Zebra Stripes with High Gyro-Harmonic Numbers

0560480 - ASÚ 2023 RIV NL eng J - Journal Article
Benáček, J. - Karlický, Marian
Zebra Stripes with High Gyro-Harmonic Numbers.
Solar Physics. Roč. 297, č. 8 (2022), č. článku 103. ISSN 0038-0938. E-ISSN 1573-093X
R&D Projects: GA ČR(CZ) GC20-09922J; GA ČR GA20-07908S; GA ČR(CZ) GC21-16508J; GA ČR(CZ) GA22-34841S
Institutional support: RVO:67985815
Keywords : radio bursts * radio emission * theory * instabilities
OECD category: Astronomy (including astrophysics,space science)
Impact factor: 2.8, year: 2022
Method of publishing: Open access

Solar radio zebras are used in the determination of the plasma density and magnetic field in solar flare plasmas. Analyzing observed zebra stripes and assuming their generation by the double-plasma resonance (DPR) instability, high values of the gyro-harmonic number are found. In some cases they exceed one hundred, in disagreement with the DPR growth rates computed up to now, which decrease with increasing gyro-harmonic number. We address the question of how zebras with high values of the gyro-harmonic numbers s are generated. For this purpose, we compute the growth rates of the DPR instability in a very broad range of s, considering a loss-cone kappa-distribution of superthermal electrons and varying the losscone angle, electron energies, and background plasma temperature. We have numerically calculated the dispersion relations and the growth rates of the upper-hybrid waves and found that the growth rates increase with increasing gyro-harmonic numbers if the loss-cone angles are similar to 80 degrees. The highest growth rates for these loss-cone angles are obtained for velocity v(k) = 0.15 c. The growth rates as a function of the gyro-harmonic number still show well distinct peaks, which correspond to zebra-stripe frequencies. The contrast between peak growth rates and surrounding growth rate levels increases as the kappa index increases and the background temperature decreases. Zebras with high values of s can be generated in regions where loss-cone distributions of superthermal electrons with large loss-cone angles (similar to 80 degrees) are present. Furthermore, owing to the high values of s, the magnetic field is relatively weak and has a small spatial gradient in such regions.
Permanent Link: https://hdl.handle.net/11104/0334207