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Aplin K.L.,University of Oxford | Goodman T.,University of Oxford | Herpoldt K.L.,University of Oxford | Herpoldt K.L.,Imperial College London | And 2 more authors.
Planetary and Space Science | Year: 2012

Electrical discharges in Martian analogue materials have previously been generated by agitation of the material in a low-pressure carbon dioxide environment. These results have led to the supposition that lightning is likely on Mars, on the basis that the surface material becomes triboelectrically charged, and the charges are then gravitationally separated in dust storms. We have reproduced one of these experiments and find that triboelectric charging of the Martian regolith simulant by the walls of the vessel used can adequately explain all the effects observed. Our results indicate that unless special care is taken to avoid wall effects, the electrostatic properties of a laboratory system cannot be extrapolated to the Martian environment. We also note that charging of the outside of the vessel used can generate transients within the vessel which could be mistaken for electrical discharge signals, unless accompanied by optical emissions. © 2012 Elsevier Ltd. All rights reserved. Source

Mostl C.,University of California at Berkeley | Mostl C.,University of Graz | Davies J.A.,RAL Space
Solar Physics | Year: 2013

The NASA Solar TErrestrial RElations Observatory (STEREO) mission offered 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 geoeffective 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. (Astrophys. J., 2012, 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 (λ). This is an extension to earlier, more simple models: fixed-Φ fitting (λ=0°) and harmonic mean fitting (λ=90°). In contrast to previous models, this approach has the advantage of allowing one to assess clearly 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-1) and arrival times (up to two days later than the apex). For very wide ICMEs (2λ>120°), the geometric correction becomes very similar to the one derived by Möstl et al. (Astrophys. J. 741, 34, 2011) 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. © 2012 Springer Science+Business Media B.V. Source

Wright I.P.,Open University Milton Keynes | Sheridan S.,Open University Milton Keynes | Morse A.D.,Open University Milton Keynes | Barber S.J.,Open University Milton Keynes | And 5 more authors.
Planetary and Space Science | Year: 2012

The Lunar Volatile Resources Analysis Package (L-VRAP) has been conceived to deliver some of the objectives of the proposed Lunar Lander mission currently being studied by the European Space Agency. The purpose of the mission is to demonstrate and develop capability; the impetus is very much driven by a desire to lay the foundations for future human exploration of the Moon. Thus, L-VRAP has design goals that consider lunar volatiles from the perspective of both their innate scientific interest and also their potential for in situ utilisation as a resource. The device is a dual mass spectrometer system and is capable of meeting the requirements of the mission with respect to detection, quantification and characterisation of volatiles. Through the use of appropriate sampling techniques, volatiles from either the regolith or atmosphere (exosphere) can be analysed. Furthermore, since L-VRAP has the capacity to determine isotopic compositions, it should be possible for the instrument to determine the sources of the volatiles that are found on the Moon (be they lunar per se, extra-lunar, or contaminants imparted by the mission itself). © 2012 Elsevier Ltd. All rights reserved. Source

Mostl C.,University of Graz | Mostl C.,Austrian Academy of Sciences | Rollett T.,University of Graz | Rollett T.,Austrian Academy of Sciences | And 14 more authors.
Astrophysical Journal | Year: 2011

One of the goals of the NASA Solar TErestrial RElations Observatory (STEREO) mission is to study the feasibility of forecasting the direction, arrival time, and internal structure of solar coronal mass ejections (CMEs) from a vantage point outside the Sun-Earth line. Through a case study, we discuss the arrival time calculation of interplanetary CMEs (ICMEs) in the ecliptic plane using data from STEREO/SECCHI at large elongations from the Sun in combination with different geometric assumptions about the ICME front shape [fixed-Φ (FP): a point and harmonic mean (HM): a circle]. These forecasting techniques use single-spacecraft imaging data and are based on the assumption of constant velocity and direction. We show that for the slow (350kms -1) ICME on 2009 February 13-18, observed at quadrature by the two STEREO spacecraft, the results for the arrival time given by the HM approximation are more accurate by 12hr than those for FP in comparison to in situ observations of solar wind plasma and magnetic field parameters by STEREO/IMPACT/PLASTIC, and by 6hr for the arrival time at Venus Express (MAG). We propose that the improvement is directly related to the ICME front shape being more accurately described by HM for an ICME with a low inclination of its symmetry axis to the ecliptic. In this case, the ICME has to be tracked to >30° elongation to obtain arrival time errors < ± 5hr. A newly derived formula for calculating arrival times with the HM method is also useful for a triangulation technique assuming the same geometry. © 2011. The American Astronomical Society. All rights reserved. Source

Xiong M.,Aberystwyth University | Xiong M.,Sigma Group | Davies J.A.,RAL Space | Bisi M.M.,Aberystwyth University | And 3 more authors.
Solar Physics | Year: 2013

Stereoscopic white-light imaging of a large portion of the inner heliosphere has been used to track interplanetary coronal mass ejections. At large elongations from the Sun, the white-light brightness depends on both the local electron density and the efficiency of the Thomson-scattering process. To quantify the effects of the Thomson-scattering geometry, we study an interplanetary shock using forward magnetohydrodynamic simulation and synthetic white-light imaging. Identifiable as an inclined streak of enhanced brightness in a time-elongation map, the travelling shock can be readily imaged by an observer located within a wide range of longitudes in the ecliptic. Different parts of the shock front contribute to the imaged brightness pattern viewed by observers at different longitudes. Moreover, even for an observer located at a fixed longitude, a different part of the shock front will contribute to the imaged brightness at any given time. The observed brightness within each imaging pixel results from a weighted integral along its corresponding ray-path. It is possible to infer the longitudinal location of the shock from the brightness pattern in an optical sky map, based on the east-west asymmetry in its brightness and degree of polarisation. Therefore, measurement of the interplanetary polarised brightness could significantly reduce the ambiguity in performing three-dimensional reconstruction of local electron density from white-light imaging. © 2012 Springer Science+Business Media B.V. Source

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