  The hairpin instability of a jet in a crossflow (JICF) for a low
jet-to-crossflow velocity ratio is investigated experimentally for a velocity
ratio range of $R\in(0.14,0.75)$ and crossflow Reynolds numbers
$Re_D\in(260,640)$. From spectral analysis we characterize the Strouhal number
and amplitude of the hairpin instability as a function of $R$ and $Re_D$. We
demonstrate that the dynamics of the hairpins is well described by the Landau
model, and, hence, that the instability occurs through Hopf bifurcation,
similarly to other hydrodynamical oscillators such as wake behind different
bluff bodies. Using the Landau model, we determine the precise threshold values
of hairpin shedding. We also study the spatial dependence of this
hydrodynamical instability, which shows a global behaviour.
