  We present UV and Ly$\alpha$ radial surface brightness (SB) profiles of
Ly$\alpha$ emitters (LAEs) at $z=2.84$ detected with the Hyper Suprime-Cam
(HSC) on the Subaru Telescope. The depth of our data, together with the wide
field coverage including a protocluster, enable us to study the dependence of
Ly$\alpha$ halos (LAHs) on various galaxy properties, including Mpc-scale
environments. UV and Ly$\alpha$ images of 3490 LAEs are extracted, and stacking
the images yields SB sensitivity of
$\sim1\times10^{-20}\mathrm{~erg~s^{-1}~cm^{-2}~arcsec^{-2}}$ in Ly$\alpha$,
reaching the expected level of optically thick gas illuminated by the UV
background at $z\sim3$. Fitting of the two-component exponential function gives
the scale-lengths of $1.56\pm0.01$ and $10.4\pm0.3$ pkpc. Dividing the sample
according to their photometric properties, we find that while the dependence of
halo scale-length on environment outside of the protocluster core is not clear,
LAEs in the central regions of protoclusters appear to have very large LAHs
which could be caused by combined effects of source overlapping and diffuse
Ly$\alpha$ emission from cool intergalactic gas permeating the forming
protocluster core irradiated by active members. For the first time, we identify
``UV halos'' around bright LAEs which are probably due to a few lower-mass
satellite galaxies. Through comparison with recent numerical simulations, we
conclude that, while scattered Ly$\alpha$ photons from the host galaxies are
dominant, star formation in satellites evidently contributes to LAHs, and that
fluorescent Ly$\alpha$ emission may be boosted within protocluster cores at
cosmic noon and/or near bright QSOs.
