Indian Journal of Radio & Space Physics Vol. 34, June 2005, pp. 153- 160

Solar EUV flux during sunspot cycles 21, 22 and 23-Correlation with proxy indices and real time prediction

K K Mahajan & A K Dwivedi

Radio & Atmosphere Science Di vision, National Physical Laboratory, New Delhi 11 0 012

Received 22 December 2004; accepted 4 March 2005

Langmuir probe on the Pioneer Venus Orbiter measured the total solar EU V flux during large portions of sunspot cycles 2 1 and 22, while CELIAS/Solar EUV Monitor (SEM) on the SOlar and Heliospheric Observatory (SOHO) measured thi s flu x in the spectral ranges 26-34 nm and 0.1-50 nm during the whole of sunspot cycle 23 . In thi s paper, daily values of these EUV flu xes are studied in relation to various often-used proxy indices to identify a single index which has the hi ghest corre lation coeffic ient with the observed values during all the three sunspot cyc les. It is observed that no single index exhibits this featu re. However. almost all the proxy indices averaged over one-half or more solar rotations show a hi gh degree of correlation with the daily EU V flu x. Further, the solar magnetic field is found to ex hibit somewhat better corre lation and it is recommended that this index averaged over Jhree previous solar rotations may be used for rea l time prediction of solar EUV flu x.

Keywords: Solar EUY fl ux, Solar indices, Sunspot cycles, Proxy indices PACS No.: 96.60 Rd, 96.60.Tf; 96.60 Qc

1 Introduction Different layers of the solar atmosphere, namely,

the photosphere, chromosphere, chromosphere-corona transition region and the corona emit a variety of electromagnetic radiations. These radiations cover a wide spectral range, from X-rays, EUV, UV , Visible, lR to radio waves. Solar radi ations with wavelength smaller than 130 nm form the X-ray and EUV part of the spectrum and originate in the chromosphere, in chromosphere-corona transition region and in the solar corona. Although this part of the solar spectrum accounts only fo r a negli gible fraction of the total solar irradi ance, it is one of the major energy input to the earth ' s thermosphere and ionosphere. The absorption of these radi ations by the major neutral constituents 0 , 0 2 and N2 above 100 km in the earth ' s atmosphere causes most of the heating in the thermosphere and results in the photo ionization of these neutral constituents thereby forming the ionosphere. Thus fo r studies dealing with the earth ' s ionosphere and thermosphere, knowledge of EUV flux becomes a bas ic requirement. These flu xes also have a practical use in ionospheri c and thermospheric models 1 which are often employed for calculating range errors and satellite drag.

In view of the importance of solar EUV for ionospheric and thermospheric stud ies as well as in their application potenti al, spectral measurements of solar EUV started way back in early sixties with

OSO- 1, and were followed by several mi ss ions li ke OS0-3, OS0-4 , AEROS-A, AE-C, AEROS-B, AE-E, SM-5 and others (see Ref. 2 for a review). These measurements undoubtedly provided the first results on short and long-term changes in · the EUV flu x related to solar rotation and solar cycle and consequently empirical relations of EUV with the well known solar activity index, F 107 (solar radi o noise at 10.7 em), were generated. Since measurements on daily basis were made only for some limited periods during these miss ions, attempts to study relationship between these fluxes and other proxy parameters of solar acti vity could not be of a very detailed nature and thus were of a limited value for real time prediction of EUV flu x. However, with the measurement of total EUV flux on dail y basis by the Langmuir probe on the Pioneer Venus Orbiter fo r the period 1979-1992 (Refs 3 and 4) and SEMI SOHO measurements of EUV in the ranges 0.1-50 nm and 26-34 nm from 1996 to date5

, the situation has greatly improved. Figure 1 shows the periods covered by these measurements as a functi on of solar acti vity . Except for the 3Y2 years "EUV hole" from Ju ly 1992 to December 1995 , there is continuous data on the solar EUV flux from January 1979 to date. Based upon these data several studies have been made in the recent past to examine the relationship of these tlu xes with other proxy parameters6

