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1.
We study variations of the lifetimes of high-ℓ solar p modes in the quiet and active Sun with the solar activity cycle. The lifetimes in the degree range ℓ=300 – 600 and ν=2.5 – 4.5 mHz were computed from SOHO/MDI data in an area including active regions and quiet Sun using the time – distance
technique. We applied our analysis to the data in four different phases of solar activity: 1996 (at minimum), 1998 (rising
phase), 2000 (at maximum), and 2003 (declining phase). The results from the area with active regions show that the lifetime
decreases as activity increases. The maximal lifetime variations are between solar minimum in 1996 and maximum in 2000; the
relative variation averaged over all ℓ values and frequencies is a decrease of about 13%. The lifetime reductions relative to 1996 are about 7% in 1998 and about
10% in 2003. The lifetime computed in the quiet region still decreases with solar activity, although the decrease is smaller.
On average, relative to 1996, the lifetime decrease is about 4% in 1998, 10% in 2000, and 8% in 2003. Thus, measured lifetime
increases when regions of high magnetic activity are avoided. Moreover, the lifetime computed in quiet regions also shows
variations with the activity cycle. 相似文献
2.
The continuous high spatial resolution Doppler observation of the Sun by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager allows us to compute a helioseismic k–ω power-spectrum diagram using only oscillations inside a sunspot. Individual modal ridges can be clearly seen with reduced power in the k–ω diagram that is constructed from a 40-hour observation of a stable and round sunspot. Comparing this with the k–ω diagram obtained from a quiet-Sun region, one sees that inside the sunspot the f-mode ridge is more reduced in power than the p-mode ridges, especially at high wavenumbers. The p-mode ridges all shift toward lower wavenumber (or higher frequency) for a given frequency (or wavenumber), implying an increase of phase velocity beneath the sunspot. This is probably because the acoustic waves travel across the inclined magnetic field of the sunspot penumbra. Line-profile asymmetries exhibited in the p-mode ridges are more significant in the sunspot than in the quiet Sun. Convection inside the sunspot is also highly suppressed, and its characteristic spatial scale is substantially larger than the typical convection scale of the quiet Sun. These observational facts demand a better understanding of magnetoconvection and interactions of helioseismic waves with magnetic field. 相似文献
3.
The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points.
These points can be found directly in a potential field extrapolation or their density can be estimated from the Fourier spectrum
of the magnetogram. The spectral estimate, in which the extrapolated field is assumed to be random and homogeneous with Gaussian
statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO’s MDI. The majority of null points
occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the
spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can
be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by
MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate
for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null
point, with altitude greater than 1.5 Mm, above every 322 Mm2 patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minima shows that the null point
density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases
approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete
elements with mean flux 〈|φ|〉=1.0×1019 Mx distributed randomly with density n=0.007 Mm−2. 相似文献
4.
We investigate the influence of the ionization of helium on the low-degree acoustic oscillation frequencies in model solar-type stars. The signature in the oscillation frequencies characterizing the ionization-induced depression of the first adiabatic exponent γ is a superposition of two decaying periodic functions of frequency ν, with 'frequencies' that are approximately twice the acoustic depths of the centres of the He i and He ii ionization regions. That variation is probably best exhibited in the second frequency difference Δ2 ν n , l ≡ν n −1, l − 2ν n , l +ν n +1, l . We show how an analytic approximation to the variation of γ leads to a simple representation of this oscillatory contribution to Δ2 ν which can be used to characterize the γ variation, our intention being to use it as a seismic diagnostic of the helium abundance of the star. We emphasize that the objective is to characterize γ, not merely to find a formula for Δ2 ν that reproduces the data. 相似文献
5.
I. Ballai 《Solar physics》2007,246(1):177-185
Following the observation and analysis of large-scale coronal-wave-like disturbances, we discuss the theoretical progress
made in the field of global coronal seismology. Using simple mathematical techniques we determine average values for the magnetic field together with a magnetic map of
the quiet Sun. The interaction between global coronal waves and coronal loops allows us to study loop oscillations in a much
wider context, i.e. we connect global and local coronal oscillations. 相似文献
6.
