and Ls 242º, when there is a positive observation) and to have continued later than Ls 296º, since it was observed in an equivalent season in previous years (Fig. 2).

[Figure: Figure 5. Daily cycle of the AMEC in MY34. Top panel: Phases of the cloud evolution and instrument coverage as a function of LTST indicated in Martian hours. Transition time between phases is represented at a fixed time for simplicity, but there is obviously a degree of sol-to-sol variability, see main text for details. Local Times of the images in the bottom panel are 11]

indicated by green marks. Bottom panel: examples showing the appearance of the cloud at different local times on cylindrical projections. Gray areas represent non-imaged regions, dark areas correspond to the night. (a) Phase 1. Cloud in twilight, around 30 km above the local areoid. 2018-10-01, Ls 261º. (VMC observation 180340). (b) Phase 2. The cloud is around 1000 km in length. 2018-10-16, Ls 270º. (VMC observation 180362) . (c) Phase 2. The cloud is around 1800 km in length.  2018-10-10, Ls 266º. (VMC observation 180352). (d) Phases 3 and 4. The cloud is detached and thin clouds are visible on the western slope of Arsia Mons:  2018-09-22, Ls 255º. (IUVS, MAVEN orbit 7757). (e) Phase 3. Detached AMEC, Arsia Mons was not imaged and its position is indicated. 2018-10-11, Ls 267º. (VMC observation 180354) . (f) Phases 3 and 4. 2018-09-21, Ls 254º. (IUVS, MAVEN orbit 7752). (g) Phase 4. 2018-10-01, Ls 260º. (MARCI product K09_057091_2603_MA_00N118W). (h) Phase 4. 2018-08-31, Ls 241º. (IUVS, MAVEN orbit 7635). (i) Phase 4. 2018-08-27, Ls 238º. (IUVS, MAVEN orbit 7613).
In what follows, we use the word “tail” to refer to the elongated part of the AMEC, and “head” to refer to the easternmost side of the cloud, next to the volcano where it starts developing. Further details on the morphology of the cloud will be given in section 5.
During the period studied, the AMEC showed remarkable repeatibility in its morphological evolution, following a well-defined daily cycle with different phases that are summarized in Figure 5. We identified the following phases, which we will describe in detail below: (1) Cloud head formation: Before sunrise, the head of the cloud forms and grows in area, probably due to ascending motions (Fig. 5a). As the observation period is around the southern solstice, there were no notable variations of sunrise time at 5.7 LTST, and thus insolation as a function of LTST was practically non-variable during the season. (2) Tail expansion: After sunrise, the cloud rapidly expands westward to generate the distinctive elongated tail (Fig. 5b,c). (3) Detachment and decay: At around 8.3-8.7 LTST, the cloud formation ceases, it detaches from its formation point, and the complete cloud moves westward due to advecting winds until it evaporates before the afternoon. Throughout the rest of the morning, the tail keeps moving to the west whilst slowly fading, probably due to evaporation (Fig. 5d-f). (4) Local clouds: At around 11 LTST, and simultaneously with the decay of the tail in (3), new local clouds, distinct from the AMEC, distributed around the volcano, and smaller in area, begin to form around Arsia Mons. The formation of these clouds always starts on the western slope, and then grows to cover the volcano. A few observations show the presence of local clouds earlier than 11 LTST (for example, in Fig 5d), but they do not grow significantly until 11 LTST. Similar clouds are observed later in the day by MARCI and IUVS (Fig. 5d, 5f-i), and are well known (Benson 2003,2006; Wang & Ingersoll, 2003), but they might not be associated with the occurrence of the AMEC elongated cloud.