6,059 research outputs found
The Cratering History of Asteroid (2867) Steins
The cratering history of main belt asteroid (2867) Steins has been
investigated using OSIRIS imagery acquired during the Rosetta flyby that took
place on the 5th of September 2008. For this purpose, we applied current models
describing the formation and evolution of main belt asteroids, that provide the
rate and velocity distributions of impactors. These models coupled with
appropriate crater scaling laws, allow the cratering history to be estimated.
Hence, we derive Steins' cratering retention age, namely the time lapsed since
its formation or global surface reset. We also investigate the influence of
various factors -like bulk structure and crater erasing- on the estimated age,
which spans from a few hundred Myrs to more than 1Gyr, depending on the adopted
scaling law and asteroid physical parameters. Moreover, a marked lack of
craters smaller than about 0.6km has been found and interpreted as a result of
a peculiar evolution of Steins cratering record, possibly related either to the
formation of the 2.1km wide impact crater near the south pole or to YORP
reshaping.Comment: Accepted by Planetary and Space Scienc
Laboratory simulations of fluid/gas induced micro-earthquakes:application to volcano seismology
Understanding different seismic signals recorded in active volcanic regions allows geoscientists to derive insight into the processes that generate them. A key type is known as Low Frequency or Long Period (LP) event, generally understood to be generated by different fluid types resonating in cracks and faults. The physical mechanisms of these signals have been linked to either resonance/turbulence within fluids, or as a result of fluids ‘sloshing’ due to a mixture of gas and fluid being present in the system. Less well understood, however, is the effect of the fluid type (phase) on the measured signal. To explore this, we designed an experiment in which we generated a precisely controlled liquid to gas transition in a closed system by inducing rapid decompression of fluid-filled fault zones in a sample of basalt from Mt. Etna Volcano, Italy. We find that fluid phase transition is accompanied by a marked frequency shift in the accompanying microseismic dataset that can be compared to volcano seismic data. Moreover, our induced seismic activity occurs at pressure conditions equivalent to hydrostatic depths of 200 to 750 meters. This is consistent with recently measured dominant frequencies of LP events and with numerous models
Multi-wavelength Diagnostics of the Precursor and Main phases of an M1.8 Flare on 2011 April 22
We study the temporal, spatial and spectral evolution of the M1.8 flare,
which occurred in NOAA AR 11195 (S17E31) on 22 April 2011, and explore the
underlying physical processes during the precursors and their relation to the
main phase. The study of the source morphology using the composite images in
131 {\deg}A wavelength observed by the SDO/AIA and 6-14 keV revealed a
multiloop system that destabilized systematically during the precursor and main
phases. In contrast, HXR emission (20-50 keV) was absent during the precursor
phase, appearing only from the onset of the impulsive phase in the form of
foot-points of emitting loop/s. This study has also revealed the heated
loop-top prior to the loop emission, although no accompanying foot-point
sources were observed during the precursor phase. We estimate the flare plasma
parameters viz. T, EM, power-law index, and photon turn-over energy by forward
fitting RHESSI spectral observations. The energy released in the precursor
phase was thermal and constituted ~1 per cent of the total energy released
during the flare. The study of morphological evolution of the filament in
conjunction with synthesized T and EM maps has been carried out which reveals
(a) Partial filament eruption prior to the onset of the precursor emission, (b)
Heated dense plasma over the polarity inversion line and in the vicinity of the
slowly rising filament during the precursor phase. Based on the implications
from multi-wavelength observations, we propose a scheme to unify the energy
release during the precursor and main phase emissions in which, the precursor
phase emission has been originated via conduction front formed due to the
partial filament eruption. Next, the heated leftover S-shaped filament has
undergone slow rise and heating due to magnetic reconnection and finally
erupted to produce emission during the impulsive and gradual phases.Comment: 16 Pages, 11 Figures, Accepted for Publication in MNRAS Main Journa
Background estimation in a wide-field background-limited instrument such as Fermi GBM
The supporting instrument on board the Fermi Gamma-ray Space Telescope, the
Gamma-ray Burst Monitor (GBM) is a wide-field gamma-ray monitor composed of 14
individual scintillation detectors, with a field of view which encompasses the
entire unocculted sky. Primarily designed as transient monitors, the
conventional method for background determination with GBM-like instruments is
to time interpolate intervals before and after the source as a polynomial. This
is generally sufficient for sharp impulsive phenomena such as Gamma-Ray Bursts
(GRBs) which are characterised by impulsive peaks with sharp rises, often
highly structured, and easily distinguishable against instrumental backgrounds.
However, smoother long lived emission, such as observed in solar flares and
some GRBs, would be difficult to detect in a background-limited instrument
using this method. We present here a description of a technique which uses the
rates from adjacent days when the satellite has approximately the same
geographical footprint to distinguish low-level emission from the instrumental
background. We present results from the application of this technique to GBM
data and discuss the implementation of it in a generalised background limited
detector in a non-equatorial orbit.Comment: Proceedings of SPIE, Vo. 8443, Paper No. 8443-3
- …