Gravitationally trapped axions on Earth
  We advocate for the idea that there is a fundamentally new mechanism for
axion production on Earth, as recently suggested in Fischer et al. (2018) and
Liang & Zhitnitsky (2018). We specifically focus on production of axions within
Earth, with low velocities such that they will be trapped in the gravitational
field. Our computations are based on the so-called Axion Quark Nugget (AQN)
dark matter model, which was originally invented to explain the similarity of
the dark and visible cosmological matter densities. This occurs in the model
irrespective of the axion mass $m_\mathrm{a}$ or initial misalignment angle
$\theta_0$. Annihilation of antimatter AQNs with visible matter inevitably
produce axions when AQNs hit Earth. The emission rate of axions with velocities
below escape velocity is very tiny compared to the overall emission, however
these axions will be accumulated over the 4.5 billion year life time of the
Earth, which greatly enhances the discovery potential. We perform numerical
simulations with a realistically modeled incoming AQN velocity and mass
distribution, and explore how AQNs interact as they travel through the interior
of the Earth. We use this to estimate the axion flux on the surface of the
Earth, the velocity-spectral features of trapped axions, the typical
annihilation pattern of AQN, and the density profile of the axion halo around
the Earth. Knowledge of these properties is necessary to make predictions for
the observability of trapped axions using CAST, ADMX, MADMAX, CULTASK, ORPHEUS,
ARIADNE, CASPEr, ABRACADABRA, QUAX, DM Radio.
