ON T HE S P E C T R A OF T Y P E - I I I S O L A R R A D I O B U R S T S

OBSERVED AT LOW F R E Q U E N C I E S

H]~CTO R A L V A R E Z

Departamento de Astronomia, Universidad de Chile, Santiago, Chile

(Received 17 November, 1980; in final form 16 April, 1982)

Abstract. The spectra of strong bursts observed at low frequencies by OGO-5 during 1968-1970 are presented. They usually exhibit an intense main peak between 100 kHz and 1 MHz, and sometimes a less intense secondary peak between 1 and 3.5 MHz. Main peaks of 10-12 Wm-2 Hz- ~ or more were obtained in very strong events, but because of antenna calibration problems those could be one or two orders of magnitude too high. Recently published work supports the finding that type III bursts at low frequencies can be at least four orders of magnitude more intense than at ground-based frequencies of observation. It is found that the energy received at the Earth increases with decreasing frequency approximately as f - ' , where 3 < n <4.

1. Introduction

The Orbiting Geophysical Observatory 5 (OGO-5) was launched on March 4, 1968. It carried, among others, the University of Michigan radio astronomy experiment whose main goal was to observe solar radio bursts at frequencies between 3.5 MHz and 50 kHz. As it turned out, the only bursts the experiment could detect were of type III. This type of bursts is produced by streams of fast particles ejected by the Sun. As the particles travel through the interplanetary plasma they excite plasma oscillations that in turn generate electromagnetic waves.

The OGO-5 results were presented in a University of Michigan internal report (Alvarez, 1971). Some of those were published in current scientific journals while others were not. Among the unpublished results were those concerning the radio bursts spectra. The spectra defined by the peaks of the burst time profiles at the different frequencies of observation gave, after applying an antenna calibration correction, flux densities considered too high to be realistic. For this reason they were not published. In spite of the uncertainties in the flux density scale the OGO-5 study revealed typical shapes in the spectra and very large fluxes.

The spectrum of type III bursts is one of the characteristics less studied and recently has received some attention theoretically (Takakura, 1979) as well as observationally (Weber, 1978a, b, 1979).

The paper by Weber (1979) was the first to come to our attention. It is based on data better than that of the OGO-5 experiment and the results obtained confirmed unpublished findings of OGO-5. On the theoretical side the work of Takakura, apparently the only one extended to low frequencies, predicts some characteristics for the type III bursts below about 10 MHz that we do find in OGO-5's data. These facts have encouraged us to publish the spectra measured by the OGO-5 experiment. The purpose of this note is to present and discuss that data.

Solar Physics 81 (1982) 355-362. 0038-0938/82/0812-0355501.20. Copyright �9 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

356 H]~CTOR ALVAREZ

2. The OGO-5 Experiment

The University of Michigan experiment consisted of a superheterodyne receiver stepping in 9.2 s through the following frequencies: 3500, 1800, 900, 600, 350, 250, 100, and 50 kHz. Each channel had 10 kHz bandwith. The antenna was a 9.15 m monopole extending out from one of the solar paddles. Electrically it was considered to behave as a center-fed dipole. The receiver was calibrated before flight using one and the same dummy load to replace the antenna at the eight frequencies. At the time of the data analysis a correction was introduced in the calibration to allow for variations of the dummy load with frequency.

Some of the initial orbital parameters of the spacecraft were: height of perigee, 292 km, height of apogee 147 000 km, period of 63 h 21 m. A more detailed description of the experiment and the data has been given elsewhere (Alvarez and Haddock, 1973).

3. The Observations

The channels at 350 kHz and below suffered from severe spacecraft interference. For this reason only 64 bursts were detected at or below 350 kHz between March 1968 and February 1970. Only one burst, that associated with the proton event of November 18, 1968, was detected down to 50 kHz (Alvarez and Haddock, 1973). Therefore, the OGO-5 results apply only to very strong events. The spectra of only five bursts are available and will be presented here.

4. Discussion of the Results

4.1. THE POWER SPECTRA

We have defined the power spectrum of a type III burst by the flux density of the peak of the time profiles obtained at the different channels. Because of the nature of the mechanism that generates type III bursts such a spectrum is different from the spectrum of ordinary radio sources in that the radiation at different frequencies is produced at different distances. There are several factors that should be considered in the inter- pretation of such a spectrum:

(a) The flux density should be corrected by the inverse-distance squared law. This requires knowing the position of the source in space.

