MEASUREMENTS OF DAYTIME SKY BRIGHTNESS WITH PHOTOELECTRIC PHOTOMETERS LAUNCHED ON ROCKETS

In connection with the discrepancy between data on the pressure (density) of the air in the upper layers of the atmosphere, obtained from measurements of twilight-sky brightness in the zenith1, and data on pressure obtained by direct methods using instruments carried on rockets23 (by one or two orders of magnitude, respectively, at altitudes of 100 and 130 km), a recently published article on measurements of daytime sky brightness with photoelectric photometers launched on rockets is of interest.4

Over a number of years, beginning in 1946, measurements of daytime sky brightness in selected regions of the spectrum at various altitudes were carried out with rockets, as a result of which data were obtained on the unexpectedly high and nearly constant magnitude of daytime-sky brightness within the altitude range from 40 to 130 km, and on the presence of clouds during the day at altitudes above 70 km.

In the flight of November 21, 1946, which began at 10:15 a.m., seven photoelectric photometers with photomultipliers were installed in the nose section of the V-2 rocket; during the rocket’s position at launch, five were directed east and west at an elevation angle of 15° above the horizon, four of them with filters \((\lambda = 4430\ \text{Å}\) and \(\lambda = 5230\ \text{Å})\).

The results of measurements of daytime sky brightness in the two spectral regions indicated above for altitudes from ground level to 27.5 km are given by the authors in absolute units (the radiation in a spectral interval 20 Å wide, arriving from a hemisphere, is expressed in watts per square meter).

During the period of engine operation, approximately up to an altitude of 27 km, the stability of the rocket was monitored and remained almost constant.

At an altitude of 25 km, the sky brightness measured by photometers with blue and green filters was, respectively, about 5.7 and 4.6% of the sky brightness at an altitude of 5 km for photometers directed westward, and about 8 and 10% for photometers directed eastward. It should be noted that the results of measurements to the east contained a certain error caused by scattering of sunlight by particles that had fallen onto the entrance protective glass of the photometers.

In the flight carried out on December 8, 1947, at 2:20 p.m., twelve photometers with photomultipliers (931A) were installed on a V-2 rocket. Two of them, with their fields of view directed downward, were intended for measuring the Earth’s albedo; five, in their launch position, were directed eastward (mainly in three directions), and five similarly westward at an elevation angle of 15° above the horizon.

The higher sensitivity of these photometers made it possible to use interference filters with transmission maxima of about 4280, 4720, 5350, 5670, and 6150 Å. The article gives the results of sky-brightness measurements by photometers directed eastward for the five spectral regions indicated above, within the range of altitudes from ground level to approximately 33 km, in relative units, which essentially repeat the results of the 1946 measurements.

The main purpose of the further experimental investigations carried out in 1950 and 1951 was to measure sky brightness at altitudes greater than 30 km.

During the flight of the V-2 rocket launched on August 31, 1950, at 10:09 a.m., the increase in sensitivity was achieved through the use of photometers with type 1P21 photomultipliers, and this in turn made it possible to reduce the size of the photometer’s field of view to 5.5°. The photometers

were located, as in the preceding flights, on the side surface of the nose section of the rocket. The voltage-divider circuit that was used made it possible to receive signals whose intensity could vary within wide limits. The interference filter 4260 Å was chosen with allowance for the \(N_2^+\) band 4278 Å, and the filter 5590 Å—for the oxygen line 5577 Å, observed in the glow of the night sky. In these measurements, as in the measurements of 1946, the photometers were calibrated in the same absolute units. Simultaneously with the results of the photometric measurements, the authors also give data on the rocket’s flight in the horizontal plane as a function of altitude, the change of altitude, and, beginning with the 72nd second of flight, the azimuth and elevation angle of the rocket as functions of time. From these data it follows that at the maximum altitude reached by the rocket (about 136 km), its longitudinal axis was directed almost horizontally (5 seconds after engine shutdown, at the 72nd second of flight, the rocket’s elevation angle was approximately 80°), and in azimuth—almost exactly in the direction opposite to that which the rocket had after the engine stopped operating. During the engine’s operation the orientation of the V-2 rocket remained almost constant, and usually the rocket rotated slowly after the engine was shut down.

