Determination of Intrinsic Mode and Linear Mode Coupling in Solar Microwave Bursts

TL;DR

This paper derives a new equation to determine the propagation angle of microwave emissions in solar observations, enabling the identification of intrinsic modes and mode coupling, with application to a specific solar event.

Contribution

A novel explicit equation for the propagational angle in solar microwave bursts is derived, aiding in mode coupling analysis and addressing magnetic field ambiguities.

Findings

The propagational angle varies around 90 degrees in one footpoint during maximum phase.

Evidence of linear mode coupling occurs when the angle is near 90 degrees.

The observed sense of circular polarization supports a transition frequency above 17 GHz.

Abstract

An explicit equation of the propagational angle of microwave emission between the line-of-sight and the local magnetic field is newly derived. The existence of the solution of propagational angle is clearly shown under a series of typical parameters in solar microwave observations. It could be used to determine the intrinsic mode and linear mode coupling in solar microwave bursts. However, a 180-degree ambiguity still exists in the direction of longitudinal magnetic field, to produce an uncertainty of the intrinsic mode. One example is selected to check the feasibility of the method in the 2001 September 25 event with a loop-like structure nearby the central meridian passage observed by Nobeyama Radio Heliograph and Polarimeters. The calculated angle in one footpoint (FP) varied around 90 degree in two time intervals of the maximum phase, which gives a direct evidence of the emission propagating through a quasi-transverse region where the linear mode coupling took place, while, the angle in another FP was always smaller than 90 degrees where the mode coupling did not happen. Moreover, the right-circular sense at 17 GHz was always observed in both two FPs during the event, which supports that the transition frequency should be larger than 17 GHz in the first FP together with strong magnetic field of over 2000 Gauses in photosphere, where the weakly coupled case should happen. Moreover, there are two possibilities of the intrinsic mode in the two FPs due to the 180-degree ambiguity.