Charge-exchange limits on low-energy α-particle fluxes in solar flares

This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyα line of He II at 304 Å, as originally suggested for hydrogen by Orrall and Zirker. Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon<sup>–1</sup>. We study 10 events in total, including the γ-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism

Determination of Material Characteristics using Electromagnetic Forming and Weak Coupled Finite Element Simulations

The aim of this approach is to determine material characteristics of aluminium alloys (in the present case: AA5747) at very high strain rates, more precisely the relationship between yield stress, plastic strain and strain rate is figured out. To achieve high strain rates up to 10^4 s^(-1) the electromagnetic forming process (EMF) is applied, where a pulsed magnetic field is used to form materials with a high electrical conductivity during a process time between 10µs - 50µs. The advantage that EMF is a non-contact forming process can be used to determine material characteristics without any influence of friction. Additionally, in contrast to other testing methods the assumption of a mean strain rate over the process time is not needed, because the evaluation is done by finite element simulations. To compute the associated flow curve array, where the strain rate is the third dimension, a method will be proposed combining an online measurement technique and iterative finite element simulations. During EMF of the tube specimen, the radial displacement of at least one significant point at the tube surface is measured online. These data are used as reference values for the iteration scheme. The iteration starts with the material data of a quasistatic tensile test. In order to minimise the deviations between online measurement and simulation result an automated data modification scheme is implemented. The kernel of this scheme consists of an optimisation algorithm and two finite element codes. The first one is used to compute the deformation process of the specimen in a conventional transient way. The second code is implemented to calculate the body force distribution by a harmonic electromagnetic analysis. These two codes are coupled in a weak staggered approach