  We present an analytical model to study the dynamics of the outer edge of
Saturn's A ring. The latter is influenced by 7:6 mean motion resonances with
Janus and Epimetheus. Because of the horseshoe motion of the two co-orbital
moons, the ring edge particles are alternately trapped in a corotation
eccentricity resonance (CER) or a Lindblad eccentricity resonance (LER).
However, the resonance oscillation periods are longer than the 4-year interval
between the switches in the orbits of Janus and Epimetheus. Averaged equations
of motion are used, and our model is numerically integrated to describe the
effects of the periodic sweeping of the 7:6 CERs and LERs over the ring edge
region. We show that four radial zones (ranges 136715-136723, 136738-136749,
136756-136768, 136783-136791 km) are chaotic on decadal timescales, within
which particle semi-major axes have periodic changes due to partial libration
motions around the CER fixed points. After a few decades, the maximum variation
of semi-major axis is about 11 km (respectively 3 km) in the case of the CER
with Janus (respectively Epimetheus). Similarly, particle eccentricities have
partial oscillations forced by the LERs every 4 yr. For initially circular
orbits, the maximum eccentricity reached is ~0.001. We apply our work to
"Peggy", an object recently discovered at the ring edge, confirming that it is
strongly perturbed by the Janus 7:6 LER. The CER has currently no effect on
that body, nevertheless the fitted semi-major axes are just outside the chaotic
zone of radial range 136756-136768 km.
