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Alexei Pevtsov   Submitted: 2008-01-24 18:31

Concept of magnetic/current helicity was introduced to solar physics about 15 years ago. Earlier studies led to discovery of such fundamental properties as hemispheric helicity rule, and role of helicity in magnetic reconnection and solar eruptions. Later, the concept was successfully applied in studies of different solar processes from solar dynamo to flare and CME phenomena. Although no silver bullet, helicity has proven to be a very useful ''tool'' in answering many stillpuzzling questions about origin and evolution of solar magnetic fields. I present an overview of some helicity studies and briefly analyze their findings.

Authors: Pevtsov, A.A.
Projects: None

Publication Status: J. Astrophys. Astron. (in press)
Last Modified: 2008-01-25 08:22
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Alexei Pevtsov   Submitted: 2008-01-24 18:28

We study the statistical significance of observed temporal variations of the solar active-region hemispheric helicity rule, as measured by the latitudinal gradient of the best-fit linear force-free-field parameter, d α /d varphi. Using data from four different vector magnetographs, we compute and compare average annual d α /d varphi values for these instruments for 19 years from solar cycles 21, 22, and 23. We find that although every instrument shows the ''wrong'' sign for the hemispheric rule in some years, there is no agreement among the instruments on which years are abnormal. None of the four data sets shows annual values of d α /d varphi departing from the hemispheric helicity rule by more than 3sigma. We conclude that because the hemispheric helicity rule is a weak tendency with significant scatter, an annual subset of active regions is likely to produce statistically unreliable results.

Authors: A. A. Pevtsov, R. C. Canfield, T. Sakurai, M. Hagino
Projects: None

Publication Status: ApJ (in press)
Last Modified: 2008-01-25 08:22
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The Relationship Between X-ray Radiance and Magnetic Flux  

Alexei Pevtsov   Submitted: 2003-07-31 10:55

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, Phi, and X-ray spectral radiance, Lx. We find that Phi and 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 ar Q sim ar B / L, where ar B is the average field strength along a closed field line and L is its length between footpoints. The Phi-Lx relationship also indicates that X-rays provide a useful proxy for the magnetic flux on stars when magnetic measurements are unavailable.

Authors: A. A. Pevtsov, G. H. Fisher, L. W. Acton, D. W. Longcope, C. M. Johns-Krull, C. C. Kankelborg, T. R. Metcalf
Projects: None

Publication Status: Astrophysical Journal, in press
Last Modified: 2003-07-31 10:55
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Chirality of Chromospheric Filaments  

Alexei Pevtsov   Submitted: 2003-05-14 09:32

We use the chromospheric full disk H_alpha observations to study 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 total number of barbs. We find that 80.2% (558 out of 696) of quiescent filaments in northern hemisphere are dextral, and 85.5% (633 out of 740) of filaments in southern hemisphere are sinistral, in agreement with 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 northern hemisphere are dextral, and 76.7% (297 out of 387) of filaments in southern hemisphere are sinistral. We show that quiescent filaments formed on leading and returning arms of the same switch-back 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.

Authors: Alexei A. Pevtsov, K. S. Balasubramaniam, Joey W. Rogers
Projects: None

Publication Status: ApJ submitted
Last Modified: 2003-05-14 09:32
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Helicity Evolution in Emerging Active Regions  

Alexei Pevtsov   Submitted: 2003-04-28 11:02

We study the evolution of twist and magnetic helicity in the coronal fields of active regions as they emerge. We use multi-day sequences of SoHO MDI magnetograms to characterize the region's emergence. We quantify the overall twist in the coronal field, α , by matching a linear force-free field to bright coronal structures in EUV images. At the beginning of emergence all regions studied have α simeq 0. As the active region grows, α increases and reaches a plateau within approximately one day of emergence. The inferred helicity transport rate is larger than differential rotation could produce. Following Longcope & Welsch (2000) we develop a model for the injection of helicity into the corona by the emergence of a twisted flux tube. This model predicts a ramp-up period of approximately one day. The observed time-history α t) is fit by this model assuming reasonable values for the sub-photospheric Alfvén speed. The implication is that helicity is carried by twisted flux tubes rising from the convection zone, and transported across the photosphere by spinning of the poles driven by magnetic torque.

Authors: Pevtsov, A.A., Maleev, V.M., Longcope, D.W.
Projects: Soho-EIT,Soho-MDI

Publication Status: submitted to ApJ
Last Modified: 2003-04-30 10:53
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The Relationship Between X-ray Radiance and Magnetic Flux
Chirality of Chromospheric Filaments
Helicity Evolution in Emerging Active Regions

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