Parametric resonance in microwave radio emission of coronal magnetic loops V.V. ZAITSEV Institute of Applied Physics, Russian Academy of Sciences Nizhny Novgorod, Russia A.G. Kislyakov, V.V. Zaitsev, A.V. Stepanov, S. Urpo, On the possible connection between photospheric 5-min oscillations and solar flare microwave emission, Solar Physics, 2006, vol. 233, pp V.V. Zaıtsev, A.G. Kislyakov, Parametric Excitation of Acoustic Oscillations in Closed Coronal Magnetic Loops, Astronomy Reports, 2006, Vol. 50, No. 10, pp. 823–833.

5-minute oscillations Leighton,1960 Brown et al., 1986 Steffens and Nurnberger, 1998 Chaplin et al., 1998

Coronal magnetic loops oscillations Fourier analysis of microwave emission from solar flares sometimes shows modulation of radio emission intensity by narrow band sinusoidal signals of frequency of the order of 1 Hz, that decreases during the burst active phase (Fig.1, Zaitsev et al., 2003) Such modulation may be caused by own oscillations of current carrying loop as an equivalent electric circuit (Fig.2, Alfven, Carlqvist, 1967; Zaitsev, Urpo, Stepanov,2000). The modulation frequency decrease during the burst corresponds to the current dissipation in the flaring loop in the course of flare development. Diagnostics: 37 GHz microwave radio emission Metsahovi 0,2-0,5 Hz

17 microwave bursts at 37 GHz observed during were investigated. In about 90% events 5-minute intensity modulation was found. In about 70% events 10-minute oscillations were observed simultaneously with 5-minute oscillations during the same events. In about 30% events simultaneous modulation by three low frequency signals with 3.3, 5 and 10-minute periods was observed.

Dynamic spectrum of solar radio emission modulation at 37 GHz and the burst profile on March 20, 2000.

Cross-section of the dynamic spectrum on over time (averaging at the time interval ~24 min). Dotted line indicates sub-harmonic frequency 1.8 MHz, corresponding to 10 minute period. Periods 5-min (3.6 MHz) and 3.3-min (5.6 MHz) are also indicated.

Parametric interaction of sound oscillations in coronal magnetic loops and 5-minute photospheric oscillations 10 min 5 min 3,3 min

Under the influence of 5-minute oscillations of the photospheric convection velocities on the footpoints of the coronal magnetic loop the effect of parametric resonance of 5- minute velocity oscillations in the solar photosphere and sound oscillations in coronal magnetic loops is possible. Such effect demonstrates itself by simultaneous excitation of 10, 5 and 3- minute period oscillations in coronal magnetic loop. These periods correspond to the sub-harmonic, pumping-up frequency, and first upper frequency of parametric resonance. Since 5-minute oscillations cannot directly penetrate into the corona, the parametric resonance may work as an effective channel for transfer the energy of photospheric oscillations to the upper layers of the solar atmosphere. It offers the challenge to understanding the heating mechanism of the solar corona. For instance, estimations show that if the current in coronal magnetic loop exceeds 7 10^9 Ampere, then the energy flux of sound oscillations arising in the coronal magnetic loop due to parametric resonance exceeds losses for optical radiation.

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Выводы -во время вспышек иногда наблюдается воздействие 5-минуитых колебаний фотосферы на собственные колебания корональных магнитных петель -при этом частота колебаний магнитных петель испытывает вариации с периодом 5 мин и относительной величиной порядка -вариации частоты происходят в фазе с вариациями интенсивности, что свидетельствует в пользу гиросинхротронного механизма излучения из оптически тонкого источника -механизм воздействия 5-минутных колебаний фотосферы на собственные колебания корональных магнитных петель связан с изменением электрического тока в петле под действием периодических вариаций скорости фотосферной конвекции

Механизмы взаимодействия 5-мин. осцилляций и собственных колебаний петли -прогрев оснований петли вследствие диссипации тока -взаимодействие 5-мин. осцилляций с электрическим током, текущим вдоль корональной магнитной петли