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Solar Zenith Angle (SZA)

When observing a given point on the Earth's surface from a satellite-based instrument, the Solar Zenith Angle (SZA) is the angle between the local zenith (i.e. directly above the point on the ground) and the line of sight from that point to the sun. This means that the higher the Sun is in the sky, to lower the SZA is. The other angle in the graph, the angle between the local zenith and the line of sight to the satellite, is called the Viewing Zenith Angle.
Solar and viewing zenith angles
Schematic illustration of the Solar Zenith Angle (SZA) and Viewing Zenith Angle (VZA) for observations from satellite-based instrument. [image taken from a NASA page]

The value of the Solar Zenith Angle depends on the position on the Earth and the local date and time. The following graphs shows the variation of the SZA for places at about the time SCIAMACHY passes directly overhead. The retrieval of SO2 data is not so reliable for very high SZA, say above 75 degrees, and becomes really unreliable at much higher SZA. That is why data presented in the maps and data files on this website is limited to SZA 85 degrees.

The time SCIAMACHY passes a place overhead is always the same local time, as the satellite orbits the Earth in a so-called sun-synchronus orbit. If a satellite in a polar orbit flies over the pole into the sunlight, the SZA can be well above 100 degrees. The minimum SZA along the centre of an orbit is about 24 degrees. While orbiting on the sun-lit side of the Earth from North to South, SCIAMACHY scans from East to West, with on the East (West) side an SZA that is a little smaller (larger) than in the centre of the orbit.

Solar zenith angle for several places
The Solar Zenith Angle as function of time in 2007 for several places at about the moment SCIAMACHY passes overhead.

place latitude longitude time (UTC)
Reykjavik 64.15 -21.97 ca. 12.35
Rome 41.88 12.50 ca. 09.40
Mexico City 19.40 -99.15 ca. 16:55
Galapagos Ils. -0.50 -91.20 ca. 16:00
Bulawayo -20.17 28.72 ca. 07:50
Wellington -41.28 174.78 ca. 21:55
South Orkney Ils. -62.00 -45.00 ca. 12:00

Note that ENVISAT, the satellite that carries SCIAMACHY, passes the equator at 10:00 a.m. local solar time. For a given logitude the time in UTC is:

UTC_time = local_solar_time - longitude/15
For example the Galapagos Islands are at about the Equator at 90 degrees West. With local_solar_time of 10:00 this gives a UTC_time for the overpass of about 16:00.

 
The following table lists the time and direction of equator passes of the different polar-orbiting nadir-viewing satellite instruments measuring SO2. All these satellites have a sun-synchronus orbit at about 790 km above the surface.

instrument satellite equator pass
local
solar time
flight
direction
SCIAMACHY ENVISAT 10:00 N -> S
OMI EOS-AURA 13:45 S -> N
GOME-2 MetOp-A 09:30 N -> S
IASI MetOp-A 09:30 & 21:30 N -> S
AIRS Aqua 00:45 & 12:45 S -> N
 
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