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Author name code: ferriz-mas
ADS astronomy entries on 2022-09-14
author:"Ferriz-Mas, Antonio"
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Title: Can Stochastic Resonance Explain Recurrence of Grand Minima?
Authors: Albert, Carlo; Ferriz-Mas, Antonio; Gaia, Filippo; Ulzega,
Simone
2021ApJ...916L...9A Altcode:
The amplitude of the 11 yr solar cycle is well known to be subject
to long-term modulation, including sustained periods of very low
activity known as Grand Minima. Stable long-period cycles found in
proxies of solar activity have given new momentum to the debate about a
possible influence of the tiny planetary tidal forcing. Here, we study
the solar cycle by means of a simple zero-dimensional dynamo model,
which includes a delay caused by meridional circulation as well as a
quenching of the α-effect at toroidal magnetic fields exceeding an
upper threshold. Fitting this model to the sunspot record, we find a
set of parameters close to the bifurcation point at which two stable
oscillatory modes emerge. One mode is a limit cycle resembling normal
solar activity including a characteristic kink in the decaying limb
of the cycle. The other mode is a weak sub-threshold cycle that could
be interpreted as Grand Minimum activity. Adding noise to the model,
we show that it exhibits Stochastic Resonance, which means that a weak
external modulation can toss the dynamo back and forth between these
two modes, whereby the periodicities of the modulation get strongly
amplified.
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Title: Response to: "Critical Analysis of a Hypothesis of the
Planetary Tidal Influence on Solar Activity" by S. Poluianov and
I. Usoskin
Authors: Abreu, J. A.; Albert, C.; Beer, J.; Ferriz-Mas, A.; McCracken,
K. G.; Steinhilber, F.
2014SoPh..289.2343A Altcode: 2014SoPh..tmp...21A
No abstract at ADS
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Title: Introduction
Authors: Ferriz-Mas, Antonio; Hollerbach, Rainer; Stefani, Frank;
Tilgner, Andreas
2013GApFD.107..383F Altcode:
No abstract at ADS
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Title: Evidence for a planetary influence on solar activity?
Authors: Beer, Jürg; Abreu, Jose A.; Ferriz-Mas, Antonio; McCracken,
Kenneth G.; Steinhilber, Friedhelm
2013EGUGA..1510282B Altcode:
In a recent paper Abreu et al. put forward the hypothesis of a planetary
influence on the solar activity. This is based on a surprisingly
good agreement between decadal to centennial solar cycles found in
proxy records of solar activity of the past 10,000 years and the
cycles present in the calculated torque exerted by the planets to a
non-spherical tachocline. The evidence and its significance for such
a planetary influence are discussed. If correct this hypothesis has
some interesting implications for space climate.
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Title: Is there a planetary influence on solar activity?
Authors: Abreu, Jose A.; Beer, Jürg; Ferriz-Mas, Antonio; McCracken,
Kenneth G.; Steinhilber, Friedhelm
2013EGUGA..1510070A Altcode:
Recently Abreu et al. have put forward the hypothesis of a planetary
influence on solar activity. They developed a simple physical model
for describing the time dependent torque exerted by the planets on
a non-spherical tachocline and compared the corresponding power
spectrum with the one obtained from a 9300 y long reconstruction
of solar activity. They found an excellent agreement between the
long-term cycles in proxies of solar activity and the periodicities
in the planetary torque If correct, this hypothesis has important
implications for solar physics and the solar terrestrial relationship.
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Title: Is there a planetary influence on solar activity?
Authors: Abreu, J. A.; Beer, J.; Ferriz-Mas, A.; McCracken, K. G.;
Steinhilber, F.
2012A&A...548A..88A Altcode:
Context. Understanding the Sun's magnetic activity is important
because of its impact on the Earth's environment. Direct observations
of the sunspots since 1610 reveal an irregular activity cycle with
an average period of about 11 years, which is modulated on longer
timescales. Proxies of solar activity such as <SUP>14</SUP>C and
<SUP>10</SUP>Be show consistently longer cycles with well-defined
periodicities and varying amplitudes. Current models of solar activity
assume that the origin and modulation of solar activity lie within
the Sun itself; however, correlations between direct solar activity
indices and planetary configurations have been reported on many
occasions. Since no successful physical mechanism was suggested to
explain these correlations, the possible link between planetary motion
and solar activity has been largely ignored. <BR /> Aims: While energy
considerations clearly show that the planets cannot be the direct cause
of the solar activity, it remains an open question whether the planets
can perturb the operation of the solar dynamo. Here we use a 9400 year
solar activity reconstruction derived from cosmogenic radionuclides to
test this hypothesis. <BR /> Methods: We developed a simple physical
model for describing the time-dependent torque exerted by the planets
on a non-spherical tachocline and compared the corresponding power
spectrum with that of the reconstructed solar activity record. <BR />
Results: We find an excellent agreement between the long-term cycles
in proxies of solar activity and the periodicities in the planetary
torque and also that some periodicities remain phase-locked over 9400
years. <BR /> Conclusions: Based on these observations we put forward
the idea that the long-term solar magnetic activity is modulated by
planetary effects. If correct, our hypothesis has important implications
for solar physics and the solar-terrestrial connection.
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Title: Coupled spin models for magnetic variation of planets and stars
Authors: Nakamichi, A.; Mouri, H.; Schmitt, D.; Ferriz-Mas, A.; Wicht,
J.; Morikawa, M.
