The effects of ionospheric horizontal electron density gradients on whistler mode signals

Journal q~ Atmospheric and Terrestrial Physics, Vol. 54, No. 7/8, pp. 1061 1074, 1992.

0021 9169/92 $5.00+ .00 Pergamon Press Ltd

Printed in Grcat Britain.

T h e effects of ionospheric horizontal electron density gradients on whistler mode signals M.A. CLILVERD,* A.J. SMITH~and N.R. THOMSON* *Department of Physics, University of Otago, Dunedin, New Zealand; tBritish Antarctic Survey (NERC), Madingley Road, Cambridge CB3 0ET, UK. (Camera-ready copy received in final form 27 November 1991; accepted 11 December 1991)

Abstract--Whistler mode group delays observed at Faraday, Antarctica (650 S, 64° W) and Dunedin, New Zealand (460 S, 1710 E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events ('discontinuities') are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.

1

INTRODUCTION

There are two large scale horizontal electron density gradients present in the ionosphere. Firstly, there is a meridional gradient observed at midand low latitudes. The average electron density gradually increases towards the equator, in response to an increasing mean solar elevation and hence photoionisation rate (CItAN and COLIN, 1969). At high latitudes the gradient is obscured by rapid temporal variations due to electron production by energetic particle precipitation. The second electron density gradient is generally oriented zonally at mid-to low latitudes and is caused by the dawn/dusk terminator. The production of electrons by photoionisation just after sunrise causes significant local time changes in the electron density values and leads to large scale gradients across the twilight region. The electron density can typically increase by a factor of 2 in the space of a few hours. Similar, though reverse, conditions occur during the period of dusk. This paper reports sunrise/sunset terminator influence on whistler mode propagation through the ionosphere, inferred from signals recorded between 1986 1990 by VLF Doppler receivers at Faraday, Antarctica (65 ° S, 64 ° W) and Dunedin, New Zealand (46 ° S, 171 ° E). At each site, whistler mode signals are received from two VLF transmitters in the northern hemisphere; these

signals have propagated along field lines near L = 2.5 (STRANGEWAYS and THOMSON, 1986). The majority of the analysis is based on data from the Faraday receivers, with additional evidence provided by the Dunedin data.

2

EXPERIMENTAL

METHOD

The VLF Doppler experiment at Faraday receives ducted whistler mode signals from two US Navy transmitters (NAA, 24.0 kHz at Cutler, Maine and NSS, 21.4 kHz at Annapolis, Maryland). Two narrow band receivers of the type described by THOMSON (1981) are able to separate the whistler mode signals from the stronger sub-ionospheric signal, and measure the group delays (tg), Doppler shifts, and arrival bearings of the whistler mode component. Results from the experiment have been published by SMITH et al. (1987), SAXTON and SMITH (1989), and SAXTON and CLILVERD (1990). Similarly, the VLF Doppler experiment at Dnnedin receives signals from NLK (24.8 kHz at Seattle, Washington) and NPM (23.4 kHz at Hawaii). The group delay times are found by crosscorrelating the whistler mode signal with the subionospheric signal and integrating over 15 minutes. Each 15-minute averaged cross-correlation function consists of 200 correlation coefficients,

1061

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M . A . CLILVERD et al.

ML

~

RR

30M

m~MPM o (3O

i

30S

GOS

FR (a)

HLKT

0

IMm

'~PM C~ (b)

(c)

Fig. 1. (a) A plot of the geographic positions of the transmitters NAA and NSS (transmissions received at Faraday), and NLK and NPM (transmissions received at Dunedin). (b) The region near the NLK and NPM transmitters, at a larger scale, showing the position of the Dunedin conjugate (DU') and the typical duct entry region for whistler mode signals received at Dunedin (DU! - - see text for details). The L-shell contours were calculated for an altitude of 100 km. (c) A similar plot of the region near the NAA and NSS transmitters, showing the position of the Faraday conjugate (FA') and the typical duct entry region (FA!).

