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Advances in Space Research 37 (2006) 234–237

Correlated variations of solar EUV line emissions

Karl Rawer a,*, Hongsheng S. Tai b

a Albert-Ludwigs-Universitat, Freiburg i.Brsg., Herrenstr.43, D 79232 March, Germanyb Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China

Received 21 September 2004; received in revised form 14 December 2004; accepted 11 February 2005

Abstract

Spectral measurements of the AEROS satellites were used to check Hinteregger�s (Hinteregger, H.E. The extreme ultraviolet solar

spectrum and its variation during a solar cycle. Ann. Geophys. 26, 547, 1970) idea that monitoring two suitably chosen EUV lines

might well allow assessing the aeronomical effects of solar EUV. Representative daily intensity values have been determined for 25

most important EUV emission lines. Cross correlations calculated for each of these with three classical activity indices (RZ, F10.7

and Ap) ended-up with mostly insignificant correlation values. Correlations were also determined for all couples of the 25 emission

lines themselves. In a two-dimensional field there appeared two distinct districts with significant correlations amongst all lines

therein, one district with mainly chromospheric emissions, the other one with those of the corona. Our results may confirm Hinte-

regger�s idea. It is concluded that an extremely simple spectrometer monitoring just two lines might be most helpful to fill the gaps

between more ambitious solar EUV-missions.

� 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar EUV radiation measurements; Solar activity indicies

1. Introduction

For aeronomical applications, it is of considerable

interest whether the intensity variations of different

emission lines are narrowly correlated. If so the EUV in-

put might be estimated from the intensities of a few lines

only as Hinteregger (1970) has proposed.

At least for the quiet thermosphere solar EUV radia-

tion is the most important energy input. Unfortunately,there are only few satellite missions in which the relevant

solar emissions were continuously monitored line by

line. Based on Hinteregger�s (1970) design of a slitless

solar spectrometer Schmidtke�s instrument consists of

a plane grating followed by a mechanical collimator

and a photomultiplier. Unfortunately, the latter�s sensi-

tivity decreases during flight. Therefore, distinct from

earlier missions the multiplier was regularly recalibrated

0273-1177/$30 � 2005 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2005.02.026

* Corresponding author.

E-mail address: 19041913rawerkarl@web.de (K. Rawer).

in flight with a beta-source. During the AEROS satellitemissions (A-1973, B-1974/75; Schmidtke et al., 1974) the

intensities of 25 most prominent lines were regularly re-

corded nearly day by day. With these spectra, a repre-

sentative daily one was derived for all observation

days (Tai, 1998). Our analysis is based on periods of

128 days in missions A and B, respectively. Mission A

happened in the downgoing part of a solar cycle (sun-

spot numbers RZ around 40), mission B during mini-mum solar activity. Cross correlations were computed

in order to find relations between EUV line intensities

and (i) three well known activity indices, and (ii) be-

tween EUV emissions themselves. In order to avoid ef-

fects of short-lived events, we excluded from the

correlograms 5% of most aberrant points and so ob-

tained ‘‘reduced’’ correlation coefficients.

Since under (ii) above some correlation coefficients(cor) were found to be quite near to one we translated

them into the so-called lgk-values defined by

lgk ¼ �2� logð1� corÞ ð1Þ

ved.

Fig. 1. (a) Cross-correlation pattern to mission AEROS-A. (b) Cross-correlation pattern to mission AEROS-B. The selected emission lines in the

order of Table 1 appear on both axes, horizontally: left to right, vertically: top to bottom. The physical identification is shown (horizontally) on top,

vertically only by identification numbers.

K. Rawer, H.S. Tai / Advances in Space Research 37 (2006) 234–237 235

In the following, we apply lgk-classes defined by

lgkcl ¼ ROUNDðlgkÞ ð2ÞNote that negative and insignificant correlation coef-

ficients <0.82 lie in lgk-classes 61. Significant positive

correlations are found in class 2 that contains cor values

from 0.822 to 0.944, class 3 from this value to 0.982,

class 4 to 0.994, and class 5 to 0.997. In Figs. 1 and 2

(see Table 1) different symbols designate significant clas-

ses P2 as specified in the captions.

