Electron density measurements in the lower D-region

Journal of Atmospheric Pergamon Press

and Temsfrial Physics in Northern

Ltd.1979.Printed

Vol. 41. PP. 1195-1200 Ireland

Electron densitymeasurementsin the lower D-region T. A. JACOBSEN* Space Research Institute of the Austrian Academy of Sciences, c/o Technical University Graz, Graz, Austria and

M. FRIEDRICH Department of Communications and Wave Propagation, Technical University Grax, Grax, Austria (Received 15 February 1979; in revised form 20 April 1979) Abatra&-Model calculations of the Faraday rotation and the absorption in the lower ionosphere are presented and the practical limits of this technique to determine the electron density are discussed. With a model and a rocket flight it is shown that probing frequencies slightly below the electron gyro-frequency are the most sensitive for measurements in the lower D-region.

1. PJTRODUCIXON

(an extrapolation of one actually measured under PCA conditions, labelled F34 in Fig. 4) together An exact knowledge of the electron density in the with a collision frequency profile considered typical lower ionosphere is not only of interest in itself, but for summer conditions. The results are presented in also of practical importance for the propagation of Fig. 1 as lines of constant Faraday rotation and very low frequencies (VLF). With the advent of differential absorption in units of deg/km and cryogenically cooled mass spectrometers which dB/km, respectively. The computations are based measure from 60 km upwards (cf. e.g. JOHANNESon the generalized magneto-ionic theory (SEN and SEN and KRANKOWSKY, 1972) the interest in good 1960) employing the Burke-Hara WYUER, electron density measurements at these heights was polynomial expansions of the semiconductor integrenewed. rals (HARA, 1963). A magnetic field strength of A radio wave propagation method has indisputa-5 x lo-’ T (gyro-frequency f_ = 1.4 MHZ) was ble advantages over -present probe techniques at applied and the angle between the geomagnetic these altitudes where one encounters high collision field lines and the propagation vector was set equal frequencies, despite the shortcomings of the integto 30”. These values are typical for launches from rating method (requirement of constant conditions northern Scandinavia. The contours are broken off during the time of the measurement, etc.). Rocket at twice the plasma frequency. From experience, a measurements making use of the differential abvalue of 0.5 deg/km and 0.5 dB/km may be consisorption and phase (Faraday rotation) appear to be dered as the detection limit for good measurethe simplest from the instrumentation point of view ments. The upper height limitation is either deter(cf. e.g. JE~PERSEN et al., 1964; MECHTLY et al., mined by signal loss indicated by A = -20 or 30 dB 1967; BENNE~ et al., 1972). In this study some relative to the 50 km value or by loss of detectable aspects of relevance to the use of the well known double spin modulation, M, e.g. MC 1 dB Faraday technique in the lower D-region will be (differential absorption >25 dB). discussed on the basis of model calculations and The figure displays the well known fact that for experience from a recent flight. frequencies above feethe Faraday rotation changes its sense of rotation from left- to right-handed 2. THEORY AND SIMULATION when the collision frequency increases above a In order to optimize the choice of sounding critical value v, (rotation sense applies for the frequencies for measurements at low altitudes, denorthern hemisphere). It is clearly demonstrated tailed simulations were carried out. To illustrate the that frequencies slightly below feewill provide useoutcome of the simulations a profile with a fairly ful data at the lowest altitudes. Data obtained from high concentration of thermal electrons was chosen frequencies above feesuffer from the phase reversal at low altitudes. The differential absorption simply * Present address: Norwegian Defence Research Estabdisplays a broad optimum frequency range near fe. lishment, Kjeller, Norway. 1195

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and

M.

FRIEDRICH

Fig. 1. Contours of constant Faraday rotation (a) and differential absorption (b) for a high density summer profile in deg/km and dB/km, respectively. A value of 0.5 deg/km and 0.5 dB/km may be considered as the detection limit for good measurements. The upper height limitation is either determined by signal loss, indicated by A = -20 or 30 dB relative to the value at 50 km, or by loss of detectable double spin modulation, M, e.g. M< 1 dB (differential absorption >25 dB). For a given collision frequency profile one can derive sensitivity curves for the electron density measurement corresponding to the detection limits quoted above. Such curves for Faraday rotation,

differential absorption (DA) and absorption of the ordinary wave (AOW) are presented in Fig. 2 for selected frequencies which have actually been used. Again, one can clearly see that 1.300 MHz not only allows continuous measurement, but also requires the lowest electron densities. At higher altitudes (lower collision frequencies) the sensitivities become inversely proportional to the square of the frequencies. 3.

