Statistical Evidence for the Existence of Alfvénic Turbulence in Solar Coronal Loops

Abstract

Recent observations have demonstrated that waves which are capable of carrying large amounts of energy are ubiquitous throughout the solar corona. However, the question of how this wave energy is dissipated (on which time and length scales) and released into the plasma remains largely unanswered. Both analytic and numerical models have previously shown that Alfvénic turbulence may play a key role not only in the generation of the fast solar wind, but in the heating of coronal loops. In an effort to bridge the gap between theory and observations, we expand on a recent study [De Moortel et al., ApJL, 782:L34, 2014] by analyzing thirty-seven clearly isolated coronal loops using data from the Coronal Multi-channel Polarimeter (CoMP) instrument. We observe Alfvénic perturbations with phase speeds which range from 250 − 750 km s −1 and periods from 140 − 270 s for the chosen loops. While excesses of high frequency wave-power are observed near the apex of some loops (tentatively supporting the onset of Alfvénic turbulence), we show that this excess depends on loop length and the wavelength of the observed oscillations. In deriving a proportional relationship between the loop length/wavelength ratio and the enhanced wave power at the loop apex, and from the analysis of the line-widths associated with these loops, our findings are supportive of the existence of Alfvénic turbulence in coronal loops.

A 17 June 2011 polar jet and its presence in the background solar wind

“…If the jet brightness does not indicate the amount of energy imparted to the coronal high‐speed patches, over the polar region, then another source of energy beyond jetting must be found for these ubiquitious coronal features. Here the polar observations from CoMP [ Tomczyk et al ., ; Tomczyk and McIntosh , ; Threlfall et al ., ; Liu et al ., ] come to mind that show apparent high‐speed enhancements which they assume are associated with an Alfvén flux in polar coronal regions. How this high‐speed magnetic change imparts energy to produce the localized high‐speed flow in patches seen in the corona must still be worked out, but some of the more popular processes include Alfvén flux heating [ Hollweg, , , ; Hollweg et al ., ; Tomczyk et al ., ; Tomczyk and McIntosh , ; McIntosh and De Pontieu , ; De Moortel et al ., ] and/or the production of shocks that drive outward small coronal regions [e.g., Lee and Wu , ].…”

“…Using Hinode X-ray Telescope (XRT) observations [Golub et al, 2007;Kosugi et al, 2007], Cirtain et al [2007] find two speeds for jet outflow components: one is slow, near the sound speed, and the faster speed is comparable to the low-corona Alfvén velocity. These two components are also mapped in Coronal Multichannel Polarimeter (CoMP) [Tomczyk et al, 2008] polar coronal hole and closed-field observations near the solar surface [Jiang et al, 2007;Tomczyk and McIntosh, 2009;Threlfall et al, 2013;Liu et al, 2014]. These high speeds are generally not associated with a jetting response.…”

On the Parallel and Perpendicular Propagating Motions Visible in Polar Plumes: An Incubator for (Fast) Solar Wind Acceleration?

“…How these ubiquitous waves relinquish their abundant energy to the heating and/or acceleration of the plasma in the closed and open magnetic regions of the outer solar atmosphere is not well established, although a considerable body of theory (focusing on the idea of turbulence) exists (e.g., Velli 1993;Verdini et al 2010;Cranmer and van Ballegooijen 2005). Recently, De Moortel et al (2014) and Liu et al (2014) presented observations from CoMP which indicated that excess high frequency power (compared to levels expected from theoretical models) was present in counter-propagating (low-frequency) Alfvénic waves near the apex of large (trans-equatorial) coronal loops. These authors proposed that the relentless counterpropagation of the waves could be a potential reservoir of energy in the coronal system through MHD turbulence and cascade of wave energy from low to high frequencies.…”

Division E Commission 10: Solar Activity

“…Observations with CoMP showing enhanced high-frequency power near the apex of coronal loops (Liu et al 2014a;De Moortel et al 2014) possibly supports this view.…”