James,
From the programme of the Mars Section:-
“Other atmospheric phenomena White clouds, frequently seen at the limb and terminator, can occur in conjunction with specific topographic features, such as the great volcanoes like Olympus Mons, those in the Tharsis region, or Elysium Mons. Such topographic or orographic clouds are carried round with the planet’s rotation, and may be enhanced with the recommended blue and green filters. The clouds over the Tharsis volcanoes can sometimes coalesce on the evening side and form a ‘W’-shaped cloud (or ‘M’ for ‘Mars’ if viewed with north up). Some basin areas act as cold-traps for volatiles: thus Argyre and Hellas can show frost patches at the appropriate season. Another interesting phenomenon is a band of equatorial white cloud, the so-called Equatorial Cloud Band, which is best seen in blue or violet light and most apparent between about Ls = 50° to 145°, and whose brighter extremities at the E and W limbs give rise to the evening and morning clouds viewed in white light.”
The last part of this probably applies to the image you have displayed as Ls at the time was 78.
I guess the reason they appear brighter at the limb and terminator is that you are viewing obliquely, looking through a greater depth of cloud than when they are on the meridian and you look straight through them.
David.