  We analyse archival Rossi X-Ray Timing Explorer (RXTE) proportional counter
array (PCA) data of thermonuclear X-ray bursts from the 2002 outburst of the
accreting millisecond pulsar SAX J1808.4-3658. We present evidence of
nonmonotonic variations of oscillation frequency during burst rise, and
correlations among the time evolution of the oscillation frequency, amplitude,
and the inferred burning region area. We also discuss that the amplitude and
burning region area evolutions are consistent with thermonuclear flame
spreading on the neutron star surface. Based on this discussion, we infer that
for the 2002 Oct. 15 thermonuclear burst, the ignition likely occured in the
mid-latitudes, the burning region took ~ 0.2 s to nearly encircle the
equatorial region of the neutron star, and after that the lower amplitude
oscillation originated from the remaining asymmetry of the burning front in the
same hemisphere where the burst ignited. Our observational findings and
theoretical discussion indicate that studies of the evolution of burst
oscillation properties during burst rise can provide a powerful tool to
understand thermonuclear flame spreading on neutron star surfaces under extreme
physical conditions.
