Goddard Space Flight Center

About: Goddard Space Flight Center is a facility organization based out in Greenbelt, Maryland, United States. It is known for research contribution in the topics: Galaxy & Solar wind. The organization has 19058 authors who have published 63344 publications receiving 2786037 citations. The organization is also known as: GSFC & Space Flight Center.

A model of the near magnetosphere with a dawn-dusk asymmetry 2. Parameterization and fitting to observations

Abstract: [1] First results are presented of an empirical modeling of the Earth's inner and near magnetosphere (X ≥ −15 RE), using a new set of data and new methods, described in a companion paper. The modeling database included 5-min average B field data, taken in a wide range of altitudes and latitudes by the International Solar Terrestrial Physics spacecraft Polar (1996–1999) and Geotail (1994–1999), as well as by earlier missions, ISEE 2 (1984–1987), Active Magnetospheric Particle Tracer Explorers (AMPTE)/CCE (1984–1988), AMPTE/Ion Release Module (1984–1986), CRRES (1990–1991), and DE 1 (1984–1990). To take into account the delayed response of the magnetosphere to the solar wind and interplanetary magnetic field (IMF), each data record in the data set was tagged by a “trail” of 5-min averages of the IMF, solar wind, and Dst field data, covering the preceding 2-hour interval. The axisymmetric ring current (SRC) and the partial one (PRC), both parameterized by the corrected Dst* index and the solar wind pressure Pd, were found to vary in strikingly different ways. While under quiet conditions the PRC is much weaker than the SRC, it dramatically grows in magnitude and rotates to the dusk sector with rising |Dst*| and Pd, significantly exceeding the SRC even during moderate storms, in excellent agreement with recent particle simulations. The innermost part of the cross-tail current is quite sensitive to the southward IMF and yields ∼90% of the tail's contribution to the Dst index, in contrast with the more distant tail current, which responds mainly to the solar wind pressure and provides no appreciable contribution to Dst. In response to southward IMF conditions, Region 1 and 2 Birkeland currents rapidly grow in magnitude and expand to lower latitudes, while their peaks shift slightly in local time toward noon. The coefficient of the IMF penetration inside the magnetosphere was found to dramatically increase with growing IMF clock angle: while quite small (∼0.1) for northward IMF, it rises to ∼0.6 as the IMF turns southward. Priorities and challenges for future data-based modeling studies are discussed.

A planetary system as the origin of structure in Fomalhaut's dust belt

Abstract: In 1983 the IRAS orbiting satellite detected excess infrared radiation from the direction of Fomalhaut, a first magnitude star in the otherwise dim constellation Piscis Austrinus. It was radiation from a huge dusty disk around the star, about four times the size of our Solar System. The Advanced Camera for Surveys onboard the Hubble Space Telescope has now detected Fomalhaut's dust complex at high resolution at optical wavelengths. The disk is offset from the star in a way that suggests the presence of several planets. The debris disks around Beta Pictoris and AU Microscopii are both edge-on, and the disk around HR 4796A has a small radius. So the Fomalhaut disk, seen on a slope rather like the ring around Saturn, older than the others and closer to us, may become the disk of choice for the study of planet formation. The Sun and >15 per cent of nearby stars are surrounded by dusty disks that must be collisionally replenished by asteroids and comets, as the dust would otherwise be depleted on timescales <107 years (ref. 1). Theoretical studies show that the structure of a dusty disk can be modified by the gravitational influence of planets2,3,4, but the observational evidence is incomplete, at least in part because maps of the thermal infrared emission from the disks have low linear resolution (35 au in the best case5). Optical images provide higher resolution, but the closest examples (AU Mic and β Pic) are edge-on6,7, preventing the direct measurement of the azimuthal and radial disk structure that is required for fitting theoretical models of planetary perturbations. Here we report the detection of optical light reflected from the dust grains orbiting Fomalhaut (HD 216956). The system is inclined 24° away from edge-on, enabling the measurement of disk structure around its entire circumference, at a linear resolution of 0.5 au. The dust is distributed in a belt 25 au wide, with a very sharp inner edge at a radial distance of 133 au, and we measure an offset of 15 au between the belt's geometric centre and Fomalhaut. Taken together, the sharp inner edge and offset demonstrate the presence of planetary-mass objects orbiting Fomalhaut.

