48,751 research outputs found
Results of long-term synoptic monitoring of Jupiter's decametric radiation
Results of the analysis of the large, homogeneous set of measurements of Jupiter's emission at 16.7 and 22.2 MHz for the apparitions during the period 1966-1974 were presented. An update of the radio rotation period determination which includes provision for beaming effects due to variations in the Jovicentric declination of the earth was presented. Some estimates of the magnitude of possible long-term variations in the rotation period were also discussed. The data clearly shows the Io-independent emission features associated with the System III central meridian longitudes of all three major Io-related source regions. There is also some evidence for heretofore unrecognized Io-related emission features which are apparently independent of the central meridian longitude. The possibility of three kinds of emission are suggested: (1) Io-stimulated, sharply beamed emission, (2) Io-independent, sharply beamed emission, and (3) Io-stimulated, broadly beamed emission
Relationship between auroral substorms and the occurrence of terrestrial kilometric radiation
The correlation between magnetospheric substorms as inferred from the AE(11) index and the occurrence of terrestrial kilometric radiation (TKR) is examined. It is found that AE and TKR are well correlated when observations are made from above the 15-03 hr local time zone and are rather poorly correlated over the 03-15 hr zone. High-resolution dynamic spectra obtained during periods of isolated substorms indicate that low-intensity, high-frequency TKR commences at about the same time as the substorm phase. The substorm expansion phase corresponds to a rapid intensification and bandwidth increase of TKR. When combined with previous results, these new observations imply that many TKR events begin at low altitudes and high frequencies (about 400-500 kHz) and spread to higher altitudes and lower frequencies as the substorm expands
Terrestrial kilometric radiation: 3-average spectral properties
A study is presented of the average spectral properties of terrestrial kilometric radiation (TKR) derived from observations made by radio astronomy experiments onboard the IMP-6 and RAE-2 spacecraft. As viewed from near the equatorial plane, TKR is most intense and most often observed in the 21-24 hr local time zone and is rarely seen in the 09-12 hr zone. The peak flux density usually occurs near 240 kHz, but there is evidence that the peak occurs at a somewhat lower frequency on the dayside. The frequency of the peak in the average flux spectrum varies inversely with increasing substorm activity as inferred from the auroral electrojet index (AE) from a maximum near 300 kHz during very quiet times to a minimum below 200 kHz during very disturbed times. The absolute flux levels in the 100-600 kHz TKR band increase significantly with increasing AE. The average power associated with a particular source region seems to decrease rapidly with increasing source altitude
Terrestrial kilometric radiation: 1: Spatial structures studies
Observations are presented of lunar occultations of the earth at 250 kHz obtained with the Radio-Astronomy-Explorer-2 satellite which were used to derive two dimensional maps of the location of the sources of terrestrial kilometric radiation (TKR). By examining the two dimensional source distributions as a function of the observer's location (lunar orbit) with respect to the magnetosphere, the average three dimensional location of the emission regions can be estimated. Although TKR events at 250 kHz can often be observed at projected distances corresponding to the 250 kHz electron gyro or plasma level (approximately 2 earth radii), many events are observed much farther from the earth (between 5 and 15 earth radii). Dayside emission apparently in the region of the polar cusp and the magnetosheath and night emission associated with regions of the magnetotail are examined. The nightside emission is suggestive of a mechanism involving plasma sheet electron precipitation in the pre-midnight sector
Learning action-oriented models through active inference
Converging theories suggest that organisms learn and exploit probabilistic models of their environment. However, it remains unclear how such models can be learned in practice. The open-ended complexity of natural environments means that it is generally infeasible for organisms to model their environment comprehensively. Alternatively, action-oriented models attempt to encode a parsimonious representation of adaptive agent-environment interactions. One approach to learning action-oriented models is to learn online in the presence of goal-directed behaviours. This constrains an agent to behaviourally relevant trajectories, reducing the diversity of the data a model need account for. Unfortunately, this approach can cause models to prematurely converge to sub-optimal solutions, through a process we refer to as a bad-bootstrap. Here, we exploit the normative framework of active inference to show that efficient action-oriented models can be learned by balancing goal-oriented and epistemic (information-seeking) behaviours in a principled manner. We illustrate our approach using a simple agent-based model of bacterial chemotaxis. We first demonstrate that learning via goal-directed behaviour indeed constrains models to behaviorally relevant aspects of the environment, but that this approach is prone to sub-optimal convergence. We then demonstrate that epistemic behaviours facilitate the construction of accurate and comprehensive models, but that these models are not tailored to any specific behavioural niche and are therefore less efficient in their use of data. Finally, we show that active inference agents learn models that are parsimonious, tailored to action, and which avoid bad bootstraps and sub-optimal convergence. Critically, our results indicate that models learned through active inference can support adaptive behaviour in spite of, and indeed because of, their departure from veridical representations of the environment. Our approach provides a principled method for learning adaptive models from limited interactions with an environment, highlighting a route to sample efficient learning algorithms
Low energy theory of disordered graphene
At low values of external doping graphene displays a wealth of unconventional
transport properties. Perhaps most strikingly, it supports a robust 'metallic'
regime, with universal conductance of the order of the conductance quantum. We
here apply a combination of mean field and bosonization methods to explore the
large scale transport properties of the system. We find that, irrespective of
the doping level, disordered graphene is subject to common mechanisms of
Anderson localization. However, at low doping a number of renormalization
mechanisms conspire to protect the conductivity of the system, to an extend
that strong localization may not be seen even at temperatures much smaller than
those underlying present experimental work.Comment: 4 page
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