We combine different measurements of the polytropic index of the proton plasma in the heliosphere: (i) near-adiabatic index in the inner heliosphere ∼1 au, (ii) subadiabatic indices in the outer heliosphere ∼20–40 au, and (iii) near-zero indices in the inner heliosheath. These observations are unified by a single theoretical model of the polytropic index throughout its radial extent in the heliosphere; the corresponding fitting reveals the decreasing trend of the polytropic index with increasing heliocentric distance R. We anticipate that with increasing R, (i) the Debye length and mean-free-path decreases; (ii) the Landau damping is less effective, transferring thus less wave energy to particles; and (iii) the collisionality degree increases, indicating that the proton plasma in the inner heliosheath might be collisionless.

我们结合了对日球质子等离子体多向性指数的不同测量结果：（i）内日球层的近绝热指数〜1 au，（ii）外日球层的近绝热指数〜20-40 au，以及（iii）内部日鞘中的指数接近零。这些观测结果是由单一理论模型在整个太阳圈的径向​​范围内统一的；相应的拟合揭示了随着中心距R的增加，多变指数的下降趋势。我们预计随着R的增加，（i）德拜长度和平均自由程减小；（ii）Landau阻尼效果较差，因此将较少的波能传递给粒子；（iii）碰撞度增加，表明内部日鞘中的质子等离子体可能是无碰撞的。