摘要:

We summarize recent theoretical investigations of low Mach number flows, that describe two distinct approaches by fluids to the incompressible regime. The first includes the effects of relatively strong density and temperature fluctuations (Type I), while the second places fluctuations in mechanical pressure, density and temperature on an equal footing (Type II). In the latter case, the pseudosound relations between density and pressure, in slightly generalized form, are recovered, whereas the former case yields entirely different small departures from incompressible behavior in that density and temperature fluctuations are predicted to be anti‐correlated. It is suggested that nearly incompressible fluids be classified as either Type I or II and it is shown that the well‐known pressure‐balanced structures represent a sub‐class of static solutions within this classification. Two examples from Voyager data illustrate the potential for observing these distinct nearly incompressible dynamical orderings in the sol

ISSN:0094-8276    

DOI:10.1029/GL017i009p01239

年代:1990

数据来源: WILEY

摘要:

This letter discusses the results produced by 3‐D hybrid particle code simulations of the solar wind interaction with unmagnetized planets such as Venus and Mars. The solar wind velocity is perpendicular to the IMF in the cases studied. It is found that there are asymmetries in both the magnetic structure and shock location for spherical obstacles ranging in radius from 1000 km to 6000 km. The asymmetries found are due to differences in the electron and ion currents paths (diamagnetic behavior). Mass loading of 0+was not included in these simulation

ISSN:0094-8276    

DOI:10.1029/GL017i009p01243

年代:1990

数据来源: WILEY

摘要:

Results are presented from two dimensional hybrid simulations of curved collisionless supercritical shocks, retaining both quasi‐perpendicular and quasi‐parallel sections of the shock in order to study the character and origin of the foreshock ion population. The simulations demonstrate that the foreshock ion population is dominated by ions impinging upon the quasi‐parallel side of the shock, while nonlocal transport from the quasi‐perpendicular side of the shock into the foreshock region is minimal. Further, it is shown that the ions gain energy by drifting significantly in the direction of the convection electric field through multiple shock enc

ISSN:0094-8276    

DOI:10.1029/GL017i009p01247

年代:1990

数据来源: WILEY

摘要:

We report here the significant occurrence of pulsating aurorae in the high latitude post‐noon auroral oval. Observations of aurorae at Davis, Antarctica (Inv. 74.5°S, Geog. 78.0°E, 68.6°S) during the 1988 Austral winter indicate pulsating auroral activity peaks at around 14.5% in the period 1330–1400 magnetic local time (MLT). Post‐noon high latitude pulsating aurora is typically a less than 250 R enhancement on a 1 kR background in the OI emission at 558 nm. Pulsation periods tend to be longer at 20–30 s compared to average auroral zone fluctuations of around 2–10 s. Similar to auroral zone stations, the optical fluctuations are always accompanied by Pi(c) magnetic pulsations. Weak Kp and IMF trends are present

ISSN:0094-8276    

DOI:10.1029/GL017i009p01251

年代:1990

数据来源: WILEY

摘要:

A self‐aligning Fabry‐Perot spectrometer has been installed at Amundsen‐Scott Station, Antarctica (Geographic South Pole) and has been used to determine neutral upper thermosphere winds and temperatures, obtained from the measurement of the Doppler shift and Doppler width of the line profiles of the [OI] 15,867 K (630 nm) line emission, during the austral winter of 1989 (April 1989 to September 1989). These first ground‐based measurements of F‐region neutral dynamics at the South Pole, show a rich variety of dynamic phenomena and strong couplings with the ionospheric plasma. Data for two contrasting days in April 1989 are presented here. The data of April 23 UT illustrate the diurnal variations of winds and temperatures during geomagnetic quiet‐to‐moderate conditions, while the data of April 27 UT illustrate diurnal variations during geomagnetically disturbed periods. These data are compared with the average pattern obtained at a similar geomagnetic latitude and conditions, but in the Northern Hemisphere (Longyearbyen, Spitsbergen) and with predictions of the average dynamics in the South Pole region made by the NCAR thermosphere‐ionosphere general circulation model (TIGCM). The measured winds at the South Pole have a stronger prevailing westward component in its diurnal cycle than is either observed at Longyearbyen (in the Northern Hemisphere) or predicted at South Po