2. Relations with activity indices

We investigated potential connections with three

classical daily activity indices RZ (Zurich sunspot

number), F10.7 (Covington index) and Ap (magnetic

disturbance index). Seventy-five percent of all values

were found insignificant or negative. For short-term

variations as are considered in this paper none ofthese three indices can therefore be taken as �leader�when the variations of unobserved EUV lines are to

be reconstructed.

3. Correlations between line intensities

Quite different is the situation with mutual correla-

tions between line intensities. Negative values are very

rare, the overall statistics has about 50% in class 0 or1 and almost 30% in higher, significant classes. So,

while not all line emission couples are significantly

correlated there exist groups of highly interrelated line

couples.

The field of cross correlations for all couples of

lines is shown in Figs. 1(a) (mission A) and (b) (mis-

sion B). Only lgk classes greater 1 are marked. Both

axes are ordered after decreasing wavelength, i.e.,increasing quantum energy. Fig. 1(a) obtained in a

period of middle solar activity clearly shows two dis-

tricts inside which quite high correlations exist. One

district (left hand on top) contains lines between

103.8 and 63.0 nm, another one (bottom right hand

corner) concerns lines from 52.1 to 28.4 nm. Appar-

ently, the first district refers to chromospheric emis-

sions of H-atoms namely Lyman beta, gamma anddelta, and of ions OIII to OVI, NIII, CIII, SVI and

NeVIII. Typical coronal emissions of higher ionized

Fig. 2. (a) Cross-correlation pattern to mission AEROS-A. (b) Cross-correlation pattern to mission AEROS-B. (Symbols are explained in Fig. 1(a).

However, the selected emission lines are ordered after increasing total energy.).

Table 1

Identification: solar EUV quanta fluxes

236 K. Rawer, H.S. Tai / Advances in Space Research 37 (2006) 234–237

ions are found in the second district, namely FeXVI,

SiXII, MgIX, MgX and NeVII. Almost all couples

outside these districts show less significant values. Just

in these corners correlations of chromospheric with

coronal emissions are noted. Evidently, these are too

low for our purpose. So, Hinteregger (1970) was right

in proposing the use of not one but two leaders,

namely a coronal and a chromospheric one.Fig. 1(b) obtained during minimum solar activity also

shows two such districts, the first one extending down to

76.0 nm being interrupted at the O-ions emissions 83.4

and 79.0 nm. The second district is also larger ranging

from 63.0 to 28.4 nm and additionally contains emis-

sions of HeI, MgX, CX and OV. Note that quite gener-

ally weaker correlations appear under solar minimum

conditions. This may be due to the fact that the fre-

quency of solar disturbances is smallest in that period.

So in both missions there appear two districts with high

correlations, one attributed to chromospheric, the other

one to coronal emissions.Feeling that instead of ordering by quantum energy

one should better order by emission temperature, i.e.,

by the sum of ionization plus quantum energy we exe-

cuted the relevant transformation on both axes and so

K. Rawer, H.S. Tai / Advances in Space Research 37 (2006) 234–237 237

obtained Figs. 2. Compared to Figs. 1, the structure in

the field has not become clearer.

4. Conclusions

Our findings are interpreted in favour of Hinteregger

(1970) idea that monitoring two suitably chosen EUV

lines might allow a good estimate of the variations of

the solar EUV input into the thermosphere. Moreover,

since the relative intensities in the solar EUV spectrum

are known from a few missions with full scale spectral

measurements, simple two lines spectrometers could fill

the long gaps in time between first class missions.

Acknowledgement

Our thank is due to G. Schmidtke for redactional and

editing help.

References

Hinteregger, H.E. The extreme ultraviolet solar spectrum and

its variation during a solar cycle. Ann. Geophys. 26, 547,

1970.

Schmidtke, G., Schweizer, W., Knothe, M. The AEROS EUV

spectrometer. J. Geophys. 40, 577, 1974.

Tai, H.-S., Personal communication, 1998.