Observation of a Metric Type N Solar Radio Burst

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 (∼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.

The Low-Frequency Solar Corona in Circular Polarization

“…Ryabov et al, 1999;Altyntsev et al, 2017;Shain, Melnikov, and Morgachev, 2017;Sharykin, Kuznetsov, and Myshyakov, 2018). At lower frequencies, QT regions are also invoked to explain the polarization properties of noise storms (Suzuki and Sheridan, 1980;White, Thejappa, and Kundu, 1992), Types U and N bursts (Suzuki and Sheridan, 1980;Kong et al, 2016), and zebra patterns in Type IV bursts (Kaneda et al, 2015). A natural place to start investigating the importance of QT regions in our observations would be by comparing the polarization sense of the active region noise storm sources from Section 4 to that expected from the magnetic field orientation assuming o-mode polarization from plasma emission.…”

“…Several others have examined source positions and structures in total intensity NRH observations, while using the polarization information to help discriminate between emission mechanisms (e.g. Gopalswamy et al, 1994;Tun and Vourlidas, 2013;Kong et al, 2016;Liu et al, 2018). The radioheliograph at Gauribidanur does not have a polarimetric capability itself, but several one-dimensional polarimeters have been installed alongside it (Ramesh et al, 2008;Kishore et al, 2015).…”

Imaging spectroscopy of type U and J solar radio bursts with LOFAR

“…This result agrees well with the observation of Aurass & Klein (1997) that accompanying type III bursts appear close to the negative frequency drift rate branch of the U-bursts. Type N bursts are believed to be signatures of electrons mirrored at the base of magnetic loops (Caroubalos et al 1987;Wang et al 2001;Kong et al 2016). The properties of the third leg in an N-burst must be significantly different from the first leg, as any mirrored electron beam would likely undergo a decrease in concentration.…”

A Review of Recent Solar Type III Imaging Spectroscopy

“…J-bursts can also occur at the same time as coronal jets, with one imaged using LOFAR by where the accelerated electron beam traveled along a large magnetic coronal loop. The electron beam can also mirror at the footpoint of magnetic loops forming what is known as an N-burst, with Kong et al (2016) reporting a wellobserved N-burst using the NRH. The bulk of magnetic flux is closed in the corona and so we might expect U-bursts and Jbursts to be observed more often than type III bursts when in fact the converse is true.…”