East-west movements of pulsating auroral-zone X-ray events

Journ& of Atmospheric and Terrestrial

Physics, 1968, Vol. 30, pp. 205-212. Pergamon Press. Printed in Northern Ireland

East-west movements of pulsating aurorahone

X-ray events

K. BR~NSTAD and H. TREFALL Universitotet i Bergen, Fysisk institutt, Bergen, Norway (Received 20 Augwt

1967)

Al&a&-During simultaneous recordings by means of balloon-borne X-ray detectors, two oases of pulsating amoral-zone electron precipitation events have been observed, which were moving from east to west along the aurora1 zone. Both events were observed between 07.00 and 09.00 hr L.T. They moved with velocities of 120 and 150 kmjmin, and had periods of 300 and 500 see, corresponding to wavelengths of about 600 and 1250 km respectively. Both events were superimposed upon the beginning phase of a more slowly varying electron precipitation event, and seem to represent modulation effects by some travelling hydxomagnetic wave upon the main precipitation event. 1. INTRODUCTION

electron precipitation events in the aurora1 zones, with pulsating or quasi-periodic time charaete~stics, are not uncommon. Such events have been observed both in riometer recordings of ionospheric absorption effects (e.g. BROWN, 1964), and in balloon recordings of X-ray bremsstrahlung from precipitating electrons (e.g. ANGER, 1963). Periods ranging from several minutes and downwards have been found. The spatial aspects of pulsating electron precipitation events are not well known, because simultaneous observations at widely separated places are then required, and riometers are not always sensitive enough nor do they have quite su~cient timeresolution for such studies. In particular it would be interesting to know whether such events are caused by a spatially stationary but time-varying electron flux, or whether they are caused by the spatial movement of a fixed precipitation pattern across the field of view of the recording instrument. Actually, pulsating events of both kinds seem to occur. EVANS(1963) concluded on the basis of the lack of time variation in the observed X-ray bremsstrahlung spectrum that an event recorded by him, with a period of about 100 see, belonged to the former type. On the other hand, ROSENBERG and BARCUSf 1965) found in~cations of a progressive spatial movement of the pattern of electron precipitation in some slowly pulsating large-scale electron precipitation events, but they were unable to form a consistent picture of such motions on the basis of their observations. In this paper we shall present the results of two sets of simultaneous balloon measurements of pulsating auroral-zone X-ray events, that can only be interpreted as being caused by a spatially progressing pattern of electron precipitation. E~ERGE~C

2. METHODSOF OBSERVATION Bremsstrahlung X-rays from precipitated electrons were recorded by means of NaI(T1) scintillation counters brought to pressure levels of about 10 mb by means of 205

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K. BR~~NSTADand H. TREFAJA

standard high-altitude polyethylene balloons. X-rays of various energies were recorded by means of pulse height discriminators set for levels between 20 and 100 keV. The first recording of a moving pulsating event was obtained in 1964 by means of 2 balloons, which had been launched from the stations Andenes and Alta about 300 km apart, and the second one in 1965 by 3 balloons, launched from Andenes, Tromso

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Fig. 1. Lomtions

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of launching sites and of the balloons during the pulsating X-ray events.

and Alta, Tromso being about midway between Andenes and Alta. All three launching sites are situated in Northern Norway at approximately equal geomagnetic latitudes along the middle of the aurora1 zone, corresponding to a magnetic shell parameter of L M 6. The positions of the launching sites and the approximate positions of the balloons at the times of the pulsating events are shown in Fig. 1. The uncertainties in the balloon positions are believed to be small compared with the fields of view of the detectors, which can be regarded as diffusely defined circles with diameters of about 150-200 km, the effective diameter depending somewhat on the energy of the X-rays being recorded and the floating level of the balloon. When the pulsating events were observed, the balloons had in both cases drifted northwards to L w 6.5, but they were still lying at approximately equal Evalues. All data were telemetered to a common receiving station at Andenes for simultaneous recording there. There is therefore no uncertainity in the relative timing of

Movements of pulsating auroral-zone X-ray avents

l?ig. 2. Observations

of X-rays

Fig. 3. Observations of X-rays

and magnetic variations August 5-6, 1964.

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during the night of

and magnetic variations during the night of 27-28, 1965.

