Earth rotation measured by lunar laser ranging

The estimated median accuracy of 194 single-day determinations of the earth's angular position in space is 0.7 millisecond (0.01 arc second). Comparison with classical astronomical results gives agreement to about the expected 2-millisecond uncertainty of the 5-day averages obtained by the Bureau International de l'Heure. Little evidence for very rapid variations in the earth's rotation is present in the data.     

Earthquakes and the rotation of the Earth

A correlation exists between long-term variations in the length of the day, Chandler wobble amplitudes, and global seismic activity. These variations may be partially due to climatic changes and ultimately to explosive volcanic activity.     

Wind velocities on venus: vector determination by radio interferometry

To determine the wind directions and speeds on Venus, as each Pioneer probe fell to the surface we tracked its motion in three dimensions using a combination of Doppler and long-baseline radio interferometric methods. Preliminary results from this tracking, coupled with results from test observations of other spacecraft, enable us to estimate the uncertainties of our eventual determinations of the velocity vectors of the probes with respect to Venus. For altitudes below about 65 kilometers and with time-averaging over 100-second intervals, all three components of the velocity should have errors of the order of 0.3 meter per second or less.     

Variations in the rotation of the Earth

Variations in the earth's rotation (UT1) and length of day have been tracked at the submillisecond level by astronomical radio interferometry and laser ranging to the LAGEOS satellite. Three years of regular measurements reveal complex patterns of variations including UT1 fluctuations as large as 5 milliseconds in a few weeks. Comparison of the observed changes in length of day with variations in the global atmospheric angular momentum indicates that the dominant cause of changes in the earth's spin rate, on time scales from a week to several years, is the exchange of angular momentum between the atmosphere and the mantle. The unusually intense El Ni?o of 1982-1983 was marked by a strong peak in the length of day.     

Global sea level and Earth rotation

Recent analyses of long time scale secular variations of sea level, based on tide gauge observations, have established that sea level is apparently rising at a globally averaged rate somewhat in excess of 1 millimeter per year. It has been suggested that the nonsteric component of this secular rate might be explicable in terms of ongoing mass loss from the small ice sheets and glaciers of the world. Satellite laser ranging and very long baseline interferometry data may be used to deliver strong constraints on this important scenario because of the information that these systems provide on variations of the length of day and of the position of the rotation pole with respect to the earth's surface geography. These data demonstrate that the hypothesis of mass loss is plausible if the Barents Sea was covered by a substantial ice sheet at the last maximum of the current ice age 18,000 years ago.     

Eavesdropping: the radio signature of the Earth

In addition to searches for purposeful signals, those attempting interstellar communication should also consider the possibility of eavesdropping on radio emissions inadvertently "leaking" from other technical civilizations. To understand better the information which might be derivable from radio leakage, the case of planet earth is considered. The most detectable and useful escaping signal arise in a few BMEWS-type radar systems and in normal television broadcasting. A model including over 2000 television transmitters is used to demonstrate the wealth of astronomical and cultural information available from a distant observer's careful monitoring of frequency and intensity variations in individual video carriers (program materials is not taken to be detectable).     

Variations in the rate of rotation of the Earth

Variations observed in the length of the day can be divided into three types: seasonal, irregular, and long-term. The astronomical data examined here for the years 1956-1969 suggest that the seasonal terms hitherto assumed are slightly in error, that the irregular variations probably share the same source with the seasonal variations, and that the long-term variations may be linked to earthquake activity.     

Solar rotation effects on the thermospheres of Mars and Earth

The responses of Earth's and Mars' thermospheres to the quasi-periodic (27-day) variation of solar flux due to solar rotation were measured contemporaneously, revealing that this response is twice as large for Earth as for Mars. Per typical 20-unit change in 10.7-centimeter radio flux (used as a proxy for extreme ultraviolet flux) reaching each planet, we found temperature changes of 42.0 +/- 8.0 kelvin and 19.2 +/- 3.6 kelvin for Earth and Mars, respectively. Existing data for Venus indicate values of 3.6 +/- 0.6 kelvin. Our observational result constrains comparative planetary thermosphere simulations and may help resolve existing uncertainties in thermal balance processes, particularly CO2 cooling.     

Venus: ionosphere and atmosphere as measured by dual-frequency radio occultation of mariner v

Venus has daytime and nighttime ionospheres at the positions probed by radio occulation. The main layers are thin by terrestrial standards, with the nighttime peak concentration of electrons being about two orders of magnitude below that of the daytime peak. Above the nighttime peak were several scale-height regimes extending to a radius of at least 7500, and probably to 9700, kilometers from the center of Venus. Helium and hydrogen at plasma temperatures of 600 degrees to 1100 degrees K seem indicated in the regimes from 6300 to 7500 kilometers, with cooler molecular ions in lower regions. Above the daytime peak a sharp plasmapause was discovered, marking a sudden transition from appreciable ionization concentrations near Venus to the tenuous conditions of the solar wind. This may be indicative of a kind of interaction of the magnetized solar wind with a planetary body that differs from the two different kinds of interaction characterized by Earth and by Moon. For Venus and probably for Mars, the magnetic field of the solar wind may pile up in front of the conducting ionosphere, form an induced magnetosphere that ends at the plasmapause, above which any ionosphere that tends to form is swept away by the shocked solar wind that flows between the stand-off bow-shock and the magnetopause. The neutral atmosphere was also probed and a surface reflection may have been detected, but the data have not yet been studied in detail. Results are consistent with a super-refractive atmosphere, as expected from Soviet measurements near the surface. Thus, two unusual features of Venus can be described in terms of a light trap in the lower atmosphere, and a magnetic trap in the conducting ionosphere.     

