2,644 research outputs found
Using Coronal Loops to Reconstruct the Magnetic Field of an Active Region Before and After a Major Flare
The shapes of solar coronal loops are sensitive to the presence of electrical
currents that are the carriers of the nonpotential energy available for
impulsive activity. We use this information in a new method for modeling the
coronal magnetic field of AR 11158 as a nonlinear force-free field (NLFFF). The
observations used are coronal images around time of major flare activity on
2011/02/15, together with the surface line-of-sight magnetic field
measurements. The data are from the Helioseismic and Magnetic Imager and
Atmospheric Imaging Assembly (HMI and AIA, respectively) onboard the Solar
Dynamics Observatory (SDO). The model fields are constrained to approximate the
coronal loop configurations as closely as possible, while also subject to the
force-free constraints. The method does not use transverse photospheric
magnetic field components as input, and is thereby distinct from methods for
modeling NLFFFs based on photospheric vector magnetograms. We validate the
method using observations of AR 11158 at a time well before major flaring, and
subsequently review the field evolution just prior to and following an X2.2
flare and associated eruption. The models indicate that the energy released
during the instability is about erg, consistent with what is
needed to power such a large eruptive flare. Immediately prior to the eruption
the model field contains a compact sigmoid bundle of twisted flux that is not
present in the post-eruption models, which is consistent with the observations.
The core of that model structure is twisted by full turns about
its axis.Comment: ApJ, in pres
Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon
Aims: We demonstrate the universal character of the quiet-Sun chromosphere
among inactive stars (solar-type and giants). By assessing the main physical
processes, we shed new light on some common observational phenomena. Methods:
We discuss measurements of the solar Mt. Wilson S-index, obtained by the
Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the
established chromospheric basal Ca II K line flux to the Mt. Wilson S-index
data of inactive ("flat activity") stars, including giants. Results: During the
unusually deep and extended activity minimum of 2009, the Sun reached S-index
values considerably lower than in any of its previously observed minima. In
several brief periods, the Sun coincided exactly with the S-indices of inactive
("flat", presumed Maunder Minimum-type) solar analogues of the Mt. Wilson
sample; at the same time, the solar visible surface was also free of any plages
or remaining weak activity regions. The corresponding minimum Ca II K flux of
the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson
sample are found to be identical to the corresponding Ca II K chromospheric
basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a
universal phenomenon among inactive stars. Its mixed-polarity magnetic field,
generated by a local, "fast" turbulent dynamo finally provides a natural
explanation for the minimal soft X-ray emission observed for inactive stars.
Given such a local dynamo also works for giant chromospheres, albeit on larger
length scales, i.e., l ~ R/g, with R and g as stellar radius and surface
gravity, respectively, the existence of giant spicular phenomena and the
guidance of mechanical energy toward the acceleration zone of cool stellar
winds along flux-tubes have now become traceable.Comment: 6 pages, 4 figures; Astronomy & Astrophysics (Research Note), in
pres
Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars
The objective of this paper is to investigate whether a convective dynamo can
account quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical simulations
of a turbulent dynamo driven by convection to characterize the dynamic
behavior, magnetic field strengths, and filling factors in a non-rotating
stratified medium, and to predict these magnetic properties at the surface of
cool stars. We use simple applications of stellar structure theory for the
convective envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0--M0 spectral range, which allows us to estimate
the unsigned magnetic flux on the surface of non-rotating reference stars. With
these estimates we use the recent results of \citet{Pevtsov03} to predict the
level of X-ray emission from such a turbulent dynamo, and find that our results
compare well with observed lower limits of surface X-ray flux. If we scale our
predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with
the observed lower limit of chromospheric emission in K dwarfs. This suggests
that dynamo action from a convecting, non-rotating plasma is a viable
alternative to acoustic heating models as an explanation for the basal emission
level seen in chromospheric, transition region, and coronal diagnostics from
late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure
Transition region fluxes in A-F Dwarfs: Basal fluxes and dynamo activity
The transition region spectra of 87 late A and early F dwarfs and subgiants were analyzed. The emission line fluxes are uniformly strong in the early F stars, and drop off rapidly among the late A stars. The basal flux level in the F stars is consistent with an extrapolation of that observed among the G stars, while the magnetic component displays the same flux-flux relations seen among solar-like stars. Despite the steep decrease in transition region emission flux for B-V less than 0.28, C II emission is detected in alpha Aql (B-V = 0.22). The dropoff in emission is inconsistent with models of the mechanically generated acoustic flux available. It is concluded that, although the nonmagnetic basal heating is an increasingly important source of atmospheric heating among the early F stars, magnetic heating occurs in any star which has a sufficiently thick convective zone to generate acoustic heating
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