Electron precipitation associated with a sudden commencement of a geomagnetic storm

Journalof Atmospheric andTerrestrial Physics,1970,Vol. 32, pp.1545-1553.Pergamon Press.Printedin Northern Ireland

Electron precipitation associated with a sudden commencement of a geomagnetic storm S. L. ULLALAND Fysisk institutt, Universitetet i Bergen, Bergen, Norway K. Max-Planck-Institut

WILHELM* fur Aeronomie, Lindau, Germany

J. KANGIAS~ Laboratoire de Physique Cosmique, Meudon, France W. RIEDLER~ Geofysiske Observatorium, Kiruna, Sweden (Received 13 _R’e7iruaq 1970) Abstract-Balloon observations of X-rays produced by precipitated electrons were made in the morning sector of the aurora1 zone at the time of the geomagnetic storm sudden commencement (SSC) of July 27, 1900. The impulsive precipitation event lasted 4 mm, both the rise and fall times being nearly 1 mm. On a shorter time scale a pronounced variation with a period of l%-I.9 set existed, which occurred together with hm-emissions of the same period range. Besides the rapid fluctuations in the precipitation a 50-set period was also present. The energy spectrum of the observed X-ray flux was rather steep, characterized by an e-folding energy of

E,, = 18-22 keV. The SSC apparently triggered a polar magnetic substorm in the midnight sector of the aurora1 zone. 1. INTRODUCTION

storm sudden commencement (SSC) itself is characterized by a sudden increase (seen on normally run magnetic records) of the North-South (N-S) component of the geomagnetic field at low latitude stations. This step-like increase amounts from a few gammas up to 100 y. ONDOH (1963) has shown that the SSC’s have rise-times of 36-130 set at noon and 200-300 set at midnight. The behaviour of the N-S component during an SSC is more complicated at stations in the aurora1 zone, SEE’s occurring during the forenoon hours normally start with a positive pulse there, followed by a negative pulse, whereas in the afternoon an increase is the prevailing feature (OBAYASI-II and JACOBS, 1957). Previous studies by means of riometer observations of the cosmic noise absorption (CNA) have shown that an SSC is associated with electron precipitation into the atmosphere along a narrow belt at about L = 6 (BROWN et al., 1961; MATSUSHITA, 1961; and ORTNER et aE., 1962). The longitudinal extension of the precipitation is very large, the flux having a maximum around noon and a minimum near midnight. The enhancement of CNA occurs as an impulsive event, lasting only a few minutes immediately after the onset of the SSC. More detailed information on the time structure and the energy spectrum during SSC precipitation events can be obtained by observing bremsstrahlung X-rays

THE UEOMAONETIC

* Now at ESRO Headquarters, Neuilly, Franoe. t Now at the Department of Physics, University of Oulu, Oulu, Finland. $ Now at the Technische Hochschule Graz, Graz, Austria.

