1,114 research outputs found
Query DAGs: A Practical Paradigm for Implementing Belief-Network Inference
We describe a new paradigm for implementing inference in belief networks,
which consists of two steps: (1) compiling a belief network into an arithmetic
expression called a Query DAG (Q-DAG); and (2) answering queries using a simple
evaluation algorithm. Each node of a Q-DAG represents a numeric operation, a
number, or a symbol for evidence. Each leaf node of a Q-DAG represents the
answer to a network query, that is, the probability of some event of interest.
It appears that Q-DAGs can be generated using any of the standard algorithms
for exact inference in belief networks (we show how they can be generated using
clustering and conditioning algorithms). The time and space complexity of a
Q-DAG generation algorithm is no worse than the time complexity of the
inference algorithm on which it is based. The complexity of a Q-DAG evaluation
algorithm is linear in the size of the Q-DAG, and such inference amounts to a
standard evaluation of the arithmetic expression it represents. The intended
value of Q-DAGs is in reducing the software and hardware resources required to
utilize belief networks in on-line, real-world applications. The proposed
framework also facilitates the development of on-line inference on different
software and hardware platforms due to the simplicity of the Q-DAG evaluation
algorithm. Interestingly enough, Q-DAGs were found to serve other purposes:
simple techniques for reducing Q-DAGs tend to subsume relatively complex
optimization techniques for belief-network inference, such as network-pruning
and computation-caching.Comment: See http://www.jair.org/ for any accompanying file
A statistical survey of dayside pulsed ionospheric flows as seen by the CUTLASS Finland HF radar
International audienceNearly two years of 2-min resolution data and 7- to 21-s resolution data from the CUTLASS Finland HF radar have undergone Fourier analysis in order to study statistically the occurrence rates and repetition frequencies of pulsed ionospheric flows in the noon-sector high-latitude ionosphere. Pulsed ionospheric flow bursts are believed to be the ionospheric footprint of newly reconnected geomagnetic field lines, which occur during episodes of magnetic flux transfer to the terrestrial magnetosphere - flux transfer events or FTEs. The distribution of pulsed ionospheric flows were found to be well grouped in the radar field of view, and to be in the vicinity of the radar signature of the cusp footprint. Two thirds of the pulsed ionospheric flow intervals included in the statistical study occurred when the interplanetary magnetic field had a southward component, supporting the hypothesis that pulsed ionospheric flows are a reconnection-related phenomenon. The occurrence rate of the pulsed ionospheric flow fluctuation period was independent of the radar scan mode. The statistical results obtained from the radar data are compared to occurrence rates and repetition frequencies of FTEs derived from spacecraft data near the magnetopause reconnection region, and to ground-based optical measurements of poleward moving auroral forms. The distributions obtained by the various instruments in different regions of the magnetosphere were remarkably similar. The radar, therefore, appears to give an unbiased sample of magnetopause activity in its routine observations of the cusp footprint
Field-aligned currents in Saturnâs magnetosphere: Observations from the F-ring orbits
We investigate the azimuthal magnetic field signatures associated with highâlatitude fieldâaligned currents observed during Cassini's Fâring orbits (October 2016âApril 2017). The overall ionospheric meridional current profiles in the northern and southern hemispheres, that is, the regions poleward and equatorward of the fieldâaligned currents, differ most from the 2008 observations. We discuss these differences in terms of the seasonal change between data sets and local time (LT) differences, as the 2008 data cover the nightside while the Fâring data cover the postâdawn and dusk sectors in the northern and southern hemispheres, respectively. The Fâring fieldâaligned currents typically have a similar four current sheet structure to those in 2008. We investigate the properties of the current sheets and show that the fieldâaligned currents in a hemisphere are modulated by that hemisphere's âplanetary period oscillationâ (PPO) systems. We separate the PPOâindependent and PPOârelated currents in both hemispheres using their opposite symmetry. The average PPOâindependent currents peak at ~1.5 MA/rad just equatorward of the open closed field line boundary, similar to the 2008 observations. However, the PPOârelated currents in both hemispheres are reduced by ~50% to ~0.4 MA/rad. This may be evidence of reduced PPO amplitudes, similar to the previously observed weaker equatorial oscillations at similar dayside LTs. We do not detect the PPO current systems' interhemispheric component, likely a result of the weaker PPOârelated currents and their closure within the magnetosphere. We also do not detect previously proposed lower latitude discrete fieldâaligned currents that act to âturn offâ the PPOs
Currents associated with Saturn's intra-D ring azimuthal field perturbations
During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturnâs D ring and the denser layers of the planetâs atmosphere, revealing the presence of an unanticipated low-latitude current system via the associated azimuthal perturbation field peaking typically at ~10-30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field-aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5â1.5 MA per radian of azimuth, similar to auroral region currents, while the field-aligned current densities above the ionosphere are typically ~5-10 nA m-2 , more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field-aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north-south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller-scale structures, possibly due to rapid temporal or local time variations
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