Ionospheric horizontal electron density gradients corresponding to group delay times between 0 s and 1 s with 5 ms resolution. Every well defined peak in the cross-correlation function is interpreted as a duct; as many as twenty ducts have been observed simultaneously, though about five is more typical. The lag value at the centre of a peak is interpreted as the t 9 for the corresponding duct. The observed difference in tg of the different frequency signals from the two transmitters, when they have propagated in the same duct, can be used to infer the L-value of the path (SAXTON and SMITtf, 1989). These authors analysed Faraday data from 8 quiet days in July 1986 and found that the average L-shell of propagation was L = 2.5, with 90% of paths lying in the range L = 2.32.7. In this paper we have assumed that the region of the plasmasphere observed by the Faraday VLF Doppler experiment is that sector of a thin (AL ~ 0.5) shell, centred at L = 2.5, which lies near the Faraday geomagnetic meridian plane. A similar study found that the average L-shell observed by the Dunedin Doppler experiment was L = 2.2, with most paths in the range L = 1.9-2.6 ( J . S T R A N G - - personal communication, 1990). The typical entry regions of the whistler mode signals for both of the experiments, deduced from the arrival bearings of the signals, are shown in Fig. 1.

3

3.1

RESULTS

Sunrise and Sunset Discontinuities

It has been shown in previous papers that there are several processes that can affect the characteristics of the whistler-mode signals, and in particular their group time delays tg - - normally an indicator of electron density levels near the plasmaspheric equator. Most changes, such as the annual, diurnal, and magnetic stdrm associated variations (CLILVERD et al., 1991; SMITH and CLILVERD, 1991) are noticeable only over time scales of hours or longer. Here, however, we are concerned with sudden changes in tg, occurring over times of order minutes, which occur near sunrise or sunset. They appear as discontinuities in the otherwise slowly varying traces normally present in tgversus UT plots (each trace corresponding to a drifting whistler duct), and we therefore term the events 'sunrise (or sunset) discontinuities'. An example of such an event is shown in Fig. 2,

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for whistler mode signals from the NSS transmitter received at Faraday on the 5th/6th June 1986; the plot shows that at about 09 UT, the range of group delays suddenly increases from 250-400 ms to 250-700 ms in one integration period (15 minutes). The high group delay signals are due to higher L-shell propagation, an increase of about 0.2 L (typically from L = 2.5 to L = 2.7). The colour scale in Fig. 2 represents the Doppler shift of the received whistler mode signals, the generally blue colour of the high group delay signals indicates large negative Doppler shifts (of order -200 mHz to -300 mHz). Negative Doppler shifts imply that the ducts either are in motion away from the Earth or are filling from the ionosphere; the fact that 09 UT is about the time of sunrise at an altitude of 100 km above Faraday's conjugate (FA t) suggests that the Doppler shifts are caused by ionospheric filling, following the onset of solar photoionisation. This relationship is seen in all of the sunrise discontinuities. There is no similar effect seen during sunrise at the Faraday end of the field line. The azimuth of the signals received at the Faraday aerials indicates that before sunrise at FAI, the whistler mode signals were received from a north-westerly direction, while the signals observed after the discontinuity, during the period of increased tg, have an easterly bearing. This effect is seen consistently in other examples of sunrise discontinuities and suggests that, following the discontinuity, the whistler mode signals have propagated to the southern hemisphere at a more easterly longitude than usual. Fig. 3 shows a sudden tg increase observed on l l t h June 1989 in whistler mode signals received at Dunedin from the NLK transmitter. In contrast to Fig. 2 the colour scale in Fig. 3 represents the amplitude of the received whistler mode signals. The change in tg occurs at 0730 UT and is coincident with the time of sunset at the duct entry region shown in Fig. 1. The effect of either sunrise or sunset conditiohs at the Faraday and Dunedin entry regions is discussed in detail in Section 5.1. The whistler mode arrival bearings at the Dunedin receiver indicate that the signals propagated at a more westerly longitude than is normal. Sunrise discontinuities occurred on approximately 11% of the days that the Faraday Doppler receivers were receiving transmitted signals in 1986. This statistic was determined from the NSS

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M. A. CLILVERDet al.

