On radio-star visibility fades and the aurora

Planet.SpaceSci. 1968.Vol. 16.pp. 1155 to 1159.

Pergamon

Press.

Printed

in Northern

ON RADIO-STAR VISIBILITY AND THE AURORA

Ireland

FADES

EDWARD J. FRRMOUW* Consultant,

Geophysical

Institute,

University of Alaska, College, Alaska 99701

(Received 17 April 1968) Ah&act--Evidence is presented showing that VHF visibility fades at high latitude are a direct manifestation of the aurora1 disturbance phenomenon, at least at night near solar minimum. Photometers were employed to record aurora1 luminosity near the line of sight to a radio star. Auroral light was detected during all visibility fades that occurred under clear-sky conditions during six and one-half months of observation. A detailed discussion is given of an event that displayed particularly close relation between development of line-of-sight luminosity and measured changes in the structure of the scattered radio wave. INTRODUCTION

The relation between radio-star scintillation and the aurora has been investigated by workers observing both from middle and aurora1 latitudes. Moorcroft and Forsyth (1963) have discussed the ambiguities and partial contradictions that have arisen in much of this work. They concluded that the difficulty lay largely in the past practice of concentrating on amplitude scintillation when aurora1 comparisons were attempted. Their own work showed a rather close relation between scintillation and aurora, especially for line-of-sight aurora. Moorcroft and Forsyth found a particularly close relation between strong line-of-sight aurora and severe phase-dominated scintillation events that they called ‘radio-star fadeouts.’ Fremouw (1968), calling these events ‘visibility fades,’ has described a technique for interpreting them quantitatively in terms of ionospheric optical depth (for scattering) and the autocorrelation function of phase in the scattered wavefront. While the former authors and the latter differ in interpretational details of the roll played by amplitude fluctuations in these events, they agree that the dominant factor is phase fluctuations (Fremouw and Lansinger, 1967). The purpose of the present paper is to explore further the relation between fades and line-of-sight aurora. GENERAL RELATIONSHIP

Moorcroft and Forsyth compared fade recordings from a phase-switch interferometer operating at 53 MHz on an East-West baseline of 50 wavelengths with all-sky camera films. They found that over 90 per cent of the I-hr periods during which strong line-ofsight aurora occurred also had fadeout periods. Thus, ‘strong’ aurora was found to produce conditions conducive to the occurrence of visibility fades. They did not comment on the converse relationship. In the present work, several East-West interferometers were used. The prime frequency was 68 MHz, with spacings of 25, 50 and 75 wavelengths employed. In addition, a 137.6 MHz instrument on a spacing of 101 wavelengths and a 223 MHz receiver on a spacing of 163 wavelengths were operated. All antennas tracked the radio star Cassiopeia A, which is circumpolar at the observing site near College, Alaska. * Permanent address: Radio Physics Laboratory, Stanford Research Institute, Menlo Park, California 94025, U.S.A. 1155

EDWARD

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J. FREMOUW

During part of the aurora1 observing season of 1964-65 and during most of the 1965-66 season, two-channel aurora1 photometers were mounted on one of the tracking antennas. In February, March, and April of 1965, one-degree circular fields of view were employed, using narrow-band filters centered at 5577 A and 4278 A. From October of 1965 through April of 1966, three-by-ten-degree rectangular fields were employed, using narrow-band filters centered at 4278 A and 4858 A (hydrogen beta). The narrow fields of view were used the first year in order to restrict reception of i~ormation as nearly as was feasible to the direction of the radio star under track. The wider fields were employed in 1965-66 in order to achieve sufhcient sensitivity for detection of the relatively weak hydrogen emission. In order to have consistent data on the two channels, identical fields were employed. In all instances, the fields of view were centered along the line of sight to the radio source. When the photometer passbands were centered at 5577 and 4278 A, essentially identical traces were obtained (aside from differences in absolute intensity~. When 4278 and El, were observed, considerable differences were noted, although the two emissions usually were detected together. For the present discussion, we shall consider only the 5577 and 4278 A observations. Records from six and one-half months of line-of-sight photometric observations have been inspected. During this period, 52 visibility fades were observed (at 68 MHz) while the photometers were in operation. The photometers detected aurora1 emission during 47 of these events. During the remaining 5 events, the sky was overcast. The College all-sky camera detected aurora through the clouds on 2 of these 5 occasions. Thus, during 90 per cent of the visibility fades, aurora1 light was detected along the radio line of sight; during an additional 4 per cent, there definitely was aurora in the sky, possibly in the line of sight; and on the remaining 6 per cent, it was impossible to detect aurora because of cloud cover. It seems very likely that aurora was present along the line of sight to the radio star during 100 per cent of the visibility fades observed. CLOSE

