118 research outputs found
Travel Time Shifts due to Amplitude Modulation in Time-Distance Helioseismology
Correct interpretation of acoustic travel times measured by time-distance
helioseismology is essential to get an accurate understanding of the solar
properties that are inferred from them. It has long been observed that sunspots
suppress p-mode amplitude, but its implications on travel times has not been
fully investigated so far. It has been found in test measurements using a
'masking' procedure, in which the solar Doppler signal in a localized quiet
region of the Sun is artificially suppressed by a spatial function, and using
numerical simulations that the amplitude modulations in combination with the
phase-speed filtering may cause systematic shifts of acoustic travel times. To
understand the properties of this procedure, we derive an analytical expression
for the cross-covariance of a signal that has been modulated locally by a
spatial function that has azimuthal symmetry, and then filtered by a phase
speed filter typically used in time-distance helioseismology. Comparing this
expression to the Gabor wavelet fitting formula without this effect, we find
that there is a shift in the travel times, that is introduced by the amplitude
modulation. The analytical model presented in this paper can be useful also for
interpretation of travel time measurements for non-uniform distribution of
oscillation amplitude due to observational effects.Comment: 17 pages, 1 figure, accepted for publication in Ap
Helioseismic Ring Analysis of CME Source Regions
We apply the ring diagram technique to source regions of halo coronal mass
ejections (CMEs) to study changes in acoustic mode parameters before, during,
and after the onset of CMEs. We find that CME regions associated with a low
value of magnetic flux have line widths smaller than the quiet regions implying
a longer life-time for the oscillation modes. We suggest that this criterion
may be used to forecast the active regions which may trigger CMEs.Comment: Accepted for publication in J. Astrophys. Astr. Also available at
http://www2.nso.edu/staff/sushant/paper.htm
A response surface analysis of critical values for the lead-lag ratio with application to high frequency and non-synchronous financial data
High Resolution Helioseismic Imaging of Subsurface Structures and Flows of A Solar Active Region Observed by Hinode
We analyze a solar active region observed by the Hinode CaII H line using the
time-distance helioseismology technique, and infer wave-speed perturbation
structures and flow fields beneath the active region with a high spatial
resolution. The general subsurface wave-speed structure is similar to the
previous results obtained from SOHO/MDI observations. The general subsurface
flow structure is also similar, and the downward flows beneath the sunspot and
the mass circulations around the sunspot are clearly resolved. Below the
sunspot, some organized divergent flow cells are observed, and these structures
may indicate the existence of mesoscale convective motions. Near the light
bridge inside the sunspot, hotter plasma is found beneath, and flows divergent
from this area are observed. The Hinode data also allow us to investigate
potential uncertainties caused by the use of phase-speed filter for short
travel distances. Comparing the measurements with and without the phase-speed
filtering, we find out that inside the sunspot, mean acoustic travel times are
in basic agreement, but the values are underestimated by a factor of 20-40%
inside the sunspot umbra for measurements with the filtering. The initial
acoustic tomography results from Hinode show a great potential of using
high-resolution observations for probing the internal structure and dynamics of
sunspots.Comment: accepted for publication in Ap
Transient downflows associated with the intensification of small-scale magnetic features and bright point formation
Small-scale magnetic features are present everywhere in the solar
photosphere. Theoretical models, numerical calculations, and simulations
describing the formation of these features have existed for a few decades, but
there are only a few observational studies in direct support of the
simulations. In this study we present the evolution of small-scale magnetic
features with a spatial resolution close to 0.15 arcsecond and compare these
observations with those predicted by numerical simulations and also with
previous observational work of a similar nature. We analyze a 40 min time
sequence of full Stokes spectropolarimetric 630.25 nm data from a plage region
near the Sun center. We use line-of-sight velocities and magnetic field
measurements obtained using Milne-Eddington inversion techniques with and
without stray-light compensation along with measured continuum and line minimum
intensities. We discuss the results in relation to earlier observations and
simulations. We present eight cases involving strong downflows and magnetic
field intensification. All cases studied are associated with the formation of a
bright point in the continuum. In three out of the eight cases we find the
presence of weak opposite polarity field in close proximity to the downflow.
Our data are consistent with earlier simulations describing flux tube collapse,
but the transition to a state with stronger field appears transient and
short-lived, rather than resulting in a permanent field intensification. Three
cases of weak opposite polarity field found adjacent to the downflows do not
appear related to reconnection but may be related to overturning convection
pulling down some field lines and leading to up/down "serpentine" field, as
seen in some simulations.Comment: Accepted for publication in Astronomy & Astrophysic
Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO
We study properties of waves of frequencies above the photospheric acoustic
cut-off of 5.3 mHz, around four active regions, through spatial maps
of their power estimated using data from Helioseismic and Magnetic Imager (HMI)
and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory
(SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known
to capture clear oscillation signals due to helioseismic p modes as well as
waves propagating up through to the chromosphere. Here we study in detail, in
comparison with HMI Doppler data, properties of the power maps, especially the
so called 'acoustic halos' seen around active regions, as a function of wave
frequencies, inclination and strength of magnetic field (derived from the
vector field observations by HMI) and observation height. We infer possible
signatures of (magneto-)acoustic wave refraction from the observation height
dependent changes, and hence due to changing magnetic strength and geometry, in
the dependences of power maps on the photospheric magnetic quantities. We
discuss the implications for theories of p mode absorption and mode conversions
by the magnetic field.Comment: 22 pages, 12 figures, Accepted by journal Solar Physic
Numerical simulation of excitation and propagation of helioseismic MHD waves: Effects of inclined magnetic field
Investigation of propagation, conversion, and scattering of MHD waves in the
Sun is very important for understanding the mechanisms of observed oscillations
and waves in sunspots and active regions. We have developed 3D linear MHD
numerical model to investigate influence of the magnetic field on excitation
and properties of the MHD waves. The results show that the magnetic field can
substantially change the properties of the surface gravity waves (f-mode), but
their influence on the acoustic-type waves (p-modes) is rather moderate.
