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1.
A clarification and discussion of the energy changes experienced by cosmic rays in the interplanetary region is presented. It is shown that the mean time rate of change of momentum of cosmic rays reckoned for a fixed volume in a reference frame fixed in the solar system is 〈p〉 =p V·G/3 (p=momentum,V is the solar wind velocity andG=cosmic-ray density gradient). This result is obtained in three ways:
- by a rearrangement and reinterpretation of the cosmic-ray continuity equation;
- by using a scattering analysis based on that of Gleeson and Axford (1967);
- by using a special scattering model in which cosmic-rays are trapped in ‘magnetic boxes’ moving with the solar wind.
2.
We characterize and analyze rotational torsional oscillations developing in a large-eddy magnetohydrodynamical simulation of solar convection (Ghizaru, Charbonneau, and Smolarkiewicz, Astrophys. J. Lett. 715, L133, 2010; Racine et al., Astrophys. J. 735, 46, 2011) producing an axisymmetric, large-scale, magnetic field undergoing periodic polarity reversals. Motivated by the many solar-like features exhibited by these oscillations, we carry out an analysis of the large-scale zonal dynamics. We demonstrate that simulated torsional oscillations are not driven primarily by the periodically varying large-scale magnetic torque, as one might have expected, but rather via the magnetic modulation of angular-momentum transport by the large-scale meridional flow. This result is confirmed by a straightforward energy analysis. We also detect a fairly sharp transition in rotational dynamics taking place as one moves from the base of the convecting layers to the base of the thin tachocline-like shear layer formed in the stably stratified fluid layers immediately below. We conclude by discussing the implications of our analyses with regard to the mechanism of amplitude saturation in the global dynamo operating in the simulation, and speculate on the possible precursor value of torsional oscillations for the forecast of solar-cycle characteristics. 相似文献
3.
In this paper we study the dependence on depth and latitude of the solar angular velocity produced by a meridian circulation in the convection zone, assuming that the main mechanism responsible for setting up and driving the circulation is the interaction of rotation with convection. We solve the first order equations (perturbation of the spherically symmetric state) in the Boussinesq approximation and in the steady state for the axissymmetric case. The interaction of convection with rotation is modelled by a convective transport coefficient k c = k co + ?k c2 P 2(cos θ) where ? is the expansion parameter, P 2 is the 2nd Legendre polynomial and k c2 is taken proportional to the local Taylor number and the ratio of the convective to the total fluxes. We obtain the following results for a Rayleigh number 103 and for a Prandtl number 1:
- A single cell circulation extending from poles to the equator and with circulation directed toward the equator at the surface. Radial velocities are of the order of 10 cm s?1 and meridional ones of the order of 150 cm s?1.
- A flux difference between pole and equator at the surface of about 5 percent, the poles being hotter.
- An angular velocity increasing inwards.
- Angular velocity constant surfaces of spheroidal shape. The model is consistent with the fact that the interaction of convection with rotation sets up a circulation (driven by the temperature gradient) which carries angular momentum toward the equator against the viscous friction. Unfortunately also a large flux variation at the surface is obtained. Nevertheless it seems that the model has the basic requisites for correct dynamo action.
4.
We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episodic polar field formation and the meridional flow. We find:
- The mean latitude of the polemost filaments tracks the boundary of the polar field cap and undergoes an equatorward dip during each arrival of additional polar field.
- Polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m s-1.
- The Mt. Wilson magnetic data, combined with a simple model calculation, show that the filament drift expected from diffusion alone would be slower than observed, and in some cases would be equatorward rather than poleward.
- The observation that filaments drift poleward along with the magnetic regions shows that fields of both polarities are carried by the meridional flow, as would be expected, rather than only the f-polarity flux which dominates the strength. This leads to the prediction that in the mid-latitudes during intervals between the passage of f-polarity regions, both polarities are present in nearly equal amounts. This prediction is confirmed by the magnetic data.
5.