2012MNRAS.423.2977N Altcode: 2011arXiv1104.5093N
Geomagnetism is characterized by intermittent polarity reversals and
rapid fluctuations. We have recently proposed a coupled macro-spin
model to describe these dynamics based on the idea that the whole
dynamo mechanism is described by the coherent interactions of many
local elements. In this paper, we further develop this idea and
construct the minimal model for magnetic variations. This simple
model naturally yields many of the observed features of geomagnetism:
its time evolution, the power spectrum, the frequency distribution
of stable polarity periods etc. This model is characterized by two
coexisting phases of spins: i.e. the cluster phase which determines
the global dipole magnetic moment, and the expanded phase which
gives random perpetual perturbations that yield the intermittent
polarity flip of the dipole moment. This model can also describe the
synchronization of the spin oscillations. This corresponds to the case
of our Sun and the model well describes the quasi-regular cycles of
the solar magnetism. Furthermore, by analysing the relevant terms of
magnetohydrodynamic equations based on our model, we have obtained a
scaling relation for the magnetism for planets, satellites and the
Sun. Comparing it with various observations, we can estimate the
relevant scale of the macro-spins.
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Title: A domino model for geomagnetic field reversals
Authors: Mori, N.; Schmitt, D.; Ferriz-Mas, A.; Wicht, J.; Mouri,
H.; Nakamichi, A.; Morikawa, M.
2011arXiv1110.5062M Altcode:
We solve the equations of motion of a one-dimensional planar Heisenberg
(or Vaks-Larkin) model consisting of a system of interacting macro-spins
aligned along a ring. Each spin has unit length and is described by its
angle with respect to the rotational axis. The orientation of the spins
can vary in time due to random forcing and spin-spin interaction. We
statistically describe the behaviour of the sum of all spins for
different parameters. The term "domino model" in the title refers to
the interaction among the spins. We compare the model results with
geomagnetic field reversals and find strikingly similar behaviour. The
aggregate of all spins keeps the same direction for a long time and,
once in a while, begins flipping to change the orientation by almost
180 degrees (mimicking a geomagnetic reversal) or to move back to
the original direction (mimicking an excursion). Most of the time
the spins are aligned or anti-aligned and deviate only slightly with
respect to the rotational axis (mimicking the secular variation of the
geomagnetic pole with respect to the geographic pole). Reversals are
fast compared to the times in between and they occur at random times,
both in the model and in the case of the Earth's magnetic field.
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Title: Past and Future Solar Activity from Cosmogenic Radionuclides
Authors: Abreu, J. A.; Beer, J.; Ferriz-Mas, A.
2010ASPC..428..287A Altcode:
The sunspot record since 1610 shows cycles of magnetic activity with
an irregular distribution of amplitudes and with a period around 11
years. They are modulated on longer timescales and were interrupted by
the Maunder minimum in the 17th century. During the past several cycles
the average solar activity was very high. This raises the question
whether the present grand maximum is likely to terminate soon or even
to be followed by another (Maunder-like) grand minimum. Cosmogenic
radionuclides stored in natural archives such as <SUP>10</SUP>Be
in ice cores and <SUP>14</SUP>C in tree rings have proven to be a
valuable tool in reconstructing past solar activity and changes in
the geomagnetic field intensity over several millennia. At present,
this is the only method to extend back the record of solar activity
beyond the instrumental period. The main properties of solar activity
will be discussed for the past 10,000 years. A detailed statistical
analysis of this record allows us to derive the life expectancy of the
present grand maximum, which will come soon to an end. By using the
same approach applied to the intervals between grand minima, we expect
a grand minimum in solar activity to occur within the next 100 years.
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Title: How to Reach Superequipartition Field Strengths in Solar
Magnetic Flux Tubes
Authors: Ferriz-Mas, A.; Steiner, O.
2007SoPh..246...31F Altcode:
A number of independent arguments indicate that the toroidal flux
system responsible for the sunspot cycle is stored at the base of the
convection zone in the form of flux tubes with field strength close
to 10<SUP>5</SUP> G. Although the evidence for such strong fields is
quite compelling, how such field strength can be reached is still a
topic of debate. Flux expulsion by convection should lead to about
the equipartition field strength, but the magnetic energy density of
a 10<SUP>5</SUP>-G field is two orders of magnitude larger than the
mean kinetic energy density of convective motions. Line stretching
by differential rotation (i.e., the "Ω effect" in the classical
mean-field dynamo approach) probably plays an important role, but
arguments based on energy considerations show that it does not seem
feasible that a 10<SUP>5</SUP>-G field can be produced in this way. An
alternative scenario for the intensification of the toroidal flux
system in the overshoot layer is related to the explosion of rising,
buoyantly unstable magnetic flux tubes, which opens a complementary
mechanism for magnetic-field intensification. A parallelism is pointed
out with the mechanism of "convective collapse" for the intensification
of photospheric magnetic flux tubes up to field strengths well above
equipartition; both mechanisms, which are fundamentally thermal
processes, are reviewed.
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Title: Flow instabilities of magnetic flux tubes. I. Perpendicular
flow
Authors: Schüssler, M.; Ferriz Mas, A.
