Lunar craters evolution and meteoroidal flux in pre-mare and post-mare times

In order to study the geomorphic evolution and lifetimes of lunar craters, data were collected from (i) 32mare andterra provinces of the nearside of the Moon using the L.P.L. catalog; (ii) amare area in Sinus Medii, using direct observations of Lunar Orbiter photos, and (iii) aterra area on the farside using direct observations of Zond-8 photos. The theory presented in a previous publication is expanded and applied to the data.The following conclusions are obtained. (1) Steady-state conditions occur on the studiedmare surfaces for craters of diameter up to approximately 220 m, and on the studiedterra surfaces for craters of diameter up to at least 50 km. (2) The average lifetime of a crater, in addition of being a function of the meteoroidal flux, is a steep function of the diameter of the crater. (3) The correlation is good between a geomorphic classification of craters based on visual comparison with standard craters and a classification of craters based on their depth-diameter ratio, resulting in a coefficient of rank correlation of 0.64. (4) When craters are classified as young, mature, and old, the length of time spent as young is less than a few percent of the total lifetime of the crater; the time spent as mature is 10 to 30%; and as much as 80% is spent as an old crater. Within the error of the calculations, these values are independent of crater diameter and apply to both pre-mare and post-mare craters, indicating that they are also independent of the intensity of the meteoroidal flux. (5) The average lifetime of a 50 km crater in pre-mare times is estimated to be less than 0.3×10⁹ years. (6) The average lifetime of a 50 km crater in post-mare times is estimated to be between 3×10¹¹ and 10¹⁴ years. (7) The average meteoroidal flux in pre-mare times is estimated to be three to six orders of magnitude more intense than in post-mare times.     

The solar corona above active regions: A comparison of extreme ultraviolet line emission with radio emission

The observation of extreme ultraviolet (EUV) emission lines of Fe ix through Fe xvi made by Orbiting Solar Observatory-1 are discussed and applied to a study of the solar corona above active regions. Ultraviolet and radio emission are determined and compared for several levels of activity classified according to the type of sunspot group associated with the active region. Both radio emission and line radiation from Fe xvi, the highest stage of ionization of Fe observed, are observed to increase rapidly with the onset of activity and are most intense over an E-spot group early in the lifetime of the active region. As activity diminishes, radiation from Fe xv and Fe xvi becomes relatively more prominent. The observations imply that the coronal temperature reaches a maximum during the period of highest activity, as indicated by sunspot-group complexity and the occurrence of chromospheric flares. A maximum coronal electron temperature of 4.0 × 10⁶ °K is estimated when taking into account the mechanism of dielectronic recombination. Concurrently, the average coronal electron density increases by a factor of 10–12. Both electron temperature and density decrease as activity subsides. The coronal temperature above the remaining Ca ii plage is estimated to be 2.5–3.0 × 10⁶ °K after flare activity has ceased and sunspots have disappeared.     

Beta decay rates forS-process studies

The rates for a variety of beta decay processes have been determined as a function of temperature for nuclei which can participate in thes-process production of heavy elements, occurring in the presence of the²²Ne(α,n)²⁵ Mg neutron source operating in the convective helium shells of thermally pulsing stars. Specifically: calculated half-lives are presented for electron emission, positron emission, and electron capture over the temperature range 10⁸–10⁹ K.     

The slowly varying component of the solar radio emission around 1 cm wavelength

The slowly varying component of the solar radio emission (S-component) has been investigated from data obtained in the United States and Japan at 35, 17, 9.4 and 4 GHz. A good correlation occurs between the 35 GHz flux of the S-component and the corresponding plage area. This is interpreted by the assumption that the 35 GHz radiation is due to pure free-free emission, and the electron density in the coronal condensation is estimated to be about 2 × 10⁹/cm³, assuming the electron temperature to be 2 × 10⁶ K and the scale height of the coronal condensation to be 3 × 10⁴ km.The S-component radiation at 17 GHz has, in turn, two components, one is due to pure free-free emission and the other is due to thermal-gyro emission. It is concluded that in the active regions a magnetic field of more than 2000 gauss extends horizontally over about 10⁴ km.     

The solar elongation distribution of low-frequency radio bursts

Over 500 days of low-frequency (<1 MHz) radio observations from the IMP-6 spacecraft have been accumulated to produce a two-dimensional map (frequency vs elongation) of solar type III burst occurrences. This map indicates that most solar bursts in this frequency range are observed at the second harmonic of the plasma frequency rather than the fundamental. The map also shows that the solar wind electron density varies as R ^?γ , where γ can be somewhat less than 2 to perhaps 3 or higher.     

