No abstract

Most asteroid discoveries consist of a few astrometric observations over a short time span, and in many cases the amount of information is too limited to compute a full orbit according to the least squares principle. We investigate whether such a Very Short Arc may nonetheless contain significant orbit information, with predictive value, e.g., allowing to compute useful ephemerides with a well defined uncertainty for some time in the future.For short enough arcs, all the significant information is contained in an attributable, consisting of two angles and two angular velocities for a given time; an apparent magnitude is also often available. In this case, no information on the geocentric range r and range-rate _ r is available from the observations themselves. However, the values of ðr; _ rÞ are constrained to a compact subset, the admissible region, if we can assume that the discovered object belongs to the Solar System, is not a satellite of the Earth and is not a shooting star (very small and very close). We give a full algebraic description of the admissible region, including geometric properties like the presence of either one or two connected components.The admissible region can be sampled by selecting a finite number of points in the ðr; _ rÞ plane, each corresponding to a full set of six initial conditions (given the four component attributable) for the asteroid orbit. Because the admissible region is a region in the plane, it can be described by a triangulation with the selected points as nodes. We show that triangulations with optimal properties, such as the Delaunay triangulations, can be generated by an effective algorithm; however, the optimal triangulation depends upon the choice of a metric in the ðr; _ rÞ plane. Each node of the triangulation is a Virtual Asteroid, for which it is possible to propagate the orbit and predict ephemerides. Thus for each time there is an image triangulation on the celestial sphere, and it can be used in a way similar to the use of the nominal ephemerides (with their confidence regions) in the classical case of a full least square orbit.

In 2019, Stromboli volcano experienced one of the most violent eruptive crises in the last hundred years. Two paroxysmal explosions interrupted the ‘normal’ mild explosive activity during the tourist season. Here we integrate visual and field observations, textural and chemical data of eruptive products, and numerical simulations to analyze the eruptive patterns leading to the paroxysmal explosions. Heralded by 24 days of intensified normal activity and 45 min of lava outpouring, on 3 July a paroxysm ejected ~6 × 107 kg of bombs, lapilli and ash up to 6 km high, damaging the monitoring network and falling towards SW on the inhabited areas. Intensified activity continued until the less energetic, 28 August paroxysm, which dispersed tephra mainly towards NE. We argue that all paroxysms at Stromboli share a common pre-eruptive weeks-to months-long unrest phase, marking the perturbation of the magmatic system. Our analysis points to an urgent implementation of volcanic monitoring at Stromboli to detect such long-term precursors.

No abstract

No abstract

[1] A new 2D/3D Lagrangian particle model (named LPAC) for the dynamics of clasts ejected during explosive eruptions is presented. The novelty of the model lies in the one-way coupling of the carrier flow field, given by a Eulerian multiphase flow code, and the particles. The model is based on a simplification of the Basset-Boussinesq-Oseen equation, expressing the Lagrangian equation of a particle as the sum of the forces exerted on it along its trajectory. It is assumed that particles are non-interacting and do not affect the background carrier flow and that the drag coefficient is constant. The model was applied to large clasts produced by Vulcanian explosions, in particular those occurring in August 1997 at Soufrière Hills Volcano, Montserrat (West Indies, UK). Simulation results allowed parametric studies as well as semi-quantitative comparisons between modeling results and field evidence. Major results include (1) the carrier flow was found to play a fundamental role even for meter-sized particles-a 1 m diameter block is predicted to reach a distance that is about 70% greater than that predicted without the effect of the carrier flow (assuming the same initial velocity), (2) assumption of the initial velocity of the particle was dropped thanks to the description of both the acceleration and deceleration phases along the particle trajectory, (3) by adopting experimentally based drag coefficients, large particles were able to reach greater distances with respect to smaller particles consistently with field observations and (4) the initial depth of the particle in the conduit was found to mainly influence the ejection velocity while the initial radial position with respect to the conduit axis was found to play a major role on the distance reached by the particle.

Numerical modeling of tephra dispersal and deposition is essential for evaluation of volcanic hazards. Many models consider reasonable physical approximations in order to reduce computational times, but this may introduce a certain degree of uncertainty in the simulation outputs. The important step of uncertainty quantification is dealt in this paper with respect to a coupled version of a plume model (PLUME‐MoM) and a tephra dispersal model (HYSPLIT). The performances of this model are evaluated through simulations of four past eruptions of different magnitudes and styles from three Andean volcanoes, and the uncertainty is quantified by evaluating the differences between modeled and observed data of plume height (at different time steps above the vent) as well as mass loading and grain size at given stratigraphic sections. Different meteorological data sets were also tested and had a sensible influence on the model outputs. Other results highlight that the model tends to underestimate plume heights while overestimating mass loading values, especially for higher‐magnitude eruptions. Moreover, the advective part of HYSPLIT seems to work more efficiently than the diffusive part. Finally, though the coupled PLUME‐MoM/HYSPLIT model generally is less efficient in reproducing deposit grain sizes, we propose that it may be used for hazard map production for higher‐magnitude eruptions (sub‐Plinian or Plinian) for what concern mass loading.

No abstract