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1.
The contributions of lunar microcrater studies to understand the overall micrometeoroid environment are summarized and compared to satellite data.In comparison with small-scale laboratory studies, most lunar crater morphologies are compatible only with impact velocities > 3·5 km/sec and projectile densities between 1–8 g/cm3; a mean value is most likely 2–4 g/cm3. The particles arenon-porous and fairly equi-dimensional; needles, platelets, rods, whiskers and other highly asymmetric particle shapes can be excluded. Data on projectile chemistry is sparse and non-diagnostic at present.The crater diameters are converted into projectile masses via small scale laboratory impact experiments. Accordingly, the observed span of crater pit diameters (0·1 μm–1 cm) corresponds to a particle mass range of ≈ 10?15–10?3 g. This large, dynamic detection range is a unique feature of the lunar rock detector. Absolute crater densities on different rocks vary from “production” to “equilibrium” conditions. After normalization of such densities, relative microcrater size frequencies are obtained to deduce a mass frequency distribution for particles 10?15–10?3 g. There is evidence that this distribution is bimodal. A radiation pressure cutoff at 10?12 g particle mass does not exist. The micrometeoroid flux obtained from lunar rocks is compatible with satellite data. There is indication that the micrometeoroid flux may have been lower in the past. Some speculative astronomical consequences concerning the origin of micrometeoroids are discussed.  相似文献   

2.
The HEOS detector measures the mass and speed of micrometeoroids in the Earth-Moon system. They are detected by the plasma produced by particle impacts on the sensor. During 2 yr of data collection 384 particles have been registered. As shown earlier (COSPAR 1973), they can be divided into 3 categories according to their temporal distribution: particles that are (1) randomly distributed or (2) appear in “groups” or (3) appear in “swarms” In this paper the origin of the groups and swarms is discussed. For this purpose the article orbits with respect to the Earth and the Moon were traced back. The results imply a lunar origin of the groups, whereas the swarms are correlated with the vicinity of the Earth. In addition, the dependence of the cumulative flux upon the detector's viewing direction indicates clearly an anisotropic particle flux.  相似文献   

3.
The experiment measures the mass, speed and approximate influx direction of the micrometeoroids. They are detected by the plasma produced during their impact on the sensor. In the first year after launch 173 particles were registrated.The temporal distribution of the particles indicates three different categories. Category 1 consists of 3 large groups of particles (“swarms”) each of which encountered the detector within some tens of minutes. Category 2 includes 8 small groups of particles each of which was detected within hours. Category 3 is made up by the remaining particles which are randomly distributed. The swarms are of very recent origin, possibly supplied by (1) meteoroids grazing the Earth's atmosphere, (2) ejecta from the Moon, and (3) collisions of dust particles. The groups may be dissipated swarms or particles from the outer zone of swarms. The randomly distributed particles consist of sporadic interplanetary particles and completely dissipated groups.The comparison of both the “particle rates” (number of detected particles per day) and the cumulative particle flux curves for the Earth's apex, antiapex, ecliptic north and south directions shows that the rates vary only within a factor of 2, whereas the particle flux is extremely anisotropic. The flux for the apex direction at 10?12 g is 7 × 10?5 m?2 s?1 which is 1 to 2 orders of magnitude higher than the flux values for the other directions.  相似文献   

4.
We surveyed craters on a space-exposed surface from the Genesis solar wind sample return mission to find new constraints on the population of micrometeoroids at the edge of the Earth's gravitational sphere of influence. The target was made of 6061-T6 aluminum, identical to the composition of the space-facing end of the Long Duration Exposure Facility satellite, which recorded micrometeoroid impacts in low Earth orbit. We use data from both locations to compare crater frequency as a function of size, with and without gravitational focussing by the Earth. We find that the cratering flux near the Earth-Sun L1 libration point is indistinguishable, within the ∼40% uncertainty of this study, from that in low Earth orbit. The small degree of gravitational focussing between the two locations indicates that particles with geocentric free-space velocities less than a few kilometers per second comprise no more than a few percent of the interplanetary dust complex.  相似文献   

5.
We present spectrophotometry between 8.0 and 13.0 microns at 2% spectral resolution for areas in and near the nuclear condensation of Comet IRAS-Araki-Alcock (1983d) on May 11 and 12, 1983. All the spectra can be fit very well by blackbody curves, and no 10 micron silicate emissions are seen. The temperature structure of the coma suggests the presence of small (radii <5 microns) dust particles within 150 km of the nucleus and larger ones further out. The change in the spatial distribution of the infrared flux between the two nights suggests that an outburst may have occurred sometime on May 11.  相似文献   

