On the correlation between pulsating aurora and cosmic radio noise absorption

Planet.

Space Sci. 1971,

VoL 19, pp. 891 to 896.

Pcrgamon

Ras.

Printed

in Northcm

IreLand

ON THE CORRELATION BETWEEN PULSATING AURORA AND COSMIC RADIO NOISE ABSORPTION A. BRFXKE The Aurora1 Observatory, TromsPr, Norway (Received in revisedform 25 June 1970)

Abstract-A statistical analysis of the correlation between aurora1 pulsations and aurora1 absorption at TromsPrhas been performed for the years 1964-68. The two phenomena seem to be completely independent except for perhaps some short periods around midnight and in the late morning hours. A timeshift for the daily maximum of pulsating aurora with the solar activity is truly seen. INTRODUCTION A study of the relationship between aurora1 pulsations and cosmic radio noise absorption may give some information concerning the properties of the precipitated particles and the acceleration mechanisms, especially may a study of this kind contribute to the understanding of pulsating aurora and knowledge about the energies involved. However, the discrepancy between the different observations concerning simultaneity of pulsating aurora and cosmic noise absorption makes it difficult to draw unambiguous conclusions. It is not clear from previous work whether the correlation between pulsating aurora and cosmic radio noise absorption is real or if it could be accidental, i.e. if there is a physical correlation between the two phenomena or if they are independent phenomena occurring at approximately the same time. The purpose of this investigation is to test the statistical correlation between the two phenomena more rigorously. OBSERVATION AND DATA REDUCTION During the four years 1964-68 observations of pulsating aurora have been performed at Tromsra (67.06”N, 116.53”E geom.). The instrument was a zenith photometer with a 5” field of view and a 5577 A interference filter. At the same time a riometer equipped with a 3 element Yagi antenna was operated to record cosmic radio noise absorption at 27.6 MHz. Both the photometer and the riometer were equipped with a 6 in./hr recorder. Due to this low speed the frequency cut-off in the pulsations was about O-1 Hz, and consequently no attempt was made to analyse the frequency. The data were selected from nights when pulsating aurora occurred at some time during the night. The observation time for each such night is divided into 23 min periods. Each period with pulsating aurora is marked 1 and periods without pulsations are marked 0. For the same nights (with pulsating aurora) the riometer absorption was calculated for each of these 2a min periods. Absorption in excess of 0.5 dB is marked 1 and less absorption is marked 0. When A represents the absorption and P the pulsating aurora, there are four possible combinations of events; (I) (2) (3) (4)

A &P-simultaneous absorption and pulsating aurora. P - A-pulsating aurora without simultaneous absorption. A - P-absorption without simultaneous pulsating aurora. ,(AuP)-neither absorption nor pulsating aurora.

The following

symbols

are now found convenient;

n(A &P),

n(P - A),

n(A -P) 891

and

nG(AuP));

A.

891

BREKKE

the total number of 24 min periods within one particular hour for one whole winter of the four combinations of A and P respectively. N = n(A &P) + n(P - ‘4) _t n(A - P) + n(l(AuP)); the total number of 24 min periods within the hour in question when observations have been included in these statistics. The occurrence frequency for one of the four cases, say A &P, is now defined as the ratio of n(A &P) to N. This definition of the occurrence frequency differs from the usual one, since only nights when pulsating aurora does occur are used. The statistical results are shown in Fig. 1 as a function of local time for all winters. Local midnight in Tromso is about 2300 GMT. The bars indicate the r.m.s. deviation which represents a lower limit for the uncertainty since the fact that a particular event usually lasts longer than the time intervals used in this analysis has not been taken into account. This means that the data from succeeding intervals are often not statistically independent. The uncertainty should rather be estimated from the number of events than from the number of measured time intervals. This will increase the uncertainties with a factor of 1-3, and therefore the indicated uncertainties possibly represent too low values. It is apparent from other data available Kvifte and Pettersen (1969, private communication) that the difference in fields of view for the two instruments does not affect the data significantly. The total frequency of pulsating aurora within a field of view between 30”

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1967164

1966/67

1965166

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FIG. 1. THE OCCURRENCEFREQUENCYFOR THE FOUR COMBINATIONS OF PULSATINGAIIRORAAND RADIO NOISE ABSORPTION EVENTSvs. LOCAL TIME, PLOTTEDFOR FOUR MNTERS N TROMSD. The bars indicate the uncorrected r.m.s. deviation.

