22,445 research outputs found
Power laws statistics of cliff failures, scaling and percolation
The size of large cliff failures may be described in several ways, for
instance considering the horizontal eroded area at the cliff top and the
maximum local retreat of the coastline. Field studies suggest that, for large
failures, the frequencies of these two quantities decrease as power laws of the
respective magnitudes, defining two different decay exponents. Moreover, the
horizontal area increases as a power law of the maximum local retreat,
identifying a third exponent. Such observation suggests that the geometry of
cliff failures are statistically similar for different magnitudes. Power laws
are familiar in the physics of critical systems. The corresponding exponents
satisfy precise relations and are proven to be universal features, common to
very different systems. Following the approach typical of statistical physics,
we propose a "scaling hypothesis" resulting in a relation between the three
above exponents: there is a precise, mathematical relation between the
distributions of magnitudes of erosion events and their geometry. Beyond its
theoretical value, such relation could be useful for the validation of field
catalogs analysis. Pushing the statistical physics approach further, we develop
a numerical model of marine erosion that reproduces the observed failure
statistics. Despite the minimality of the model, the exponents resulting from
extensive numerical simulations fairly agree with those measured on the field.
These results suggest that the mathematical theory of percolation, which lies
behind our simple model, can possibly be used as a guide to decipher the
physics of rocky coast erosion and could provide precise predictions to the
statistics of cliff collapses.Comment: 20 pages, 13 figures, 1 table. To appear in Earth Surface Processes
and Lanforms (Rocky Coast special issue
Observational evidence for gravitationally trapped massive axion(-like) particles
Unexpected astrophysical observations can be explained by gravitationally
captured massive particles, which are produced inside the Sun or other Stars
and are accumulated over cosmic times. Their radiative decay in solar outer
space would give rise to a `self-irradiation' of the whole star, providing the
time-independent component of the corona heating source. In analogy with the
Sun-irradiated Earth atmosphere, the temperature and density gradient in the
corona - chromosphere transition region is suggestive for an omnipresent
irradiation of the Sun. The same scenario fits other astrophysical X-ray
observations. The radiative decay of a population of such elusive particles
mimics a hot gas. X-ray observatories, with an unrivalled sensitivity below ~10
keV, can search for such particles. The elongation angle relative to the Sun is
the relevant new parameter.Comment: 35 pages, LaTeX, 9 figures. Accepted by Astroparticle Physic
Kinetic Electron and Ion Instability of the Lunar Wake Simulated at Physical Mass Ratio
The solar wind wake behind the moon is studied with 1D electrostatic
particle-in-cell (PIC) simulations using a physical ion to electron mass ratio
(unlike prior investigations); the simulations also apply more generally to
supersonic flow of dense magnetized plasma past non-magnetic objects. A hybrid
electrostatic Boltzmann electron treatment is first used to investigate the ion
stability in the absence of kinetic electron effects, showing that the ions are
two-stream unstable for downstream wake distances (in lunar radii) greater than
about three times the solar wind Mach number. Simulations with PIC electrons
are then used to show that kinetic electron effects can lead to disruption of
the ion beams at least three times closer to the moon than in the hybrid
simulations. This disruption occurs as the result of a novel wake phenomenon:
the non-linear growth of electron holes spawned from a narrow dimple in the
electron velocity distribution. Most of the holes arising from the dimple are
small and quickly leave the wake, approximately following the unperturbed
electron phase-space trajectories, but some holes originating near the center
of the wake remain and grow large enough to trigger disruption of the ion
beams. Non-linear kinetic-electron effects are therefore essential to a
comprehensive understanding of the 1D electrostatic stability of such wakes,
and possible observational signatures in ARTEMIS data from the lunar wake are
discussed.Comment: 9 pages, 10 figure
Research in space physics at the University of Iowa
Various research projects in space physics are summarized. Emphasis is placed on: (1) the study of energetic particles in outer space and their relationships to electric, magnetic, and electromagnetic fields associated with the earth, the sun, the moon, the planets, and interplanetary medium; (2) observational work on satellites of the earth and the moon, and planetary and interplanetary spacecraft; (3) phenomenological analysis and interpretation; (4) observational work by ground based radio-astronomical and optical techniques; and (5) theoretical problems in plasma physics. Specific fields of current investigations are summarized
Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1
Significant proton fluxes were detected in the near wake region of the Moon
by an ion mass spectrometer on board Chandrayaan-1. The energy of these
nightside protons is slightly higher than the energy of the solar wind protons.
The protons are detected close to the lunar equatorial plane at a
solar zenith angle, i.e., ~50 behind the terminator at a height of
100 km. The protons come from just above the local horizon, and move along the
magnetic field in the solar wind reference frame. We compared the observed
proton flux with the predictions from analytical models of an electrostatic
plasma expansion into a vacuum. The observed velocity was higher than the
velocity predicted by analytical models by a factor of 2 to 3. The simple
analytical models cannot explain the observed ion dynamics along the magnetic
field in the vicinity of the Moon.Comment: 28 pages, 7 figure
- âŠ