The heliospheric magnetic field through several solar cycles.

Open magnetic flux of the Sun, carried out by the solar wind has been investigated as a function of space and time. Magnetic field measurements by several spacecraft, including Helios 1 and 2, Ulysses and ACE have been used to compare the number density of field lines in various locations in the heliosphere. Special care has been taken to eliminate the effect of magnetic field fluctuations which would prevent us to make realistic assessment of the open magnetic flux. It has been found that the magnetic flux density is remarkably independent of heliographic latitude and longitude. The very long duration of OMNI data base (continuous from 1964) has been used to compare the southern and northern magnetic field over four solar cycles. No significant difference has been found in the magnetic flux density in the two hemispheres, which significantly limits any possible north-south offset of the location of heliospheric current sheet. As for the evolution of the magnetic flux with distance from the Sun, the number of field lines crossing a sphere around the Sun increases with the radius of the sphere. However, this is due to the folding back of the field lines, which is more frequent in the outer heliosphere where the fluctuations in the direction of the magnetic field tend to increase. Apart from this effect, the investigation of the radial evolution of the magnetic field has given no evidence for any flux excess in the outer heliosphere. The long time scale variations of the open magnetic flux has also been studied from the 1960s to the present. The results have been compared to the open magnetic flux of the Sun, determined from the solar magnetograms, by applying model calculations.