<p>Impact craters are common geomorphological features on Mars. The density of craters is different among various regions. Higher crater density means older terrain. Craters can be divided into two types by the interior morphology: simple and complex. The cavity of Simple craters is bowl-shape, and complex craters display various interior features, such as central peaks. The depth/diameter ratio (d/D) of simple craters is larger than that of complex craters. The transition diameter from simple to complex morphologies ranges between 5 and 10 km, and is commonly cited to be about 7 km in the equatorial regions and 6 km near the poles, but the exact value also could vary with terrain type. In this research, seven regions, Amazonis Planitia, Arabia Terra, Chryse Planitia, Hesperia Planum, Isidis Planitia, Solis/Syria/Sinai Planum, and Terra Sirenum, were selected to investigate the onset diameter of complex craters and the relationship of crater diameter and depth in these regions on Mars in order to understand how the geology affects crater d/D. The analysis revealed that the slopes of the d/D relations are different, and these are linked to the surface material in different regions. The onset diameters in young volcanic regions with stronger material are slightly higher than older volcanic regions, and much higher than that of volatile regions. The research proves the different geological units can affect the morphology and morphometry of craters. </p>

<p> Debris disk candidates around nearby stars were detected with the <i> Spitzer Space Telescope</i> using the Multiband Imaging Photometer (MIPS) at 24&mu;m (MIPS24) and 70&mu;m (MIPS70). A survey sample of 978 stars within 25pc of the Sun was observed with MIPS24, with 438 stars solely observed with MIPS70. Candidates were defined as target stars with excess infrared emission above the stellar photospheric emission at both 3&sigma; and 5&sigma; levels. At 24&mu;m, debris disks were identied around 262 stars, yielding a detection frequency of 26.8% at the 3&sigma; level. A detection rate of 22.1% is deduced from 216 stars at the 5&sigma; level. The detection frequency at 24&mu;m shows a decreasing trend with late spectral types. A detection rate of 26.5% is implied by 128 MIPS70 candidates at the 3&sigma; level, and 11.8% is revealed by 57 candidates at the 5&sigma; level. At 70&mu;m, our 3&sigma; and 5&sigma; detections correspond to fractional luminosities of 10^-3.8 and 10^-3.4, respectively. Both detection rates increase with later spectral types. </p>

Cosmic-ray secondaries and tertiaries have been studied in a series of experiments. It has been shown that the secondaries from lead have little power to produce detectable tertiaries or showers from lead or aluminum. The secondaries from aluminum have been found to produce more showers in lead than the secondaries from air. The absorption coefficient of the secondaries from aluminum has been found to be 0.7 cm-1 Pb and the absorption coefficient of their lead tertiaries 2.0 cm-1 Pb. The values for secondaries from air and a heavy roof, and their lead tertiaries were previously found to be 0.5 cm-1 Pb and 2.58 cm -1 Pb. The values obtained from Rossi's and Funfer's data are 0.32 cm-1 Pb for the air secondaries and 1.18 cm -1 Pb for their lead tertiaries. It follows that the air secondaries and their lead tertiaries have greater energies than the aluminum secondaries and their lead tertiaries. It has been found that a component of the cosmic rays even softer than the corpuscular component is probably the chief source of the secondaries producing the showers. There is some evidence that non-ionizing particles produce a portion of the showers and that possibly non-ionizing secondaries are produced in lead and that these particles can produce ionizing particles in aluminum.

Solar filaments have long been associated with coronal mass ejections and Space Weather phenomena. Filaments are dynamcal, and changes in their overall mass or its distribution could trigger an eruption. However, a comprehensive study of the elemental composition, abundance, and dynamics in a variety of filaments had not yet been conducted. The research work presented here is a detailed study of twenty filaments with various properties, which was conducted by analyzing the absorption due to the filaments in cotemporal He I (10830 A) and Haalpha (6563 A) images. The results show bands of relative helium deficits and surpluses that imply a stratification of filament material, possibly due to neutral atom cross-field diffusion. For more highly-varying material; there is a weak direct relation between a change in absorption and a change in the relative abundance of helium, and erupting filaments exhibit increasing absorption and a spatial homogenization of material in the pre-erupting filament section.

I present an analysis of over 300 spectra of the photospheres of young stars in order to address three outstanding questions of current interest: (1) whether or not the youngest stars are accompanied by massive substellar companions, (2) the degree to which starspots influence radial velocity measurements in young stars, and (3) establishment of a scale to convert spectral observations to effective temperatures and determining effective temperature for a large sample of stars. The first question is essential for understanding the star and planet formation process, the second question sets fundamental limits on ability of radial velocity surveys to detect young planets, and the third is a key for getting accurate mass and age estimates. Results from a new radial velocity survey of T Tauri stars focus on three specific stars---BP Tau, DN Tau, and LkCa19---with the largest number of observations. All the objects show far greater radial velocity variability than our radial velocity standards. Using a new simulation of the effect of star spots on the photometric and radial velocity variability of T Tauri star, I show the data for LkCa19 is fully consistent with variations caused by large star spots present on this rapidly rotating young star. In the case of BP Tau and DN Tau, the origin of the velocity variability is currently not well established. I also present the results of a new method to determine the effective temperature for weak-lined T Tauri stars, from the direct fitting of TiO bands near 7000 to NextGen synthetic spectra. The new temperatures are consistently warmer (by as much as 400K) than previous determinations based on spectral type. For the lowest mass young stars, the revised effective temperature doubles the mass inferred from evolutionary models.

