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List of Figures

View Image Figure 1:
A sketch of the steady-state solar magnetic field in the ecliptic plane. Close to the Sun, in a spatial region approximately bounding the solar corona, the magnetic field dominates the plasma flow and undergoes significant non-radial (or super-radial) expansion with height. At the source surface, typically taken to be a few solar radii, the pressure-driven expansion of the solar wind dominates and both the field and flow both become purely radial. In the heliosphere, rotation of the HMF footpoints within a radial solar wind flow generates an azimuthal component of the HMF, Bϕ, leading to a spiral geometry. Regions of opposite HMF polarity, shown as red and blue lines, are separated by the heliospheric current sheet (HCS), shown as the green dashed line. Image adapted from Schatten et al. (1969).
View Image Figure 2:
Probability distribution functions of heliospheric magnetic field angles to the radial direction for different solar wind speed intervals. The solid curves show hourly OMNI observations of the near-Earth HMF, covering the period 1965 – 2012. Vertical dashed lines show the equivalent ideal Parker spiral angles for the centre of the speed bins.
View Image Figure 3:
Ideal Parker spiral magnetic field lines between 0 and 25 AU for a solar wind speed of 450 km s–1. Black, blue, and red lines show heliographic latitudes of 0, 30, and 60 degrees, respectively.
View Image Figure 4:
A summary of the Ulysses observations. The white line in the left-hand panel shows the heliographic latitude of the spacecraft, overlaid on the sunspot number. The centre and right-hand columns show latitude-longitude maps of Ulysses scan observations made during the three fast-latitude scans, mapped back to the source surface in the same manner as Jones et al. (2003). The centre column shows magnetic field polarity, with blue/red dots as inward/outward field. The right-hand column shows solar wind speed, with blue through red showing 200 to 800 km s–1. Image adapted from Owens et al. (2011a).
View Image Figure 5:
Maps of the observed (top) and PFSS-reconstructed (middle) in-ecliptic HMF polarity as a function of Carrington longitude and time. Blue/red indicates inward/outward sectors, respectively. The HMF observed in near-Earth space has been ballistically mapped back to 2.5 RS for direct comparison with the PFSS output. The bottom panel shows sunspot number.
View Image Figure 6:
The heliospheric current sheet obtained from a coupled corona-heliosphere MHD simulation (Odstrčil et al., 2004) of Carrington rotation 1912, close to the solar minimum at the start of solar cycle 23. At this time, the HCS was primarily about the heliographic equator, but red/blue colours show warps of the HCS which extend approximately 10 degrees above/below the equator. The thick black line shows Earth’s path through the HCS. Warping of the HCS results in six major HCS crossings during this Carrington rotation. Note that the thinning of the black line in the upper-left corner indicates a period when Earth skims the HCS for an extended period, which may result in numerous HCS crossings from fine scale structure not revealed by these simulation results.
View Image Figure 7:
A sketch of a stream interaction region. Left: Looking down on the ecliptic plane. Magnetic field lines within fast (slow) wind, shown in red (blue), become aligned with the stream interface by the reverse (forward) wave. Right: a view from Earth. The magnetic axis, M, and therefore the wind speed belts, are inclined to the rotation axis, R. The point in the heliosphere at which fast wind is able to catch up to the slow wind ahead of it is the stream interface (SI), which forms a spiral front in the heliosphere, shown as the black-outlined curved surface. In the frame of reference of the SI, both fast and slow wind flow toward the SI. Fast (slow) wind, shown by the red (blue) arrow, is slowed (accelerated) and deflected along the SI in the direction counter to (along) solar rotation. Right panel adapted from Pizzo (1991).
View Image Figure 8:
A corotating interaction region observed by Ulysses just below the ecliptic plane (–20° latitude) at 5.1 AU. Panels, from top to bottom, show: (a) magnetic field intensity, the angle of the magnetic field (b) in the R-T plane (i.e., the plane containing the ideal Parker spiral magnetic field, in this case, close to the ecliptic plane) and (c) out of the R-T plane, (d) the solar wind speed, the angle of the solar wind flow (e) in and (f) out of the R-T plane, (g) the proton density, and (h) proton temperature. The black vertical line shows the stream interface. The red (blue) vertical line shows the reverse (forward) shock propagating into the fast (slow) solar wind behind (ahead) of the SI. The green dashed line shows the location of the heliospheric current sheet.