whistler mode signals; the corresponding figure for NAA was only 7% of the days. Both transmitters were operational for about 250 days during the period that the experiment was running in 1986. A seasonal variation in the occurrence of sunrise discontinuities was observed. There appear to be no observations of discontinuities between the months of October and March, while 80% of them occurred during the months of June, July and August. The mean and standard deviation of the solar zenith angle (SZA) for 0 km altitude at FA', at times of the NSS sunrise discontinuities observed at Faraday (during 1986), was 94 ° -4- 6 ° . A second feature observable in Fig. 2 occurs in the majority of cases: the high group delay signals which follow the discontinuity disappear, often equally suddenly, after about 2 hours. The average SZA at the end of the high tg's was 69 ° + 7°. A class of events in which the return to presunrise group delays does not occur is described in Section 3.2. Sunset (near the Dunedin receiver's conjugate) discontinuities are observed at Dunedin for both NLK and NPM simultaneously. Four sunset events were observed in 1989, with a similar number in 1990, representing approximately 4% of the number of observing days throughout the year. All of the sunset discontinuities occurred in the months May-July. Sunrise discontinuities have also been observed at Dunedin although they occur infrequently < 1%), and only between the months November-February. The probable cause for the seasonality of these events is discussed in Section 5.1. 3.2

Post-discontinuity Plateaux

Usually, when a sunrise discontinuity is observed, there is a sudden increase in the group delay times of the whistler mode signals, shortly followed by return to the pre-sunrise values. In some cases, however, there is an enduring presence of high tg signals, with no return to pre-sunrise levels until about local midnight. We term these events 'post-discontinuity plateaux' (PDPs). An example is shown in Fig. 4, in which the colour scale represents signal intensity. The plot shows whistler mode signals from the 2nd/3rd July 1986; there is a sudden increase in the group delay times of the signals at 10 UT on the 2nd, followed by a period of both high and normal group delay times throughout the day. The systematic changes in

the delays of each individual duct throughout the recording period are known to be due to plasma convection (SAXTON and SMITH, 1989). It is the aim of this paper to study the sudden discontinuities rather than these more gradual changes. There is a decrease in tg around local midnight (04 UT) followed by another sunrise discontinuity at 10 UT on the 3rd, after which the signals return to their normal t a values. Note that the effect is asymmetrical, i.e. the reversal of the change at sunrise occurs near midnight, not sunset. The arrival bearing of the high ts signals indicate that they are arriving at the aerials from a region to the east and south. The direction of arrival of the signals is consistent with propagation at high L-shells (since for Faraday, L "- 2.5); analysis of the signals from both transmitters, using the SAXTON and SMITH (1989) method, indicates that the waves have propagated at L = 2.6. The lower group delay signals are generally from the west and north of Faraday, consistent with propagation at lower L. Analysis of the data from both transmitters during this period confirms that the waves have propagated at L = 2.4. The conditions that produce the sunrise discontinuities and PDPs can prevail for more than one day; examples of PDPs occurring on two or three consecutive days have been observed at Faraday. 4

IONOSPHERIC CONTROL OF THE L-SHELL OF PROPAGATION

At a sunrise discontinuity, a sudden change to higher group delay times occurs. The higher group delays correspond to whistler mode signals propagating at higher L-shells, usually up to the half gyro-frequency cut-off (L=2.7 for NSS). Fig. 1 shows that the NAA transmitter is situated to the north of the typical duct entry region, while NSS is to the west. From the relative positions of the transmitters it would be expected that whistler mode signals would be observed up to the half gyro-frequency cut-off almost all of the time. The change in the L-shell of propagation of the whistler mode signals during sunrise discontinuities indicates that just before Faraday conjugate sunrise the normal propagation path of the NAA and NSS signals is at an L (about L = 2.5) that is significantly lower than the cut-off. A possible mechanism that could influence the L-shell of propagation of whistler mode signals is the refraction of the transmitter signal in the lower ionosphere (90-250 km). It is normally

Ionospheric horizontal electron density gradients

1065

NSS 21.4 kHz 1986 June 5/6 Doppler shift (N-S loop)

900

df (mHz)

400 e~

200

0

-200

-400

18

20

22

0

2

4

6

8

10

12

14

16

tit Fig. 2. A colour plot of whistler mode signals received at Faraday on the 5th/6th June 1986. The colour scale represents the Doppler shift of the signals, in a t 9 versus UT plot. The horizontal traces near the top of the figure are calibration signals. The sudden increase in group delay times at 09 UT is coincident with large negative Doppler shifts, indicating filling of the ducts due to the effects of sunrise.

