5,347 research outputs found
Dynamo cycles in global convection simulations of solar-like stars
Several solar-like stars exhibit cyclic magnetic activity similar to the Sun
as found in photospheric and chromospheric emission. We want to understand the
physical mechanism involved in rotational dependence of these activity cycle
periods. We use three-dimensional magnetohydrodynamical simulations of global
convective dynamos models of solar-like stars to investigate the rotational
dependency of dynamos. We further apply the test-field method to determine the
effect in these simulations. We find dynamo with clear oscillating
mean magnetic fields for moderately and rapidly rotating runs. For slower
rotation, the field is constant or exhibit irregular cycles. In the moderately
and rapidly rotating regime the cycle periods increase weakly with rotation.
This behavior can be well explained with a Parker-Yoshimura dynamo wave
traveling equatorward. Even though the effect becomes stronger for
increasing rotation, the shear decreases steeper, causing this weak dependence
on rotation. Similar as other numerical studies, we find no indication of
activity branches as suggested by Brandenburg et al. (1998). However, our
simulation seems to agree more with the transitional branch suggested by
Distefano et al. (2017) and Olspert et al. (2017). If the Sun exhibit a dynamo
wave similar as we find in our simulations, it would operate deep inside the
convection zone.Comment: 11 pages, 10 figures, accepted for publication in A&
Data-driven model of the solar corona above an active region
We aim to reproduce the structure of the corona above a solar active region
as seen in the extreme ultraviolet (EUV) using a three-dimensional
magnetohydrodynamic (3D MHD) model. The 3D MHD data-driven model solves the
induction equation and the mass, momentum, and energy balance. To drive the
system, we feed the observed evolution of the magnetic field in the photosphere
of the active region AR 12139 into the bottom boundary. This creates a hot
corona above the cool photosphere in a self-consistent way. We synthesize the
coronal EUV emission from the densities and temperatures in the model and
compare this to the actual coronal observations. We are able to reproduce the
overall appearance and key features of the corona in this active region on a
qualitative level. The model shows long loops, fan loops, compact loops, and
diffuse emission forming at the same locations and at similar times as in the
observation. Furthermore, the low-intensity contrast of the model loops in EUV
matches the observations. In our model the energy input into the corona is
similar as in the scenarios of fieldline-braiding or flux-tube tectonics, that
is, energy is transported to the corona through the driving of the vertical
magnetic field by horizontal photospheric motions. The success of our model
shows the central role that this process plays for the structure, dynamics, and
heating of the corona.Comment: 5 pages, 3 Figures, published in A&A letter
The singular kernel coagulation equation with multifragmentation
In this article we prove the existence of solutions to the singular
coagulation equation with multifragmentation. We use weighted -spaces to
deal with the singularities and to obtain regular solutions. The Smoluchowski
kernel is covered by our proof. The weak compactness methods are applied
to suitably chosen approximating equations as a base of our proof. A more
restrictive uniqueness result is also given.Comment: 16 pages. arXiv admin note: substantial text overlap with
arXiv:1210.150
Recurrent flux emergence from dynamo-generated fields
We investigate the emergence of a large-scale magnetic field. This field is
dynamo-generated by turbulence driven with a helical forcing function. Twisted
arcade-like field structures are found to emerge in the exterior above the
turbulence zone. Time series of the magnetic field structure show recurrent
plasmoid ejections.Comment: 2 pages, 2 figures, conference proceedin
Magnetic twist: a source and property of space weather
We present evidence for finite magnetic helicity density in the heliosphere
and numerical models thereof, and relate it to the magnetic field properties of
the dynamo in the solar convection zone. We use simulations and solar wind data
to compute magnetic helicity either directly from the simulations, or
indirectly using time series of the skew-symmetric components of the magnetic
correlation tensor. We find that the solar dynamo produces negative magnetic
helicity at small scales and positive at large scales. However, in the
heliosphere these properties are reversed and the magnetic helicity is now
positive at small scales and negative at large scales. We explain this by the
fact that a negative diffusive magnetic helicity flux corresponds to a positive
gradient of magnetic helicity, which leads to a change of sign from negative to
positive values at some radius in the northern hemisphere.Comment: 7 pages, 12 Figures, accepted in Journal of Space Weather and Space
Climat
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