Dependence of the Interplanetary Magnetic Field on Heliocentric Distance at 0.3-1.7 AU: A Six-Spacecraft Study

We use magnetometer data taken simultaneously by MESSENGER, VEX, STEREO and ACE to characterize the variation of the interplanetary magnetic field (IMF) with heliocentric distance, r(h), for r(h) less than or similar to 1 AU. Power law fits (a r(h) (b)) to the individual IMF components and magnitude indicate that, on average, the IMF is more tightly wound and its strength decreases less rapidly with r(h) than the Parker spiral prediction. During Solar Cycle 24, temporal changes in b were insignificant, but changes in amplitude, a, were correlated with sunspot number, up to sunspot number similar to 84. MAVEN data taken at 1.4-1.7 AU since late 2014 broadly confirm and extend these results in space and time. Our study demonstrates the importance of simultaneous observations from multiple spacecraft to separate heliocentric distance and temporal variations in the IMF and provides a modern benchmark for comparison with data from the Parker Solar Probe Mission.