1,704 research outputs found
The Role of Helicity in Magnetic Reconnection: 3D Numerical Simulations
We demonstrate that conservation of global helicity plays only a minor role
in determining the nature and consequences of magnetic reconnection in the
solar atmosphere. First, we show that observations of the solar coronal
magnetic field are in direct conflict with Taylor's theory. Next, we present
results from three-dimensional MHD simulations of the shearing of bipolar and
multi-polar coronal magnetic fields by photospheric footpoint motions, and
discuss the implications of these results for Taylor's theory and for models of
solar activity. The key conclusion of this work is that significant magnetic
reconnection occurs only at very specific locations and, hence, the Sun's
magnetic field cannot relax completely down to the minimum energy state
predicted by conservation of global helicity.Comment: AGU LaTeX manuscript, 17 pages, 2 b-w figures, 4 color plate
Chirality of Intermediate Filaments and Magnetic Helicity of Active Regions
Filaments which form either between or around active regions (ARs) are called
intermediate filaments. In spite of various theoretical studies, the origin of
the chirality of filaments is still uncovered. We investigated how intermediate
filaments are related to their associated ARs, especially from the point of
view of magnetic helicity and the orientation of polarity inversion lines
(PILs). The chirality of filaments has been determined based on the
orientations of barbs observed in BBSO full-disk Halpha images taken during the
rising phase of solar cycle 23. The sign of magnetic helicity of ARs has been
determined using S/inverse-S shaped sigmoids from Yohkoh SXT images. As a
result, we have found a good correlation between the chirality of filaments and
the magnetic helicity sign of ARs. Among 45 filaments, 42 filaments have shown
the same sign as helicity sign of nearby ARs. It has been also confirmed that
the role of both the orientation and the relative direction of PILs to ARs in
determining the chirality of filaments is not significant, against a
theoretical prediction. These results suggest that the chirality of
intermediate filaments may originate from magnetic helicity of their associated
ARs.Comment: 13 pages, 7 figures, Accepted for Ap
On the origin of the 1/f spectrum in the solar wind magnetic field
We present a mechanism for the formation of the low frequency 1/f magnetic
spectrum based on numerical solutions of a shell reduced-MHD model of the
turbulent dynamics inside the sub-Alfv\'enic solar wind. We assign reasonably
realistic profiles to the wind speed and the density along the radial
direction, and a radial magnetic field. Alfv\'en waves of short periodicity
(600 s) are injected at the base of the chromosphere, penetrate into the corona
and are partially reflected, thus triggering a turbulent cascade. The cascade
is strong for the reflected wave while it is weak for the outward propagating
waves. Reflection at the transition region recycles the strong turbulent
spectrum into the outward weak spectrum, which is advected beyond the
Alfv\'enic critical point without substantial evolution. There, the magnetic
field has a perpendicular power-law spectrum with slope close to the Kolmogorov
-5/3. The parallel spectrum is inherited from the frequency spectrum of large
(perpendicular) eddies. The shape is a double power-law with slopes of -1 and
-2 at low and high frequencies respectively, the position of the break
depending on the injected spectrum. We suggest that the double power-law
spectrum measured by Helios at 0.3 AU, where the average magnetic field is not
aligned with the radial (contrary to our assumptions) results from the
combination of such different spectral slopes. At low frequency the parallel
spectrum dominates with its characteristic 1/f shape, while at higher
frequencies its steep spectral slope (-2) is masked by the more energetic
perpendicular spectrum (slope -5/3).Comment: 5 pages, 4 figures, accepted for publication in ApJL, V2: typo
corrected in eq.1, color figure
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