DETAILED

RELATION

DURING

A PARTICULAR

EVENT

The detailed relationship between radio-star scintillation activity and individual aurora1 forms appears to be quite complicated, on the whole. In individual cases, however, it can be rather close. Fremouw and Lansinger (1968a) have reported two radio-star refraction events directly related to passage of aurora1 arcs through the line of sight to the source. In addition, Benson (1960) described two cases of close temporal relationship between scintillation events and passage of active amoral arcs through the line of sight. One of Benson’s events was a visibility fade, which he termed a ‘long-duration fade.’ In a number of instances, the photometric observations reported here also have revealed a close relation between development of fade-producing irregularities and development of line-of-sight aurora1 luminosity. One of the clearest of these is shown in Fig. 1, which displays the photometer records for the period of interest along with records from several of the radio interferometers. The top strip is a real amplitude record obtained by simple detection of the audio signal from the 68 MHz phase-sweep inte~erometer operating with a spacing of 220 m (50 2). Next are shown the two photometer records. Three complex-amplitude interferometer records appear below the photometer records. They are, from top to bottom, from the 223, 137 and 68 MHz coherently detecting interferometers having baselines of approximately 220 m. (Note that the time scale on the top three records is different from that on

FIG.

1. J%,&wLB

OF CLOSE ASSOCIATION OF A VISIBILITY FADE TO LINE-OF-SIGHT AURORAL IIWENSITYr NIGaT OF

17/18 .‘iPU,

l%i5,

1156

ON RADIO-STAR

VISIBILITY FADES AND THE AURORA

1157

the bottom three. The chart speed for the radio amplitude and the photometer traces was about three times that for the coherently detected interferometer traces. The records are aligned in time at 2336). The 68 MHz records display considerable scintillation activity throughout the period shown. Between about 2330 and 2400 (150° WMT), the interference fringes in the bottom strip are essentially lost, which constitutes the visibility fade in question. The time of

I! 0’

I 2320

Autorol Peak I II 2330 2340

‘n I 2350

I 0000

I 0010

I 0020

I 0030

150” WMT FIG.

2.

DEVELOPMENT OF THE PHASE AUTOCOFCRELATIONFUNCTION, P&E),

AND THE 68 MHz

OPTICAL DEPTH, @, FOR THE EVENT OF FIG. 1.

commencement of the fade is most readily determined from the 223 MHz record, it being the least sensitive to prior scintillation activity. The 223 MHz interference fringe starting approximately at 2332 was the first one to be reduced in average amplitude. The time at which the fringe reduction began is marked by the left-most arrow on the 4278 A photometer trace. This is seen to correspond closely to a brightening of the line-of-sight aurora1 intensity. (The photometer outputs were related approximately logarithmically to aurora1 intensity.) The next 223 MHz fringe recovered amplitude, but within a few seconds of 2335 the major fade effect commenced. The major effect lasted for 1Qmin, the period between the center and right-most arrows on the photometer trace. This period corresponded closely to the period of brightest line-of-sight aurora. Simultaneously, the 68 MHz real-amplitude trace shows a very noticeable decrease in fluctuation. When taken together with the decrease in fringe visibility, this fact implies a loss of correlation between amplitude fluctuations at the two antennas of the 68 MHz, 220-m interferometer. There is an attendent small decrease in the average level of the realamplitude trace, in accord with a prediction of the amplitude distributions developed numerically by Fremouw and Lansinger (1967) under the assumption of a randomly phased angular spectrum. Figure 2 shows the development of ionospheric optical depth, @, and the one-dimensional

