Correlated variations of solar EUV line emissions
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most important EUV emission lines. Cross correlations calculated for each of these with three classical activity indices (RZ, F10.7
put might be estimated from the intensities of a few lines
tivity decreases during ight. Therefore, distinct from
earlier missions the multiplier was regularly recalibrated
days (Tai, 1998). Our analysis is based on periods of
tained reduced correlation coecients.
Since under (ii) above some correlation coecients(cor) were found to be quite near to one we translated
them into the so-called lgk-values dened by
lgk 2 log1 cor 1
* Corresponding author.
E-mail address: email@example.com (K. Rawer).
Advances in Space Research 370273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rightsonly 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 Hintereggers (1970) design of a slitlesssolar spectrometer Schmidtkes instrument consists ofa plane grating followed by a mechanical collimator
and a photomultiplier. Unfortunately, the latters sensi-
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 nd 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-and Ap) ended-up with mostly insignicant correlation values. Correlations were also determined for all couples of the 25 emission
lines themselves. In a two-dimensional eld there appeared two distinct districts with signicant correlations amongst all lines
therein, one district with mainly chromospheric emissions, the other one with those of the corona. Our results may conrm Hinte-
reggers idea. It is concluded that an extremely simple spectrometer monitoring just two lines might be most helpful to ll the gapsbetween more ambitious solar EUV-missions.
2005 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Solar EUV radiation measurements; Solar activity indicies
For aeronomical applications, it is of considerable
interest whether the intensity variations of dierent
emission lines are narrowly correlated. If so the EUV in-
in ight 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 observationCorrelated variations of
Karl Rawer a,*, H
a Albert-Ludwigs-Universitat, Freiburg i.Bb Center for Space Science and Applied Resea
Received 21 September 2004; received in revised
Spectral measurements of the AEROS satellites were used to
spectrum and its variation during a solar cycle. Ann. Geophys.
might well allow assessing the aeronomical eects of solar EUVdoi:10.1016/j.asr.2005.02.026lar EUV line emissions
gsheng S. Tai b
Herrenstr.43, D 79232 March, Germany
Chinese Academy of Sciences, Beijing, China
14 December 2004; accepted 11 February 2005
Hintereggers (Hinteregger, H.E. The extreme ultraviolet solar47, 1970) idea that monitoring two suitably chosen EUV lines
presentative daily intensity values have been determined for 25
K. Rawer, H.S. Tai / Advances in Space Research 37 (2006) 234237 235In the following, we apply lgk-classes dened by
lgkcl ROUNDlgk 2Note that negative and insignicant correlation coef-
n Spa236 K. Rawer, H.S. Tai / Advances iions are found in the second district, namely FeXVI,
SiXII, MgIX, MgX and NeVII. Almost all couples
outside these districts show less signicant 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 rst 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
Fig. 2. (a) Cross-correlation pattern to mission AEROS-A. (b) Cross-correla
However, the selected emission lines are ordered after increasing total energ
Identication: solar EUV quanta uxesce Research 37 (2006) 234237from 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
tion pattern to mission AEROS-B. (Symbols are explained in Fig. 1(a).
obtained Figs. 2. Compared to Figs. 1, the structure in
the eld has not become clearer.
Our ndings 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 ll
the long gaps in time between rst class missions.
Our thank is due to G. Schmidtke for redactional and
Hinteregger, H.E. The extreme ultraviolet solar spectrum and
its variation during a solar cycle. Ann. Geophys. 26, 547,
Schmidtke, G., Schweizer, W., Knothe, M. The AEROS EUV
spectrometer. J. Geophys. 40, 577, 1974.
Tai, H.-S., Personal communication, 1998.
K. Rawer, H.S. Tai / Advances in Space Research 37 (2006) 234237 237
Correlated variations of solar EUV line emissionsIntroductionRelations with activity indicesCorrelations between line intensitiesConclusionsAcknowledgementReferences