  Two dimensional Fourier spectra of near-infrared images of galaxies provide a
powerful diagnostic tool for the detection of spiral arm modulation in stellar
disks. Spiral arm modulation may be understood in terms of interference
patterns of outgoing and incoming density wave packets or modes. The brightness
along a spiral arm will be increased where two wave crests meet and
constructively interfere, but will be decreased where a wave crest and a wave
trough destructively interfere. Spiral arm modulation has hitherto only been
detected in grand design spirals (such as Messier 81). Spiral arm amplitude
variations have the potential to become a powerful constraint for the study of
galactic dynamics. We illustrate our method in two galaxies: NGC 4062 and NGC
5248. In both cases, we have detected trailing and leading m=2 waves with
similar pitch angles. This suggests that the amplification mechanism is the
WASER type II. In this mechanism, the bulge region reflects (rather than
refracts) incoming waves with no change of pitch angle, but only a change of
their sense of winding. The ratio between the amplitudes of the leading and the
trailing waves is about 0.5 in both cases, wherein the higher amplitude is
consistently assigned to the trailing (as opposed to leading) mode. The results
are particularly significant because NGC 5248 is an optically grand design
galaxy, whereas NGC 4062 is optically flocculent. NGC 4062 represents the very
first detection of spiral arm modulation in the stellar disk of an optically
flocculent galaxy.
