SOLAR SECTOR BOUNDARY CROSSING
On Sept. 3rd, Earth will cross a fold in the heliospheric current sheet

On Sept. 3rd, Earth will cross a fold in the heliospheric current sheet--a vast wavy structure in interplanetary space separating regions of opposite magnetic polarity. This is called a "solar sector boundary crossing," and it could trigger geomagnetic activity around Earth's poles. Arctic sky watchers should be alert for auroras when the crossing occurs.

Many people have never heard of the heliospheric current sheet, yet it is one of the biggest things in the solar system–a vast undulating system of electrical currents shaped like the skirt of a ballerina. Earth dips in and out of it all the time. These crossings are called “solar sector boundary crossings,” and they occasionally spark high-latitude aurora.
The heliospheric current sheet separates regions of the solar wind where the magnetic field points toward or away from the Sun. The complex field structure in the photosphere simplifies with increasing height in the corona until a single line separates the two polarities at about 2.5 solar radii. That line is drawn out by the radially accelerating solar wind to form a surface similar to the one shown in this idealized picture. The surface is curved because the underlying magnetic pattern rotates every 27 days with the Sun.
It would take about 3 weeks for material near the current sheet traveling at 400 km/s in the solar wind to reach the orbit of Jupiter, as depicted here. In reality, the surface becomes increasingly distorted because of variations in the solar wind speed along the surface and other dynamic effects operating in the interplanetary medium.
The shape of the current sheet usually evolves slowly – over months – as the large-scale pattern of the Sun’s field changes in response to the emergence and decay of solar active regions. Coronal mass ejections often disrupt the background pattern temporarily, but sometimes the changes are permanent.
During most of the solar cycle, the source of the heliospheric current sheet resembles a slightly tilted dipole with varying degrees of quadrupole distortion. Near solar maximum the polar dipole decays, leaving a much more complicated structure. This picture shows the heliospheric current sheet as it might appear during the rising phase of the cycle, when the dipole and quadrupole components are balanced; at this point the neutral line at the base of the sheet resembles the seam on a baseball.
Prof. John M. Wilcox was one of the discoverers of the heliospheric current sheet and did much to develop our understanding of it during the 1960s and 1970s. He worked with NASA artist Werner Heil to create this picture, first published in Science in 1980. An earlier 3D rendering of the HCS appeared in a 1976 paper by L. Svalgaard and J.M. Wilcox in Nature. The shape for this image was inferred from observations of the corona and solar wind and also based on coronal field models like those of Wilcox’s former student, Ken Schatten.

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