59 research outputs found
Subsurface Flows in and Around Active Regions with Rotating and Non-rotating Sunspots
The temporal variation of the horizontal velocity in subsurface layers
beneath three different types of active regions is studied using the technique
of ring diagrams. In this study, we select active regions (ARs) 10923, 10930,
10935 from three consecutive Carrington rotations: AR 10930 contains a
fast-rotating sunspot in a strong emerging active region while other two have
non-rotating sunspots with emerging flux in AR 10923 and decaying flux in AR
10935. The depth range covered is from the surface to about 12 Mm. In order to
minimize the influence of systematic effects, the selection of active and quiet
regions is made so that these were observed at the same heliographic locations
on the solar disk. We find a significant variation in both components of the
horizontal velocity in active regions as compared to quiet regions. The
magnitude is higher in emerging-flux regions than in the decaying-flux region,
in agreement with earlier findings. Further, we clearly see a significant
temporal variation in depth profiles of both zonal and meridional flow
components in AR 10930, with the variation in the zonal component being more
pronounced. We also notice a significant influence of the plasma motion in
areas closest to the rotating sunspot in AR 10930 while areas surrounding the
non-rotating sunspots in all three cases are least affected by the presence of
the active region in their neighborhood.Comment: Solar Physics (in press), includes 11 figure
Two-Dimensional Helioseismic Power, Phase, and Coherence Spectra of {\it Solar Dynamics Observatory} Photospheric and Chromospheric Observables
While the {\it Helioseismic and Magnetic Imager} (HMI) onboard the {\it Solar
Dynamics Observatory} (SDO) provides Doppler velocity [], continuum
intensity [], and line-depth [] observations, each of which is
sensitive to the five-minute acoustic spectrum, the {\it Atmospheric Imaging
Array} (AIA) also observes at wavelengths -- specifically the 1600 and 1700
Angstrom bands -- that are partly formed in the upper photosphere and have good
sensitivity to acoustic modes. In this article we consider the characteristics
of the spatio--temporal Fourier spectra in AIA and HMI observables for a
15-degree region around NOAA Active Region 11072. We map the
spatio--temporal-power distribution for the different observables and the HMI
Line Core [], or Continuum minus Line Depth, and the phase and coherence
functions for selected observable pairs, as a function of position and
frequency. Five-minute oscillation power in all observables is suppressed in
the sunspot and also in plage areas. Above the acoustic cut-off frequency, the
behaviour is more complicated: power in HMI is still suppressed in the
presence of surface magnetic fields, while power in HMI and the AIA bands
is suppressed in areas of surface field but enhanced in an extended area around
the active region, and power in HMI is enhanced in a narrow zone around
strong-field concentrations and suppressed in a wider surrounding area. The
relative phase of the observables, and their cross-coherence functions, are
also altered around the active region. These effects may help us to understand
the interaction of waves and magnetic fields in the different layers of the
photosphere, and will need to be taken into account in multi-wavelength local
helioseismic analysis of active regions.Comment: 18 pages, 15 figures, to be published in Solar Physic
Local helioseismology of sunspot regions: comparison of ring-diagram and time-distance results
Local helioseismology provides unique information about the subsurface
structure and dynamics of sunspots and active regions. However, because of
complexity of sunspot regions local helioseismology diagnostics require careful
analysis of systematic uncertainties and physical interpretation of the
inversion results. We present new results of comparison of the ring-diagram
analysis and time-distance helioseismology for active region NOAA 9787, for
which a previous comparison showed significant differences in the subsurface
sound-speed structure, and discuss systematic uncertainties of the measurements
and inversions. Our results show that both the ring-diagram and time-distance
techniques give qualitatively similar results, revealing a characteristic
two-layer seismic sound-speed structure consistent with the results for other
active regions. However, a quantitative comparison of the inversion results is
not straightforward. It must take into account differences in the sensitivity,
spatial resolution and the averaging kernels. In particular, because of the
acoustic power suppression, the contribution of the sunspot seismic structure
to the ring-diagram signal can be substantially reduced. We show that taking
into account this effect reduces the difference in the depth of transition
between the negative and positive sound-speed variations inferred by these
methods. Further detailed analysis of the sensitivity, resolution and averaging
properties of the local helioseismology methods is necessary for consolidation
of the inversion results. It seems to be important that both methods indicate
that the seismic structure of sunspots is rather deep and extends to at least
20 Mm below the surface, putting constraints on theoretical models of sunspots.Comment: 10 pages, 10 figures, submitted to Journal of Physics: Conference
Series (JPCS) GONG 2010 - SoHO 24 "A new era of seismology of the Sun and
solar-like stars", June 27 - July 2, 2010 Aix-en-Provence, Franc
Time-Distance Helioseismology of Deep Meridional Circulation
A key component of solar interior dynamics is the meridional circulation
(MC), whose poleward component in the surface layers has been well observed.
