current, and then outward at the dawn meridian. Closure of

640

P. R N

this loop is made by current flow along the flanks of the magnetosphere. The Pc.5 pulsations are the signature of the combined fluctuations of current in these two loops. Even if the conductivity is relatively constant, oscillations of the magnetosphereionosphere electric field (which is latitudinally contined) are sufiicient to cause a fluctuating electrojet. Despite the existence of a high conductivity channel in the pre-dawn morning sector aurora1 oval ionosphere, caused by precipitating energetic particles, the associated current loop will not oscillate at PCS frequencies, but rather will oscillate at much lower frequencies due to its large scale size. One of the prerequisites for the generation of Pc5 pulsations in the Rostoker and Lam (1978) theory then, is that there be a discontinuity in ionospheric conductivity in the vicinity of the dawn meridian. They suggested that local sunrise would provide such a discontinuity. The present analysis has demonstrated that Pc5 activity is indeed enhanced at the time of local sumisc at ionospheric heights, thus lending support to their suggestion. In addition to the enhancement of PCS activity following sunrise, we also found an enhancement commencing at about 0730 L.T. The question arises whether the theory for Pc5 generation proposed by Rostoker and Lam can support such an occurrence. The answer will be affirmative if we can demonstrate and explain the existence of a diswntinuity in ionospheric conductivity at about 0730 L.T. with the increased conductivity stretching approximately to local noon. A number of observational results have shown that a region of increased ionospheric conductivity which fits our requirements does develop at certain times: (1) Hartz and Brice (1967) examined statistical data for a variety of high latitude phenomena to establish the latitude versus time distribution of particle precipitation at auroral zone latitudes. Their study revealed two distinct zones of energetic electron precipitation partially separated in latitude, as shown diagramatically in their Fig. 19. The tlrst zone, which lies along the auroral oval, results from the irdlux of soft electrons (energies of a few keV). The sewnd zone, which is of concern to us here, lies approximately along lines of wnstant geomagnetic latitude and is due to the precipitation of hard electrons (energies of a few tens of keV). A region of particularly intense electron precipitation occurs after dawn, with a maximum at about 08OOM.L.T. (-0700-0730L.T. at URAN, depending on season), at latitudes slightly equatorward of the auroral oval.

and G. Rosroxna (2) Kremser et al. (1973) used Bremsstrahhmg X-ray measurements to study the amoral zone electron precipitation in the morning sector. They identified two separate zones of electron precipitation. The one zone, which they called the direct precipitation zone, is spatially associated with the westward electrojet and lies in the poleward part of the auroral zone. The second zone, which they called the drift zone, is located equatorward of the electrojet position. Both kinds of precipitation are associated with substorm activity; however, the Xray fluxes in the direct zone are observed at the same time as substorm events, while the X-ray fluxes in the equatonvard drift zone are delayed in time with respect to the substorm onsets by amounts consistent with drift times of the injected electrons. (3) Rostoker and I-Iron (1975) demonstrated the existence of eastward electrojet flow in the dawn sector in the latitude regime normally occupied by the westward convection electrojet. They found that this eastward electrojet may w-exist with the westward electrojet and lies equator-ward of it. They claimed that this eastward electrojet consists of Pedersen current driven by an eastward electric field and that it flows in the high conductivity charmel whose presence is suggested by the observations of Hartz and Brice (1967) and Kremser et al. (1973). The above findings provide wnvincing evidence that a region of increased ionospheric conductivity, situated at auroral zone latitudes and with the increased conductivity occurring after 0730L.T. develops in association with substorm activity, and further, that this region of increased conductivity will persist for a time after the substorm activity has died down. Consequently, the fluctuating electrojet currents associated with oscillations of the magnetosphere-ionosphere electric field wiIl be enhanced in the local time sector between 0730L.T. and noon. The signature of the enhanced current will be observed at the ground as an increase in Pc5 power, thus accounting for the enchancement in Pc5 activity which we observed to develop at 0730 L.T. We now look at some of the morphological features of Pc5 pulsations and consider how they fit in with the findings and explanation which we have presented here. Rao and Gupta (1978) made a statistical study of certain Pc5 characteristics using 11 yr of data from Fort Churchill; their Fig. 10 shows a clear minimum in Pc5 occurrence during the summer months. Of wutse, since the solar zenith angle x