I

CORRELATION

BETWEEN

AURORAL

PULSATIONS

AND

ABSORPTION

893

south and 45” high to the north is only 1.25 times the frequency in the zenith. A rather odd correlation between these pulsating auroras and absorption would be necessary to inlluence the results. ‘0 1 are combined such that the A-curves are the sum of In the Fig. 2 the curves from Fi,. the (A &P)- and (A - P)-curves, while the P-curves are the sum of the (A &P)- and (P - A)-curves for corresponding years. These A- and P-curves then show the total occurrence frequency of aurora1 absorption and pulsating aurora events separately. The bars also here show the uncorrected r.m.s. deviation. To get a measure of the correlation between aurora1 pulsation and radio noise absorption, the two ratios n(A & P)/n(P - A) and n(A - P)/n(,(AuP)) have been calculated. The physical interpretation of n(A & P)/n(P - A) is the relative occurrence frequency of events with absorption and events without absorption when pulsations occur, while the interpretation of n(A - P)/n(,AuP)) is the correspondin g relative occurrence frequency of events when pulsations do not occur. These ratios are plotted as a function of time for all four winters in Fig. 3. The bars represent the uncorrected r.m.s. deviation. high

to the

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2. THE

TOTAL ABSORPTION The P ctuves are the of the (A - P)- and

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1966167

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OCCURRENCE FREQUENCY OF PULSATING AURORA (P) AND RADIO NOISE (A) VS. LOCAL TIME, PLO’ITED FOR FOLIX WINTERS IN TROMSPI. sum of the (A & P)- and (P - &-curves, while the A curVes are the sum (A &P&curves from Fig. 1. for corresponding years. The bars indicate the uncorrected r.m.s. deviation.

f

a06 0.04 i

1966167

1965166

1967/66

FlG. 3. THE RELATIVEOCCURRENCE FREQUENCY FORTHEFOURCOMBINATIONS OF PULSATING AURORAAND RADIO NOISEABsORP~-~ON EVENTS vs. LOCALnm, PLANKED FOR FOURWINTERS n-4 T~0Ms0. The dots represent the n(A &P)/n(P - A) ratios, while the rings represent the n(A - P)/ n(l(AuP)) ratios and the bars indicate the uncorrected r.m.s. deviation. DISCUSSION Statistical results are often dependent on the method of data reduction.

When the mean

values of two phenomena which usually occur at largely the same times of the night are derived for time intervals substantially longer than the duration of the single events, then a correlation may be found which has no physical significance. tion, the time intervals must be less than the deviation times. generally the case. The curves in Fig. 1 show that absorption

To test true physical correlaIn the present study this is

is a far more frequent phenomenon

than

pulsating aurora in Tromso. This result is also demonstrated in Fig. 2, where the total occurrence frequencies of absorption and pulsating aurora are shown. Figure 3 deserves particular attention. If the ratios n(A & P)/n(P - A) and n(A - P)/n(r(AvP)) were equal, absorption would occur with the same probability, independent of whether or not pulsation would occurs. A greater value of n(A & a)jn(P - A) compared to n(A - P)/n(l(A~P)) mean that absorption

is more probable

when pulsations occur than when they do not.

Figure 3 shows no significant difference between the two curves n(A & P)/n(P - A) and MA - ~)~~(~(~U~)) except perhaps for a short post midnight period during the winters 1964/G and 1966167 when the n(A - P)~~~(AvP)) curve exceeds the n(A & P)jn(P - A) curve, and in the late morning hours during the winters 1965166 and 1967168 when the contrary is seen.

