Volume 601, May 2017
|Number of page(s)||6|
|Published online||26 April 2017|
Oscillation of solar radio emission at coronal acoustic cut-off frequency
1 Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
2 Poltava Gravimetrical Observatory at Institute of Geophysics, National Academy of Sciences of Ukraine, Myasoedova str., 27/29, 36014 Poltava, Ukraine
3 IGAM, Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
4 Institute of Radio Astronomy, National Academy of Sciences of Ukraine, Chervonopraporna st., 4, 61002 Kharkiv, Ukraine
5 Abastumani Astrophysical Observatory at Ilia State University, Kakutsa Cholokashvili Ave 3/5, 0162 Tbilisi, Georgia
Received: 30 June 2016
Accepted: 14 March 2017
Recent SECCHI COR2 observations on board STEREO-A spacecraft have detected density structures at a distance of 2.5–15 R0 propagating with periodicity of about 90 min. The observations show that the density structures probably formed in the lower corona. We used the large Ukrainian radio telescope URAN-2 to observe type IV radio bursts in the frequency range of 8–32 MHz during the time interval of 08:15–11:00 UT on August 1, 2011. Radio emission in this frequency range originated at the distance of 1.5–2.5 R0 according to the Baumbach-Allen density model of the solar corona. Morlet wavelet analysis showed the periodicity of ~80 min in radio emission intensity at all frequencies, which demonstrates that there are quasi-periodic variations of coronal density at all heights. The observed periodicity corresponds to the acoustic cut-off frequency of stratified corona at a temperature of 1 MK. We suggest that continuous perturbations of the coronal base in the form of jets/explosive events generate acoustic pulses, which propagate upwards and leave the wake behind oscillating at the coronal cut-off frequency. This wake may transform into recurrent shocks due to the density decrease with height, which leads to the observed periodicity in the radio emission. The recurrent shocks may trigger quasi-periodic magnetic reconnection in helmet streamers, where the opposite field lines merge and consequently may generate periodic density structures observed in the solar wind.
Key words: Sun: corona / Sun: oscillations / Sun: radio radiation
© ESO, 2017
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