Effects of Fully Relativistic Condition on Electron Cyclotron Maser Emission

The electron cyclotron maser (ECM) instability is a very important nonthermal radiation mechanism. It has been developed by proposing various electron distribution functions as well as the relativistic resonance condition, called the semirelativistic correction. Taking account of the relativistic effects of both the velocity distribution of energetic electrons and the resonance condition, called the fully relativistic correction, the present paper investigates the ECM instability driven by a power-law electron distribution with a low-energy cutoff. The results show that (1) both in the semirelativistic and fully relativistic cases, the growth rate and relative frequency bandwidth of ordinary ( O ) and extraordinary ( X ) modes show a positive correlation with cutoff energy E c , i.e., the peak frequency decreases with increasing E c ; (2) the peak frequency ratio ( H peak / F peak ) of the harmonic and fundamental waves is always ∼2; (3) compared with the semirelativistic case, the fully relativistic case has a larger growth rate (for both the O and X mode) and a smaller peak frequency (only for the O mode) for energy > 50 keV, and there is almost no difference at lower energy for the two cases; (4) the peak frequency of the X1 mode can be higher than its cutoff frequency in a strongly magnetized plasma, implying that the X1 mode emission may escape more easily for a higher E c and stronger magnetic field. These results can be helpful for us to understand better the physics of radio bursts from the Sun and other objects.