20 research outputs found
Substorm Effects in MHD and Test Particle Simulations of Magnetotail Dynamics
Recent magnetohydrodynamic simulations demonstrate that a global tail instability, initiated by localized breakdown of MHD, can cause plasmoid formation and ejection as well as dipolarization and the current diversion of the substorm current wedge. The connection between the reconnection process and the current wedge signatures is provided by earthward flow from the reconnection site. Its braking and diversion in the inner magnetosphere causes dipolarization and the magnetic field distortions of the current wedge. The authors demonstrate the characteristic properties of this process and the current systems involved. The strong localized electric field associated with the flow burst and the dipolarization is also the cause of particle acceleration and energetic particle injections. Test particle simulations of orbits in the MHD fields yield results that are quite consistent with observed injection signatures
Structural Characteristics and Stellar Composition of Low Surface Brightness Disk Galaxies
We present UBVI surface photometry of a sample of low surface brightness
(LSB) disk galaxies. LSB disk galaxies are fairly well described as exponential
disks with no preferred value for either scale length, central surface
brightness, or rotational velocity. Indeed, the distribution of scale lengths
is indistinguishable from that of high surface brightness spirals, indicating
that dynamically similar galaxies (e.g., those with comparable Rv^2) exist over
a large range in surface density.
These LSB galaxies are strikingly blue. The complete lack of correlation
between central surface brightness and color rules out any fading scenario.
Similarly, the oxygen abundances inferred from HII region spectra are
uncorrelated with color so the low metallicities are not the primary cause of
the blue colors. While these are difficult to interpret in the absence of
significant star formation, the most plausible scenario is a stellar population
with a young mean age stemming from late formation and subsequent slow
evolution.
These properties suggest that LSB disks formed from low initial overdensities
with correspondingly late collapse times.Comment: Astronomical Journal, in press 45 pages uuencoded postscript (368K)
including 9 multipart figures also available by anonymous ftp @
ftp.ast.cam.ac.uk /pub/ssm/phot.uu CAP-30-210442962983742937
Suprathermal Magnetospheric Atomic and Molecular Heavy Ions at and Near Earth, Jupiter, and Saturn: Observations and Identification
We examine longâterm suprathermal, singly charged heavy ion composition measured at three planets using functionally identical chargeâenergyâmass ion spectrometers, one on Geotail, orbiting Earth at ~9â30 Re, the other on Cassini, in interplanetary space, during Jupiter flyby, and then in orbit around Saturn. O+, a principal suprathermal (~80â220 keV/e) heavy ion in each magnetosphere, derives primarily from outflowing ionospheric O+ at Earth, but mostly from satellites and rings at Jupiter and Saturn. Comparable amounts of Iogenic O+ and S+ are present at Jupiter. Ions escaping the magnetospheres: O+ and S+ at Jupiter; C+, N+, O+, H2O+, 28M+ (possibly an aggregate of the molecular ions, MI, CO+, N2+, HCNH+, and/or C2H4+), and O2+ at Saturn; and N+, O+, N2+, NO+, O2+, and Fe+ at Earth. Generally, escaped atomic ions (MI) at Earth and Saturn have similar (higher) ratios to O+ compared to their magnetospheric ratios; Saturn's H2O+ and Fe+ ratios are lower. At Earth, after O+ and N+, ionospheric origin N2+, NO+, and O2+ (with proportions ~0.9:1.0:0.2) dominate magnetospheric heavy ions, consistent with recent highâaltitude/latitude ionospheric measurements and models; average ion count rates correlate positively with geomagnetic and solar activity. At ~27â33 amu/e, Earth's MIs dominate over lunar pickup ions (PUIs) in the magnetosphere; MIs are roughly comparable to lunar PUIs in the magnetosheath, and lunar PUIs dominate over MIs beyond Earth's bow shock. Lunar PUIs are detected at ~39â48 amu/e in the lobe and possibly in the plasma sheet at very low levels