6.
In this paper we consider the dynamics of the electrostatic disruption products of fragile interplanetary dust aggregates which are initially electrically charged on entering the Jovian plasmasphere. On account of their large specific charges, these small dust fragments are strongly effected both by the Lorentz electric force as well as by the polarization electric force resulting from the corotation of the Jovian plasmasphere. The detailed orbits of these charged dust fragments, which are shown to be confined to the equatorial plane, are computed for various launch angles. It is established that the fragments with radii typically around 1 are magneto-gravitationally trapped within the plasma sphere due to the velocity induced oscillation of their surface potentials. The spatial distribution of these fragments are evaluated and the time evolution of the distributions followed. On this basis it is argued that the distribution of micrometeoroid dust within the Jovian magnetospheres, observed by the Pioneer 10 and the recent Voyager spacecraft, is a result of this magneto-gravitational trapping and subsequent orbital evolution of these charged dust fragments. Our discussion includes both the sudden increase, by over an order of magnitude, of the micrometeoroid dust flux at about 30R J observed by Pioneer 10, and the thin inner dust ring recently observed by the Voyaer spacecraft. The observed brightness asymmetries between the leading and trailing sides of the Galilean satellites appears to be a natural consequence of the impact geometries of these charged dust grains with the satellite surfaces.  相似文献   

7.
In this study, we try to refine the relation existing between the exobase temperature and density distributions of atomic hydrogen around the Earth (assuming that the zero net ballistic flux condition is satisfied all over the critical level). We find essentially that neither local heating in high latitude regions, nor the addition of proton fluxes around the Earth, induce large perturbations in the equatorial density distribution (less than 10 per cent). On the other hand, certain local heating can give large perturbations in the global density distribution (more than 50 per cent).The effect of the Earth's rotation is also studied. We find that at the exobase the density distribution of atomic hydrogen lags about one hour behind the temperature distribution. At higher altitudes this time lag increases, reaching 5–6 hr at 20 Earth radii.We show also that, due to a density inversion which takes place at 2 Earth radii, if the minimum of density at the exobase is on the dayside, above 2 Earth radii, a maximum of density is then on the dayside when going higher, due to the rotational effect, that density maximum shifts towards the evening, reaching early parts of the night at 20 Earth radii.  相似文献   

8.
The composition of the impact plasma produced by fast dust particles (v > 1 km/sec) hitting an Au or W target was measured both with a model of the HELIOS micrometeoroid experiment (low electric field at the target) and a high field detector. The plasma composition and the total plasma charge depend strongly on the impact velocity and the electric field strength at the target. Spectra of 9 different projectile-target combinations were analysed. Two types of spectra could be observed, depending on the projectile material. (1) Spectra of metals and hard dielectrics (Mohs' hardness ? 5). Particle constituents of low ionisation energy (e · u ? 7eV, e.g. Na, K, Al) dominate the spectra of these materials at impact velocities below 10 km/sec. At higher speed the relative intensities change and new ions with higher ionisation energies appear. (2) Spectra of soft dielectrics (Mohs' hardness < 3). Below 9 km/sec these materials produced less total charge than did the others. The highest masses were detected at 74 amu. The relative abundance of ions with low ionization energies such as Li, Na, K, etc. is comparatively small. Negative ions were also observed in the impact plasma. Their total number was found to be approximately 3–6% of that of the positive ions at 6 km/sec particle speed.  相似文献   

9.
We present the observations of cosmic gamma-ray bursts (GRBs) with the main detector of the SIGMA telescope onboard the Granat Observatory from January 1990 through September 1994. The observations were carried out in the energy range 35–1300 keV. We detected 36 GRBs and 31 high-energy solar flares during this period. No GRB fell within the main field of view; they were all recorded by the “secondary optics” of the telescope. The SIGMA telescope recorded relatively bright bursts with peak fluxes of 10?6–10?4 erg s?1 cm?2 in the 100–500-keV energy band. Stable detector background allows the long-term variability of GRB sources on a time scale of ～1000 s to be studied. The results of our search for early afterglows of GRBs are presented. The flux averaged over all bursts in the interval 100–800 s after the main event is 0.36±0.14 counts s?(35–300 keV), suggesting that there is soft gamma-ray emission on this time scale after a considerable number of GRBs.  相似文献   