CORRELATION

BETWEEN

AURORAL

PULSATIONS

AND

ABSORF’TION

895

As pulsating aurora often occurs after a break-up around midnight in Tromso, but also occurs without break-up taking place, it is possible that the difference between the n(A -P)/ n(,(AvP)) and n(A & P)/,(P - A) curves around midnight if it is real, is due to the aurora1 absorption related to this breakup displays. If the difference in the morning hours during the one winter 1965/66 is real, it indicates a greater probability for the two phenomena to occur simultaneously, in agreement with Johansen’s results from 1961/62 and 1963/64 in Tromso, Johansen (1965, 1966). These differences, however, are rather doubtful since the estimated correction will increase the uncertainties with a factor of 2 and 3 for the n(A & P)/jz(P - A)- and the n(A - p)/n(r(AuP))-curves respectively. The big differences between the two curves for the morning hours during the winter 1964165 and for the evening hours durin g the winter 1967168 are not significant, as the relative uncertainties for the n(A & P)/n(P - A) points not provided with bars are more than a hundred per cent. We therefore conclude that the two phenomena, pulsating aurora and aurora1 absorption , generally are independent except perhaps for the short periods mentioned above. Figure 2 shows that the occurrence frequency for radio noise absorption is rather constant during the night. However, during the morning hours in the years 1964-66 the occurrence frequency is decreasing, while it increases rather slowly towards a new maximum for the years 1966-68. Since the sunspot number increases from about 10 for the years 1964/65 to about 100 for the years 1967/68, and also the magnetic activity increases with the sunspot number (Chamberlain, 1961), the tendency for the aurora1 absorption to occur in one broad maximum during the first two winters and in two separate ones for the two last winters is felt to be in good qualitative agreement with the recent work of Shepherd and Ecklund (1969). The occurrence frequency of pulsating aurora in Fig. 2 shows a clear maximum for all the winters, but the time of the maximum is delayed from one winter to the next by about one hour. A possible explanation of this may be found in the dependency between the solar sunspot cycle and the aurora1 oval. It has been known for a long time that the aurora1 oval expands and its southern edge moves toward lower latitudes, while the northern border remains almost constant in position during increasing solar activity (Feldstein et al., 1967). To illustrate the problem Fig. 4 shows the position of the aurora1 oval in geomagnetic time at average solar activity, Feldstein et al. (1967) together with the position of Tromss for each hour GMT. The delay of the occurrence maximum with increasing solar activity indicates that pulsating aurora occurs in a pattern which depends on the solar activity like the ordinary aurora1 oval, and which is situated in the southern edge of this between 2400 and 0500 MET, but which is narrower in latitudinal extent. A site passing under the oval will therefore see a sharper maximum in the occurrence frequency of pulsating aurora than in phenomena associated with the general appearance of aurora. That the variation from year to year is systematic and connected to the solar cycle is of course a tentative conclusion only. The explanation given for the maximum in the morning hours is, however, in agreement with Heppner’s analysis (1954) and Kvifte and Pettersen (1969). The opposite time shift of the radio noise absorption and pulsating aurora occurrence

A. BREKKE

896

1200

FIG. 4. THE

l

TROMSd

AURORAL OVAL IN GEOMAGNETIC COORDINATES AT

MEDIUMMAGNETICACrMrY, FELDS~EINet al. (1967). The crossed circles indicate the position of Tromse relative to the oval for each hour GMT.

frequency maxima indicates that the two phenomena are statistically independent phenomena, but since they can both be seen in the night, they are often observed to coincide. CONCLUSION

Absorption of radio noise and pulsating aurora are found to be statistically independent phenomena except perhaps in the earliest morning hours and during the break-up period when absorption is connected to aurora1 displays. The time variation during the night is consistent with the previous work reporting that pulsations, located at the equator-ward side of the oval, mainly occur in the post-midnight hours. Acknowledgements-I am greatly indebted to Professor A. Omholt for helpful advice through all stages o f this work. I would also like to express my gratitude to Amanuensis G. Kvifte for enlighten@ discussions and Amanuensis H. Pettersen for making a computer program available. REFERENCES CHAMBERLAIN,

J. W. (1961). Physics of the Aurora and the Airglow, Znternafional Series, Vol. 2. Academic

Press. FELDSTEIN,Y. I. and STARKOV,G. V. (1967). Planet. space Sci. 15,209. H.EPPNER, J. P. (1954). Thesis Calif. Inst. of Tech. JOHANSEN, 0. E. (1965). Planet. Space Sri. 13,225-235. JOHANSEN, 0. E. (1966). Planet. Space Sci. 14,217-219. Kvrm, G. J. and PETTERSEN, H. (1969). Planet. Space Sci. 17,1599-1607. SHEPHERD, D. C. and ECKLUND, W. L. (1969). J.geophys. Res. 74,2891-2898.