2007A&A...463...23S Altcode:
Context: The stability properties of filamentary magnetic structures
are relevant for the storage and dynamics of magnetic fields in
stellar convection zones and possibly also in other astrophysical
contexts. <BR />Aims: In a series of papers we study the effect
of external and internal flows on the stability of magnetic flux
tubes. In this paper we consider the effect of a flow perpendicular
to a straight, horizontal flux tube embedded in a gravitationally
stratified fluid. The flow acts on the flux tube by exerting an
aerodynamic drag force and by modifying the pressure stratification
in the background medium. <BR />Methods: We carry out a Lagrangian
linear stability analysis in the framework of the approximation of
thin magnetic flux tubes. <BR />Results: The external flow can drive
monotonic and oscillatory instability (overstability). The stability
condition depends on direction and magnitude of the external velocity as
well as on its first and second derivatives with respect to depth. The
range of the flow-driven instabilities typically extends to modes with
much shorter wavelengths than for the buoyancy-driven undulatory Parker
instability. <BR />Conclusions: .Perpendicular flows with Alfvénic
Mach number of order unity can drive monotonic as well as oscillatory
instability of thin magnetic flux tubes. Such instability can affect
the storage of magnetic flux in stellar interiors.
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Title: Connecting solar radiance variability to the solar dynamo
with the virial theorem
Authors: Steiner, O.; Ferriz-Mas, A.
2005AN....326..190S Altcode:
The variability of solar radiance over a solar cycle is thought to
result from a delicate balance between the radiative deficit of sunspots
and the extra contribution of plage and network regions. Although the
net effect is tiny, it must imply structural and thermal changes in
the Sun or in partial layers of it as an unavoidable consequence of
the virial theorem. Using the virial theorem for continua--including
the magnetic field--it can be shown how solar radiance variability
might be connected to a deeply seated flux-tube dynamo and how this
connection is established on a hydrodynamical time scale.
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Title: The deep roots of solar radiance variability .
Authors: Steiner, O.; Ferriz-Mas, A.
2005MmSAI..76..789S Altcode:
The variability of solar radiance over a solar cycle is thought to
result from a delicate balance between the radiative deficit of sunspots
and the extra contribution of plage and network regions. Although the
net effect is tiny, it implies structural and thermal changes in the
Sun or in partial layers of it as an unavoidable consequence of the
virial theorem. Using the virial theorem for continua--including the
magnetic field--it can be shown how solar radiance variability might be
connected to a deeply seated flux-tube dynamo and how this connection
is established on a hydrodynamical time scale.
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Title: Where does the solar dynamo operate?
Authors: Ferriz-Mas, Antonio
2003ESASP.535...99F Altcode: 2003iscs.symp...99F
The Sun's large-scale magnetic field shows a cyclic behaviour with a
period of about 22 years, but the exact origin of this cycle is not well
understood yet. The observed properties of the magnetic field in the
solar photosphere along with theoretical studies of magneto-convection
in electrically well-conducting fluids suggest that the magnetic
field in stellar convection zones is quite inhomogeneous: magnetic
flux is concentrated into flux tubes embedded in a significantly less
magnetized plasma. This concentration of magnetic flux has important
consequences for magnetic flux storage and for modeling the solar
dynamo, since dynamical aspects such as buoyancy and drag force must
be taken into account.
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Title: Magnetic flux tubes and the dynamo problem
Authors: Schüssler, Manfred; Ferriz-Mas, Antonio
2003and..book..123S Altcode: 2003eclm.book..123S
The observed properties of the magnetic field in the solar photosphere
and theoretical studies of magneto-convection in electrically
well-conducting fluids suggest that the magnetic field in stellar
convection zones is quite inhomogeneous: magnetic flux is concentrated
into magnetic flux tubes embedded in significantly less magnetized
plasma. Such a state of the magnetic field potentially has strong
implications for stellar dynamo theory since the dynamics of an ensemble
of flux tubes is rather different from that of a more uniform field
and new phenomena like magnetic buoyancy appear. <P />If the diameter
of a magnetic flux tube is much smaller than any other relevant length
scale, the MHD equations governing its evolution can be considerably
simplified in terms of the thin-flux-tube approximation. Studies of
thin flux tubes in comparison with observed properties of sunspot
groups have led to far-reaching conclusions about the nature of the
dynamo-generated magnetic field in the solar interior. The storage
of magnetic flux for periods comparable to the amplification time of
the dynamo requires the compensation of magnetic buoyancy by a stably
stratified medium, a situation realized in a layer of overshooting
convection at the bottom of the convection zone. Flux tubes stored
in mechanical force equilibrium in this layer become unstable with
respect to an undular instability once a critical field strength is
exceeded, flux loops rise through the convection zone and erupt as
bipolar magnetic regions at the surface. For parameter values relevant
for the solar case, the critical field strength is of the order of
10<SUP>5</SUP> G. A field of similar strength is also required to
prevent the rising unstable flux loops from being strongly deflected
poleward by the action of the Coriolis force and also from `exploding'
in the middle of the convection zone. The latter process is caused by
the superadiabatic stratification. <P />The magnetic energy density of
a field of 10<SUP>5</SUP> G is two orders of magnitude larger than the
kinetic energy density of the convective motions in the lower solar
convection zone. This raises serious doubts whether the conventional
turbulent dynamo process based upon cyclonic convection can work on the
basis of such a strong field. Moreover, it is unclear whether solar
differential rotation is capable of generating a toroidal magnetic
field of 10<SUP>5</SUP> G; it is conceivable that thermal processes like
an entropy-driven outflow from exploded flux tubes leads to the large
field strength required. <P />The instability of magnetic flux tubes
stored in the overshoot region suggests an alternative dynamo mechanism
based upon growing helical waves propagating along the tubes. Since
this process operates only for field strengths exceeding a critical
value, such a dynamo can fall into a `grand minimum' once the field
strength is globally driven below this value, for instance by magnetic
flux pumped at random from the convection zone into the dynamo region
in the overshoot layer. The same process may act as a (re-)starter of
the dynamo operation. Other non-conventional dynamo mechanisms based
upon the dynamics of magnetic flux tubes are also conceivable.