Comparison our simulations with the time-distance helioseismology results from
SOHO/MDI shows that the travel time variations caused by the inclined magnetic
field do not exceed 25% of the observed amplitude even for strong fields of
1400-1900 G. This can be an indication that other effects (e.g. background
flows and non-uniform distribution of magnetic field) can contribute to the
observed travel time variations. The travel time variations caused by the wave
interaction with magnetic field are in phase with the observations for strong
fields of 1400-1900 G if Doppler velocities are taken at the height of 300 km
above the photosphere where plasma parameter beta<<1. The simulations show that
the travel times only weakly depend on the height of velocity observation. For
the photospheric level the travel times are systematically smaller on
approximately 0.12 min then for the hight of 300 km above the photosphere for
all studied ranges of the magnetic field strength and inclination angles. The
numerical MHD wave modeling and new data from the HMI instrument of the Solar
Dynamics Observatory will substantially advance our knowledge of the wave
interaction with strong magnetic fields on the Sun and improve the local
helioseismology diagnostics.Comment: 24 pages, 10 figures, submitted to Ap
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The Propagation of Coherent Waves Across Multiple Solar Magnetic Pores
Solar pores are efficient magnetic conduits for propagating magnetohydrodynamic wave energy into the outer regions of the solar atmosphere. Pore observations often contain isolated and/or unconnected structures, preventing the statistical examination of wave activity as a function of the atmospheric height. Here, using high-resolution observations acquired by the Dunn Solar Telescope, we examine photospheric and chromospheric wave signatures from a unique collection of magnetic pores originating from the same decaying sunspot. Wavelet analysis of high-cadence photospheric imaging reveals the ubiquitous presence of slow sausage-mode oscillations, coherent across all photospheric pores through comparisons of intensity and area fluctuations, producing statistically significant in-phase relationships. The universal nature of these waves allowed an investigation of whether the wave activity remained coherent as they propagate. Utilizing bisector Doppler velocity analysis of the Ca ii 8542 Å line, alongside comparisons of the modeled spectral response function, we find fine-scale 5 mHz power amplification as the waves propagate into the chromosphere. Phase angles approaching zero degrees between co-spatial line depths spanning different line depths indicate standing sausage modes following reflection against the transition region boundary. Fourier analysis of chromospheric velocities between neighboring pores reveals the annihilation of the wave coherency observed in the photosphere, with examination of the intensity and velocity signals from individual pores indicating they behave as fractured waveguides, rather than monolithic structures. Importantly, this work highlights that wave morphology with atmospheric height is highly complex, with vast differences observed at chromospheric layers, despite equivalent wave modes being introduced into similar pores in the photosphere
Validation of shoot fly resistance introgression lines using SNP markers in sorghum
Shoot fly is a major pest in sorghum production globally. Shoot fly management using insecticides is expensive and environmentally un-safe. Therefore, host plant resistance is exploited to develop shoot fly resistance (SFR) lines including transfer of shoot fly resistance QTLs using marker assisted backcrossing. QTLs controlling SFR component traits, glossiness, trichome density, ovipositional non-preference were used for introgression in this stud
Meridional circulation dynamics in a cyclic convective dynamo
Surface observations indicate that the speed of the solar meridional circulation in the photosphere varies in anti-phase with the solar cycle. The current explanation for the source of this variation is that inflows into active regions alter the global surface pattern of the meridional circulation. When these localized inflows are integrated over a full hemisphere, they contribute to slowing down the axisymmetric poleward horizontal component. The behavior of this large-scale flow deep inside the convection zone remains largely unknown. Present helioseismic techniques are not sensitive enough to capture the dynamics of this weak large-scale flow. Moreover, the large time of integration needed to map the meridional circulation inside the convection zone, also masks some of the possible dynamics on shorter timescales. In this work we examine the dynamics of the meridional circulation that emerges from a 3D MHD global simulation of the solar convection zone. Our aim is to assess and quantify the behavior of meridional circulation deep inside the convection zone where the cyclic large-scale magnetic field can reach considerable strength. Our analyses indicate that the meridional circulation morphology and amplitude are both highly influenced by the magnetic field via the impact of magnetic torques on the global angular momentum distribution. A dynamic feature induced by these magnetic torques is the development of a prominent upward flow at mid-latitudes in the lower convection zone that occurs near the equatorward edge of the toroidal bands and that peaks during cycle maximum. Globally, the dynamo-generated large-scale magnetic field drives variations in the meridional flow, in stark contrast to the conventional kinematic flux transport view of the magnetic field being advected passively by the flow.Centra-ISTGRPS-UdeMNatural Sciences and Engineering Research Council of CanadaNational Science FoundationUniversity of the Algarveinfo:eu-repo/semantics/publishedVersio
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