Joint probability distribution of parallel beta and temperature anisotropy for protons (blue) and alphas (green) at radial distances 0.3–0.4 AU (top panel) and 0.6–0.7 AU (bottom panel) in fast solar wind. The contours are logarithmically spaced from the maximum bin count to 0.1 times the maximum bin count. The dashed lines show the alpha firehose instability threshold for normalised growth rates of γ/Ω_α = 10⁻³ (left-hand curve) and 10⁻² (right-hand curve). See Appendix B for details of the instability curve calculation. The large spread in alpha particle values is likely partly due to experimental uncertainties in fitting alpha number densities and parallel temperatures.