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

    reserved.

    * Corresponding author.

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

    1. Introduction

    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

    Abstract

    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

    www.elsevier.com/locate/asr

    (2006) 234237

  • 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-

    cients

  • 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

    Table 1

    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).

    y.).

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

    the eld has not become clearer.

    4. Conclusions

    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.

    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.

    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