The occultation technique for atmospheric remote sounding

As a natural extension of the observation of light attenuation from the ground, the occultation technique consists of observing a light source (mostly the Sun, but it can also be the Moon, a star or a planet) from a low orbit satellite at an altitude between 500 and 1000 km. Such a satellite will perform a revolution in about 90-100 minutes and, depending on the inclination of its orbit plane,occulprincipfinal1.jpg it will cross the boundary between sunlight and shadow of the Earth about 30 times per day. In other words, the satellite will experience 15 sunsets and 15 sunrises each day. Each of these events is called an « occultation » of the Sun by the Earth. In such an orbital sunset for instance, the occultation is preceded by a short period (about 1 minute) during which the Sun rays are grazing the Earth’s surface through the atmosphere, before hitting the satellite (see figure).

As in ground observations, recording the attenuation of light at different wavelengths will permit the retrieval of the quantity of some absorbing gas like ozone. Furthermore, we can calibrate the instrument by looking at the Sun when the line-of-sight is well above the atmosphere. Also, the method is more sensitive for the detection of small quantities of atmospheric trace gases because the trajectory of the rays through the atmosphere is longer than if the instrument was at ground level.

The occultation technique has been used by several space instruments hosting a spectrometer (that measure the light intensity at different wavelenghts). In particular, the Belgian Institute for Space Aeronomy developed an occultation radiometer named ORA that was looking at the full solar disk during the occultation (see figure). The experiment, operating in 1992-1993, was successful and allowed to detect the very important quantity of volcanic aerosols released in the stratosphere by the cataclysmic eruption of Mount Pinatubo in June 1991.

Notice that the atmosphere is acting as a divergent lens in the lower layers leading to an apparent flattening of the Sun. This effect, although weaker at ground level, can be observed during a sunset above the ocean.

oraocculfinal1red.pngHowever, the excellent ability of the occultation method to obtain a good vertical resolution for the retrieved atmospheric profiles (about 1 km) has a price : the effective region sampled along the line-of-sight is rather extended : 500 km. Also, the number of occultations is limited to about 30 per day. During two sunset occultations for instance, the satellite takes about 100 minutes to come back and the Earth has rotated by 25° eastward (because the Earth rotates by 360° in one day of 24 hours=1440 minutes). So the observed region is now 25° westward to the prior one, roughly at the same latitude because the relative position of Earth and Sun did not change more than a fraction of a degree. After one day, the instrument has sampled two latitudes in about 15 sites. Finally, on a weekly basis, the sampled latitudes will slowly drift as the relative positions of Sun and Earth are changing. Depending on the orbital plane inclination, it will take two months to get a global coverage of the world.

The insufficient coverage in the Sun occultation technique can be addressed by using other light sources. This was the goal of the GOMOS (Global Ozone Monitoring by Occultation of Stars) experiment, an instrument dedicated to the observation of star occultations. Clearly, there are much more stars to observe during one orbit meaning that 30-50 different occultations can be performed for at each revolution. However, stars are faint objects of different brightnesses and colors, leading to larger uncertainties in the retrieved concentration profiles.

As opposed to occultation experiments, other instruments are looking to the nadir (downward to the ground) with a reduced vertical resolution. Mostly, they provide an « ozone column » which equals the total ozone quantity above some location. Placed on a quasi-polar satellite, such instruments are capable of achieving a full coverage of the globe in about 3 days, a considerable advantage on the Sun occultation sensors.

The goal of the limb scattering method is precisely to combine a good vertical resolution with an extended coverage.