S. A. Pope

The University of Sheffield, Sheffield, England, United Kingdom

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Publications (19)69.87 Total impact

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    ABSTRACT: Low frequency waves in the foot of a supercritical quasi-perpendicular shock front have been observed since the very early in situ observations of the terrestrial bow shock (Guha et al., 1972). The great attention that has been devoted to these type of waves since the first observations is explained by the key role attributed to them in the processes of energy redistribution in the shock front by various theoretical models. In some models, these waves play the role of the intermediator between the ions and electrons. It is assumed that they are generated by plasma instability that exist due to the counter-streaming flows of incident and reflected ions. In the second type of models, these waves result from the evolution of the shock front itself in the quasi-periodic process of steepening and overturning of the magnetic ramp. However, the range of the observed frequencies in the spacecraft frame are not enough to distinguish the origin of the observed waves. It also requires the determination of the wave vectors and the plasma frame frequencies. Multipoint measurements within the wave coherence length are needed for an ambiguous determination of the wave vectors. In the main multi-point missions such as ISEE, AMPTE, Cluster and THEMIS, the spacecraft separation is too large for such a wave vector determination and therefore only very few case studies are published (mainly for AMPTE UKS AMPTE IRM pair). Here we present the observations of upstream low frequency waves by the Cluster spacecraft which took place on 19 February 2002. The spacecraft separation during the crossing of the bow shock was small enough to determine the wave vectors and allowed the identification of the plasma wave dispersion relation for the observed waves. Presented results are compared with whistler wave dispersion and it is shown that contrary to previous studies based on the AMPTE data, the phase velocity in the shock frame is directed downstream. The consequences of this finding for both types of models that were developed to explain the generation of these waves are discussed.
    Annales Geophysicae 08/2013; 31(8):1387-1395. DOI:10.5194/angeo-31-1387-2013 · 1.68 Impact Factor
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    ABSTRACT: The magnetic field experiment onboard Venus Express presents a new age in the accurate measurement of the magnetic field in space. This paper highlights the major stages in going from an unclean data set (i.e. one that is contaminated with a large spacecraft generated field) to a usable data set for scientific analysis. The evolution of these methods over the 5 years from pre-launch to the current state of the experiment is presented.
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    ABSTRACT: Typically multi-spacecraft missions are ideally suited to the study of shock spatial scales due to the separation of temporal and spatial variations. These missions are not possible at all locations and therefore in-situ multi-spacecraft measurements are not available beyond the Earth. The present paper presents a study of shock spatial scales using single spacecraft measurements made by the Venus Express spacecraft. The scales are determined based on previous knowledge of shock overshoot scales measured by the ISEE and Cluster missions. The study encompasses around 60 crossings of the Venusian bow shock from 2006 to 2009. The statistical relationship between the shock ramp spatial scales, overshoot and upstream shock parameters are investigated. We find that despite somewhat different solar wind conditions our results are comparable with those based on multi-spacecraft missions at the terrestrial bow shock.
    Annales Geophysicae 11/2011; 29(11):2081-2088. DOI:10.5194/angeo-29-2081-2011 · 1.68 Impact Factor
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    ABSTRACT: In situ measurements of the magnetic field are vital to the study of many fundamental problems in planetary research. Therefore the magnetometer experiment is a key element of the payload of Venus Express. In addition to the interaction of the solar wind with Venus, these measurements are crucial for the study of atmospheric escape and detection of lightning. However, the methodology for the magnetic field measurements had to be different to the traditional approach, because Venus Express is not a magnetically clean spacecraft. A technique based on two-point simultaneous measurements of the magnetic field and systems identification software is used to separate the natural magnetic field from the spacecraft generated interference. In this paper an overview of the techniques developed to separate these two field types and the results achieved for 1 Hz Venus Express data are presented. Previous publications suggest that the resulting Venus Express cleaned data is of comparable quality to measurements made from onboard magnetically clean spacecraft (Zhang et al., 2008a, b; Slavin et al., 2009).
