Radio Star Scintillations Research Articles

A comparison of radio star scintillations at 1390 and 79 Mc/s at low angles of elevation

Abstract Observations of radio star scintillations have been made on a radio frequency of 1390 Mc/s. Evidence is presented which indicates that they are ionospheric in origin and a value of 100 km has been set for the altitude at which the scintillations are impressed upon the incoming radio waves. The 1390 Mc/s records have been compared with routine 79 Mc/s scintillation records and it is found that the theoretical laws governing the variation of scintillation amplitude with observing frequency do not hold over this large frequency range. This cannot be explained in terms of a diffraction effect and it is suggested that the breakdown is due to absorption of 79 Mc/s radiation from sources at low angles of elevation.

Diffraction by a Screen Causing Large Random Phase Fluctuations

ABSTRACTThe propagation of a scalar wave is studied, after it has suffered changes of phase in its passage through an irregular refracting medium, such as the ionosphere. The rms phase fluctuations may be large; i.e. greater than one radian. The paper is in three parts. In the first, the receiver is at a great distance from the screen (Fraunhofer diffraction), and a formula is found for all even order moments of the amplitude, and for the correlation function of the even powers of the amplitude. The probability distribution of the amplitude is proved to be a Rice distribution. The multiple integrals expressing the moments of the amplitude reduce to sums of correlations of the original phase fluctuations amongst points over the screen, which are evaluated using combinatorial arguments.In the second part the variance of the square of the amplitude is calculated numerically for points near the screen (Fresnel diffraction). When the rms phase fluctuation is greater than one the variance is maximum near the point where the irregularities in the screen would bring the rays to a focus.In the third part radio star scintillation and scatter propagation are discussed in the light of the theory.

The Correlation of Radio Star Scintillations with Geomagnetic Disturbances

The correlation between the degree of scintillation of the radio source Cassiopeia A and the local magnetic K-index is studied. It is shown that the correlation coefficient between these quantities depends upon the epoch of the solar cycle at which the observations are made, on the position of the source in the sky, and on the radio frequency used. For observations on 38 Mc/s made at sunspot minimum, the correlation coefficient is small but positive for all positions of the source. At sunspot maximum, the correlation coefficient is positive when the source is near upper transit and negative when it is near lower transit; the average value of the correlation coefficient for all positions of the source is approximately zero. It is shown that these results can be explained when the finite angular diameter of the radio source is taken into account. For some conditions an increase in the degree of irregularity of the ionosphere can result in a decrease in the degree of scintillation which is observed. It is suggested, therefore, that the degree of irregularity of the ionosphere always increases during magnetic disturbances in medium latitudes, and that the negative values of the correlation coefficient between scintillation index and magnetic K-index which are sometimes obtained are due to a secondary effect related to the finite size of the source. The results of the present paper explain the inconclusive and conflicting results of earlier workers.

Height of Night-time F Layer Irregularities at the Equator

IT has been established by observations at low latitudes that there exists near the equator a belt in the F region in which large irregularities in electron density give rise to equatorial spread F1 severe radio star scintillations2 and the scatter of radio waves3. Work by Hewish4 and Briggs5 on extra-terrestrial radio sources shows how the height of the irregularities may be inferred from observations of the diffraction pattern on the ground. But the methods used are rather indirect, and therefore subject to some uncertainty. More recently Cohen and Bowles6, using radar techniques, arrived at an estimation of irregularities of height for equatorial locations. It is not certain that these irregularities are to be identified with those responsible for satellite and radio star scintillation.

Scintillations and the Latitude Distribution of Ionospheric Irregularities

KENT1 has reported a series of observations on the 40 Mc./s. transmissions from Sputnik 1 made at Cambridge during October 1957. He found that the transmissions always underwent rapid fading (scintillation) when the satellite was north of Cambridge but only occasionally when south of Cambridge. He concluded that the scintillations were due to ionospheric irregularities which are north of the observing station but not south of it. Further, since the southward extent of the scintillation region was greater for satellite passes to the west of Cambridge than to the east, he postulated that the density of the irregularities is controlled by the magnetic rather than geographic latitude (Cambridge's declination is about - 9°). Mawdsley2 objected to Kent's seemingly arbitrary assumption that the irregularities do not occur south of the observer, on the grounds that radio star scintillations are observed at lower latitudes than that of Cambridge. He suggested an alternative interpretation of Kent's results in terms of preferential forward scattering by the irregularities, assumed to be field aligned, when the line of sight from the observer to the source is normal to the field lines. Afterwards, Bain 3 and Frihagen and Troin4, working at Slough and Kjeller respectively, observed scintillations when the satellite under observation was south of the observing station. However, they both found that the scintillation activity increased for geomagnetic latitudes greater than that of Cambridge, that is, 55°. This led Frihagen and Troin to support Kent's interpretation. It is the purpose of this communication to point out the existence of further pieces of evidence which support Kent's point of view rather than Mawdsley's.