(b) It is necessary to know whether the received radiation was emitted as fundamental or second harmonic, since this will affect (a).

(c) The effect of the receiver bandwidth is especially important at low frequencies (km 2). The radiation entering the band has a contribution from different plasma levels and does not represent the emitted spectrum at the plasma level corresponding to the center of the band. For example, assuming reasonable electron densities for the interplanetary plasma and a 10 kHz band, the plasma levels corresponding to the ends of the band centered at 50 kHz are separated by distances of the order of some tens

SPECTRA OF TYPE-III SOLAR RADIO BURSTS 357

of solar radii. S aid in a different way, the amplitude of a burst profile at a given frequency and at a given time has contributions from radiation emitted at different places and at different times.

(d) The directivity of the emitted radiation should be known. This depends on (b). (e) The directivity of the receiving antenna should be known. (f) The complexity of the burst time profile may be due to successive particle

injections, to fundamental and/or second harmonic radiation or to radio wave reflec- tions.

Since these factors were poorly known or unknown we decided not to correct the data for them.

Figure 1 shows four of the five available power spectra corresponding to simple radio bursts. The background noise was subtracted and the calibration correction was applied. It was found that most of the time the spectra were single or double-peaked with the principal maximum reaching fluxes of the order of 10-12W m -2 Hz-1. The secondary peak could be several orders of magnitude less intense than the main peak.

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December 24, 68, May 6, 69, and June 18, 69, respectively.

358 H~CTOR ALVAREZ

The main peak occurred usually between 100 kHz and 1 MHz while the secondary peak ocurred between 1 and 3.5 MHz. It was impossible to locate more precisely the position of these peaks due to the crude frequency resolution of the radiometer, especially at the high frequency end. The same double-peaked pattern is seen on the event of August 4, 1972 obtained by Weber (1979) showing maxima at about 300 kHz and 3 MHz, and a through close to i MHz. It would be of interest to investigate if the double peak is associated with the radiation change from fundamental to second harmonic exhibited by some bursts at these frequencies (Haddock and Alvarez, 1973).

Figure 2 shows the November 18, 1968 event. This event was associated with a complex solar proton event. The radio burst peaked at 50 kHz, reaching about 5 x 10-11 Wm-2 Hz- 1 Considering that the associated flare was on the west limb and that the emission was most probably second harmonic it is possible that the 50 kHz source was very close to the Earth. In that case the inverse-distance squared correction would substantially decrease the intensity at the lowest frequency. From the values of typical starting frequencies of type III bursts (Malville, 1967) we could safely assume that the spectrum below about 200 MHz belongs to a type III. Between 10 and

Fig. 2.

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SPECTRA OF TYPE-III SOLAR RADIO BURSTS 359

100 MHz the data suggest a secondary peak and above about 100 MHz the spectrum foolows the same pattern found by Castelli et al. (1967) in other proton events.

A comparison of the spectra obtained by OGO-5 with those of other observers indicates that the flux densities determined by OGO-5 are not unrealistically high. The flux density of the November 18 event at 3.5 MHz is 2 x 10-15 Win-2 H z - l ; this order of magnitude can be found in Weber's (1978a) data at similar frequencies. The flux density at 50 kHz is four orders &magnitude higher than that at 3.5 MHz; increases as large as this are found in Weber's data and are predicted by Takakura's (1979) simulation. There is no record of a flux density as high as 10-11 W m -2 H z - 1, as shown in Figure 2, and we believe that this value could be too high but not more than about two orders ofmaguitude for the following reasons: First, since the event was associated with a west limb flare the trajectory of the exciter particles probably passed close to the Earth. Second, the November 18 event was extremely strong. Third the burst occurred at the maximum of a solar cycle. Fourth, Figure 2 gives 8 x 10 -13 W m -2 Hz -1 at 600 kHz while Evans et al. (1971) give lower limits as high as 10-14W m -z Hz-1 at frequencies close to 700 kHz.