Several candidates for low-order p modes (n 5) and possibly g modes were found by applying mode-detection techniques such as multivariate spectral regression analysis and time-frequency analysis to the VIRGO full-disc solar irradiance data. Three out of the candidates for low-order p modes could be confirmed by significant peaks in the un-treated power spectra in good agreement with theoretical predictions. The frequency of a fourth candidate for a low-order p mode lies some 2.8 Hz below the predicted frequency. The candidates found for g modes are less reliable, since none of them could be confirmed neither by significant peaks in the un-treated power spectra nor by the detection of multiplets. 相似文献
6.
We study the effect of localized sound-speed perturbations on global mode frequencies by applying techniques of global helioseismology to numerical simulations of the solar acoustic wave field. Extending the method of realization-noise subtraction (e.g., Hanasoge, Duvall, and Couvidat, Astrophys. J. 664, 1234, 2007) to global modes and exploiting the luxury of full spherical coverage, we are able to achieve very highly resolved frequency differences that are then used to study sensitivities and the signatures of the thermal asphericities. We find that i) global modes are almost twice as sensitive to sound-speed perturbations at the bottom of the convection zone in comparison to anomalies well inside the radiative interior (r?0.55R ⊙), ii) the m degeneracy is lifted ever so slightly, as seen in the a coefficients, and iii) modes that propagate in the vicinity of the perturbations show small amplitude shifts. Through comparisons with error estimates obtained from Michelson Doppler Imager (MDI; Scherrer et al., Solar Phys. 162, 129, 1995) observations, we find that the frequency differences are detectable with a sufficiently long time series (70?–?642 days). 相似文献
7.
Fundamental standing modes and their overtones play an important role in coronal seismology. We examine the effects of a significant field-aligned flow on standing modes that are supported by coronal loops, which are modeled here as cold magnetic slabs. Of particular interest are the period ratios of the fundamental to its (n?1)th overtone [P 1/nP n ] for kink and sausage modes, and the threshold half-width-to-length ratio for sausage modes. For standing kink modes, the flow significantly reduces P 1/nP n in general, the effect being particularly strong for higher n and weaker density contrast [ $\rho_{0}/\rho_{\rm e}$ ] between loops and their surroundings. That said, even when $\rho_{0}/\rho_{\rm e}$ approaches infinity, this effect is still substantial, reducing the minimal P 1/nP n by up to 13.7?% (24.5?%) for n=2 (n=4) relative to the static case, when the Alfvén Mach number [M A] reaches 0.8, where M A measures the loop flow speed in units of the internal Alfvén speed. Although it is not negligible for standing sausage modes, the flow effect in reducing P 1/nP n is not as strong. However, the threshold half-width-to-length ratio is considerably higher in the flowing case than in its static counterpart. For $\rho_{0}/\rho_{\rm e}$ in the range [9,1024] and M A in the range [0,0.5], an exhaustive parameter study yields that this threshold is well fitted by $(d/L)_{\rm cutoff, fit} = \frac{1}{2}\sqrt{\frac{1}{\rho_{0}/\rho_{\rm e}-1}} \exp (3.7 M_{\mathrm{A}}^{2} )$ , which involves the two parameters in a simple way. This allows one to analytically constrain the combination $(\rho_{0}/\rho_{\rm e}, M_{\mathrm {A}})$ for a loop with a known width-to-length ratio when a standing sausage oscillation is identified. It also allows one to examine the idea of partial sausage modes in more detail, and the flow is found to significantly reduce the spatial extent where partial modes are allowed. 相似文献
8.