Phase-correlation statistics comparing acoustic radiation coming out of a particular point on the solar photosphere with acoustic radiation going into it show considerably reduced sound travel times through the subphotospheres of active regions. We have now applied techniques in phase-sensitive seismic holography to data from the Solar Oscillations Investigation – Michelson Doppler Imager (SOI-MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft to obtain high resolution phase-correlation maps of a large, complex active region and the `acoustic moat' which surrounds it. We report the following new results: First, the reduced sound travel-time perturbations in sunspots, acoustic moats, and isolated plages increase approximately in proportion to the logarithm of the surface magnetic flux density, for flux densities above 10 G. This is consistent with an interpretation of the travel-time anomalies, observed with holographic and other local-helioseismic procedures, as caused by acoustic Wilson-like depressions in photospheres of magnetic regions. Second, we find that, compared with isolated plages, the acoustic moats have an additional sound travel-time reduction on the order of 3–5 s which may be explained by a thermal excess due to the blockage of convective transport by the sunspot photosphere. Third, the combined effect of the Wilson depression in plages, acoustic moats, and sunspots may explain the observed variation of global p-mode frequencies with the solar cycle. Fourth, we find that active regions, including sunspots, acoustic moats, and plages, significantly reflect p modes above the acoustic cut-off frequency, where the surface of the quiet Sun acts as a nearly perfect absorber of incident acoustic radiation. 相似文献
7.
This paper presents general relations for the intensity of the resonant transition radiation (RTR) and their detailed analysis.
This analysis shows that the spectrum amplitude of the x-mode at some frequencies for high-energy electrons can grow with
the magnetic field increase in some interval from zero value; it can even dominate over that for the o-mode. With further
magnetic field increase, the intensity of the RTR x-mode decreases in comparison with the intensity of the o-mode and this
decrease is higher for higher velocities of energetic electrons. The polarization of the RTR depends on the velocity of energetic
electrons, too. For velocities lower than some velocity limit v<v
i
the RTR emission is unpolarized in a broad interval of magnetic field intensities in the radio source. For reasonable values
of indices of the power-law distribution functions of energetic electrons, the RTR is broadband in frequencies (df/f≈0.2−0.4). Furthermore, we show various dependencies of the RTR and its spectral characteristics. Assuming the same radio
flux of the transition radiation and the gyro-synchrotron one at the Razin frequency, we estimate the limit magnetic field
in the radio source of the transition radiation. Then, we analyze possible sources of small-scale inhomogeneities (thermal
density fluctuations, Langmuir and ion-sound waves), which are necessary for the transition radiation. Although the small-scale
inhomogeneities connected with the Langmuir waves lead to the plasma radiation, which is essentially stronger than RTR, the
inhomogeneities of the ion-sound waves are suitable for the RTR without any other radiation.
We present the relations describing the RTR for anisotropic distribution functions of fast electrons. We consider the distribution
functions of fast electrons in the form of the Legendre polynomials which depend on the pitch-angle. We analyze the influence
of the degree of the anisotropy (an increase of the number of terms in the Legendre polynomial) on spectral characteristics
of the RTR. A comparison with previous studies is made. As an example of the use of the derived formulas for the RTR, the
24 December 1991 event is studied. It is shown that the observed decimetric burst can be generated by the RTR in the plasma
with the density inhomogeneities at the level 〈ΔN
2〉/N
2=2.5⋅10−5. 相似文献
8.
An attempt is made to infer the structure of the solar convection zone from observedp-mode frequencies of solar oscillations. The differential asymptotic inversion technique is used to find the sound speed in
the solar envelope. It is found that envelope models which use the Canuto-Mazzitelli (CM) formulation for calculating the
convective flux give significantly better agreement with observations than models constructed using the mixing length formalism.