A numerical test of the quasi-equilibrium approximation for stellar silicon burning

A measure of the range of the validity of the nuclear quasi-equilibrium approximation, employed byBodansky, Clayton andFowler (1968) for the treatment of stellar silicon burning, is obtained by comparison of the predicted abundances with the results of a numerical solution of the equations for the time rates of change of the nuclear abundances. The results of these calculations, performed for a temperatureT=3×10⁹K, are compared with those obtained by Bodanskyet al. for a temperatureT=5×10⁹K. The time required for the realization of the quasi-equilibrium condition at high temperatures is found to comprise a more substantial fraction of the silicon burning lifetime. This behavior is found to be attributable to the relative temperature sensitivities of the nuclear photodisintegration rates which determine the silicon burning lifetime and the charged particle reaction rates (largely Ti⁴⁴(,p) V⁴⁷) which determine the rate of buildup of iron peak nuclei.     

On the lifetime of bright X-ray sources

The lifetime of massive X-ray binaries is about 2–5×10⁵ yr, close to the nuclear time scale. The lifetime of nonmassive X-ray binaries close to the thermal one is about 0.5–1×10⁷ yr. Massive systems may be conserved at supernova explosion, the probability of the conservation of nonmassive systems being 1–3×10^–3.     

The coronal and transition region temperature structure of a solar active region

Using measurements of EUV and X-ray spectral lines we derive the differential emission measure vs electron temperature T from the transition region to the corona of an active region (10⁵ T <5 × 10⁶ K). The total emission measure and radiative losses are of order 3 × 10⁴⁸ cm^–3 and 4 × 10²⁶ ergss^–1 respectively. The emission measure at T > 10⁶ K (i.e. that mainly responsible for the X-ray emission) is about 75% of the total. We also examine the use of Mg x 625 Å as an indicator of coronal electron density. A set of theoretical energy balance models of coronal loops in which the loop divergence is a variable parameter is presented and compared with the observations. Particular attention is given to the limitations inherent in any such comparison.     

Properties of coronal arches

The properties of coronal arches located on the peripheries of active regions, observed during a sounding rocket flight on March 8, 1973, are discussed. The arches are found to overlie filament channels and their footpoints are traced to locations on the perimeters of supergranulation cells. The arches have a wide range of lengths although their widths are well approximated by the value 2.2 × 10⁹ cm. Comparison of the size of the chromospheric footprint with the arch width indicates that arches do not always expand as they ascend into the corona. The electron temperatures and densities of the plasma contained in the arches were measured and the pressure calculated; typical values are 2 × 10⁶ K, 1 × 10⁹ cm^–3, and 2 × 10^–1 dyne cm^–2, respectively. The variation of these parameters with position along the length of the arch indicates that the arches are not in hydrostatic equilibrium.     

Transient brightenings of interconnecting loops

We study sudden brightenings of coronal loops that interconnect active regions. Such brightenings often occur within one or two days after the birth of a new interconnecting loop, as well as in some old interconnections. The brightenings of young loops are obviously associated with the emergence of new magnetic flux near their footpoints, whereas some enhancements of old loops may be triggered by slowly moving disturbances propagating from other centers of activity. A few loop brightenings are associated with flares, but the loop does not brighten in consequence of energy supply from the flare. Both the flare and the loop brightening are independent consequences of one common agent, presumably newly emerging flux.Temperatures in brightened loops are between 3 and 4 × 10⁶ K and densities are < 2 × 10⁹ cm^–3, probably < 5 × 10⁸ cm^–3 in some old loops. The top part of a loop is the site of the most intense brightening in the initial phase of a loop enhancement. The most frequent lifetime of these brightenings is 6 to 7 hr.Hale Observatories are operated jointly by the Carnegie Institution of Washington and the California Institute of Technology.     

Detection of Titan's Ionosphere from Voyager 1 Radio Occultation Observations

Evidence for a marginal detection of the Titan ionosphere has been obtained from a new analysis of the dual-frequency Doppler data recorded during theVoyager 1occultation in 1980. The original report by Lindalet al.(1983,Icarus53,348–363) gave only upper bounds on the peak electron density of 3000 cm⁻³during ingress (evening terminator) and 5000 cm⁻³during egress (morning terminator). The dual-frequency ingress data imply a maximum electron density of 2400 ± 1100 cm⁻³for Titan's upper ionosphere at an altitude of 1180 ± 150 km. The egress data were determined to be of limited use for this analysis because the X-band signal was received for only a few seconds. Nevertheless, a distinct ionospheric peak is revealed in the S-band data for both ingress and egress. The height and peak density of this ionized layer are in good agreement with expectations from numerical models that invoke photoionization and energetic electron impacts.     