In local helioseismology, numerical simulations of wave propagation are useful to model the interaction of solar waves with perturbations to a background solar model. However, the solution to the linearised equations of motion include convective modes that can swamp the helioseismic waves that we are interested in. In this article, we construct background solar models that are stable against convection, by modifying the vertical pressure gradient of Model S (Christensen-Dalsgaard et al., 1996, Science 272, 1286) relinquishing hydrostatic equilibrium. However, the stabilisation affects the eigenmodes that we wish to remain as close to Model S as possible. In a bid to recover the Model S eigenmodes, we choose to make additional corrections to the sound speed of Model S before stabilisation. No stabilised model can be perfectly solar-like, so we present three stabilised models with slightly different eigenmodes. The models are appropriate to study the f and p 1 to p 4 modes with spherical harmonic degrees in the range from 400 to 900. Background model CSM has a modified pressure gradient for stabilisation and has eigenfrequencies within 2% of Model S. Model CSM_A has an additional 10% increase in sound speed in the top 1 Mm resulting in eigenfrequencies within 2% of Model S and eigenfunctions that are, in comparison with CSM, closest to those of Model S. Model CSM_B has a 3% decrease in sound speed in the top 5 Mm resulting in eigenfrequencies within 1% of Model S and eigenfunctions that are only marginally adversely affected. These models are useful to study the interaction of solar waves with embedded three-dimensional heterogeneities, such as convective flows and model sunspots. We have also calculated the response of the stabilised models to excitation by random near-surface sources, using simulations of the propagation of linear waves. We find that the simulated power spectra of wave motion are in good agreement with an observed SOHO/MDI power spectrum. Overall, our convectively stabilised background models provide a good basis for quantitative numerical local helioseismology. The models are available for download from http://www.mps.mpg.de/projects/seismo/NA4/ . 相似文献
9.
Gravity modes are the best probes to infer the properties of the solar radiative zone, which represents 98% of the Sun’s total mass. It is usually assumed that high-frequency g modes give information about the structure of the solar interior whereas low-frequency g modes are more sensitive to the solar dynamics (the internal rotation). In this work, we develop a new methodology, based on the analysis of the almost constant separation of the dipole gravity modes, to introduce new constraints on the solar models. To validate this analysis procedure, several solar models – including different physical processes and either old or new chemical abundances (from, respectively, Grevesse and Noels (Origin and Evolution of the Elements 199, Cambridge University Press, Cambridge, 15, 1993) and Asplund, Grevesse, and Sauval (Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis CS-336, Astron. Soc. Pac., San Francisco, 25?–?38, 2005)) – have been compared to another model used as a reference. The analysis clearly shows that this methodology has enough sensitivity to distinguish among some of the models, in particular, among those with different compositions. The comparison of the models with the g-mode asymptotic signature detected in GOLF data favors the ones with old abundances. Therefore, the physics of the core – obtained through the analysis of the g-mode properties – is in agreement with the results obtained in the previous studies based on the acoustic modes, which are mostly sensitive to more external layers of the Sun. 相似文献
10.
The underlying physics that generates the excitations in the global low-frequency (<?5.3?mHz) solar acoustic power spectrum is a well-known process that is attributed to solar convection; however, a definitive explanation as to what causes excitations in the high-frequency regime (>?5.3?mHz) has yet to be found. Karoff and Kjeldsen (Astrophys. J. 678, 73??C?76, 2008) concluded that there is a correlation between solar flares and the global high-frequency solar acoustic waves. We have used Global Oscillation Network Group (GONG) helioseismic data in an attempt to verify the Karoff and Kjeldsen (2008) results as well as compare the post-flare acoustic power spectrum to the pre-flare acoustic power spectrum for 31 solar flares. Among the 31 flares analyzed, we observe that a decrease in acoustic power after the solar flare is just as likely as an increase. Furthermore, while we do observe variations in acoustic power that are most likely associated with the usual p-modes associated with solar convection, these variations do not show any significant temporal association with flares. We find no evidence that consistently supports flare-driven high-frequency waves. 相似文献
11.
After adding the data observed in the years from 1979 to 1982 to those obtained earlier (Ding et al., 1981), we re-examine the previous results and conclude:
- The longitudinal distribution of spiral spots on the solar disc is generally the same as that of sunspot groups with areas of S p ≥ 400, but their active longitudes seem to be more concentrated.
- The distribution of spiral patterns in the southern and northern hemispheres shows that the differential rotation may be a fundamental solar dynamo for the formation of the spiral spots.
- The statistical directions of the emerging twisted magnetic vectors in the active regions in the southern and northern hemispheres are synchronously inverse with a period of about two years. This period seems to be detected in other solar observations.
12.