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Title: Advances in Nonlinear Dynamos
Authors: Ferriz-Mas, Antonio; Núñez, Manuel
2003and..book.....F Altcode:
No abstract at ADS
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Title: Variable Solar and Stellar Activity by a Flux Tube Dynamo
Authors: Schmitt, D.; Ferriz-Mas, A.
2003PADEU..13...89S Altcode:
The dynamo action of unstable magnetic flux tubes due to magnetic
buoyancy in a rotating stellar convection zone is summarized and the
implications of a flux tube dynamo with a threshold in field strength
for dynamo action is discussed in connection with the observed
variability of solar and stellar magnetic activity.
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Title: The need for very high resolution spectroscopy for the study
of hot subdwarfs
Authors: Ulla, A.; Manteiga, M.; Thejll, P.; Saffer, R. A.; Pérez
Hernández, F.; MacDonald, J.; Ferriz-Mas, A.; Elkin, V.; Oreiro
Rey, R.
2003RMxAC..16..313U Altcode:
Hot subdwarf stars (hot sds) are blue subluminous objects at high
galactic latitudes. They split into two well-separated spectroscopic
sequences: the O (sdOs) and B-type (sdBs), according to composition and
effective temperature. As they are immediate progenitors of white dwarfs
(WDs), this resembles the spectroscopic H (DA)/He (DB) distinction
between these. Among the various theories for the origin and final
fate of hot sds, both single and close binary evolution have been
suggested but the issue is still debated. Only a few determinations are
available to date regarding the study of such aspects as rotation and
microturbulent velocities (Heber et al.~2000) or the magnetic nature
of these objects (Elkin 1996).
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Title: Hot Subdwarfs: Magnetic, Oscillatory and Other Physical
Properties
Authors: Oreiro, R.; Pérez Hernández, F.; Manteiga, M.; Ulla, A.;
González Pérez, J. M.; Zapatero Osorio, M. R.; GarcÍa López, R.;
MacDonald, J.; Thejll, P.; Ferriz-Mas, A.; Saffer, R. A.; Elkin, V.
2003Ap&SS.284..269O Altcode:
Hot subdwarf stars (hot sds) are blue subluminous objects. Only a
few determinations are available to date regarding the study of such
aspects as rotation, microturbulent velocities or the magnetic nature
of these objects. Over 26 sdBs are known to date to be multiperiodic
rapid oscillators. This project presents preliminary results of new
observations and models of a sample of pulsating and non-pulsating
hot sds, including considerations on mass loss and eventual magnetic
properties.
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Title: Studying the Asymmetry of Bipolar Active Regions by Means of
the Thin Flux-Tube Approximation
Authors: Ferriz-Mas, Antonio
2002smra.progE...4F Altcode:
No abstract at ADS
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Title: Solar Interior: Convection Zone Flux Tubes
Authors: Ferriz-Mas, A.
2000eaa..bookE2244F Altcode:
The magnetic field at the solar surface reflects the presence of
isolated flux tubes below the photosphere. The magnetic field is not
diffusively distributed over the entire surface but is concentrated
into regions of rather intense field, ranging from SUNSPOTS—with
diameters of approximately 20 000 km and field strengths of 3000
G—down to smaller magnetic elements (100-200 km and 1000 G). The e...
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Title: Stellar Dynamos: Nonlinearity and Chaotic Flows
Authors: Nunez, Manuel; Ferriz-Mas, Antonio
1999ASPC..178.....N Altcode: 1999sdnc.conf.....N
No abstract at ADS
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Title: On the Asymmetry of Bipolar Active Regions
Authors: Ferriz-Mas, A.; Schüssler, M.
1998ASPC..155...14F Altcode: 1998sasp.conf...14F
No abstract at ADS
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Title: Variability of Solar and Stellar Activity by Two Interacting
Hydromagnetic Dynamos
Authors: Schmitt, D.; Schussler, M.; Ferriz-Mas, A.
1998ASPC..154.1324S Altcode: 1998csss...10.1324S
We propose a combination of two dynamos for solar and stellar magnetic
activity. A strong-field dynamo operating in the overshoot layer at
the base of the convection zone generating superequipartition fields
concentrated in isolated flux tubes is responsible for cyclic activity
(e.g., sunspots), while a turbulent weak-field dynamo in the convection
zone produces a more irregular field. The combination of a threshold in
field strength for dynamo action due to instability of magnetic flux
tubes in the overshoot layer and random fluctuations due to magnetic
fields from the turbulent convection zone leads, in the case of the
Sun and solar-type stars, to activity cycles with strong amplitude
variations and the occasional appearance of grand minima. Stronger
fluctuations may destroy the cyclic behaviour of the overshoot layer
dynamo and lead to increased but irregular activity. Such activity is
observed in fast rotating cool stars. On the other hand, stars with
low and non-variable magnetic activity may be in a state with only
the turbulent convection zone dynamo active.
