The study of the internal structure of the Sun entered a new age with helioseismology. Several ground based networks, as well as the space mission SoHO, continuously observe the solar oscillations. In particular GONG, the “Global Oscillation Network Project”, gathers six sites around the world with six identical Doppler instruments. These instruments observe the phase shift of the Ni 676.8nm line with 3 images every minute, 1.8 sites observing simultaneously. Millions of solar p-modes have been detected. The inversion of the measured frequencies yields accurate and detailed information about the sound velocity in the Sun's interior, which in turn leads to constraints on the equation of state, opacities, chemical composition. Precise informations on the differential rotation inside the Sun have also been obtained. (See the special Science issue on GONG, vol 272, 31 May 1996).
<p><strong>We present a statistical study on the early evolution of coronal mass ejections (CMEs), to better understand the effect of CME (over)- expansion and how it relates to the production of Solar Energetic Particle (SEP) events. We study the kinematic CME characteristics in terms of their radial and lateral expansion, from their early evolution in the Sun&#8217;s atmosphere as observed in EUV imagers and coronagraphs. The data covers 72 CMEs that occurred in the time range of July 2010 to September 2012, where the twin STEREO spacecraft where in quasiquadrature </strong><strong>to the Sun-Earth line. From the STEREO point-of-view, the CMEs under study were observed close to the limb. We calculated the radial and lateral height (width) versus time profiles and derived the corresponding peak and mean velocities, accelerations, and angular expansion rates, with particular emphasis on the role of potential lateral overexpansion in the early CME evolution. We find high correlations between the radial and lateral CME velocities and accelerations. CMEs that are associated tend to be located at the high-value end of the distributions of velocities, widths, and expansion rates compared to nonSEP associated events.<br></strong></p>
Outer convection zones of white dwarfs in the range 5800 K ≤ Teff ≤ 30000 K have been studied assuming that they have the same chemical composition as determined by Weidemann (1960) for van Maanen 2. Convection is important in all these stars. In white dwarfs Teff < 8000 K the adiabatic temperature gradient is strongly influenced by the pressure ionization of H, HeI and HeII which occurs within the convection zone. Partial degeneracy is also important.Convective velocities are very small for cool white dwarfs but they reach considerable values for hotter objects. For a white dwarf of Teff = 30000 K a velocity of 6.05 km/sec and an acoustic flux (generated by the turbulent convection) of 1.5 × 1011 erg cm−2 sec−1 is reached. The formation of white dwarf coronae is briefly discussed.
AbstractIn addition to the well-known granulation and supergranulation of the solar convection zone (the “SCZ”), the presence of so-called “giant cells” has been postulated. These are supposed span the entire thickness of the SCZ and to stretch from pole to pole in a sequence of elongated cells like a “cartridge belt” or a bunch of “bananas” strung uniformly round the Sun. Conclusive evidence for the existence of such giant cells is still lacking, despite strenuous observational efforts to find them. After analyses of sunspot motion, Ribes and others believe that convective motions near the solar surface occurs in a pattern that is the antithesis of the cartridge belt: a system of “toroidal” or “doughnut” cells, girdling the Sun in a sequence that extends from one pole to the other. Galloway, Jones and Roberts have recently tried to meet the resulting theoretical challenge, with the mixed success reported in this paper.
The 5 minute oscillations in a sunspot umbra are the response of the sunspot to forcing by the 5 minute p-modes in the surrounding convection zone (Thomas 1981). This interaction of solar p-modes with a sunspot can be used to probe the structure of a sunspot beneath the visible surface of the Sun (Thomas, Cram, and Nye 1982). Here we report briefly the results of both an observational study and a simple theoretical analysis of this interaction. A full account of these results will be published elsewhere (Abdelatif, Lites, and Thomas 1986; Abdelatif and Thomas 1987).
Helioseismic Constraints on the Solar Structure: Evidences of Element Segregation and Mild Mixing Below the Convection Zone