M. A. CLILVERD et al.

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NLK 24.8 kHz 1989 June 11 Unealibrated power (N-S loop)

900

800

700

600

dB

500

1°I

o L.

400

300

5 200

100

0 0 5

7

9

11

13

15

17

19

UT Fig. 3. A colour plot of whistler mode signals received at Dunedin on the l l t h June 1989. The colour scale represents the amplitude of the signals. T h e discontinuity occurred at 0730 UT.

>-,

10(

20{]

300

400

500

500

700

800

900

t3

15

17

19

21

UT

23

1

3

5

7

9

Fig~ 4. A colour plot of two consecutive days of whistler mode signals received at Faraday on the 2nd/3rd July 1986, The colour scale represents the amplitude of the whistler mode signals. The plot shows that the high group delay times were observed throughout the day.

11

NSS 21.4 kHz 1986 July 2-3 Uncallbrated power (N-S loop)

11

13

15

10'

dB

*n

,<

r~

O

o

O

O

5"

Ionospheric horizontal electron density gradients assumed t h a t the up-going wave in the ionosphere has been refracted into an almost vertical p a t h (CAIROand CERISIER,1976) and that transmission to the opposite hemisphere is caused by t r a p p i n g through the side of a field-aligned duct (STRANGEWAYS, 1981). W h e n the transmitted sub-ionospheric signal originates at large distances from the duct entry region then the wave angle of incidence with the atmosphere-ionosphere b o u n d a r y will be large, and non-vertical propagation up through the ionosphere m a y occur. If the up-going wave is tilted in the same sense as the field-aligned duct then t r a p p i n g is more likely to occur. Snell's law of refraction at a b o u n d a r y can be applied to this problem: sin 0~ = n sin 0~

(1)

where 0i is the angle of incidence in the Earthionosphere waveguide and 0~ is the angle of refraction corresponding to a refractive index of n. Taking the real part of Appleton's equation shows t h a t the refractive index reaches a value of about n = 3.5 in a typical nighttime D-region for which HELLIWELL (1965) gives electron plasma, gyro-, and collision frequencies as being approximately f0 = 0.4 MHz, fh = 1 MHz, v = 0.1 MHz at a height of 100 kin. A sub-ionospheric signal from either N A A or NSS has to propagate about 700 k m in order to enter the ionosphere at L = 2.5 in the F a r a d a y duct entry region of Table 1. The variation of the angles of incidence and refraction at the lower ionosphere (90 km), for signals travelling from the NAA VLF transmitter to different L-shells of the duct entry region, with the curvature of the Earth taken into account.

Distance (south) from NAA (km) 1000 900 800 700 600 500 400 300 200

L

2.20 2.30 2.40 2.45 2.53 2.60 2.70 2.80 2.90

Oi

0,.

(deg.)

(deg.)

