1 Introduction

A Martian cloud can have one of two forms: volatile (or white) clouds and dust (or yellow) clouds. Some volatile clouds may consist of CO₂ ice particles, but most of them in low to mid latitudes in northern summer may be water-ice clouds. Hereafter the term "cloud'' refers to a water-ice cloud. The investigation of Martian clouds is important for the study of Martian climate. Since the Viking era, the Martian climate has been simulated by global circulation models (e.g., Pollack et al. 1990). Seasonally appearing Martian clouds may be one constraint for the circulation models. Morphological features and optical characteristics of Martian clouds have been secured by several techniques (Table 1). Slipher (1962) and Martin et al. (1992) summarize the morphological features of clouds observed by ground-based telescopes.

   

Table 1: Cloud observations (some examples).

Visual observations	Miyamoto (1965, 1968, 1974)
Imaging observations
Earth based	Slipher (1962), Martin & Baum (1969);
	Smith & Smith (1972); Martin & McKinney (1974);
	Hattori & Akabane (1974); Akabane et al. (1980, 1987, 1995);
	Parker et al. (1999)
Mariner 9	Leovy et al. (1972, 1973); Curran et al. (1973); Briggs & Leovy (1974)
Viking 1, 2	Briggs et al. (1977); Piekersgill & Hunt (1981); French et al. (1981); Christensen & Zurek (1984)
HST$^\ast$	James et al. (1994, 1996); Wolff et al. (1999)
Polarimetric observations	Dollfus (1961); Dollfus et al. (1989, 1996); Lee et al. (1990)

$^\ast$ Hubble Space Telescope.

Owing to those and other observations it is known that three types of cloud appear in low latitudes of both hemispheres and mid latitudes of the northern hemisphere in northern spring to summer. They are morning, evening, and afternoon clouds. Morning and evening clouds extend frequently from southern low latitudes to northern mid latitudes, covering wide areas. Afternoon clouds are visible as bright spots in the afternoon, but are much smaller than morning and evening clouds. They appear on large volcanoes such as Elysium Mons and Olympus Mons in the northern hemisphere. Afternoon clouds begin to be visible in early spring of the northern hemisphere. First they are visible only near the evening limb or terminator, and their brightness increases as the season advances towards late spring.

In spring to summer of the northern hemisphere, clouds in low latitudes form a seasonal cloud belt (Clancy et al. 1996). The cloud belt is called the "equatorial cloud belt'' (James et al. 1996), "low latitude cloud belt'' (Clancy et al. 1996), or "aphelion cloud belt'' (Wolff et al. 1999). In this paper we use the term "low latitude cloud belt''. The most active period of the cloud belt is late spring to early summer and the cloud belt disappears by early autumn (Wolff et al. 1999; Tamppari et al. 2000). Most of the morning, evening, and afternoon clouds lie in the cloud belt. Regions in which all of the three types of cloud appear are restricted to two large orogenic areas in northern low latitudes: Elysium and Tharsis. The afternoon clouds on Elysium Mons (Elysium afternoon cloud) is as bright as those on Olympus Mons (Olympus afternoon cloud), but morning and evening clouds in Elysium are much less active than those in Tharsis. Clouds in Tharsis are bright and conspicuous, and have been morphologically studied. The classical "W cloud'' is characteristic of Tharsis (Slipher 1962). The morning, evening, and afternoon clouds in Tharsis repeat the diurnal variation for a long period in northern spring to summer (Slipher 1962; Smith & Smith 1972; Briggs et al. 1977). The regular appearance of clouds in the cycle of a day may suggest that meteorological conditions repeat in the same period. However, quantitative investigations of the diurnal variation of the cloud activity are few. We quantitatively show the cloud activity in recent apparitions and compare it to our earlier observations.

The vertical optical thickness may be one of criterions of cloud activity. But optical thicknesses of Martian clouds have not yet been observed directly from the Martian ground. Curran et al. (1973) derived the optical thickness of clouds over Tharsis to be about 0.4 in early summer from the Mariner 9 data in 1972. It was the average over the field of view of the Mariner 9 instrument. They suggest that the optical thickness of the brightest region of the cloud may be four times larger. From the Viking data at $L{\rm s}=29^\circ$ in 1979 (Ls is the areocentric orbital longitude of the Sun), Christensen & Zurek (1984) estimated the optical thickness of a cloud over Olympus Mons to be of the order of unity or larger. Narumi et al. (1987) derived the optical thickness of a morning cloud at 90 $^\circ$W longitude on the equator around $L{\rm s}=116^\circ$ from imaging observations in 1982. The optical thickness was about 3 near 7h MLT (Martian local time) and decreased to about 0.2 around local noon. The cloud repeated almost the same diurnal variation during their observations from April 18 to 25, 1982. Clancy et al. (1996) derived optical thicknesses of clouds in low latitudes from the Hubble Space Telescope (HST) observations. The optical thickness of a cloud at 120 $^\circ$W and 10 $^\circ$N in Tharsis was 0.7 in the mid morning at $L{\rm s}=94^\circ$ in 1993. Analyzing the HST images, Wolff et al. (1999) derived the optical thickness of a morning cloud at 33 $^\circ$W and 0$^\circ$-30 $^\circ$N in Chryse at $L{\rm s}=98^\circ$ (1997). It was 0.3 at 10.5h MLT. Wolff et al. (1999) also showed seasonal variations of cloud opacities in the low latitude cloud belt near Syrtis Major, in Amazonis, and in Chryse. Their Figs. 7-9 suggest that the cloud belt had not yet existed in the Syrtis Major region at $L{\rm s}=40^\circ$, and that it was visible at $L{\rm s}=65^\circ$, although the cloud belt appeared in the Amazonis region at $L{\rm s}=45^\circ$.

In this paper we investigate the diurnal variation of the optical thickness of morning clouds centered at 120 $^\circ$W and 10 $^\circ$N in Tharsis and afternoon clouds on Olympus Mons in the 1995 to 1999 apparitions. It was mid spring to early summer in the northern hemisphere. We represent the transient features from morning to afternoon clouds in Tharsis in Sect. 3, and diurnal variations of morning clouds over Tharsis and of afternoon clouds over Olympus Mons are represented in Sect. 4. The distribution of the optical thickness of clouds on a parallel of 10 $^\circ$N in the low latitude cloud belt is shown in Sect. 5.