Magnetic Observations during the Recent Declining Phase of Solar Activity

Changes in the heliosphenc magnetic field during the rcccnt declining phase in solar activity are reviewed and compared with observations during past sunspot cycles. The stud y is based pnncipall y on data obtained by IMP-8 and Ulysses. The field magnitude is found to have increased during the declining phase until it reached a maximum value of 11 .5nT in = 1991.5, approximately two years after sunspot maximum. The dominant polarity of the field as see~l by Ulysses from 1991.8 onward is consistent with the field of the sun’s south pole which became negative after a reversal in early 1990. The sector structure disappeared at Ulysses in April 1993 when the latitude of the spacecraft was -300 revealing a low inclination of the heliosphenc current sheet. The solar wind structure evident in the magnetic field and plasma measurements shows a decline in the number and effectiveness of Coronal Mass Ejections. A large outburst of solar activity in h4arch 1991 caused 4 CMES and numerous shocks at the location of Ulysses. Following a delay of more than a year, a series of recurrent high speed streams and Corotating Interaction Regions commenced in July 1992 which were observed by IMP-8, Ulysses and Voyager 2. In all these respects, the behavior of the magnetic field mimics that seen in the two earlier sunspot cycles. The comprehensive data set suggests a correlation between /B/ and sunspot number. The major solar cycle variations in the radial component (and magnitude) of the field have been successfully reproduced by a recent model consisting of a tilted solar dipole, whose strength and tilt undergo characteristic changes over the sunspot cycle, and the heliosphenc curmt sheet. The large outbursts of activity in mid-1972, mid1982 and the first quarter of 1991 may represent a characteristic last “gasp” of solar activity before the sun evolves to a different state. The recurrent high speed streams in 1973, 1984 and 1992 accompany the development of large asymctncal polar coronal holes and the growth in intensity of the polar cap fields. After they endure for about one year, the polar coronal holes recede and the high speed streams are replaced by weaker streams more characteristic of solar minimum.