摘要:

The polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes to the magnetosphere along geomagnetic field lines. The polar wind plasma consists mainly of H+, He+, and O+ions and electrons. Although it was initially believed that O+ions play a major role only at low altitudes, it is now clear from observations that relatively large amounts of suprathermal and energetic O+ions are present in the polar magnetosphere. Recently, thermal O+outflow has been observed at altitudes of 5000–10,000 km together with H+and He+ions. The polar wind undergoes four major transitions as it flows from the ionosphere to the magnetosphere: (1) from chemical to diffusion dominance, (2) from subsonic to supersonic flow, (3) from collision‐dominated to collisionless regimes, and (4) from heavy to light ion composition. The collisions are important up to about 2500 km, after which the ions and electrons exhibit temperature anisotropies. The direction of the anisotropy varies with geophysical conditions. The polar wind outflow varies with season, solar cycle, and geomagnetic activity. The O+flux exhibits a summer maximum, while the H+flux reaches a maximum in the spring. The He+flux increases by a factor of 10 from summer to winter. At both magnetically quiet and active times the integrated H+ion flux is largest in the noon sector and smallest in the midnight sector. The integrated upward H+ion flux exhibits a positive correlation with the interplanetary magnetic field. In the sunlit polar cap the photoelectrons can increase the ambipolar electric field, which in turn increases the polar wind ion outflow velocities. The outflowing polar wind plasma flux tubes also convect across the polar cap. When the flux tubes cross the cusp and nocturnal auroral regions, the plasma can be heated and become unstable. Similar mixing of hot magnetospheric plasma with cold polar wind may result in instabilities. A number of free energy sources in the polar wind, including temperature anisotropy, relative drift between species, and spatial inhomogeneities, feed various fluid and kinetic instabilites. The instabilities can produce plasma energization and cross‐field transport, which modify the large‐scale polar wind

ISSN:8755-1209    

DOI:10.1029/96RG00497

年代:1996

数据来源: WILEY

摘要:

The importance of mid‐ocean ridge hydrothermal systems has been recognized for their role in the regulation of ocean and sediment chemistry, as well as for providing a chemosynthetic source of carbon which drives a unique population of animals found at hydrothermal vents. Despite the importance of these systems the rates, length, and depth scale of submarine hydrothermal processes are not precisely known because they are, for the most part, inaccessible to observational tools. We must therefore rely on indirect methods to quantify these processes. One way of investigating the rates, or timescales, of processes in a hydrothermal (or any natural) system is through the study and modeling of naturally occurring radioisotopes. Disequilibria among the naturally occurring radioactive decay series in vent fluids, associated mineral deposits, and overlying effluent plume have provided geochemical tools to investigate the rates of various processes occurring in submarine hydrothermal systems. Because the half‐lives of the radioisotopes vary from days to many years, processes which encompass a wide range of spatial and temporal scales can be studied. This paper presents a review of methods that estimate the residence time of hydrothermal fluids in the ocean crust, establish the geochronology of seafloor sulfide deposits, investigate the rates of chemical reactions within hydrothermal effluent plumes, and derive the heat and mass flux from seafloor hydrothermal ar

ISSN:8755-1209    

DOI:10.1029/96RG01762

年代:1996

数据来源: WILEY

摘要:

The water cycle regulates and reflects natural variability in climate at the regional and global scales. Large‐scale human activities that involve changes in land cover, such as tropical deforestation, are likely to modify climate through changes in the water cycle. In order to understand, and hopefully be able to predict, the extent of these potential global and regional changes, we need first to understand how the water cycle works. In the past, most of the research in hydrology focused on the land branch of the water cycle, with little attention given to the atmospheric branch. The study of precipitation recycling, which is defined as the contribution of local evaporation to local precipitation, aims at understanding hydrologic processes in the atmospheric branch of the water cycle. Simply stated, any study on precipitation recycling is about how the atmospheric branch of the water cycle works, namely, what happens to water vapor molecules after they evaporate from the surface, and where will they precipitate? Estimation of precipitation recycling over any large basin, such as the Mississippi or the Amazon, is a necessary step before developing a quantitative description of the regional water cycle. This paper reviews the research on the concept of precipitation recycling and emphasizes the basic role of this process in defining the different components of the atmospheric branch in any regional water cycle. To illustrate the assumptions and limitations involved in estimation of precipitation recycling, we present and discuss a general formula for estimation of precipitation recycling. The recent estimates of annual precipitation recycling ratio from different regions are reviewed and compared. Finally, the dependence of precipitation recycling over any region on the spatial scale is discussed and illustrated by the example of the Amazon basi

ISSN:8755-1209    

DOI:10.1029/96RG01927

年代:1996

数据来源: WILEY

摘要:

Variations in interplanetary plasma and magnetic field parameters over the course of a solar cycle affect a broad range of physical phenomena, such as planetary magnetospheres, cosmic ray modulation, and the interaction of the solar wind with the local interstellar medium. For more than 3 decades a succession of spacecraft have provided in situ measurements of these parameters. These measurements span a range of heliocentric distances from 0.3 to 61 AU and provide an explicit picture of the three‐dimensional structure of the inner and outer heliosphere in the vicinity of the ecliptic plane. In addition, ground‐based interplanetary scintillation measurements and observations from the Ulysses spacecraft have extended our knowledge to smaller heliocentric distances and higher heliographic latitudes. The structure of the heliosphere varies dramatically over the course of a solar cycle. Much of this variation can be related to the changes in the structure and inclination of the coronal magnetic field. The coronal neutral line, which separates the Sun's magnetic hemispheres, is associated with a belt of low‐speed and high‐density solar wind. Its inclination varies over the course of a solar cycle, from a low value near solar minimum to a high value near solar maximum. When the inclination of the coronal neutral line is low, its signature is evident in observations near the solar equator. When the inclination of the coronal neutral line is high, the heliosphere at moderate heliographic latitudes is swept by a succession of high‐ and low‐speed streams and is shaped by their i

ISSN:8755-1209    

DOI:10.1029/96RG00892

年代:1996

数据来源: WILEY