Solar type V radio bursts are broad-band continua associated with type III bursts, which are generally believed to be caused by coronal electron beams. Type V bursts appear sometimes for 0.2 to 3 minutes as a continuum following an intense type III burst or group of bursts.
The spectral peak of type V bursts is generally below 100 MHz. The high-frequency edge is below the start frequency of the associated type III burst, and the low-frequency limit is often less than that of ground-based spectrometers (20 – 40 MHz). Both starting and trailing edges drift usually but not always from high to low frequencies. Thus, the combination of a type V and III burst has the shape of a flag on a pole in the spectrogram. Stewart (1978) reports that type III/V bursts are better correlated with hard X-ray flares than single type III bursts (80% vs. 20%), suggesting more powerful events. In some cases, there is a time gap between the type III burst and the start of the type V emission. Such cases are referred to as detached events. The circular polarization of type V bursts is weak (of order 10% or less). Dulk, Gary, and Suzuki (1980) report that it is usually reversed compared to the preceding type III burst.
Electron beams are two-stream unstable and drive Langmuir waves that couple into radio waves (type III burst). In the non-linear phase, the electron distribution becomes quasi-stationary and elongated in forward direction. This distribution has been predicted to be unstable towards the electron firehose instability involving resonant protons and for large anisotropies also resonant electrons (Pilipp and Benz 1977). The secondary instability then scatters the beam electrons into transverse direction. It may be speculated that the isotropic electrons develop a loss-cone by precipitation and become unstable to masering radio emission.
Che et al. (2014) have simulated the non-linear evolution of electrons beams by particles-in-cell (PIC) and found evidence for the sequence of instabilities leading to type III radio emission and scattering. After an early phase dominated by growing Langmuir waves, the electron two-stream instability drives non-propagating Weibel-like waves that excite both kinetic Alfvén waves and whistler waves by wave–wave coupling. These kinetic processes scatter beam electrons into transverse direction.
Here we present an extraordinary type V event and ask the question: Can all of this type III/V event be explained by phenomena originating from the non-linear evolution of the two-stream instability?
Observations
The flare SOL2021-05-07T03:39 was selected for its combination of a meter wave type U burst followed by a type V burst. The event was observed by several stations of the e-CALLISTO network. We used the data from ASSA in Sunnydale (Australia). The spectrometer was programmed to observe the frequency range 15 – 87 MHz with a resolution of 0.375 MHz. Calibration is not available.
Figure 1 shows an overview of the radio emission. The background was determined in time intervals of low fluctuations during 30 minutes before and after the bursts. The 5%-quantile of the flux was used as background and subtracted.
The structures of the emission in the spectrogram are characterized by their peak flux in time. The peak time is determined from Gaussian fits at each frequency. The highest peak at each frequency is selected, and its peak time is recorded. The values define a curve in the spectrogram (ν,t)-space). Results are shown in Figure 1 (bottom) and used in Figure 1 (top) to identify structures.
Figure 1: Bottom: Spectrogram showing radio flux with U burst (event 3) and type V burst (event 4) in uncalibrated units. Black crosses “x” mark a local maximum of the flux in time for type III und U bursts. For the type V emission, the “x” indicates the starting point of the emission at a given frequency. Top: Ratio between left and right circular polarization.
Figure 2: U Burst: Blue dots: peak times in frequency channel. Red-dashed curves: fitted to peak.
Figure 3: Type V: Blue dots indicate times of 8% increase above background. Red-dashed curves: fitted to starting edge.
Results and Discussion
- Drift rate
The drift of the descending branch of the U-burst is -0.30 [s-1]. For a density scale height of 1010cm (2 MK) and plasma emission, the beam velocity is 6·109 cm/s. The starting edge of the type V burst drifts with -0.15 [s-1]. If the emission process is plasma emission, the drift velocity of the type V starting edge is half of that of the U-burst. If the emission is electron cyclotron maser and the magnetic scale height is the same as the density scale height, the velocity is 1/4 of the U-burst beam. For both cases of emission, the data suggest that the Type V causing electrons have lower velocity than the type III causing electrons.
- Start of events
The starting edge of the type V emission begins at about 03:40:23 UTC, coinciding with the
loop apex of the U-burst. Thus the type V emission starts when the electron beam passes the apex of the loop.
- Circular Polarization
Figure 1 top indicates that the polarization of the type U burst is compatible with zero,
The polarization of the type V burst, however, is left circular (green). Thus the type III and type V emissions may be caused by different mechanisms.
Scenario and Conclusions
The data of the U burst (Event 3) and type V burst (Event 4) can be combined into a
coherent scenario:
- Electrons are accelerated near the footpoints of a magnetic loop. A beam of electrons propagates along a coronal loop.
- The beam is two-stream unstable, exciting Langmuir waves. They collapse in the nonlinear phase exciting ion acoustic waves. Wave-wave coupling produces radio emission, which is observed as type III und U bursts.
- The velocity distribution of a beam is preferentially parallel, driving the electron Firehose instability. In the nonlinear phase, it corresponds to exciting Weibel and whistler waves, which scatter energetic electrons into perpendicular velocity. The non-thermal electrons form an isotropic halo.
- After the beam has passed, the quasi-isotropic electron distribution deforms, losing electrons in parallel direction that escape and precipitate.
- The energetic electrons develop a loss-cone distribution. It becomes unstable to electron maser emission, and causes a type V radio emission at frequencies above the plasma frequency.
This scenario suggests a complex interplay of various kinetic plasma processes.
PIC simulations have been inspiring, but are too limited yet to confirm the scenario.
More imaging observations of the spatial relation between III/U and V bursts are also necessary.
Based on the recent paper: Benz, A.O., ·Huber, C.R., Timmel, V., Monstein, C. Observation of an Extraordinary Type V Solar Radio Burst: Nonlinear Evolution of the Electron Two-Stream Instability: Solar Physics, 299, 146 (2024). https://doi.org/10.1007/s11207-024-02395-8
References
Che, H., Goldstein, M. L., Vinas, A. F. 2014, Phys. Rev. Lett., 112, 061101.
Dulk, G.A., Gary, D.E., Suzuki, S. 1980, A&A, 88, 218.
Pilipp, W.G., Benz A.O. 1977, A&A, 56, 39.
Stewart R.T. 1978, Solar Phys,. 58, 121.