10.
Abstract— Photometric observations of the twilight sky were carried out during Leonids 1998. The obtained vertical distributions of aerosol between 20 and 140 km demonstrate the processes of the intrusion of fine meteor dust and its subsequent intra-atmospheric dynamics. The characteristic radii of two fractions of the meteor dust particles were estimated by their sedimentation velocities. They varied within rp = 0.006–0.06 μm and rp = 19–81 μm limits depending on an assumed particle density within ρp = 0.4–4.0 g cm?3. The assumption of ρp = 2.0 g cm?3 gave radii of the two fractions to be 0.01 and 30 μm, respectively.  相似文献   

11.
Detailed mapping in a 14,000,000 km2 area of northwestern Guinevere Planitia and northern Beta Regio bounded by 264°-312°E, 24°-60°N has revealed thousands of long extensional lineaments (graben, fissures and related fractures). These can be grouped into radiating, circumferential and linear systems. Thirty four radiating systems have been identified, of which 16 have radii greater than 300 km and eight have radii greater than 1000 km. Twenty six linear (straight) systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km. Linear systems are generally associated with rifts, although some may represent distal portions of radiating systems. In addition, 19 circumferential systems, some associated with coronae, have been identified. The distribution of each system is compared with the host geology in order to place the graben-fissure systems in a regional stratigraphic framework. The majority of systems are: (1) younger than tesserae, ridge belts and densely fractured plains, (2) coeval with, and in many cases, define fracture belts, (3) partially flooded by wrinkle-ridged plains units, and (4) older than smooth and lobate plains units and young rifts. The inventory of radiating graben-fissure systems that we catalogue represents a database of tectono-magmatic centers that complements the centers defined using other criteria, e.g., large volcanoes, coronae, and shield fields. We have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes. At least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms.  相似文献   

12.
The speeds of coronal mass ejection events   总被引：2，自引：0，他引：2  
The outward speeds of mass ejection events observed with the white light coronagraph experiment on Skylab varied over a range extending from less than 100 km s–1 to greater than 1200 km s–1. For all events the average speed within the field of view of the experiment (1.75 to 6 solar radii) was 470 km s–1. Typically, flare associated events (Importance 1 or greater) traveled faster (775 km s–1) than events associated with eruptive prominences (330 km s–1); no flare associated event had a speed less than 360 km s–1, and only one eruptive prominence associated event had a speed greater than 600 km s–1. Speeds versus height profiles for a limited number of events indicate that the leading edges of the ejecta move outward with constant or increasing speeds.Metric wavelength type II and IV radio bursts are associated only with events moving faster than about 400 km s–1; all but two events moving faster than 500 km –1 produced either a type II or IV radio burst or both. This suggests that the characteristic speed with which MHD signals propagate in the lower (1.1 to 3 solar radii) corona, where metric wavelength bursts are generated, is about 400 to 500 km s–1. The fact that the fastest mass ejection events are almost always associated with flares and with metric wavelength type II and IV radio bursts explains why major shock wave disturbances in the solar wind at 1 AU are most often associated with these forms of solar activity rather than with eruptive prominences.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

13.
《Planetary and Space Science》1999,47(3-4):557-576
A significant flux enhancement in energetic particles (E ∼ 60–⩾260 keV),showing internal fine structure interpreted to represent signatures produced during the traversalof various cometary boundaries in P⧸Grigg-Skjellerup, was recorded by the EPONA instrumentaboard spacecraft Giotto on 10 July 1992. A further internally structured flux enhancement withabout the same amplitude, recorded by EPONA in the energy range ∼60–100 keV but detected90×103 km further on along the Giotto trajectory, is herein compared with theP⧸Grigg-Skjellerup record. Possible explanations for the second flux enhancement areindividually considered and it is suggested, on the basis of the available evidence, that itconstituted the signature of another smaller comet, either having a separate genesis from, ororiginating in a splitting of, the P⧸Grigg-Skjellerup nucleus.  相似文献   

14.
We have observed an eruptive prominence at the east solar limb on 25 January 1991 which started earlier than 0623 UT and was associated with a limb flare (S16 E90) of class 1B/ X10.0. We have recorded a huge mass ejection in the corona by the limb flare associated eruptive prominence. The eruptive prominence ejected a part of the loop in the corona with maximum speed of about 1280 km/sec. The ejected material attain height upto 150,000 km in the corona and finally faded/disappeared in the corona. During the ascending phase of the prominence material in the corona there was a unscrewing of the loop system associated with the eruptive prominence. The type II, III, and IV radio bursts were also reported by a number of Radio Observatories during observation of the eruptive prominence. The high flux of sudden ionospheric disturbances and the solar radio emissions on fixed frequencies (245–80000 MHz) were also recorded. The eruptive prominence associated with limb flare also shows increased proton flux (>10 MeV) during its occurence. The flare was classified as X10.0 flare. In this paper we have analysed the observed data and compared it with the theoretical model of the solar flare.On leave from his original Institute  相似文献   

