Variability of solar ultraviolet irradiance

J. M. PAP.* R. F. DONNELLY,+ H. S. HuDwN.S G. J. ROTTMAN~ and R. C. WILLSON:

*University ofColorado/CIRES and NOAAjSEL. 325 Broadway. Boulder, CO 80303, U.S.A. : I-NOAAISEL, 325 Broadway, Boulder. CO 80303. U.S.A. ; $University of California. San Diego.

CA 92109, U.S.A. : $University of Colorado/LASP. Boulder, CO 80309. U.S.A. ; ‘Jet Propulsion Laboratory. Pasadena, CA 91 103. U.S.A.

(Recriwd itljtzal fiwnt I6 April 1991 )

Abstract-A model of solar Lyman alpha irradiance developed by multiple linear regression analysis, including the daily values and Xl-day running means of the full disk equivalent width of the Helium line at 1083 nm, predicts reasonably well both the short- and long-term variations observed in Lyman alpha. In contrast, Lyman alpha models calculated from the 10.7 cm radio flux overestimate the observed variations in the rising portion and maximum period of solar cycle. and underestimates them during solar minimum. We show models of Lyman alpha based on the He line equivalent width and 10.7 cm radio flux for those time intervals when no satellite observations exist, namely back to 1974 and after April 1989, when the measurements of the Solar Mesosphere Satellite were terminated.

INTRODUCTION

Solar irradiance. both in the ultraviolet and integrated

over all wavelengths, has been monitored from space

by a variety of independent instruments, during solar

cycles 21 and 22. These observations show that both total and UV irradiances vary over the solar cycle

(WILLSON. 1984; DONNELLY. 1989) and short-term

changes, related to the evolution of active regions, are superposed on the long-term trend (DONNELLY, 1989).

Since solar irradiance is the main driver of the climatic and photochemical processes of the Earth’s atmo-

sphere. it is of great interest to understand and predict its variability. In this paper we examine the variations of UV irradiance at Lyman alpha (121.6nm) and

present a two-parameter model calculated from two

ground based activity indices. the 10.7cm radio flux and the full disk equivalent width of the He line at

1083 nm. The Lyman alpha radiation (BARTH rt ul., 1990)

was measured on aboard the Solar Mesosphere

Explorer (SME) satellite and spans the time interval

I January 1983~ I3 April 1989. The total irradiance

(S), the 10.7cm radio flux (FIO) and the full disk

equivalent width of the He line at 1083nm (EWHe) are used as indices of solar activity. Data for the total irradiance come from the ACRIM radiometer on the

Solar Maximum Mission satellite (WILLSON. 1984). In order to compare the variations of Lyman alpha and total irradiance, the effect of sunspots has been

removed from S by means of the ‘Photometric Sun- spot Index (PSI)’ (HUIISON 1’1 rrl., 1982). The total

irradiance corrected for sunspot darkening (SC) is

therefore calculated as SC = Sf PSI. Daily values of FlO have been published in the Solar Geophysical

Data catalogue. The He 1083 nm line equivalent width

has been measured at the Kitt Peak National Solar

Observatory since 1974 (HARVEY, 1980). It correlates well with the full disk Ca-K index (HARVEY, 1980);

thus, it is used as a proxy for the emission of bright

magnetic clemcnts. including plagcs and the acti\c

network (LE-AS. 1987).

MULTIPLE LINEAR REGRESSION ANALYSIS

The method of multiple linear regression analysis is used to estimate both the short- and long-term

variations observed in Lyman alpha irradiance (PAP

er trl., 1990). A proxy Lyman alpha irradiance can be

calculated from the formula : y = o+b.r+cz, where (1, h. and c are the partial regression coefficients. derived from the comparison of the SME Lyman

alpha data and the selected solar index. ‘I’ is the daily value of the selected solar index and ‘-r’ is its 81- day running mean. The parameter b characterizes the

variations related to the rapid fluctuations of the given index, while c describes the slower fluctuation evident in the 8 1 -day smoothing. Therefore, ~1 gives the best-fit relationship between Lyman alpha and solar activity indices, considering both the short- and long-term variations.