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Title: Long-term Variation of Solar Activity by a Dynamo Based on
Magnetic Flux Tubes
Authors: Schussler, M.; Schmitt, D.; Ferriz-Mas, A.
1997ASPC..118...39S Altcode: 1997fasp.conf...39S
We show that cyclic activity and Maunder-type grand minima can be
explained by a dynamo driven by the instability of magnetic flux tubes
in the overshoot layer at the bottom of the solar convection zone. The
combination of a threshold in field strength for dynamo action and
random fluctuations due to magnetic fields from the turbulent convection
zone leads to activity cycles with strong amplitude variations and
the occasional appearance of grand minima.
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Title: Distribution of starspots on cool stars. I. Young and main
sequence stars of 1M<SUB>sun</SUB>_.
Authors: Schuessler, M.; Caligari, P.; Ferriz-Mas, A.; Solanki, S. K.;
Stix, M.
1996A&A...314..503S Altcode:
Sunspots are restricted to a latitude band within 30degof the solar
equator. In contrast, the latitudes of spots on the surfaces of
rapidly rotating cool stars can range from their polar regions, for
RS CVn systems and for T Tauri stars leaving the Hayashi track, to
mid latitudes for stars close to or on the main sequence. In order to
find an explanation for these observed spot latitudes we have applied
the criteria for the undulatory instability (Parker instability) of
a toroidal magnetic flux tube embedded in the convective overshoot
layer below the outer convection zone and calculated the non-linear
evolution of the rising magnetic loops formed by this instability. We
describe the results for a star of one solar mass in different phases
of its evolution before and on the main sequence. We find that there
usually is a range of latitudes at which magnetic flux can emerge on
the stellar surface. The mean latitude of emergence shifts towards
the poles for increasingly rapid rotation. The internal structure
of the star, however, plays an almost equally important role in
determining the latitude of magnetic emergence. For stars of solar
mass only the youngest objects, with extremely deep convection zones,
should show spots emerging at the stellar poles. Pre-main sequence
stars at an age of 10^7^ y (convection zone reaching down half-way to
the centre) exhibit high latitude, but not truly polar spots, while
a main sequence star of one solar mass, even at high rotation rates,
only shows intermediate latitude spots. These results are found to be
in good agreement with Doppler images of young rapid rotators.
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Title: Enhanced inertia of thin magnetic flux tubes.
Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A.
1996A&A...312..317M Altcode:
Accelerated bodies immersed in a fluid experience enhanced inertia
due to the associated co-acceleration of a certain volume of fluid in
their environment. We discuss the concept of enhanced inertia in the
framework of the approximation of thin flux tubes, which is widely
used to describe the dynamics of concentrated magnetic structures in
astrophysical objects. Previous attempts to incorporate this effect
have used a local approach, in which the reaction force of the external
medium on a given tube mass element solely depends on the relative
acceleration of tube and environment at that element. We show that
those previous formulations are inconsistent (either on physical
or geometrical grounds). We present here an alternative derivation
of the enhanced inertia term by geometrical means, still within a
local treatment of the problem but avoiding the pitfalls of previous
works. Our formulation, on the other hand, reveals a basic problem: all
local approaches are bound to give incorrect answers for the reaction
force in as far as they disregard the variation of the external flow in
the direction parallel to the flux tube: in doing so, they generally
fail to provide for global momentum conservation. An exact solution
and detailed analysis for an instance of this failure is given. The
discussion of this paper may be of use also in the hydrodynamical
framework of vortex tube dynamics.
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Title: Intermittent solar activity by an on-off dynamo.
Authors: Schmitt, D.; Schuessler, M.; Ferriz-Mas, A.
1996A&A...311L...1S Altcode:
We show that the alteration between intervals of cyclic activity and
grand minima like the Maunder minimum in the 17th century, which is
characteristic for the long-term variation of the solar activity,
can be described as on-off intermittency arising in a dymano driven
by the instability of magnetic flux tubes in the overshoot layer below
the convection zone. The combination of a threshold in field strength
for dynamo action and random fluctuations due to magnetic fields from a
turbulent convection zone dynamo leads to activity cycles with strong
amplitude variations and the occasional appearance of grand minima of
very low activity. The resulting pattern is in qualitative agreement
with the long-term records of solar activity.
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Title: On the Storage of Magnetic Flux Tubes at the Base of the
Solar Convection Zone
Authors: Ferriz-Mas, Antonio
1996ApJ...458..802F Altcode:
The study of the storage of toroidal flux tubes at the base of the
convection zone is of interest in connection with the operation of a
solar αΩ-type dynamo. The natural equilibrium state for an isolated
flux tube is that of neutral buoyancy (so that there is a mass flow
along the tube in the direction of solar rotation); flux tubes in
thermal equilibrium must be ruled out for magnetic fluxes Φ ≳
10<SUP>19</SUP> Mx. <P />In this paper we study the equilibrium and
stability properties of toroidal flux tubes in the equatorial plane of
the Sun. To that end, we use a solar model which includes a consistently
calculated overshoot region at the bottom of the convection zone, based
on a nonlocal mixing length formalism. As a consequence of the nonlocal
treatment, the superadiabaticity already becomes negative in the lowest
part (≃26,000 km) of the convection zone proper, which is defined as
the region in which the convective flux, is positive. In the present
model, the underlying overshoot region extends over some 10,000 km, and
the total extent of the subadiabatic layer is about 36,000 km. For the
angular velocity distribution, we use a semiempirical formula based on
helioseismological results. Flux tubes with field strengths of about
10<SUP>5</SUP> G can be stored only in the overshoot region, while
tubes with equipartition field strength could be kept in a subadiabatic
layer at the bottom of the convection zone proper. The results of the
stability analysis are compared with those of previous studies.
