ON THE INTERPRETATION OF THE RESULTS OF RADAR OBSERVATIONS OF A METEOR SHOWER

E. I. FIALKO

(Presented by the scientific seminar of the Radio Engineering Faculty)

In Astronomical Circular No. 173, a paper by Yu. A. Pugyshev was published, devoted to radar observations of the Perseid meteor shower of 1956, carried out at the Engelhardt Astronomical Observatory [1].

The experimental results presented are of definite interest. However, some statements made by the author of [1] raise objections.

In a series of curves depicting the mean hourly numbers of echoes on August 10–13, three maxima are clearly visible, corresponding to approximately \(22^h\), \(\simeq 1^h\), and \(\simeq 6^h\) sidereal time (Fig. 1 gives the results of observations on August 10–12). In connection with this, Yu. A. Pugyshev believes that “it is possible to distinguish three streams (I, II, III), the determination of whose radiants is as yet difficult [1].”

In our opinion, in explaining the cause of the appearance of several peaks in the number of meteor echoes, it is necessary to take into account the lobing of the directional pattern, caused by reflection of radio waves from the surface of the Earth (Fig. 2).

Fig. 1. Variation of the mean hourly number of meteor echoes on August 10–12, 1956 (according to data from the Engelhardt Astronomical Observatory [1]).
\(N\) — mean hourly number of detected meteors; \(t\) — sidereal time; 1 — August 10–11; 2 — August 11–12.

Consideration of the pattern of the diurnal motion of the Perseid radiant and of the antenna directional pattern (which during the observations was directed to the west and inclined to the plane of the horizon at an angle of \(22^\circ\)) under conditions of normality of meteor reflections\(^1\) leads to

\(^1\) Meteor reflections are observed in the case of a sufficiently high electron density in the section of the trail adjacent to the base of the perpendicular dropped from the point at which the radar is located onto the meteor trajectory.

confirms this assumption (Fig. 3). At a wavelength \(\lambda \simeq 4\) m, mainly those meteors will be detected whose ionized trails are formed in the regions most intensely irradiated by the radio waves. Consequently, an increase in the number of detected meteors will be observed when the radiant crosses planes perpendicular to the axes of the partial lobes (Fig. 3).

As is seen from Fig. 3, for approximately half a day there were favorable conditions for detecting meteors (when the radiant moved along the arc \(\mathrm{III}_1-\mathrm{II}_1-\mathrm{I}_1-\mathrm{I}-\mathrm{II}-\mathrm{III}\)); during the remaining part of the day the number of detected meteors should have decreased substantially. These assumptions agree with the observational results (Fig. 1).

From comparison of Figs. 1, 2, and 3 it follows that peak II corresponds to reception on lobe 3, peak I to reception on lobe 4, and peak III to reception on lobe 2.

In reception on the lower lobe 1, meteor echoes practically could not be detected because of the large slant range (\(R \simeq 1000\) km).

The considerations presented do not exclude the presence of inhomogeneities (branches) in the Perseid stream; however, the assertion that three streams can be distinguished [1] is untenable, since the influence of the properties of the antenna-system radiation pattern on the number of detected meteors was not taken into account.

Fig. 2. Radiation pattern of the antenna system (illustration): \(a\)—radiation pattern in the vertical plane (in free space), main lobe; \(b\)—radiation pattern of the antenna system (in the vertical plane), taking into account reflection of radio waves from the Earth’s surface; \(\beta_0\)—angle of inclination of the antenna arrow relative to the horizontal plane. The angle at which the direction of the most intense radiation of a partial lobe is inclined,

\[ \beta_{k(\max)} \]

is determined from

\[ \sin \beta_{k(\max)} \simeq \frac{\lambda}{4h}(2k-1), \]

where \(\lambda\) is the wavelength, \(k\) is the lobe number, and \(h\) is the height of the antenna above the Earth’s surface.

For \(h \simeq 3\lambda\),

\[ \beta_{1(\max)} \simeq 5^\circ,\quad \beta_{2(\max)} \simeq 15^\circ,\quad \beta_{3(\max)} \simeq 25^\circ,\quad \beta_{4(\max)} \simeq 35^\circ. \]

To test the validity of Yu. A. Pupyshev’s assertion concerning the presence of three streams in the Perseid stream, it would be advisable to carry out a simultaneous observation of the stream using two antennas: a directional antenna and a half-wave dipole located at a height of

\[ \simeq \frac{\lambda}{4} \div \frac{\lambda}{3} \]

above the Earth’s surface. In the latter case, lobing of the radiation pattern is absent and

the presence of significant peaks on the curve \(N(t)\) would indicate an inhomogeneity of the stream.

Fig. 3. Path of the Perseid radiant (illustration). 1, 2, 3, 4—lines of intersection of the sphere with planes normal, respectively, to the axes of the 1st, 2nd, 3rd, and 4th lobes of the antenna directivity pattern; the dashed line is the trajectory of the Perseid radiant. When the radiant is located at points III and III\(_1\), reception is carried out mainly on the 2nd lobe.

Conclusions

1. The peculiarities in the variation of the mean hourly numbers of detected meteor trails, noted during radar observations of the Perseid stream in 1956 [1], can be explained by the peculiarities of the antenna directivity pattern (and not only by the presence of three meteor streams within the Perseid stream).

2. In interpreting the results of radar observations of meteors, it is necessary to take into account the form of the directivity pattern of the antenna system.

3. To reveal inhomogeneity of a meteor stream, observations should be carried out using an antenna with a directivity pattern not subject to lobing.

References

1. Pulyaev Yu. A., Radar observations of meteor activity at the Engelhardt Astronomical Observatory, August 10–17, 1956, Astronomical Circular No. 173, 1956.