Metric type II radio bursts are a type of solar radio emission characterized by a slowly drifting radio emission observed in dynamic spectra. They are generated by the interaction of a coronal shock wave with the ambient solar wind plasma.

We present our recent study, where we have compiled a comprehensive catalog of metric type II radio bursts using data from the Radio Solar Telescope Network (RSTN). This study gives the occurrence, associations, and properties of radio emissions and their parent solar activity phenomena.

Our team has carefully categorized the m-type II radio bursts into two distinct qualitative groups: certain and uncertain, based on the intensity and clarity of the radio emission features. Analyzing data from Solar Cycle 24 (2009 – 2019), available from RSTN stations at Learmonth, Sanvito, Sagamore Hills, and Palehua, we have identified all metric type II bursts (429) detected within the 25 – 180 MHz range through thorough visual inspection.

Figure 1 The location of the four RSTN stations on the world map and their time of observation given in Universal Time (shown below the world map).

We have explored the relationships between the radio bursts and solar eruptive events like solar flares and coronal mass ejections (CMEs). Our study presents and discusses these connections, offering valuable insights into solar and space-weather activities from a ground-based radio perspective.

The newly assembled catalog of metric type II bursts and their associated solar events is a valuable resource for the solar scientific community, and is freely available online: https://catalogs.astro.bas.bg/

Figure 2 Examples of m-type II radio bursts for certain (left) and uncertain (right) events on 04 November 2015 and 02 March 2015, respectively. The spectra are chosen from the Learmonth Hill station of the RSTN network. The faint m-type II radio bursts in the uncertain case are enclosed inside the black oval.

We identified 17% (out of 429) new bursts and found that 61% are short-lived, under 15 minutes. The temporal offsets between bursts and solar events varied, with certain bursts being more common and associated with stronger solar eruptions.

This study has developed a comprehensive catalog of m-type II radio bursts from Solar Cycle 24, utilizing data from the RSTN. The catalog comprises 429 events, and the solar activities related to these bursts, such as solar flares and coronal mass ejections (CMEs), were investigated. The main findings of the study are as follows:

1. New Identifications: 17% (74 out of 429) of the m-type II bursts were new identifications made after a careful visual inspection of the RSTN dynamic spectra.
2. Occurrence Trends: The occurrences of m-type II bursts, along with their parent solar flares and CMEs, follow the solar cycle trend. About 61% (261 out of 429) of these events are relatively short-lived, lasting under 15 minutes.
3. Association with Solar Flares: 74% (318 out of 429) of the m-type II bursts are associated with solar flares. Most of these flares are classified as M and C class based on their GOES SXR intensity. The rise times of these flare-associated bursts are typically between 6 and 10 minutes. The active region belts related to these events are situated around the solar equator with no significant hemispheric asymmetry.
4. Association with CMEs: 73% (315 out of 429) of the m-type II bursts are associated with CMEs. About half (49%) of these CMEs have speeds less than 500 km/s, and only one-third are halo CMEs.
5. Speed Calculations: The speed of the m-type II bursts was calculated using three different density models. There was no significant correlation between the speeds of CMEs and m-type II bursts, although fast CMEs generally tend to be associated with fast shock drivers.
6. Solar Drivers: m-type II bursts associated with only solar flares or only CMEs each account for about 10% of the cases. The majority of the events (63%) involve both solar flares and CMEs. When both solar drivers are present, the associated m-type II bursts tend to have longer durations and are accompanied by stronger and faster solar eruptions. However, the deduced shock speeds are intermediate in value.

These findings provide valuable insights into the characteristics and drivers of m-type II radio bursts during Solar Cycle 24.

Based on a recent paper: Bendict Lawrance, Pooja Devi, Ramesh Chandra, Rositsa Miteva A Catalog of Metric Type II Radio Bursts Detected by RSTN During Solar Cycle 24, Solar Physics, Volume 299, Issue 6, id.75. (2024)  DOI:10.1007/s11207-024-02317-8