Dependence of Pc5 micropulsationpower on conductivityvariations increases toward 90” as one moves towards the nightside, there will be a gradient in conductivity due to solar U.V. radiation even if x never reaches 90” (i.e. there is no terminator) since the rate of ionization in the ionosphere due to solar radiation varies roughly as cos’” x. However. the conductivity gradient will be much less for values of x < 70” than for the cases of x + 90“. Thus, one would expect less electrojet current to diverge up the field lines across the morning sector during the summer months compared to the winter months and accordingly weaker micropulsation activity in agreement with the results of Rao and Gupta (1978). As a further consequence of the absence of a sunrise terminator at high latitudes during part of the year, we might expect that the incidence of PCS pulsations at local dawn (i.e. at 0600 L.T.) when averaged over a year will decrease in going from lower to higher latitudes. However, the occurrence of the pulsations which develop at 0730 L.T. should not be affected. The combined effect of these two factors will be to move the occurrence peak of Pc5 pulsations to slightly later times in going from lower to higher latitudes. This, in fact, appears to be the case, as shown by Fig. 4 of Gupta (1976) where the time for peak occurrence of Pc5 changes from approximately 0600 to 0900 L.T. in going from a latitude of 55”N to 64”N. This is even better illustrated by Gupta (1976) (his Fig. 6) which shows the morning type Pc5 pulsations to have maximum amplitude at 0900 L.T. at Baker Lake, a broad peak from 0600 to 0900L.T. at Churchill, and a large peak just before 0600 L.T. (with a secondary peak at 0900 L.T.) at Great Whale River. At Baker Lake, and Churchill there are, respectively, 99 and 52 days per year in which there is no sunrise terminator (viz. there is continuous sunshine) at a height of lOOkm, while there is always a sunrise terminator at Great Whale River. In addition to the time of peak Pc5 occurrence changing with latitude, we might expect it to change with season, or at least to occur at a slightly later time during the summer months, due to the decrease of the SR enhancement. Figure 6 of the paper by Rao and Gupta (1978) shows that the time of peak occurrence during the summer months (0830 L.T.) is about an hour later than during the winter and equinoctial months (0730 L.T.). Rao and Gupta (1978) also studied the effect of increasing magnetic activity on the diurnal variation of the occurrence of Pc5 pulsations (see Fig. 3 of their paper). They found that for magnetically quiet peroids (I&, IO+) pulsation activity occurred most frequently at 0600 L.T. For low K, values (1-s:

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K, I 2+) the occurrence frequency peaked around 0700 L.T., while for moderately strong activity (4- 5 & 5 4+) the peak occurred at 0800 L.T. Once again, the mechanism we have proposed for Pc5 generation is in accord with these observations. During magnetically quiet intervals, the precipitation of energetic electrons and, consequently, also the region of increased conductivity necessary for the enhancement of Pc5 activity after 0730L.T., will be at a very low level. However, the process producing pulsations at sunrise will still be operative, and when averaged over a long period of time they will be observed to occur near 0600 L.T. As the degree of magnetic activity increases, the process responsible for enhancing pulsations at 0730L.T. will become operative and increase in effectiveness, thus causing the Pc5 occurrence peak to move to later times as observed by Rao and Gupta (1978).

We have shown that at auroral zone observatories a marked increase in Pc5 activity is detectable at local sunrise. Furthermore, a second more clearly defined enhancement is observed close to 0730 L.T. These enhancements of Pc5 activity can be explained in terms of the resonant oscillations of a three-dimensional current system as proposed by Rostoker and Iam (1978). However, we should like to emphasize that the role of ionospheric conductivity can also be couched in the format of hydromagnetic wave theory. In this format, one would consider the change in the transmission coefficients of the waves through the ionosphere. Alternatively, one would consider the changes in current flow associated with ionospheric screening (Hughes and Southwood, 1976). In any event, the ultimate explanation of the effects discussed in this paper should be equally viable in terms of oscillating current configuration and hydromagnetic wave theory as applied to resonance regions. Acknowledgements-We are indebted to Dr. J. V. Olson for useful discussions and for his help in the utilization of the computer software. We are grateful to the Department of Fisheries and the Environment (Atmospheric Environment Service) and to Transport Canada (Telecommunications Branch) for their help in the acquisition of the data on which this study was based. One of us (P.R.S.) is indebted to the Department of Physics for providing computing support during his stay at the University of Alberta. This research is part of the International Magnetospheric Study and was supported by the National Research Council of Canada.

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