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produced by the precipitated electrons. Up to now only four observations of SSCassociated X-ray events have been reported, two of them belonging to the same event (ANDERSON, 1958; BROWN et al., 1961; KIEPPLERet al., 1962; and HOFMANNand WINCKLER,1963). Since X-ray observations during SSC conditions are rare, two balloon measurements shall be mentioned that revealed enhanced X-ray fluxes associated with magnetic sudden impulses (S1). The first event occurred at about local noon and was relatively uns~uetured in time. The X-ray energy spectrum was steep with an e-folding energy &, ~zi13 keV (BROWN, 1967). The other observation was performed during sudden impulses in the initial phase of a sudden commencement storm at about 0400 LT (BARCUS,1968). The X-ray flux was softer, characterized by .EOr? 10 keV. It is remarkable that on this occasion no conspicuous precipitation was observed at the time of the SSC. Satellite measurements have by their very nature contributed little to our knowledge of SSC-associated electron precipitation, There exist however some observations of energetic electrons during SSC’s made far away from the Earth. KONRADI(1968) reported an increase of trapped > 20 keV electrons at the time of an SSC near L = 8 at 0600 LT in the equatorial plane. The e-folding energy, & cri.15 keV, did not change during the event. Most of the SSC’s are not only accompanied by electron precipitation, but also by hydromagnetic (hna) emissions in the dayside aurora1 zone (TROITSKAYAet at., 1962; TEPLEYand WENTWORTH,1962; and HEACOCKand HESSLER,1965). These observations have been confirmed by KOKUBUNand OGUTI(1968), who also found that the emission of electromagnetic radiation in the VLF band <4 kHz is closely related to the SSC. 2. ORSERVATIONS High-altitude balloon flights were performed from Andenes, Kiruna and Sodankylg in Northern Scandinavia during July and August 1966 (WILHELMet al., 1967). Each payload contained a NaI(T1) scintillation detector (crystal thickness and diameter each 2.54 cm), coupled to a B-channel integral pulse-height analyser, and in addition a Geiger-Miiller telescope. Two of these detectors were at ceiling altitude, when the SSC occurred on July 27 at 0603 UT. The balloon launched from Andenes was located at L cz 6 and floated at 4.5 g/cm2 atmospheric depth, the Sodankyla balloon at L ~1 5 and 9 g/cm 2. The East-West separation amounted to approximately 600 km. The SSC of July 27 cannot be associated with any known solar flare. The solar activity on the whole was low and consequently the period before the SSC was geomagnetically quiet with &, = 2 and 1 in the two 3-hr intervals before the SSC, and lir, = 3 in the subsequent interval. In Fig. 1 the li or X components are shown of magnetograms recorded at four aurora1 zone stations and one equatorial station, covering the time 0200-1200 UT. At the time of the SSC a 15 y increase in the H component in 1 min was observed at Guam, which was located near local noon. The magnetograms of the aurora1 zone stations Tromsla and Kiruna near 0700 LT both show a positive impulse of 25 y and 30 y, followed by a negative-going impulse of 40 y and 25 y, respectively, whereas in the evening sector an increase of

Electron p~i~it&t~on

aesociated with a sudden co~encement

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Fig. I. Records of the N-S component of the geomagnetio field from various stations covering the time around the SSC on July 27, 1966.

observed at College, in accordance with the findings of OBAPASHI and In the mid-night sector of the aurora1 zone represented by Fort Churchill a polar magnetic substorm was already in progress. Therefore it is not possible to identify the SSC at this station, but apparently triggered by the i3S’C is a sudden decrease of the X component, which we tend to interpret as a sharp intenaifitioationof the existing aurora1 substorm or a aharp onset of a new one. In Fig. 2 are given the gross feature of the X-ray flux with energies >25 keV as recorded by the two detectors. Until about 0430 UT only cosmic ray background radiation was recorded during both flights. Thereafter both detectors responded 80

y was

JACOBS (1957).

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K. WIL.HELM,J. KANGAS and W. RIEDLEH.

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Fig. 2. Gross feature of the X-ray event on July 27, recordedby the Andenes flight (L N 6) and the Sodankylii flight (L N 5).

to a very weak electron precipitation event lasting for several hours. This event is most clearly seen on the northern flight. In the light of recent studies on slowly varying absorption (SVA) events (BEWERSDORFF et al., 1966; JELLY and BRICE, 1967) we regard this observed increase of the X-ray flux as being caused by the morning precipitation connected with the magnetospheric substorm, which is indicated by the above mentioned polar magnetic substorm, and not with the SSC. Superimposed on this weak precipitation an impulsive X-ray burst occurred at 0603 UT, coinciding with the SSC. The burst lasted nearly 4 min and had rise and fall times of approximately 1 min. The SSC precipitation was very weak at the location of the Sodankyla flight. This indicates that the precipitation did not extend much below L = 6, which agrees with the previous observations mentioned above. In Fig. 3 are shown high resolution plots of the E > 25, > 50, > 75 and > 100 keV X-ray energy channels of the Andenes recordings during the interval 06020608 UT. Also plotted are the geomagnetic pulsation measurements from Tromss and Kiruna. The traces labelled Tromser and Kiruna show the N-S component in the passband from 0.05 to 10 set and 1 to 200 set, respectively. We determined the local onset time (from the Kiruna plot) of the SSC to be at 0603: 12 UT f 3 set, and the beginning of the SSC-associated precipitation to be at 0603:06 UT & 6 sec. Therefore, the onset time of the precipitation burst coincides within the uncertainty limits with the onset of the local magnetic disturbance. The precipitation event has a pronounced temporal structure with temporal distances between individual peaks of about 2 sec. Beside these fast pulsations a slow variation with a period of approximately 50 set can be seen. The shortperiod pulsations of the X-ray intensity were further investigated by means of power spectrum analysis. The event shows the strongest and most regular pulsations during the minute around 0605 UT. The stationarity turned out to be fairly good

