Radio-astronomical Observations of the Sun at 2.7 GHz

Susanne Schell, Michael Mittelstaedt, Michael Winter

During the spring of 1987 a group of students from our school gave us a partially assembled radio telescope (a 1.75m parabolic mirror with a receiver). We repaired the transmission, improved the control electronics and aligned the equatorial mount of the parabolic reflector using our "dipole shadow method". After determining the antenna's radiation pattern, we were able to determine sources of measurement errors and eliminate systematic errors. Thereafter, we conducted regular solar measurements by tracking the sun with the telescope every day (including holidays). This way, we were able to detect several radio bursts. The most remarkable one was observed on 24th July 1987 and it is discussed in our report in greater detail.

1987 was a quiet year in terms of solar activity. Therefore we calibrated the charts using calculations of the sun's thermal emission as a first approximation. We were able to improve this calibration when professional radio observatories published their findings for our period of observation.

During the summer holidays (24th July 1987), we observed a very remarkable radio burst that consisted of overlapping bursts of radiation. The radio observatories in San Vito (Italy) and Ondregov (Czechoslovakia) confirmed that this was not local interference from a source near Bad M√ľnstereifel. They also informed us about the size of the corresponding Hα-flares. This allowed us to analyse our measurement of the radio burst in great detail. Initially, we looked for mathematical characteristics. By plotting the radio burst on a logarithmic scale we were able to show that the radiation flux of our "double burst" decayed exponentially (S1 = 131.8 ⋅ e-0.006 ⋅ t; S2 = 59.3 ⋅ e-0.008 ⋅ t. By integrating these functions over time, we were able to calculate the energy that our telescope collected during the radio burst (W = 8.25 ⋅ 10-10 J).

We tried to understand the nature of the radiation and the underlying mechanism that caused the emission on the sun based on the measured radiation flux. Using Rayleigh-Jeans law, Wien's displacement law, the Stefan-Boltzmann law and Einstein's famous equation E = mc2 we were able to show that the detected radiation could not be of thermal origin. However, also the common view that radio bursts result form synchrotron radiation of highly relativistic electrons (v > 0.9c) seems questionable based on our frequency-flux-diagram. In our opinion, the radio burst that we detected was caused by magnetobremsstrahlung of moderately relativistic electrons (0.2c < v < 0.8c). Furthermore, our time-frequency-diagram shows that the emission region moved outwards in the solar corona during the observation.

We hope that our observations make a small contribution to solving the scientific questions surrounding the origin of radio bursts. We are planning to start collaborating with a radio observatory near Zurich (Switzerland) soon, as the observations of a single radio telescope cannot yield certain results. One of the goals of this collaboration is to participate in the current discussion about the existence of periodic solar intensity variations on a time scale of minutes. This requires cross correlation of our measurements with those from Zurich in order to determine the pure solar intensity variations. We have already written a computer program to perform this cross correlation. We have also written a program that performs Fourier analysis of the complicated solar radiation charts and that is easy to use for 11th grade students.

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