-8

. Viereck et al. 7, for

example, compared the SEM/SOHO measurements of

154 INDIAN J RADIO & SPACE PHYS, JUNE 2005

180

180

140

120

100 .... ;

80

60

40

20

0 1979 1984 1989 1994 1999 2004

YEAR

Fig. !- Solar EUV measurements during solar cycles 21, 22 and 23. [Langmuir probe on PYO measured total solar EUV flux during major portions of solar cycles 21 and 22, while SOHO/SEM measured the EUV flux in the range 0.1-50 nm and 26-34 nm during solar cycle 23.]

daily solar EUV flux at 30.4 nm during the rising phase of solar cycle 23 with proxy parameters F 10.7

and Mg lJ ratio and found an excellent correlation with these parameters. They also found that Mg II index tracked the solar EUV flux better than F1 07 and that the correlation coefficient between daily solar EUV and Mg Tl improved to 0.996 wh en 13-day running average of the Mg II index was employed. For real time prediction, Viereck et a/. 7 generated the empirical relationship (EUV PrOXYreahime = 0.6 Mg Il cta ily + 0.4 Mg lh9-ctay avg) after obtaining a correlation of 0.99 1 between the time series calculated with this formula and the SEM 30.4 data for the 4-yr period starting from January 1996.

The He 30.4 nm radiations analyzed by Viereck et a/. 7 are of coronal origin. Solar EUV radiations of wavelengths less than 50 nm which originate from corona contribute less than 10% to the total solar EUV flux ( 10-130 nm), the remainder of the flux originates from chromosphere and the transition region. For aeronomical applications, knowledge of EUV flux from all the radiations (i.e. total EUV flux) is more meaningful. Therefore, in this paper we re­visit the total EUV flux measured by the Langmuir Probe on PVO during solar cycles 21 and 22 and examine its relationship with the often-used proxy parameters like F 10.7, Ly-a, Mg II index and solar magnetic field. In the present analysis SEM/SOHO

measurements of EUV flux during the ri sing as we: as the declining part of solar cycle 23 are als included and an attempt is made to identify a prox index which can be used for real time prediction o solar EUV flux. It is found that mostly all of th often-used proxy indices, averaged over one-half o more solar rotations, show a high degree o correlation with the daily EUV flux measured 01 PVO as well as on SOHO and that solar magnet i' field, Bs, exhibits somewhat better correlation.

2 Solar EUV data sets of solar EUV flux

2.1 PVO Langmuir probe data The Pioneer Venus Orbiter (PVO) employed ;

rhenium-coated Langmuir probe to measure th1 temperature and concentration of electrons in th1 ionosphere of Venus9

. However, when the probe wa outside the ionosphere and was in the solar wind. i measured photoelectron current and thus acted as ; photo diode3

. The solar EUV radiations between )( and 130 nm, on striking the rhenium surface, ejecte< photoelectrons which produced current at the level o 5-20 nA. The current (/pe) so produced is proportiona to the total solar EUV flux for wavelengths whid create the ionosphere and heat the thermosphere. /­daily value of / pe was obtained for each pass by notin: the maximum level of / pe from measurements take1

MAHAJAN & DWIYEDI: SOLAR EUY FLUX & CORRELATION WITH PROXY INDICES 155

about one hour before periapsis. That the lpe was indeed proportional to solar EUV flux was first demonstrated by Elphic et al. 10 while studying the ionospheric variability at Venus who found that probe current very faithfully tracked the solar-rotation­related cyclic changes in the Venus ionosphere. Subsequently Brace et a/ .3 derived the equation

VEuv = 1.53xl011 /pephotonscm-2 s- 1

(where /pe is in 10-9 A) to calculate EUV flux at Venus and studied its morphology during the declining part of solar cycle 21. Hoegy and Mahajan 11

, by translating thi s EUY flux to the solar longitude of earth, demonstrated its application for aeronomic studies at the earth. Hoegy et a/.4 examined the Ire measurements over the 13'12-yr period from January 1979 to June 1992 and reported that solar EUY flux behaved differently during the two solar cycles 21 and 22, in consonance with solar magnetic field 12

• They further discussed at length any possibility of instrumental errors due to calibration problems during the long 13'12-yr period and concluded that no such errors existed and that the probe gave reliable values of the EUY flux during the whole period.