PRACFKAL

RESULT

the authors’ knowledge, a sounding frequency below fee was employed for the first time in August 1971 (JOHANNESSENet al., 1972). The rocket was launched under fairly quiet conditions (0.6 dB on the 27.6 MHz riometer) at local noon from the To

And%ya Rocket Range, Norway. An electron density profile could be derived from 62 to 100 km with the probing frequencies 1.300, 2.200 and 3.883MHz. A more recent flight is that of the rocket 518-2 (Nike-Apache) which was launched from Esrange, Sweden, on 21 February 1976, at 19-42-32 UT under rather strong aurora1 absorption conditions (2.7 dB on the riometer, cf. BJ~RN et al., 1979). The Faraday experiment was provided by the Wppsala Ionospheric Observatory, Uppsala, Sweden. For comparison, the data were also processed at the Technical University Graz. The frequencies employed were 1.300, 3.883 and 7.835 MHz, the radiated power was between 80 and 1.50 W. The receiver outputs were telemetered by an S-bit PCM system with a sampling rate of 500 s-‘. A possibility to reduce the raw data onboard by electronic means (cf. FRANCE and WILLIAMS, 1976) had been applied earlier (FRIEDRICH, 1974), but

Electron density measurements

I-

2.200

/ -

3483

in the lower D-region

74B3C

1.300 \ \ \ \ \ \

I-

\ ~

t \

‘\ ,

\ \

-

-

LEFT

---

RIGHT

ELECTRON

I-

DA

---

AOW

DENSITY,

m-’

/

Fig. 2. Electron densities required to produce a Faraday rotation of 0.5 degikm (a) and an absorption of 0.5 dB/km (b), considered as the detection limits of these two quantities, respectivefy. The acronyms DA and AOW in Fig. 2b refer to differential absorption and absorption of the ordinary wave, respectively. The calculations have been carried out for five different frequencies commonly used in rocket experiments.

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has not been pursued further due to the successful application of a computerized raw data processing scheme developed for post flight analysis. The signal received by the aerials on the spinning payload is ideally of the form: f(t)= Ex'l + (2D cos 20,)/( 1 + D’),

(1)

where E is a function of both wave components, D contains the differential absorption and o, is the angular spin frequency. Twice every spin period all telemetered values are fitted to the above function by an r.m.s. method (TORKAR and FRIEDRICH, 1976). The obtained Faraday rotation, differential absorption and the absorption of the ordinary wave

and M. FRIEDRICH

(the latter corrected for geometric decrease with slant range) are shown in Fig. 3. The full lines are the simulated curves using the electron density profile depicted in Fig. 4 (labelled S18-2) which has been derived from the processed raw data (TORKAR et al., 1976). Both the programme to compute the electron density and the collision frequency are based on the generalized magneto-ionic equations. The geomagnetic field model applied is the one described by CAIN et al. (1967). Changes of the magnetic field due to vertical and horizontal movements of the vehicle are included. The profile in Fig. 4 may be termed ‘most probable’ since the simulated rotation and absorption values yield the

AUKUDE, km

Fig.3. Observed

and simulated values of Faraday rotation and absorption of the rocket Bight S18-2. For graphical reasons the rotation of the 7.835 MHz signal is broken off after 81 s.

Electron density measurements

Fig. 4. Ebctron

in the lower D-region

1199

density profile obtained from the rocket flight SlS-2 together with the idealized profile used for the model computations (F 34).

