Band Splitting in m-Type II radio Bursts and their Role in Coronal Parameter Diagnostics

TL;DR

The paper analyzes 44 m-type II radio bursts with band splitting observed by RSTN during solar cycle 24 to diagnose coronal parameters. By applying the BDW-based interpretation and a hybrid coronal density model, it derives shock speeds/heights, density jumps, Alfvén Mach numbers, Alfvén speeds, and magnetic fields, and identifies power-law relationships such as $BDW \propto f_L^{-0.4}$ and $B \propto R^{\sim-3}$. The results show BDW is largely invariant with frequency and height, CME-driven shocks dominate, and coronal magnetic fields decrease with height, yielding $B \propto R^{-2.7}$ to $R^{-3.1}$ across cases. Together, these findings advance coronal diagnostics by linking observable band-splitting features to fundamental plasma parameters across a substantial, systematic data set. The study thus strengthens the use of type II band splitting as a diagnostic tool for coronal magnetism and shock physics in the lower corona and beyond.

Abstract

Type II radio bursts are signatures of shock waves generated by solar eruptions, observed at radio wavelengths. While metric (m) type II bursts originate in the lower corona, their longer-wavelength (up to kilometers) counterparts extend into interplanetary space. A rare but valuable feature observed in some type II bursts is band splitting in their dynamic spectra, which provides crucial insights into physical parameters such as shock speed, Alfvén Mach number, Alfvén speed, and coronal magnetic field strength (B). In this study, we investigate band-splitting in 44 m-type II radio bursts observed by the Radio Solar Telescope Network during solar cycle 24 (2009 -- 2019). These events exhibit splitting in both fundamental and harmonic bands and are analyzed under both perpendicular and parallel shocks. All events are associated to solar flares and 41 (93 \%) with the coronal mass ejections. Shock speeds, derived using a hybrid coronal density model proposed by \cite{Vrsnak2004}, range from $\approx$ 350 to 1727 \kms. The relative bandwidth (BDW) of the split bands remains constant with frequency and height. Alfvén Mach numbers indicate moderate shock strength (1.06 -- 3.38), while Alfvén speeds and $B$ vary from $\approx$ 230 -- 1294 \kms\ and $\approx$ 0.48 -- 7.13 G, respectively. Power-law relationships are established as $BDW \propto f_L^{-0.4}$ and $BDW \propto R^{\sim1}$, while the coronal magnetic field decreases with height as $B \propto R^{\sim-3}$. These results enhance our understanding of shock dynamics and magnetic field structures in the solar corona.

Figures (14)

• Figure 1: An example of m-type II radio burst from Learmonth Hill on 08 November 2013 showing the fundamental and harmonic bands. The UFBs and LFBs are shown with red and green dashed curves, respectively.
• Figure 2: m-type II radio bursts associated flare parameters namely rise time, decay time, and duration of flares in panels (a), (b), and (c), respectively.
• Figure 3: Distribution of location of the sources of m-type II radio bursts on the solar disk. The bars with "vertical lines" and "crosses" represent the latitude and longitude of the source location, respectively.
• Figure 4: Locations of m-type II burst sources shown on the solar disk with start symbols.
• Figure 5: Histograms of speed, acceleration, and angular width of the CMEs associated with m-type II radio bursts.