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Origin of the Modulation of the Radio Emission from the Solar Corona by a Fast Magnetoacoustic Wave
Kolotkov, Dmitrii Y.,Nakariakov, Valery M.,Kontar, Eduard P. American Astronomical Society 2018 The Astrophysical journal Vol.861 No.1
<P>Observational detection of quasi-periodic drifting fine structures in a type III radio burst associated with a solar flare SOL2015-04-16T11:22, with the LOw Frequency ARray (LOFAR), is presented. Although similar modulations of the type III emission have been observed before and were associated with the plasma density fluctuations, the origin of those fluctuations was unknown. Analysis of the striae of the intensity variation in the dynamic spectrum allowed us to reveal two quasi-oscillatory components. The shorter component has an apparent wavelength of similar to 2 Mm, phase speed of similar to 657 km s(-1), which gives an oscillation period of similar to 3 s, and a relative amplitude of similar to 0.35%. The longer component has a wavelength of similar to 12 Mm and relative amplitude of similar to 5.1%. The short frequency range of the detection does not allow us to estimate its phase speed. However, the properties of the shorter oscillatory component allowed us to interpret it as a fast magnetoacoustic wave guided by a plasma nonuniformity along the magnetic field outwards from the Sun. The assumption that the intensity of the radio emission is proportional to the amount of plasma in the emitting volume allowed us to show that the superposition of the plasma density modulation by a fast wave and a longer-wavelength oscillation of an unspecified nature could readily reproduce the fine structure of the observed dynamic spectrum. The observed parameters of the fast wave give an absolute value for the magnetic field in the emitting plasma of similar to 1.1 G, which is consistent with the radial magnetic field model.</P>
Non-stationary quasi-periodic pulsations in solar and stellar flares
Nakariakov, V M,Kolotkov, D Y,Kupriyanova, E G,Mehta, T,Pugh, C E,Lee, D-H,Broomhall, A-M IOP 2019 Plasma physics and controlled fusion Vol.61 No.1
<P>Often the enhanced electromagnetic radiation generated in solar and stellar flares shows a pronounced (quasi)-oscillatory pattern—quasi-periodic pulsations (QPP), with characteristic periods ranging from a fraction of a second to several tens of minutes. We review recent advances in the empirical study of QPP in solar and stellar flares, addressing the intrinsic non-stationarity of the signal, i.e. the variation of its amplitude, period or phase with time. This non-stationarity could form a basis for a classification of QPP, necessary for revealing specific physical mechanisms responsible for their appearance. We could identify two possible classes of QPP, decaying harmonic oscillations, and trains of symmetric triangular pulsations. Apparent similarities between QPP and irregular geomagnetic pulsations Pi offer a promising avenue for the knowledge transfer in both analytical techniques and theory. Attention is also paid to the effect of the flare trend on the detection and analysis of QPP.</P>
Nakariakov, V. M.,Pilipenko, V.,Heilig, B.,Jelí,nek, P.,Karlický,, M.,Klimushkin, D. Y.,Kolotkov, D. Y.,Lee, D. H.,Nisticò,, G.,Doorsselaere, T. D. Reidel Pub. Co 2016 Space science reviews Vol.200 No.1
<P>Magnetohydrodynamic (MHD) oscillatory processes in different plasma systems, such as the corona of the Sun and the Earth's magnetosphere, show interesting similarities and differences, which so far received little attention and remain under-exploited. The successful commissioning within the past ten years of THEMIS, Hinode, STEREO and SDO spacecraft, in combination with matured analysis of data from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI) makes it very timely to survey the breadth of observations giving evidence for MHD oscillatory processes in solar and space plasmas, and state-of-the-art theoretical modelling. The paper reviews several important topics, such as Alfv,nic resonances and mode conversion; MHD waveguides, such as the magnetotail, coronal loops, coronal streamers; mechanisms for periodicities produced in energy releases during substorms and solar flares, possibility of Alfv,nic resonators along open field lines; possible drivers of MHD waves; diagnostics of plasmas with MHD waves; interaction of MHD waves with partly-ionised boundaries (ionosphere and chromosphere). The review is mainly oriented to specialists in magnetospheric physics and solar physics, but not familiar with specifics of the adjacent research fields.</P>