80.4 80.2 80.0 79.4 78.8 77.0 75.6 72.0 65.0

16.4 16.4 16.3 16.3 16.3 16.2 16.1 15.8 15.0

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Fig. 1, giving it an angle of incidence of about 79 ° at the night D-region's lower b o u n d a r y (90 km) when the E a r t h ' s curvature is taken into account. Snell's law indicates that the angle of refraction is about 16 °. Table 1 shows the variation with distance of travel of the sub-ionospheric signal from N A A to the Faraday conjugate region, of the angle of incidence of the wave on the lower b o u n d a r y of the ionosphere, and of the angle of refraction after transmission through the boundary. The table shows t h a t the angle of refraction becomes less as the distance travelled in the Earth-ionosphere waveguide decreases. The angle rapidly becomes smaller at distances of only a few hundred kilometres, corresponding to an L-shell of about L = 2.8. The position of NSS with respect to the duct entry region is very different from t h a t of NAA; NSS is situated to the west of the duct entry region and at a similar L-shell (L = 2.7). Table 2 shows the variation of distance, and bearing, from NSS to selected L-shells in the centre of the duct entry region. A fourth column shows the sign of the N - S component of the refracted wave and indicates that at about L = 2.5 the tilt in the wave normals, produced by refraction at the atmosphere-ionosphere boundary, changes from having a southerly component to having a northerly one. The difference in having a southerly component to having a northerly one m a y influence the likelihood of waves becoming e n t r a p p e d in ducts. This possibility is discussed in section 5.3. Table 2. The distance and bearing from NSS (with the curvature of the Earth taken into account) of points on the meridian of FA~ at different L-values, and the sign of the N-S component of the transmitter signal at the entry region (N positive).

2.80 2.70 2.60 2.50 2.40 2.30 2.20

Distance (east) from NSS (km)

Bearing. (deg.)

N-S component

840 850 800 680 690 740 810

65 70 77 88 98 110 119

+ve +ve +ve +ve -ve -ve -ve

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M.A. CLILVERDet al. ~uU

~ss ~' V

~ Fn

Fig. 5. A sketch of the dawn/dusk terminator as it passed over the duct entry region at an altitude of 100 km at the time of the occurrence of the sunrise discontinuity (0830 UT) on the 6th June 1986 (see Fig. 2), the angle between the great circle tangent to the terminator and the N-S direction is shown.

5

5.1

DISCUSSION

Sunrise and Sunset Discontinuities

Sunrise discontinuities occur at the time of sunrise in the conjugate region to Faraday. Because no similar effects are seen at the time of sunrise at F a r a d a y itself, it m a y be assumed that the mechanism which produces the discontinuities is active in the northern hemisphere. The rapid time scale of the tg change ( < 15 minutes) suggests that the process responsible operates at ionospheric altitudes, since the magnetosphere would be expected to have longer time constants. This hypothesis is confirmed by the observation of a sudden change of propagation path, determined by L-shell analysis. The arrival azimuth d a t a indicate t h a t after sunrise discontinuities the whistler mode signals p r o p a g a t e at more easterly longitudes than before sunrise. Easterly tilting of the up-going waves in the ionosphere would cause the wave to be t r a p p e d at more easterly longitudes. Tilting of signals at similar frequencies to those of N A A and

NSS during the passage of the sunrise terminator was observed by the FR-1 satellite (CAIRO and CERISIER, 1976). The wave normals of signals from a V L F t r a n s m i t t e r in western Europe were studied at an altitude of 750 km. Cairo and Cerisier concluded that the wave normals are always tilted towards the sun, eastward in the morning and westward in the evening. The east-west component of the wave normal exhibited a large change in the morning, at about SZA = 95 °, when the wave normal was tilted towards the east; it returned to normal when SZA decreased to about 65 ° . These observations agree well with the average SZA at the start and end of the post-sunrise high-tg events (94 ° and 69 ° respectively). There was a similar, though less extreme, effect on the E - W component at sunset, when the up-going wave was tilted towards the west. The tilting of the wave normals is caused by the presence of large scale horizontal electron density gradients in the ionosphere (JAMES, 1972; FISZLEIBER et al., 1975; CAIRO and CERISIER, 1976), in this case produced by the n i g h t / d a y terminator. The ar-

