observations of pulsating aurora in the day sector auroral zone

PIwe. Spacr Sci., Vol. 26, pp. 635 to 650 0032~33/78/0?01~35$02.oo1o

Q %q$amon Press Ltd., 1978. Printed in Northern lrcland

OBSERVATIONS OF PULSATING AURORA IN THE RAY SECTOR AURORAL ZONE

F. T. BERKEY*

The Aurora1 Observatory, University of Tromse, N-9001 Tromsti, Norway

{Received in fina! form 20 December 1977)

Abstract-Utilizing a unique set of photometric and riometer observations obtained just after twilight (~30~1600 M.L.T.) at College, Alaska, the occurrence of pulsating auroras in the afternoon sector of the aurora1 zone is documented for the first time. A consistent tendency for the aurora1 pulsations to terminate as the Earth’s shadow passes through the E- and F-layers is interpreted in terms of the precipitation pulsation mechanism proposed by Coroniti and Kennel (1970) and the cold plasma inje&on theory of Brice and Lucas-( 1971).

INTRODUCTION

Statistical studies of particle precipitation indicate the existence of two distinct zones in the day sector (Hartz and Brice, 1967). A high latitude zone occurs near the locus of the aurora1 oval and appears to be caused by low energy or “soft” particle precipitation. The second zone is observed at a nearly constant magnetic latitude, essentially coin- ciding with the aurorai zone, and is dominated by “hard” particle precipitation. A study of the temporal and geographical development of individual aurora1 absorption events (Berkey et al., 1971, 1974) has shown that the hard particle precipitation expands into the dayside sector, along the aurora1 zone, from a source region located in the night sector. According to this study, the absorption tends to persist in the day sector and is often observed more than 2-3 hours after the onset of precipitation in the night sector.

The ISIS-2 scanning aurora1 photometer data show two distinct belts of difIuse emission on the dayside (Lui and Anger, 1973; Lui, 1974) however, it has not been determined if the outer of these two belts coincides with the zone of hard precipitation.

The impulsive nature of precipitation occurring within the early morning aurora1 zone is well documented by the observations of pulsating aurora (see e.g. Omholt, 1971). Observations of pulsating X-ray events (Brenstad and Trefall, 1968) indicate that modulation of higher energy (EZ 25 keV) electrons also occurs. Rosenberg et af. (1971) have observed a positive correlation between X-ray pulsations and pulsations of the 557713 aurora1 emission and have concluded that a

* Present address: Department of Physics, University of Catgary, Caigary, Alberta, Canada T2N lN4.

single process is modulating both the low and high- energy primary electron flux.

X-ray pulsations recorded in the dayside aurora1 zone (Ullaland et al., 1967) suggests that the modulation of energetic particle fluxes occurs through- out the aurora1 zone. However, the limitations to measuring aurora1 emissions in the day sector are obvious, and only a few observations of simultaneous hard and soft particle precipitation have been reported.

In the midday sector, Snyder et al. (1972) have observed an enhancement of fmin and occasional total ionosonde blackout simultaneously with the appearance of patchy aurora1 forms on all-sky camera (ASC) data. Their observations were made from a high altitude jet aircraft, which traversed a region equatorward of the midday aurora1 oval between approximately 71 and 73”N corrected geomagnetic (CC) latitude, from 1230 to 1430 L.M.T. The patches were initially observed some two hours after the onset of an aurora1 substorm in the midnight sector.

Montbriand (1969) discussed observations of a “very faint quiet patch aurora” which he observed on 41 occasions during the late afternoon hours in Canadian ASC data. He concluded that such dusk sector patches spread westward, equato~ard of the region in which the westward travelling surge (Akasofu et al., 1965) is observed. He also noted that aurora1 absorption sometimes occurs in the dusk sector and thereby may be associated with the patchy aurora.

Moshupi et al. (1977) have observed patches of aurora1 luminosity equatorward of the diffuse aurora in the ISIS-2 scanning photometer data. These patches were typically less than 10 kR (at 3914 A) in intensity and occurred predominantly in the 1800-2100 magnetic time sector.