2.2 SOHO/SEM data The solar EUY Monitor (SEM) experiment has

been described by Ogawa et a/.13 and its first results have been reported by Judge et a/.5 The SEM is a highly stable transmission grating EUY spectrometer which is a part of the Charge, Element and Isotope Analysis System (CELIAS) instrument aboard the SOlar and Heliospheric Observatory (SOHO) satellite. There are three highly efficient aluminium coated silicon photo diodes, which measure the solar radiance at the zero order and 30.4 nm first order positions. The zero order detector (channel 2) primarily measures the solar irradiance within the nominal aluminium band pass (17-77 nm), while the first order detectors (channels 1 and 3) measure the irradiance within an 8 nm bandpass centered about the 30.4 nm solar He fl emission . There are two sets of SEM EUY data, one centered about 30.4 nm and the other for the range 0.1-50 nm. The He 30.4 nm line contributes about 50% of the total signal to the SEM data which matches with the portion of atmospheric heating contributed by He II 30.4 line7

• Mostly He II 30.4 data have been used in the present analysis. The SEM/SOHO solar EUV data are available on http:!!www. usc.edu/dep/space-science/semdata.htm

and the data for the period January 1996-June 2003 have been employed in the present analysis, thus covering all the rising part and a very large part of the declining phase of sunspot cycle 23 .

3 Broad features of solar EUV flux during sunspot cycles 21, 22 and 23

Dai ly values of solar EUV flux for sunspot cycles 21, 22 and 23 are plotted in Figs 2, 3 and 4 , respectively. A major feature, also reported earlier by several workers, is the large variance in the flux on day-to-day basis during the solar maxima. Thi s variance is rather small during the solar minima. Another important feature is that while the total solar EUY flux (1 0-130 nm) measured by the Langmuir probe changed by factors of 1.6 and 2.0 from solar

cycle 21 ~ 2110 E ~ 9 150 0

'X ..., 100

" ,..:;

50

0.280 l = .. ~ ::0

' ~ ~ s ~ c 0 Q ~ Q.

.; 6£+11 ;:, <ol

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~ c 0 Q .. ~

~ Q. c.... ·= SE.

; .. 19110 1982 1984 1986

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Fig. 2-Plot of daily values of total EUY flux measured on PYO during solar cycle 21. [Proxy indices F 10_7, Mg II , Ly-a and 8 ,. averaged over three previous solar rotations are also plotted fo r the same period. Average values ver)' fa ithfully track the daily values of EUV flux (F10.7 is in units of 10-22 W m- 2

; EUY is in units of photons cm-2 s- 1

; Ly-a is in units of 10 11 photon cm-2 s- 1;

and 8 , is in units of gauss).]

156 INDIAN J RADIO & SPACE PHYS, JUNE 2005

~~ 2DO

E ~ 150 ~

;: X 100 "" :;; .;:

50

= .. =-

' ~ ~

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Fig. 3-Same as Fig. 2, but fo r solar cycle 22

min imum to solar maximum in sunspot cycles 2 1 and 22 respecti vely, this fac tor was about 3 in the 30.4 nm and 0.1-50 nm channels in sunspot cycle 23 . This is an important result which has aeronomical implications in modelling solar cycles changes in thermospheri c temperature and composition and thus in the ionospheric behav iour. Most studi es in the pas t have adopted a factor of two change in the EUV flu x responsible for thermospheric heating during a solar cycle.