1158

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J. FREMOUW

autocorrelation function of phase in the scattered wavefront, p&). These quantities were deduced from the 68 MHz observations by the method described by Fremouw (1968). The scale of phase i~e~l~ti~ in the wavefront varied from about 50 to about 290 m during the time that it could be measured. The present event showed a decrease in scale size during the period of peak optical depth, in contrast to some visibility fades (Fremouw, 1968). Quasi-periodic phase structure was present during at least a portion of the event, for it would be impossible to draw monotonically decreasing autoco~elation functions (such as the Gaussian) through the data points of the third and fourth periods without departing from the probable-error bars. Quasi-periodicity could have been considerably more well developed in periods two through four than that shown in the solid autocorrclation functions, Alternative functions, displaying greater periodicity, are indicated by the broken lines for instance. The spatial periods associated with the suggested quasi-periodicity are on the order of 300 m. Within the time resolution allowed by the analysis technique, Fig. 2 shows that the peak in optical depth coincided with the peak in line-of-sight aurora1 intensity. The original records shown in Fig. 1 indicate that the time correspondence between aurora1 luminosity and the radio effects actually was much closer still. It appears that the optical depth increased essentially as an impulse (as indicated by the dashed alternative curve for p in Fig. 2) upon arrival of a strong burst of auroral electrons in the line of sight, The increase in optical depth implies either an increase in the geometric thickness of the scattering layer, an increase in the strength of ion-density irregularities contained in the layer, or both. The College all-sky camera film exposed on the night in question shows considerable aurora through varying thin overcast during much of the night. Immediately prior to the visibility fade under discussion, the aurora was concentrated in a band near the zenith. The fade coincided with a no~hward sweep of the aurora, followed by a sudden b~ghtening of a large part of the sky. The radio star under observation was located north of College in the suddenly luminous part of the sky. While a large portion of the sky was involved in the aurora1 activity, the photometer records shown in Fig. 1 relate to a one-degree cone centered on the radio star. The effective radio resolution is determined by the apparent angular size of the source under observation. Under undisturbed conditions, the angular diameter of Cas A is about four minutes of arc. During the visibility fade, ionospheric scattering increased the apparent diameter to a value comparable with the fringe width of the interferometers employed. This means that radio information was being received from an East-West angle also on the order of a degree. Thus, the radio and optical information relate to the same small region of the ionosphere. CONCLUSION

The prime objective here has been to establish whether high-latitude visibility fades constitute an aurorally produced or associated phenomenon, per se, or rather simply represent a general characteristic of the ionosphere in the aurora1 zones. At least for nighttime fades near solar minimum, we conclude that the first alternative is the correct one. Near solar minimum, night-time formation of ionospheric irregularities small enough, strong enough, and numerous enough to produce visibility fades at 68 MHz and above, with interferometer spacings of a few hundred meters, requires the influx of auroralproducing electrons.

ON RADIO-STAR

VISIBILITY

FADES

AND THE AURORA

1159

The scale sizes measured in the present work (see Fremouw and Lansinger, 1968b, for summary) are comparable to the lateral dimensions of certain aurora1 rays (Davis, 1967). It seems likely that the fade-producing structure is related rather closely to the visible structure of aurora1 forms. Acknowledgements-This under grant GP-5540.

work was supported

by NASA under research contract NASS-3940 and by NSF

REFERENCES BENSON,R. F., Effect of line-of-sight aurora on radio-star scintillations, J. geophys. Res. 65, 1981-1985, 1960. DAW, T. N., Cinematographic observations of fast aurora1 variations, Aurora and Airglow, pp. 133-141. Ed. McCormac, Reinhold, 1967. FREMOUW, E. J., Measurement of phase variance and autocorrelation during radio-star visibility fades, J. geophys. Res. 73, No. 11, 1968. FREMOUW,E. J. and J. M. LANSINGER,Interferometer phase and amplitude measurements for determining coherence ratio and wavefront correlation. Rad. Sci. 2.947-954. 1967. FREMOUW,E. J. and J. M. LANSINGER,VHF refraction by visible &oral forms, J.geophys. Res. 73, No. 9, 1968a. FREMOUW,E. J. and J. M. LANSINGER,Radio-star visibility fades in Alaska near solar minimum, J.geophys. Res. 73, No. 11, 1968b. M~~RCROF~, D. R. and P. A. FORSYIX, On the relation between radio-star scintillations and auroral and magnetic activity, J. geophys. Res. 68,117-124, 1963.