On the temperature distribution in the solar corona

An improved value of coronal temperature is obtained by the degree of ionization method taking various processes into consideration. Comparison with some of the existing results has also been made.     

On a cold emission in the solar corona

The faint emission of hydrogen, helium and metals in the corona which appeared near an active prominence is studied. The calculations showed that the temperature of the emission region is in the limits from 10 000 K to 30 000 K and the electron density is between 10⁹-10¹⁰ cm^-3, respectively.     

On green-to-red line intensity ratio in the solar corona

An improved formula for the green-to-red line intensity ratio in the solar corona is proposed. The results are compared with those given by the previous expression.     

On the geometric flattening index of the solar corona

Values of the Nikol??skii geometric flattening index of the solar corona, H, have been collected for 77 total solar eclipses from 1860 to 2010. The dependence of the H index on the Wolf number and the phase of solar activity is studied. The H index is found to take values in the range 0.9 to 2.5 and to anticorrelate with solar activity: the maximum values of the index are observed at solar minima and the minimum values are observed at solar maxima. In addition, the correlations between the H index and the Ludendorff photometric flattening index a + b and between the H index and extent of polar ray systems along the limb are investigated.     

Electrons in the solar corona

During a balloon flight in France on September 13, 1971, at altitude 32 000 m, the solar corona was cinematographed from 2 to 5R during 5 hr, with an externally occulted coronagraph.Motions in coronal features, when they occur, exhibit deformations of structures with velocities not exceeding a few 10 km s^–1; several streamers were often involved simultaneously; these variations are compatible with magnetic changes or sudden reorganizations of lines of forces.Intensity and polarization measurements give the electron density with height in the quiet corona above the equator. Electron density gradient for one of the streamers gives a temperature of 1.6 × 10⁶ K and comparisons with the on-board Apollo 16 coronal observation of 31 July, 1971 are compatible with the extension of this temperature up to 25 R _bd.Three-dimensional structures and localizations of the streamers are deduced from combined photometry, polarimetry and ground-based K coronametry. Three of the four coronal streamers analysed have their axis bent with height towards the direction of the solar rotation, as if the upper corona has a rotation slightly faster than the chromosphere.     

Electrons in the solar corona

We discuss a model for the formation of the chromospheric Ca ii K line which does not make the usual assumption of complete redistribution. Using a physically reasonable scattering model, we find significant departures due to the frequency dependence of the line source function, particularly in the relative intensity and centre-to-limb behaviour of the K₁ parts of the line and in the asymmetry produced by differential velocity fields. We conclude that the frequency dependence of the K line source function must be considered in quantitative models for the formation of the K line.     

On the reality of potential magnetic fields in the solar corona

Global magnetic field calculations, using potential field theory, are performed for Carrington rotations 1601–1610 during the Skylab period. The purpose of these computations is to quantitatively test the spatial correspondence between calculated open and closed field distributions in the solar corona with observed brightness structures. The two types of observed structures chosen for this study are coronal holes representing open geometries and theK-coronal brightness distribution which presumably outlines the closed field regions in the corona. The magnetic field calculations were made using the Adams-Pneuman fixed-mesh potential field code based upon line-of-sight photospheric field data from the KPNO 40-channel magnetograph. Coronal hole data is obtained from AS&E's soft X-ray experiment and NRL's Heii observations and theK-coronal brightness distributions are from HAO'sK-coronameter experiment at Mauna Loa, Hawaii.The comparison between computed open field line locations and coronal holes shows a generally good correspondence in spatial location on the Sun. However, the areas occupied by the open field seem to be somewhat smaller than the corresponding areas of X-ray holes. Possible explanations for this discrepancy are discussed. It is noted that the locations of open field lines and coronal holes coincide with the locations ofmaximum field strength in the higher corona with the closed regions consisting of relatively weaker fields.The general correspondence between bright regions in theK-corona and computed closed field regions is also good with the computed neutral lines lying at the top of the closed loops following the same general warped path around the Sun as the maxima in the brightness. One curious feature emerging from this comparison is that the neutral lines at a given longitude tend systematically to lie somewhat closer to the poles than the brightness maxima for all rotations considered. This discrepancy in latitude increases as the poles are approached. Three possible explanations for this tendency are given: perspective effects in theK -coronal observations, MHD effects due electric currents not accounted for in the analysis, and reported photospheric field strengths near the poles which are too low. To test this latter hypothesis, we artificially increased the line-of-sight photospheric field strengths above 70° latitude as an input to the magnetic field calculations. We found that, as the polar fields were increased, the discrepancy correspondingly decreased. The best agreement between neutral line locations and brightness maxima is obtained for a polar field of about 30 G.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

On the abundance of deuterium in the solar atmosphere

The newly determined upper limit abundance of deuterium from the spectrograms of the fourth flight of the SSO, D/H < 10^–5, confirms the data obtained from the spectrograms of the third flight.     

Electrons in the solar corona

The polarimetric survey of electrons in the K-corona initiated at Pic-du-Midi and Meudon Observatories in 1964 now covers a full solar cycle of activity. The measurements are photometrically calibrated in an absolute scale.In June 1967 a persistent coronal feature was fan-shaped as a lame coronale above quiescent prominences. We deduce an electron density of N ₀ = 1.5 × 10⁸ at 60 000 km above the photosphere, a total number of 14 × 10³⁹ electrons, a hydrostatic temperature of 1.7 × 10⁶ K, and a total thermal energy 3N _eKT = 1.0 × 10³¹ ergs. When a center of activity appeared, a major localized condensation developed to replace the old elongated feature, with N ₀ = 4.5 × 10⁸, a total of 4.5 × 10³⁹ electrons and the same temperature of 1.7 × 10⁶ K.Also, a fan-shaped feature of exceptional intensity was analysed on 8 September 1966, with N ₀ = 6 × 10⁸ and a total of 24 × 10³⁹ electrons.Fan-shaped features are frequent above quiescent prominences. They degenerate above a height of 2R into thinner isolated columns or blades with temperatures also around 1.7 × 10⁶ K.     

