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Solar Wind Speed - ThumbnailSolar Wind Speed from South Pole to North Pole: The upper panel contains an X-ray image of the Sun obtained by the Soft X-ray Telescope on the Japanese Yohkoh spacecraft; the lower panel shows the solar wind speed and density observed by the Ulysses spacecraft from the South Pole to the North Pole. The latitudinal region indicated by the yellow bar in the lower panel is the region previously explored by in-ecliptic spacecraft. Hot, trapped coronal plasma (ionized gas) radiates in the X-ray and appears in the Yohkoh image as the bright regions. The high speed solar wind is known to originate in coronal holes (the dark areas of the image) from which coronal plasma can readily escape. Two distinct plasma regimes can be seen in the solar wind data. (1) At high latitudes the velocity (red line in lower panel) is high and the density (blue line in lower panel) is low. (2) Near the equator, the velocity is low and the density is high. A significant qualitative feature at high latitude is the steady solar wind speed (approximately 750 km/sec). Prior to these observations, a continuous increase of velocity toward the pole had been expected. (SWOOPS experiment, J. Phillips, Los Alamos National Laboratory).

Solar Magnetic Field Strength - ThumbnailSolar Magnetic Field Strength: Many models of the solar magnetic field used prior to Ulysses assumed that the solar magnetic field was similar to that of a dipole; field lines near the solar equator were thought to form closed loops whereas field lines from the poles were dragged far into interplanetary space by the solar wind. For a dipole, the field strength over the poles is twice that at the equator. Ulysses found that the amount of outward magnetic flux in the solar wind did not vary greatly with latitude, indicating the importance of pressure forces near the sun for evenly distributing magnetic flux. (MAG experiment, A. Balogh, Imperial College; E. Smith, Jet Propulsion Laboratory).

Compositional Differences - ThumbnailCompositional Differences of Fast and Slow Streams: Three parameters are shown over an interval of one and a half solar rotations, using a superposed epoch method of nine solar rotations of 26 days duration. The solar wind speed, V, (dashed line, derived from the measurement of alpha particles (doubly ionized He)), shows alternating high speed streams (from high latitude) and low speed solar wind (from low latitude). The abundances of two heavy ions in the solar wind, magnesium and oxygen, are shown as their ratio, Mg/O, in red. The anticorrelation with V shows that Mg/O is larger in low speed streams than in high speed streams. The third parameter (dark blue) is the so-called "freezing-in" temperature. It is derived from the relative abundances of O7+ and O6+, i.e., seven and six times ionized oxygen atoms. The ratio is a measure of the temperature in the solar atmosphere at the location where the ions were created: T(O7/O6). In the low speed solar wind, this temperature is high, over 1.6 million degrees, indicative of a hot coronal source. The compositional and temperature boundaries between the two types of plasma are very sharp, much better defined than by speed. A further point of interest is the value of Mg/O which lies between 6% in the fast streams and 13% in the slow wind (on the red scale; more recent results yield somewhat higher values of 8% and 17%, respectively). Such values are higher than the abundance ratios typically observed in the photosphere. The surprising conclusion is that chromospheric ionization and transport processes are influencing coronal composition, and that the strength of these processes is correlated to the temperature in the corona. Hence there exists a close, causal relationship between chromospheric and coronal conditions.(SWICS experiment, J. Geiss, University of Ben; G. Gloeckler, University of Maryland; figure from J. Geiss, G. Gloeckler and R. von Steiger, Origin of the Solar Wind From Composition Data, Space Science Rev. 72, 49-60, 1995).

Diffusion of Cosmic Rays - ThumbnailDiffusion of Cosmic Rays in the Heliosphere: Cosmic rays are very energetic atomic nucleiaccelerated in interstellar space; they diffuse inward in the heliosphere through the solar wind's electric and magnetic fields and are observed on the Earth and by spacecraft in interplanetary space. The magnetic fields of the solar wind are known to be wound up in a spiral pattern due to the rotation of the Sun; the winding is analogous to the spiral pattern made by a rotating lawn sprinkler. Since the solar rotation velocity is lower at high latitudes, the azimuthal magnetic fields at high latitudes are weaker, and the length along the magnetic field to the boundary of the heliosphere where cosmic rays enter the solar system is less. Since cosmic rays tend to follow magnetic field lines, many thought that cosmic rays would have easier access at high latitudes and that cosmic ray fluxes would be higher in this region. Ulysses established that cosmic ray fluxes are not greatly enhanced in the polar regions because the cosmic rays traveling through the polar regions are scattered by large-amplitude magnetic waves (not shown in the figure) that Ulysses discovered in this region.

 
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