Ionospheric horizontal electron density gradients rival azimuths of the whistler mode signals after sunrise discontinuities are consistent with the upgoing waves having been tilted towards the sun. As the t e r m i n a t o r leaves the duct e n t r y region, the associated horizontal electron density gradient decays; the wave normal tilting is removed, and pre-sunrise propagation paths and tg values are restored. We have already noted that the occurrence of the discontinuity and the passage of the sunrise t e r m i n a t o r across FA' are closely spaced in time, and by implication, also causally. As an example, we show in Fig. 5 a plot of the position of the sunrise t e r m i n a t o r (at 100 k m altitude), in the vicinity of the duct entry region in the northern hemisphere on 6th June 1986, at the time of the sunrise discontinuity (09 UT). The positions of the two transmitters are also indicated. The seasonal variation in the occurrence of sunrise and sunset discontinuities, in b o t h of the experimental d a t a sets, is probably caused by the variation in the declination of the sun and therefore the angle of approach of the t e r m i n a t o r to the duct e n t r y region. At the Faraday conjugate region, during the northern hemisphere summer months ( M a y - A u g u s t ) , the t e r m i n a t o r line lies N W - S E and produces a tilt in the wave normals towards the NE; consequently if trapping occurs, it occurs at higher L-shells. In the northern hemisphere winter months ( O c t o b e r - F e b r u a r y ) , the terminator line lies N E - S W and would produce a tilt in the wave normals towards the SE, and possible trapping at lower L-shells. Sunrise discontinuities are not observed during the period O c t o b e r - F e b r u a r y as no whistler mode signals are received around the time of sunrise; this is assumed to be because of the increased D-region absorption of the whistler mode signals in the southern hemisphere due to the constant photoionisation t h a t occurs in summer at high geographic latitudes, at the time of northern hemisphere sunrise. It is apparent that the geometry of the terminator orientation and the duct entry region are i m p o r t a n t , as well as the positions of the transmitters. W h e n discussing the possible tilting of up-going whistler mode signals, with consequent change in duct t r a p p i n g efficiency, with reference to the position of the transmitter, the local magnetic field declination is important. The I G R F 1985 ( u p d a t e d to 1986) model was used to estimate the angle between the horizontal com-

1071

ponent of the magnetic field and true north at 40 ° N, 290 ° E (the duct entry region for the Faraday experiment); it was 15 ° W (see Fig. 5). W h e n the angle of the terminator, with respect to true north, is greater t h a n 15 ° W it would be possible to tilt the up-going waves to higher L-shells.

(a) 50'

*~ ~ a0.

10. -10

~

-30 -so ,00

20°

3o0

Day of Year

(b) so

~

a0 10

-~ -10 -a0 ~z

-s0

,00

200

3~o

Dny of Year

Fig. 6. (a) The variation of the angle of the tangent to the sunrise terminator at 400 N, 2900 E with a north-south line. The horizontal dashed line represents the angle of the horizontal component of the magnetic field lines with respect to north-south (i.e. the declination). (b) As (a) but for 470 N, 2040 E (the Dunedin typical duct entry region). See text for details.

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M. A. CLILVERDet al.

Fig. 6(a) shows the variation of the angle of the terminator at 40 ° N, 290 ° E throughout the year. The figure indicates that between days 100-250 the angle is greater than 15 ° W and thus the possibility exists of trapping the whistler mode signals on higher L-shell paths. The figure indicates that sunrise discontinuities could occur from April-September, a result that agrees well with the observations. Similar arguments can be used to explain the occurrence of discontinuities in the Dunedin data set. The sunset terminator line lies N E - S W during the months May-September and would produce a tilt in the wave normal to the NW and thus higher L-shells. The IGRF 1985 (updated to 1986) model indicates that the angle between the horizontal component of the magnetic field and true north is 17 ° E at the duct entry region for the Dunedin experiment (47 ° N, 156 ° W). Fig. 6(b) shows the variation of the angle of the sunset terminator at the duct entry region throughout the year and indicates that sunset discontinuities would expected to be observed between April and September; this is in agreement with observations. The observation of sunrise discontinuities in November February is consistent with the passage of the terminator across the duct exit region. This suggests a degree of symmetry between the transmitter and receiver ends of the path that is not observed in the Faraday data. This could be due to the greater geographic/geomagnetic symmetry of the Dunedin paths as compared to the Faraday paths. Sunrise discontinuities seen in the data set from the NSS transmitter occurred on approximately 11% of the days that the experiment was operational in 1986. This figure is perhaps surprisingly small when it is considered that the sunrise terminator passes the duct entry region once a day. The majority of the discontinuities were seen during the May-August period but only about once every five days. There is a day to day variability in the ionospheric response to the passing of the terminator, as seen by ionosondes (A.S. RODGER, 1988 --- private communication) but this is small compared to the main effects of the terminator. It is possible that the terminator is not the only significant factor in determining whether a discontinuity will occur. The presence of high L-shell (L = 2.6) ducts will be important in the observation of the discontinuity. No estimates of the frequency of occurrence of ducts at L = 2.6 can be made but

