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Dive into the research topics where Gareth D. Dorrian is active.

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Featured researches published by Gareth D. Dorrian.


The Astrophysical Journal | 2012

USE OF INCIDENT AND REFLECTED SOLAR PARTICLE BEAMS TO TRACE THE TOPOLOGY OF MAGNETIC CLOUDS

Lun C. Tan; Donald V. Reames; Chee K. Ng; Linghua Wang; Gareth D. Dorrian

Occasionally, large solar energetic particle (SEP) events occur inside magnetic clouds (MCs). In this work, the onset time analysis, the peak intensity analysis, and the decay phase analysis of SEPs are used to investigate two large SEP events inside MCs: the 1998 May 2 and 2002 April 21 events. The onset time analysis of non-relativistic electrons and ∼MeV nucleon −1 heavy ions shows the stability of the magnetic loop structure during a period of a few hours in the events examined. The joint analysis of pitch-angle distributions and peak intensities of electrons exhibits that, depending on the particle pitch angle observed at 1 AU, in the April event the reflection point of particles may be distributed along a wide spatial range, implying that the magnetic loop is a magnetic bottle connected to the Sun with both legs. In contrast, in the May event particle reflection occurs abruptly at the magnetic mirror formed by a compressed field enhancement behind the interplanetary shock, consistent with its open field line topology.


The Astrophysical Journal | 2008

The Solar Eruption of 2005 May 13 and Its Effects: Long-Baseline Interplanetary Scintillation Observations of the Earth-Directed Coronal Mass Ejection

A. R. Breen; R. A. Fallows; M. M. Bisi; R. A. Jones; Bernard V. Jackson; M. Kojima; Gareth D. Dorrian; H. R. Middleton; P. Thomasson; Gudmund Wannberg

Long-baseline observations of interplanetary scintillation (IPS) provide a unique source of information on solar wind speed and meridional direction across the inner regions of the solar system. We report the results of a series of coordinated IPS observations of an Earth-directed CME. A significant development in the interpretation of these data is the use of 3D tomographic reconstructions of solar wind structure derived from STELab IPS data to better constrain the analysis of extremely long baseline observations from EISCAT and MERLIN. The combination of these two approaches leads to a significantly better understanding of the interaction of the CME with the background solar wind than would be possible with either technique alone, revealing a significant rotation in the meridional flow direction of the background wind associated with the passage of the CME. The CME itself is decelerated significantly between its emergence through the corona and its arrival in the IPS ray path, with comparatively little change in speed from then until arrival at ACE.


Solar Physics | 2013

Effects of Thomson-scattering geometry on white-light imaging of an interplanetary shock:synthetic observations from forward magnetohydrodynamic modelling

Ming Xiong; J. A. Davies; M. M. Bisi; M. J. Owens; R. A. Fallows; Gareth D. Dorrian

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.


Geophysical Research Letters | 2008

Simultaneous interplanetary scintillation and Heliospheric Imager observations of a coronal mass ejection

Gareth D. Dorrian; A. R. Breen; Daniel Stephen Brown; J. A. Davies; R. A. Fallows; A. P. Rouillard

We describe simultaneous Interplanetary Scintillation (IPS) and STEREO Heliospheric Imager (HI) observations of a coronal mass ejection (CME) on 16 May 2007. Strong CME signatures were present throughout the IPS observation. The IPS raypath lay within the field-of-view of HI-1 on STEREO-A and comparison of the observations shows that the IPS measurements came from a region within a faint CME front observed by HI-1A. This front may represent the merging of two converging CMEs. Plane-of- sky velocity estimates based on time-height plots of the two converging CME structures were 325 kmsﰀ1 and 550 kmsﰀ1 for the leading and trailing fronts respectively. The plane- of-sky velocities determined from IPS ranged from 420 ± 10 kmsﰀ1 to 520 ± 20 kmsﰀ1. IPS results reveal the presence of micro-structure within the CME front which may represent interaction between the two separate CME events. This is the first time that it has been possible to interpret IPS observations of small-scale structure within an interplanetary CME in terms of the global structure of the event.


Solar Physics | 2010

Transient Structures and Stream Interaction Regions in the Solar Wind: Results from EISCAT Interplanetary Scintillation, STEREO HI and Venus Express ASPERA-4 Measurements

Gareth D. Dorrian; A. R. Breen; J. A. Davies; Alexis Rouillard; R. A. Fallows; Ian Whittaker; Daniel Stephen Brown; Richard A. Harrison; C. J. Davis; M. Grande

We discuss the detection and evolution of a complex series of transient and quasi-static solar-wind structures in the days following the well-known comet 2P/Encke tail disconnection event in April 2007. The evolution of transient solar-wind structures ranging in size from <105 km to >106 km was characterised using one-minute time resolution observation of Interplanetary Scintillation (IPS) made using the European Incoherent SCATter (EISCAT) radar system. Simultaneously, the global structure and evolution of these features was characterised by the Heliospheric Imagers (HI) on the Solar TERrestrial RElations Observatory (STEREO) spacecraft, placing the IPS observations in context. Of particular interest was the observation of one transient in the slow wind, apparently being swept up and entrained by a Stream Interaction Region (SIR). The SIR itself was later detected in-situ at Venus by the Analyser of Space Plasma and Energetic Atoms (ASPERA-4) instrument on the Venus Express (VEX) spacecraft. The availability of such diverse data sources over a range of different time resolutions enables us to develop a global picture of these complex events that would not have been possible if these instruments were used in isolation. We suggest that the range of solar-wind transients discussed here may be the interplanetary counterparts of transient structures previously reported from coronagraph observations and are likely to correspond to transient magnetic structures reported in in-situ measurements in interplanetary space. The results reported here also provide the first indication of heliocentric distances at which transients become entrained.


