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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
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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 |
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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
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Publication Status: Astrophysical Journal, in press
Last Modified: 2003-07-31 10:55
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Helicity Evolution in Emerging Active Regions |
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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
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Publication Status: submitted to ApJ
Last Modified: 2003-04-30 10:53
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