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Radar echoes from the sun

The nature of solar radar echoes was a surprise in almost every aspect

Volker Grassmann, DF5AI

April 2005

The first solar radar echoes were detected by a research group at Stanford University in 1959. Transmitting 40 kW at 25.6 MHz by using an antenna of 3.450 sqm (square meters) aperture, the radar echoes were marginally significant with an integration time of 18 minutes. Regular observations were made between 1960 and 1969 by a MIT group using the 38.25 MHz solar radar near El Campo, Texas, which delivered 500 kW to a phased array of 18.000 sqm. In 1965, solar radar echoes were also detected at 40 MHz in Arecibo, attempts to detect echoes even at 2380 MHz with the Arecibo radar were however unsuccessful [1].

Before launchings research campaigns and before financing the instrumentation and the experimental setup, scientists develop theoretical models to analyse the scientific requirements, benefits and, most important, to analyse what information may be actually expected in an experiment ­ the same was true with the solar radar experiments. However, the observations of the El Campo radar "were a surprise in every respect" [1]. The echo strength was weaker and much more variable than expected, the mean Doppler shift was larger than expected, the Doppler broading was stronger than expected and also highly variable and there were significant anomalous events and their properties were also quite variable. All this findings attracted the interest of many solar physicists but "they were unable to explain even the gross features of the observations" [2] - the El Campo findings are therefore considered "one of the enduring mysteries of solar physics" [2] and "the observations have actually not contributed much to our present understanding of the sun and its corona" [1]. Is this a reason to be amused about the scientists’ inability to interpret the data? No, not at all. This is the way how science works and this is the reason why scientists need to carry out experiments. In this particular case there is actually a reason to take the hat off to the El Campo research group because it was indeed "ahead of its time” [2]. In fact, the phase of intense solar research hasn’t yet commenced in those days, i.e. the features of "coronal holes" and "coronal mass ejections" were not yet discovered and no satellite was available monitoring the solar wind and the solar surface ­ and this is the reason why today solar physicists wish to conduct solar radar experiments again, see, e.g., [2] and [4].

References and recommended readings

[1] Solar radar, Coles W. A., Solar and Space Weather Radiophysics, ch. 16
[2] A proposal to establish a solar radar at Arecibo, Coles W. A., Harmon J., Sulzer K., Isham B.
[3] Solar radiophysics with HF radar, Rodriguez P., Naval Research Laboratory
[4] Solar-terrestrial relationships at the LOFAR webpage
The El Campo Solar Radar, from [3].
Long-term average of the echo delay time,

Probing the solar corona

Probing the solar corona by using radiowaves is possible at frequencies between, say 15 MHz and 50 MHz. The reflection process may be compared to shortwaves reflected by the topside of the Earth's ionosphere: in terrestrial shortwave communication, we are located within a plasma sphere (i.e. the ionosphere) which reflects short radiowaves back to the Earth surface. Moving the transmitter outside of the spherical ionosphere (e.g. by placing the transmitter on a high flying satellite), radiowaves are now reflected by the topside of the ionosphere which is not accessible in terrestrial HF communication. In fact, scientists have carried out this type of experiments by using the so-called topside sounders. Detecting solar radio echoes is indeed similar to topside sounding - at a much longer distance though and by targeting a much more turbulent and violent plasma sphere with temperatures, magnetic fields and other effects much more complicated compared to the Earth ionosphere, i.e. the sun's corona.


Copyright (C) of Volker Grassmann. All rights reserved. The material, or parts thereof, may not be reproduced in any form without prior written permission of the author.