View Image Figure 9:
A cartoon of the global structure of the heliosphere. The solar wind flows radially away from the Sun. As the flow is supersonic, a termination shock forms inside the heliopause, to slow and deflect the solar wind inside the heliosheath. Outside the heliopause, the very local interstellar medium (VLISM) is deflected around the heliosphere. Depending on the strength and orientation of the magnetic field within the VLISM, this interaction may or may not involve a standing bow shock.
View Image Figure 10:
A sketch of heliospheric magnetic topology inferred from suprathermal electron observations. Left panel: A view of the ecliptic plane, with magnetic field lines shown as black arrows and the anti-sunward suprathermal electron flux shown as red arrows. Right panel: The STE pitch-angle distribution seen by a spacecraft in near-Earth orbit. At (a), the field is part of an inward-polarity sector, so the STE strahl is anti-parallel to the field. Similarly, at (c), the outward sector results in a parallel strahl. At (b), the magnetic field is connected to the Sun at both ends, resulting in both parallel and anti-parallel strahls, or counterstreaming. At (d), there is no solar connection, so no strahl is seen.
View Image Figure 11:
An example of a magnetic cloud, observed by the Wind and ACE spacecraft in August 1998. Approximately 5 days of data are shown, with the MC boundaries shown as the solid vertical lines. The panels, from top to bottom, show: the suprathermal electon pitch-angle distribution, the magnetic field magnitude, the in- and out-of-ecliptic magnetic field angles, the solar wind flow speed, the in- and out-of-ecliptic flow angles, the proton number density, and the proton temperature.
View Image Figure 12:
Sketches of proposed mechanisms for coronal heating and subsequent HMF braiding. On the left, the corona is heated by reconnection between open solar flux and closed loops emerging through the photosphere. In this model, the heliospheric magnetic field is likely to become tangled due to foot point motions. In the right-hand sketch, the corona is heated by waves or turbulence. The heliospheric magnetic field can then become tangled by turbulent motions, either propagating directly from the corona or generated in transit. Image reproduced by permission from Owens et al. (2011b), copyright by Springer.
View Image Figure 13:
The location of the heliospheric current sheet as a function of solar cycle. The grey shaded area shows the latitudinal extent of the HCS estimated by a potential-field source-surface solution to the observed photospheric magnetic field. There is a strong solar cycle variation. Over plotted in red (blue) are the latitudes at which Ulysses encountered unipolar (bipolar) magnetic fields for whole Carrington rotations. Unipolar fields are expected polewards of the HCS, thus the Ulysses observations agree well with the PFSS reconstructions.
View Image Figure 14:
The heliospheric magnetic field during the space age. Top: Carrington-rotation averages (white) and annual averages (red) of the near-Earth HMF scalar magnetic field intensity from the OMNI dataset. The dark background shows the monthly sunspot number, scaled to fit the same axis. Bottom: Carrington-rotation (white) and 1-year (red) averages of 1-AU open solar flux, computed from the 1-day modulus of 1-hour measurements of the near-Earth radial magnetic field.
View Image Figure 15:
The heliospheric magnetic field over the last century. Top: The scalar magnetic field intensity, B. Spacecraft observations are shown in red, with reconstructions from geomagnetic activity data shown in white (Lockwood et al., 2013a,b) and yellow (Svalgaard and Cliver, 2010). Sunspot number is shown as the dark background, scaled to fit the same axes. Bottom: 1-AU open solar flux (OSF), in the same format. Note that the geomagnetic reconstructions of B have been converted to OSF using an observationally constrained non-linear relation.
View Image Figure 16:
Reconstructions of the total OSF (bottom) from 1610 to present. White: The Lockwood et al. (2013a,b) geomagnetic reconstructions shown in Figure 15. Green: Group sunspot number-based reconstructions (see Owens and Lockwood, 2012, for more detail). Blue (red): Cosmogenic isotope reconstructions using 14C (10Be) (see Lockwood and Fröhlich, 2008, for more detail).