39 research outputs found
The Sub-Surface Structure of a Large Sample of Active Regions
We employ ring-diagram analysis to study the sub-surface thermal structure of
active regions. We present results using a large number of active regions over
the course of Solar Cycle 23. We present both traditional inversions of
ring-diagram frequency differences, with a total sample size of 264, and a
statistical study using Principal Component Analysis. We confirm earlier
results on smaller samples that sound speed and adiabatic index are changed
below regions of strong magnetic field. We find that sound speed is decreased
in the region between approximately r=0.99R_sun and r=0.995R_sun (depths of 3Mm
to 7Mm), and increased in the region between r=0.97R_sun and r=0.985R_sun
(depths of 11Mm to 21Mm). The adiabatic index is enhanced in the same deeper
layers that sound-speed enhancement is seen. A weak decrease in adiabatic index
is seen in the shallower layers in many active regions. We find that the
magnitudes of these perturbations depend on the strength of the surface
magnetic field, but we find a great deal of scatter in this relation, implying
other factors may be relevant.Comment: 16 pages, 11 figures, accepted for publication in Solar Physic
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
Seeing-Induced Errors in Solar Doppler Velocity Measurements
Imaging systems based on a narrow-band tunable filter are used to obtain
Doppler velocity maps of solar features. These velocity maps are created by
taking the difference between the blue- and red-wing intensity images of a
chosen spectral line. This method has the inherent assumption that these two
images are obtained under identical conditions. With the dynamical nature of
the solar features as well as the Earth's atmosphere, systematic errors can be
introduced in such measurements. In this paper, a quantitative estimate of the
errors introduced due to variable seeing conditions for ground-based
observations is simulated and compared with real observational data for
identifying their reliability. It is shown, under such conditions, that there
is a strong cross-talk from the total intensity to the velocity estimates.
These spurious velocities are larger in magnitude for the umbral regions
compared to the penumbra or quiet-sun regions surrounding the sunspots. The
variable seeing can induce spurious velocities up to about 1 km/s It is also
shown that adaptive optics, in general, helps in minimising this effect.Comment: 14 page
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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
Physical Properties of Wave Motion in Inclined Magnetic Fields Within Sunspot Penumbrae
At the surface of the Sun, acoustic waves appear to be affected by the
presence of strong magnetic fields in active regions. We explore the
possibility that the inclined magnetic field in sunspot penumbrae may convert
primarily vertically propagating acoustic waves into elliptical motion. We use
helioseismic holography to measure the modulus and phase of the correlation
between incoming acoustic waves and the local surface motion within two
sunspots. These correlations are modeled assuming the surface motion is
elliptical, and we explore the properties of the elliptical motion on the
magnetic field inclination. We also demonstrate that the phase shift of the
outward propagating waves is opposite to the phase shift of the inward
propagating waves in stronger, more vertical fields, but similar to the inward
phase shifts in weaker, more inclined fields.Comment: 22 pages, 13 figure
Seismic Emissions from a Highly Impulsive M6.7 Solar Flare
On 10 March 2001 the active region NOAA 9368 produced an unusually impulsive
solar flare in close proximity to the solar limb. This flare has previously
been studied in great detail, with observations classifying it as a type 1
white-light flare with a very hard spectrum in hard X-rays. The flare was also
associated with a type II radio burst and coronal mass ejection. The flare
emission characteristics appeared to closely correspond with previous instances
of seismic emission from acoustically active flares. Using standard local
helioseismic methods, we identified the seismic signatures produced by the
flare that, to date, is the least energetic (in soft X-rays) of the flares
known to have generated a detectable acoustic transient. Holographic analysis
of the flare shows a compact acoustic source strongly correlated with the
impulsive hard X-ray, visible continuum, and radio emission. Time-distance
diagrams of the seismic waves emanating from the flare region also show faint
signatures, mainly in the eastern sector of the active region. The strong
spatial coincidence between the seismic source and the impulsive visible
continuum emission reinforces the theory that a substantial component of the
seismic emission seen is a result of sudden heating of the low photosphere
associated with the observed visible continuum emission. Furthermore, the
low-altitude magnetic loop structure inferred from potential--field
extrapolations in the flaring region suggests that there is a significant
inverse correlation between the seismicity of a flare and the height of the
magnetic loops that conduct the particle beams from the corona.Comment: 16 pages, 7 figures, Solar Physics Topical Issue: SOHO 19/GONG 2007
"Seismology of Magnetic Activity", Accepte
Determining Absorption, Emissivity Reduction, and Local Suppression Coefficients inside Sunspots
The power of solar acoustic waves is reduced inside sunspots mainly due to
absorption, emissivity reduction, and local suppression. The coefficients of
these power-reduction mechanisms can be determined by comparing time-distance
cross-covariances obtained from sunspots and from the quiet Sun. By analyzing
47 active regions observed by SOHO/MDI without using signal filters, we have
determined the coefficients of surface absorption, deep absorption, emissivity
reduction, and local suppression. The dissipation in the quiet Sun is derived
as well. All of the cross-covariances are width corrected to offset the effect
of dispersion. We find that absorption is the dominant mechanism of the power
deficit in sunspots for short travel distances, but gradually drops to zero at
travel distances longer than about 6 degrees. The absorption in sunspot
interiors is also significant. The emissivity-reduction coefficient ranges from
about 0.44 to 1.00 within the umbra and 0.29 to 0.72 in the sunspot, and
accounts for only about 21.5% of the umbra's and 16.5% of the sunspot's total
power reduction. Local suppression is nearly constant as a function of travel
distance with values of 0.80 and 0.665 for umbrae and whole sunspots
respectively, and is the major cause of the power deficit at large travel
distances.Comment: 14 pages, 21 Figure
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
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
Time--Distance Helioseismology Data Analysis Pipeline for Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results
The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory
(SDO/HMI) provides continuous full-disk observations of solar oscillations. We
develop a data-analysis pipeline based on the time-distance helioseismology
method to measure acoustic travel times using HMI Doppler-shift observations,
and infer solar interior properties by inverting these measurements. The
pipeline is used for routine production of near-real-time full-disk maps of
subsurface wave-speed perturbations and horizontal flow velocities for depths
ranging from 0 to 20 Mm, every eight hours. In addition, Carrington synoptic
maps for the subsurface properties are made from these full-disk maps. The
pipeline can also be used for selected target areas and time periods. We
explain details of the pipeline organization and procedures, including
processing of the HMI Doppler observations, measurements of the travel times,
inversions, and constructions of the full-disk and synoptic maps. Some initial
results from the pipeline, including full-disk flow maps, sunspot subsurface
flow fields, and the interior rotation and meridional flow speeds, are
presented.Comment: Accepted by Solar Physics topical issue 'Solar Dynamics Observatory