Spatially resolved microwave pulsations of a flare loop

Abstract

Abstract.A microwave burst with quasi-periodic pulsations was studied with high spatial resolution using observations with the Nobeyama Radioheliograph (NoRH). We found that the time profiles of the microwave emission at 17 and 34 GHz exhibit quasi-periodic (with two well defined periods P 1 = 14-17 s and P 2 = 8-11 s) variations of the intensity at different parts of an observed flaring loop. Detailed Fourier analysis shows the P 1 spectral component to be dominant at the top, while the P 2 one near the feet of the loop. The 14-17 s pulsations are synchronous at the top and in both legs of the loop. The 8-11 s pulsations at the legs are well correlated with each other but the correlation is not so obvious with the pulsations at the loop top. For this P 2 spectral component, a definite phase shift, P 2 /4 ≈ 2.2 s, between pulsations in the northern leg and loop top parts of the loop have been found. The length of the flaring loop is estimated as L = 25 Mm (≈34 ) and its average width at half intensity at 34 GHz as about 6 Mm (≈8 ). Microwave diagnostics shows the loop to be filled with a dense plasma with the number density n 0 ≈ 10 11 cm −3 , penetrated by the magnetic field changing from B 0 ≈ 100 G near the loop top up to B 0 ≈ 200 G near the north footpoint. A comparative analysis of different MHD modes of the loop demonstrates the possibility of the simultaneous existence of two modes of oscillations in the loop: the global sausage mode, with the period P 1 = 14-17 s and the nodes at the footpoints, and a higher harmonics mode (possibly with the radial wave number l > 1), with P 2 = 8-11 s.

Energy and energy flux in axisymmetric slow and fast waves

“…A&A 578, A60 (2015) In this paper we focus on axisymmetric slow and fast sausage modes, which have recently been discovered in numerous structures, but have not been adequately modelled to estimate their energy content. They have been detected in coronal loops (De Moortel et al 2000;Wang et al 2002Wang et al , 2003Krishna Prasad et al 2012), in photospheric pores (Dorotovič et al 2008(Dorotovič et al , 2014Morton et al 2011;Moreels et al 2015), in network bright points (McAteer et al 2003;Bharti et al 2006;Martínez González et al 2011), in flares (Nakariakov et al 2003;Melnikov et al 2005;Van Doorsselaere et al 2011), and in the chromosphere (Morton et al 2012a). Some authors have attempted to estimate the amount of energy present in these observations.…”

Study of multi-periodic coronal pulsations during an X-class solar flare

“…According to Aschwanden (1987), the short QPPs are probably connected with the interaction of electromagnetic, plasma, or whistler waves with accelerated particles detected in the radio emission. Medium QPPs are likely associated with magnetohydrodynamic (MHD) processes in the solar flaring loops as detected in radio, microwave, white light, and X-ray emissions (Inglis et al, 2008Jakimiec & Tomczak, 2008;Asai et al, 2001;Melnikov et al, 2005;Reznikova et al, 2007;McAteer et al, 2005;Reznikova & Shibasaki, 2011, and references cited therein). On the other hand, the long ones are usually relevant to active region (AR) dynamics (Sych et al, , 2015 and global oscillations of the Sun (Tan et al, 2010;Srivastava & Dwivedi, 2010).…”

“…Different periods of QPPs may be associated either with different MHD modes simultaneously present in a flaring loop or with different harmonics of the same MHD mode (Melnikov et al, 2005;Van Doorsselaere et al, 2007;Srivastava et al, 2008;Andries et al, 2009;Srivastava & Dwivedi, 2010, e.g. ).…”

Perturbations of gyrosynchrotron emission polarization from solar flares by sausage modes: forward modeling

“…Observational evidence of the sausage mode of a single flare loop has mostly come from radio instruments: the Nobeyama Radioheliograph (Stepanov 2004;Melnikov et al 2005;Reznikova et al 2007;Inglis et al 2008) and the Owens Valley Solar Array (Mossessian & Fleishman 2012). Microwave emission from solar flares detected by these instruments is mainly generated by gyrosynchrotron (GS) radiation of trapped nonthermal electrons.…”