plasmaspheric structures can be observed almost continuously from L = 2.3 to L = 2.5, suggesting that structure at L = 2.6 is common. A study of the sunset discontinuities that were observed at Dunedin and the sunrise discontinuities observed at Faraday in 1989 is summarised in Table 3. The table shows the number of discontinuity events that occurred (with the maximum possible in brackets), sorted by the maximum I(p value during the preceding 24 hours. Although the numbers in the table are small there is possibly an indication that discontinuities are more common at Faraday than Dunedin, and that they are more likely to occur on the same day at the two stations than would be expected by chance if the two events were unconnected. The greater incidence of discontinuities observed at Faraday is probably due to the lower propagational losses attributable to the proximity of the transmitters to the Faraday conjugate region, and to more extreme tilting of the waves by the sunrise terminator than the sunset terminator (CAIRO and CERISIER, 1976). Coincident discontinuity events typically follow within 1 day of moderate/high magnetic activity (Kp >_ 5). A study of the 1986 Faraday discontinuities alone, does not suggest a similar link to magnetic activity. A more detailed study of the discontinuity events, including mathematical modelling, could lead to a more detailed knowledge of the probably multiple causes of the phenomenon. 5.2

Post-discontinuity Plateaux

Post-discontinuity plateaux (PDPs) are char-

Table 3. The occurrence of sunrise dicontinuities at Faraday and sunset discontinuities at Dunedin during 1989, sorted by the maximum Kp value during the preceding 24 hours. The maximum number of events that were possible for each Kp range is shown in brackets. The coincident events typically occurred within one or two hours of each other.

Occurrence of discontinuities

0-2

Kp Range 3-5

6 8

Dunedin Faraday Coincident events

0 (3) 0 (3) 0 (3)

1 (23) 3 (23) 1 (23)

3 (7) 3 (7) 2 (7)

Ionospheric horizontal electron density gradients acterised by sudden changes in the group delay times of whistler mode signals during sunrise at the conjugate region to Faraday and with the high L-shell signals remaining observable for long periods, typically until after local midnight. The PDPs are seen to occur on consecutive days, indicating that the mechanism which causes them may persist for up to 3 days. No link with periods of magnetic activity could be found. Horizontal electron density gradients that influence the L-shell propagation of whistler mode signals can be separated into two components. The E - W component caused by variations in the ionospheric structure with longitude or local time (like the E W component of the sunrise terminator) and the N S component caused by latitudinal variations (CAIRO and CEIUSlI~R, 1976). The N S component of the horizontal electron density gradient will exert a greater influence on the latitudinal propagation of the whistler mode signals than the E - W component. Generally the N-S component of the wave normal is directed southwards (CAIRO and CEPdSIEIt, 1976) because of the steady increase in electron density towards the equator (CHAN and COLIN, 1969) during summer days at American longitudes. The directing of the up-going whistler mode signals toward higher L-shells could be caused by a reversal of the typical N-S component of the electron density gradient. This reversal could be caused by an unusual thermospheric neutral wind configuration which produces a region of lower electron densities at lower latitudes, or higher electron densities at higher latitudes, thus reversing the N S gradient. 5.3

Ionospheric Limits on the L-shell of Propagation

The whistler mode signals from the NSS transmitter would be expected to propagate at all Lshells up to the half gyro-frequency limit (for NSS at 21.4 kHz the limit is L = 2.73). The results from the Faraday VLF Doppler experiment indicate that the usual limit of propagation of the signals from the NSS transmitter just before sunrise is about L = 2.5, approximately 0.2 L lower than expected. Using Snell's Law it has been shown that tilting of the wave normals can occur at the atmosphere-ionosphere boundary, especially when the waves have propagated over long distances in the Earth-ionosphere waveguide. Table 1 shows

