Type II Solar Radio Bursts and You

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Title: A Type II Radio Burst Driven by a Blowout Jet on the Sun
Authors: Zhenyong Hou et al.
First Author’s Institution: Peking University
Status: Published in ApJ

The Background

One of the most exciting qualities of the Sun is its magnetic activity that can manifest in a variety of ways and across the entire electromagnetic spectrum. X-ray photons reveal flares produced through magnetic reconnection, microwaves arise from synchrotron and gyrosynchrotron emissions from electrons accelerated along the Sun’s magnetic field, and low-frequency radio waves provide information on the plasma properties in the upper atmosphere of the Sun, called the corona. Magnetic activity also involves particle motion; plasma ejected by this activity can take many forms, and, if the plasma is energetic enough, it can escape from the Sun entirely as a coronal mass ejection.

Because there are so many aspects to how magnetic activity manifests and evolves, it’s difficult to piece together a cohesive picture for how it all works. One phenomenon that has eluded explanation for a long time is the origin of Type II radio bursts. Type II bursts are an example of plasma emission — emission from the coherent oscillation of electrons in a plasma that then produces coherent emission. Like laser light, the coherent emission is bright only at specific frequencies, namely, the plasma frequency, which depends on the plasma density. As the accelerator of the burst moves outwards through the corona, the ambient plasma density decreases and so the plasma emission frequency drifts to lower frequencies, giving Type II bursts a very distinctive “sweeping” signature in time–frequency space, as shown in the bottom frame of Figure 1.

solar magnetic field, X-ray flux, and dynamic spectrum

Figure 1: The solar magnetic field as seen by the Helioseismic and Magnetic Imager (top left) and the jet as seen by the Atmospheric Imaging Assembly and the Solar Upper Transition Imager, (top middle and right, respectively). The middle plot is the light curve of the flare in the X-ray as seen by the Geostationary Operational Environmental Satellite, and the bottom plot is the dynamic spectrum (brightness as function of time and frequency) including the Type II burst as seen by the Chashan Solar Radiospectrograph. The Solar Upper Transition Imager images of the jet are from soon after the peak of the flare’s intensity, by which time the jet was in its ejection phase, traveling at almost 600 km/s. [Hou et al. 2023]

For a long time, folks believed that all Type II bursts are associated with coronal mass ejections. This is because coronal mass ejections are one of the few phenomena from the Sun that are capable of generating the necessary shock in the corona for producing plasma emission. Producing a coronal shock requires moving faster than the Alfvén speed — which helps define the sound speed in a magnetized plasma — which can be hundreds to thousands of kilometers per second (km/s) in the Sun’s corona. However, there have been observations in recent years that suggest there are other processes on the Sun that can produce Type II bursts. This article presents one such case, where the Type II burst may be associated with a jet instead of a coronal mass ejection.

The Events

Because different parts of the electromagnetic spectrum are sensitive to different components of magnetic activity, properly associating two events or phenomena with each other requires collecting simultaneous data across multiple instruments. Data from nine instruments were used in this study! A few of the major* contributors were:

  1. the Solar Upper Transition Imager (SUTRI), Atmospheric Imaging Assembly (AIA), and Extreme Ultraviolet Imager (EUVI), all of which take pictures of the Sun at various extreme-ultraviolet wavelengths and from different satellites;
  2. the Helioseismic and Magnetic Imager (HMI), aboard the same satellite as AIA, which is responsible for measuring the magnetic field of the Sun;
  3. the Geostationary Operational Environmental Satellite (GOES) for observing X-ray photons; and
  4. the Chashan Solar Radiospectrograph (CBSm), operating at about 100–500 megahertz (or wavelengths of about 1–3 meters!).

The story of this event starts with AIA and SUTRI detecting a solar jet — a narrow burst of plasma from the Sun’s atmosphere — during its initial phase. During this phase, the jet moves at about 370 km/s (about 1,000 times faster than the speed of sound on Earth)! Soon after, a flare is detected in the X-ray by GOES, as shown in the middle frame of Figure 1. Following this, the jet accelerates to 560 km/s and transitions to the ejection phase.

extreme-ultraviolet intensity of the jet and the wave

Figure 2: Extreme-ultraviolet intensity (color scale) as a function of time and distance from the flare’s base along the direction of the jet’s path, as observed by AIA at 211 Angstroms (Å). The jet is the bright feature outlined by the cyan lines. The extreme-ultraviolet wave is the bright crest outlined by the bright green line, preceded by a dark feature thought to be the trough of the wave. [Adapted from Hou et al. 2023]

Things get extra interesting after the ejection phase has begun. At this point, AIA now detects a wave-like structure propagating through the Sun’s corona in the same direction and at the same speed as the jet during its initial phase (see Figure 2). Not even a minute later, a Type II burst is detected by CBSm. By the time that the wave structure and the Type II burst are no longer detectable by their respective instruments, they are moving at the same speed as (or somewhat faster than) the jet.

The Big Picture

The authors claim that the similarities of the jet, flare, wave structure, and Type II burst in terms of occurrence time, location, and speed suggest that the three phenomena are related to one another. The data paint a picture of a flare causing the eruption of material in the form of a jet. The jet excites the surrounding material as it moves through the corona, producing the extreme-ultraviolet waves and the Type II burst. All of this happens without any evidence of a coronal mass ejection.

This is significant in several ways. It’s amazing 1) to have simultaneous data spanning so many observing methods and, by extension, 2) to be able to analyze the flare, the jet, and the coronal excitation (the extreme-ultraviolet wave and Type II burst) for a single event, and 3) to see a Type II burst without any evidence of a coronal mass ejection! This article represents an exciting step towards understanding how our Sun’s activity, and the emission from it, are produced and evolve.

* Unfortunately, there’s not enough space in a single article to meaningfully reference all nine instruments and their results (although all were important). For those interested in the other instruments that were used, they were the COR2 coronagraph on the STEREO satellite, LASCO on the Solar and Heliophysics Observatory satellite, and the H-alpha imaging system on the New Vacuum Solar Telescope.

Original astrobite edited by Lynnie Saade.

About the author, Ivey Davis:

I’m a third-year astrophysics grad student working on the radio and optical instrumentation and science for studying magnetic activity on stars. When I’m not crying over radio frequency interference, I’m usually baking, knitting, harassing my cat, or playing the banjo!