The solid lines of Fig. ia+ present the 77-da) running of means of the SME Lyman alpha obser-

999

1000 J. M. PAP et al

Table 1. Correlation and partial regression coefficients of multiple linear regression analysis between Lyman alpha and

its models from the listed solar indices

Regression coefficients Indices r a b c

EWHe 0.96 1.38E-3 5.37E-5 7.79E-6 SC 0.94 -1.46 3.13E-4 6.99E-4 FlO 0.91 3.12E-3 7.60E-6 8.12E-6

Units: EWHE: mA; SC: W/m’; FIO: 10” W/m*/Hz.

vations, the dashed lines give the Lyman alpha values

estimated from the EWHe (la), SC (lb) and FlO (1~). The partial regression coefficients and correlation coefficients between the observed and modeled Lyman

alpha are given in Table 1. Figure 1 and Table 1 indicate that the model calculated from the EWHe gives a somewhat better fit to the observed Lyman alpha (I = 0.96). Total irradiance corrected for sun-

_ 6 7 ‘0 5.5 c : 5

z 4.5

4

3.5 I I I I I / -

6.5 _ Lyman a/Se b

3.2 I I I I I I,

Lyman a/F10.7 c 6.5 _ : :

“? 9: 1990

Fig. 1. The 27-day running means of SME/Lyman alpha are given (solid lines). Dashed lines show the 27-day running means of estimated Lyman alpha from the EWHe (a), SC (b)

and FlO (c).

spot effect also has a high correlation with Lyman

alpha (r = 0.94). A phase shift is recognized in Fig. 1 between

Lyman alpha and its model from FlO. During the

declining portion of solar cycle 21 the FlO model peaks and decreases earlier than the actual Lyman alpha. It reaches a minimum level in early 1985 and

shows a relatively flat background level during the two years of solar minimum (BARTH et al., 1990 ; PAP et al., 1990). Furthermore, the Lyman alpha estimated

from F10 increases faster than the SME data during the rising portion of cycle 22 and it overestimates the observed values near the solar maximum, at the

beginning of 1989. The Lyman alpha model, cal- culated from SC, also shows a higher value than the measured data at the beginning of 1989, although this

is not so pronounced than for the FlO model. To demonstrate the correlation between Lyman

alpha data and its model estimates, we have plotted the scatter diagram of the observed Lyman alpha vs the modeled values. The dots show the data for

the descending phase and minimum of solar cycle 21, while crosses show the data points for the rising portion of cycle 22. Figure 2a gives additional evi- dence for the strong correlation between Lyman alpha

and its model based on the EWHe. For the model on SC (Fig. 2b) the scatter increases towards the

maximum of cycle 22. The structure of Fig. 2c (FlO model) is more complicated, the dots indicate a reasonably good linear relationship between Lyman alpha and FlO during the descending portion of solar cycle 21 (BARTH et al., 1990) which breaks down during solar minimum (BARTH et al., 1990 ; PAP et al., 1990). The crosses lie above the dots, showing that FlO increases faster than Lyman alpha during the rise of cycle 22.

MODELS OF LYMAN ALPHA IRRADIANCE

The relation between SME/Lyman alpha and the

EWHe makes it possible to estimate the Lyman alpha variability for those time intervals when no satellite

measurements exist, that is back to the beginning of the EWHe measurements in 1974 and to the end of 1989 when the EWHe data are available. In order to get more information on the phase shift between Lyman alpha and FlO, we have also calculated Lyman alpha from FlO for the same time interval, that is back to 1974 and to the end of 1989. Figure 3a and b shows the 27-day running means of Lyman alpha derived from models using the EWHe and FlO, respectively. In order to test our models, we have also plotted (Fig. 3~) the 27-day running means of the Mg II core-to-

Variability of solar ultraviolet irradiance 1001

0

3 31 ? 8 r F10.7 ^, c

3 33_1 8

Fig. 2. The scatter diagram of daily values of Lyman alpha estimated from the EWHe (a), SC (b). and FIO (c) vs its measured values. The periods show data for the descending phase and minimum of solar cycle 21, the crosses show the

data for the ascending phase of cycle 22.

wing ratios (R(c/w)), derived from the UV irradiance

measurements made by the SBUVl (HEATH and SCHLESINGER, 1986) and SBUVZ (DONNELLY. 1990) monitors on the Nimbus-7 and NOAA9 satellites, respectively.