635

636 F. T. BERKEY

Quantitative measurements of evening (- 2000 local magnetic time) pulsating aurora have been reported by Sawchuk and Anger (1976). The observations were carried out in Western Canada at L = 5.0.

In this note, several incidences of simultaneous hard and soft particle precipitation in the dayside aurora1 zone are reported. On each occasion the low-energy precipitation was characterized by high- frequency temporal variations.

B’JSl’RUMBNTATION AND OBSERVATIONS

An observational programme to simultaneously monitor HF radiowave absorption and aurora1 emissions over a relatively small angular cone was initiated at College, Alaska (64.8”N CG) in 1963. (Ansari, 1963, 1964) and carried out through 1968 (Berkey, 1968, 1971). The observations to be reported here were made over an angular cone of 7” (centred on the zenith) and the Nz+ band emission at 4278 A and the variation of cosmic radio noise at 36 mHz were the measured parameters.

In late 1967, the output of the 4278 8, photometer was paralled with a differentiator circuit, which amplified that component of the signal having a high rate of change and excluded the D.C. level. A similar system was used by Campbell and Rees (1961), who pointed out that such a system made it possible to observe during the presence of moon, under overcast sky conditions and much farther into twilight than possible with an undifferentiated system. The latter point is illustrated in Fig. 1, which shows the twilight variation during an undisturbed interval on 5 February 1968. The photometer shutter opened at 0251 U.T. (7” solar depression angle (SDA) but twilight dominated the undifferentiated signal until approximately 0350 U.T. (13”SDA). The undifferentiated output was use- able from 0300 U.T. (8” SDA), only a few minutes after the shutter opened.

Three of the observations reported here (18 and 20 January and 4 February 1968) were im- mediately obvious due to the large amplitude of the events. A subsequent, detailed examination of data obtained during January, February and December 1968 yielded seven additional events, of which two were eliminated due to overcast sky conditions and therefore unreliable data. Table 1 summarizes weather and sky conditions (as determined from visual, ASC observations and weather records during these events. Cold and clear conditions pre- vailed during the time the observations were made, so that the possibility that the variations were due

TABLE 1

Weather Date conditions All-sky camera observations

12 January 1968 -30°F AX saturated by moonlight 13 January 1968 -35” ASC saturated by moonlight 18 January 1968 -52” no moon 20 January 1968 -41” no moon 4 February 1968 -45” no moon 5 February 1968 -45” 6 February 1968 -42” moon -60” south of zenith

6 December 1968 -39” moon - 75’ north of zenith 9 December 1968 -41” no data

to a combination of moonlight and clouds can be ruled out.

Pulsating auroras at College occur predominantly after local magnetic midnight (- 1115 U.T.). Figure 2 shows the occurrence of pulsating auroras at College during the winter months of 1967 and 1968. In the morning sector, pulsating auroras tend to occur on a background of several kilorayleigh in 4278 A, the most intense modulating a level up to 10 kR (Cresswell, 1968). An example of the large amplitude and background level usually associated with pulsating auroras in the morning sector at College is given in Fig. 3. This is shown in contrast with the observations which follow.

The photometer shutter was controlled by a photocell and adjusted so that the shutter opened at about 7” SDA. The differentiated signal output was stable approximately 7 minutes after the shutter opened. Note that timing and resolution of the data are accurate to within 30 seconds in the origi- nal data.

Although twilight saturated the photometer re- sponse until approximately 8” SDA, pulsations were present on several occasions as soon as the differentiating amplifier stabilized, implying their presence prior to the time of shutter opening. Dur- ing the events of 18 January (Fig. 6), 20 January (Fig. 5) and 4 February (Fig. 4) increased radiowave absorption was evident both before and after the time the pulsations were initially detected. This indicates that a background precipitation of relatively energetic electrons was present.

The event of 4 February is interesting as the radiowave absorption recovered to the undisturbed level very rapidly after the cessation of pulsations. During this event there are evidences of non- modulated increases of 4278 A intensity (see 0456U.T.) which are not accompanied by increased absorption. Figure 4 also shows the undifferentiated output for the 4 February event; the amplitude of the modulation is on the order of a

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few hundreds of Rayleighs. This shouid be con- trasted with the intensity of morning sector pulsating auroras shown in Fig. 3.

