Triggering of a magnetospheric substorm by a sudden commencement of a geomagnetic storm observed by the x-ray experiments of SBARMO-79 and the charged particle spectrometer on GEOS-2

Adv. Space Res. Vol. 1, pp. 273—278. © COSPAR, 1981.

0273—1177/81/OIOI--0273$O5.OO/O

Printed in Great Britain.

TRIGGERING OF A MAGNETOSPHERIC SUBSTORM BY A SUDDEN COMMENCEMENT OF A GEOMAGNETIC STORM OBSERVED BY THE X-RAY EXPERIMENTS OF SBARMO-79 AND THE CHARGED PARTICLE SPECTROMETER ON GEOS-2 S. Ullaland,* J. Bjorda1,~L. P. Block,** K. Brønstad,* I. B. Iversen,*** J. Kangas,÷A. Korth,+ + G. Kremser,~+ M. M. Madsen,*** T. Moe,* W. RiedIer,~+ + J. Stadsnes,~P. Tanskanen, + and K. M. Torkar* * University of Bergen, Bergen, Norway **Royal Institute of Technology, Stockholm, Sweden ~ Space Research Institute, Lyngby, Denmark + University of Oulu, Oulu, Finland + ~Max-Planck Institutfür Aeronomie, Katlenburg-Lindau, FRG + + + Technical University Graz, Graz, Austria I Space Research Institute of the Austrian Academy of Sciences, Graz, Austria

ABSTRACT Balloon observations of X—rays produced by electron precipitation were made in the evening sector of the auroral zone at the time of the geomagnetic storm sudden commencement (SSC) of July 6, 1979. Seven instrumented balloons floated in an area reaching from 2~°E to 200 W through L—values from 5.2 to 11.0. X—ray observations with high tine and spatial resolution together with the charged particle observations are analysed showing the dynamics of the precipitation following the SSC. INTRODUCTION Energetic particle precipitation in the auroral zone has been observed by polar orbiting satellites and rockets, by balloons via X—ray measurements and by ground based instruments such as riometers and photometers. Most of the precipitation is associated with magnetospheric subatorms, (see review paper by McPherron [1]). It is well known that substoruis are controlled by the interplanetary magnetic field (IMF), although the causal relationship between the triggering of a substorm expansion phase and changes in the IMF is not fully understood. SSC pressure pulses, however, are known to be able to trigger substorms [2] and [3]. In the present investigation we will use data obtained during the onset of the expansion phase of a substorm apparently triggered by a SSC. We will use data from the balloonprogram SBARMO—79 [4) and the geostationary satellite GEOS—2 [5]. OBSERVATIONS Magnetospheric electrons and ions were observed by the charged particle spectrometer on GEOS—2 (6]. Auroral X—ray fluxes were measured by omnidirectional X—ray spectrometers as well as with high spatial resolution by the Multi—Experiment Payload (MEP) [4]. In this payload auroral X—rays are measured by an array of seven collimated detectors. 273

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A geomagnetic storm sudden commencement (SSC) occurred at 1930:00 UT on July 6, 1979. During that period seven instrumented balloons of the SBARMO—79 program were aloft in the area from 25°E to 20°W (1900 to 220.0 MLT) through L—values from 5.2 to 11.0. Figure 1 shows the gross features of the X—ray events on July 6, 1979, from 1200 to 2400 UT together with ground observations from Andenes. As seen an expansion of a magnetospheric substorm started close to the occurrence of the SSC. It can also be seen that the situation during the 2—3 hours before the SSC was quiet except for the minor electron precipitation over 11o07067911 and Ka07067909, a feature suggesting prebay precipitation [7]. Figure 2 shows details of X—ray recordings together with GEOS—2 electrons and ions and the TromsØ pulsation magnetometer. Balloon positions marked as fields of view of the X—ray detectors and the nominal foot print of the geomagnetic field lines through GEOS—2 at 1930 UT are also shown. Note that on the map in figure 2 we have marked the MEP—experiment Ka0706 7909 with the fields of view (at the X—ray production layer) of the seven collimated X—ray detectors. The field of view of the zenith detector is 80 km in diameter and that of the six tilted ones covers the surrounding region within a radius of 150 km. The pulsation magnetogram defines the onset of the SSC to be at 1930:00 UT for this region. An increase of electron precipitation can be observed over the Ka07067909 balloon from about 1931:00 UT. The ion flux at synchronous orbit responded almost directly to the SSC. The increase of electron flux at synchronous orbit began somewhat later, at about 1931:30 UT. The GEOS—2 electron and proton flux reached a plateaulike level about two minutes after the SSC. The precipitating electrons first reached a maximum at about 1934 UT (Ka—balloon) and a minimum at about 1934:20 UT. Thereafter an impulsive event started at about 1934 :42 UT on the Ho07057909 balloon and at about 1935:00 UT on the Ka07067909 and Ho0706791l balloons. These features indicate auroral fading (the temporary minimum) [7, 8] and onset of expansion phase (the impulsive event) [9]. The longitudinal separation between Ho07057909 and Ka07067909 / }1o070679ll is 700 km . Assuming that the time difference of about 20 s in onset times of the impulsive event reflects an eastward expansion of the precipitation region, the speed is in the order of 30—40 km/s . Very weak traces of the impulsive precipitation event can also be seen on the three So—balloons. The electron flux at synchronous orbit increased strongly from about 1943 UT onwards and exhibited strong temporal variations.

