Simultaneous measurements of electron density and temperature in the northern auroral zone

Planet. Space Sci. 1967, Vol. 15. pp. 1049 to 1054.

Pergamon Press Ltd.

Printed in Northern Ireland

SIMULTANEOUS MEASUREMENTS OF ELECTRON DENSITY AND TEMPERATURE IN THE NORTHERN AURORAL ZONE J. J. BERTHELIER Groupe de Recherches Ionospheriques, St. Maur-des-Foss&s, and Department

France

D. J. STURGES of Electron Physics, The University of Birmingham (Received 8 February 1966)

Abstract-Measurements of electron density using an R.F. capacitance probe, and of electron temperature using a new type of double Langmuir probe, are reported. Data were obtained over an altitude range from about 65 to 240 km, during a moderately strong zenithal aurora. The probes were launched from Andoya, Norway, using a Dragon rocket. INTRODUCTION

This paper is a preliminary report of results obtained during the first part of a programme of studies of ionospheric phenomena being made by the Groupe de Recherches Ionospheriques (G.R.I.) under the patronage of the French Centre Nationale des etudes Spatiales (C.N.E.S.). Two Dragon rockets have been launched in the northern aurora1 zone, and it is intended to launch four more similarly instrumented rockets to study some aspects of the diurnal magnetic activity in the southern polar region. The purpose of these first two launches was to study with fairly high spatial and temporal resolution the fluctuations in intensity, angular distribution, and energy spectrum, of energetic proton and electron fluxes during a zenithal aurora: and to investigate a possible relationship between these observations and the local plasma density and temperature. Simultaneous ground measurements made included ionospheric sounding, ionospheric absorption, magnetic field variations, and VLF recording. Dragon rocket D20 was launched from Andoya, Norway, at 19.0512 hours U.T. (20.0512 hours L.T.) on May 26th 1966. The ground magnetometers indicated strong magnetic activity, with large scale fluctuations up to 509 in the horizontal field. The 27.6 MC/Sriometer indicated an absorption of approximately 1.9 db at the time of launch, increasing to 2.1 db after 2 min and to 2.3 db after 5 min, and then decreasing to 2 db after 8 min. No visual observations of the aurora were possible, because of the prevailing daylight. Dragon rocket D19 was launched at 00.14.36 hours U.T. on 15th June, 1966, during similar conditions : no experimental data were obtained, due to vehicle failure. For D20, incomplete deployment of the proton and electron spectrometers has made interpretation of the data obtained very complicated. The rocket attitude and trajectory have also been difficult to calculate, due to a large precession angle. It is intended to publish a complete analysis of all experimental data as soon as these calculations have been completed. Meanwhile the electron density and electron temperature results are presented here for their own interest. INSTRUMENTATION

The scientific payload of the rockets consisted of 6 scintillation counters, for the detection of protons and electrons in the energy range 15-180 keV, provided by G.R.I. : and electron 7

1049

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J. J. ~ERT~L~R

md 1). 3. STURGES

density and electron temperature probes, provided by Brmingham University, In order to determine the attitude of the rocket with respect to the local magnetic vector, a tfiaxial magnetometers provided by C.N.E.S., was also installed. The electron density experiment consists of the measurement of the reactive impedance formed between grid-like electrodes of well-defined geometry, at a probing frequency of 39 MC/S. This frequency, being much greater than either the electron gyro-frequency in the Earth’s magnetic field, or the local electron collision frequency (at least for altitudes above 70 km”‘), atlows the derivation of the efectron density from a simplified plasma permittivity equation, in which terms involving the gyro-frequency or collision frequency may be neglected. This method, and an earlier version of the apparatus, have been described in more 26300 P A. Electron

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detail elsewhere:@) the presently incorporated refinements make no essential difference to the method of operation or analysis of the results. Electron densities in the range 2 x lOa to 5 x lo5 GrnJ can be measured with approximately &5 per cent accuracy. The electron temperature experiment uses what is basically a double Langmuir probe. Two spherical electrodes, carefuhy machined to have identical areas and cleaned to eliminate contact-Fotential ~~eren~s, are used to investigate the d~eren~al Langmuir chara~~stic. The electronic system is designed so that the data can be reduced in real time to produce a measurement which is proportional to electron temperature, and which does not require any further calibration. An experimental accuracy of approximately &100X! is possible. The technique will be described in detail in a forthcoming publication.(3) The operation of the density and temperature probes was synchronised, each making one measurement every 250 msec (i.e. about every 400 m at II- and E-region altitudes with the Dragon rocket). The sensing electrodes for each probe were deployed at a distance of 90 cm from the rocket axis, to avoid the disturbing influence of the rocket body, and were swept through space potential by sawtooth voltage waveforms applied between the electrodes and the rocket body, to ensure that any ion sheath around the electrodes was completely