On 20 January (see Fig. 5) the radiowave absorption also recovered shortly after the cessation of pulsations, but further absorption increases were associated with non-modulated increases in the 4278 8, signal. During the event of 18 January, the amplitude of the pulsations decreased markedly after about 0330 U.T., but the data indicate the signal was modulated after that time, albeit with a much smaller amplitude (see Fig. 6). The data also indicate that variations in the overall 4278 A intensity occurred during this time.

The remaining five events, extracted by a closer examina~on of the data, differed from the three events discussed previously in either amplitude or character. During the events of 12 and 13 January and that of 9 December 1968, pulsations were not evident until some time after the photometer shutter had opened (see Fig. 7). The pulsation amplitude was quite small, although there was increased absorption associated with each event.

The event of 6 December 1968, (Fig. 8a) was unique in that the period of pulsation was signifi- cantly longer (- 30 s) than any of the other events. As shown in Fig. 8b, the pulsations observed on 6 February 1968 were also unique, appearing more as bursts than pulsations. These bursts tended to occur at random time intervals up to about 0450U.T. when a train of bursts occurred for approximately 25 minutes. Again, the bursts were associated with increased radiowave absorption. The nature of these two events, particularly the latter, suggest that they may represent a separate class of events from those considered earlier.

All-sky camera data were available for only the event of 4 February 1968. In most cases the camera had not been turned on during the time pulsations occurred. During the time the pulsations occurred on 4 February, it was possible to detect very faint “arc segments”. Through application of cinematic projection, it was determined that these arc segments moved equatorward during the period of inter- est. As these aurora were very near the threshold sensitivity of the ASC film, photographic reproduc- tion was not attempted.

Using the magnetometer recordings from the College Observatory, it was established that there were no substorm activations at the time when the pulsating events were observed. However, an examination of magnetic data from observatories located near the midnight sector at the time of the observations, revealed that substorm disturbances had occurred some hours prior to that time. This is shown in Fig. 9, where the mean value of the hourly AE index for the 8 events discussed here is plotted. This curve shows that a high level of activity generally preceded the events, suggesting that energetic particles may have been injected into the magnetosphere prior to that time. The hourly AE values for the 5 most disturbed and quietest days, and the mean AE amplitude for the month of January 1968 are also plotted. These curves show that the time interval in which the pulsating events occurred was not anomalous and that disturbances were noted on other occasions within this time interval.

DISCUSSION

The space and time variation of time-varying aurora1 phenomena are often neglected when the

Pulsating aurora observations 639

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640 F. T. BERKEY

4 Febmny /968

16” 13” 10” ‘0 7 SDA

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FIG. 4. OBSERVAIIONS OF PULSATING AURORA IN THE EVENING TWILIG~ AT COLLEGE ON 4 FEBRUARY 1968. THE UPPER TWO PANELS SHOW THE UNDIFFERENTIATED AND DIPPERENTIATED 4278 A RECORDINGS, WHILE THE 36 MHZ RIOMETER TRACE IS SHOWN IN THE BOOM PANEL. THE UNDISTURBED OR QUIET-DAY

LEVEL IS INDICATED BY A DASHED LINE. LOCAL hlAGNJ%TIC TIh4!3 (L.h’fT) AND THE SOLAR DEPRESSION

ANGLE (SDA) ARE ALSO INDICATED.

substorm process as a whole is considered. Al- though it is beyond the scope of this paper to consider these variations, it should be noted that the substorm is virtually dominated by time-varying processes from about the time of the recovery phase. The observations reported here suggest that low energy electron fluxes drifting eastward from the midnight sector, under the influence of gradient and curvature drift, undergo temporal variations as they drift through the dayside magnetosphere.