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Title: Instabilities of Magnetic Flux Tubes in a Stellar Convection
Zone
Authors: Ferriz-Mas, A.; Schüssler, M.
1996ApL&C..34....1F Altcode:
No abstract at ADS
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Title: Instabilities of magnetic flux tubes in a stellar convection
zone II. Flux rings outside the equatorial plane
Authors: Ferriz-Mas, A.; Schüssler, M.
1995GApFD..81..233F Altcode:
Motivated by the problems of magnetic flux storage and dynamo action in
stars with convection zones, we study the equilibrium and stability of
magnetic flux tubes under the influence of differential rotation and
stratification. The formalism developed in the first paper in this
series is applied to axisymmetric, toroidal flux tubes (flux rings)
lying in planes parallel to the equator at an arbitrary latitude. We
assume mechanical force equilibrium, which requires neutral buoyancy
of the flux tube and a longitudinal internal flow in the direction
of stellar rotation. Stability against isentropic perturbations is
investigated by considering both axisymmetric and non-axisymmetric,
three-dimensional displacements of the equilibrium configuration. For
axisymmetric modes, we find qualitative differences between the
stability criteria for flux tubes within and outside the equatorial
plane, where instability is generally easier to excite and overstable
modes appear. In the case of non-axisymmetric perturbations, the
results of a numerical study with parameter values corresponding to
the bottom of the solar convection zone are discussed. The stability
properties depend in a complicated way on the various parameters (e.g.,
latitude, magnetic field, superadiabaticity of the stratification,
angular velocity and its gradient). While the magnetic field value for
the onset of undulatory (Parker) instability with large growth rates
is mainly determined by the stratification and the rotation rate,
instabilities at somewhat lower field strengths with relatively small
growth rates depend strongly on the sign and the value of the angular
velocity gradient.
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Title: Waves and Instabilities of a Toroidal Magnetic Flux Tube in
a Rotating Star
Authors: Ferriz-Mas, A.; Schuessler, M.
1994ApJ...433..852F Altcode:
The oscillation modes and instabilities of a toroidal flux tube
lying in the equatorial plane of a differentially rotating star are
investigated using the thin flux-tube approximation. The behavior
of the frequencies as functions of the magnetic field strength
and of the superadiabaticity is explored for both axisymmetric and
nonaxisymmetric modes. In limiting cases, the modes can be identified
with known normal modes of simple configurations (e.g., a plane-parallel
atmosphere with constant gravity or a magnetic flux tube embedded in a
homogeneous medium). The axisymmetric modes are essentially radial modes
oscillating with a magnetically modified Brunt-Vaisala frequency. As
to the nonaxisymmetric modes in the limit of rapid rotation (or of
weak magnetic field) there is one pair of stable inertial waves, which
are due to the Coriolis force, and one pair of magnetospheric modes,
which are longitudinal slow modes modified by rotation. In the general
case (intermediate rotation rates), the modes do not have a definite
character, but their properties can be investigated analytically. The
corresponding bifurcation diagrams are discussed.
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Title: A dynamo effect due to instability of magnetic flux tubes.
Authors: Ferriz-Mas, A.; Schmitt, D.; Schuessler, M.
1994A&A...289..949F Altcode:
We show that a non-axisymmetric instability of toroidal magnetic
flux tubes in a rotating star provides a dynamo effect. The
instability occurs in the form of propagating helical waves; their
growth in amplitude causes a phase shift between the perturbations
of magnetic field and velocity, which leads to an electric field
(anti)parallel to the direction of the unperturbed field. Together with
differential rotation, this effect is capable of driving a dynamo of the
α{OMEGA}-type. In contrast to the conventional α-effect in cyclonic
convection, this dynamo effect operates in strong (super-equipartition)
magnetic fields which resist against distortion by convective flows. We
calculate the induced electric field using results from linear stability
analysis and a model of the solar convection zone which consistently
includes an overshoot layer. We find that for growing magnetic field
the dynamo effect occurs first in high latitudes (near the poles) in
a region of weak instability which moves towards the equator as the
field strength increases further. The dependence of the dynamo effect
on the location of the flux tubes (in depth and latitude) and on the
rotation rate is discussed.
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Title: Forces on Magnetic Flux Tubes Moving in Inhomogeneous Flows
Authors: Moreno-Insertis, F.; Ferriz-Mas, A.; Schussler, M.