4.2. T H E E N E R G Y S P E C T R U M

Instead of plotting the peak of the burst profile versus frequency we can plot the area under the profile as a function of frequency. This area represents the energy Esper unit frequency interval that is received per unit surface at the Earth during the burst lifetime at a given frequency. This energy spectrum was first defined by Wild (1950) who obtained it between 70 and 130 MHz. He found that the energy increases as the frequency decreases according to a inverse power law E F ~ f - n with plausible values for n between 2.8 and 3.6. A best fit line through his data gives

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with frequency in MHz.

We obtained the energy spectrum for eight events having the simplest time profiles and, as before, no correction was applied. Figure 3 shows the spectrum resulting from the average of those eight bursts. Also shown are the results from Wild (1950) and the points deduced from the data of other observers at different times. An inverse power law with n ~ 3.6 fits the ground-based and the space data down to about 350 kHz where we see an indication that the spectrum bends down. Figure 4 shows the energy spectrum of the November 18, 1968 event which is striking because the ground-based and space data fit very well a line corresponding to

Ef = 10- lO .5Of-3 .oo Joule m -2 H z - 1 (2)

from about 270 MHz down to 50 kHz. A comparison of the space data of Figures 3 and 4 reveals that the average values of Figure 3 practically coincide with those of the November 18 event between 3 .5MHz and 350kHz. This indicates that the November 18 event was different only in that it continued to radiate intensely down to 50 kHz.

360 H~CTOR ALVAREZ

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the references. The OGO-5 data are uncorrected.

If we integrate Ef between the starting, f l , and the cut-off, fo, frequencies of the event we have the total energy E received per unit surface at the Earth during the lifetime of

the whole event. Because of the form found for Ef

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That is, the total energy is practically determined by the low frequency radiation. Melrose (1970) integrated the expression (1) assuming a cut-off frequency of approxi- mately 35 MHz and that the radiation was isotropically emited. He obtained 1018-1019 erg per event. (The actual values he gives are 102~ erg due to an error

in the exponent o f f in his Equation (8). The error originates in a misprint in Kundu's book (1965) where the exponent appears as - 3.5 instead of - 3.25). A similar calculation using the data of Figure 3 with a cut-off of 350 kHz gives 1026 erg. We believe that the correction of the space data for the effects mentioned earlier would not decrease

S P E C T R A O F T Y P E - I I I S O L A R R A D I O B U R S T S 361

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significantly the large difference between the ground-based and space data. The low frequency results indicate that, accepting that the number of particles in the beam is constant, the efficiency of the conversion of kinetic into electromagnetic energy increases fast with decreasing frequency. This result should be of interest to the theories of type III solar bursts extended to low frequencies.

Acknowledgements

The author thanks Prof. T. Takakura for his comments on an early manuscript and J. May for a helpful discussion. The original OGO-5 data analysis was funded by N A S A under contract N A S 5-9099.

References

Alvarez, H.: 1971, UM/RAO Report 71-9, Doctoral Dissertation. Alvarez, H. and Haddock, F. T.: 1973, Solar Phys. 29, 197. Castelli, J. P., Aarons, J., and Michael, G. A.: 1967, J. Geophys. Res. 72, 5491. Evans, L. G., Fainberg, J., and Stone, R. G.: 1971, Solar Phys. 21, 198.

362 H~CTOR ALVAREZ

Feix, G.: 1970, J. Geophys. Res. 75, 211. Goldstein, S. J.: 1959, Astrophys. J. 130, 393. Haddock, F. T. and Alvarez, H.: 1973, Solar Phys. 29, 183. Kai, K.: 1970, Solar Phys. 11,456. Krtiger, A.: 1970, World Data Center, A, Report UAG-9. Kundu, M. R.: 1965, Solar Radio Astronomy, Interscience, New York. Malville, J. M.: 1967, Solar Phys. 2, 484. Maxwell, A., Howard, W. E., III, and Garmire, G.: 1960, J. Geophys. Res. 65, 3581. Melrose, D. B.: 1970, Australian J. Phys. 23, 885. Slysh, V. I.: 1967, Soviet Astron. 11, 389. Takakura, T.: 1979, Solar Phys. 61, 161. Weber, R. R.: 1978a, NASA/GSFC X-692-78-12. Weber, R. R.: 1978b, Solar Phys. 59, 377. Weber, R. R.: 1979, Nature 280, 570. Wild, J. P.: 1950, Australian J. Sci. Res. A3, 541.