A possible mechanism for modulating electron

fluxes has been proposed by Coroniti and Kennel (1970). They have suggested that micropulsations can cause pulsations in electron precipitation when trapped particle fluxes have been increased above the critical flux limit for the whistler instability. Central to their theory is that such precipitation pulsations should occur only superposed on an already enhanced background precipitation. They also point out that pulsations may be most pro- nounced at high energies, since low energy particles have the largest fluxes and are thereby more likely


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to be strongly unstable. (In the strong diffusion case, the precipitation rate is not increased by increases in the diffusion coefficient .)

An explanation for the maximum in precipitation during the late morning hours has been set forth by Brice and Lucas (1971). According to their theory, energetic electrons injected into the midnight sector undergo gradient-curvature drift to reach the day sector, where they are precipitated when the trapped flux increases and the stable trapping limit (Kennel and Petschek, 1966) is violated as a result of the flux of cold plasma from the ionosphere into the magnetosphere. The origin of the cold plasma is the result of solar illumination of the ionosphere and the resultant photoionization and photodetach- ment processes.

The mechanism proposed by Brice and Lucas does not account for the impulsive nature of day sector precipitation, but does suggest a means by which the limit of stable trapping may be decreased below the existing flux levels. On the other hand, the micropulsation theory of Coroniti and Kennel provides a means of modulating particle fluxes which exceed the stable trapping limit, but does not indicate the source of the particle fluxes. For the purposes of the discussion that follows, let it be assumed that the flow of cold plasma from the sunlit ionosphere increases the trapped particle flux and in this manner the critical flux limit for the whistler instability is violated. Furthermore, let it

be conjectured that the whistler instability is mod- ified by magnetic micropulsations, thereby leading to pulsations in the electron flux, as described by Coroniti and Kennel.

Upward streaming ions of ionospheric origin have been detected by an ion mass spectrometer aboard the S3-3 spacecraft (Johnson et al., 1977). The ions had energies over the range of the detec- tor (OS-16 keV) and were observed both during and after geomagnetic storms at latitudes of 60- 80”.

It should also be noted that Haugstad (1975) has criticized the Coroniti-Kennel theory on the grounds of inconsistencies within the theory. He finds, however, that precipitation pulsations may still occur although the conditions under which they might occur are more restrictive than postulated by Coroniti and Kennel.

During the recovery phase of the aurora1 substorm, the area in which micropulsations are observed expands along the aurora1 zone into the noon sector (Akasofu, 1968). Since the sunlit ionosphere provides a source of cold plasma, it might be expected that the dayside aurora1 zone is characterized by impulsive precipitation, according to the assumptions made above. Therefore, at the boundaries of the day sector, sunrise and sunset, we should observe an increase and a subsequent decrease in the occurrence of modulated fluxes.

The results of Berkey et al. (1974) show that

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DISTURBED AND 5 QUIET DAYS AS WELL AS THE MEAN HOURLY AE INDEX ARE ALSO PLOTI-ED.

648 F. T. BERKEY

there is no asymmetry in the precipitation of energetic electrons across the day-night boundary in the morning sector. Therefore, they argue that the theory of Brice and Lucas cannot account for the morning maximum observed in riometer data. Note that the discussion here does not necessarily lend support to the Brice-Lucas mechanism, but merely incorporates the concept of cold plasma escape into the magnetosphere.

Figure 10 shows the variation of the solar zenith angle as a function of Universal Time for the events illustrated in the previous section. The arrows indicate the approximate time of cessation of pulsating aurora for each of the events. With the exception of the rather anomalous event of 6 February, all of the aurora1 pulsations terminated between approximately 101 and 106” zenith angle. A calculation of the corresponding shadow heights (Lloyd, 1968: shows that these solar zenith angles correspond to heights between 132 and 267 km, as indicated in Fig. 10. This calculation indicates that the Earth’s shadow was passing through the ionospheric E- and F-layers at times corresponding to the cessation of the modulation of low energy particle fluxes.

Taking into consideration the effect of a finite transport velocity for the cold plasma, the effective height regime indicated in Fig. 10 is decreased. Assuming a transport velocity of - 13 km s-* (Wes- cott et al., 1975) and that the region where the modulation occurs lies in the equatorial plane, the

height decrease would be on the order of 60 km. Obviously, if the source of the modulation is located closer to the ionosphere, the decrease of height from that indicated in Fig. 10 would be even less.