Markus J. Aschwanden 《Journal of Astrophysics and Astronomy》2008,29(1-2):3-16
Celebrating the diamond jubilee of the Physics Research Laboratory (PRL) in Ahmedabad, India, we look back over the last six decades in solar physics and contemplate on the ten outstanding problems (or research foci) in solar physics:
- The solar neutrino problem
- Structure of the solar interior (helioseismology)
- The solar magnetic field (dynamo, solar cycle, corona)
- Hydrodynamics of coronal loops
- MHD oscillations and waves (coronal seismology)
- The coronal heating problem
- Self-organized criticality (from nanoflares to giant flares)
- Magnetic reconnection processes
- Particle acceleration processes
- Coronal mass ejections and coronal dimming
13.
Alfvén Reflection and Reverberation in the Solar Atmosphere 总被引：1，自引：0，他引：1
P. S. Cally 《Solar physics》2012,280(1):33-50
Magneto-atmospheres with Alfvén speed [a] that increases monotonically with height are often used to model the solar atmosphere, at least out to several solar radii. A?common example involves a uniform vertical or inclined magnetic field in an isothermal atmosphere, for which the Alfvén speed is exponential. We address the issue of internal reflection in such atmospheres, both for time-harmonic and for transient waves. It is found that a mathematical boundary condition may be devised that corresponds to perfect absorption at infinity, and, using this, that many atmospheres where a(x) is analytic and unbounded present no internal reflection of harmonic Alfvén waves. However, except for certain special cases, such solutions are accompanied by a wake, which may be thought of as a kind of reflection. For the initial-value problem where a harmonic source is suddenly switched on (and optionally off), there is also an associated transient that normally decays with time as $\mathcal{O}(t^{-1})$ or $\mathcal{O}(t^{-1}\ln t)$ , depending on the phase of the driver. Unlike the steady-state harmonic solutions, the transient does reflect weakly. Alfvén waves in the solar corona driven by a finite-duration train of p-modes are expected to leave such transients. 相似文献
14.
Markus J. Aschwanden 《Solar physics》2013,287(1-2):323-344
We derive an analytical approximation of nonlinear force-free magnetic field solutions (NLFFF) that can efficiently be used for fast forward-fitting to solar magnetic data, constrained either by observed line-of-sight magnetograms and stereoscopically triangulated coronal loops, or by 3D vector-magnetograph data. The derived NLFFF solutions provide the magnetic field components B x (x), B y (x), B z (x), the force-free parameter α(x), the electric current density j(x), and are accurate to second-order (of the nonlinear force-free α-parameter). The explicit expressions of a force-free field can easily be applied to modeling or forward-fitting of many coronal phenomena. 相似文献
15.
We examine the effect of random horizontal flows on the dispersion relation of high-degree solar f modes. We follow the approach of Murawski and Roberts (Astron. Astrophys. 272, 595, 1993), addressing some limitations of that paper, and extending the results to include damping and to apply for a general turbulent spectrum. We find a reduction in frequency below the classical result that is about three times that observed. For large wavenumber the damping rate is larger than the frequency correction by a factor of the order of the nondimensional wavenumber, which appears to be consistent with observation. 相似文献
16.
G. B. Gelfreikh Yu. T. Tsap Yu. G. Kopylova T. B. Goldvarg Yu. A. Nagovitsyn L. I. Tsvetkov 《Astronomy Letters》2004,30(7):489-495
Based on observational data obtained with the RT-22 Crimean Astrophysical Observatory radio telescope at frequencies of 8.6 and 15.4 GHz, we investigate the quasi-periodic variations of microwave emission from solar active regions with periods Tp<10 min. As follows from our wavelet analysis, the oscillations with periods of 3–5 min and 10–40 s have the largest amplitudes in the dynamic power spectra, while there are virtually no oscillations with Tp<10 s. Our analysis shows that acoustic modes with Tp?1 min strongly dissipate in the lower solar corona due to thermal conduction losses. The oscillations with Tp=10–40 s are associated with Alfvén disturbances. We analyze the influence of acoustic and Alfvén oscillations on the thermal mechanisms of microwave emission in terms of the homogeneous model. We discuss the probable coronal heating sources. 相似文献
17.