July

data from the different balloons. The absolute timing may not be better than a few seconds, but this is quite sufficient for comparison with recordings from groundbased instruments like riometers, magnetometers and earth current recorders. 3. OBSERVATIONS AND

RESULTS In Figs. 2 and 3 are shown the balloon recordings of all X-ray events which were observed during the nights of 5-6 August 1964 and 27-28 July 1965, respectively, with the intervals of pulaating X-ray fluxes marked off. Magnetic recordings from Andenes are included in the same figures. The density-versus-time curves have been

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Fig. 4. Details of X-ray, CNA and magnetic recordingsduring the 1964 event.

plotted on a logarithmic scale and displaced with respect to each other so as to avoid overlapping of the curves. There is considerable similarity between the X-ray and magnetic activities during the two nights. In both cases a negative magnetic bay occurred around midnight, with associated midnight-type X-ray events. Then there was a period of relative calm, lasting for 3-4 hours, whereafter typical morning X-ray events were observed, during which there was only medium or little local geomagnetic activity. Both pulsating events occurred during the later part of the morning activity, between 0630 and 0830 hr local time, corresponding to local geomagnetic times from approximately 8-10 hours. Figures 4 and 5 show in greater detail the X-ray intensity variations during the intervals of interest, together with details of riometer recordings from nearby stations, and simultaneous magnetic variations as observed at Tromso. During the 1964 event three fairly well-defined intensity peaks, marked A, B and C, can be recognized in the recordings of both balloons as well as in the riometer recordings from Andenes and Tromd, and apparently associated pulsations are seen in the Tromsij recordings of all three geomagnetic field components. During the

Movementsof pulsating amoral-zone X-ray events

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Fig. 5. Details of X-ray, CNA and magnetic recordingsduring the 1965 event.

1965 event, six different intensity peaks, marked A to F, are clearly seen in the recording from the Andenes balloon. Intensity peaks corresponding to A, B, C and D are aIso easily seen in the recordings from the Tromsii and A&a balloons, whereas the existence of peaks corresponding to E and F is more doubtful. Peaks corresponding to A, B, C and D can be seen in the riometer recordings from Andenes and Tromsij too. However, in the 1965 magnetic recordings from Tromso only the H-component shows some slight indication of simultaneous pulsations. Both pulsating events are seen to be fairly periodic. By measuring time separations between successive intensity maxima and successive intensity minima and averaging over all recordings incIu~g the riometers, we find periods of about 500 set and 300 set for the 1964 and 1965 events, respectively. It is also clear that in both cases the intensity peaks appear first in the recordings from the easternmost balloon or riometer. What are corresponding intensity peaks in the different recordings is not open to doubt, for in both cases the pulsating event marks the beginning of a new general increase of X-ray flux, and the first intensity 3

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K. BR~NSTADand H. TREFALL

peak of each event can easily be identified. Also, maximum similarity between the shape of corresponding peaks is obtained with the chosen identification. On the basis of the balloon recordings alone, we find an east-west velocity component of 150 km/min for the 1964 event, and 120 km/min for the 1965 event. Comparison of the riometer recordings from Andenes and Tromsij gave 95 km/min in 1965, but in 1964 the timing of the Andenes riometer recordings was somewhat uncertain, and not good enough for any velocity determination. Only in 1965, when the balloons lay significantly north of the riometer stations (at L M 6.5 as compared with L M 6 for the riometers), could any north-south velocity component have been determined with a reasonable accuracy, but this was found to be very much smaller than the east-west component and possibly non-existent. It thus appears that in 1965 we had a nearly periodic pattern of electron precipitation which was moving from east to west along the aurora1 zone, and we shall assume the same to hold for the 1964 event too. This is also supported by the fact that in neither case did the Kiruna riometer show intensity variations corresponding to those observed by the balloons and the Andenes and Tromsij riometers. It thus appears that these pulsating events were limited to and moved along the northern part of the aurora1 zone. On the basis of the observed periods and speeds of propagation, the corresponding wavelengths are found to be about 1250 km and 600 km for the 1964 and 1965 events respectively. Both pulsating events were superimposed on more slowly varying precipitation events, which suggests that the pulsating events are the results of intensity modulation of the electron precipitation. This is supported by the fact that the X-ray energy spectrum varied only little during the pulsating events and also during the rest of the associated slowly varying precipitation event. On the other hand, a marked softening of the energy spectrum was observed when the pulsating events began, indicating that these do indeed mark the beginning of fresh electron precipitation events, which continued after the observed pulsations had died away. 4. DISCUSSION The two events show many similarities and appear to be of basically the same type. They had comparable periods and speeds of propagation, they both occurred on the morning side of the earth, they were both associated with similar longerlasting precipitation events, and both appeared to move westwards along the aurora1 zone. The apparently associated geomagnetic pulsations which in 1964 started at the same time as the pulsating electron precipitation event, suggest that the modulation of the electron precipitation event may have been caused by some travelling hydromagnetic wave. In 1965 the situation is less clear, since there is only a slight indication of associated geomagnetic variations. This may however be due to the fact that the balloons were then much farther away from Tromsij than in 1964, and that TromsG may have been lying near the border of the event, which agrees with the fact that the pulsations observed by the Andenes and Tromsij riometers were much less pronounced in 1965 than in 1964. If we are really seeing the effect of some travelling hydromagnetic wave, it might

