Alexei A. Pevtsov

Latitudinal variation of helicity of photospheric magnetic fields

Using a 1988-1994 data set of original photospheric vector magnetograms as well as published data, we have studied the average magnetic helicity of 69 diverse active regions, adopting the linear force-free field parameter alpha as a measure. This average value was determined by minimizing the differences between the computed constant-alpha force-free and observed horizontal magnetic fields. The average magnetic helicity shows a sign difference at the 2 sigma level in opposite hemispheres. In our data set, 76% of the active regions in the northern hemisphere have negative helicity, and 69% in the southern hemisphere, positive. Although the data show considerable variation from one active region to the next, the data set as a whole suggest that the magnitude of the average helicity increases with solar latitude, starting at zero near the equator, reaches a maximum near 15 deg - 25 deg in both hemispheres, and drops back toward smaller values avove 35 deg - 40 deg. Qualitative comparison with published models shows that such latitudinal variation of the average magnetic helicity may result from either turbulent convective motions or differential rotation, although our studies of rotating sunspots lead us to favor the former.

Flux-Tube Twist Resulting from Helical Turbulence: The Σ-Effect

Recent observational studies suggest that active region magnetic flux emerges in a twisted state and that the sense of twist depends weakly on solar hemisphere. We propose that this twist is imparted to the flux through its interaction with turbulent velocities in the convection zone. This process, designated the Σ-effect, operates on isolated magnetic flux tubes subjected to buffeting by turbulence with a nonvanishing kinetic helicity

The Relationship Between X-Ray Radiance and Magnetic Flux

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Patterns of helicity in solar active regions

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ON THE SUBPHOTOSPHERIC ORIGIN OF CORONAL ELECTRIC CURRENTS

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Chirality of Chromospheric Filaments

-->. The Σ-effect leads to twist of the same sense inferred from observation and opposite to that predicted by the α-effect. A series of numerical calculations are performed to estimate the magnitude of the Σ-effect in the solar convective zone. The results compare favorably with observations in both mean value and statistical dispersion. We find a further relationship with total magnetic flux that can be tested in future observations. The model also predicts that twist is uncorrelated with the tilt angle of the active region.

Hemispheric Helicity Trend for Solar Cycle 23

We use soft X-ray and magnetic field observations of the Sun (quiet Sun, X-ray bright points, active regions, and integrated solar disk) and active stars (dwarf and pre-main-sequence) to study the relationship between total unsigned magnetic flux, � , and X-ray spectral radiance, LX. We find thatand LX exhibit a very nearly linear relationship over 12 orders of magnitude, albeit with significant levels of scatter. This suggests a universal relationship between magnetic flux and the power dissipated through coronal heating. If the relationship can be assumed linear, it is consistent with an average volumetric heating rate � Q � �=L, where � B is the average field strength along a closed field line and L is its length between footpoints. The �- LX relationship also indicates that X-rays provide a useful proxy for the magnetic flux on stars when magnetic measurements are unavailable. Subject headings: stars: coronae — stars: magnetic fields — Sun: corona — Sun: magnetic fields — Sun: X-rays, gamma rays

Reconnection and Helicity in a Solar Flare

Using 46 vector magnetograms from the Stokes Polarimeter of Mees Solar Observatory (MSO), we studied patterns of local helicity in three diverse solar active regions. From these magnetograms we computed maps of the local helicity parameter alpha = J(sub z)/B(sub z). Although such maps are noisy, we found patterns at the level approximately 2 to 3 sigma(sub J(sub z)), which repeat in successive magnetograms for up to several days. Typically, the alpha maps of any given active region contain identifiable patches with both positive and negative values of alpha. Even within a single sunspot complex, several such alpha patches can often be seen. We followed 68 alpha patches that could be identified on at least two successive alpha maps. We found that the persistence fraction of such patches decrease exponentially, with a characteristic time approximately 27 hr.

Transequatorial Loops in the Solar Corona

Using photospheric vector magnetograms from the Haleakala Stokes Polarimeter and coronal X-ray images from the Yohkoh Soft X-Ray Telescope (SXT), we infer values of the force-free field parameter α at both photospheric and coronal levels within 140 active regions. We determine the value of α for a linear force-free field that best fits each magnetogram in a least-squares sense. We average values from all available magnetograms to obtain a single mean photospheric α-value αp for each active region. From the SXT images we estimate α in the corona by determining (π/L) sin γ for individual loops, where γ is the observed shear angle of X-ray loops of length L. We then average these values of α to obtain a single coronal α value, αc, for each active region. In active regions for which the photospheric α-map is predominantly of one sign, we find that the values of αp and αc are well correlated. Only for active regions in which both signs of α are well represented, and in which our method of analysis therefore breaks down, are the values of αp and αc poorly correlated. The former correlation implies that coronal electric currents typically extend down to at least the photosphere. However, other studies imply subphotospheric origin of the currents, and even current systems, that are observed in the photosphere. We therefore conclude that the currents responsible for sinuous coronal structures are of subphotospheric origin.

Magnetic twist and writhe of active regions On the origin of deformed flux tubes

We use the chromospheric full-disk Hα observations to study the chirality of 2310 filaments from 2000-2001. For each filament, we identify the spine and its barbs and determine the filament chirality as fraction of dextral/sinistral barbs of the total number of barbs. We find that 80.2% (558 out of 696) of quiescent filaments in the northern hemisphere are dextral and 85.5% (633 out of 740) of filaments in southern hemisphere are sinistral, in agreement with the well-known hemispheric helicity rule. Our data also show that the active-region filaments follow the same rule, though the hemispheric dependence is weaker: 74.9% (338 out of 451) of active-region filaments in the northern hemisphere are dextral, and 76.7% (297 out of 387) of filaments in the southern hemisphere are sinistral. We show that quiescent filaments formed on leading and returning arms of the same switchback exhibit the same chirality. We also investigate a possible change in the hemispheric rule with polarity reversal of the polar field, and we find no such change.