SOHO-Ulysses quadratures are explained by this figure which shows the
orbits of Ulysses and SOHO/Earth.  SOHO orbits the Sun with the Earth
so no distinction is made between the two. Ulysses' orbit is inclined
to the heliographic equator by 80 degrees, has an aphelion of 5.4 AU,
a perihelion of 1.34 AU, and is essentially stationary with respect
to the Sun. Since it takes the Earth and SOHO 1 year to circumnavigate
the Sun, the included angle between Ulysses and SOHO, as measured with
respect to the Sun, is generally 90 degrees twice a year.
The locations where this occurs in the SOHO orbit are marked here
with triangles and these times are when Ulysses and SOHO are in
quadrature.  The May 1997 quadrature geometry is indicated with the
red lines/triangle and the December 1998 quadrature geometry is
indicated with the blue lines/triangle.  In May 1997 SOHO was on the
near side of the Sun, at the upward pointing red triangle, observing
10 degrees north of the equator off the east limb.  Ulysses was at
5.1 AU, 10 degrees north of the equator on the east limb.  In December
1998 SOHO was on the far side of the Sun, at the downward pointing
blue triangle, observing 17 degrees south of the equator off the
west limb.  Ulysses was at 5.4 AU, 17 degrees south of the equator
on the west limb.  These configurations give us an opportunity to
sample the same plasma parcel first in the corona and later in situ
at Ulysses, relating the physical conditions and processes at the
two locations.  We give in the following an example of the kinds of
studies/results one can obtain with this research.
(Suess et al., JGR, 105 (A11), 25033-25051, 2000).