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with

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S. L. ULLALAND, IL WIFE,

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and thus the power spectrum from 0603:50 to 0606: 10 UT is presented in Fig. 4. This shows a highly significant peak at a period of l-8-1.9 sec. The minor peaks at about 0.8 and O-6 set are hardly significant as they are very near the limit of the time resolution at these counting rates. The magnetic pulsation activity at Tromss and Kiruna was very low during the night before the SSC. At the time of the SSC a slow pulsation train of 5-6 min duration commenced. One minute later the pulsation agitation in the Pcl band increased and became more regular until 0606 UT. A sona~am analysis of these pulsations shows periods of 1.5-3 sec. The relatively regular pulsations lasted for

0

3.6

1.8

1.2 PERIOD

(SECONDS)

Fig. 4. Power spectrum of the counting rate of the >25 keV ohannel of the Andenesflight from 0603 : 60 to 0606 : 10 UT.

about 140 set during the maximum of the precipitation. At 0606 UT, when the precipitation event began to decay, a stronger Pi1 activity started, which went on for several tens of minutes. Concerning the energy spectrum we must distinguish between the X-ray burst associated with the SSC and the weak slowly varying X-ray event upon which it was superimposed. The energy spectrum of the latter is rather hard with an efolding energy about 50 keV, whereas for the SSC-associated X-rays an e-folding energy of 18-22 keV is found. A more detailed examination of several separate intervals in the period 0603-0608 UT shows no significant spectral variations with time. 3.

DISCUSSION

As there was no PCA-event in progress around the time of the SSC, the geomagnetic conditions were relatively quiet, there was a near absence of electron precipitation in the time before the SSC, and the SSC itself was of moderate strength, the associated electron precipitation may be regarded as a ‘clean’ event with little contam~ation from other effects. CNA recordings from the auroralzone stations like Andenes, Kiruna, Reykjavik and College put the event into the

Efectron precipitation associratedwith a sudden co~enc0ment simple 8iW precipitation class described

