Equatorwards Expansion of Unperturbed, High-Latitude Fast Solar Wind
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Equatorwards Expansion of Unperturbed, High-Latitude Fast Solar Wind G.D. Dorrian · A.R. Breen · R.A. Fallows · M.M. Bisi
Received: 22 November 2011 / Accepted: 17 July 2012 / Published online: 23 August 2012 © Springer Science+Business Media B.V. 2012
Abstract 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. Keywords Solar wind · Interplanetary scintillation · Fast Solar Wind · Parker Spiral · Radio observations · Heliosphere · Interplanetary magnetic field · IPS
1. Introduction The rapid variation of the apparent intensity of distant, small-diameter radio sources was first observed in the early 1960s and it was soon realised that observations of this interplanetary scintillation (IPS) could provide information on the solar wind (Hewish, Scott, and Wills, 1964; Dennison and Hewish, 1967). Density irregularities in the solar wind induce phase variations along the raypath of a radio beam as it passes close to the Sun,
A.R. Breen is deceased. Observations and Modelling of the Inner Heliosphere Guest Editors: Mario M. Bisi, Richard A. Harrison, and Noé Lugaz G.D. Dorrian () Institute of Astronomy and Astrophysics, National Observatory of Athens, Athens, Greece e-mail: [email protected] G.D. Dorrian · A.R. Breen · R.A. Fallows · M.M. Bisi Institute of Mathematics and Physics, Aberystwyth University, Aberystwyth, Wales, UK R.A. Fallows ASTRON, The Netherlands Institute for Radio Astronomy, Dwingeloo, The Netherlands
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Table 1 Summary of the ELB observations of IPS discussed in this paper. The P-point is the point of closest approach of the IPS raypath to the Sun. Included is the PoS velocity for each solar wind stream, confirming that it is the fast solar wind which is being sampled in each case. Date
Radio source
Latitude of P-point (heliographic)
Heliocentric distance of P-point (R )
Solar wind PoS velocity at maximum cross-correlation (km s−1 )
15 May 2002
J0319+415
60.0◦ N
82.9
800
12 May 2004
J0319+415
56.2◦ N
84.7
785
13 May 2005
J0319+415
57.4◦ N
84.1
733
10 June 2006
J0521+166
82.5◦ S
24.9
791
J0521+166
80.9◦ S
24.3
842
11 June 2006
leading to rapid variations in the amplitude recorded at an Earth based receiver. Developments in the technique led to the use of two-station observations, in which the scintillation pattern is measured at two widely separated receivers (Armstrong and Coles, 1972). Given that the distance to the radio source is much greater than the distance between the receivers, the two raypaths can be assumed to be parallel. In such a case the correlation between the signals detected at the two sites should be a maximum w
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