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Introduction to Special Issue on High Speed Solar Wind Streams and
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Geospace Interactions (HSS-GI).
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M. H. Denton1, J. E. Borovsky2, R. B. Horne3, R. L. McPherron4, S. K. Morley5, and B. T. Tsurutani6.
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Department of Communication Systems, Lancaster University, Lancaster, UK.
Space Science and Applications, Los Alamos National Laboratory, Los Alamos, USA.
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3 British
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4 IGPP,
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Antarctic Survey, Cambridge, UK.
University of California at Los Angeles, Los Angeles, USA.
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of Newcastle, Newcastle, Australia.
Propulsion Laboratory, California Institute of Technology, Pasadena, USA.
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1. Introduction
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This special issue of the Journal of Atmospheric and Solar-Terrestrial Physics is devoted to research into high
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speed solar wind streams (HSSs) and their effects on the region of near-Earth space commonly known as
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‘geospace’. Interest in the effects of HSSs has increased during the last solar cycle and, following the successful
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meeting focusing on corotating solar wind streams in Manaus, Brazil [Tsurutani et al., 2006], we recognised the
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need for further work on the topic, with particular focus on HSSs and their effects in the inner magnetosphere,
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ionosphere, and neutral atmosphere. As a result the High Speed Solar Wind Streams and Geospace Interactions
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(HSS-GI) Workshop was held at Hilltop, St. Martin’s College in Ambleside, UK, from 2 nd to 7th September,
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2007 [Kavanagh and Denton, 2007; Denton et al., 2008], sponsored by the Department of Communication
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Systems at Lancaster University. The majority of the work presented in this special issue was prompted by
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discussion and interaction at the workshop. It is indeed an indication of the importance of HSSs that the papers
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in this issue cover the entire region from the Sun and solar wind, through the magnetosphere, and into the
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ionosphere, thermosphere, and down to the stratosphere. It is hoped that this research will stimulate more
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understanding, appreciation, and research, into these important drivers of physical phenomena within geospace.
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2. High Speed Solar Wind Streams - Recent Progress
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Periodic high speed solar wind streams have been known to exist since research by Snyder et al. [1963], with the
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source of such streams being traced to solar coronal holes by Krieger et al. [1973]. Since this early work,
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further links between HSSs and recurrent activity within geospace have been reported by numerous authors, and
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the relationship between such events and phenomena within the magnetosphere has become increasingly
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apparent. Superposed epoch analyses of the solar wind characteristics of HSSs reveal their broad features, the
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response of geomagnetic indices during their passage of the magnetosphere, and the Russell-McPherron
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dependence of the coupling between the solar wind and the magnetosphere [McPherron et al., 2008].
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As is widely known, HSSs typically produce less significant deflections in geomagnetic indices such as Dst than
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transient phenomena such as coronal mass ejections (CMEs). As a result HSSs are sometimes regarded as less
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important drivers of geomagnetic activity. This has led to many studies, where event-selection is based solely on
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the Dst signature, overlooking HSS-events and hence neglecting their significance. At the workshop, this was
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referred to as ‘the Dst mistake’. Comparisons between HSS-driven events and CME-driven events do reveal
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important differences in their effects within all regions of geospace (e.g. Lindsay et al. [1995]; Borovsky and
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Denton [2006]) and whilst CME-events certainly produce the largest geomagnetic storms, HSS-events are
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particularly important for phenomena such as relativistic electron acceleration, radiation belt enhancements and
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satellite anomalies (e.g. Wrenn et al. [2008]). In terms of the energy input to the magnetosphere, HSSs are
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found to input a total-energy-per-event which is similar in magnitude to that during CMEs; indeed the ratio of
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the energy-deposited-in-the-magnetosphere to the energy-input-to-the-magnetosphere appears to be greater for
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HSSs than for CMEs (e.g. Turner et al. [2006; 2008]; Kozyra et al. [2006]; Guarnieri et al. [2006]).
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It is known that HSSs are particularly important for acceleration and loss processes in the Earth’s radiation belts
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and in recent years much interest in the community has focused on determining the principal acceleration and
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loss mechanisms (e.g. Tsurutani et al. [2006]; Horne, [2007]; Liemohn and Chen, [2007]; Reeves [2008]). An
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important source population for the ring current/radiation belts is the Earth’s plasma sheet and in this issue the
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timescales for delivery of plasma sheet material to the inner magnetosphere during HSSs are evaluated by
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Denton and Borovsky [2008]. Results indicate a super-hot and super-dense plasma sheet forms at the onset of a
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HSS-driven storm and is convected Earthwards in a period of a few hours. Energisation of this plasma sheet
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population during HSSs leads to formation of a enhanced ring current [Jordanova et al., 2008; Sørbø et al.,
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2008] which is subsequently energised leading to enhanced fluxes in the radiation belts. Fluxes within the belts
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may fluctuate rapidly during HSSs revealing the balance between sources and sinks, and also our ability to
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reproduce such fluctuations using theoretical models [Lam et al., 2008]. The derivation of radiation belt radial
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diffusion coefficients during periods of high speed solar wind is discussed by Rae et al. [2008] who also
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examine the solar wind speed dependence of ULF wave power using ground-based magnetometer data.
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Recently, a number of authors have revealed the importance of a 9-day periodicity in the solar wind speed and
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consequences for the lower atmosphere (e.g. Temmer et al., [2007]; Lei et al. [2008]; Mlynczak et al. [2008];
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Thayer et al. [2008]). In this issue Emery et al. [2008] show that the global electron hemispheric power (a proxy
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for the strength of auroral activity) also exhibits a 9-day periodicity. Loss of energetic particles from the
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magnetosphere into the atmosphere induces detectable changes in the Earth’s ionosphere where an
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decrease/increase in spectral hardness is evident during HSSs [Birch et al., 2008]. Such energetic particle
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precipitation (particularly relativistic electron precipitation during HSSs) may induce the generation of NOx
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(odd nitrogen) at lower altitudes, with possible subsequent links to ozone depletion in the stratosphere [Turunen
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et al., 2008].
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The large amplitude Alfvénic fluctuations which occur during HSSs have implications for the generation of
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recurrent substorms [Lyons et al., 2008; Morley et al., 2008], HILDCAAs [Tsurutani and Gonzalez, 1987],
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source and loss processes within the radiation belts [Sandanger et al., 2008], and for resultant auroral activity
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[D’Amicis et al., 2008]. Whilst it is clear that the southward-IMF coupling between the solar wind and the
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magnetosphere during Alfvénic fluctuations is important, it is still not clear whether there is a significant
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difference between the driving of the magnetosphere in an on-off fashion versus a steady average fashion
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[Denton et al., 2008]. In the future we plan to determine whether or not such on-off driving is important.
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Although the papers contained in this special issue highlight the importance of high speed solar wind streams,
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and also reveal how physical processes within geospace respond to HSS-events, it is clear that much remains to
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be discovered about these fascinating phenomena.
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3. Acknowledgements
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The authors would like to thank everyone who helped make the 2007 Ambleside workshop such a successful and
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productive meeting. Thanks are due to the Faculty of Science and Technology, the Photonic Band Gap Research
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Group, and InfoLab21 at Lancaster University, all of whom provided funding in support of the workshop.
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Particular thanks are due to Jill Greenwood and Joanna Denton for assistance in organising the workshop and to
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our hosts at St. Martin’s College in Ambleside. The authors also express their thanks to Meta Ottevanger, Tim
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Horscroft, William Lokto, and the staff at Elsevier for making this special issue possible.
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