Title: Large-Scale Magnetic Structures - Inverse Cascade of Magnetic Helicity 

　Speaker: Shiva Kumar Malapaka 

　Date & Time: Tuesday 12/22, 3pm 

　Venue: Middle conference room 

　Abstract: 
Magnetic helicity is defined as the volume integral of the dot product of the Magnetic field and the magnetic vector potential. In 3D-MHD turbulence, it exhibits an interesting property in the spectral space called 'inverse cascade',  i.e.  transfer of this  quantity to large scales with a constant flux.  It is believed that  this property of magnetic helicity is one of the important  ways through which large-scale magnetic structure in the Universe is formed. We report some of the important results from the spectral and structural study of this property using high resolution Direct Numerical Simulations (DNS) in two setups namely, forced 3D-MHD turbulence and decaying 3D-MHD turbulence. A new relation involving magnetic helicity, magnetic energy, kinetic helicity and kinetic energy; has been obtained from the dimensional analysis of the eddy damped quasi normal Markovian (EDQNM) approximation equation for magnetic helicity and is tested using the data from the above mentioned numerical simulations. Role of both non-linear as well as non-local effects is discussed in the context of the obtained relation. Further, this relation can also be written in a form to represent the alpha effect. Some of the statistical properties [structure functions, flatness, correlation functions] of the magnetic field obtained from our simulations bear a striking similarity to the statistical properties of the magnetic field of the Active Regions (ARs) of the Sun. We also proposed a mechanism that involves both forced and decaying 3D-MHD turbulences through which large-scale magnetic structure could form in some of the astrophysical environments like radio relics. 
This talk summarizes 4 of the published papers S. K. Malapaka & Mueller.W.-C., APJ, 2013, 778, 21;  S. K. Malapaka, Mueller W-C., APJ, 2013, 774, 84; Mueller. W.-C. & S.K. Malapaka, GAFD, 2013, 107, Nos: 1-2,
93;  and  Muller, W.-C., S.K. Malapaka & Busse, A., Physical Review E 2012, 85, 015302.