    Annales Geophysicae 01/2011; 29:639-647. DOI:10.5194/angeo-29-639-2011 · 1.68 Impact Factor
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    ABSTRACT: Within the Venusian magnetosheath, Venus Express has observed the existence of high-amplitude, nonlinear waves which have been interpreted as large-scale nonlinear rotational structures within the magnetic field. A number of mechanisms have been proposed that can be used to explain their origin. Venus Express data have been searched for other examples of the similar structures. It appears that such structures are quite rare, being observed during only 8 of the dayside magnetosheath crossings among 676 that took place between April 2006 and February 2008. All together, only 50 such waves have been identified during these 8 crossings. The properties of these structures were studied. After considering a number of possible generation mechanisms, it is concluded that the most probable generation mechanism is the Kelvin-Helmholtz Instability; however, other mechanisms such as shock-related processes cannot be completely discounted.
    Journal of Geophysical Research Atmospheres 01/2011; 116. DOI:10.1029/2010JA015916 · 3.44 Impact Factor
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    ABSTRACT: Venus Express is a magnetically unclean spacecraft. Thus far only low-resolution (1Hz) data has been successfully cleaned of spacecraft disturbances. New software has been developed specifically for high-resolution (32Hz) data cleaning where spacecraft events are identified via a statistical approach and field corrections are implemented by a combination of empirical and statistical models. This poster gives a summary of the cleaning methodology. The cleaned high-resolution data is used to examine the rotation of the magnetic field observed during several large vortices in the Venusian magnetosheath. During an encounter with the vortex three types of rotations are observed, namely gradual rotation, reverse rotation and a rapid rotation. These characteristics are investigated in greater detail.
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    ABSTRACT: The interaction of the solar wind with Venus has a significant influence on the evolution of its atmosphere. Due to the lack of an intrinsic planetary magnetic field, there is direct contact between the fast flowing solar wind and the Venusian ionosphere. This leads to a number of different types of atmospheric escape process. Using Venus Express observations, we show that such contact leads to the formation of global vortices downstream of the Venusian bow shock. These vortices accelerate heavy ionospheric ions such as oxygen, leading to their escape. We argue that these vortices are the result of the Kelvin-Helmholtz instability excited by the shear velocity profile at the boundary between the solar wind and ionospheric plasma. These vortices also help to repopulate the night-side ionosphere during solar minimum, when the ionospheric flow from day to night is restricted by the lowered ionopause altitude at the terminator.
    Geophysical Research Letters 04/2009; 36(7). DOI:10.1029/2008GL036977 · 4.46 Impact Factor
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    ABSTRACT: The solar wind interacts directly with the ionosphere of Venus. This is due to the lack of an intrinsic planetary magnetic field. This is significantly different to the terrestrial case where the ionosphere is protected by the Earth's magnetic field. The shear velocity profile at the boundary between the solar wind and Venusian ionosphere can lead to the formation of nonlinear waves along the boundary. High temporal resolution (32Hz) magnetic field data collected by Venus Express is used to analyse the structure, location and rate of occurrence of nonlinear waves on the ionopause. The implications of these observations, with respect to mass loading of the solar wind with ionospheric material and the redistribution of dayside ionospheric plasma to maintain the nightside ionosphere are discussed.
