THE FALL 2000 AND FALL 2001 SOHO-ULYSSES QUADRATURES

S. SUESSNASA Marshall Space Flight Center/SD50, Hunstville, AL 35812, U.S.A.

G. POLETTOOsservatorio Astrofisico di Arcetri, Firenze, Italy

Abstract. SOHO-Ulysses quadrature occurs when their included angle with the Sun is 90◦. At thesetimes the same plasma leaving the Sun in the direction of Ulysses can first be remotely analyzed withSOHO and then later be sampled in situ at Ulysses. Quadratures in Fall 2000/2001 are of specialinterest because Ulysses will be near the south and north heliographic poles, respectively, and it willbe near sunspot maximum. But, the quadrature geometry is complex – Ulysses is not in a true polarorbit and the orbital speed of Ulysses and SOHO about the Sun will be comparable. In neither caseis true quadrature achieved, but this works to the observer’s advantage. Here we show plots of therelative positions of SOHO and Ulysses throughout the two quadrature intervals.

1. Introduction

The Fall 2000/2001 SOHO-Ulysses quadratures present a unique opportunity be-cause it will be near sunspot maximum (Hathaway et al., 2000) and Ulysses willbe over the poles. SOHO can be used to analyze the same plasma at the Sun thatlater reaches Ulysses (Suess et al., 2000). To do this, a detailed description ofthe positions of Ulysses and SOHO is required. Our purpose here is to give thisdescription.

2. Relative Positions of Ulysses and SOHO

Ulysses’ orbit is inclined at 80◦ to the heliographic equator, with a 5.4 AU aphelion,a 1.34 AU perhelion, and 6.2 year period. The plane of the orbit is essentially fixedin the heliographic-inertial frame of reference. SOHO is at the L1 Lagrangian pointbetween Earth and the Sun, about 1% of the distance towards the Sun (see, e.g.,Figure 1 in Suess et al., 2000). Because of the relative positions of these orbits andthe relatively fast motion of SOHO around the Sun, the SOHO-Sun-Ulysses in-cluded angle is 90◦, or at quadrature, about twice a year. When Ulysses is below 60◦latitude true quadrature is achieved but above 60◦ latitude the included angle maynot reach 90◦ because the orbit is not perpendicular to the equator. Nevertheless werefer to both cases as quadrature.

Space Science Reviews 97: 59–62, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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Figure 1. The Ulysses-SOHO longitudinal angle and Ulysses-Sun-SOHO included angle in 2000 and2001. In 2000 the included angle is between 100◦ and 105◦ from 1 November – 31 December. In2001 it is greater than 75◦ from 1 October – 21 December.

The high latitude Fall 2000 and 2001 quadratures require calculating the in-cluded angle, as seen in Figure 1 where both the longitude difference and includedangle are shown. Quadratures are at A and B, on 14 June and 14 December in2000 and on 30 May and 9 November in 2001. On 14 June 2000 and 30 May 2001the longitude difference is −90◦ (Ulysses off the east limb) and 90◦, respectively,and the included angle is 90◦. Conversely, on 14 December 2000 the longitudedifference is ∼ 160◦ while the included angle is ∼ 100◦. The included angle doesnot fall to 90◦ because Ulysses will be behind the solar south pole. Similarly, on 9November 2001 the longitude difference is 60◦ while the included angle is ∼ 80◦.The included angle does not rise to 90◦ because Ulysses will be in front of thenorth pole. In both years the included angle changes by ∼ 1◦ day−1 in the springbut more slowly in the fall.

Figure 2 shows the position of Ulysses for the year 2000 in coordinates fixedrelative to the SOHO-Sun line. The bottom panels show only the Fall quadratureinterval. The left two panels show that Ulysses is off the east limb for the Springquadrature (A) but appears to hang over the south pole for more than two monthsaround the Fall quadrature. The center two panels show that Ulysses is actuallydropping back towards the Sun. These and the right two panels show that Ulyssesis behind the Sun from spring quadrature to the end of the year. Ulysses appearsto hang over the pole because SOHO, in front of the Sun, is moving west whileUlysess, behind the Sun, is moving to the east. Because Ulysses is actually behindthe pole in Fall 2000 it will be impossible to look directly at its footpoint on the Sun.Instead, understanding what is at the Ulysses footpoint can be done by trackingfeatures in EIT and LASCO to determine which features are behind the limb andwhich are in front. Those moving to the right will be in front of the limb and thosemoving to the left will be behind the limb.

THE FALL 2000 AND 2001 SOHO-ULYSSES QUADRATURES 61

Figure 2. Orbital motion of Ulysses in 2000 in a coordinate system fixed to the Earth (SOHO)-Sunline. E: location of Earth. S: Sun. Days of year 305, 335, and 365 are identified in the bottom threeexpanded-scale panels, where every fifth DoY is marked with an ‘o’.

Figure 3. Orbital motion of Ulysses in 2001 in the same format as Figure 2.

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Figure 3 shows the relative positions and motions of SOHO and Ulysses in 2001,in the coordinates used in Figure 2. The left pair of panels shows that Ulysses neverappears to be directly over the pole. The center pair of panels shows that Ulysseswill be in front of the Sun and, with the left panels, shows that the motion is aspiral rising away from the plane of the ecliptic. The right pair of panels is a viewdown on this spiral and shows that the motion of Ulysses relative to the SOHO-Sunline changes direction slightly more than a month prior to the Fall quadrature. Thischange in direction is the reason Ulysses again appears to hang over the pole duringthe Fall 2001 quadrature. In Fall 2001 Ulysses will always be in front of the limb ofthe Sun. Nevertheless, it will be necessary to continuously follow coronal featuresin EIT and LASCO to distinguish between structures in front of and behind thelimb.

Acknowledgements

The research of STS was partially supported by the Ulysses/SWOOPS and SOHO/UVCS projects. The research of GP was partially supported by the Italian SpaceAgency (ASI). Orbital data was provided by NASA-National Space Science DataCenter.

References

Hathaway, D. H., Wilson, R. M., and Reichmann, E. J.: 1999, ‘A Synthesis of Solar Cycle PredictionTechniques’, J. Geophys. Res. 104, 22 375.

Suess, S. T., Poletto, G., Romoli, M., Neugebauer, M., Goldstein, B. E., and Simnett, G.: 2000, ‘TheMay 1997 SOHO-Ulysses Quadrature’, J. Geophys. Res. 105, 25 033.