3.2. Spiral-shaped Interplanetary Magnetic Field (~ 108 km; ~ 10-6 pc) (~ 10-5 Gauss)
Figure 5 shows a sketch of the interplanetary magnetic field, as measured between the planets by the spacecrafts launched from Earth. The interplanetary magnetic field takes its source from the Sun. As the Sun rotates, the escaping magnetic field takes a spiral shape - also called Archimedean, akin to a water jet escaping from a rotating carrousel.
Figure 5. Interplanetary magnetic field, as originating in the Sun and as deformed by the effects of the solar rotation and solar wind.
The interplanetary magnetic field strength is about 50 microGauss near the Earth (1 AU from the Sun), and near 20 microGauss at a distance of 2 AU from the Sun. The solar wind near 2 AU is about 1 proton / cm3, and flows at ~ 300 km/s (e.g., Baumgärtel et al. 1994). The solar wind has a permanent slow component and an irregular gusty fast component (up to ~ 800 km/s). The slow component seems to come from long narrow structures towering over the arched magnetic fields of the streamer belt near the solar equator. The fast component emerges from isolated patches of open field lines over most of the solar surface and including coronal holes near the two solar poles, allowing the hot gas to rush out unimpeded (e.g., Glanz 1997).
The Ulysses spacecraft, going out of the ecliptic plane after an encounter with Jupiter, was deflected into a polar orbit around the Sun. It confirmed that the overall interplanetary magnetic field was dipolar, with the same polarity as that at the Sun's surface outward for both the Northern hemisphere of the Sun and for the interplanetary space; inward for both the Southern hemisphere of the Sun and for the interplanetary space, in 1995 (e.g., Forsyth et al. 1996). The solar equator is inclined by 7° from the ecliptic plane (Earth's orbital plane), so the Earth rises above and below the solar equator.
The interplanetary magnetic field lines with their origin in the Sun are swept out by the solar wind. This sweeping creates two broad regions of opposite magnetic polarity (outward for 180°, inward for 180°) in solar longitudes, separated by a current 'sheet' (often warped) near the plane of the solar equator (latitude 0°).
There is a single warp of the equatorial sheet. The Ulysses spacecraft, while in the ecliptic plane, observed the current 'sheet' lying close to the equator, but being warped up to +20° and down to -20° in solar latitudes, much like a double-peaked cosine function as one goes around the solar equatorial longitudes (e.g., Forsyth et al. 1996). Near the equator, outward magnetic field lines were found in 1995 near solar longitudes 285° and 105° (due to a current 'sheet' warped down to ~ - 20° latitude); inward magnetic field lines were found near longitudes 195° and 15° (due to a current 'sheet' warped up to ~ + 20° latitude). Thus four 'magnetic sectors' were encountered in the equatorial plane going around the solar longitudes. Hence as the Earth orbits in a year around the Sun, it encounters an outward-going interplanetary magnetic field (for ~ 3 months), then an inward-going field (for ~ 3 months), then an outward-going field (for ~ 3 months), and finally an inward-going field (for ~ 3 months).
Figure 6 shows a simplified sketch of the heliospheric magnetism in 1995, as seen by Ulysses. As the Earth orbits a full circle around the Sun, these four interplanetary magnetic sectors are crossed, leading to observational effects on Earth such as geomagnetic storms (e.g., Parker 1958; Vallée 1969; Lapointe and Vallée 1970; Vallée 1982). Geomagnetic activity can thus change significantly, depending on which magnetic sector the Earth is in (i.e., depending on the number of sunspots and the particle-emitting activity of the sunspots, in that magnetic sector).
Figure 6. A sketch of the heliospheric magnetic field in 1995, with the location of the neutral sheet (double-cosine curve, also shown dashed on the back side of the Sun), adapted from Forsyth et al. (1996). When moving along the equatorial plane (dots), one encounters 4 magnetic polarities, hence 4 magnetic sectors in interplanetary space due to the solar wind.