Heating of the solar corona

The suggestion is advanced that heating of the solar corona results from Landau damping of ion-acoustic waves generated in the motion of photospheric granules. Laboratory experiments relevant to the question of corona heating are discussed, together with the available observational information on the extent of energy deposition in the corona.Of the European Space Research Organization (ESRO).     

Green corona emission,calcium plage activity and solar sector magnetism

The relationship between coronal green line emission and solar sector magnetism has been studied statistically for the years 1965–1969. This period includes the rising portion and the maximum phase of solar cycle no. 20. In the years around solar maximum the results suggest the existence of longitudinal magnetic arcades at the solar sector boundaries. The arcades extend from at least 50°N to 50°S and are flanked by north-south oriented coronal holes about 90° apart. In the rising portion of the cycle the general picture consists of a high green line intensity structure to the west of the boundary and a region of low intensity several days wide to the east of it.Analyses of the calcium plage distribution in the years 1962–1969 show that, on the average, there is a tendency for the plage activity to peak near the sector boundaries. It is further concluded that the activity distribution suggested by Wilcox (1971a, b) is not typical of the behaviour of solar activity relative to the sector boundaries.     

Siphon flows in the solar corona

The flows in a coronal magnetic arch associated with a pressure difference between the footpoints are investigated. Steady flows are of different types: always subsonic; subsonic in one branch of the arch, supersonic in the second; subsonic-supersonic with stationary shocks which adjust the flow to the boundary conditions in the second footpoint. The large velocity increase along the loop in subsonic-supersonic flows is associated with a large density decrease. A velocity drop and a density jump occur across the shock. The emission of such arches in coronal lines (625 of Mg x and 499 of Si xii) is calculated. It is suggested that the intensity drop along the axis observed in some UV loops is due to the density drop associated with subsonic-supersonic flows.     

Plasma drift in the solar corona

Model calculations of plasma drifts in the solar corona were performed. We established that only drifts in crossed fields could result in velocities V of several hundred kilometers per second. Such velocities are typical of coronal mass ejections (CMEs). We derived an analytic expression for V where n, the expansion harmonic of the magnetic-field strength, varies with time. As follows from this expression, V is a power function of the distance with index (2?n) and the radial component changes sign (n?1) times in the latitude range from ?π/2 to +π/2. We found that if the magnetic dipole moment varies with time, the similarity between the spiral structures of coronal plasma is preserved when they displace within several solar radii and the density gradient at the conical boundaries increases (the apparent contrast is enhanced). There is a correspondence between the inferred model effects and the actually observed phenomena that accompany CMEs.     

On the oscillatory motions of the principal plane of the solar corona

We define the principal plane of the solar corona as the equatorial plane of the dipole component of the solar magnetic field at the source surface. The position of this plane defines the orientation of the heliospheric current sheet and the outer solar corona in 3D space, in particular, their inclination with respect to the solar equatorial plane. Their tilt varies from almost zero at solar minimum to almost 90° at solar maximum. But this change is not monotonic; more or less regular oscillations are superimposed onto the average curve. Data on the inclination of the coronal principal plane during 1994–2000 were analyzed. Oscillations with a quasiperiod of 1.3 years were clearly revealed. These oscillations were found to be correlated with variations of the solar rotation rate at the base of the solar convection zone. The latter variations have the same quasi-period of 1.3 years, but they are 1 month out of phase.     

On the observation of scattered radio emission from sources in the solar corona

The observations of a brief flaring region between two plages on the eastern limb of the Sun and the subsequent coronal transient are reported for June 16, 1972 by Koomen et al. (1974). Both of these events have unambiguous and closely timed associations with the solar noise bursts observed at 2800 and 2700 MHz and are also accompanied with good X-ray and SID effects but faint subflare (Solar-Geophysical Data). The two frequencies are those monitored at widely separated stations operated by the Astrophysics Branch of the National Research Council at Lake Traverse, Ontario and at Penticton, B.C.     

On acceleration and motion of ions in corona and solar wind

Assuming a stationary, radial, spherically symmetric solar wind and a radial magnetic field direction in the vicinity of the sun, an equation of motion for ions heavier than protons in the solar wind is derived. The general properties of this equation are discussed and the results of numerical integrations are given. These results are based on the assumption of maxwellian velocity distribution functions for electrons, protons and ions, but the effects of first order deviations from such distributions are also presented and discussed. It is shown that dynamical friction, i.e. momentum transfer from protons to heavier ions accounts for the observed fact that heavier ions - if accelerated at all - normally reach the same velocity as the protons in the solar wind. Because of the non-linear relation between dynamical friction and proton-ion velocity difference a minimum proton flux is required to carry a certain ion species in the solar wind. Formulae comparing the minimum fluxes for different ions are given. It is shown that elements up to and beyond iron will be carried along in the solar wind as long as helium is carried along. Substantial isotopic fractionation is possible, in particular in the case of helium. The effects of ion motion and escape on abundances in the corona and in the outer convective zone of the sun are discussed.     

Electron-cyclotron heating of the solar corona

A model of heating of the solar corona is proposed using electron-cyclotron resonance heating.