Grain alignment and rotational disruption by radiative torques in
  exoplanet atmospheres
  Dust clouds are ubiquitous in the atmospheres of hot Jupiters and affect
their observable properties. The alignment of dust grains in the clouds and
resulting dust polarization is a promising method to study magnetic fields of
exoplanets. Moreover, the grain size distribution plays an important role in
physical and chemical processes in the atmospheres, which is rather uncertain
in atmospheres. In this paper, we first study grain alignment of dust grains in
the atmospheres of hot Jupiters by RAdiative Torques (RATs). We find that
silicate grains can be aligned by RATs with the magnetic fields (B-RAT) due to
strong magnetic fields of hot Jupiters, but carbonaceous grains of diamagnetic
material tend to be aligned with the radiation direction (k-RAT). At a low
altitude of $r<2R_{\rm p}$ with $R_{\rm p}$ being the planet radius, only large
grains can be aligned, but tiny grains of $a\sim 0.01\mu$m can be aligned at a
high altitude of $r>3R_{\rm p}$. We then study rotational disruption of dust
grains by the RAdiative Torque Disruption (RATD) mechanism. We find that large
grains can be disrupted by RATD into smaller sizes. Grains of high tensile
strength are disrupted at an altitude of $r>3R_{\rm p}$, but weak grains can be
disrupted at a lower altitude. We suggest that the disruption of large grains
into smaller ones can facilitate dust clouds to escape to high altitudes due to
lower gravity and may explain the presence of high-altitude clouds in hot
Jupiter as well as super-puff atmospheres.
