75 research outputs found
Explosive Events and the Evolution of the Photospheric Magnetic Field
Transition region explosive events have long been suggested as direct
signatures of magnetic reconnection in the solar atmosphere. In seeking further
observational evidence to support this interpretation, we study the relation
between explosive events and the evolution of the solar magnetic field as seen
in line-of-sight photospheric magnetograms. We find that about 38% of events
show changes of the magnetic structure in the photosphere at the location of an
explosive event over a time period of 1 h. We also discuss potential
ambiguities in the analysis of high sensitivity magnetograms
Dynamics of active regions observed with TRACE
I present results of an international joint observing campaign, which was carried out in September 2000, to study the oscillatory behaviour of active regions. In this contribution I will concentrate on oscillations in the higher layers of the solar photosphere as observed with the UV filters of the Transition Region and Coronal Explorer (TRACE). I study the distribution of oscillatory power in an
extended active region. I can find a number of well-known chromospheric dynamic phenomina like running penumbral waves, enhanced 5 min power in the plage and network regions and strong 3 min power in the internetwork. In addition, I find that the 3 min power in the surroundings of the active region is decreased, an effect that has not been observed before. From the topology of the magnetic field I infer that this can be explained by an interaction of the acoustic wave field with the expanding magnetic field of the active region
Application of artificial neural networks to solar infrared Stokes spectra
Inthis paper we have for the first time applied the new Stokes inversion approach of using artificial neural networks to measured Stokes data of a solar pore. We have demonstrated that this method is capable to produce the same
results as other conventional methods, but at a much faster speed
Evidence for Two Separate but Interlaced Components of the Chromospheric Magnetic Field
Chromospheric fibrils are generally thought to trace out low-lying, mainly horizontal magnetic elds that fan out from flux concentrations in the photosphere. A high-resolution (approximately 0.1" per pixel) image, taken in the core of the Ca II 854.2 nm line and covering an unusually large area, shows the dark brils within an active region remnant as fine, looplike features that are aligned parallel to each other and have lengths comparable to a supergranular diameter. Comparison with simultaneous line-of-sight magnetograms confirms that the fibrils are centered above intranetwork areas (supergranular cell interiors), with one end rooted just inside the neighboring plage or strong unipolar network but the other endpoint less clearly defined. Focusing on a particular arcade-like structure lying entirely on one side of a lament channel (large-scale polarity inversion), we find that the total amount of positive-polarity flux underlying this "fibril arcade" is approximately 50 times greater than the total amount of negative-polarity flux. Thus, if the brils represent closed loops, they must consist of very weak fields (in terms of total magnetic flux), which are interpenetrated by a more vertical field that contains most of the flux. This surprising result suggests that the fibrils in unipolar regions connect the network to the nearby intranetwork flux, while the bulk of the network flux links to remote regions of the opposite polarity, forming a second, higher canopy above the fibril canopy. The chromospheric field near the edge of the network thus has an interlaced structure resembling that in sunspot penumbrae
The three-dimensional structure of sunspots II. The moat flow at two different heights
Many sunspots are surrounded by a radial outflow called the moat flow. We
investigate the moat flow at two different heights of the solar atmosphere for
a sunspot whose magnetic properties were reported in the first paper of this
series. We use two simultaneous time series taken with the Transition Region
And Coronal Explorer (TRACE) in white light and in the UV at 170 nm. The
field-of-view is centered on the small sunspot NOAA 10886 located near disk
center. Horizontal velocities are derived by applying two different local
correlation tracking techniques. Outflows are found everywhere in the moat. In
the inner moat, the velocities from the UV series are larger than those from
white light, whereas in the outer part of the moat we find the converse result.
The results imply that the white light velocities represent a general outflow
of the quiet sun plasma in the moat, while UV velocities are dominated by small
bright points that move faster than the general plasma flow.Comment: Manuscript accepted by Astronomy & Astrophysic
Evidence For Mixed Helicity in Erupting Filaments
Erupting filaments are sometimes observed to undergo a rotation about the
vertical direction as they rise. This rotation of the filament axis is
generally interpreted as a conversion of twist into writhe in a kink-unstable
magnetic flux rope. Consistent with this interpretation, the rotation is
usually found to be clockwise (as viewed from above) if the post-eruption
arcade has right-handed helicity, but counterclockwise if it has left-handed
helicity. Here, we describe two non--active-region filament events recorded
with the Extreme-Ultraviolet Imaging Telescope (EIT) on the {\it Solar and
Heliospheric Observatory} ({\it SOHO}), in which the sense of rotation appears
to be opposite to that expected from the helicity of the post-event arcade.
Based on these observations, we suggest that the rotation of the filament axis
is in general determined by the net helicity of the erupting system, and that
the axially aligned core of the filament can have the opposite helicity sign to
the surrounding field. In most cases, the surrounding field provides the main
contribution to the net helicity. In the events reported here, however, the
helicity associated with the filament ``barbs'' is opposite in sign to and
dominates that of the overlying arcade.Comment: ApJ, accepte
Newly identified properties of surface acoustic power
The cause of enhanced acoustic power surrounding active regions, the acoustic
halo, is not as yet understood. We explore the properties of the enhanced
acoustic power observed near disk center from 21 to 27 January 2002, including
AR 9787. We find that (i) there exists a strong correlation of the enhanced
high frequency power with magnetic-field inclination, with greater power in
more horizontal fields, (ii) the frequency of the maximum enhancement increases
along with magnetic field strength, and (iii) the oscillations contributing to
the halos show modal ridges which are shifted to higher wavenumber at constant
frequency in comparison to the ridges of modes in the quiet-Sun.Comment: 16 pages, 10 figures, submitted to solar physic
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