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The temporary maximum at about 1934 UT and the impulsive event starting at about 1935 UT will now be studied in more details, especially with the aid of the MEP experiment on Ka07067909. I’’’’’ c/s

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Figure 3 shows the count rates (6 s averages) of the six tilted collimated X—ray detectors on Ka07067909 together with the orientation of their fields of view at 1933:15 UT, 1933:51 UT, 1934:21 UT and 1935:03 UT. At the temporary maximum the electron precipitation over the Ka—balloon was stronger to the south and southeast (detectors 2, 3 and 4) than to the north and west (detectors 1, 6 and 5) where approximately cosmic ray background level was observed. At the time of auroral fading this spatial asymmetry disappeared and the sky over the Ka—balloon was homogeneously X—ray illuminated with a flux of about two times the background level. Later on, during the beginning of the impulsive event over the Ka—balloon, the precipitation was strongest on the northern sky. If this south—north change in the electron precipitation over the Ka—balloon can be related to a poleward expansion

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of the precipitation region, the speed is of the order of 100 km/20 s = 5 km/s Figure 4 shows the 0.5 s average counting rates of the same detectors as in figure 3 from 1934:58 UT until the maximum of the impulsive event. During the upleg of the event detector 1 (in NW) leads detectors 6 and 2 with about 2 s and detectors 3, 4 and 5 with another 2 s. The time differences between the flux increases of the different detectors could be explained by the arrival of an elongated precipitation region approaching from northwest with a speed of the order of 70 krn/2 s — 35 km/s. INTERPRETATION

The main features of the observations seem to be as follows: At the balloons 81) The magnetospheric substorm was triggered by the SSC. 82) The speed of the poleward expansion around the auroral fading was of the order of S km/s B3) The speed of the eastward movement at the onset of substorm expansion was of the order of 35 km/s At synchronous orbit Si) The ions respond nearly directly to the SSC S2) The electrons respond to the SSC about 90 s after the ions. The intensity of the electron flux with 90°pitch angles Ce 1) only, is affected in the beginning of the event. S3) The electron flux increased strongly about 8 minutes after the substorm onset. There was no ion flux increase at that time. The result under B2 is in good agreement with the results of Pytte et al. [9] and reflects processes at the outer boundary of the expanding plasma sheet. The observations which led to the eastward movement of the precipitation regions (83) indicates that in this case the injection of energetic electrons began in the evening sector around 1900 MLT and expanded towards midnight with a speed of about 30—40 km/s . The increase in e1 only, indicates betatron acceleration shortly after the SSC at synchronous orSit. We think that the whole electron precipitation which followed the SSC was associated with the magnetospheric substorm. The strong increase of electron precipitation at substorm expansion onset and the strong increase in electron flux at GEOS— 2 about 8 minutes later, probably indicates that the balloon and GEOS—2 observed processes originating from different regions of the magnetosphere. We assume that the electron precipitation at the substorm onset originated from the outer boundary of the plasma sheet. GEOS—2 (at RE 6.6) on the other hand could not see the outer boundary of the plasma sheet. During the beginning of the expansion phase the inner boundary of the plasma sheet may have moved inward and encountered GEOS—2 about 8 minutes after the substorm onset. Pellinen and Heikkila [8] suggested that the fading of electron precipitation just prior to the breakup initiates the substorm expansion onset, implying that the behaviour of the magnetospheric electric field can be a trigger mechanism. As an SSC is known to be able to trigger substorms, the present observations indicate that the SSC event may have triggered the magnetospheric substorm via an influence on the magnetospheric electric field. This possibility will be a subject for further investigation.

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5. Ullaland et at. REFERENCES

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R. L. McPherron, Rev. Geophys. Space Res. 17, 657 (1979).

2.

S. Kokubun, R. L. McPherron, C. T. Russell, J. Geophys. Res. 82, 74 (1977).

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R. R. Brown, J. Geophys. Res. 83, 1169 (1978).

4.

S. Ullaland, J. Bjordal, L. P. Block, K. Br~nstad,A. Egeland, J. Holtet, J. Kangas, G. Kremser, M. M. Madsen, T. Moe, W. Riedler, H. Slamanig, J. Stadsues, K. H. Saeger, P. Tanskanen, E. Thrane, Proc. Vth ESA—PAC Symp. 235 (1980).

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K. Knott, ESA/ASE Sci. Tech. Rev. !, 173 (1975).

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B. Wilken, A. Korth, E. Keppler, Z. Flugwiss. Weltraumforschungi, 298 (1977).

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T. Pytte, H. Trefall, G. Kremser, L. Jalonen, W. Riedler, J. Atuos. Terr. ~

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R. J. Pellinen, W. J. Heikkila, J. Geophys. Res. 83,4207 (1978).

9.

T. Pytte, H. Trefall, G. Kremser, P. Tanskanen, W. Riedler, J. Atmos. Terr. ~ 38, 757 (1976). ACKNOWLEDGEMENTS

The SBARNO—79 program was supported financially by the Austrian Science Research Council, the Finnish National Research Council of Sciences, the Royal Norwegian Council for Scientific and Industrial Research (NTNF), Space Activity Division, the Swedish Board for Space Activities, the Austrian Academy of Sciences and the Deutschen Forschungsgemeinschaft. The GEOS—experiment was supported by the Max—Planck—Gesellschaft zur F’6rderung der Wissenschaften and by the Ministerium f~rForschung und Technologie through the DFVLR—BPT under contract No. RV 14—812/73 (WRK 243) — SF21.