Ionospheric observations during the annular solar eclipse of 20 May 1966—III Four D-region electron density profiles measured by rocket techniques

Journalof Atmosphedc and TerrestrialPhysics,1970, vol. 32, pp.1859-1863. PergamonPress. Printedin Northern

Ireland

Ionospheric observations during the annular solar eclipse of 20 May 1966IU Four D-region electron density profiles measured by rocket techniques M. JESPERSENand B. MILLER PEDERSEN* Danish Space Research Institute,

Lyngby, Denmark

(Received 29 Julte 1970) Ab&&--Measurements of Faraday rotation and of differential absorption on frequencies 2.2, 3.9 rend 7.3 MHz have been used to determine the height distribution of collision frequency and electron concentration during the annular solar eclipse of May 20, 1966 and on a cornparison day.

1. I~~R~Du~TIo~ DURING the 15 and 20 May, 1966, six Sparrow/Areas sounding rockets were launched by ESRO (project A23) from a provisional launching site established at Karystos in Greece. The payloads were instrumented for studying effects in the lower ionosphere caused by the annular solar eclipse present on the 20 May. Results on electron densities and collision frequencies were obtained on four flights: AEOl launched on the 15 May for test and calibration purposes, AE03, AEO4 and AE06 launched during the solar eclipse on the 20 May. Due to rocket failures no data are available from the flights AE02 and AEO5. Table 1. Data for the ESRO rockets Payload No.

AEOl

Date of lwnch: May 1966 Time of launch: LMT Per cent visible sun at F, =75km Time at ?z = 70km: LMT Time at h = 80 km: LMT Time at h = 90 km: LMT

15 11 51 27.2

AE03

100 11 53 06.8

AE04

AE06

20 110051

20 11 41 00-4

20 12 40 00.3

44 110216 11 02 29.7 11 02 485

15 11 42 223 11 42 36.4 11 42 53.4

84 12 41 245 12 41 38.6 12 41 56.6

2. EXPERIMENTAL

METHOD

The results were obtained by studying the Faraday rotation of the plane of polarization and the differential absorption effects on propagating waves. The frequencies used were: 2.2 MHz, 3.9 MHz and 7-8 MHz. Linearly polarized oont~uous waves were transmitted from ground based, horizontal dipoles and received on a common IinearIy polarized antenna on board the rocket. The rocket borne antenna was mounted perpendicular to the rocket spin axis, and so it was rotated by the mechanical spin through the polarization patterns of the sounding waves. * Present address:

Observatoire de Meudon, Meudon, France. 1859

M. JESPERSEN and B. MBLLER PEDERSEN

1860

The received RF signals were amplified, rectified, and filtered (7 w 1 ms) and the resulting signaIs equal to the envelope curves were telemetered to ground. When recorded as a function of time, an envelope curve exhibits a characteristic modulation pattern equal to the projection on the rotating antenna of the gradually rotating polarization ellipse of the corresponding sounding wave. The repetition frequency in the modulation pattern is twice the sum of the rocket spin frequency and the Faraday rotation frequency. Thus, by comparing corresponding time intervals of the modulation patterns at kwo different frequencies, it is possible to exclude the spin frequency from the data and to determine the difference in Faraday rotation of the planes of pofarization as a function of height. The amplitude maxima and minima in an envelope curve represent the major and the minor axis respectively of the polarization ellipse. Consequently, the amplitudes of maxima and minima are proportional respectively to the sum and the difference of the signal strengths of the two characteristic waves (ordinary and extraordinary), which constitute the propagating wave at a given height. Thus, from the calibrated amplitude values it is possible to cafculate differential absorption data as a function of height. From the observed data on difference Faraday rotation

(0 and x indicate the characteristic waves) it is possible to determine the electron density N, and collision frequency vX by use of the generalized Appleton-Nartree theory (SEN and WYLLER, 1960; KANE, 1962). Assuming quasi-longitudinal

propagation the theory yields:

w1 and ~0~are the sounding frequencies and UIHthe electron gyro fraqusncy. tions of ox’ 02, ox and vx which can be calculated. Above a certain height where &h)

I

m+

P and Q!are func-

equation 12) simplifif3sto

(3) where k

e2

2cq

==2E,rrLe wp

-cl+

2Wf.r

-

wg -w&J

Ionospheric observations-III

1861

and co = permittivity

of free space

m = electron mass e = electron charge c = velocity of light. Substitution of N in equation (1) by equation (2) yields 2

((V, - R&J, - (cp, - VOLJ,),,) F = Y,V G & (In E, - In E,)

(4)

where the left side is a function of height to be calculated from observed data, and the right side is a theoretical function, which can be calculated as a fun&ion of v~$ when w and WE are known. Consequently, VH can be determined as a function of height by use of equation (4). Knowing VM = v~(h), the function F(vM) is also known as a function of height, and N(h) can be determined at lower altitudes by use of equation (2). At higher altitudes N(h) can be determined by equation (3), independent of v~ as mentioned above, only involving observed phase data.

3. RESULTS The values of collision frequency YM are calculated from ~~erential absorption data at 2-2 MHz and 344 MEz and difference Faraday rotation derived from data at 2-2 MHz and 3.9 MHz compared with data at 7.8 MHz. Figure 1 shows the obtained collision frequency profile yILl= v&h).

Fig. 1. Profile of collision frequency VM obtained and AE06.

from AEOl,

AE03,

AE04

M. JESPERSEN and B. MPILLER PEDERSEN

1862

AE 01

1153 LMT

AE 03

1102 LMT

AE 06

1241 LMT

103 N

Fig. 2. Electron

I 102

-

density profiles obtained

I

1

,

I

103 N A

105

104

cms3

I11111 IOL

from AEOl,

I

I

I

I

AE03,

11111 I05

I

AE04

and AE06.

I Illllll I06

crK3

Fig. 3. Comparison of electron density profiles from AE03,

AE04

and AE06.

Ionospheric o~serv~tio~-I~

1863

We have assumed that the collision frequency profile does not change with time, and the profile in Fig. 1 is based on data from all flights. The electron density results are based on the difference Faraday rotation data and, at lower altitudes, on the rlld profile shown in Fig. 1. Figures 2 and 3 show the electron density profiles N = N(h). On Figs. 1 and 2 the uncertainties are indicated by horizontal bars. A bar indicates maximum deviations possible on a single number density, taking into account the different sets of data available for the calculations. However, the plotted curves are the most probable, when consistency with the observed integrated effects shall be maintained. The method of calculation used assumes a ho~zontally stratified ionosphere unchanged during the recording periods of the single flights. To improve this approximation only data recorded during the ascending parts of the trajectories have been used. Further, vertical propagation is assumed, and u)~ as well as the observed Faraday rotation and absorption data have been corrected accordingly. 4. CONCLUDING REMARKS The results obtained on the flights AE03, AE04 and AEO6 indicate a minimum in electron density around maximum eclipse at all altitudes. As for the calibration flight AEOl, a much lower ionization than on the day of the solar eclipse is observed. This is in good agreement with observations by other experimenters, who find a particularly high ionization on the day of the eclipse. A more detailed discussion and comparison of results obtained will take place in Part V. REFERENCES KANE J. A.

1962

SEN H. K. and WYLLER A. A.

1960

“Re-evaluation of Ionosplwic Electron Densities and Collision Frequencies Derived from Rocket Measurements of Refractive Index and Attenuation, (Edited by N. C. GERSON),AGARDograph 53. Pergamon Press, Oxford. J. geopQ8. Res. 65, 3931.