ISSN:0094-8276    

DOI:10.1029/GL017i009p01255

年代:1990

数据来源: WILEY

摘要:

The first stage of a new midlatitude facility at the Hardware Ranch Observatory near Bear Lake (41.93°N, 111.42°W, 1970 M elevation), for studies of the aeronomy of the middle and upper atmosphere, was completed during early September 1989. An Imaging Fabry‐Perot interferometer (IFPI) (Rees et al., 1982; Rees et al., 1990), was commissioned with a special Imaging Photon Detector (IPD) (McWhirter et al., 1982), equipped with a Gallium Arsenide (GaAs) photocathode. Wind and temperature structure can be deduced from observations of the Doppler shift and Doppler broadening of airglow and auroral emissions from the mesosphere and thermosphere. The near infra‐red Meinel bands from near 86 km in the upper mesosphere were observed with the IFPI and show 40 ms−1amplitude wind oscillations, consistent with a semi‐diurnal t

ISSN:0094-8276    

DOI:10.1029/GL017i009p01259

年代:1990

数据来源: WILEY

摘要:

An investigation is conducted to determine the influence of changes in the upper stratospheric mean zonal wind on the circulation of the lower atmosphere. In addition to observed data, results of numerical experiments with a general circulation model are used, in which the solar ultraviolet heating rate is varied to force changes in the mean zonal wind in the upper stratosphere.It is found that when the upper stratospheric mid‐latitude westerlies are strong during December, lower stratospheric polar night jet is persistent and the westerlies in the polar region of the troposphere become stronger in the following February. These results are common to both the observations and the numerical experiment

ISSN:0094-8276    

DOI:10.1029/GL017i009p01263

年代:1990

数据来源: WILEY

摘要:

In 1989 the Total Ozone Mapping Spectrometer (TOMS) aboard the Nimbus 7 satellite observed the springtime decrease in Antarctic total ozone for the 11th consecutive year. The 1989 minimum values of total ozone measured by TOMS declined throughout the month of September at a rate nearly identical to 1987. The National Meteorological Center analysis of lower stratospheric temperatures in August and September 1989 also showed conditions similar to those observed in 1987. A minimum in total ozone of 111 DU was reached on October 7, 1989. Within uncertainties this is the same as the previously observed minimum on October 5, 1987. The area of the ozone hole as defined by the 220 DU contour grew rapidly during early September. It reached a mid‐September peak of 7.5% of the southern hemisphere or 19 million square kilometers, essentially the same as observed in 1987. From mid October through November 1989, minimum polar total ozone values increased and the area within the 220 DU contour decreased more rapidly than during the comparable period of 1987. The more rapid erosion of the 1989 ozone hole resulted from strong wave number one perturbations of the vortex dynamics in late Octobe

ISSN:0094-8276    

DOI:10.1029/GL017i009p01267

年代:1990

数据来源: WILEY

摘要:

Near‐simultaneous soundings of backscatter, particle size distribution and frost point were obtained in north polar stratospheric clouds (PSCs) near the center of the vortex. The measured particle sizes and concentration in type I PSCs tend to confirm earlier predictions based on remotely sensed propertie

ISSN:0094-8276    

DOI:10.1029/GL017i009p01271

年代:1990

数据来源: WILEY

摘要:

Measurements of the altitude distributions of gaseous nitric acid and aerosols were made in the winter arctic stratosphere using balloon‐borne mass spectrometer‐ and aerosol counter techniques. The measurements took place under conditions of very low stratospheric temperatures, with minimum values of 186 K around 24 km altitude being about 6 degrees below the existence temperature (T(NAT)) for NAT (Nitric Acid Trihydrate) aerosols. At these heights, a local depletion of gaseous nitric acid by up to about 60% was observed suggesting the occurrence of nitric acid incorporation into aerosols, which were in fact observed and whose inferred nitric acid content is roughly consistent with the missing gaseous nitric acid. Our data suggest that rapid formation of NAT‐aerosols associated with substantial condensational depletion of gaseous nitric acid occurred only at temperatures of about 3 to 4 degrees below T(NAT) corresponding to large HNO3‐supersaturations, which are probably required for activation of a major fraction of preexisting condensation

ISSN:0094-8276    

DOI:10.1029/GL017i009p01275

年代:1990

数据来源: WILEY