Observation of a Metric Type N Solar Radio Burst

TL;DR

This paper reports detailed observations of a rare N-shaped solar radio burst, providing insights into coronal magnetic structures and electron beam dynamics through polarization, timing, and spectral analysis.

Contribution

It presents the first comprehensive analysis of a well-observed N-shaped burst, confirming its origin from a single electron beam and exploring its polarization and timing characteristics.

Findings

The N-shaped burst has three branches with increasing duration, indicating beam dispersion.

The source is near large-scale coronal loops with opposite polarization on each leg.

A significant delay exists between fundamental and harmonic components, challenging classical emission models.

Abstract

Type III and type-III-like radio bursts are produced by energetic electron beams guided along coronal magnetic fields. As a variant of type III bursts, Type N bursts appear as the letter "N" in the radio dynamic spectrum and reveal a magnetic mirror effect in coronal loops. Here, we report a well-observed N-shaped burst consisting of three successive branches at metric wavelength with both fundamental and harmonic components and a high brightness temperature ($>$10$^9$ K). We verify the burst as a true type N burst generated by the same electron beam from three aspects of the data. First, durations of the three branches at a given frequency increase gradually, may due to the dispersion of the beam along its path. Second, the flare site, as the only possible source of non-thermal electrons, is near the western feet of large-scale closed loops. Third, the first branch and the following two branches are localized at different legs of the loops with opposite sense of polarization. We also find that the sense of polarization of the radio burst is in contradiction to the O-mode and there exists a fairly large time delay ($\sim$3-5 s) between the fundamental and harmonic components. Possible explanations accounting for these observations are presented. Assuming the classical plasma emission mechanism, we can infer coronal parameters such as electron density and magnetic field near the radio source and make diagnostics on the magnetic mirror process.