  The space-borne missions CoRoT and Kepler have revealed numerous mixed modes
in red-giant stars. These modes carry a wealth of information about red-giant
cores, but are of limited use when constraining rapid structural variations in
their envelopes. This limitation can be circumvented if we have access to the
frequencies of the pure acoustic dipolar modes in red giants, i.e. the dipole
modes that would exist in the absence of coupling between gravity and acoustic
waves. We present a pilot study aimed at evaluating the implications of using
these pure acoustic mode frequencies in seismic studies of the helium
structural variation in red giants. The study is based on artificial seismic
data for a red-giant-branch stellar model, bracketing seven acoustic dipole
radial orders around vmax. The pure acoustic dipole-mode frequencies are
derived from a fit to the mixed-mode period spacings and then used to compute
the pure acoustic dipole-mode second differences. The pure acoustic dipole-mode
second differences inferred through this procedure follow the same oscillatory
function as the radial modes second differences. The additional constraints
brought by the dipolar modes allow us to adopt a more complete description of
the glitch signature when performing the fit to the second differences. The
amplitude of the glitch retrieved from this fit is 15% smaller than that from
the fit based on the radial modes alone. Also, we find that thanks to the
additional constraints, a bias in the inferred glitch location, found when
adopting the simpler description of the glitch, is avoided.
