  We present an analysis of asteroid (3200) Phaethon using coronagraphic
observations from 2008 to 2022 by the COR2 cameras onboard the twin Solar
TErrestrial RElations Observatory (STEREO) spacecraft. Although undetected in
individual images, Phaethon was visible in stacks combined from the same
perihelion observations, yet only at small ($\lesssim$30\deg) but not large
($\gtrsim$150\deg) phase angles. The observations are in line with the
contribution from a bare nucleus, thereby seriously contradicting the
interpretation based on HI-1 observations that attributes the perihelion
activity to the ejection of \micron-sized dust. We obtained an upper limit to
the effective cross-section of \micron-sized dust to be $\lesssim \! 10^{5}$
m$^{2}$, at least three orders of magnitude smaller than earlier estimates
based on HI-1 data. On the contrary, the COR2 observations cannot rule out the
existence of mm-sized or larger debris around Phaethon. However, the fact that
no postperihelion debris tail has ever been detected for Phaethon suggests the
unimportance of such dust in the perihelion activity. We thus conclude that the
perihelion activity of Phaethon is highly unlikely relevant to the ejection of
dust. Rather, we deduce that the activity is associated with gas emissions,
possibly Fe I and/or Na D lines. To verify our conjecture and to fully
understand the perihelion activity of Phaethon, more observations at small
heliocentric distances are desired. We compile a list of observing windows
ideal for the search of gas emissions of the asteroid from ground telescopes.
The best opportunities will be during total solar eclipses.
