13,133 research outputs found
Generation of twist on magnetic flux tubes at the base of the solar convection zone
Using two-dimensional magnetohydrodynamics calculations, we investigate a
twist gen- eration mechanism on a magnetic flux tube at the base of the solar
convection zone based on the idea of Choudhuri, 2003, Sol. Phys., 215, 31 in
which a toroidal mag- netic field is wrapped by a surrounding mean poloidal
field. During generation of the twist, the flux tube follows four phases. (1)
It quickly splits into two parts with vortex motions rolling up the poloidal
magnetic field. (2) Owing to the physical mechanism similar to that of the
magneto-rotational instability, the rolled-up poloidal field is bent and
amplified. (3) The magnetic tension of the disturbed poloidal magnetic field
re- duces the vorticity, and the lifting force caused by vortical motion
decreases. (4) The flux tube gets twisted and begins to rise again without
splitting. Investigation of these processes is significant because it shows
that a flux tube without any initial twist can rise to the surface in
relatively weak poloidal fields.Comment: 10 pages, 6 figur
Fine strand-like structure in the solar corona from MHD transverse oscillations
Current analytical and numerical modelling suggest the existence of
ubiquitous thin current sheets in the corona that could explain the observed
heating requirements. On the other hand, new high resolution observations of
the corona indicate that its magnetic field may tend to organise itself in fine
strand-like structures of few hundred kilometres widths. The link between small
structure in models and the observed widths of strand-like structure several
orders of magnitude larger is still not clear. A popular theoretical scenario
is the nanoflare model, in which each strand is the product of an ensemble of
heating events. Here, we suggest an alternative mechanism for strand
generation. Through forward modelling of 3D MHD simulations we show that small
amplitude transverse MHD waves can lead in a few periods time to strand-like
structure in loops in EUV intensity images. Our model is based on previous
numerical work showing that transverse MHD oscillations can lead to
Kelvin-Helmholtz instabilities that deform the cross-sectional area of loops.
While previous work has focused on large amplitude oscillations, here we show
that the instability can occur even for low wave amplitudes for long and thin
loops, matching those presently observed in the corona. We show that the
vortices generated from the instability are velocity sheared regions with
enhanced emissivity hosting current sheets. Strands result as a complex
combination of the vortices and the line-of-sight angle, last for timescales of
a period and can be observed for spatial resolutions of a tenth of loop radius.Comment: Accepted for publication in ApJ
Recording Leaf Movements with a Strain Gauge
Lightweight strain gauge sensing unit for recording circadian leaf movements in space environmen
Detection of flux emergence, splitting, merging, and cancellation of network field. I Splitting and Merging
Frequencies of magnetic patch processes on supergranule boundary, namely flux
emergence, splitting, merging, and cancellation, are investigated through an
automatic detection. We use a set of line of sight magnetograms taken by the
Solar Optical Telescope (SOT) on board Hinode satellite. We found 1636 positive
patches and 1637 negative patches in the data set, whose time duration is 3.5
hours and field of view is 112" \times 112". Total numbers of magnetic
processes are followed: 493 positive and 482 negative splittings, 536 positive
and 535 negative mergings, 86 cancellations, and 3 emergences. Total numbers of
emergence and cancellation are significantly smaller than those of splitting
and merging. Further, frequency dependences of merging and splitting processes
on flux content are investigated. Merging has a weak dependence on flux content
only with a power- law index of 0.28. Timescale for splitting is found to be
independent of parent flux content before splitting, which corresponds to \sim
33 minutes. It is also found that patches split into any flux contents with a
same probability. This splitting has a power-law distribution of flux content
with an index of -2 as a time independent solution. These results support that
the frequency distribution of flux content in the analyzed flux range is
rapidly maintained by merging and splitting, namely surface processes. We
suggest a model for frequency distributions of cancellation and emergence based
on this idea.Comment: 32 pages, 10 figures, 1 table, accepted to Ap
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