1994ApJ...422..652M Altcode:
The back-reaction of an incompressible two-dimensional flow of constant
strain to the accelerated motion of a straight cylinder is calculated
in the general case that the flow is inhomogeneous, nonstationary, and
with nonvanishing (although constant) vorticity. The resulting enhanced
inertia of the cylinder is basically given by its relative acceleration
with respect to the background flow. Further force terms that appear
because of the nonstationary and inhomogeneity of the background flow
are the following: a force identical to that experienced by the mass
elements of the unperturbed flow at the position of the axis of the
cylinder; the customary lift force because of the circulation around
the body; finally, a force term that appears only with nonvanishing
relative speed between the body and the background flow and which
is related to the energy which has to be imparted by the body to the
surrounding fluid in order to adapt the perturbation to the new local
velocity. The results have application to the dynamics of magnetic
flux tubes in the convection zone and atmosphere of the Sun as well
as to other astrophysical problems.
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Title: Alpha-effect due to instability of magnetic flux tubes and
the solar dynamo
Authors: Schmitt, D.; Ferriz-Mas, A.; Schüssler, M.
1994smf..conf..101S Altcode:
No abstract at ADS
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Title: Instability and eruption of magnetic flux tubes
Authors: Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis, F.;
Schüssler, M.
1994smf..conf..139C Altcode:
No abstract at ADS
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Title: Instability and eruption of magnetic flux tubes in the solar
convection zone.
Authors: Schussler, M.; Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis,
F.
1994A&A...281L..69S Altcode:
We present a consistent model of storage, instability and dynamical
eruption of magnetic flux tubes in the solar convection zone
and underlying overshoot region. Using a convection zone model
with self-consistent overshoot layer, we calculate equilibrium
configurations of magnetic flux tubes and determine their linear
stability properties, taking into consideration the effects of
stratification and rotation. Instability of flux tubes stored in
the overshoot layer with growth times below one year requires field
strengths of the order of 10<SUP>5</SUP> G; in many cases, the dominant
mode has an azimuthal wave number of m = 2. Numerical simulations are
used to follow the nonlinear evolution of such unstable flux tubes and
their rise through the convection zone, from which they emerge to form
active regions. The results are in accordance with the following two
requirements, based on observational facts: (a) the upward motion of
the tubes is not significantly deflected by the Coriolis force so that
they can emerge at low latitudes, (b) their inclination with respect
to the East-West direction (tilt angle) as a function of the latitude
of emergence is consistent with observations of acitve regions.
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Title: Storage of Magnetic Flux in the Overshoot Region
Authors: Moreno-Insertis, F.; Schussler, M.; Ferriz-Mas, A.
1993IAUS..157...41M Altcode:
No abstract at ADS
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Title: Instabilities of magnetic flux tubes in a stellar convection
zone I. Equatorial flux rings in differentially rotating stars
Authors: Ferriz-Mas, A.; Schüssler, M.
1993GApFD..72..209F Altcode:
The stability properties of magnetic flux tubes in stellar
convection zones including overshoot regions is of considerable
interest in connection with the problems of magnetic flux storage
and hydromagnetic dynamo action in the Sun and other cool stars. We
have developed a general formalism based on the approximation of thin
flux tubes which provides a basis for a linear stability analysis
of arbitrary flux tube equilibria. As a first application, the
stability of axisymmetric, toroidal flux tubes (flux rings) located
in the equatorial plane of a star under the influence of differential
rotation and stratification has been considered. Arbitrary angular
velocity differences between the interior of the flux ring and its
environment are permitted. It is found that the linear evolution of
radial and azimuthal perturbations (i.e., within the equatorial plane)
is decoupled from that of latitudinal perturbations (perpendicular to
the plane). The latitudinal instability ('poleward slip') is found to
be suppressed if the matter within the flux tube rotates faster than
its environment by a sufficient amount. For perturbations within the
equatorial plane, both stratification (sub-order superadiabatic) of the
external gas and rotation are crucial. Angular momentum conservation
tends to suppress axisymmetric modes. This effect is enhanced by a
faster rotation of the gas within the flux tube. Non-axisymmetric modes
are more unstable since the constraint of angular momentum conservation
is broken. For these modes, a slower internal rotation rate has a
stabilizing effect. Within a certain range of magnetic field strengths,
a second region of stability exists within the region of unstable
configurations, which can extend into the superadiabatically stratified
(convectively unstable) region. The character of the different modes
is discussed in conjunction with the topology of the stability diagram.
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Title: On the Stability of Magnetic Flux Tubes in the Equator of
a Star
Authors: Ferriz-Mas, A.; Schussler, M.
1993IAUS..157...45F Altcode:
No abstract at ADS
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Title: Modes of a flux ring lying in the equator of a star.
Authors: Ferriz-Mas, A.; Schüssler, M.
1993spd..conf...69F Altcode:
No abstract at ADS
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Title: Shock wave propagation in a magnetic flux tube
Authors: Ferriz-Mas, A.; Moreno-Insertis, F.
1992PhFlA...4.2700F Altcode: 1992PhFl....4.2700F
The propagation of a shock wave in a magnetic flux tube is studied
within the framework of the Brinkley-Kirkwood theory adapted to
a radiating gas. Simplified thermodynamic paths along which the
compressed plasma returns to its initial state are considered. It is
assumed that the undisturbed medium is uniform and that the flux tube
is optically thin. The shock waves investigated, which are described
with the aid of the thin flux-tube approximation, are essentially
slow magnetohydrodynamic shocks modified by the constraint of lateral
pressure balance between the flux tube and the surrounding field-free
fluid; the confining external pressure must be balanced by the internal
gas plus magnetic pressures. Exact analytical solutions giving the
evolution of the shock wave are obtained for the case of weak shocks.
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Title: Storage of magnetic flux tubes in a convective overshoot region
Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A.