15.
Data processing and interpretation of the nephelometer measurements made in the Venus atmosphere aboard the Venera 9, 10 and 11 landers in the sunlit hemisphere near the equator are discussed. These results were used to obtain the aerosol distribution and its microphysical properties from 62 km to the surface. The main aerosol content is found in the altitude range between 62 km (where measurements began) and 48 km, the location of the cloud region. Three prominent layers labeled as I (between 62 and 57 km), II (between 57 and 51 km) and III (between 51 and 48 km), each with different particle characteristics are discovered within the clouds. The measured light-scattering patterns can be intrepreted as having been produced by particles with effective radii from 1 to 2 μm depending on height and indices of refractivity from 1.45 in layer I to 1.42 in layer III. These values do not contradict the idea that the droplets are made of sulfuric acid. In layers II and III the particle size distribution is at least bimodal rather than uni-modal. The index of refraction is found to decrease to 1.33 in the lower part of layer II, suggesting a predominant abundance of larger particles of different chemical origin, and chlorine compounds are assumed to be relevant to this effect. In the entire heightrange of the Venera 9–11 craft descents, the clouds are rather rarefied and are characterized by a mean volume scattering coefficient σ ～ 2 × 10?5 cm?1 that corresponds to the mean meteorological range of visibility of about 2 km. The average mass content of condensate is estimated to be equal to 4 × 10?9 g/cm3, and the total optical depth of clouds to τ ～ 35. Near the bottom of layer III clouds are strongly variable. In the subcloud atmosphere a haze was observed between 48 and 32 km; that haze is mainly made of submicron particles, reff ～ 0.1μm. The atmosphere below that is totally transparent but separate (sometimes possibly disappearing) layers may be present up to a height of 8 km above the surface. A model of this region with a very low particle density (N ? 2–3 cm?3) strongly refractive large particles (reff ? 2.5 μm; 1.7 < n < 2.0) provided satisfactory agreement. The optical depth of aerosol in the atmosphere below the subcloud haze does not exceed 2.5.  相似文献   

16.
“The investigation into the possible effects of cosmic rays on living organisms will also offer great interest.” – Victor F. Hess, Nobel Lecture, December 12, 1936High-energy radiation bursts are commonplace in our Universe. From nearby solar flares to distant gamma ray bursts, a variety of physical processes accelerate charged particles to a wide range of energies, which subsequently reach the Earth. Such particles contribute to a number of physical processes occurring in the Earth system. A large fraction of the energy of charged particles gets deposited in the atmosphere, ionizing it, causing changes in its chemistry and affecting the global electric circuit. Remaining secondary particles contribute to the background dose of cosmic rays on the surface and parts of the subsurface region. Life has evolved over the past ∼3 billion years in presence of this background radiation, which itself has varied considerably during the period [1], [2], [3]. As demonstrated by the Miller–Urey experiment, lightning plays a very important role in the formation of complex organic molecules, which are the building blocks of more complex structures forming life. There is growing evidence of increase in the lightning rate with increasing flux of charged particles. Is there a connection between enhanced rate of cosmic rays and the origin of life? Cosmic ray secondaries are also known to damage DNA and cause mutations, leading to cancer and other diseases. It is now possible to compute radiation doses from secondary particles, in particular muons and neutrons. Have the variations in cosmic ray flux affected the evolution of life on earth? We describe the mechanisms of cosmic rays affecting terrestrial life and review the potential implications of the variation of high-energy astrophysical radiation on the history of life on earth.  相似文献   