Figure 4 contains EUV flu x measured in 30 .4 nm channel as well as the rati o of the EUV flu x in the 0. 1-50 nm to the one in the 30 .4 nm channel. It can be noted that the flux in the range 0.1-50 nm is larger by about a fac tor of two as compared to the one in the range 26-34 nm. This fac tor changes marg inally from 1.9 to 2. 1 during the solar cycle, but there is a very large day-to-day fluctuation in thi s fac tor which is associated with the 27-day solar rotation, especially, at shorter wavelengths. Further compari son of PVO measured total EUV flu x (pl otted in Figs 2 and 3)

Cycle 23 ~ E ~ 250

'I 2DO C>

s 150

:; 100 .;:

= .. =-'~

YEAR

Fig. 4-Same as Fig. 2, but for solar cyc le 23 [Add it ionally ratio of EUV flu xes in the ranges 0. 1-50 nm and 26-34 nm (X UV /EUV) are al so plotted.]

with the SEM/SOHO measured fl ux from wavelengths less than 50 nm (which orig inate from coronal regions) shows that the coronal EUV flu x contributes about 4% during sol ar minimum and about 7% during solar max imum to the total solar E UV flux . An examination of Figs 2-4 also reveals that, in addition to the well known solar rotation and solar cycle changes, the 7-month peri odicity in the PVO solar EUV flux 3

·14 is also present in the

SEM/SOHO solar EUV flu x. This peri odicity is seen more clearly when 8 1-day averaged va l es are pl otted (not shown in Fig . 4).

Figures 2-4 also contain plots of important solar EUV proxy indices F 10_7, Mg II, Ly-a and so lar magnetic field, averaged over three prev ious solar rotations. One can note that all the average proxy indices, very fa ith full y, track the daily EUV flu x.

3.1 Correlation with proxy indices As po inted out earlier, in a recent study Viereck

et al.1 fo und a very high degree of correlation (correlation coefficient 0 .981 ), on dai ly basi s. between SEM/SOHO solar EU V flu x measured in the 30.4 nm channe l and the Mg fl index during the rising

MAHAJAN & DWIVEDI : SOLAR EUV FLUX & CORRELATION WITH PROXY INDICES 157

phase of solar cycle 23 . The/ have, therefore , recommended the use of Mg II as a proxy index for realti me prediction of solar EUV flux . In view of thi s important result, we have extended this analysis to the total solar EUV measured on PVO during the two previous sunspot solar cycles and also to al l the SEM/SOHO EUV data avai lable to date. Figure 5 shows a scatter plot of daily values of these two parameters for each of the sunspot cycles 21 , 22 and 23. As reported by Viereck et aC one can see that there is nearly one-to-one relationship between the SEM 30.4 and Mg II in solar cycle 23. However, this relationship is not as good as that between the PVO total EUV flu x and Mg II for cycles 21 and 22. In fact, there is a large scatter near the solar maxima.

The correlation of the daily val ues of EUY flux with each of the four proxy parameters, averaged over solar rotations has also been presented in this study. In Figs 6-9 are shown the scatter plots for these proxy parameters averaged over three previous rotations.

1[+12 ,-,-,1-,-,-1 .--.--l,-,1-.--.-\.--.--\-,--,

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Fig. 5-Plots of daily values of solar EUV fl ux and Mg II index for solar cyc les 2 I. 22 and 23 [There is a large scatter in solar cycle 21 and 22, especially, at solar maxima; but scatter is rather small in solar cycle 23. Cycles 2 1 and 22 have total EUV flux, while for cycle 23, the flux is for 26-34 nm range, which is less than 10% of the total flux . The units of EUV are given in Fig. 2. )

1£+12 ,--,-,1-r--r-1.--,1-r---rl--,-,--.,--,.,--~

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Fig . 6-Piots of dai ly values of EUV flux with Mg II index averaged over 3 previous solar rotat ions for solar cycles 21. 22 and 23 [The units of EUV are given in Fig. 2.]

