Fig. 1.
From: Ballooning modes and their stability in a near-Earth plasma
Dispersion curves 
(solid line) and 
(dashed line) for dimensionless frequency Ω = ω/(κ
c
V
a
sinα) depending on parameter β (shown near curves): (left-hand panel) at Λ = −1 (stable plasma); (right-hand panel) Λ = 1. At Λ = 1 the maximal cut-off frequency Ω+(0) is reached at β = 0.991. In the region 
the slow branch is unstable.