Time-distance helioseismic studies of the deep structure of MC, however, have
yielded conflicting inferences. Here, following a summary of existing results
we show how a large center-to-limb systematics (CLS) in the measured travel
times of acoustic waves affect the inferences through an analysis of frequency
dependence of CLS, using data from the Helioseismic and Doppler Imager (HMI)
onboard Solar Dynamics Observatory (SDO). Our results point to the residual
systematics in travel times as a major cause of differing inferences on the
deep structure of MC.Comment: 6 pages, 3 figures, to appear in the Springer series Astrophysics and
Space Science Proceedings of "Dynamics of the Sun & Stars: Honoring the Life
& Work of Michael Thompson" (2020
Helioseismology of Sunspots: A Case Study of NOAA Region 9787
Various methods of helioseismology are used to study the subsurface
properties of the sunspot in NOAA Active Region 9787. This sunspot was chosen
because it is axisymmetric, shows little evolution during 20-28 January 2002,
and was observed continuously by the MDI/SOHO instrument. (...) Wave travel
times and mode frequencies are affected by the sunspot. In most cases, wave
packets that propagate through the sunspot have reduced travel times. At short
travel distances, however, the sign of the travel-time shifts appears to depend
sensitively on how the data are processed and, in particular, on filtering in
frequency-wavenumber space. We carry out two linear inversions for wave speed:
one using travel-times and phase-speed filters and the other one using mode
frequencies from ring analysis. These two inversions give subsurface wave-speed
profiles with opposite signs and different amplitudes. (...) From this study of
AR9787, we conclude that we are currently unable to provide a unified
description of the subsurface structure and dynamics of the sunspot.Comment: 28 pages, 18 figure
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Angular-momentum coupling through the tachocline
Astronomical observation of stellar rotation suggests that at least the
surface layers of the Sun have lost a substantial amount of the angular
momentum that they possessed at the beginning of the main-sequence phase of
evolution; and solar-wind observations indicate that magnetic coupling is still
draining angular momentum from the Sun today. In addition, helioseismological
analysis has shown that the specific angular momentum at the top of the almost
uniformly rotating radiative interior is approximately (although not exactly)
the same as the spherically averaged value at the base of the (differentially
rotating) convection zone, suggesting that angular momentum is being
transported through the tachocline. The mechanism by which that transport is
taking place is not understood. Nor is there a consensus of opinion. I review
some of the suggestions that have been put forward, biassing my discussion, no
doubt, according to my own opinions.Comment: 19 pages, 7 figures, conference on `Magnetic coupling between the
interior and the atmosphere of the Sun' ed. S. S. Hasan and R. J. Rutten,
Bangalore, December 200
Local Helioseismology of Sunspots: Current Status and Perspectives (Invited Review)
Mechanisms of the formation and stability of sunspots are among the
longest-standing and intriguing puzzles of solar physics and astrophysics.
Sunspots are controlled by subsurface dynamics hidden from direct observations.
Recently, substantial progress in our understanding of the physics of the
turbulent magnetized plasma in strong-field regions has been made by using
numerical simulations and local helioseismology. Both the simulations and
helioseismic measurements are extremely challenging, but it becomes clear that
the key to understanding the enigma of sunspots is a synergy between models and
observations. Recent observations and radiative MHD numerical models have
provided a convincing explanation to the Evershed flows in sunspot penumbrae.
Also, they lead to the understanding of sunspots as self-organized magnetic
structures in the turbulent plasma of the upper convection zone, which are
maintained by a large-scale dynamics. Local helioseismic diagnostics of
sunspots still have many uncertainties, some of which are discussed in this
review. However, there have been significant achievements in resolving these
uncertainties, verifying the basic results by new high-resolution observations,
testing the helioseismic techniques by numerical simulations, and comparing
results obtained by different methods. For instance, a recent analysis of
helioseismology data from the Hinode space mission has successfully resolved
several uncertainties and concerns (such as the inclined-field and phase-speed
filtering effects) that might affect the inferences of the subsurface
wave-speed structure of sunspots and the flow pattern. It becomes clear that
for the understanding of the phenomenon of sunspots it is important to further
improve the helioseismology methods and investigate the whole life cycle of
active regions, from magnetic-flux emergence to dissipation.Comment: 34 pages, 18 figures, submitted to Solar Physic
Concern with COVID-19 pandemic threat and attitudes towards immigrants: The mediating effect of the desire for tightness
Tightening social norms is thought to be adaptive for dealing with collective threat yet it may have negative consequences for increasing prejudice. The present research investigated the role of desire for cultural tightness, triggered by the COVID-19 pandemic, in increasing negative attitudes towards immigrants. We used participant-level data from 41 countries (N = 55,015) collected as part of the PsyCorona project, a crossnational longitudinal study on responses to COVID-19. Our predictions were tested through multilevel and SEM models, treating participants as nested within countries. Results showed that peopleâs concern with COVID19 threat was related to greater desire for tightness which, in turn, was linked to more negative attitudes towards immigrants. These findings were followed up with a longitudinal model (N = 2,349) which also showed that peopleâs heightened concern with COVID-19 in an earlier stage of the pandemic was associated with an increase in their desire for tightness and negative attitudes towards immigrants later in time. Our findings offer insight into the trade-offs that tightening social norms under collective threat has for human groups
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