Solar UV, stratospheric ozone and UVB

Abstract

The variations of stratospheric ozone and UVB observed at ground were compared with those of solar indices. During sequences of 27-day oscillations, X-ray had very large amplitudes (exceeding 50%), EUV near        30 nm had ~20%, Lyman alpha 121.6 nm ~10%, Mg II 280 nm ~5%, and continua 180-210 nm ~2%, 210-240 nm ~1.5%, 240-300 nm ~1%, 300-350 nm ~0.5%. There were distinct peaks seen in all of these, with spacing of roughly about 27 days. For effects on stratospheric ozone, the relevant wavelengths are mostly in the continua. In the 27-day sequences, the Dobson ozone at Arosa (47° N, 10° E) showed large fluctuations (~50%) with many peaks, but the peaks did not match with the peaks of solar indices. During one such 27-day sequence, the TOMS ozone overpass over Kagoshima, Japan (32° N, 121° E) and the UVB (295-325 nm) observed at Kagoshima showed large day-to-day fluctuations, ~20% for ozone, ~50% or UVB, but the peaks of ozone and UVB did not match with each other, nor with the peaks of solar indices, thus indicating altogether different mechanisms as their causes. In long-term changes for Arosa Dobson ozone, the largest fluctuations were the seasonal variation (~40% range, peak to trough) and the next largest were the QBO (Quasi-biennial oscillation, ~7%, peak to trough). Solar cycle variation ranges of ozone (peak to trough) were only ~2.6% in cycle 19 (sunspot maximum number 210), ~3.5% in cycle 20 (sunspot maximum number 111) and uncertain thereafter, because of the global depletion which started in 1980. For 1991-1997, the monthly values of ozone at Thessaloniki, Greece (40° N, 23° E) were anti-correlated with 305 nm UVB (1% ozone decrease corresponded to ~2% UVB increase), but poorly correlated with 325 nm UVB, indicating that ozone-UVB relationship was highly wavelength-dependent. However, whereas solar indices had a maximum in 1991 and a minimum in 1996, neither ozone nor any of the two UVB 305 nm and 325 nm showed resemblance with the solar index variations. In particular, ozone level was high in 1991 but decreased considerably within two years (by 1993). A comparison with ozone at Arosa (47° N) and Syowa (69° S) showed that this drop is not related to solar cycle but is due to the global depletion of ozone.