MACROSCALE COHERENCE OF THE HELIOSPHERIC CURRENT SHEET - PIONEERS 10 AND 11 COMPARISONS

We use the near radial alignments of Pioneers 10 and 11 during 1974 to study the macroscale geometry of the heliospheric current sheet (HCS) . The interval of near alignment gave eight analyzable cases of encoun ters of both spacecraft with the same HCS and one case in which the IM P and Pioneer 11 spacecraft, while nearly radially aligned, encountere d the same current sheet. The degree of macroscale coherence of the HC S was judged by comparing observed solar wind speeds against solar win d speeds calculated on the bases of HCS encounter times and ideal Park er spiral geometry. The correlation coefficient between the two sets o f speeds is 0.53. The difference between the calculated and observed s peeds can be understood in terms of observed deviations from ideal spi ral geometry in the ecliptic plane or in terms of typical corrections to the calculations from small latitudinal factors. One case, however, defies explanation in these terms. This range of behaviour demonstrat es that the HCS is a useful probe of heliospheric dynamics.