A statistical study of ionospheric drifts measured by the radio star scintillation technique

Ionospheric drifts have been measured by the radio-star scintillation spaced receiver technique and histograms are presented showing the magnitudes and directions of the drifts observed. The direction of drift is commonly found to reverse suddenly near 2300 hours local time, but the complementary reversal occurs at a less definite time during the following day.

Ionospheric Scattering of Satellite Transmissions

RADIO waves from satellites may fade in both regular and random fashion when received with a linearly polarized aerial. Fading due to Faraday rotation or satellite rotation is an example of the former type. Random fades, up to several per second, are sometimes observed, and these have been attributed to scattering from the same kind of ionospheric irregularities as produce radio star scintillation. This fading is more rapid for satellites than for radio stars since it depends on satellite velocity rather than ionospheric drift velocity. Henceforth we shall be concerned only with the randomly fading component.

Statistical analysis of radio star scintillation

Radio star scintillation data are analysed by statistical techniques to find the autocorrelation functions and power density spectra of the fluctuations in amplitude and phase. Interpretation is given in terms of a drifting phase-changing screen as postulated by Booker, Ratcliffe and Shinn. It is shown how the drift velocity of the irregularities in ionospheric electron density that lead to the scintil lations, as well as the mean square amplitude and phase of the fluctuations, may be obtained from these records.

Radio star scintillation and multiple scattering in the ionosphere

Recent experimental evidence of radio star scintillation indicates that multiple scattering effects are of importance in the ionosphere. It is therefore of interest to apply the transport equation for the expectation of the photon density function to this problem. The solution of the transport equation is used to predict the mean-squared scattering angle and corresponding size of the ionospheric irregularities as measured on the earth. The particular example discussed in detail is based on a Gallet model for turbulence in the underside of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</tex> layer under nighttime conditions. However, if should be noted that the general theoretical results can be applied to any other model as well.

The magnetic storm-time variation of radio star scintillations and auroral radio echoes

The magnetic storm-time variation of radio star scintillations and auroral radio echoes

Amplitude Scintillation of Radio Star at Ultra-High Frequency

Observations of radio star scintillation at a frequency of 915 Mc over a 12-month period (October, 1957-September, 1958) are described. By daily observations of Cygnus A (IAU 19N4A) over a wide range of elevation angles, both the amplitude and rate of scintillation were studied. It was found that the amplitude scintillation is strongest near the horizon and decreases rapidly as the altitude increases. The amount of the intensity fluctuation ranges from less than 3 per cent to over 50 per cent of the undisturbed intensity, while the rate of fluctuation varies from ? to 8 peaks per minute. A fluctuation amplitude greater than 10 per cent was observed at low-altitude angles for 71 per cent of the total observations. The frequency of occurrence of scintillation follows a seasonal variation having two maxima; one in the winter and the other in the summer. During these two maximum periods, the mean fluctuation amplitude was found to be higher than during other periods. It was also found that the mean fluctuation rate in the winter is about three times faster than that in the summer. It was also observed that the scintillation characteristics are markedly affected by the presence of auroras and/or geomagnetic disturbances. During the winter months, the fluctuation rate is roughly proportional to the three-hour geomagnetic K index.

Observed variations in the amplitude scintillations of the Cassiopeia (23N5A) radio source

Continuous monitoring of the radio source in Cassiopeia (23N5A) on a frequency of 79 Mc/s has made possible an analysis of the variations of radio star scintillations observed from Jodrell Bank (latitude 53°N). Monthly average daily variations are presented as functions of solar time, and the mean daily variations averaged over a year are evaluated as functions of both solar and sidereal time. An effect due to the geomagnetic latitude at which the observations were made, is demonstrated. Since records are available for over 4 years, the variation of scintillations with the sunspot cycle has been studied, and it has been found that in years of high solar activity scintillations show a strong seasonal dependence with maximum activity at the equinoxes. When examined over this long period, there is a strong negative correlation between scintillations and the occurrence of spread- F echoes. Evidence has been obtained that the angular size of the Cassiopeia source is sufficiently large to produce scintillations different from those expected of a point source.

