This Brief Communication presents a quantitative investigation for the effect of electron holes on electron-cyclotron maser (ECM) driven by horseshoe distributions. The investigation is based on an integrated distribution function for the horseshoe distributions with electron holes. Results show that the presence of electron holes can significantly enhance the ECM growth rate by 2-3 times in a very narrow waveband. The present study suggests that these electron holes probably are responsible for some fine structures of radiations, such as narrowband events in auroral kilometric radiation and solar microwave spikes.
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