The Solar Atmosphere at Radio Wavelengths

We briefly review the status of radio observations of the solar atmosphere with an emphasis on relevance to stellar observations. 1. Radio Emission Mechanisms Solar radio observations are sensitive to virtually every feature of the solar atmosphere, covering a temperature range from the temperature minimum (5000 K) up to the soft Xray-emitting plasma produced by solar flares (> 107 K) and the most energetic electrons accelerated by the Sun (energies > 1 MeV). The reason for this broad sensitivity lies in the fact that four very different emission mechanisms, each of them well understood, contribute to opacity at radio wavelengths: (i) free–free or bremsstrahlung emission is the ubiquitous mechanism of solar radio astronomy, present even when the others are absent. The opacity is proportional to NeNi= f 2T 1:5 where Ne;Ni are the electron and ion number densities, f the frequency and T the temperature. It is therefore particularly effective at high densities, low frequencies and low temperatures. (ii) “H opacity” is the misnomer applied to the mechanism that replaces bremsstrahlung at the very low temperatures in the lower chromosphere at which H and He are neutral. Free electrons due to ionization of light metals such as sodium can polarize an H atom and the interaction between the electrons and the dipolar atoms provides opacity. This mechanism is responsible for the quiet–Sun contribution to radio emission at very high frequencies where the optically thick layer in the solar atmosphere lies low in the chromosphere or temperature minimum where the temperature is well below 104 K. (iii) Gyroresonance opacity is due to the acceleration experienced by an electron moving through a magnetic field under the Lorentz force. The acceleration associated with the gyromotion provides opacity in the radio regime at frequencies that are integer multiples (harmonics) of the electron gyrofrequency, fB = 2:8 106 B Hz, where the magnetic field B is measured in gauss (G). This is the dominant source of opacity in regions of strong magnetic fields in the corona. The nonthermal version of gyroemission (called “gyrosynchrotron” emission when mildly relativistic electrons dominate, or “synchrotron” when ultrarelativistic electrons dominate) is responsible for most flare radio emission. (iv) Plasma emission is commonly found to be produced by energetic electrons at low frequencies. It involves the production of electrostatic Langmuir waves at the plasma frequency fp = 9000 p Ne (Ne measured in cm 3), and the subsequent conversion of these electrostatic waves to propagating electromagnetic waves at fp and its harmonics. This mechanism produces the well-known “zoo” of radio bursts at low frequencies where electron beams and shocks can radiate efficiently and free–free absorption is low.