Journal of Atmospheric and Solar-Terrestrial Physics | 2011

Forward modelling to determine the observational signatures of white-light imaging and interplanetary scintillation for the propagation of an interplanetary shock in the ecliptic plane

Ming Xiong; A. R. Breen; M. M. Bisi; M. J. Owens; R. A. Fallows; Gareth D. Dorrian; J. A. Davies; P. Thomasson

Recent coordinated observations of interplanetary scintillation (IPS) from the EISCAT, MERLIN, and STELab, and stereoscopic white-light imaging from the two heliospheric imagers (HIs) onboard the twin STEREO spacecraft are significant to continuously track the propagation and evolution of solar eruptions throughout interplanetary space. In order to obtain a better understanding of the observational signatures in these two remote-sensing techniques, the magnetohydrodynamics of the macro-scale interplanetary disturbance and the radio-wave scattering of the micro-scale electron-density fluctuation are coupled and investigated using a newly constructed multi-scale numerical model. This model is then applied to a case of an interplanetary shock propagation within the ecliptic plane. The shock could be nearly invisible to an HI, once entering the Thomson-scattering sphere of the HI. The asymmetry in the optical images between the western and eastern HIs suggests the shock propagation off the Sun–Earth line. Meanwhile, an IPS signal, strongly dependent on the local electron density, is insensitive to the density cavity far downstream of the shock front. When this cavity (or the shock nose) is cut through by an IPS ray-path, a single speed component at the flank (or the nose) of the shock can be recorded; when an IPS ray-path penetrates the sheath between the shock nose and this cavity, two speed components at the sheath and flank can be detected. Moreover, once a shock front touches an IPS ray-path, the derived position and speed at the irregularity source of this IPS signal, together with an assumption of a radial and constant propagation of the shock, can be used to estimate the later appearance of the shock front in the elongation of the HI field of view. The results of synthetic measurements from forward modelling are helpful in inferring the in-situ properties of coronal mass ejection from real observational data via an inverse approach.


Proceedings of SPIE | 2007

Combined STELab, EISCAT, ESR, and MERLIN IPS observations of the solar wind

M. M. Bisi; Bernard V. Jackson; R. A. Fallows; Andrew Robert Breen; P. Paul Hick; Gudmund Wannberg; P. Thomasson; C. A. Jordan; Gareth D. Dorrian

The technique of interplanetary scintillation (IPS) can be used to probe interplanetary space between the Sun and Earth most-commonly in terms of speed and also by using the scintillation-level (g-level) as a proxy for density. We combine the large spatial-scale 3D tomographic techniques previously only applied to IPS data from the Solar Terrestrial Environment Laboratory (STELab) array, Nagoya University in Japan, and the previously operational Cambridge IPS system in England, with the finer-scale capabilities of the longer baselines between the systems of the Multi-Element Radio-Linked Interferometer Network (MERLIN) in the UK, and the European Incoherent SCATter (EISCAT) radar and the EISCAT Svalbard Radar (ESR) in northern Scandinavia. Using the UCSD 3D reconstruction technique, we present results of detailed measurements of speed in the solar wind and also those of solar wind flow-directions, constrained by the large-scale density tomography through the use of a kinematic model, as well as applying this tomographic technique for the first time to the MERLIN, EISCAT, and ESR IPS solar wind speed observations in terms of velocity.


Solar Physics | 2013

Equatorwards Expansion of Unperturbed, High-Latitude Fast Solar Wind

Gareth D. Dorrian; A. R. Breen; R. A. Fallows; M. M. Bisi

We use dual-site radio observations of interplanetary scintillation (IPS) with extremely long baselines (ELB) to examine meridional flow characteristics of the ambient fast solar wind at plane-of-sky heliocentric distances of 24 – 85 solar radii (R⊙). Our results demonstrate an equatorwards deviation of 3 – 4∘ in the bulk fast solar wind flow direction over both northern and southern solar hemispheres during different times in the declining phase of Solar Cycle 23.


Solar Physics | 2009

Two Years of the STEREO Heliospheric Imagers

Richard A. Harrison; J. A. Davies; A. P. Rouillard; C. J. Davis; C. J. Eyles; Danielle Bewsher; S. R. Crothers; Russell A. Howard; N. R. Sheeley; Angelos Vourlidas; David F. Webb; Daniel Stephen Brown; Gareth D. Dorrian


Annales Geophysicae | 2008

Developments in the use of EISCAT for interplanetary scintillation

R. A. Fallows; A. R. Breen; Gareth D. Dorrian

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A. R. Breen

Aberystwyth University

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M. M. Bisi

Aberystwyth University

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J. A. Davies

Science and Technology Facilities Council

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P. Thomasson

University of Manchester

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A. P. Rouillard

University of Southampton

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R. A. Jones

Aberystwyth University

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