Movements of pulsating amoral-zone X-ray events

211

be possible to find related effects in recordings from other stations within the aurora1 zone. We have examined magnetic recordings from Juliannehaab (Greenland), Fort Churchill (Canada) and College (Alaska), but only in the recordings from College are there indications of geomagnetic pulsations which may be associated with the event which we observed in Northern Norway. In 1965 pronounced pulsations were observed in the H-component at College, starting at about 0615 U.T. At the same time a sharp increase in cosmic noise absorption was also observed by the College riometer, and very intense earth current ALASKAN

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Fig. 6. Riometer recordings from Alaskan stations around the time of the 1964 pulsating event.

activity suddenly started. In 1964, there is no such clear indication of magnetic pulsations in College, but a significant increase in cosmic noise absorption was observed at 0505 U.T., and earth current activity started this time too. Riometer recordings from several Alaskan stations during the relevant time intervals are shown in Figs. 6 and 7. However, in both cases the activity at College started approximately 30 minutes before the pulsating event was observed by our balloons. Rather than being associated with these, the College activity may have been associated with the slowly varying p~cipitation events that preceded the pulsat~g events, as there is close agreement in time between the onsets of these events in Norway and the absorption events in Alaska. This is in agreement with previous findings about SVA-events (BEWERSDORFFet al. 1966).

On the other hand, in 1965 pulsations of roughly 300 set period did occur in the College riometer recording between 0630 and 0650 U.T., or just 10 minutes before similar pulsations were observed by the Norwegian balloons, and it seems remarkable that the period as we11as the duration of these pulsations were the same in Alaska as in Norway. This could be explained by assuming that somewhere in the aurora1zone between Alaska and Norway a source of hydromagnetic waves had been active for a limited period and emitted waves in both directions, which gave rise to nearly periodic modulation of the electron precipitation both over College and over Northern

K.

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BR~NSTAD and H. TREFA.LL

Norway. With a speed of propagation of 120 kmlmin, and an east-west direction on the morning side of the Earth, the source must have been located on the dayside at 13-1400 hours L.T., or not far from the noon meridian. If this tentative suggestion is correct, the reason why no similar pulsations were observed by Alaskan riometers in 1964 may simply be that on that occasion there was no electron precipitation over Alaska which could be modulated, as is apparent from Fig. 6. These observations may also be connected with the previously observed type of stationary large-scale pulsating X-ray events around local noon (ULLA~~D et al., 1967), as they might be the direct manifestation of the primary hydromagnetic disturbance. I,

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Fig. 7. Riometer recordings from AIaskan stations around the time of the 1965 pulsating event.

Acknowledgements- The authors are indebted to the Directors of the Aurora1 Observatory in Tromso, the Kiruna Geophysical Observatory and the Geophysical Institu~ of the University of Alaska for permission to use magnetic and ionospheric recordings. Nr. J. BJORDALconstructed the payloads for the 1964 baboon flights, and we are also indebted to 8Ir. S. ULLALAND, Mr. J. STADSNES and Mr. H. HANSSEN for significant contributions to the balloon programmes of 1964 and 1965. The balloon flights were carried out with financial support from the NATO Science Committee in Paris, Norges Teknisk-Naturvitenskape~ge Forskningsrad, and U.S. Air Force Office of Aerospace Research under Contract No. AF 61(052)-~69. ANGER C. D., BARCUS J. It., BROW R. R. and EVANS D. S. BARCUSJ. R. and ROSENBERGT. J. BE~SDO~FF A., KREXSER G,, RIEDLER W. and LEGRANDJ. P. BEOWN R. R, EVANS D.S. ULLALAND S. L., TREFALLH., KREMSERG. and BEWERSDORFF A.

REFERENCES 1963 J. Geophys. Res. 68, 3307-3310. 1965 1966

J. Geophys. Res. ‘PO, 1707-1716. Ark. fiir i?eo&q8. 5,mr. 16, 115-127.

1964 1963 1967

J. Geophys. Res. 69, 2315-2321, J. Geophys. Res. 68, 395400. J. dtrnosph. Xew. Phye. 29, 395410.