J Todd Hoeksema
Wilcox Solar Observatory
Stanford University
The computed location of the heliospheric current sheet (HCS) near the Sun generally evolves slowly in time. The computation, made with a potential field source surface model, is not sensitive to transient events, but does reflect changes in the underlying photospheric field due to active regions.
Daily magnetograms at the Wilcox Solar
Observatory began near solar minimum in
May 1976 and now span 3.5 solar cycles

over 39 years. The figures show for each
Carrington Rotation (CR) the photospheric synoptic charts (left column) and the PFSS computation of the coronal field (right).
The structure in the corona is simple compared with the photosphere.
A BLACK neutral line separates positive
(blue) and negative (red) magnetic field regions. Each panel is two rotations wide centered on the labeled rotation. Green lines indicate 0 and 360 longitude and horizontal lines show the equator and 70 degrees north and south. In each group of
15 rotations the calendar year is indicated in the center at the top and in each
January.
There is far too much information here to provide a complete description, but some gross features to notice are the changes associated with the solar cycle.
At minima in 1976, 1986, 1996, and 2008 the photospheric field is weak and the corona is simple, with the neutral line close to the equator.
Because the polar field in the most recent minimum was
~50% weaker, the HCS was not as flat.

The polar field reverses near solar maxima, as shown most clearly in the coronal field plots. Both field strength and complexity increase toward maximum and gradually decrease during the declining phase.
Despite the sporadic appearance of active regions, the structure of the coronal field computed at 2.5 R s
usually evolves slowly and looks much the same for several rotations or even for a year or more.
Occasionally more dramatic changes occur. These happen when active regions emerge in particular locations that disrupt the existing structure.
Some Examples:
Year Rotation Description
1978 CR 1644 New sector
1979 Most Continuous change
1980 CR 1695 New + sector in S
1981 CR 1712 Decay of structure

1988 CR 1802 Phase change
1989 CR 1818 Reconfiguration
1990 CR 1836 Complexification
1992 CR 1856 Decay of – structure
1994 CR 1887 Hint of new + sector
1998 CR 1940 Emerging/shifting
1998 CR 1949ff Major disruption
2000 CR 1970 Major change
2001 CR 1975 Strong new – sector
2004 CR 2020 – Sector expansion
2005 CR 2036 Minor sector decay
2007 CR 2057 – first emerges in N
2010 CR 2091 New – sector in N
2011 CR 2116 Complexification
2012 CR 2126ff Weakens & changes
2013 CR 2142 Strong – in S
2014 CR 2146 Strong! + Eq 1 st app.*
2014 CR 2154 New – region
*Intriguing! big! strong! enduring! unipolar region
Next steps: associate with active regions, nests, CMEs, flares, Hale boundaries, and subsurface origins.