  Sunskirting asteroid (3200) Phaethon has been repeatedly observed in STEREO
HI1 imagery to anomalously brighten and produce an antisunward tail for a few
days near each perihelion passage, phenomena previously attributed to the
ejection of micron-sized dust grains. Color imaging by the SOHO LASCO
coronagraphs during the 2022 May apparition indicate that the observed
brightening and tail development instead capture the release of sodium atoms,
which resonantly fluoresce at the 589.0/589.6 nm D lines. While HI1's design
bandpass nominally excludes the D lines, filter degradation has substantially
increased its D line sensitivity, as quantified by the brightness of Mercury's
sodium tail in HI1 imagery. Furthermore, the expected fluorescence efficiency
and acceleration of sodium atoms under solar radiation readily reproduce both
the photometric and morphological behaviors observed by LASCO and HI1 during
the 2022 apparition and the 17 earlier apparitions since 1997. This finding
connects Phaethon to the broader population of sunskirting and sungrazing
comets observed by SOHO, which often also exhibit bright sodium emission with
minimal visible dust, but distinguishes it from other sunskirting asteroids
without detectable sodium production under comparable solar heating. These
differences may reflect variations in the degree of sodium depletion of
near-surface material, and thus the extent and/or timing of any past or present
resurfacing activity.
