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Magnetic and Kinetic Power Spectra as a Tool to Probe the Turbulent Dynamo  

Valentyna Abramenko   Submitted: 2011-12-16 10:27

Generation and diffusion of the magnetic field on the Sun is a key mechanismresponsible for solar activity on all spatial and temporal scales - from thesolar cycle down to the evolution of small-scale magnetic elements in the quietSun. The solar dynamo operates as a non-linear dynamical process and is thoughtto be manifest in two types: as a global dynamo responsible for the solar cycleperiodicity, and as a small-scale turbulent dynamo responsible for theformation of magnetic carpet in the quiet Sun. Numerous MHD simulations of thesolar turbulence did not yet reach a consensus as to the existence of aturbulent dynamo on the Sun. At the same time, high-resolution observations ofthe quiet Sun from Hinode instruments suggest possibilities for the turbulentdynamo. Analysis of characteristics of turbulence derived from observationswould be beneficial in tackling the problem. We analyse magnetic and velocityenergy spectra as derived from Hinode/SOT, SOHO/MDI, SDO/HMI and the New SolarTelescope (NST) of Big Bear Solar Observatory (BBSO) to explore thepossibilities for the small-scale turbulent dynamo in the quiet Sun.

Authors: Valentyna Abramenko, Vasyl Yurchyshyn, Philip Googe
Projects: Hinode/SOT

Publication Status: accepted in ASP Conference Series
Last Modified: 2011-12-19 09:07
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The Rate of Emergence of Magnetic Dipoles in Coronal Holes and Adjacent Quiet-Sun Regions  

Valentyna Abramenko   Submitted: 2006-10-17 11:18

Observations from the Michelson Doppler Imager and the EUV Imaging Telescope on the Solar and Heliospheric Observatory are analyzed to determine the rate of emergence of new magnetic flux in coronal holes and in surrounding regions on the quiet Sun. Coronal holes are regions where the open magnetic flux of the Sun, the component that forms the heliospheric magnetic field, is concentrated. It is determined that the rate of emergence of new magnetic flux is systematically lower, by a factor of ~2, in coronal holes relative to the surrounding quiet Sun. This result is consistent with a prediction in a recent model for the transport of open magnetic flux on the Sun, which demonstrated that open flux tends to accumulate and concentrate in regions where the rate of emergence of new magnetic flux is a local minimum.

Authors: Abramenko, V. I.; Fisk, L. A.; Yurchyshyn, V. B.
Projects: SoHO-MDI

Publication Status: The Astrophysical Journal, Volume 641, Issue 1, pp. L65-L68
Last Modified: 2006-10-17 18:52
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Coronal Heating and Photospheric Turbulence Parameters: Observational Aspects  

Valentyna Abramenko   Submitted: 2006-10-17 11:15

In this study, the soft X-ray luminosity of the solar corona, measured by the Yohkoh spacecraft for 104 well-developed and decaying active regions, is compared to the magnetic field parameters determined from SOHO MDI high-resolution magnetograms. We calculate and compare (1) two area-independent characteristics of the magnetic field: the index (α) of the magnetic power spectrum, E(k)~k-α, and the magnetic energy dissipation rate (ε̄/η), which is a proxy for the energy of random footpoint motions induced by turbulent convection in the photosphere and below; and (2) four area-independent parameters of the soft X-ray emission: the area-normalized flux in Yohkoh Al.1 and AlMgMn channels, and the emission measure and temperature of the coronal plasma. Here we report that the area-normalized soft X-ray flux correlates with both the power index α (Pearson correlation coefficient ρ=0.72/Al.1 and 0.73/AlMgMn) and the magnetic energy dissipation rate ε̄/η (ρ=0.68/Al.1 and 0.70/AlMgMn). Also, both magnetic parameters are well-correlated with the logarithm of the emission measure (ρ=0.72) and the logarithm of temperature [ρ=0.59/α and 0.63/(ε̄/η)]. Our results present strong observational support to those coronal heating models that rely on random footpoint motions as an energy source to heat the corona above active regions.

Authors: V.I. Abramenko, Alexei P. Pevtsov and R. Romano
Projects: SoHO-MDI

Publication Status: The Astrophysical Journal, Volume 646, Issue 1, pp. L81-L84
Last Modified: 2006-10-17 18:52
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Distribution of the magnetic flux in elements of the magnetic field in active regions  

Valentyna Abramenko   Submitted: 2004-10-13 10:06

The unsigned magnetic flux content in flux concentrations in two active regions are calculated by using a set of 248 high resolution SOHO/MDI magnetograms for each active region. Data for flaring active region NOAA 9077 (July 14, 2000) and non-flaring active region NOAA 0061 (Aug 09, 2002) were analyzed. We present an algorithm to automatically select and quantify magnetic flux concentrations above a threshold p. Each active region is analyzed using 4 different values of the threshold, p (p=25, 50, 75 and 100 G). Probability distribution functions (PDFs) and cumulative distribution functions (CDFs) of the magnetic flux were calculated and approximated by the log-normal, exponential and power law functions in the range of flux Phi > 1019 Mx. The Kolmogorov-Smirnov test applied for each of the approximations, showed that the observed distributions are consistent with the log-normal approximation only. Neither exponential, nor power law functions can approximate satisfactory the observed distributions. The parameters of the log-normal distribution do not depend on the threshold value, however they are different for the two active regions. For flaring active region 9077, the expectation value of the magnetic flux content is mu=28.1 imes 1018 Mx and the standard deviation of the log-normal distribution, sigma=79.0 imes 1018 Mx. For non-flaring active region NOAA 0061 these values are: mu=23.8 imes 1018 Mx and sigma=29.6 imes 1018 Mx. The log-normal character of the observed distribution functions suggests that the process of fragmentation dominates over the process of concentration in the formation of the magnetic structure in an active region.

Authors: V.I. Abramenko and D.W. Longcope
Projects: Soho-MDI

Publication Status: Astrophys. J., accepted
Last Modified: 2004-10-13 10:06
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Abstracts by Author
Magnetic and Kinetic Power Spectra as a Tool to Probe the Turbulent Dynamo
The Rate of Emergence of Magnetic Dipoles in Coronal Holes and Adjacent Quiet-Sun Regions
Coronal Heating and Photospheric Turbulence Parameters: Observational Aspects
Distribution of the magnetic flux in elements of the magnetic field in active regions

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