Conversion Gain and Interpixel Capacitance of Cmos Hybrid Focal Plane Arrays

The conversion gain of optical and infrared focal plane CMOS hybrid arrays is a fundamental parameter, whose value computes into the derivation of other parameters characterizing the performance of a detector. The widespread “noise squared versus signal” method used to obtain the conversion gain can overestimate the nodal capacitance of the detector pixel by more than 20% for infrared arrays and by more than 100% for Si-PIN diode arrays. This is because this method does not take account of the capacitive coupling between neighboring pixels. A simple technique has been developed to measure the nodal capacitance directly by comparing the voltage change of an external calibrated capacitor with the voltage change on the nodal capacitor of the detector pixel. The method is elaborated in detail and has been verified with a Si-PIN diode array hybridized to a Hawaii-2RG multiplexer using an Fe ⁵⁵ X-ray source. It is also in good agreement with a stochastic method based on 2D autocorrelation.     

Electron density and temperature structure of two limb active regions observed by SOHO-CDS

The analysis of two active regions on the limb using observations from SOHO-CDS allows us to determine the electron density and temperature distribution of the coronal emission. We find that the active regions have hot cores (3×10⁶ K) with larger cooler (10⁶ K) loop structures extending above the limb. The electron number density, determined using the Si X diagnostic line ratio, is found to be highest in the active region core (greater than 2.3×10⁹ cm^–3). Electron number density values are determined for a range of spectral lines from different ions and are found to increase with temperature between 0.8 and 2.5×10⁶ K. These results are consistent with recent models of enhanced heating along the compact core of active regions, where the magnetic field shear is strongest.     

On the lifetimes of galactic clusters

From the observed age distribution of galactic clusters within 1 kpc we deduce that the typical total lifetime of a galactic cluster is about 2×10⁸ yr. The individual lifetimes vary between 10⁸ and 10¹⁰ yr. The observed lifetimes are compared with the evaporation times which are found from numerical experiments with star cluster models. These models contain up to 250 stars with a realistic mass spectrum. The effect of the galactic tidal field is taken into account and enhances the rate of escape significantly. Escapers are identified by using the Jacobian integral. We give the evaporation time in years as a function of the median radius for different values of the total mass of a cluster. The agreement between the resulting theoretical lifetimes and the observed values is sufficiently good. We estimate that the tidal field of passing interstellar clouds should be in most cases less efficient in dissolving a galactic cluster than the internal evaporation process combined with the effect of the general galactic field.     

Gaseous electron multiplier gain characteristics using low-pressure Ar/CO<Subscript>2</Subscript>

Gaseous Electron Multiplier detectors, or GEMs, show promise for use on space-based X-ray missions. Operating pressure strongly affects the gain of the detector and must be optimized for best performance. We have measured the gain characteristics of a GEM detector at various pressures below atmosphere using a mixture of Ar:CO₂ with the goal of maximizing gain to push GEM capabilities to the lowest energies possible. This paper discusses our tests, results, and their implications for choosing a detector pressure. We found that at any operating pressure the detector voltage can be adjusted to achieve roughly the same maximum gain prior to the onset of electrical discharges. We also find that the gain varies substantially by spatial location across the detector, but this variation is insensitive to changes in pressure allowing it to be calibrated and corrected if necessary. The detector pressure can therefore be optimized in the interest of other performance parameters such as leak rate, window stress, power requirements, or quantum efficiency without concern for negatively affecting the gain. These results can inform the choice of operating pressure and voltage for GEMs used onboard future space missions.     