~~ [__L _ _L____JL___L. __ J____l _ _L_J..____l

40 110 120 2110 240

Fig. 7- Piots of daily values of EUV flu x wi th F 10_7 averaged over 3 prev ious solar rotations for solar cycles 21. 22 and 23 [The units of EUV and F 10.7 are given in Fig. 2.]

!58 INDIAN J RADIO & SPACE PHYS, JUNE 2005

~·u ~~~~--~--~--.--.1--.--.1--.-~

§ BE+ll i­o .c Q.

~ '::. BE+ll l-­;;.. ;;,

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75 Cycle 22 ~ lE+U i-c .. 0 .c Q.

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Fig . 8-Piots of daily values of EUV flux with 8 , averaged over 3 previous solar rotations for solar cycles 2 1, 22 and 23 [The units of EUY and B, are given in Fig. 2.]

One can note the reduction in the scatter, though the scatter is still appreciable at solar maxima. It is interesting to note that daily values of 30.4 nm EUV show less scatter with daily Mg II data than with the one averaged over three previous solar rotations. Table I g ives the correlation coefficients between the EUV flux and the proxy parameters averaged over 13, 27, 40, 54, 67 and 81 previous days. The other commonly used index, the sunspot number, has a lso been included in this analysis. It can be noted that no single index shows the highest corre lation coefficient in all the three solar cycles. In cycle 21, all the proxy indices have almost the same correlation coefficient with the EUV flux, in cycle 22 solar magnetic field has an edge over the other indices and in cycle 23, daily values of Mg II have the highest correlation coefficient, although solar magnetic field is quite close.

It is to be pointed out that the correlation coefficients between He 30.4 and Mg II shown in Table 1 are somewhat smaller than those reported by

' ~+U I I 1 ~

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Fig. 9-Piots of daily values of EUV flux wi th Ly-a averaged over 3 previous solar rotations for solar cycles 21. 22 and 23 ]The units of EUV and La are g iven in Fig. 2.]

Viereck et a/7• This is due to the fact that we have

included data from the declining phase of cycle 23 also. As pointed out by Viereck et a/.7

, the large value of the correlation coefficient obtained by them was due to the large dynamic range of EUV flux in the 4-yr period which dominated over the short-term correlations. Correlation coefficient between SEM 30.4 nm and Mg II was comparatively small during the solar minimum periods.

The correlation coefficients of solar EUV flux with these proxy indices averaged over 13, 27, 40, 54, 67 and 81 current and future days have also been examined. Tables 2 and 3 contain these coefficients. As can be noted, these are not very different from those given in Table I . A very surpri sing result is that sunspot number shows up as good a proxy index as any other index when averaged over one-half or more solar rotations.

A comment on the data gaps in the proxy indices is in order. Although there were no data gaps in FHn,

MAHAJAN & DWIVEDI: SOLAR EUV FLUX & CORRELATION WITH PROXY INDICES 159

Ly-a and sunspot number, large gaps were found in Mg li and 8 ,. This resulted in loss of data points when values averaged over several rotations were required. However, data gaps for periods less than 5 days were filled by cubic spline method. Correlations coefficients in Tables 1-3 are based upon a couple of thousand or more data points and we feel that these data gaps may not have much effect on the correlation coefficients given in Tables 1-3.

4 Real time prediction of solar EUV flux and conclusion

Index

Mgll

Fw.7 Mag. field Sunspot Ly-a

Mgll

F 10.1

Mag. field Sunspot Ly-a

Mgll

F 10.1

Mag. field Sunspot Ly-a

Index

Mgll F 10.1

Mag. field Sunspot Ly-a

Mgll

F 10.1

Mag. field Sunspot Ly-a

Mgll

F 10.1

Mag. field Sunspot

Ly-a

From the above analysis, it is quite clear that real time prediction of solar EUY flux, and especially the total solar EUY flux, cannot be made on the basis of daily values of any of the proxy indices including the Mg II index. From an examination of Tables 1-3 it seems that solar magnetic field has an edge over the other indices and, therefore, it is recommend that B,