1992A&A...264..686M Altcode:
Consideration is given to the suppression of the radial and polar escape
of magnetic flux in the form of toroidal flux tubes (flux rings) from
low latitudes in the overshoot region below the solar convection zone
through the combined action of the subadiabatic ambient stratification
and the rotationally induced forces. It is shown that a flux ring which
is initially in thermal equilibrium with its environment and rotates
with the ambient angular velocity moves radially and latitudinally
towards an equilibrium configuration of lower internal temperature
and larger internal rotation rate with respect to the surrounding
nonmagnetic gas. Flux rings perform superposed buoyancy and inertial
oscillations around their equilibrium positions. From a study of the
frequencies and amplitudes of these oscillations, it is concluded that
flux rings with B of less than about 100,000 G can be kept within the
overshoot region if the superadiabaticity is sufficiently negative,
i.e., less than about -0.00004.
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Title: Damping of Shocks in Magnetic Flux Tubes
Authors: Ferriz Mas, A.; Moreno Insertis, F.
1991mcch.conf..417F Altcode:
No abstract at ADS
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Title: Dynamics of magnetic flux concentrations - The second-order
thin flux tube approximation
Authors: Ferriz-Mas, A.; Schuessler, M.; Anton, V.
1989A&A...210..425F Altcode:
The thin flux tube approximation for the dynamics of magnetic flux
concentrations is extended up to second order in the radial expansion
to consistently include azimuthal velocities and twisted magnetic
fields. The linear wave modes of a flux tube as described by the
new set of equations are calculated and compared with the results
of the conventional zeroth-order thin flux tube approximation. By
comparison with exact solutions which are available for a uniform
flux tube in a non-stratified medium, it is shown that the thin flux
tube approximation (based on an expansion approach) is best suited to
describe surface modes of a magnetic structure while body waves are
not so well represented.
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Title: Radial expansion of the magnetohydrodynamic equations for
axially symmetric configurations
Authors: Ferriz-Mas, A.; Schüsler, M.
1989GApFD..48..217F Altcode:
We introduce a general expansion approach to obtain a fully consistent
closed set of magnetohydrodynamic equations in two independent
variables, which is particularly useful to describe axially symmetric,
time-dependent problems with weak variation of all quantities in
the radial direction. This is done by considering the hierarchy
of expanded magnetofluid equations in cylindrical coordinates and
equating terms with equal powers in the radial coordinate r. From
geometrical considerations it is shown that the radial expansions
of the pertaining physical quantities are either even series or odd
series in r; this introduces a significant reduction in the number
of variables and equations. The closure of the system is provided by
appropriate boundary conditions. Among other possible applications,
the method is relevant for the analysis of structure and dynamics of
magnetic field concentrations in stellar atmospheres.
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Title: Estudio de la dinámica de tubos de flujo magnético mediante
el desarrollo en serie de las ecuaciones magnetohidrodinámicas Title:
Estudio de la dinámica de tubos de flujo magnético mediante el
desarrollo en serie de las ecuaciones magnetohidrodinámicas Title:
Study of the dynamics of magnetic flux tubes through series expansion
of the magnetohydrodynamic equations;
Authors: Ferriz Mas, Antonio
1989PhDT.......119F Altcode:
No abstract at ADS
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Title: Nonlinear flows along magnetic flux tubes: Mathematical
structure and exact simple wave solutions
Authors: Ferriz-Mas, A.
1988PhFl...31.2583F Altcode:
The mathematical structure of nonlinear, isentropic longitudinal flows
of an ideal magnetohydrodynamic plasma confined to a magnetic flux
tube embedded in a quiescent nonmagnetic fluid is investigated. Exact
analytical solutions are derived for a special type of nonlinear
flow in the absence of gravity: the simple waves, for which all the
unknowns depend on single functions of space and time. These exhibit
analytically the formation of shock waves. Emphasis is placed on
new features introduced by the magnetic distensibility, which acts
as an additional restoring force, as compared with the hydrodynamic
flow of gas along a rigid tube. Introducing a series expansion in
a suitable parameter, it is shown that the hydrodynamic problem
can be considered as the zeroth-order approach to the magnetic flux
tube problem. Finally, the motion in a magnetic flux tube under the
action of a piston advancing in a prescribed manner has been briefly
considered. This problem is of current interest in relation to the
generation of tube-guided waves in the solar atmosphere.
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Title: An analytical study of shock waves in thin magnetic flux tubes
Authors: Ferriz-Mas, A.; Moreno-Insertis, F.
1987A&A...179..268F Altcode:
The jump conditions across a shock front in a thin magnetic flux
tube are studied by purely analytical means. Some properties of
magnetohydrodynamic shock waves in extended media are shown also to
hold in the more complicated case of thin magnetic flux tubes. It is
shown that flux tube shock waves are always compressive, thus being
accompanied by a weakening of the magnetic field strength and increase
of the tube radius. Some consequences of this are examined, such as
the sub- or supercritical character of the flow velocity with respect
to the Alfvén, sound and "tube" speeds. The range of variation of the
ratios of the different variables across the shock front is determined
along with the equivalent of the Hugoniot curve and further properties
of the shocks in magnetic flux tubes. The analogies and differences
with HD and MHD shocks in extended media are pointed out. Finally,
some implications of the shock wave structure for the solar atmosphere
are briefly discussed.
