291 research outputs found
Analysis of time dependent phenomena observed with the LPSP OSO-8 instrument
Data obtained by the Laboratoire de Physique Stellaire et Planetaire's ultraviolet spectrometer onboard the OSO-8 spacecraft were analyzed in an effort to dynamically model the solar chromosphere as an aid in enhancing knowledge of the dynamical processes themselves and of spectral line formation in the dynamic chromosphere. Repeated spectral scans of strong, optically thick resonance lines formed in the solar chromosphere were examined for indications of oscillatory velocities and intensities among other indications of velocity which were studied, the blue peak is reasonably well defined, and the position of a parabolic filter fitted by the least squares method was used to define it. Observed chromospheric oscillation periods are discussed as well as the variations in altitude of the emitting region which result primarily from the motion up and down during the oscillation
Analysis of time dependent phenomena observed with the LPSP OSO-8 instrument
The dynamics of the solar photosphere and chromosphere are studied. Observations obtained by the Laboratorie de Physique Stellaire et Planetaire's (LPSP) ultraviolet spectrometer onboard the OSO-8 spacecraft are analyzed, and dynamic models of the chromosphere and the emitted resonance line spectrum are calculated. Some of the unpublished data analysis and theoretical modeling which are being prepared for publication are discussed. A discussion of the state of the theory of velocity fields in the solar atmosphere is also presented. An invited review presented at the OSO-8 Workshop on the topic of oscillatory motions in the quiet sun is included. The results of the OSO-8 data analysis prepared in close collaboration with LPSP scientists are presented. Material for two articles is also presented
On the flare induced high-frequency global waves in the Sun
Recently, Karoff and Kjeldsen (2008) presented evidence of strong correlation
between the energy in the high-frequency part (5.3<\nu<8.3 mHz) of the acoustic
spectrum of the Sun and the solar X-ray flux. They have used disk-integrated
intensity observations of the Sun obtained from the VIRGO (Variability of solar
IRradiance and Gravity Oscillations) instrument on board SOHO (Solar and
Heliospheric Observatory) spacecraft. Similar signature of flares in velocity
observations has not been confirmed till now. The study of low-degree
high-frequency waves in the Sun is important for our understanding of the
dynamics of the deeper solar layers. In this paper, we present the analysis of
the velocity observations of the Sun obtained from the MDI (Michelson and
Doppler Imager) and the GOLF (Global Oscillations at Low Frequencies)
instruments on board SOHO for some major flare events of the solar cycle 23.
Application of wavelet techniques to the time series of disk-integrated
velocity signals from the solar surface using the full-disk Dopplergrams
obtained from the MDI clearly indicates that there is enhancement of
high-frequency global waves in the Sun during the flares. This signature of
flares is also visible in the Fourier Power Spectrum of these velocity
oscillations. On the other hand, the analysis of disk-integrated velocity
observations obtained from the GOLF shows only marginal evidence of effects of
flares on high-frequency oscillations.Comment: 20 pages, 5 figures, To appear in the APJ Letter
Sub-Surface Meridional flow, Vorticity and the life time of Solar Active Regions
Solar sub-surface fluid topology provides an indirect approach to examine the
internal characteristics of active regions (ARs). Earlier studies have revealed
the prevalence of strong flows in the interior of ARs having complex magnetic
fields. Using the Doppler data obtained by the Global Oscillation Network Group
(GONG) project for a sample of 74 ARs, we have discovered the presence of steep
gradients in meridional velocity at depths ranging from 1.5 to 5 Mm in flare
productive ARs. The sample of these ARs is taken from the Carrington rotations
1980--2052 covering the period August 2001-January 2007. The gradients showed
an interesting hemispheric trend of negative (positive) signs in the northern
(southern) hemisphere, i.e., directed toward the equator. We have discovered
three sheared layers in the depth range of 0 - 10 Mm, providing an evidence of
complex flow structures in several ARs. An important inference derived from our
analysis is that the location of the deepest zero vertical vorticity is
correlated with the remaining life time of ARs. This new finding may be
employed as a tool for predicting the life expectancy of an AR.Comment: 20 pages, 4 figures, To be appear in the Astrophysical Journal
Letter
Variations in -Mode Parameters with Changing Onset-Time of a Large Flare
It is expected that energetic solar flares releasing large amount of energy
at the photosphere may be able to excite the acoustic (-) modes of
oscillations. We have determined the characteristic properties of mode
parameters by applying the ring diagram technique to 3-D power spectra obtained
for solar active region NOAA 10486 during the long duration energetic X17.2/4B
flare of October 28, 2003. Strong evidence of substantial increase in mode
amplitude and systematic variations in sub-surface flows, i.e., meridional and
zonal components of velocity, kinetic helicity, vorticity, is found from
comparison of the pre- to the post-flare phases.Comment: 14 pages, 4 figure
Proper orthogonal decomposition of solar photospheric motions
The spatio-temporal dynamics of the solar photosphere is studied by
performing a Proper Orthogonal Decomposition (POD) of line of sight velocity
fields computed from high resolution data coming from the MDI/SOHO instrument.
Using this technique, we are able to identify and characterize the different
dynamical regimes acting in the system. Low frequency oscillations, with
frequencies in the range 20-130 microHz, dominate the most energetic POD modes
(excluding solar rotation), and are characterized by spatial patterns with
typical scales of about 3 Mm. Patterns with larger typical scales of 10 Mm, are
associated to p-modes oscillations at frequencies of about 3000 microHz.Comment: 8 figures in jpg in press on PR
Mode excitation by turbulent convection in rotating stars. I. Effect of uniform rotation
We focus on the influence of the Coriolis acceleration on the stochastic
excitation of oscillation modes in convective regions of rotating stars. Our
aim is to estimate the asymmetry between excitation rates of prograde and
retrograde modes. We extend the formalism derived for obtaining stellar -
and -mode amplitudes (Samadi & Goupil 2001, Belkacem et al. 2008) to include
the effect of the Coriolis acceleration. We then study the special case of
uniform rotation for slowly rotating stars by performing a perturbative
analysis. This allows us to consider the cases of the Sun and the CoRoT target
HD 49933. We find that, in the subsonic regime, the influence of rotation as a
direct contribution to mode driving is negligible in front of the Reynolds
stress contribution. In slow rotators, the indirect effect of the modification
of the eigenfunctions on mode excitation is investigated by performing a
perturbative analysis of the excitation rates. It turns out that the excitation
of solar modes is affected by rotation with excitation rates asymmetries
between prograde and retrograde modes of the order of several percents. Solar
low-order modes are also affected by uniform rotation and their excitation
rates asymmetries are found to reach up to 10 %. The CoRoT target HD 49933 is
rotating faster than the Sun () and we show
that the resulting excitation rates asymmetry is about 10 % for the excitation
rates of modes. We have then demonstrated that and mode excitation
rates are modified by uniform rotation through the Coriolis acceleration. Study
of the effect of differential rotation is dedicated to a forthcoming paper.Comment: 9 pages, 4 figures, accepted in A&
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