Although there appeared to be a consistency among the data relative to the time of pulsation cessation, the relationship between the pulsating aurora and the accompanying radiowave absorption was not as consistent. During the 20 January, 4 February and 9 December events, the riometer attained the quiet-day (undisturbed) level approximately 7 minutes after the pulsations terminated. This suggests that the precipitation mechanism modulating the low energy fluxes may also have been modulating higher energy (E > 25 keV) fluxes as well. Riometer data cannot provide a unique evidence for the presence of such modulation, at best it can only be inferred from the correlation between pulsating auroras and increased radiowave absorption. However, this inference is supported by the good correlation between pulsating auroras and pulsating X-ray events reported by Rosenberg et al. (1971) and Serensen et al. (1973).

Radiowave absorption occurred both before and after a decrease in pulsation amplitude at approximately 16” SDA during the 18 January event. The remaining events were generally quite weak and therefore it was difficult to determine the variations in radiowave absorption. It should be noted that in

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FIG. 10. A PLOT OF SOLAR ZENITH ANGLE (AND EARTH SHADOW HEIGHT) vs UNIVERSAL TIME FOR

CCXLEGE, ALASKA. CURVES FOR THE DATES DISCUSSED IN THE TEXT ARE PLOTI-ED: FOR EVENTS ON

CONSECWTIVE DAYS, ONLY ONE CURVE HAS BEEN DRAWN.


none of the events did the radiowave absorption recover to an undisturbed level before the termina- tion of the aurora1 pulsations. The variation of the radiowave absorption appears to be consistent with the theory of Coroniti and Kennel in that it is more persistent than the modulation of the lower energy fluxes.

The occurrence of micropulsations is a necessary condition for the Coroniti-Kennel model. Although it is unknown if micro-pulsations occurred during the observations presented here, there are a number of observations of the correlation between aurora1 pulsations and both radio-wave absorption (Campbell and Leinbach, 1961) and micropulsations (Campbell and Rees, 1961). Such micropulsations are typically of type Pi 1 whose origin is most likely due to the interaction of the solar wind and the magnetic field in the equatorial plane at several R, (Jacobs, 1970).

SUMMARY AND CONCLUSIONS

The observations reported here can be interpreted in the following manner: precipitation pulsations are generated as a result of micro-pulsations which modulate the whistler mode wave particle interaction as proposed by Coroniti and Kennel (1970). The transport of cold plasma from the sunlit ionosphere into the magnetosphere (Brice and Lucas, 1971) decreases the limit of stable trapping, thereby subjecting additional populations of electrons to the whistler mode instability.

In the morning sector, large fluxes of energetic particles exist having been injected into the magnetosphere as a result of the substorm process. The addition of cold plasma particles at the night-day boundary on the morning side may be insignificant relative to existing populations due to substorm processes. On the other hand, particle populations can become depleted as a result of precipitation along the dayside aurora1 zone and the removal of cold plasma particles could reduce the level of precipitation.

This interpretation is consistent with the observations, assuming that the flow of cold particle fluxes terminates when the ionizing source is removed from the upper ionosphere.

The observation of pulsating aurora in the noon- evening sector of the aurora1 zone is the first such report, although Brekke (1969) has observed pulsating auroras in the day sector from a location near the polar cleft. In a qualitative manner, the correlation between pulsating auroras and increased radiowave absorption is also shown.

Acknowledgements-The observational programme which supplied the data used in this study was-supported by the National Science Foundation under Grant No. 10127 to the Geophysical Institute of the University of Alaska. Much of the analysis was carried out under the auspices of the Royal Norwegian Council for Scientific and Industrial Research while the author held a post-doctoral fellowship at The Aurora1 Observatory of the University of Tromspr. At the University of Calgary this research was supported by National Research Council Grant A-3131.

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650 F. T. BERKEY

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observations of pulsating aurora in the day sector auroral zone

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