Scintillation, spreadF, and transequatorial scatter

Radio-star scintillations of great intensity are a common occurrence at the equator. Near the minimum of sunspot activity they exhibit little dependence on magnetic activity, but as sunspot maximum approaches there is a marked negative correlation between the occurrence of scintillation and the degree of magnetic disturbance. Radio-star scintillation is found to be much greater at sunspot maximum than at sunspot minimum.

Design and Performance of V.H.F. Receiving Equipment for Reception of Extremely Weak Signals of Solar and Galactic Origin in the Metre Wavelength Region

Three V.H.F. receivers of high gain stability and low internal noise designed and constructed at the Kodaikanal Observatory are described. The receivers which operate at 1½, 3 and 5 metre wavelengths are being used for the measurement of extra-terrestrial emission of solar and galactic origin and also for observations of radio-star scintillation. A report on the performance of one of the receivers over long periods together with details of aerial systems used and calibrating equipment is also given.

Effect of line-of-sight aurora on radio star scintillations

Evidence is presented to show that active auroral forms that pass across the line of sight to a radio star can have a direct effect on the scintillations of the signal received on a frequency of 223 Mc. This effect is most pronounced if the level of scintillation activity is relatively low before the aurora crosses the star position. Two examples are presented: in the first, an intense auroral band between the radio source in Cassiopeia (angular diameter, 4′) and the receiver disrupts the characteristic sinusoidal trace produced by the radio star signal in the output of the phase switch interferometer. This long-duration fade lasted for a period of about 8 minutes. In the second example, the aurora is accompanied by strong angular scintillation coincident with strong absorption of 27.6 Mc galactic radio waves. In both examples the radio star was within 10° of the geomagnetic zenith.

Ionospheric drifts determined from radio-star scintillation observations

Spaced-receiver observations of radio-star scintillation were made at the Royal Society Base, Halley Bay, during the I.G.Y. A technique is described whereby ionospheric velocities can be deduced from the motion of the diffraction pattern on the ground. These velocities were approximately towards the west in the first part of the night, and towards the east for the rest of the night and most of the next day. The reversals in direction of motion occurred at 0030 and 1630 L.M.T. Drift speeds were of the order of 400 m/s in the evening, falling to 100 m/s after easterly motions had been established. Both the time of midnight reversal and the drift speed correlate well with Evans’s (1959) measurements of motions of auroral forms.

The simultaneous observation of radio star scintillations on different radio-frequencies

A comprehensive study of the phenomenon of radio star scintillations has been made over the range 26 Mc/s to 408 Mc/s, by recording simultaneously on at least two frequencies within this frequency range. With records obtained over a wide range of scintillation conditions, it has been possible to test the theoretical laws governing the frequency dependence of scintillation amplitude and rate. It is established that under extreme conditions these laws no longer hold. A study of the probability amplitude distribution of scintillations covers a wide range of mean scintillation amplitudes. The smaller scintillations have a Gaussian or Rayleigh amplitude distribution, but with increasing scintillation activity, discontinuities appear in the distribution obtained. Under the most intense conditions a nett attenuation of the source is observed. The cross-correlation coefficient between records has been determined over a frequency ratio of 3:1 and the results are compatible with those of other workers.

Geomorphology of spread F and characteristics of equatorial spread F

Between magnetic latitudes 20°N and 20°S there is a well defined region where spread F is a normal occurrence on magnetically quiet days. Equatorial spread F is a night-time phenomenon that begins between 1900 and 2200 by a characteristic doubling of the layer and an increase in virtual height, indicating a vertical velocity. Later in the night, after 2300, the records show group retardation and no stratification. The occurrence of equatorial spread F is decreased by magnetic activity. Radio-star scintillations in equatorial regions are correlated strongly with spread F and have the same diurnal variation and the same anticorrelation with magnetic activity. The phenomenon of ‘flutter’ (rapid fading of long-distance, high-frequency stations) shows the same variations. Measurements using the spaced receiver method indicate that, in the equatorial region, the irregularities in the F layer are greatly elongated along the lines of the earth's magnetic field.

Large-scale movements of ionization in the ionosphere

The complexity of the causes of variations in and motions of ionization in the ionosphere is noted, as is the difficulty of differentiating real and virtual motions. An instability mechanism for deviations in ionization density is suggested, for which the predicted temporal and spatial morphologies appear to be consistent with those of the occurrence of sporadic E, spread F, and radio-star scintillations.

Magnetohydrodynamics of the small-scale structure of the F region

The influence of the E region on Martyn's model for the explanation of radio-star scintillations and spread F is briefly discussed.