ELECTRON

DENSITY

AND

TEMPERATURE

DURING

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1051

collapsed once during each measurement period. Two independent sawtooth waveforms were used so that it could be arranged that both sets of electrodes were not driven above space potential at the same time, thus reducing the depression of the rocket potential relative to the plasma by the electron current drawn to the electrodes. RESULTS AND DISCUSSION

The results obtained from both density and temperature measurements exhibit fluctuations of amplitude at a frequency corresponding to the roll rate of the rocket, approximately 1 c/s. This is shown in Fig. 1, which represents an excerpt from the recorded data for both Envelopes of data for: -Upward trajectory ---Downward trajectory

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electron density and temperature. These fluctuations are believed to be mainly due to the periodic motion of the electrodes through the wake of the rocket, and to wake effects of one electrode on another. Such effects are possible throughout most of the rocket trajectory due to the large precession angle of this rocket. A consistent explanation of the fluctuations is possible in these terms, but confirmation of this explanation must await completion of the attitude calculations for the rocket. If, following Samir and Willmore,(4) it is suggested that the density data maxima correspond to undisturbed density measurements, when the electrodes are free from wake effects, then, due to the relation between the roll rate and the data measuring periods, the most accurate temperature measurements should be those occurring 500 msec after each density maximum. The density and temperature profiles obtained in this way are indicated on Fig. 1. A more detailed discussion of these wake effects is planned when attitude information becomes available. The data presented in Figs. 2 and 3 correspond to the upper and lower envelopes of the roll-modulated profiles such as those shown in Fig. 1. The data presented in Fig. 1 are for

1052

J. J. BERTHELIER

and D. J. STURGES

altitudes 160-174 km on the upward trajectory. Figure 2 shows the complete electron density profiles observed on both upward and downward trajectories; Fig. 3 shows the corresponding temperature profiles. The altitude scales used for these figures are subject to errors of f 15 km: it is hoped to improve on this accuracy when the data reduction problems mentioned above have been solved. Analysis of the data in Figs. 2 and 3 is further complicated by variations occurring at the precession frequency of the rocket (ca. 0.02 c/s), but some features of the profiles are easily 250Envelopes 230 -

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separated from the influence of these variations. * During the ascent both density and temperature rose very rapidly with altitude. The density increased 3 orders of magnitude in only 15 km, and though the lowest part of this profile must be viewed with some caution since information is not available on whether the collision frequency was sufficiently high to invalidate the simplifying assumptions made in deriving the theory of the experiment,(2) previous measurements of collision frequency(1*5) suggest that any correction necessary is likely to be small. Although unusual, such rapidly rising profiles are not without precedent, similar profiles of electron densityt6) and temperature (‘) having been previously observed * For the density profiles these variations occur as changes in the value of both upper and lower envelopes, and can be most clearly seen for the downward leg data of Fig. 2. For the temperature profiles of Fig. 3 the variation can be seen as a periodic reduction in the amplitude of the roll-dependent fluctuations, i.e. as a reduction in the separation between the upper and lower envelopes.

