Speeds and arrival times of solar transients approximated by self-similar expanding circular fronts

The NASA STEREO mission opened up the possibility to forecast the arrival times, speeds and directions of solar transients from outside the Sun-Earth line. In particular, we are interested in predicting potentially geo-effective Interplanetary Coronal Mass Ejections (ICMEs) from observations of density structures at large observation angles from the Sun (with the STEREO Heliospheric Imager instrument). We contribute to this endeavor by deriving analytical formulas concerning a geometric correction for the ICME speed and arrival time for the technique introduced by Davies et al. (2012, ApJ, in press) called Self-Similar Expansion Fitting (SSEF). This model assumes that a circle propagates outward, along a plane specified by a position angle (e.g. the ecliptic), with constant angular half width (lambda). This is an extension to earlier, more simple models: Fixed-Phi-Fitting (lambda = 0 degree) and Harmonic Mean Fitting (lambda = 90 degree). This approach has the advantage that it is possible to assess clearly, in contrast to previous models, if a particular location in the heliosphere, such as a planet or spacecraft, might be expected to be hit by the ICME front. Our correction formulas are especially significant for glancing hits, where small differences in the direction greatly influence the expected speeds (up to 100-200 km/s) and arrival times (up to two days later than the apex). For very wide ICMEs (2 lambda > 120 degree), the geometric correction becomes very similar to the one derived by M\"ostl et al. (2011, ApJ, 741, id. 34) for the Harmonic Mean model. These analytic expressions can also be used for empirical or analytical models to predict the 1 AU arrival time of an ICME by correcting for effects of hits by the flank rather than the apex, if the width and direction of the ICME in a plane are known and a circular geometry of the ICME front is assumed.Comment: 15 pages, 5 figures, accepted for publication in "Solar Physics

The use of GPS-arrays in detecting shock-acoustic waves generated during rocket launchings

This paper is concerned with the form and dynamics of shock-acoustic waves (SAW) generated during rocket launchings. We have developed a method for determining SAW parameters (including angular characteristics of the wave vector, and the SAW phase velocity, as well as the direction towards the source) using GPS-arrays whose elements can be chosen out of a large set of GPS-stations of the global GPS network. The application of the method is illustrated by a case study of ionospheric effects from launchings of launch vehicles (LV) Proton and Space Shuttle from space-launch complexes Baikonur and Kennedy Space Center (KSC) in 1998 and 1999 (a total of five launchings). The study revealed that, in spite of a difference of LV characteristics, the ionospheric response for all launchings had the character of an N - wave corresponding to the form of a shock wave, regardless of the disturbance source (rocket launchings, industrial explosions). The SAW period T is 270--360 s, and the amplitude exceeds the standard deviation of TEC background fluctuations in this range of periods under quiet and moderate geomagnetic conditions by factors of 2 to 5 as a minimum. The angle of elevation of the SAW wave vector varies from 30 degree to 60 degree, and the SAW phase velocity (900-1200 m/s) approaches the sound velocity at heights of the ionospheric F-region maximum.Comment: EmTeX-386, 23 pages, 6 figure

Traveling wave packets of total electron content disturbances as deduced from global GPS network data

We identified a new class of mid-latitude medium-scale traveling ionospheric disturbances (MS TIDs), viz. traveling wave packets (TWPs) of total electron content (TEC) disturbances. For the first time, the morphology of TWPs is presented for 105 days. Using the technique of GPS interferometry of TIDs we carried out a detailed analysis of the spatial-temporal properties of TWPs by considering an example of the most conspicuous manifestation of TWPs on October 18, 2001 over California, USA. The velocity and direction of TWPs correspond to those of mid-latitude MS TIDs obtained previously from analyzing the phase characteristics of HF radio signals as well as signals from geostationary satellites and discrete cosmic radio sources.Comment: LaTeX2.09, 28 pages, 9 figure

Composition measurement of epitaxial Sc<inf>x</inf>Ga<inf>1-x</inf>N films

Four different methods for measuring the compositions of epitaxial ScxGa1-xN films were assessed and compared to determine which was the most reliable and accurate. The compositions of epitaxial ScxGa1-xN films with 0 ≤ x ≤ 0.26 were measured directly using Rutherford backscattering (RBS) and x-ray photoelectron spectroscopy (XPS), and indirectly using c lattice parameter measurements from x-ray diffraction and c/a ratio measurements from electron diffraction patterns. RBS measurements were taken as a standard reference. XPS was found to underestimate the Sc content, whereas c lattice parameter and c/a ratio were not reliable for composition determination due to the unknown degree of strain relaxation in the film. However, the Sc flux used during growth was found to relate linearly with x and could be used to estimate the Sc content