We examine the possibility that the observed cosmic-ray protons are of primary extragalactic origin. The present \(\bar p\) data are consistent with a primary extragalactic component having \(\bar p\) /p?3.2±0.7 x 10-4 independent of energy. Following the suggestion that most extragalactic cosmic rays are from active galaxies, we propose that most of the observed \(\bar p\) 's are alos from the same sites. This would imply the possibility of destroying the corresponding \(\bar \alpha \) 'sat the source, thus leading to a flux ratio \(\bar \alpha \) /α< \(\bar p\) /p. We further predict an estimate for \(\bar \alpha \) α～10-5, within the range of future cosmic-ray detectors. the cosmological implications of this proposal are discussed. 相似文献
18.
A study is made of equilibrium and stability properties of a semi-toroidal current loop imbedded in a high temperature plasma. The loop carries a toroidal current density J t and poloidal current density J p. By explicity including the global curvature of the loop, the net Lorentz and pressure forces acting along the major radius are calculated. Requirement of equilibrium force-balance gives rise to conditions that must be satisfied by the physical parameters and geometry. On the basis of these conditions, we deduce a class of equilibrium semi-toroidal current loops satisfying c #X2212;1 J × B ? ▽p = 0. It is found that the averge pressure inside the loop is less than the ambient coronal pressure in equilibrium. Furthermore, this class of equilibria is shown to be stable to a number of destructive MHD modes. The theoretical results are discussed in the context of solar bipolar current loops. 相似文献
19.
A statistical study is carried out on the photospheric magnetic nonpotentiality in solar active regions and its relationship with associated flares. We select 2173 photospheric vector magnetograms from 1106 active regions observed by the Solar Magnetic Field Telescope at Huairou Solar Observing Station, National Astronomical Observatories of China, in the period of 1988??C?2008, which covers most of the 22nd and 23rd solar cycles. We have computed the mean planar magnetic shear angle ( $\overline{\Delta\phi}$ ), mean shear angle of the vector magnetic field ( $\overline{\Delta\psi}$ ), mean absolute vertical current density ( $\overline{|J_{z}|}$ ), mean absolute current helicity density ( $\overline{|h_{\mathrm{c}}|}$ ), absolute twist parameter (|?? av|), mean free magnetic energy density ( $\overline{\rho_{\mathrm{free}}}$ ), effective distance of the longitudinal magnetic field (d E), and modified effective distance (d Em) of each photospheric vector magnetogram. Parameters $\overline{|h_{\mathrm{c}}|}$ , $\overline{\rho_{\mathrm{free}}}$ , and d Em show higher correlations with the evolution of the solar cycle. The Pearson linear correlation coefficients between these three parameters and the yearly mean sunspot number are all larger than 0.59. Parameters $\overline {\Delta\phi}$ , $\overline{\Delta\psi}$ , $\overline{|J_{z}|}$ , |?? av|, and d E show only weak correlations with the solar cycle, though the nonpotentiality and the complexity of active regions are greater in the activity maximum periods than in the minimum periods. All of the eight parameters show positive correlations with the flare productivity of active regions, and the combination of different nonpotentiality parameters may be effective in predicting the flaring probability of active regions. 相似文献
20.
We study the influence of horizontal and vertical random flows on the solar f mode in a plane-parallel, incompressible model that includes a static atmosphere. The incompressible limit is an adequate approximation for f-mode type of surface waves that are highly incompressible. The paper revisits and extends the problem investigated earlier by Murawski and Roberts (Astron. Astrophys. 272, 601, 1993). We show that the consideration of the proposed velocity profile requires several restrictive assumptions to be made. These constraints were not recognised in previous studies. The impact of the inconsistencies in earlier modelling is analysed in detail. Corrections to the dispersion relation are derived and the relevance of these corrections is analysed. Finally, the importance of the obtained results is investigated in the context of recent helioseismological data. Detailed comparison with our complementary studies on random horizontal flows (Mole, Kerekes, and Erdélyi, Solar Phys., accepted, 2008) and the random magnetic model of Erdélyi, Kerekes, and Mole (Astron. Astrophys. 431, 1083, 2005) is also given. In particular, for realistic solar parameters we find significant frequency reduction and wave damping, both of which increase with the characteristic thickness of the random layer. 相似文献