777 research outputs found
Helioseismic response to X2.2 solar flare of February 15, 2011
The X2.2-class solar flare of February 15, 2011, produced a powerful sunquake
event, representing a helioseismic response to the flare impact in the solar
photosphere, which was observed with the HMI instrument on the Solar Dynamics
Observatory (SDO). The impulsively excited acoustic waves formed a compact
wavepacket traveling through the solar interior and appearing on the surface as
expanding wave ripples. The initial flare impacts were observed in the form of
compact and rapid variations of the Doppler velocity, line-of-sight magnetic
field and continuum intensity. These variations formed a typical two-ribbon
flare structure, and are believed to be associated with thermal and
hydrodynamic effects of high-energy particles heating the lower atmosphere. The
analysis of the SDO/HMI and X-ray data from the Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) shows that the helioseismic waves were initiated
by the photospheric impact in the early impulsive phase, observed prior to the
hard X-ray (50-100 keV) impulse, and were probably associated with atmospheric
heating by relatively low-energy electrons (~6-50 keV) and heat flux transport.
The impact caused a short motion in the sunspot penumbra prior to the
appearance of the helioseismic wave. It is found that the helioseismic wave
front traveling through a sunspot had a lower amplitude and was significantly
delayed relative to the front traveling outside the spot. These observations
open new perspectives for studying the flare photospheric impacts and for using
the flare-excited waves for sunspot seismology.Comment: 11 pages, 5 figures, accepted for ApJL, on-line movie:
http://soi.stanford.edu/~sasha/Sunquakes
Effects of Solar Active Regions on Meridional Flows
The aim of this paper is to extend our previous study of the solar-cycle
variations of the meridional flows and to investigate their latitudinal and
longitudinal structure in the subphotospheric layer, especially their
variations in magnetic regions. Helioseismology observations indicate that mass
flows around active regions are dominated by inflows into those regions. On
average, those local flows are more important around leading magnetic
polarities of active regions than around the following polarities, and depend
on the evolutionary stage of particular active regions. We present a
statistical study based on MDI/SOHO observations of 1996-2002 and show that
this effect explains a significant part of the cyclic change of meridional
flows in near-equatorial regions, but not at higher latitudes. A different
mechanism driving solar-cycle variations of the meridional flow probably
operates.Comment: 4 pages, 5 figures, accepted for publication in ApJ
Optimal Masks for Low-Degree Solar Acoustic Modes
We suggest a solution to an important problem of observational
helioseismology of the separation of lines of solar acoustic (p) modes of low
angular degree in oscillation power spectra by constructing optimal masks for
Doppler images of the Sun. Accurate measurements of oscillation frequencies of
low-degree modes are essential for the determination of the structure and
rotation of the solar core. However, these measurements for a particular mode
are often affected by leakage of other p modes arising when the Doppler images
are projected on to spherical-harmonics masks. The leakage results in
overlaping peaks corresponding to different oscillation modes in the power
spectra. In this paper we present a method for calculating optimal masks for a
given (target) mode by minimizing the signals of other modes appearing in its
vicinity. We apply this method to time series of 2 years obtained from
Michelson Doppler Imager (MDI) instrument on board SOHO space mission and
demonstrate its ability to reduce efficiently the mode leakage.Comment: to be published in Astrophys.J. Letter
Imaging the Solar Tachocline by Time-Distance Helioseismology
The solar tachocline at the bottom of the convection zone is an important
region for the dynamics of the Sun and the solar dynamo. In this region, the
sound speed inferred by global helioseismology exhibits a bump of approximately
0.4% relative to the standard solar model. Global helioseismology does not
provide any information on possible latitudinal variations or asymmetries
between the Northern and Southern hemisphere. Here, we develop a time-distance
helioseismology technique, including surface- and deep-focusing measurement
schemes and a combination of both, for two-dimensional tomographic imaging of
the solar tachocline that infers radial and latitudinal variations in the sound
speed. We test the technique using artificial solar oscillation data obtained
from numerical simulations. The technique successfully recovers major features
of the simplified tachocline models. The technique is then applied to SOHO/MDI
medium-l data and provides for the first time a full two-dimensional
sound-speed perturbation image of the solar tachocline. The one-dimensional
radial profile obtained by latitudinal averaging of the image is in good
agreement with the previous global helioseismology result. It is found that the
amplitude of the sound-speed perturbation at the tachocline varies with
latitude, but it is not clear whether this is in part or fully an effect of
instrumental distortion. Our initial results demonstrate that time-distance
helioseismology can be used to probe the deep interior structure of the Sun,
including the solar tachocline.Comment: accepted for publication by Ap
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
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