Radio method for the precise measurement of the rotation period of the Earth

Radio interferometry with independent high-precision clocks, without a high-frequency communication channel between the stations, is now a possibility. It allows the stations to be as far apart as the earth can accommodate. This then makes the radio band from 10- to 60-centimeters wavelength the best frequency range for high-precision angular measurements, since the variability of the atmosphere is less disturbing there than in the optical band.     

An el nino signal in atmospheric angular momentum and Earth rotation

Anomalously high values of atmospheric angular momentum and length of day were observed in late January 1983. This signal in the time series of these two coupled quantities appears to have been a consequence of the equatorial Pacific Ocean warming event of 1982-1983.     

Venus: mass, gravity field, atmosphere, and ionosphere as measured by the mariner 10 dual-frequency radio system

Analysis of the Doppler tracking data near encounter yields a value for the ratio of the mass of the sun to that of Venus of 408,523.9 +/- 1.2, which is in good agreement with prior determinations based on data from Mariner 2 and Mariner 5. Preliminary analysis indicates that the magnitudes of the fractional differences in the principal moments of inertia of Venus are no larger than 10(-4), given that the effects of gravity-field harmonics higher than the second are negligible. Additional analysis is needed to determine the influence of the higher order harmonics on this bound. Four distinct temperature inversions exist at altitudes of 56, 58, 61, and 63 kilometers. The X-band signal was much more rapidly attenuated than the S-band signal and disappeared completely at 52-kilometer altitude. The nightside ionosphere consists of two layers having a peak density of 10(4) electrons per cubic centimeter at altitudes of 140 and 120 kilometers. The dayside ionosphere has a peak density of 3 X 10(5) electrons per cubic centimeter at an altitude of 145 kilometers. The electron number density observed at higher altitudes was ten times less than that observed by Mariner 5, and no strong evidence for a well-defined plasmapause was found.     

Earth as an intense planetary radio source: similarities to jupiter and saturn

Observations from spacecraft have revealed naturally occurring radio emission emanating from two regions near Earth. The characteristics of these two sources suggest a correlation with areas of known electron precipitation. The possibility of a similar production mechanism for observed nonthermal radio emissions from other planetary magnetospheres permits the polar magnetic field strengths of Jupiter and Saturn to be predicted.     

Venus: mapping the surface reflectivity by radar interferometry

The surface reflectivity of Venus obtained by radar interferometry at a wavelength of 3.8 centimeters has been mapped for a region extending approximately from -80 degrees to 0 degrees in longitude (Carpenter's definition) and from -50 degrees to +40 degrees in latitude. The map is free from the twofold range-Doppler ambiguity because the interferometer fringe pattern makes possible the separation of two points of equal range and Doppler shift. The map presents many new features and clearly delineates features already observed. Most notably, the map shows large circular regions of significantly lower reflectivity than their surroundinigs.     

Temperature exposure measured by the use of thermoluminescence

A method has been developed to measure temperature exposure based on the use of the temperature-dependent property of thermoluminescence fade. This property provides a relatively simple and inexpensive method for measuring both average and accumulated temperature exposure without the need for sophisticated temperature-recording equipment.     

Monitoring Earth and sun by satellite

Researchers attending the spring meeting of the American Geophysical Union held 18 to 21 May in Baltimore have grown familiar with doing geophysical studies by satellite. Here are three current examples discussed at the meeting: gauging the output of the sun, measuring crustal movement, and deciphering the mineral composition of surface rocks.     

Capture resonance of the asteroid 1685 toro by the Earth

The asteroid 1685 Toro has its perihelion inside the Earth's orbit and a period which is 8/ 5 of that of the Earth. A calculation of Toro's orbit covering 200 years shows that this asteroid at present is captured in resonance with the Earth. The capture is due to the gravitational interaction at close encounters between the bodies.     

用模拟酸雨淋溶法研究红壤和红色石灰土的盐基迁移状况

用模拟酸雨对红壤和红色石灰土的原状土柱进行连续淋溶,探讨土壤盐基的淋溶强度、容量以及土壤pH和交换性盐基的变化规律。结果指出,酸雨对红色石灰土的盐基淋溶强度和容量大于红壤。交换性盐基的淋溶速率也有同样趋势;土层厚度的增加能大大降低酸雨对盐基的淋溶强度、容量和淋溶速度;红壤的pH对酸雨淋溶较敏感。而红色石灰土的交换性盐基对酸雨的反应较敏感。所以就土壤酸化速率而言,以土壤pH为指标时,则红壤大于红色石灰土;但以盐基淋失量为指标时则相反。     

Simulations of jets driven by black hole rotation

The origin of jets emitted from black holes is not well understood; however, there are two possible energy sources: the accretion disk or the rotating black hole. Magnetohydrodynamic simulations show a well-defined jet that extracts energy from a black hole. If plasma near the black hole is threaded by large-scale magnetic flux, it will rotate with respect to asymptotic infinity, creating large magnetic stresses. These stresses are released as a relativistic jet at the expense of black hole rotational energy. The physics of the jet initiation in the simulations is described by the theory of black hole gravitohydromagnetics.