Secondary Antiprotons and Propagation of Cosmic Rays in the Galaxy and Heliosphere

Abstract: High-energy collisions of cosmic-ray nuclei with interstellar gas are believed to be the mechanism producing the majority of cosmic-ray antiprotons. Because of the kinematics of the process, they are created with a nonzero momentum; the characteristic spectral shape with a maximum at D2 GeV and a sharp decrease toward lower energies makes antiprotons a unique probe of models for particle propagation in the Galaxy and modulation in the heliosphere. On the other hand, accurate calculation of the secondary antiproton —ux provides a ii background ˇˇ for searches for exotic signals from the annihilation of supersymmetric particles and primordial black hole evaporation. Recently, new data with large statistics on both low- and high-energy antiproton —uxes have become available which allow such tests to be performed. We use our propagation code GALPROP to calculate interstellar cosmic-ray propagation for a variety of models. We show that there is no simple model capable of accurately describing the whole variety of data: boron/carbon and sub-iron/iron ratios, spectra of protons, helium, antiprotons, positrons, electrons, and diUuse c-rays. We —nd that only a model with a break in the diUusion coefficient plus convection can reproduce measurements of cosmic-ray species, and the reproduction of primaries (p, He) can be further improved by introducing a break in the primary injection spectra. For our best-—t model we make predictions of proton and antiproton —uxes near the Earth for diUerent modulation levels and magnetic polarity using a steady state drift model of propagation in the heliosphere.

The first hundred brown dwarfs discovered by the wide-field infrared survey explorer (wise)

Abstract: We present ground-based spectroscopic verification of 6 Y dwarfs (see also Cushing et al.), 89 T dwarfs, 8 L dwarfs, and 1 M dwarf identified by the Wide-field Infrared Survey Explorer (WISE). Eighty of these are cold brown dwarfs with spectral types ≥T6, six of which have been announced earlier by Mainzer et al. and Burgasser et al. We present color-color and color-type diagrams showing the locus of M, L, T, and Y dwarfs in WISE color space. Near-infrared and, in a few cases, optical spectra are presented for these discoveries. Near-infrared classifications as late as early Y are presented and objects with peculiar spectra are discussed. Using these new discoveries, we are also able to extend the optical T dwarf classification scheme from T8 to T9. After deriving an absolute WISE 4.6 μm (W2) magnitude versus spectral type relation, we estimate spectrophotometric distances to our discoveries. We also use available astrometric measurements to provide preliminary trigonometric parallaxes to four of our discoveries, which have types of L9 pec (red), T8, T9, and Y0; all of these lie within 10 pc of the Sun. The Y0 dwarf, WISE 1541–2250, is the closest at 2.8^(+1.3)_(–0.6) pc; if this 2.8 pc value persists after continued monitoring, WISE 1541–2250 will become the seventh closest stellar system to the Sun. Another 10 objects, with types between T6 and >Y0, have spectrophotometric distance estimates also placing them within 10 pc. The closest of these, the T6 dwarf WISE 1506+7027, is believed to fall at a distance of ~4.9 pc. WISE multi-epoch positions supplemented with positional info primarily from the Spitzer/Infrared Array Camera allow us to calculate proper motions and tangential velocities for roughly one-half of the new discoveries. This work represents the first step by WISE to complete a full-sky, volume-limited census of late-T and Y dwarfs. Using early results from this census, we present preliminary, lower limits to the space density of these objects and discuss constraints on both the functional form of the mass function and the low-mass limit of star formation.