ELECTRON

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1053

during magnetically disturbed conditions. However, this is believed to be the first simultaneous observation, and it is interesting to compare the density and temperature measurements on the upward and downward trajectories. Both density profiles indicate a peak density occurring at about 90-100 km, with densities first decreasing, and then remaining fairly constant, at higher altitudes: a quite similar profile was obtained by Lacey and Matthew@ up to 120 km during a 1 db aurora at Fort Churchill. The occurrence of an E-region peak at about 100 km, and the high densities as low as 80 km, suggest that this was predominantly an electron aurora: the G.R.I. particle detectors confirmed that electron fluxes were very much larger than proton fluxes during the flight. A few minutes before the launch, the ionosonde indicated a large electron density in the upper D-region, but during the flight strong absorption made it impossible to obtain a density profile, so that no check on the values indicated by these profiles is possible. A fairly sharply defined layer of increased electron density is seen between 166 and 180 km. This did not appear during descent: although there is apparently evidence of some continuation of this layered structure at a slightly lower altitude, the observed decrease in density seems to be largely due to the precession-dependent fluctuations in the data. Densities during descent were consistently lower than during ascent, and this observation is consistent with the particle detector measurements of electron fluxes. There seems to be some correlation between high density and high temperature measurements : temperatures are generally higher on the upward than on the downward trajectory, and this is particularly noticeable in the E-region and in the layer commencing at 166 km. The correlation suggests that the rocket ascended through a region of quite highly disturbed conditions, and descended through a region where the disturbance was much less pronounced: and that there was, associated with the disturbance, an increase in the density and temperature of the electrons that was especially pronounced in the lower E-region, and in a 10 or 20 km thick region at about 170 km altitude. The temperature profile during ascent indicates a rapid rise in electron temperature above 82 km. The fall in temperature from 78 to 81 km may be partly real, but it is not yet definitely established at exactly what density level the temperature equipment ceases to operate accurately, and the density is varying very rapidly at these altitudes. The temperature at 90 km and above is higher than most previous measurements have suggested, but there is wide divergence in such measurements,(7) with strong dependence on solar activity, magnetic disturbances, latitude, and time of day, so that the high temperature found here might plausibly be tentatively attributed to the presence of high-energy aurora1 electrons in this high-density region. Data for the descent do not show this high-temperature region, but instead indicate a more customary profile,“) with a fairly steady decrease of temperature with decreasing altitude. Above about 160 km there is sensible agreement between the ascent and descent profiles for both density and temperature, but below this altitude the difference between the temperature profiles becomes increasingly marked. The profile for descent falls to impossibly low values between 75 and 95 km, and indicates an error in the measuring system. This is believed to be a systematic error, perhaps due to contamination of the electrode surfaces producing a contact-potential difference between the electrodes, which would result in a data zero-offset. Such contamination would be difficult to account for during flight in the ionosphere, and it is therefore most likely that both ascent and descent temperature profiles are uniformly too low by at least 100°K on this account. (It is intended to eliminate any such error on future flights by interchanging the electrical role of the electrodes during ilight.)

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J. J. BERTHELIER

and D. J. STURGES

The temperature profiles are thus believed to indicate relative temperature changes accurately, but not the absolute values. In summary, although it is not possible to comment on aurora1 mechanisms on the basis of the present incompleted analysis of data from a single rocket flight, several interesting features have been observed. These are, principally, an unusually rapid rise of density through the D- and lower E-regions, together with a density peak in the E-region at approximately 100 km and a layer of enhanced density in the vicinity of 170 km, and a correlation of these high density regions with marked increases in the electron temperature. Acknowledgements-We should like to acknowledge the contributions to this programme of many of our colleagues in both France and Britain, and in particular to Prof. J. Sayers, Dr. J. W. G. Wilson, Mr. J. H. Wager, and Mr. R. Godard. Financial support was provided by C.N.E.S. and the British Science Research Council. REFERENCES 1. M. JESPERSEN, J. K. OLE~ENand B. LANDMARK,High LatitudeParticles andthe Ionosphere (ed. B. Maehlum), p. 63. Logos Press (1965). 2. E. C. MACKENZIEand J. SAYERS,PIanet. Space Sci. 14,731 (1966). 3. J. W. G. WILSONand G. GARSIDE(To be published). 4. U. SAMIRand A. P. WILLMORE,Planet. Space Sci. 13,285 (1965). 5’. E. V. THRANEand W. R. PIGGO~~,J. atmos. terr. Phys. 28,721 (1966). 6. W. K. LACEYand D. L. MATTHEWS,Private communication. 7. L. H. BRACE,N. W. SPENCERand G. R. CARIGNAN,J, geophys. Res. 68,5397 (1963). PeareM%Coo6maercn 06 naMepenasx 3neKTpOHHOi8IIJIOTHOCTH IIOCPOACTBOM aorrnnpoBaHHR p3JHO%WTOTHOi% eMKOCTH, a TaKXEe 3JIeKTpOHHOti TeMIWpaTypbI IIpH IIOMOqIl HBO~HOFO aonna JIanrMtopa, HOBOPO o6paana. Bbrna nosryrenbr Aamrbre no nnanaaony BbICOTbl OT IIpElMOpHO 65 A0 240 KM., BO BPOMR IlOJlRpHOI'OCElflAElRyMOpWiHOi8 CHJIJJ. ~OHAJJ 6b1nn nyqeabr 113 k1~0ti14 (Hopsernrf), npu IIOMOIIJHpaKeTbI aflparton*.