  Determining the properties of solar-like oscillating stars can be subject to
many biases. A particularly important example is the helium-mass degeneracy,
where the uncertainties regarding the internal physics can cause a poor
determination of both the mass and surface helium content. Accordingly, an
independent helium estimate is needed to overcome this degeneracy. A promising
way to obtain such an estimate is to exploit the so-called ionisation glitch,
i.e. a deviation from the asymptotic oscillation frequency pattern caused by
the rapid structural variation in the He ionisation zones.
  Although progressively becoming more sophisticated, the glitch-based approach
faces problems inherent to its current modelling such as the need for
calibration by realistic stellar models. This requires a physical model of the
ionisation region explicitly involving the parameters of interest such as the
surface helium abundance, $Y_s$.
  Through a thermodynamic treatment of the ionisation region, an analytical
approximation for the first adiabatic exponent $\Gamma_1$ is presented.
  The induced stellar structure is found to depend on only three parameters
including the surface helium abundance $Y_s$ and the electron degeneracy
$\psi_\textrm{CZ}$ in the convective region. The model thus defined allows a
wide variety of structures to be described and, in particular, is able to
approximate a realistic model in the ionisation region. The modelling work
conducted enables us to study the structural perturbations causing the glitch.
More elaborate forms of perturbations than the ones usually assumed are found.
It is also suggested that there might be a stronger dependence of the structure
on both the electron degeneracy in the convection zone and on the position of
the ionisation region rather than on the amount of helium itself.