This inference can be drawn from both the scaled frequency differences and the sound speed difference. The sound speed in
the CM envelope model is within 0.2% of that in the Sun except in the region withr > 0.99R
⊙. The envelope models are extended below the convection zone, to find some evidence for the gravitational settling of helium
beneath the base of the convection zone. It turns out that for models with a steep composition gradient below the convection
zone, the convection zone depth has to be increased by about 6 Mm in order to get agreement with helioseismic observations. 相似文献
9.
Solar p modes are one of the dominant types of coherent signals in Doppler velocity in the solar photosphere, with periods showing
a power peak at five minutes. The propagation (or leakage) of these p-mode signals into the higher solar atmosphere is one of the key drivers of oscillatory motions in the higher solar chromosphere
and corona. This paper examines numerically the direct propagation of acoustic waves driven harmonically at the photosphere,
into the nonmagnetic solar atmosphere. Erdélyi et al. (Astron. Astrophys.
467, 1299, 2007) investigated the acoustic response to a single point-source driver. In the follow-up work here we generalise this previous
study to more structured, coherent, photospheric drivers mimicking solar global oscillations. When our atmosphere is driven
with a pair of point drivers separated in space, reflection at the transition region causes cavity oscillations in the lower
chromosphere, and amplification and cavity resonance of waves at the transition region generate strong surface oscillations.
When driven with a widely horizontally coherent velocity signal, cavity modes are caused in the chromosphere, surface waves
occur at the transition region, and fine structures are generated extending from a dynamic transition region into the lower
corona, even in the absence of a magnetic field. 相似文献
10.
Helioseismic techniques such as ring-diagram analysis have often been used to determine the subsurface structural differences
between solar active and quiet regions. Results obtained by inverting the frequency differences between the regions are usually
interpreted as the sound-speed differences between them. These in turn are used as a measure of temperature and magnetic-field
strength differences between the two regions. In this paper we first show that the “sound-speed” difference obtained from
inversions is actually a combination of sound-speed difference and a magnetic component. Hence, the inversion result is not
directly related to the thermal structure. Next, using solar models that include magnetic fields, we develop a formulation
to use the inversion results to infer the differences in the magnetic and thermal structures between active and quiet regions.
We then apply our technique to existing structure inversion results for different pairs of active and quiet regions. We find
that the effect of magnetic fields is strongest in a shallow region above 0.985R
⊙ and that the strengths of magnetic-field effects at the surface and in the deeper (r<0.98R
⊙) layers are inversely related (i.e., the stronger the surface magnetic field the smaller the magnetic effects in the deeper layers, and vice versa). We also find that the magnetic effects in the deeper layers are the strongest in the quiet regions, consistent with the
fact that these are basically regions with weakest magnetic fields at the surface. Because the quiet regions were selected
to precede or follow their companion active regions, the results could have implications about the evolution of magnetic fields
under active regions. 相似文献
11.
Joseph B. Gurman 《Solar physics》1987,108(1):61-75
Time series observations of the profile of the Mgii k line 2795.52 have been obtained in five sunspots with the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission. The three sunspots with umbrae larger than the 3 × 3 pixel size show significant oscillations in integrated line intensity and line centroid, with frequencies in the range 5.29–7.55 mHz (periods of 132–190 s).The frequencies of significant peaks in average umbral power spectra agree well with the frequencies of the three lowest-frequency transmission peaks predicted by a model of resonant transmission of acoustic waves. If radiative delays are unimportant, and the line centroid can be interpreted straightforwardly as a Doppler shift, the measured velocity-intensity phase differences indicate the superposition of upward- and downward-propagating waves in the umbral chromosphere; this is further evidence for the resonant transmission model.A single, quiet Sun time series of k core profiles yields power spectra and a phase difference consistent with the existence of a chromospheric p-mode.The SMM data used in this work were available only because of the repair of the SMM spacecraft by the crew of Challenger on mission 41-C. The pilot for that mission and the commander of Challenger's last mission was Francis R. Scobee.This work is dedicated to his memory. 相似文献
12.
