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Solar Physics

, Volume 291, Issue 12, pp 3637–3658 | Cite as

Is Cyclotron Maser Emission in Solar Flares Driven by a Horseshoe Distribution?

  • D. B. Melrose
  • M. S. Wheatland
Article

Abstract

Since the early 1980s, decimetric spike bursts have been attributed to electron cyclotron maser emission (ECME) by the electrons that produce hard X-ray bursts as they precipitate into the chromosphere in the impulsive phase of a solar flare. Spike bursts are regarded as analogous to the auroral kilometric radiation (AKR), which is associated with the precipitation of auroral electrons in a geomagnetic substorm. Originally, a loss-cone-driven version of ECME, developed for AKR, was applied to spike bursts, but it is now widely accepted that the measured distribution function is horseshoe-like (an isotropic distribution with a one-sided loss cone), and that a horseshoe-driven version of ECME applies to AKR. We explore the implications of the assumption that horseshoe-driven ECME also applies to spike bursts. We develop a 1D model for the acceleration of the electrons by a parallel electric field, and show that under plausible assumptions it leads to a horseshoe distribution of electrons in a solar flare. A second requirement for horseshoe-driven ECME is an extremely low plasma density, referred to as a density cavity. We argue that a coronal density cavity should develop in association with a hard X-ray burst, and that such a density cavity can overcome a long-standing problem with the escape of ECME through the second-harmonic absorption layer. Both the horseshoe distribution and the associated coronal density cavity are highly localized, and could not be resolved in the statistically large number of local precipitation regions needed to explain a hard X-ray burst. The model highlights the “number problem” in the supply of the electrons needed to explain a hard X-ray burst.

Keywords

Solar flares Electron acceleration 

Notes

Acknowledgements

We acknowledge support from an Australian Research Council Discovery Project grant. D.B. Melrose acknowledges support from the International Space Science Institute, Bern, Switzerland, and discussions with members of the team on “Magnetic Waves in Solar Flares.” We thank an anonymous referee for helpful suggestions.

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Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.Sydney Institute for Astronomy, School of PhysicsUniversity of SydneyNSWAustralia

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