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排序方式: 共有310条查询结果,搜索用时 15 毫秒
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An experiment has been performed in interplanetary space which closely simulates the observations that would be madef by an ultraviolet spectrometer observing the atmospheres of the jovian planets. A mixture of ammonia, nitrogen, and hydrogen was released from the Mariner spacecraft, and spectra were recorded while these gases were illuminated by sunlight. The principal emissions observed were the HI 1216-angstrom Lyman-alpha line, the H(2) B-X Lyman bands, and the NH c-a and A-X bands. 相似文献
303.
Pearce JB Riddle AC Warwick JW Alexander JK Desch MD Kaiser ML Thieman JR Carr TD Gulkis S Boischot A Leblanc Y Pedersen BM Staelin DH 《Science (New York, N.Y.)》1979,206(4421):991-995
The Voyager 2 Planetary Radio Astronomy experiment to Jupiter has confirmed and extended to higher zenomagnetic latitudes results from the identical experiment carried by Voyager 1. The kilometric emissions discovered by Voyager 1 often extended to 1 megahertz or higher on Voyager 2 and often consisted of negatively or, less frequently, positively drifting narrowband bursts. On the basis of tentative identification of plasma wave emissions similar to those detected by Voyager 1, the plasma torus associated with Io appeared somewhat denser to Voyager 2 than it did to Voyager 1. We report here on quasiperiodic sinusoidal or impulsive bursts in the broadcast band range of wavelengths (800 to 1800 kilohertz). A Faraday effect appears at decametric frequencies, which probably results from propagation of the radiation near its sources on Jupiter. Finally, we discuss the occurrence of decametric emission in homologous arc families. 相似文献
304.
RW Graham EL Lundelius MA Graham EK Schroeder RS Toomey E Anderson AD Barnosky JA Burns CS Churcher DK Grayson RD Guthrie CR Harington GT Jefferson LD Martin HG McDonald RE Morlan HA Semken SD Webb L Werdelin MC Wilson 《Science (New York, N.Y.)》1996,272(5268):1601-1606
Analyses of fossil mammal faunas from 2945 localities in the United States demonstrate that the geographic ranges of individual species shifted at different times, in different directions, and at different rates in response to late Quaternary environmental fluctuations. The geographic pattern of faunal provinces was similar for the late Pleistocene and late Holocene, but differing environmental gradients resulted in dissimilar species composition for these biogeographic regions. Modern community patterns emerged only in the last few thousand years, and many late Pleistocene communities do not have modern analogs. Faunal heterogeneity was greater in the late Pleistocene. 相似文献
305.
The paired claws and closer muscles of the lobster, Homarus americanus, are identical in the early juvenile stages, but subsequently differentiate into a stout crusher claw with only slow fibers and a slender cutter with largely fast fibers. Rearing with different substrates or exercise of the claws revealed that claw laterality is determined in the central nervous system by differential reflex activity in the paired claws; the side with greater activity becomes the crusher, while the contralateral side becomes the cutter. 相似文献
306.
Okkenhaug K Bilancio A Farjot G Priddle H Sancho S Peskett E Pearce W Meek SE Salpekar A Waterfield MD Smith AJ Vanhaesebroeck B 《Science (New York, N.Y.)》2002,297(5583):1031-1034
Class IA phosphoinositide 3-kinases (PI3Ks) are a family of p85/p110 heterodimeric lipid kinases that generate second messenger signals downstream of tyrosine kinases, thereby controlling cell metabolism, growth, proliferation, differentiation, motility, and survival. Mammals express three class IA catalytic subunits: p110alpha, p110beta, and p110delta. It is unclear to what extent these p110 isoforms have overlapping or distinct biological roles. Mice expressing a catalytically inactive form of p110delta (p110delta(D910A)) were generated by gene targeting. Antigen receptor signaling in B and T cells was impaired and immune responses in vivo were attenuated in p110delta mutant mice. They also developed inflammatory bowel disease. These results reveal a selective role for p110delta in immunity. 相似文献
307.
Warwick JW Pearce JB Evans DR Carr TD Schauble JJ Alexander JK Kaiser ML Desch MD Pedersen M Lecacheux A Daigne G Boischot A Barrow CH 《Science (New York, N.Y.)》1981,212(4491):239-243
The Voyager 1 planetary radio astronomy experiment detected two distinct kinds of radio emissions from Saturn. The first, Saturn kilometric radiation, is strongly polarized, bursty, tightly correlated with Saturn's rotation, and exhibits complex dynamic spectral features somewhat reminiscent of those in Jupiter's radio emission. It appears in radio frequencies below about 1.2 megahertz. The second kind of radio emission, Saturn electrostatic discharge, is unpolarized, extremely impulsive, loosely correlated with Saturn's rotation, and very broadband, appearing throughout the observing range of the experiment (20.4 kilohertz to 40.2 megahertz). Its sources appear to lie in the planetary rings. 相似文献
308.
Bartonella henselae is occasionally associated with neurological dysfunction in people and some experimentally infected cats. The purpose of this study was to determine whether B henselae seroprevalence or titer magnitude varies among cats with neurological disease, cats with non-neurological diseases, and healthy cats while controlling for age and flea exposure. There was no difference in B henselae seroprevalence rates between cats with seizures and cats with other neurological diseases. Cats with non-neurological disease and healthy cats were more likely than cats with neurological disease to be seropositive. While the median B henselae antibody titer was greater in cats with seizures than in cats with other neurological disease, the median B henselae antibody titer was also greater in healthy cats than cats with seizures. The results suggest that titer magnitude cannot be used alone to document clinical disease associated with B henselae infection and that presence of B henselae antibodies in serum of cats with neurological disease does not prove the clinical signs are related to B henselae. 相似文献
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