Solar radio astronomy

The solar radio astronomy deals with the radio emission from the sun and their causes.

overview

The sun is not just a radiator for light in the optical range. Rather, their electromagnetic spectrum covers a wide range of wavelengths and part of this spectrum is formed by radio radiation . With different antennas, such as B. in a facility belonging to the Astrophysical Institute Potsdam (AIP) , the radio waves emitted by the sun are captured, recorded and evaluated. An overview of the data is available to everyone publicly live on the Internet.

The radiation from the sun depends heavily on solar activity . If there is little solar activity (few sunspots ), their radio spectrum is constant over time. When there is high solar activity, the constant contributions are supplemented by time-variable components that clearly exceed the constant in intensity.

Radio emission from the less active sun

The refraction of the radio waves in the solar corona means that at certain wavelengths only radio waves can be measured that come from a wavelength-dependent minimum height. In detail this means:

Radio waves in the meter range come from areas far above the visible surface of the sun. Its intensity is greatest in the center of the solar disk and decreases towards the edge. 5 m waves are visible up to a distance of about two solar radii from the center of the solar disk.
At wavelengths between about 10 cm and 3 m, the radio waves come from a narrow, luminous area around the optically visible sun. The solar disk itself appears dark.
At wavelengths below 10 cm, the refraction of radio waves is similar to that of visible light, and the low density of the corona also has an effect. As a result, radio waves pass through the corona approximately in a straight line and the radiation from the corona is so weak that no bright, narrow edge is visible.

Mechanism of origin of radio radiation

In the non-thermal radio radiation which mainly from the corona of the sun is emitted is plasma radiation, which due to the collective oscillations of the plasma produced. Therefore, conclusions can be drawn from the frequency of the radio radiation on the frequency of the local oscillations . With a more precise calculation it can be shown that the local electron plasma oscillation frequency is proportional to the square root of the particle number density of the electrons : ${\ displaystyle f _ {\ text {radio}}}$ ${\ displaystyle f _ {\ text {plasma}}}$ ${\ displaystyle f _ {\ text {plasma}}}$ ${\ displaystyle N_ {e}}$

{\ displaystyle f _ {\ text {radio}} \ approx f _ {\ text {plasma}} = {\ frac {1} {2 \ pi}} {\ sqrt {\ frac {e ^ {2} N_ {e} } {\ varepsilon _ {0} m_ {e}}}}}

( Elementary charge , electric field constant , electron mass , particle number density of electrons ) ${\ displaystyle e}$ ${\ displaystyle \ varepsilon _ {0}}$ ${\ displaystyle m_ {e}}$ ${\ displaystyle N_ {e}}$

The solar atmosphere is barometric due to solar gravity, which means that the particle density increases towards the solar center. There is therefore a clear correlation between the electron plasma frequency of the emitted radio waves and the distance of the radio source from the center of the sun. This makes it possible to unambiguously assign an altitude of the radio source to each radio frequency (provided one has a density model of the sun): Radio radiation of high frequencies is therefore generated from the radio sources near the center of the sun. With the help of radio spectra, the movements of radio sources within the solar atmosphere can be observed. ${\ displaystyle N_ {e}}$

Radio emissions from the active sun

If the sun is in a phase of high activity, there are two additional components of solar radio waves:

The slowly variable component: These radio waves come from discrete areas of the visible solar disk, mostly from spots. The intensity correlates with the relative sunspot number . The wavelength of this component is 1 to 100 cm with a maximum at 15 cm.
Radiation bursts: The wavelength is between 1 cm and 15 m. The intensity during the bursts of radiation is 20 to 40 times the normal value for the short wavelengths and 100,000 times for the long wavelengths. As described above, the frequency depends on the properties of the corona, so that the migration of the eruption through the solar atmosphere can be followed from the time course of the frequency.

Trivia

The radio radiation from the sun contributes as sun noise to the noise temperature at the entrance of radio receivers . Directional radio links and satellite reception can be disturbed if the receiving antenna looks into the sun.

Individual evidence

↑ Current solar activity
↑ Kristen Rohlfs, TL Wilson et al. a .: Tools of radio astronomy. Springer, Berlin 1999. ISBN 3-540-66016-X , pp. 254-257
^ Arnold Hanslmeier : Introduction to Astronomy and Astrophysics . Spektrum Verlag, 2nd edition 2007. pp. 227-229, ISBN 978-3-8274-1846-3
^ Hans Dodel, Sabrina Eberle: Satellitenkommunikation . Springer, 2007, ISBN 978-3-540-29575-4 , page 29.

Web links

Solar radio astronomy at the AIP

[1] Current solar activity

[2] Kristen Rohlfs, TL Wilson et al. a .: Tools of radio astronomy. Springer, Berlin 1999. ISBN 3-540-66016-X , pp. 254-257

[3] Arnold Hanslmeier : Introduction to Astronomy and Astrophysics . Spektrum Verlag, 2nd edition 2007. pp. 227-229, ISBN 978-3-8274-1846-3

[4] Hans Dodel, Sabrina Eberle: Satellitenkommunikation . Springer, 2007, ISBN 978-3-540-29575-4 , page 29.