ON THE SOLAR ULTRAVIOLET SPECTRAL IRRADIANCE DURING THE MAUNDER MINIMUM

In the historical record of solar activity the period from 1645 to 171 5 is a singular epoch during which the number of sunspots decreased ma rkedly for a generation. Known as the Maunder Minimum, this solar epoc h coincided with the coldest part of the Little Ice Age (circa 1450 to 1850). We estimate the change at this time in the output of solar ult raviolet (UV) radiation at wavelengths from 120 to 300 nm, relative to contemporary observations. Since this portion of the solar UV spectru m determines ozone composition in the stratosphere, our results bear o n the historical variability of ozone and its potential climatic effec ts. Between the Maunder Minimum and 1986 (the present day solar activi ty minimum between cycles 21 and 22) we estimate reductions of 64% in the irradiance of the Lyman alpha line of neutral hydrogen (at 121.6 n m), 8% at 200 nm, and 3.5% in the wavelength range from 210 to 250 mt. The reduction in the solar output from the entire spectral band betwe en 120 and 300 nm is estimated to be 0.17 W/m(2), which is approximate ly 6% of the change in the total Solar irradiance of 2.7 W/m(2) previo usly estimated by us (Lean et al., 1992a) over the same time span. Bec ause of this diminished UV output due to very low solar activity the M aunder Minimum total ozone concentration may have been 4% below its 19 80 level. While the climatic consequences of such a change have yet to be determined, recent work by Haigh (1994) on modulation of radiative climate forcing by stratospheric ozone emphasizes the need to underst and the role of UV irradiance variability as one forcing mechanism.