1,406 research outputs found
Distributions of Birkeland current density observed by AMPERE are heavy-tailed or long-tailed
No full textAbstract We analyze probability distributions of Birkeland current densities measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We find that the distributions are leptokurtic rather than normal and they are sometimes heavy-tailed. We fit q-exponential functions to the distributions and use these to estimate where the largest currents are likely to occur. The shape and scale parameters of the fitted q-exponential distribution vary with location: The scale parameter maximises for current densities with the same polarity and in the same location as the average Region 1 current, whereas the shape parameter maximises for current densities with the same polarity and in the same location as the average Region 2 current. We find that current densities |J|≥ 0.2 μA m−2 are most likely to occur in the average Region 1 current region, and second most likely to occur in the average Region 2 current region. However, for extreme currents (|J|≥ 4.0 μA m−2), we find that the most likely location is colocated with the average Region 2 current region on the dayside, at a colatitude of 18° − 22°
Identifying the magnetotail lobes with Cluster magnetometer data
No full textWe describe a novel method for identifying times when a spacecraft is in Earth’s magnetotail lobes solely using magnetometer data. We propose that lobe intervals can be well identified as times when the magnetic field is strong and relatively invariant, defined using thresholds in the magnitude of BX and the standard deviation σ of the magnetic field magnitude. Using data from the Cluster spacecraft at downtail distances greater than 8 RE during 2001–2009, we find that thresholds of 30 nT and 3.5 nT, respectively, optimize agreement with a previous, independently derived lobe identification method that used both magnetic and plasma data over the same interval. Specifically, our method has a moderately high accuracy (66%) and a low probability of false detection (11%) in comparison to the other method. Furthermore, our method identifies the lobe on many other occasions when the previous method was unable to make any identification and yields longer continuous intervals in the lobe than the previous method, with intervals at the 90th percentile being triple the length. Our method also allows for analyses of the lobes outside the time span of the previous method
Tailward propagation of magnetic energy density variations With respect to substorm onset times
No full textDuring geomagnetic substorms, around 1015 J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail ( urn:x-wiley:jgra:media:jgra54303:jgra54303-math-0001) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail ( urn:x-wiley:jgra:media:jgra54303:jgra54303-math-0002). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time
Recurrent substorm activity during the passage of a corotating interaction region
No full textRecent observations of magnetospheric dynamics driven by interaction with both high-speed solar wind streams (HSSs) and interplanetary coronal mass ejections (ICMEs) have shown periodic substonns to be common under strong driving, but with different periods of similar to 4 and 2-3 h, respectively. However, it is unclear what causes these substorms and what determines their periodicity. Observations during the passage of a corotating interaction region preceding a high-speed stream are presented here and the observed enhancements in the AE index are shown to be a quasi-periodic sequence of substorms. The occurrence times of these substorms and the variation of the AL index are shown to be consistent with a simple loading-unloading model without the need for external triggers. A possible explanation is given for the lengthening of the inter-substorm period during HSS relative to that of sawtooth events during ICMEs. (C) 2009 Elsevier Ltd. All rights reserved
A pilgrimage to Rubidoux : [a poem] / Elizabeth Anderson Freeman.
No full text[20] p. on double leaves, [9] leaves of plates
No evidence for externally triggered substorms based on superposed epoch analysis of IMF Bz
No full textSuperposed epoch analyses have shown that, on average, the interplanetary magnetic field (IMF) turns northward close to substorm onset. This has been commonly accepted as evidence for the substorm onset being triggered by a rapid northward turning of the IMF. Here we show that the tendency arises in any superposed epoch analysis of the IMF in which event onset is biased to occur for southward IMF, irrespective of a coincident rapid northward turning of the IMF. The overall IMF variation found in the largest superposed epoch analysis of this kind is also well reproduced using a Minimal Substorm Model in which substorm onsets are determined without the requirement of a northward IMF turning trigger. We discuss the explanation underlying these results and conclude that there is no conclusive evidence in favour of the hypothesis that substorm onsets are triggered by a rapid northward turning of the IMF. Citation: Freeman, M. P., and S. K. Morley (2009), No evidence for externally triggered substorms based on superposed epoch analysis of IMF B-z, Geophys. Res. Lett., 36, L21101, doi: 10.1029/2009GL040621
Timescales of Birkeland currents driven by the IMF
No full textWe obtain current densities from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), alongside By and Bz from the Interplanetary Magnetic Field (IMF) for March 2010. For each AMPERE spatial coordinate, we cross‐correlate current density with By and Bz, finding the maximum correlation for lags up to 360 min. The patterns of maximum correlation contain large‐scale structures consistent with the literature. For the correlation with By, the lags on the dayside are 10 min at high latitudes but up to 240 min at lower latitudes. Lags on the nightside are 90–150 min. For Bz, the shortest lags on the dayside are 10–20 min; on the equatorward edge of the current oval, 60–90 min; and on the nightside, predominantly 90–150 min. This novel approach enables us to see statistically the timescales on which information is electrodynamically communicated to the ionosphere after magnetic field lines reconnect on the dayside and nightside
Conference in Honour of the 90th Birthday of Freeman Dyson
No full textProfessor Freeman Dyson, a great physicist, thinker and futurist, has been very active in scientific, literary and public policy activities throughout his career. As a tribute to him on the occasion of his 90th birthday and to celebrate his lifelong contributions in physics, mathematics, astronomy, nuclear engineering and global warming, a conference covering a wide range of topics was held in Singapore from 26 to 29 August 2013. Distinguished scientists from around the world, including Nobel Laureate Professor David Gross, joined Professor Dyson in the celebration with a festival of lectures. This memorable volume collects an interesting lecture by Professor Dyson, Is a Graviton Detectable?, contributions by speakers at the conference, as well as guest contributions by colleagues who celebrated Dyson's birthday in Princeton. About Freeman Dyson Freeman John Dyson FRS, born December 15, 1923, is an eminent English-born American physicist, mathematician, and futurist. He is famous for his work in quantum electrodynamics, solid-state physics, mathematics, astronomy and nuclear engineering, as well as a renowned and best-selling author. He has spent most of his life as a professor of physics at the Institute for Advanced Study in Princeton, taking time off to advise the US government and write books for the public. He has won numerous notable awards including the Enrico Fermi Award, Templeton Prize, Wolf Prize, Pomeranchuk Prize, and Henri Poincaré Prize
A FREEMAN ION-SOURCE FOR OXYGEN IONS IN AN IMPLANTER
No full textA Freeman ion source for the LC-2F implanter was improved. In this source the filament is a 2-mm-diam tungsten wire covered with a LaB6 cylinder. The discharge chamber is made of antioxidation and high temperature resistant stainless steel. The source lifetime is more than 10 h in pure oxygen discharges, and no oxidation phenomenon occurs on the discharge chamber during long time operation. The source discharge and extraction characteristics are similar to those used before for BF3 and P discharge. So the source can be used in a wide range of microelectronics and material science.Instruments & InstrumentationPhysics, AppliedSCI(E)
Some of the exercises of a believing soul described; in a short answer to twelve serious and important questions. See contents in the next page. / By Eunice Smith, of Ashfield. Author of Mary and Martha.
No full text24 p. ; 18 cm. (8vo)"To fill up a vacant page, the editor has inserted The spiritual soldier's uniform. By Mr. F-----n H----y [i.e., Freeman Hearsey] of Boston, aet. 19."--p. [23]-24.Printer's advertisement, p. 24
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