17.
We report on the coronal hole (CH) influence on the 54 magnetic cloud (MC) and non-MC associated coronal mass ejections (CMEs) selected for studies during the Coordinated Data Analysis Workshops (CDAWs) focusing on the question if all CMEs are flux ropes. All selected CMEs originated from source regions located between longitudes 15E?–?15W. Xie, Gopalswamy, and St. Cyr (2013, Solar Phys., doi: 10.1007/s11207-012-0209-0 ) found that these MC and non-MC associated CMEs are on average deflected towards and away from the Sun–Earth line, respectively. We used a CH influence parameter (CHIP) that depends on the CH area, average magnetic field strength, and distance from the CME source region to describe the influence of all on-disk CHs on the erupting CME. We found that for CHIP values larger than 2.6 G the MC and non-MC events separate into two distinct groups where MCs (non-MCs) are deflected towards (away) from the disk center. Division into two groups was also observed when the distance to the nearest CH was less than 3.2×105 km. At CHIP values less than 2.6 G or at distances of the nearest CH larger than 3.2×105 km the deflection distributions of the MC and non-MCs started to overlap, indicating diminishing CH influence. These results give support to the idea that all CMEs are flux ropes, but those observed to be non-MCs at 1 AU could be deflected away from the Sun–Earth line by nearby CHs, making their flux rope structure unobservable at 1 AU.  相似文献   

18.
This paper presents a time-dependent two-dimensional numerical model of the impact phenomena. The model deals with formation and evolution of a crater, formation of an impact jet, and with global deformation and dynamical parameters of the impacted body. The model is applied to study the problem of deformation of the Earth when impacted by an asteroid. A hydrodynamical code of the free particle numerical method (HEFP) is applied to a silicate asteroid (impactor) and to the multilayered spherical Earth (target) described by the PREM model. The asteroids radii are within a range between 5 and 800 km. The velocity range is 20–30 km s−1. Calculations cover the time intervals up to 2000 s.

Each of the material points of the bodies under consideration (the Earth and an asteroid) is described by its time-dependent position, velocity, specific internal energy, pressure and density. The global results, among others, are as follows: (i) deformation of the Earth's surface; (ii) position of the shock wave within the Earth; (iii) deformation of consecutive layers within the Earth's interior, and (iv) morphology of the crater including behavior of the impact jet and deformation of the impactor.  相似文献   


19.
From the UCSD OSO-7 X-ray experiment data, we have identified 54 X-ray bursts with 5.1–6.6 keV flux greater than 103 photon cm?2 keV?1 which were not accompanied by visible Hα flare on the solar disk. By studying OSO-5 X-ray spectroheliograms, Hα activity at the limb and the emergence and disappearance of sunspot groups at the limb, we found 17 active centers as likely seats of the X-ray bursts beyond the limb. We present the analysis of 37 X-ray bursts and their physical parameters. We compare our results with those published by Datlowe et al. (1974a, b) for disk events. The distributions of maximum temperature, maximum emission measure, and characteristic cooling time of the over-the-limb events do not significantly differ from those of disk events. We show that of conduction and radiation, the former is the dominant cooling mechanism for the hot flare plasma. Since the disk and over-the-limb bursts are similar, we conclude that the scale height for X-ray emission in the 5–10 keV range is large and is consistent with that of Catalano and Van Allen (1973), 11000 km, for primarily 1–3 keV emission. Twenty-five or about 2/3 of the over-the-limb events had a non-thermal component. The distribution of peak 20 keV flux is not significantly different from that of disk events. However, the spectral index at the time of maximum flux is significantly different for events over the limb and for events near the center of the disk; the spectral index for over-the-limb events is larger by about δγ = 3/4. If hard X-ray emission came only from localized sources low in the chromosphere we would expect that hard X-ray emission, would be occulted over the limb; on the contrary, the observation show that the fraction of soft X-ray bursts which have a nonthermal component is the same on and off of the disk. Thus hard X-ray emission over extended regions is indicated.  相似文献   

20.
Eddington-limited X-ray bursts from neutron stars can be used in conjunction with other spectroscopic observations to measure neutron star masses, radii and distances. In order to quantify some of the uncertainties in the determination of the Eddington limit, we analysed a large sample of photospheric radius-expansion thermonuclear bursts observed with the Rossi X-ray Timing Explorer . We identified the instant at which the expanded photosphere 'touches down' back on to the surface of the neutron star and compared the corresponding touchdown flux to the peak flux of each burst. We found that for the majority of sources, the ratio of these fluxes is smaller than ≃1.6, which is the maximum value expected from the changing gravitational redshift during the radius expansion episodes (for a  2 M⊙  neutron star). The only sources for which this ratio is larger than ≃1.6 are high-inclination sources that include dippers and Cyg X-2. We discuss two possible geometric interpretations of this effect and show that the inferred masses and radii of neutron stars are not affected by this bias. On the other hand, systematic uncertainties as large as ∼50 per cent may be introduced to the distance determination.  相似文献   

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