Turbulent power spectrum in warm and cold neutral medium using the
  Galactic HI 21 cm emission
  Small-scale fluctuations of different tracers of the interstellar the medium
can be used to study the nature of turbulence in astrophysical scales. Of
these, the `continuum' emission traces the fluctuations integrated along the
line of sight whereas, the spectral line tracers give the information along
different velocity channels as well. Recently, Miville-Desch\^enes et al.
(2016) have measured the intensity fluctuation power spectrum of the continuum
dust emission, and found a power law behaviour with a power law index of $-2.9
\pm 0.1$ for a region of our Galaxy. Here, we study the same region using
high-velocity resolution 21-cm emission from the diffuse neutral medium, and
estimate the power spectrum at different spectral channels. The measured 21-cm
power spectrum also follows a power law, however, we see a significant
variation in the power law index with velocity. The value of the power-law
index estimated from the integrated map for different components are quite
different which is indicative of the different nature of turbulence depending
on temperature, density and ionization fraction. We also measure the power
spectra after smoothing the 21 cm emission to velocity resolution ranging from
$1.03$ to $13.39~{\rm km~s^{-1}}$, but the power spectrum remains unchanged
within the error bar. This indicates that the observed fluctuations are
dominantly due to density fluctuations, and we can only constrain the power-law
index of velocity structure function of $0.0 \pm 1.1$ which is consistent with
the predicted Kolmogorov turbulence $(\gamma=2/3)$ and also with a
shock-dominated medium $(\gamma=1.0)$.