The Lyman alpha models show variation over the

solar cycle similar to the R(c/w) calculated from the Nimbus-7 and NOAA9 measurements. The peaks in

the UV irradiance observed by these two satellites align well with the peaks in the calculated Lyman alpha. The correlation coefficient between Lyman alpha estimates based on the EWHe and R(c/w) derived from the Nimbus-7/SBUVl measurements is 0.97 and it is 6.98 for the NOAA9 R(c/w) data. The correlation coefficient between Lyman alpha estimate based on FIO and the two Mg core-to-wing ratios is

0.97. There is also a high correlation between the SMElLyman alpha and the Mg core-to-wing ratios

(r = 0.97 in both cases). The fact that Lyman alpha models correlate well with the R(c/w) values gives a

confidence that our models predict successfully the variations observed in the UV irradiance.

Nevertheless, some differences remain. The model calculated from FlO gives higher values for Lyman

alpha during solar maximum than estimates based on the EWHe. This FlO-based model also shows (Fig. 3)

a faster increase during the rising portions of cycles 2 1 and 22 than the model calculated from the EWHe,

which more closely track the Lyman alpha obser- vation. During the rising portion and maximum of

solar cycle the Lyman alpha model based on FlO

overestimates, and during solar minimum it under-

estimates, the real irradiance variations.

Our interpretation is as follows: the variation of Lyman alpha is primarily caused by the bright plages

and the active network (LEAN, 1987), while FlO includes a strong gyroresonance component related

to the strong magnetic fields concentrated in sunspots

(TAPPING, 1987). When active regions first form, the sunspots are relatively more important than the

associated plages, causing an early peak in FlO. As active regions evolve, the plage area continues to expand and may remain large for several rotations, thereby continuing to emit Lyman alpha. Therefore,

during solar minimum the continuously decreasing

emission from the plage remnants and network can be detected in the UV spectra1 bands, but not in FlO.

Furthermore. the fast rise of FlO during the ascending phase of the solar cycle indicates that the newly emerg-

ing magnetic fields. which lead also to the formation of sunspots, provide a signiiicant contribution to the

FlO variation. This also indicates that the phase shift between Lyman alpha and FlO is primarily related to

the gyroresonance component of FlO, rather than to the transition-region and coronal component (see also DONNELLY. 1990).

CONCLUSIONS

Irradiance models developed with multiple linear

regression analysis. including daily values and 8 1 -day running means of solar indices, predict reasonably well the variations observed in Lyman alpha. The full disk equivalent width of the He line at 1083 nm offers a good proxy for Lyman alpha, and total solar irradiance corrected for the sunspot effect also has a high correlation with Lyman alpha.

A phase shift is found between Lyman alpha and 10.7cm radio flux. During the rising portion and

J. M. PAP ~1 nf.

lrrodiance model on EWHE

I I I I I I I I s-

lrradiance model on FI 0.7 b zz : 7_ 0

: 6__

g 5__ u

: 4_ E 3

0.293 - I I I I I I I i

- 1.1

.12 C

7 < ,:G.

.F 0.28 - ::- * _ 1.05

I I .A’ 0 7

I

? 0.27 _ . ..“’ , ,. _ 1

iz - Nimbus-7/R(c/w) ZZ

’ 0.26 L

NOAAS/Rfc/w)

I I I I I 1 I 0.95 1974 1976 1978 1980 1982 1984 1986 1988 1990

Fig. 3. The 77~day running means of Lyman alpha modeled from the EWHe (a), FtO (b) are shown. (c) gives the 27-day running means of the Mg core-to-wing ratio calculated from the measurements of the

Nimbus-7 (solid line) and NOAA9 (dashed line) satellites.

maximum of the solar cycie the model of Lyman alpha based on FIO overestimates, and during solar mini- mum it underestimates, the actual variations of UV irradiance. Our view is that a significant part of the variation of FIO is related to its gyroresonance com- ponent, which is associated with the newly emerging magnetic fields, while variation of ultraviolet irradiance is more directly related to the changing emission of @ages and the active network (LEAN. 1987). A correction for the gyroresonance component of FIO (DONNELLY. 1990) may significantly improve

the models of Lyman alpha. This has a great import- ance, considering that daily values of FlO are available back to 1947, and would provide estimates of Lyman alpha in time intervals when no satellite observations exist.

Ac,ltrzollkeulye,,2ents--The authors express their gratitude to Dr J. Harvey for providing the Mc 1083 data. which are produced cooperatively by NSOINOAO, NASAjCSFC and NOAA/SEL. This research was supported by a NOAA grant. The SME data are available from NSSDC and from the SME databases at the University of Colorado.

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