The Visible Signature of Ozone at Twilight

In preparation for future „transit spectroscopy”—analyzing the light transmitted by a planet’s atmosphere as it passes in front of its star—researchers have conducted spectrophotometry of the twilight sky. These observations can be compared with those of the eclipsed Moon, where light grazes Earth’s atmosphere during a lunar eclipse, resulting in strong suppression of the blue end of the spectrum due to Rayleigh and aerosol extinction.

Using an Ocean Optics Jaz spectrometer covering 350-1000 nm and a single optical fiber input pointed about 10° above the western sky, twilight spectra were obtained in overcast conditions at an altitude of 560 meters and a latitude of +47.8°. The experimental setup included the Jaz spectrometer, a fiber feed, and a data-taking laptop, as shown in Figure 1.

Figure 1. The experimental setup for the twilight observa- tions comprises the Jaz spectrometer, the fiber feed and the data-taking laptop. These photographs, taken around the time of sunset with a “daylight” white balance setting on a Canon 5D Mark II, illustrate the high color temperature of the ambient light at sunset, which is caused predominantly by the ozone absorption (Figure 2).

The spectra revealed variations in sky brightness at 700 nm and color temperature measured with a Gossen Colormaster 2F color meter. This simple experiment highlighted the significant impact of ozone absorption on twilight colors, with the color temperature of the western sky ranging from 6,000 K to 14,000 K (Figure 2). These findings suggest a strong visible spectral signature of an oxygen-rich exoearth atmosphere during transit spectroscopy.

Figure 2. These spectra reveal variation of sky brightness at 700 nm, in units of counts per 8192 ms, and color temperature measured with a Gossen Colormaster 2F color meter.

Results

The absence of strong O2 and H2O signatures in the spectral ratios likely indicates that most light entered the spectrometer from above the cloud layer, where ozone dominates absorption. The twilight spectra, captured at approximately 10-minute intervals from an hour before to 15 minutes after sunset, demonstrated the development of ozone Chappuis absorption centered at 600 nm. The data also showed the dramatic bluish tint of the sky during this period, despite variable cloud thickness (Figure 3).

Figure 3. A sequence of relative irradiance spectra is captured under a cloudy western sky at approximately 10-minute intervals from an hour before, to 15 minutes after, sunset. Also plotted is the lunar eclipse spectrum from [3]. The rapid development of the ozone Chappuis absorption, centered at 600 nm, is apparent together with the dramatic bluish tint of the sky color during this period. The somewhat irregular behavior of the blue end of the spectrum is due to the variable cloud thickness during these measurements. The reference spectrum is taken with a solar altitude of +13° and the final spectrum of the sequence is with an altitude of -3°.

An interesting aspect of this data is the potential to derive the ozone Chappuis absorption cross-section multiplied by the column density of the atmospheric pathlength characteristic of these twilight observations. The observed ratio, compared to atmospheric models using the HITRAN database, illustrated the pure ozone absorption and combined ozone, O2, and H2O absorption (Figure 4).

Figure 4. The observed ratio (dark blue), with a normal- ized continuum, of a spectrum taken with a solar altitude of-3° is compared to one at +13°. Atmospheric models [4] using the HITRAN database (cfa.harvard.edu/hitran) [5], with similar but pure transmission geometry, are over plotted (red: pure ozone absorption; light blue: ozone + O2 + H2O). The models have been scaled in intensity by a factor of 1.7 as a way of accounting for the simplification of the model geometry.