PSDs of δB measurements (upper panel) on board C3 on 2008 March 17 at 12:30–12:35. The solid lines indicate the observed magnetic field spectra (top panel) and the dashed lines indicate the compensated (﹩\delta B\times {f}^{| \alpha | }﹩) spectra (middle panel). The different components of ﹩\delta {\boldsymbol{B}}﹩ are color-coded (top panel) along with the absolute value of their corresponding spectral index ﹩| \alpha | ﹩ (middle panel) and its uncertainty (calculated from the power-law fit with 95% confidence bounds), computed in the frequency range delimited by gray vertical lines. The gyrofrequency is indicated by the vertical black line. The thresholds for mirror (red) and proton cyclotron (black) instability are shown (as T_⊥/T_∥ vs. β_i∥) in the bottom panel, for a relatively strong growth rate (for details see Samsonov et al. 2001): γ = 10⁻². Every 4 s of data a circle is obtained, the color of which indicates the magnetic compressibility (see the color bar).