The readings of the photometers in two spectral regions, recorded continuously, indicate an almost exponential decrease in the brightness of the sky from ground level up to an altitude of about 35 km. The brightness of the sky at an altitude of 30 km amounted to about one to two percent of the brightness of the sky measured at ground level. During the remainder of the flight the photometer readings proved to be largely independent of altitude (some cyclic changes in the photometer readings could be connected, as the authors point out, with the rotation of the rocket, namely with: (1) the photometers passing, during rotation, alternately through the solar and shadow sides, (2) the rocket’s precession, and (3) changes in the brightness of the sky with direction).

Taking the minimum values of the photometer readings corresponding to altitudes near 85 km and 115 km as quantities representing the amount of light entering the photometers at these altitudes, the authors obtain the data shown in Table I.

Table I

Radiation in a spectral interval 20 Å wide, arriving from the hemisphere
(in milliwatts per square meter)

In the flight of the “Aerobee” rocket on July 25, 1951 (which began at 09 h 25 m), two photometers (with a field of view equal to \(4^\circ\)), located in the upper part of the rocket, were used. In this case, unlike the previous ones, the photometers were directed along the longitudinal axis of the rocket. Measurements of brightness

sky in different portions of the spectrum were carried out by using a rotating disk in which eight interference filters were mounted. In the ninth aperture of the disk there was a special lamp for checking the calibration of the photometers during flight. During each half-second (the period of rotation of the disk), measurements in the entire series of light filters were made by each photometer. The transmission limits of the filters corresponded to well-known lines and bands of the visible region; some, however, fell on portions of the spectrum between them. The results of measurements in three portions of the spectrum, 5910 Å, 6150 Å, and 6360 Å (see Table II), confirmed for these wavelengths the presence of a large and constant sky brightness, measured in 1950 for 4260 Å and 5590 Å.

For portions of the spectrum located in the interval from 4200 Å to 5900 Å, the sky brightness turned out to be greater than the maximum values that could still be measured by the photometers operating in this interval. Therefore, for wavelengths located within the limits from 4200 to 5900 Å, the authors were able only to establish that the light intensities must be greater than the values indicated in the third column of Table II.

Table II

Radiation in a spectral interval 20 Å wide, coming from the hemisphere (in milliwatts per square meter)

In the flight of the 1951 rocket, in addition to measurements of sky brightness by photoelectric photometers, the photographing of the sky was carried out by means of a camera. A modified GSAP camera was used, with a focal length of 25 mm, relative aperture 4.5, exposure of 1/400 sec., and a frame frequency of 6.67 per sec.; it was mounted in the same direction as the photometers. When the rocket reached an altitude of 70 km, several photographs of the clouds that, according to the authors’ supposition, were comparatively not far from the rocket, in a layer of minimum temperature, at an altitude of about 80 km, where, as is known, noctilucent clouds are observed under twilight illumination conditions, were obtained. The authors note rapid changes in the clouds observed over three frames taken during half a second.

In interpreting the results of the measurements made with the aid of photoelectric photometers, the authors consider that the brightness of the sky in the troposphere and the lower layers of the stratosphere is due chiefly

Rayleigh scattering of sunlight. Rayleigh scattering, with increasing altitude and decreasing air density, decreases almost exponentially and becomes negligibly small compared with the daytime sky glow beginning at approximately an altitude of 35–40 km, where the total brightness of the sky is about 2–3% (depending on direction) of the brightness of the sky measured at the Earth. The daytime sky glow that remains at the greatest accessible altitude (135 km) is constant; in their opinion, it is caused by resonance radiation, or fluorescence, or both together, and by the scattering of light by clouds in the upper layers of the atmosphere. This daytime sky glow, as the authors indicate, is by order of magnitude ten thousand times greater than the night-sky glow and is caused, in their supposition, by a region lying above the maximum altitude reached by the rocket.

V. Morozov

REFERENCES CITED

1. T. G. Megrelishvili and I. A. Khvostikov, DAN 59, 1283 (1948).
2. G. R. UFN 50, 145 (1953).
3. G. Newell and J. Siry, Problems of Rocket Engineering, no. 4, 3 (1953).
4. N. A. Miley, E. H. Cullington and J. F. Bedinger, Trans. Am. Geophys. Un. 34, 680 (1953).