We use the method of time – distance analysis to measure lifetimes of solar p modes in the range ℓ=100 − 600 and ν=3.0 − 4.5 mHz with data taken with the Taiwan Oscillation Network (TON). The lifetimes of p modes are determined by the changes in the amplitude and width of the cross-correlation function of a wave packet with the
number of skips. The amplitude of the cross-correlation function decreases exponentially with the number of skips as in previous
work. This decrease has been interpreted as the effect of the finite p-mode lifetime. In this study, we find that the width of the cross-correlation function increases with the number of skips.
We interpret this phenomenon as the effect of the dispersion of the wave packet. We include this effect in the determination
of the lifetime of the wave packet. The lifetime increases after the dispersion is taken into account. We also study the change
in lifetime between solar minimum and maximum. 相似文献
13.
Given the complexity involved in a flux-transport-type dynamo driven by both Babcock – Leighton and tachocline α effects, we present here a step-by-step procedure for building a flux-transport dynamo model calibrated to the Sun as a guide
for anyone who wishes to build this kind of model. We show that a plausible sequence of steps to reach a converged solution
in such a dynamo consists of (i) numerical integration of a classical α – ω dynamo driven by a tachocline α effect, (ii) continued integration with inclusion of meridional circulation to convert the model into a flux-transport dynamo
driven by only a tachocline α effect, (iii) final integration with inclusion of a Babcock – Leighton surface α effect, resulting in a flux-transport dynamo that can be calibrated to obtain a close fit of model output with solar observations. 相似文献
14.
Paul S. Cally 《Monthly notices of the Royal Astronomical Society》2009,395(3):1309-1318
Phase perturbations due to inclined surface magnetic field of active region strength are calculated numerically in quiet Sun and simple sunspot models in order to estimate and compare the direct and indirect (thermal) effects of the fields on helioseismic waves. It is found that the largest direct effects occur in highly inclined field characteristic of penumbrae, and scale roughly linearly with magnetic field strength. The combined effects of sunspot magnetic and thermal anomalies typically yield negative travel-time perturbations in penumbrae. Travel-time shifts in umbrae depend on details of how the thermal and density structure differs from the quiet Sun. The combined shifts are generally not well approximated by the sum of the thermal and magnetic effects applied separately, except at low field strengths of around 1 kG or less, or if the thermal shift is small. A useful rule-of-thumb appears to be that travel-time perturbations in umbrae are predominantly thermal, whereas in penumbrae they are mostly magnetic. 相似文献
15.
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°. 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. 相似文献
16.
Sh. S. Kholikov O. S. Burtseva A. V. Serebryanskiy Sh. A. Ehgamberdiev 《Astronomy Letters》2004,30(8):577-581
We analyze the pattern of behavior of p-mode wave packets with solar cycle using TON one-day helioseismic data with a high spatial resolution. The time—distance method is used to perform this task. We make an attempt to determine the variations in the travel time of acoustic waves at maximum and minimum solar activity; at maximum activity, this time decreases by 2 s compared to that at minimum activity to a depth of 0.8R⊙. In addition, the correlation amplitudes of acoustic wave packets from minimum to maximum solar activity were found to decrease by 10–20% for all angular distances. 相似文献
17.