Narrow-band split frequency decimeter solar burst

A wide-band digital decimetric spectroscope is in the process of development at INPE (Brazil), in conjunction with a 9-m diameter polar mounted antenna. Initially, this spectroscope was operating over a narrow-band 1.6 ± 0.05 MHz in an analogue mode. Here we report a slow drifting 5.0 MHz s^-1, narrow-band 1630-1580 MHz, split frequency solar burst lasting for about 15 s, observed on 15 June, 1991. The separation between the split components is 30 MHz, and the upper split frequency component is more intense than the lower split frequency component. These observed characteristics favour the hypothesis of conversion of plasma waves by combinational scattering on upgoing ion-sound waves in a magnetic loop. Existing strong electron density gradients across the magnetic field in the source region will reduce the free-free absorption of radiation at the fundamental frequency. In order to explain the observed temporal characteristics of the upper and lower split frequency components, the radiation has to propagate at some angle to the electron density gradient in the source region. Estimated physical parameters of the source and exciters are as follows: (i) maximum source size (height) 2 × 10⁷ cm; (ii) velocity and length of ion-sound pulse 3.1 × 10⁶ cm s^-1 and 5 × 10⁷ cm, respectively, and (iii) velocity of the exciter of plasma waves (electron or proton beam) 5 × 10⁸ cm s^-1.On leave from the Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia.     

The mean lifetime of interstellar molecular clouds

We describe the results of a study of the mean lifetime of molecular clouds based on actual observations. Using the model of growth from cloud-cloud collision and observations of ¹³CO along the galactic equator between longitudes 27°.85 and 40°, we derive a lower limit of 1 × 10⁹ yr for the lifetime of the interstellar molecular clouds     

Hilda asteroids among Jupiter family comets

Hilda asteroids and comets are similar from the compositional point of view. The D-taxonomic class prevailing among Hildas has all the characteristics found in cometary spectra. Jupiter Family Comets (JFCs) coming from the trans-neptunian region are under the gravitational control of Jupiter, making them a dynamically unstable population with a mean dynamical lifetime of 10⁴ to ¹⁰⁵ years. In contrast, Hilda asteroids residing in the 3:2 mean motion resonance with Jupiter are a very stable population. But once they escape from the resonance, they are dynamically controlled by Jupiter, and in this sense their behavior resembles that of JFC. We performed a numerical simulation to analyze the dynamical evolution that Hildas follow after escaping from the resonance, and their contribution to the JFC population. We found that 8% of the particles leaving the resonance end up impacting Jupiter. 98.7% of the escaped Hildas live at least 1000 years as a JFC, with a mean lifetime of 1.4×¹⁰⁶ years. In particular, escaped Hildas stay mainly in the region of perihelion distances greater than 2.5 AU. On the other hand, the number of escaped Hildas reaching the inner Solar System (q<2.5 AU) is negligible. So, there are almost no Hilda asteroids among the NEO population. We also analyzed the possibility that the Shoemaker-Levy 9 were an escaped Hilda asteroid. In this case, it would be possible to give stronger constraints to its pre-capture orbital elements.     

The transfer of Lyman continuum radiation in chromospheric flares

We study the time evolution of a layer of the middle or lower chromosphere being heated by a stream of energetic particles during a solar flare. The region, which is not in LTE, is allowed to cool by the transfer of Lyman continuum radiation, with collisional as well as radiative processes being considered. The resulting time dependence of the electron density and the effective thickness of the layer are in good agreement with values derived from observations. We assume the supply of energetic particles to be cut off when the central electron density of our model layer reaches the peak value of n _e = 4.4 × 10¹³ cm^–3 derived from observations of an importance 3 flare. Depending on the total hydrogen density assumed, the central electron temperature reaches a value ranging from 8000 to 10000 K. These quantities decrease by 20% during the following minute and at a slower rate thereafter.     

Etoile centrale et condition d'ionisation de l'enveloppe de la nova Delphini 1967

After correcting the observed flux of the forbidden lines for the supplementary reddening (due to the circumstellar envelope), we have recalculated the electron density and temperature of the envelope of the Nova. We have determined the temperature and radius of the Nova and obtained values of 1.03×10⁵ K, 1.15×10⁵ K and 1.21×10⁵ K for the temperature and values of 8.49×10¹⁰ cm, 4.32×10¹⁰ cm and 1.18×10¹⁰ cm for the radius of the Nova for 1968, 1969 and 1970, respectively. Using the temperature and radius of the Nova the electron density and temperature of the envelope and the degree of ionization, we have determined the optical depth of the envelope in the Lyman continuum, and found that it is of the order of 10 during the nebular stage.Considering the stratification of the envelope in different regions of ionization, we have determined the radius of the inner and outer edge and the electron temperature of every region. We found that the electron temperature of the inner part of the envelope is at least three times greater than that of the outer part. The variation of electron temperature as a function of the radius of the ionization region considered shows an abrupt increase of theT _e whenR/R _out(env) is between 0.44 and 0.54.