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Dense cores are the final place where turbulence is dissipated. It has beenproposed from theoretical arguments that the non-thermal velocity dispersionshould be narrower both for molecular ions (compared to neutrals) and fortransitions with higher critical densities. To test these hypotheses, wecompare the velocity dispersion of N$_2$H$^+$ (1--0) (n$_{\rm crit}$ =$6\times10^4$ cm$^{-3}) and NH$_3$ (n$_{\rm crit}=2\times10^3$ cm$^{-3}), inthe dense core Barnard 5. We analyse well resolved and high signal-to-noiseobservations of NH$_3$ (1,1) and (2,2) obtained with combining GBT and VLAdata, and N$_2$H$^+$ (1--0) obtained with GBT Argus, which present a similarmorphology. % Surprisingly, the non-thermal velocity dispersion of the ion issystematically higher than that of the neutral by 20\%. The derived sonic Machnumber, $\mathcal{M}_s = \sigma_{\rm NT}/c_s$, has peak values $\mathcal{M}_{s,{\rm N_2H^+}} = 0.59$ and $\mathcal{M}_{s, {\rm NH}_3} = 0.48$ for N$_2$H$^+$and NH$_3$, respectively. % This observed difference may indicate that themagnetic field even deep within the dense core is still oscillating, as it isin the turbulent region outside the core. The ions should be more stronglydynamically coupled to this oscillating field than the neutrals, thusaccounting for their broader linewidth. If corroborated by furtherobservations, this finding would shed additional light on the transition toquiescence in dense cores.

Neutral vs Ion Linewidths in Barnard 5: Evidence for Penetration by MHD Waves