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Coronal Mass Ejection-driven Type II solar radio burst structure with LOFAR and radio-wave scattering by Nicolina Chrysaphi et al.*

Intriguingly, Smerd et al. (1974, 1975) were aware of previously-observed large separations between sub-band sources (approximately 0.06 – 0.25 Rsun) challenging their model which requires the sub-band sources to originate from virtually the same region. They provided several reasons as to why these observed separations might not be representative of the true physical relation of the sub-band sources. Suggestions included the time delay due to the inability to image the two sub-bands at the same time, meaning that a source propagated with the shock during that time, and the frequency-dependent shift that sources could experience due to radio-wave scattering effects in the corona. Hence, the dominant mechanism of band splitting is a topic of ongoing discussion in solar physics.

We utilised the imaging capabilities of the LOw-Frequency ARray (LOFAR) to study the spatial relation of the higher- and lower-frequency sub-band sources. LOFAR has the unprecedented capability to image the emissions at any and all frequency and time points. We have thus obtained, for the first time, simultaneous images of the two sub-bands enabling us to estimate the locations of the sources without any time-delay ambiguities.

 Observations & Results

A split-band Type II burst was observed by LOFAR between 30 and 40 MHz (see Figure 1). The separation between the sources of the two sub-bands was estimated to be approximately 0.2+/-0.05 Rsun. The locations of both sub-band sources were compared to the Newkirk (1961) coronal density model and it was found that both sub-bands are described by the 4.5xNewkirk model. The description of both sub-bands by the same density model is inconsistent with the Smerd et al. (1974, 1975) model as a density jump between the sub-band sources is not observed.

Figure 1. Top panel: a combination of data from SDO/AIA, SOHO/LASCO/C2, and LOFAR. Blue crosses represent centroids of the higher-frequency sub-band sources, red crosses represent centroids of the lower-frequency sub-band sources, and green crosses represent Type III centroids. Yellow and magenta lines indicate fits through the Type II and Type III centroids, respectively. Bottom panel: dynamic spectrum recorded by LOFAR on 25 June 2015 around 10:45 UT showing the split-band Type II burst between 30 and 42 MHz. Black dashed lines indicate moments of equal time, red crosses indicate points from the higher- and lower-frequency Type II sub-bands, and black crosses indicate points from a Type III burst. (Figure adapted from Chrysaphi et al. (2018).)

In light of recent investigations (Kontar et al. (2017)) that highlighted the dominance of scattering effects on low-frequency observations, we quantitatively estimated, for the first time, the extent of scattering on split-band Type II bursts. Scattering shifts sources radially father from the Sun, with lower frequencies experiencing a higher degree of shift. We find that a 32 MHz source appears as radially shifted by ~0.3 Rsun from a 40 MHz source. Therefore, the imaged separation of 0.2+/-0.05 Rsun is consistent with radio-wave scattering effects, indicating that the sub-band sources could in reality be virtually co-spatial.

Since scattered sources are observed radially farther from their true locations, the deduced coronal density will appear to be larger than its true value. We find that coronal densities are exaggerated by a factor of ~4.3, meaning that the 4.5xNewkirk model obtained from the observations is an over-estimation. We also find that once scattering effects are considered, the sub-band sources are no longer described by the same density model, but a density jump between the sub-bands occurs. These effects are schematically illustrated in Figure 2.

Figure 2. Schematic illustration of the effects of scattering on radio-wave emission sources. True sources (shown in circles) are shifted radially outwards from the Sun, with the higher frequency source (red) shifted more than the lower frequency source (blue). This frequency-dependent shift causes the apparent emission sources (shown in crosses) to appear as spatially separated, despite that they could originate from virtually co-spatial regions on the shock. The radial displacement of the sources also causes the apparent coronal density to appear larger than its true value. (Figure adapted from Chrysaphi et al. (2018).)

Conclusions

We showed that the large separations observed between the components of a split-band Type II burst are consistent with radio-wave scattering effects, highlighting the importance of considering radio-wave propagation effects in the analysis of radio emissions. As a consequence, the true emission sources of a split-band Type II burst could originate in virtually co-spatial regions, and hence the emission could be produced in the upstream and the downstream shock front parts.

Based on the recently published paper: Nicolina Chrysaphi et al., 2018, ApJ, 868, 79, DOI: 10.3847/1538-4357/aae9e5

References

Holman, G. D., & Pesses, M. E. 1983, ApJ, 267, 837

Kontar, E. P., Yu, S., Kuznetsov, A. A., et al. 2017, NatCo, 8, 1515

Newkirk, G., Jr. 1961, ApJ,133, 983

Smerd, S. F., Sheridan, K. V., & Steward, R. T. 1974, in IAU Symp. 57, 389

Smerd, S. F., Sheridan, K. V., & Steward, R. T. 1975, ApL, 16, 23

*Full list of authors: Nicolina Chrysaphi, Eduard P. Kontar, Gordon D. Holman, and Manuela Temmer



This post first appeared on Solar Radio Astronomy, please read the originial post: here

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Coronal Mass Ejection-driven Type II solar radio burst structure with LOFAR and radio-wave scattering by Nicolina Chrysaphi et al.*

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