Effects of hot isostatic pressing on the elastic modulus and tensile properties of 316L parts made by powder bed laser fusion

The microstructure and mechanical properties of 316 L steel have been examined for parts built by a powder bed laser fusion process, which uses a laser to melt and build parts additively on a layer by layer basis.Relative density and porosity determined using various experimental techniques were correlated against laser energy density. Based on porosity sizes, morphology and distributions, the porosity was seen to transition between an irregular, highly directional porosity at the low laser energy density and a smaller, more rounded and randomly distributed porosity at higher laser energy density, thought to be caused by keyhole melting. In both cases, the porosity was reduced by hot isostatic pressing (HIP).High throughput ultrasound based measurements were used to calculate elasticity properties and show that the lower porosities from builds with higher energy densities have higher elasticity moduli in accordance with empirical relationships, and hot isostatic pressing improves the elasticity properties to levels associated with wrought/rolled 316 L. However, even with hot isostatic pressing the best properties were obtained from samples with the lowest porosity in the as-built condition.A finite element stress analysis based on the porosity microstructures was undertaken, to understand the effect of pore size distributions and morphology on the Young's modulus. Over 1–5% porosity range angular porosity was found to reduce the Young's modulus by 5% more than rounded porosity. Experimentally measured Young's moduli for samples treated by HIP were closer to the rounded trends than the as-built samples, which were closer to angular trends.Tensile tests on specimens produced at optimised machine parameters displayed a high degree of anisotropy in the build direction and test variability for as-built parts, especially between vertical and horizontal build directions. The as-built properties were generally found to have a higher yield stress, but lower upper tensile strength and elongation than published data for wrought/hot-rolled plate 316 L. The hot isostatically pressed parts showed a homogenisation of the properties across build directions and properties much more akin to those of wrought/hot-rolled 316 L, with an increase in elongation and upper tensile strength, and a reduction in yield over the as-built samples

ChatGPT sits the DFPH exam: large language model performance and potential to support public health learning

Background Artificial intelligence-based large language models, like ChatGPT, have been rapidly assessed for both risks and potential in health-related assessment and learning. However, their applications in public health professional exams have not yet been studied. We evaluated the performance of ChatGPT in part of the Faculty of Public Health’s Diplomat exam (DFPH). Methods ChatGPT was provided with a bank of 119 publicly available DFPH question parts from past papers. Its performance was assessed by two active DFPH examiners. The degree of insight and level of understanding apparently displayed by ChatGPT was also assessed. Results ChatGPT passed 3 of 4 papers, surpassing the current pass rate. It performed best on questions relating to research methods. Its answers had a high floor. Examiners identified ChatGPT answers with 73.6% accuracy and human answers with 28.6% accuracy. ChatGPT provided a mean of 3.6 unique insights per question and appeared to demonstrate a required level of learning on 71.4% of occasions. Conclusions Large language models have rapidly increasing potential as a learning tool in public health education. However, their factual fallibility and the difficulty of distinguishing their responses from that of humans pose potential threats to teaching and learning

Heliospheric Observations of STEREO-Directed Coronal Mass Ejections in 2008--2010: Lessons for Future Observations of Earth-Directed CMEs

We present a study of coronal mass ejections (CMEs) which impacted one of the STEREO spacecraft between January 2008 and early 2010. We focus our study on 20 CMEs which were observed remotely by the Heliospheric Imagers (HIs) onboard the other STEREO spacecraft up to large heliocentric distances. We compare the predictions of the Fixed-Phi and Harmonic Mean (HM) fitting methods, which only differ by the assumed geometry of the CME. It is possible to use these techniques to determine from remote-sensing observations the CME direction of propagation, arrival time and final speed which are compared to in situ measurements. We find evidence that for large viewing angles, the HM fitting method predicts the CME direction better. However, this may be due to the fact that only wide CMEs can be successfully observed when the CME propagates more than 100 deg from the observing spacecraft. Overall eight CMEs, originating from behind the limb as seen by one of the STEREO spacecraft can be tracked and their arrival time at the other STEREO spacecraft can be successfully predicted. This includes CMEs, such as the events on 4 December 2009 and 9 April 2010, which were viewed 130 deg away from their direction of propagation. Therefore, we predict that some Earth-directed CMEs will be observed by the HIs until early 2013, when the separation between Earth and one of the STEREO spacecraft will be similar to the separation of the two STEREO spacecraft in 2009--2010.Comment: 21 pages, accepted to Solar Physic