Statistics and Classification of the Microwave Zebra Patterns Associated With Solar Flares

2015

A&A

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Aims. We present a new method for determining the magnetic field strength and plasma density in the solar zebra radio sources. Methods. Using the double plasma resonance (DPR) model of the zebra emission, we analytically derived the equations for computing the gyroharmonic number s of selected zebra lines and then solved these equations numerically. Results. The method was successfully tested on artificially generated zebras and then applied to observed ones. The magnetic field strength and plasma density in the radio sources were determined. Simultaneously, we evaluated the parameterwhere L n and L b are the characteristic scale-heights of the plasma density and magnetic field strength in the zebra source, respectively. Computations show that the maximum frequency of the low-polarized zebras is about 8 GHz, in very good agreement with observations. For the high-polarized zebras, this limit is about four times lower. Microwave zebras are preferentially generated in the regions with steep gradients of the plasma density, such as in the transition region. In models with smaller density gradients, such as those with a barometric density profile, the microwave zebras cannot be produced owing to the strong bremsstrahlung and cyclotron absorptions. We also show that our DPR model is able to explain the zebras with frequency-equidistant zebra lines.

“…Furthermore, Tan et al (2014) mentioned only one zebra in the 7−8 GHz range. Therefore, we explain this high-frequency limit using our model.…”

Section: High-frequency Limit Of Microwave Zebrasmentioning

confidence: 99%

Section: Zebras With Equidistant Zebra Linesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Determination of plasma parameters in solar zebra radio sources

2015

A&A

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“…Zebra patterns (ZPs), appearing as regular stripes in emission and absorption in the dynamic spectra of solar radio bursts, are commonly known, and their observed characteristics and their models have been described in many papers (Slottje 1972;Kuijpers 1975;Zheleznyakov & Zlotnik 1975;Chernov 1976Chernov , 1990LaBelle et al 2003;Kuznetsov 2005;Bárta & Karlický 2006;Ledenev et al 2006;Tan 2010;Karlický 2013;Zlotnik 2013;Tan et al 2014;Chernov et al 2018), see also the reviews by Chernov (2011Chernov ( , 2014. The most frequently discussed mechanism in the literature is that based on the double plasma-resonance (DPR) instability (Kuijpers 1975(Kuijpers , 1980Zheleznyakov & Zlotnik 1975;Mollwo 1983Mollwo , 1988Winglee & Dulk 1986;Yasnov & Karlický 2004;Kuznetsov & Tsap 2007;Chen et al 2011;Karlický & Yasnov 2015;Yasnov et al 2016Yasnov et al , 2017Benáček et al 2017).…”

Section: Introductionmentioning

confidence: 99%

Double plasma-resonance surfaces in flare loops and radio zebra emission

2018

A&A

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Aims. The zebra structures observed in radio waves during solar flares are some of the most important structures used as diagnostics of solar flare plasmas. We here not only analyze the so-called double plasma-resonance (DPR) surfaces, but also estimate the effects of their form on the size of the zebra sources and brightness temperature. Methods. To compute the DPR surfaces, we used numerical and analytical methods. Results. We found that except for the case of a constant magnetic field across the loop, the DPR surfaces deviate from the constant plasma density surfaces. We found that the regime with a finite height scale has three forms of resonance surfaces depending on the magnetic field variation across the loop. This magnetic field variation also determines if in the generated zebra structure, an increase in gyro-harmonic number leads to an increase or decrease of the zebra stripe frequency. In the case with an infinite height scale, the resonance surfaces are parallel to the loop axis. Furthermore, we found that for highly polarized zebra structures that are generated at DPR surfaces close to the plasma frequency, the zebra emission is limited to the narrow escaping cone and the emitting source area increases with increasing viewing angle compared to the loop axis. Moreover, with increasing deviation of the DPR surfaces from those of constant density surfaces, the frequency bandwidth of the DPR emission increases and can cause the zebra stripes to overlap, which limits the zebra generation. For the zebra structures observed on 14 February 1999, 6 June 2000, and 1 August 2010 and the observed view perpendicular to the loop axis, we estimated that the brightness temperature is 3.67 × 1014 K, 6.58 × 1013 K, and 7.35 × 1015 K, respectively. These brightness temperatures are much lower than those derived for the view along the loop axis (up to 1017 K), and thus are more realistic. The area of the emitting source for coronal loops in the view perpendicular to the loop axis can be larger by several orders of magnitude than that in the view along the loop axis.

“…Several mechanisms explaining zebras have been proposed (Zheleznyakov and Zlotnik, 1975;Winglee and Dulk, 1986;Bárta and Karlický, 2006;LaBelle et al, 2003;Tan, 2010;Karlický, 2013); see also reviews of Chernov (2010) and Chernov, Yan, and Fu (2014). Based on statistical analysis, it was recently found that zebras observed in different flare phases have different parameters (Tan et al, 2014b). This indicates that zebras in different flare phases are generated by different mechanisms.…”

Section: Introductionmentioning

confidence: 99%

Regions of Generation and Optical Thicknesses of dm-Zebra Lines

2015

Sol Phys

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Using a new model based on the double plasma resonance (DPR), we show that the zebra structure seen in solar radio bursts is generated in the transition region and at the tops of the magnetic arcade. The magnetic field in zebra sources is probably weaker than 150 gauss. According to this model, a generation of zebras in stronger magnetic fields is improbable. The high-frequency boundary of decimetric zebras depends on the background electron plasma density, but not on the magnetic field strength in the generation regions. The bremsstrahlung absorption in atmospheric layers above the DPR zebra generation region and the cyclotron absorption in the DPR region and in the gyroresonance layers at higher altitudes limit the spectrum of zebras from both high-frequency and low-frequency sides. While the bremsstrahlung reduces the emission from the high-frequency side, the cyclotron absorption limits the low-frequency side. The observed frequency range and the number of observed zebra lines are determined not only by these absorptions, but also by appropriate distribution functions of superthermal electrons and plasma conditions in this region. Lowfrequency (metric) zebra emissions can be generated at high altitudes. Computations show that such emissions can escape from the DPR generation region only at high gyro-harmonics (s > 10) and with many zebra lines.