of a geoma~etic

storm

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by ORTNERet al. (1962). The study of such an event may lead to an understanding of the involved magnetospheric processes. An important feature of SSC- or SI-associated precipitation events appears to be their soft energy spectrum, which is atypical for the times of day when they have been observed (BROWN, 1967; BARCUS, 1968). In the present case an efolding energy of 18-22 keV was found for the bremsstrahlung X-rays, whereas a simultaneously occurring back~ound X-ray event had &, = 40-50 keV, the latter being typical for the morning sector. Barcus concluded that the local release of quasi-trapped electrons through the impulsive disturbance of the field could not be responsible for the observed soft electron precipitation. This is a most important result since SSC-associated precipitation events observed in the morning and day sectors occur on closed field lines, and as BROW et al. (1961) have shown that the supply of electrons trapped on such field lines would be sufficient to produce the observed X-ray events. The energy spectrum of such precipitation events actually resembles closely that of substorm-associated precipitation events observed in the mi~ight sector, which suggests that the acceleration mechanism might be more or less similar in the two eases (BARGUS,1968). It is even possible that there might be a direct relationship between SSC-precipitation and bay-associated precipitation, as both in the present case and in the one reported by Barcus, there started a polar magnetic substorm with a very sharp onset just at the time of the SSC. A similar sharp onset of a polar magnetic substorm happened at the SSC-time for the previously mentioned satellite observation by KONRADI(1968). It thus looks as if a world-wide sudden disturbance in the magnetic field (SSC or SI), presumably caused by a change in the solar wind, may trigger a sharp bay onset within an existing ~sturbance pattern (HEPP~ER, 1968), which together cause the bay-associated acceleration and precipitation mechanisms to extend over to the dayside of the Earth. The near periodic character of the SSC electron precipitation is another interesting feature of this event, in particular the 1%-X*9 set period as it influences a major fraction of the precipitation. The 1.8-1.9 sea period is equal to the bounce period of 40 keV electrons mirroring close to the atmosphere on L N 6, This period is also nearly equal to the period of hm-emissions, which are known to be associated with the SSC-events. One other case of oscillating electron precipitation with the electron bounce period has been observed by WI~CKLERet al. (1962). W~~TWORTHand TEPL~Y (1962) used their observations on ~~-emissions and the observation of Winckler et al. to develop a model for hrra-emissions,and identified the electron bounce period with the period of hm-emissions. In recent years one has however come to the conclusion that electrons can hardly be directly responsible for the generation of hna-emissions. (For a review of theoretical and experimental studies see TROITSKAYA(1967)) One can speculate whether the model of a bouncing bundle of electrons can at all be correct. The model assumes that the electron bundle shall exist over a number of bounce periods, which is difficult to explain since a velocity dispersion of the particles within the bundle would cause it to diffuse along the field lines.

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s. L. u LLALAND, K. WILHELM, J. KANUAS and W. RIEDLER

More experimental results on the time structure and energy spectrum of SSCassociated precipitation events are obviously needed before it will be possible to decide what are the typical and fundamental characteristics of these events, and what are more superficial features that may vary from event to event in a more or less random manner, depending on the instantaneous conditions in the magnetosphere. In particular, it will be very interesting to see whether the 1.8-1.9 set period turns out to be a fundamental feature of these events, or whether the period of existing rapid variations will vary from event to event. Also interesting is to identify the mechanisms, probably of a wave-particle association type (ANDERSON, 1968), which give rise to the precipitation of electrons with the observed time structure and energy spectrum. The assumption of a wave-particle association mechanism is further strengthened as KOKUBUN and O~UTI (1968) found that this particular event was accompanied by VLF emission in the 2 kHz band. Acknowledgements-We are greteful to IGY-WDC-Geomagnetism C 1, Meteorological Institute, Charlottenlund, Denmark, for copies of the Fort Churchill, College, and Guam magnetograms, and to the Director of the Aurora1 Observatory, Tromse, Norway, for the Tromse magnetograms. We wish to thank the Director of Group de Recherchez Ionospherique, CNET, 3 Avenue de la Republique, 92 Issy-les-Moulineaux, France, for permission to use the pulsation magnetograms recorded in Tromse, and Dr. E. FELLMANN of the same institute who reduced this recording. We acknowledge valuable discussion with Professor H. TREFALLand Dr. G. KREMSER. One of us (S.L.U.) is indebted to the ALEXANDERVON HUMBOLDT STIFTUNGand Norges Almenvitenskapelige Forskningsr&d, for research scholarships during his stay at the Max-PlanckInstitut fur Aeronomie, at the time when the present recordings were performed. The balloon flights from Andenes were supported financially by Norges Teknisk-Naturvitenskapelige Forskningsr&d, by the U.S.A.F. Office of Aerospace Research under Contract No. AF-61(052)-869, and by the Max-Planck-Gesellschaft, Germany. The balloon flights from Kiruna were supported financially by the Max-Planck-Gesellschaft, Germany, and in part by the Swedish Natural Science Research Council. The balloon flights from Sodankylli were supported financially by the Centre National d’Etudes Spatiales (CNES) and partly by the Finnish National Research Council for Sciences. REFERENCES ANDERSONK. A. ANDERSONK. A.

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Electron precipitation associ&ed with a sudden commencement

of a geomagnetic storm

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1962

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