Narrowband dm-spikes come from the most fascinating great structures of solar radio bursts that are closely linked to main flare energy-release procedures (Krueger 1979) and observed in many cases near the Type III burst beginning frequency. They happen in clouds of narrowband bursts with a typical duration less than 100 ms, frequency relative bandwidth 1-3 %, and brightness temperature approximately $10 ^ $ K.
For them a number of radio emission models were suggested. Earlier designs recommended emission by the electron cyclotron maser system for $Y= omega _ pe/ omega _ ce < < 1$, where $omega _ pe $ is the electron plasma frequency, $omega _ $ is the electron cyclotron frequency (Melrose & & Dulk 1982). In this model, emission happens near the harmonics of the electron cyclotron frequency. Non-integer ratios between spike bands were observed by Krucker & & Benz (1994 ). A design based on emission at Bernstein mode frequencies was proposed by Willes & & Robinson (1996 ). The model of emission at upper-hybrid frequencies was suggested for $Y > > 1$ by Stepanov et al. (2001 ). In this design, emission regions are separated in height, and hence they discharge at different frequencies.
We investigate the 2013 November 7 spike occasion showing unique really narrow bands with non-integer frequency ratios. In analysis of this occasion we consider 2 models: (1 ) with the emission from one emission source at Bernstein mode frequencies and (2 ) with the emission from separated emission sources at upper-hybrid frequencies.
We study the 2013 November 7 spike occasion where spikes were clustered in special really narrow bands (Figure 1). This spike occasion belongs to 53 spike events that were signed up by Ondřejov radiospectrographs in the range 0.8-2 GHz during 1992-2020 years. The frequencies of its 4 narrow bands are 1003, 1276, 1572, and 1877 MHz, giving ratios between surrounding bands as 1.275, 1.23, and 1.19. Figure 2 reveals the radio flux evolution of these bands and their cross-correlations. Figure 1– Observed spike event on 2013 November 7. (a) Broadband cloud of spikes followed by unique four really narrow bands of spikes. (b) Detail of the radio spectrum in 1 — 2 GHz range with 1 s duration. The arrow reveals the band frequency variation for which the form is integrated with the comparable types in all 4 bands. The spectrum is not smoothed and thus shows bins in the record.
We computed growth rates for both our models. These modes have a non-integer frequency ratios, and their positions fit well with the observed band frequencies.
Figure 2–( a) Evolution of radio flux at 1877 MHz (black line), at 1572 MHz (red line) +400 au, at 1276 MHz (blue line) +800 au, and at 1003 MHz (orange line) +1200 au, starting at 12:26:55 UT and lasting 1 s. (b) Cross-correlations of the radio flux profiles on 1877 and 1572 MHz (black line), 1572 and 1276 MHz (red line), and 1276 and 1003 MHz (blue line)
Figure 3– Growth rates of the Bernstein modes as a function of the frequency and perpendicular wavenumber for specifications $omega _ pe/ omega _ = 2.7$, $f _ pe = 855$ MHz, $v _ tb/ c = 0.02$, $v _ t/ c = 0.25$, $n _ e/ n _ h = 10$. Frequency corresponds to the radio emission on Bernstein mode frequencies. Green lines: dispersion modes, $s$ is the gyro-harmonic variety of each branch. White dashed horizontal line denotes the plasma frequency
Figure 4– Growth rate of the upper-hybrid waves vs. the upper-hybrid to electron cyclotron frequency ratio; $s$ represents the gyro-harmonic number of the most unstable wave.
We found that the Bernstein mode model better explains the observed spikes than the upper-hybrid model, at least in the studied occasion. It is not extremely possible that 4 apart regions give these ratios in the upper-hybrid design.
Based on the current paper.
Karlický, M., Benáček, J., & & Rybák, J. 2021, ApJ, 910, 108. doi:10.3847/ 1538-4357/ abe62b.
Krucker, S. & & Benz, A. O. 1994, A&A, 285, 1038..
Krueger, A. 1979, Geophysics and Astrophysics Monographs, Dordrecht: Reidel, 1979.
Melrose, D. B. & & Dulk, G. A. 1982, ApJ, 259, 844. doi:10.1086/ 160219.
Stepanov, A. V., Kliem, B., Krüger, A., et al. 1999, ApJ, 524, 961. doi:10.1086/ 307835.
Willes, A. J. & & Robinson, P. A. 1996, ApJ, 467, 465. doi:10.1086/ 177620.
* Full list of authors: Marian Karlický, Jan Benáček and Ján Rybák.
In this model, emission happens near the harmonics of the electron cyclotron frequency. In this model, emission regions are separated in height, and hence they discharge at separate frequencies.
The arrow reveals the band frequency variation for which the kind is integrated with the similar kinds in all 4 bands. These modes have a non-integer frequency ratios, and their positions fit well with the observed band frequencies. Frequency corresponds to the radio emission on Bernstein mode frequencies.