482 research outputs found
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
The Cause of Photospheric and Helioseismic Responses to Solar Flares: High-Energy Electrons or Protons?
Analysis of the hydrodynamic and helioseismic effects in the photosphere
during the solar flare of July 23, 2002, observed by Michelson Doppler Imager
(MDI) on SOHO, and high-energy images from RHESSI shows that these effects are
closely associated with sources of the hard X-ray emission, and that there are
no such effects in the centroid region of the flare gamma-ray emission. These
results demonstrate that contrary to expectations the hydrodynamic and
helioseismic responses (''sunquakes") are more likely to be caused by
accelerated electrons than by high-energy protons. A series of multiple
impulses of high-energy electrons forms a hydrodynamic source moving in the
photosphere with a supersonic speed. The moving source plays a critical role in
the formation of the anisotropic wave front of sunquakes.Comment: 13 pages, 5 figures, ApJL in pres
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
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
Properties of Flares-Generated Seismic Waves on the Sun
The solar seismic waves excited by solar flares (``sunquakes'') are observed
as circular expanding waves on the Sun's surface. The first sunquake was
observed for a flare of July 9, 1996, from the Solar and Heliospheric
Observatory (SOHO) space mission. However, when the new solar cycle started in
1997, the observations of solar flares from SOHO did not show the seismic
waves, similar to the 1996 event, even for large X-class flares during the
solar maximum in 2000-2002. The first evidence of the seismic flare signal in
this solar cycle was obtained for the 2003 ``Halloween'' events, through
acoustic ``egression power'' by Donea and Lindsey. After these several other
strong sunquakes have been observed. Here, I present a detailed analysis of the
basic properties of the helioseismic waves generated by three solar flares in
2003-2005. For two of these flares, X17 flare of October 28, 2003, and X1.2
flare of January 15, 2005, the helioseismology observations are compared with
simultaneous observations of flare X-ray fluxes measured from the RHESSI
satellite. These observations show a close association between the flare
seismic waves and the hard X-ray source, indicating that high-energy electrons
accelerated during the flare impulsive phase produced strong compression waves
in the photosphere, causing the sunquake. The results also reveal new physical
properties such as strong anisotropy of the seismic waves, the amplitude of
which varies significantly with the direction of propagation. The waves travel
through surrounding sunspot regions to large distances, up to 120 Mm, without
significant decay. These observations open new perspectives for helioseismic
diagnostics of flaring active regions on the Sun and for understanding the
mechanisms of the energy release and transport in solar flares.Comment: 12 pages, 4 figures, submitted to Ap
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