A coded aperture mask has been designed for Prometheus I, a Rice University gamma-ray telescope sensitive to energies ranging from 0.03 MeV to 6.5 MeV. The mask will allow Prometheus I to record images at those energies. If it is successful, these will be the first astronomical MeV gamma ray images ever recorded. To create the mask, first a basic 9 x 7 uniformly redundant array was generated using the m-sequences method described by E. C. Fenimore (1983). A two by two mosaic of that pattern produced the final 18 x 14 array. The mask is passive rather than active; the opaque sections are two inches thick and made of tungsten. The distance from the center of one square to the center of an adjacent one is 5/8 inch, which is equal to the distance from the center of one detector element to a neighboring one. With 128 of the 252 mask elements transparent, the mask will have an open fraction of 50.7%. When it is installed on Prometheus I, the mask will be located 60 inches from the detector, giving the instrument an angular resolution of 28.6 arc minutes. Its fully coded field of view will be 5.4$\sp\circ$ and its partially coded field of view will be 16$\sp\circ$. The first flight of Prometheus I is scheduled for summer 1994.

A magnetohydrodynamic (MHD) theory has been developed for the motion of a thin magnetic flux tube through a two-dimensional stationary medium that is in MHD equilibrium. The flux tube is represented as a one-dimensional filament. Simple properties of the computed time development of the filament have been explored analytically, including linear intermediate and slow-mode waves and corresponding MHD shock solutions. Numerical solutions for filaments in the tail display the strong earthward flow and dipolar shapes that are characteristic of bursty bulk flows that are frequently observed in the plasma sheet of the Earth's magnetosphere.

Geospace is a vast complex system. A portable quantitative model of the geospace electromagnetic structure is the foundation of many researches in the area of space plasma physics. Work described in this dissertation uses both the analytical and numerical approaches to bring the existing models several steps closer to the ultimate model. An analytical model is developed to simulate the magnetic effect of the magnetopause current by solving the Laplace equation in a complex geometry. The procedure involves the minimization of an integral quantity to determine the coefficients of a harmonic expansion for the magnetic scalar potential of the magnetopause field. This procedure can shield any kind of interior magnetic field, or open the closed magnetosphere up with an arbitrary normal component distribution over the magnetopause. The Toffoletto and Hill 1993 open magnetosphere model is improved by adding a new physical module of the shielded ring current field, and modifying its additional tail field to eliminate the shortcomings that are evident. The new improved version has a better representation of the magnetic field in the inner magnetosphere. A numerical model for the electric potential on closed field lines is developed by extending the polar-cap potential into the low-latitude ionosphere using a finite element method. This significantly enhances the model's capability. By mapping the electric potential into the magnetosphere, the associated magnetospheric plasma convection can be examined to assess the reasonability of the normal component distribution on the magnetopause.

We study the origin, configuration and effects of magnetic fields in protoplanetary disks. Standard accretion disk models are adopted for protoplanetary disks to determine their electrical properties. A new computational approach to calculate the two-dimensional large scale magnetic field in thin disks, is used to study two scenarios previously proposed for the origin of magnetic fields in protoplanetary disks. We first consider the possibility that the accretion flow in a protoplanetary disk drags an external, uniform and vertical magnetic field to the required configuration to launch winds centrifugally. Results depend strongly on the magnetic Prandtl number of the prescribed turbulent motions in the disk. For fiducial values of such parameter, magnetic field dragging is unlikely to yield a configuration capable of driving cold winds centrifugally. For this to happen, the Prandtl number must be reduced by almost two orders of magnitude from the expected value, and still dragging in the disk's outer portions will be ineffective. In the second part of this thesis we calculate the magnetic configuration from an $\alpha\Omega$ dynamo operating inside a protoplanetary disk. A vacuum is assumed outside the disk. We incorporate a saturation mechanism for the dynamo instability to model the back reaction of the Lorentz force on the turbulent motions. This allows us to study the feasibility of achieving a wind-conducive magnetic configuration from the interaction of the dynamo field and a weak, externally generated, magnetic field. In general, our results indicate that some combinations of disk models and exterior magnetic field strengths result in portions of the disk being threaded by open field lines with the right configuration to drive winds. In summary, dynamo magnetic fields may be sustained in extended portions of protoplanetary disks for times comparable to the lifetime of the disk. However, the existence of an intermediate region, where the low ionization does not allow the field to be regenerated, is a general feature of viscous protoplanetary disk models. The contribution of the generated magnetic fields to the transport of angular momentum through the accretion disk, can be comparable to the effect of the turbulent viscosity.

Rice University has been developing a gamma ray telescope to be used for the detection of spectral gamma ray lines from Type I supernovae within one megaparsec in the energy range from 100 keV to 10 MeV. The detector is an actively shielded array of 121 0.5$\sp{\prime\prime}$ x 0.5$\sp{\prime\prime}$ x 2.0$\sp{\prime\prime}$ NaI(Tl) crystals, each optically separated from the remainder of the array independently viewed. Inflight calibration of the 121 individual PMTs will be handled by an on board computer in conjunction with LEDs attached to the crystals. The active shield is constructed of individual blocks viewed separately by one or more photomultiplier tubes acting in anticoincidence with the central detector, providing excellent isolation from non-source gamma ray detections. The telescope uses a coded aperture approach yielding an overall geometrical spatial resolution of 2.6$\sp\circ$ x 2.6$\sp\circ$ FWHM. The energy resolution of the detector is expected to be 12% FWHM at 0.661 MeV. Simulations of the detector/shield assembly yield a projected sensitivity of 1 $\times$ 10$\sp{-4}$ photons cm$\sp{-2}$ sec$\sp{-1}$ for a 3 sigma detection at 1 MeV. (Abstract shortened with permission of author.)