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the resultant angle of refraction (caused by the sharp refractive index boundary) varying with the distance from the transmitter. The table shows that no significant variation in the angle of refraction of the up-going wave occurs within the limits of whistler mode propagation for NAA (at 24.0 kHz the half gyro-frequency cut-off limits propagation to less than L = 2.63). But the positions of the NAA transmitter and the duct entry region (Fig. 1) determine that the refracted wave normals are tilted in the same sense as the fieldaligned ducts, probably assisting the wave trapping mechanism. The N-S component of the refracted wave normals from the NSS transmitter shown in Table 2 indicates that tilting of the waves, in the opposite sense to the field-aligned ducts at higher L-shells, reduces the probability of the signals becoming entrapped in ducts. It is possible, therefore, that a factor limiting the L-shell range in propagation of the NSS whistler mode signals is the refraction of the waves by the atmosphere-ionosphere boundary and the relative positions of the transmitter and the duct entry region. Refraction of whistler mode signals at the atmosphere-ionosphere boundary can explain the L-shell propagation limits that are observed on signals from the NSS transmitter, but possibly not those from the NAA transmitter which appear to be limited by the half gyro-frequency cut-off. When the sunrise terminator passes through the duct entry region, the horizontal electron density gradients tilt the up-going waves to higher latitudes where they may become trapped into fieldaligned ducts. The difference between the data sets from the two transmitters in the occurrence of sunrise discontinuities can be explained by noting that the NAA signals at L-shells above L = 2.6 will become unducted and will not propagate to the opposite hemisphere.

6

CONCLUSIONS

Sudden increases in the group delay times of whistler mode signals occur at the same time as the occurrence of sunrise at Faraday's conjugate region. These events ('discontinuities') are observed on 11% of days. During the high-tg regime following a discontinuity, the whistler mode signals propagate at more easterly longitudes than at other times.

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M. A. CLILVERDet al.

The sudden changes in group delay can be understood in terms of a tilting of the up-going wave towards the sun by horizontal electron density gradients. The gradients are caused by the passage of the d a w n / d u s k terminator, and result in the waves becoming trapped at higher L-shells. The solar zenith angles of the beginning and the end of the Faraday sunrise discontinuities were found to be 95 ° and 69 ° respectively, values that agree well with the results from FR-1 satellite observations. A seasonal variation in the occurrence of sunrise discontinuities is observed. No examples are seen by the Faraday experiment during the October-February months, while 80% of the examples are seen during J u n e - A u g u s t . The seasonal dependence can be understood in terms of the orientation of the terminator with respect to the duct entry region. During J u n e - A u g u s t there is a significant polewards component in the tilt of the up-going wave produced by the horizontal electron density gradients. Sunset discontinuities are observed by the Dunedin experiment during the M a y - J u l y months and can be explained by the same terminator geometry reasoning that was applied to the Faraday events. The Dunedin sunset discontinuities occurred on the same days as the Faraday sunrise events, although not necessarily vice versa, and may possibly be linked to moderate/high magnetic activity that typically occurred 1 day before. High L-shell signals that last throughout the day, beginning at sunrise, can be explained by a continued presence of horizontal electron density gradients, possibly driven by unusual configurations of thermospheric neutral winds. When the lower ionosphere is considered as a refractive index boundary, Snell's Law can explain the L-shell distribution of the whistler mode signals from the NSS transmitter. The L-shell distribution of the whistler mode signals from the NAA t r a n s m i t t e r is determined by the half equatorial electron gyro-frequency limit of propagation. The relative positions of the transmitters and the duct entry region are important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.

Acknowledgements--The authors would like to thank

J.M. Saxton, A.S. Rodger, K. Bullough, S.S. Sahzin, H.J. Strangeways and K.It. Yearby for their constructive discussions concerning this paper. M.A. Clilverd would also like to acknowledge support from the Science and Engineering Research Council in the form of a Postgraduate Studentship at Sheffield University and, more recently, support from the Royal Society in the form of an Overseas Fellowship. REFERENCES

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