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    ABSTRACT: Although there is no intrinsic magnetic field at Venus, the convected interplanetary magnetic field piles up to form a magnetic barrier in the dayside inner magnetosheath. In analogy to the Earth's magnetosphere, the magnetic barrier acts as an induced magnetosphere on the dayside and hence as the obstacle to the solar wind. It consists of regions near the planet and its wake for which the magnetic pressure dominates all other pressure contributions. The initial survey performed with the Venus Express magnetic field data indicates a well-defined boundary at the top of the magnetic barrier region. It is clearly identified by a sudden drop in magnetosheath wave activity, and an abrupt and pronounced field draping. It marks the outer boundary of the induced magnetosphere at Venus, and we adopt the name “magnetopause” to address it. The magnitude of the draped field in the inner magnetosheath gradually increases and the magnetopause appears to show no signature in the field strength. This is consistent with PVO observations at solar maximum. A preliminary survey of the 2006 magnetic field data confirms the early PVO radio occultation observations that the ionopause stands at ∼250 km altitude across the entire dayside at solar minimum. The altitude of the magnetopause is much lower than at solar maximum, due to the reduced altitude of the ionopause at large solar zenith angles and the magnetization of the ionosphere. The position of the magnetopause at solar minimum is coincident with the ionopause in the subsolar region. This indicates a sinking of the magnetic barrier into the ionosphere. Nevertheless, it appears that the thickness of the magnetic barrier remains the same at both solar minimum and maximum. We have found that the ionosphere is magnetized ∼95% of the time at solar minimum, compared with 15% at solar maximum. For the 5% when the ionosphere is un-magnetized at solar minimum, the ionopause occurs at a higher location typically only seen during solar maximum conditions. These have all occurred during extreme solar conditions.
    Planetary and Space Science 05/2008; 56(6-56):790-795. DOI:10.1016/j.pss.2007.10.013 · 1.63 Impact Factor
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    ABSTRACT: In this study, magnetic field measurements obtained by the Venus Express spacecraft are used to determine the bow shock position at solar minimum. The best fit of bow shock location from solar zenith angle 20-120 degrees gives a terminator bow shock location of 2.14 R(V) (1 R(V) = 6052 km) which is 1600 km closer to Venus than the 2.40 R(V) determined during solar maximum conditions, a clear indication of the solar cycle variation of the Venus bow shock location. The best fit to the subsolar bow shock is 1.32 R(V), with the bow shock completely detached. Finally, a global bow shock model at solar minimum is constructed based on our best-fit empirical bow shock in the sunlit hemisphere and an asymptotic limit of the distant bow shock which is a Mach cone under typical Mach number of 5.5 at solar minimum. We also describe our approach to making the measurements and processing the data in a challenging magnetic cleanliness environment. An initial evaluation of the accuracy of measurements shows that the data are of a quality comparable to magnetic field measurements made onboard magnetically clean spacecraft. (c) 2008 Elsevier Ltd. All rights reserved.
    Planetary and Space Science 05/2008; 56(6-6):785-789. DOI:10.1016/j.pss.2007.09.012 · 1.63 Impact Factor
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    ABSTRACT: [1] Magnetometer data from two Venus years of the Venus Express mission in orbit are investigated for the occurrence of ion cyclotron waves. Proton cyclotron waves were recently detected in the upstream region of Venus by the spacecraft, indicating pickup of planetary protons from Venus's exosphere by the solar wind and loss of hydrogen to interplanetary space. A study of representative cases illustrates the waveform, spectrum, duration, and higher-order resonances of the transverse waves with left-hand circular polarization and propagation nearly along the magnetic field; their properties in the magnetic field principal axes system are determined. A statistical approach studies the wave properties as a function of the angle between the solar wind and magnetic field direction, as a function of their occurrence in space, and with respect to the motional solar wind electric field. Proton cyclotron waves are found up to 9 Venus radii from the planet, for a large range of angles between the solar wind and magnetic field direction, independent from foreshock geometry and independent from the direction of the motional electric field. This reveals that cyclotron wave generation from local pickup of neutral hydrogen is efficient over a large volume of space upstream of the planet and imposes the existence of an extended reservoir of planetary neutral hydrogen at Venus.
    Journal of Geophysical Research Atmospheres 01/2008; 113. DOI:10.1029/2008je003148 · 3.44 Impact Factor
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    ABSTRACT: Collisionless shocks are present in the vicinity of many astrophysical objects such as supernova remnants, space jets, stars and planets immersed in the supersonic flow of stellar winds. Understanding the shock structure is crucial for understanding the processes of the redistribution of the upstream flow energy into accelerated particles and formation of downstream thermalized distribution. We report first observations (by Venus Express) of subcritical shocks that do not fit into the well-established classical structure classification. It is shown that its abnormal structure is due to kinematical collisionless relaxation of downstream ions. The spatial gyrophase mixing leads to formation of a downstream thermalized distribution, instead of various instabilities. This type of subcritical shock with kinematic relaxation has never been discussed before in theoretical models (e.g., C. F. Kennel et al., 1985).
