Radial increase of the compensated interplanetary magnetic field magnitude as a result of the growth of spherically polarized Alfv'en waves

Measurements from the NASA Parker Solar Probe (PSP) mission demonstrate that inside about20 solar radii the magnitude of the interplanetary magnetic field does not fall off simply as 1/R squared, with,R squared 15--20% below its value at about60,. Here we use PSP magnetic field measurements over 10.6 -- 56.8, to show that the outward rise of,R squared is not due to the global structure of the heliospheric magnetic field or proximity to the heliospheric current sheet but is the direct result of the growth of large-amplitude, spherically polarized Alfvén waves (so-called `switchbacks'). Over the same distance range,,R squared stays constant, so that the sector-signed radial flux is conserved. The wave amplitude ε= B, rms/ grows from 0.26 to 0.57 as R 1/2 α, and / rises by 20%. A simple model parametrized by the broadening of the distribution of deflection angles reproduces the effect to high accuracy. These results demonstrate that as the relative wave amplitudes grow with increasing solar distance, a dynamo-like process operates to enhance the field magnitude by up to 20%. By enhancing magnetic pressure this has implications for the large scale solar wind structure in the low-beta inner heliosphere, as well as partially resolving the long-standing discrepancy between remote measurements of the solar magnetic field and those made in situ by earlier missions farther from the Sun.