Science Highlights from the First Solar Orbit
An obvious question to be asked is: do the data from the first polar passes reveal significant north-south asymmetries? This is clearly one aspect where the out-of-ecliptic studies would have benefited from the simultaneous observations available with the original two-spacecraft mission. Nevertheless, the rapid pole-to-pole scan, together with the slowly changing activity conditions characteristic of solar minimum, have enabled a comparison of the two hemispheres to be made whereby, at least to first order, temporal effects can be neglected.
The above comparison reveals modest north-south asymmetries in a number of the data sets. For example, an examination of spatial gradients in solar wind parameters reveals that the average solar wind speed at latitudes greater than 40° is ~15-25 kms-1 higher in the north than in the south, qualitatively consistent with corresponding open coronal field line expansion factors computed using magnetogram data. Another solar wind asymmetry, presently not understood, is found in the radial proton temperature gradient, which is steeper in the north than in the south.
North-south asymmetries are also reported in the low-energy particle measurements. The recurrent increases in the flux of 50 keV electrons with a period of ~26 days referred to above were seen up to 80° south latitude, but not at high latitudes in the northern hemisphere. Comparison with in-ecliptic data reveals that solar and interplanetary activity persisted through 1995, implying that the lack of variations seen in the north is a spatial rather than temporal effect.
None of the models proposed to date to explain the ~26-day recurrent behaviour would predict an intrinsic north-south asymmetry, leading to the suggestion that the observations result from differences in the physical conditions in the two hemispheres. At higher energies, the cosmic ray flux observed by Ulysses, including the ACR component, was higher at a given heliographic latitude in the north than in the south by up to 50 percent.
Not all data show north-south asymmetries. For example, the magnitude of the radial magnetic field, when corrected for heliocentric distance, and the amplitude and radial gradient of the field variances, are the same in both hemispheres. Measurements from the Ulysses plasma wave experiment reveal no significant asymmetry between the solar wind electron temperature and density profiles derived from thermal noise spectroscopy for the northern and southern hemispheres. Similarly, a comparison of directional discontinuities (DDs) and tangential discontinuities (TDs) in the north and south polar wind reveals no differences in the rate of occurrence of DDs and TDs between the two hemispheres.