Radial dependence of magnetic compressibility of magnetic field fluctuations near Mercury’s orbit. (a)–(f) show the distributions of the magnetic compressibility ﹩{C}_{\parallel }\,﹩ as functions of frequency for six heliocentric distance ranges from 0.31 to 0.47 au. The horizontal dashed line at ﹩{C}_{\parallel }\,﹩= 1/3 indicates isotropic fluctuations. (g) and (h) present the distributions of ﹩{C}_{\parallel }\,﹩ at fixed frequencies of 0.5 and 5.0 Hz, respectively, for different heliocentric distances.