We present new results of heliographic observations of quiet‐Sun radio emission fulfilled by the UTR‐2 radio telescope. The solar corona investigations have been made close to the last solar minimum (Cycle 23) in the late August and early September of 2010 by means of the two‐dimensional heliograph within 16.5–33 MHz. Moreover, the UTR‐2 radio telescope was used also as an 1‐D heliograph for one‐dimensional scanning of the Sun at the beginning of September 2010 as well as in short‐time observational campaigns in April and August of 2012. The average values of integral flux density of the undisturbed Sun continuum emission at different frequencies have been found. Using the data, we have determined the spectral index of quiet‐Sun radio emission in the range 16.5–200 MHz. It is equal to –2.1±0.1. The brightness distribution maps of outer solar corona at frequencies 20.0 MHz and 26.0 MHz have been obtained. The angular sizes of radio Sun were estimated. It is found that the solar corona at these frequencies is stretched‐out along equatorial direction. The coefficient of corona ellipticity varies slightly during above period. Its mean magnitudes are equal to ≈ 0.75 and ≈ 0.73 at 20.0 MHz and 26.0 MHz, respectively. The presented results for continuum emission of solar corona conform with being ones at higher frequencies. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
Based on the data on a spectral dependence of the geometric albedo of giant planet discs, we obtained depth variations in
the optical thickness τ
a
of the aerosol component and relative concentration γ of methane (Uranium, Neptune) lnτ
a
= −0.720 + 1.507Δlnp (for −2.2085 ≤ lnp ≤ −1.0018), lnτ
a
= +1.224 + 1.160Δlnp (for −1.0018 ≤ lnp ≤ −0.0595), lnτ
a
= +2.318 + 0.192Δlnp (for −0.0595 ≤ lnp), γ = 0.0027 for Jupiter; lnτ
a
= −0.846 + 1.598Δlnp (for −3.3619 ≤ lnp ≤ −2.0575), lnτ
a
= +1.238 + 1.342Δlnp (for −2.0575 ≤ lnp ≤ −1.2074), lnτ
a
= +2.379 + 0.722 (for −1.2074 ≤ lnp ≤ −0.6501), lnτ
a
= +2.781 + 0.326Δlnp (for 0.6501 ≤ lnp), γ = 0.0027 for Saturn; lnτ
a
= −2.694 + 0.087Δlnp (for +0.3685 ≤ lnp ≤ +1.2314), lnτ
a
= −2.619 + 7.341Δlnp (for +1.2314 ≤ lnp ≤ +1.7556), lnτ
a
= +1.229 + 0.956Δlnp (for +1.7556 ≤ lnp) for Uranium; lnτ
a
= −1.861 + 1.248Δlnp (for +0.3204 ≤ lnp ≤ +0.9051), lnτ
a
= −1.131 + 0.347Δlnp (for +0.9051 ≤ lnp) for Neptune; depth-averaged relative methane concentration lnγ = −9.982 + 2.676Δlnp(0.3584 ≤ lnp ≤ 1.5445); ln γ = −9.738 + 2.561Δlnp(0.3237 ≤ lnp ≤ 1.6156) and γ = 0.00382(lnp ≥ 1.6156); 0.00554(lnp ≥ 1.6156) for Uranium and Neptune, respectively (p is in bar). 相似文献
19.
We investigate how helioseismic waves that originate from effective point sources interact with a sunspot. These waves are
reconstructed from observed stochastic wavefields on the Sun by cross-correlating photospheric Doppler-velocity signals. We
select the wave sources at different locations relative to the sunspot, and investigate the p- and f-mode waves separately. The results reveal a complicated picture of waveform perturbations caused by the wave interaction
with the sunspot. In particular, it is found that for waves originating from outside of the sunspot, p-mode waves travel across the sunspot with a small amplitude reduction and slightly higher speed, and wave amplitude and phase
get mostly restored to the quiet-Sun values after passing the sunspot. The f-mode wave experiences some amplitude reduction passing through the sunspot, and the reduced amplitude is not recovered after
that. The wave-propagation speed does not change before encountering the sunspot and inside the sunspot, but the wavefront
becomes faster than the reference wave after passing through the sunspot. For waves originating from inside the sunspot umbra,
both f- and p-mode waves show significant amplitude reductions and faster speed for all propagation paths. A comparison of positive and
negative time lags of cross-correlation functions shows an apparent asymmetry in the waveform changes for both the f- and p-mode waves. We suggest that the waveform variations of the helioseismic waves interacting with a sunspot found in this article
can be used for developing a method of waveform heliotomography, similar to the waveform tomography of the Earth. 相似文献
20.
We present a new approach for the precise and accurate forward modeling of the solar oscillation ℓ−ν power spectrum. The approach
is designed to provide the basis for a streamlined solar seismic inversion without measurements of the p-mode frequencies. The new strategy represents a paradigm change in how information is extracted from the oscillation spectrum.
It also represents a step toward the ideal case of inferring the Sun's properties directly from the raw observations. 相似文献