  We explore what dominant physical mechanism sets the kinetic energy contained
in neutral, atomic (HI) gas. We compare the HI line widths predicted from
turbulence driven by supernova (SN) explosions and magneto-rotational
instability (MRI) to direct observations in 11 disk galaxies. We use
high-quality maps of the HI mass surface density and line width, obtained by
the THINGS survey. We show that all sample galaxies exhibit a systematic radial
decline in the HI line width, which appears to be a generic property of HI
disks and also implies a radial decline in kinetic energy density of HI. At a
galactocentric radius of r25 there is a characteristic value of the HI velocity
dispersion of $10\pm2$ \kms. Inside this radius, galaxies show HI line widths
above the thermal value expected from a warm HI component, implying that
turbulence drivers must be responsible for maintaining this line width.
Therefore, we compare maps of HI kinetic energy to maps of the star formation
rate (SFR) and to predictions for energy generated by MRI. We find a positive
correlation between kinetic energy of HI and SFR. For a given turbulence
dissipation timescale we can estimate the energy input required to maintain the
observed kinetic energy. The SN rate implied by the observed recent SFR is
sufficient to maintain the observed velocity dispersion, if the SN feedback
efficiency is at least \epsilon_SN\simeq0.1. Beyond r25, this efficiency would
have to increase to unrealistic values, $\epsilon>1$, suggesting that
mechanical energy from young stars does not supply most energy in outer disks.
On the other hand, both thermal broadening and turbulence driven by MRI can
produce the velocity dispersions and kinetic energies that we observe in this
regime.