best agreement with the raw data of Fig. 3. Similarly, the best agreement for the collision frequency, v, was obtained in the form v = Kxp (pressure p taken from CIRA, 1972). The best fit of a height dependent propo~onaIi~ factor was: K = 0.35 x 10’ exp (0.035 x h), h in km,

m’N_’ s-r

(2)

which is within the range of vaiues reported by M.ucHTLY (1974). It is clearly demonstrated in Fig. 3 that the determination of the electron density below 65 km was made possible primarily by the 1.300 hGIz signal. Since the rocket S18-2 was flown in winter, the collision frequency was lower than the one in the model computations (F 34 in Figs. 1 and 2). Hence, the phase reversal of the two higher fre-

quencies (3.883 and 7.835 MI-Ix) occurred below the height region of the measurement. Both the auroral emission on 557.7 nm observed on the ground and the flux of energetic electrons measured on the rocket payload were stable up to about 100 s flight time (-86 km). From there on they decreased with a time constant on the order of 50-100s. The resulting change of the ion density can clearly be seen in the difference between the up- and downleg data of the electrostatic probe (e.g. about a factor of 5 within I90 s at 60 km, BJ&N et al, 1979). Time constants of the plasma density decay at every height were thus derived and the Faraday rotation simulated for both a steady-state ionosphere and for the case of an exponentially decaying electron density underneath the altitudes under investigation The increase of the Faraday

1200

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and M.

rotation by the altitude increment was reduced by 1.5, 4.1 and 2.2% at 86, 94 and 100 km, respectively, due to the decline of the accumulated rotation underneath in the corresponding time intervals. Consequently, the computed electron densities can also be expected to be too low by the same percentages. This systematic error was considered negligible except near the apogee where the vertical velocity is small and more weight must be given to the (normalized) probe data. The error bars every 5 km were computed on the basis of data scatter and assuming the collision frequency to be reliable within a factor of 2.

FRIEDFUCH

will provide useful data in height regions where other Faraday frequencies would suffer from a phase reversal. This change of sign of the Faraday rotation makes the data interpretation ambiguous. It could furthermore be shown that although a wave propagation experiment is an integrating method, systematic errors introduced by an electron density varying with time are rather small, provided there is a steadily increasing density profile, a condition which is usually fulfilled in the D- and E-region.

Acknowledgements-This study has been made possible by several grants of the Austrian Council for Scientific Research. The numerical calculations were carried out at the Graz Computing Centre (RZG). Thanks are particularly due to L. BJ~RN from the Uppsala Ionospheric Observatory for allowing us to work with their data and to Dr K. M. TORKARfor his expert programming of the rocket data. The discussions with Dr E. V. THRANEwere extremely valuable.

4. CONCLJJSIONS

The sensitivity of various frequencies in the range used for Faraday experiments has been studied on the basis of model calculations. It has been demonstrated that at low altitudes it is advisable to have one probing frequency below feewhich

RFSEIIENCES BENNE~

F. D. G., HALL J. E. and DICKINSON P. H. G. BJ~RN L. G., ARNOLDF., KRANKOWSKY D., GRANDALB., HAGEN0. and THRANEE. V. CAIN S. H., HENDRICKS S. J., LANGELR. A. and HUDSON W. V. CIRA

FRANCE

L. A. and WILLIAMS E. R. HARA E. H. JE+YPERSEN M., PETERSENO., RYBNERJ., BIELLANDB., HOLT0. and LANDMARK B. JOHANNE!SSEN A. and KRANKOWSKY D. JOHANNE~~EN A., KRANKOWSKY D., ARNOLDF., RIEJXERW., FRIEDRICH M., FOLKESTAD K., THRANEE. V. and TRP)IMJ. MECHTLY E. A., BOWHILLS. A., SMITHL. G. and KNOEBELH. W. MECHTLY E. A. SEN H. K. and WKLLERA. A. Reference

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J. atmos. terr. Phys. 34, 1321.

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1972 1972

COSPAR International Reference Atmosphere, Akademie, Berlin. J. atmos. terr. Phys. 38, 957. J. geophys. Res. 68, 4388. In: Electron Density Distribution in Ionosphere and Exosphere, p. 22, North-Holland, Amsterdam. J. geophys. Res. 77, 2888. Nature, Land. 235, 215.

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J. geophys. Res. 72, 5239.

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Radio Sci. 9, 373. J. geophys. Res. 65, 3931.

1976 1963 1964

is also made to the following unpublished material:

FRIEDRICH M. TORKARK. and FRIEDRICH M. TORKARK., JACOBSEN T. A.. BAUER A. and FRIEDRICH M.

1974 1976 1976

Thesis, Techn. Univ. Graz. internal Report JNW 7602, Techn. Univ. Graz. Internal Report NW 7603, Techn. Univ. Graz.