    Geophysical Research Letters 01/2008; 35(1). DOI:10.1029/2007GL032495 · 4.46 Impact Factor
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    ABSTRACT: On 10-11 September 2006 the Venus Express magnetometer detected a very strong Interplanetary Coronal Mass Ejection (ICME) event with an average field about 2 times higher than that of a typical ICME at 0.72 AU. While the effective obstacle to the solar wind is compressed to a smaller dimension during this ICME event, the bow shock is located far upstream of its nominal location. The observed shocks are weak and appear very dynamic. The location of the shock crossing can be found all along the Venus Express trajectory, which has an apocenter of 12 R V . We attribute the atypical distant bow shock location as an effect of the extremely low Mach number during the ICME.
    Journal of Geophysical Research Atmospheres 01/2008; 113. DOI:10.1029/2008JE003128 · 3.44 Impact Factor
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    ABSTRACT: Collisionless shocks are present in the vicinity of many astrophysical objectssuch as supernova remnants, space jets, stars and planets immersed in the supersonic flow of star winds. Collisionless shocks are the most powerful cosmic rays accelerators and are responsible for the emission of gamma-ray bursts. Understanding of the shock structure is crucial for understanding of the processes of the redistribution of the upstream flow energy into accelerated particles and formation of downstream thermalised distribution. We report first observations (by Venus Express) of subcritical shocks that does not fit the well-established classical structure classification. It is shown that its abnormal structure is due to kinematic collisionless relaxation of downstream ions. The spatial gyrophase mixing leads to the formation of a downstream thermalised distribution, instead of various instabilities.This type of subcritical shock with kinematic relaxation was never discussed before in theoretical models.
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    ABSTRACT: Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.
    Nature 12/2007; 450(7170):654-6. DOI:10.1038/nature06026 · 42.35 Impact Factor
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    ABSTRACT: The interaction of the solar wind with Venus differs from the terrestrial case because Venus does not posses intrinsic magnetic field. This leads to the direct interaction between the fast flowing solar wind and the Venusian ionosphere at the ionopause. Venus Express(VEX) magnetometer data show nonlinear steepened surface waves at the ionopause. In some crossings it shows that nonlinear evolution of these waves leads to the formation of vortices. We argue that these waves and vortex structures are the result of the Kelvin-Helmholtz instability excited due to the shear velocity profile at the ionopause. Analysis of VEX observations show that these vortices should contribute to the ionospheric and atmospheric escape and the formation of plasma clouds observed in previous experiments.
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    ABSTRACT: It is a challenging problem to make accurate in situ measurements of the local magnetic field vector from a non-magnetically clean spacecraft. It is shown how fuzzy logic and nonlinear identification techniques can be applied to two point measurements to identify and correct for spacecraft contributions in the measured field vector. The techniques developed have been applied to data returned from the VEXMAG instrument onboard Venus Express.
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    ABSTRACT: It is extremely important to have magnetometer experiments on planetary spacecraft in particular to study the interaction between a planet and the solar wind In general this would require a magnetically clean spacecraft However for some planetary missions this requirement cannot be fulfilled for a number of reasons such as limited resources As a consequence any magnetic field measurements made will contain interference from the spacecraft This is the case with Venus Express and an automated method that allows the natural magnetic field to be distinguished from the spacecraft induced interference using two point measurements will be presented
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    ABSTRACT: Planetary missions often involve spacecraft that are not magnetically clean. The Venus EXpress (VEX) mission, that is going to be launched in October 2005 is one such spacecraft. VEX will utilise two point magnetic field measurements to distinguish between natural and spacecraft contributions to the magnetic field measurements. It is shown how neural networks can be employed to determine the natural magnetic field using two sets of data measured at different distances from the spacecraft.