Table I -Correlation coefficient of solar EUV flux with proxy indices

Daily Correlation coefficient averaged over days before 13 27 40 54 67 81

For cycle 21

0.838 0.858 0.888 0.884 0.888 0.886 0.888 0.806 0.832 0.874 0.875 0.886 0.886 0.891 0.778 0.829 0.857 0.86 1 0.866 0.868 0.872 0.762 0.806 0.861 0.865 0.877 0.878 0.884 0.812 0.827 0.853 0.849 0.853 0.851 0.852

For cycle 22

0.805 0.861 0.882 0.895 0.898 0.903 0.903 0.778 0.828 0.863 0.876 0.884 0.892 0.893 0.840 0.890 0.927 0.922 0.927 0.932 0.935 0.721 0.784 0.825 0.842 0.853 0.858 0.857 0.847 0.871 0.880 0.882 0.882 0.884 0.883

For cycle 23

0.966 0.935 0.942 0.935 0.933 0.928 0.925 0.943 0.933 0.947 0.947 0.944 0.941 0.938 0.841 0.950 0.959 0.961 0.957 0.955 0.949 0.894 0.911 0.935 . 0.938 0.938 0.937 0.937 0.962 0.933 0.944 0.940 0.938 0.935 0.934

Table 2-Correlation coefficient of solar EUV flux with proxy indices

Daily Correlation coefficient averaged over days after 13 27 40 54 67 81

For cycle 21

0.838 0.888 0.893 0.904 0.904 0.915 0.915 0.806 0.862 0.876 0.894 0.897 0.912 0.916 0.778 0.855 0.869 0.881 0.887 0.899 0.905 0.762 0.834 0.855 0.877 0.881 0.898 0.902 0.812 0.854 0.861 0.872 0.973 0.885 0.887

For cycle 22

0805 0.835 0.863 0.861 0.864 0.864 0.863 0.778 0.813 0.844 0.847 0.851 0.853 0.856 0.840 0.894 0.916 0.922 0.931 0.939 0.945 0.721 0.772 0.816 0.835 0.851 0.864 0.874 0.847 0.861 0.874 0.874 0.875 0.875 0.875

For cycle 23

0.966 0.936 0.949 0.946 0.946 0.944 0.945 0.943 0.908 0.927 0.925 0.927 0.925 0.927 0.841 0.908 0.939 0.939 0.942 0.944 0.946 0.894 0.844 0.906 0.905 0.909 0.909 0.913 0.962 0.940 0.944 0.944 0.938 0.935 0.933

160 INDIAN J RADIO & SPACE PHYS, JUNE 2005

Table 3-Correlation coefficient of solar EUV flux with proxy indices

Index Daily Correlation coeffi c ient averaged over current days 13 27 40 54 67 8 1

For cycle 21

Mgll

F 10.1

Mag. fie ld Sunspot Ly-a

0.838 0 .806 0.778 0.762 0.8 12

0.870 0.892 0.843 0.874 0.843 0.889 0.822 0.858 0.838 0.858

0.893 0.896 0.898 0.902 0.884 0.89 1 0.896 0.902 0.871 0.874 0.877 0.909 0.869 0 .878 0.885 0.890 0.859 0 .862 0.864 0.867

For cycle 22

Mg fl

F 10.1

Mag. fi e ld Sunspot Ly-a

0.805 0 .778 0.840 0.721 0.847

0.845 0.873 0.819 0.852 0.901 0 .923 0.777 0 .822 0.863 0.876

0.878 0.884 0.889 0.897 0.859 0.845 0 .869 0.874 0.9 18 0.921 0.925 0.893 0.83 1 0.844 0 .851 0.855 0.878 0.879 0.880 0.882

For cycle 23

Mgll

F 10.1

Mag. fi eld Sunspot Ly-a

0.966 0.943 0.84 1 0.894 0.962

0.973 0.955 0.961 0.951 0.910 0.953 0 .938 0.937 0 .968 0.953

averaged over three previous so lar rotations may be used as a proxy index fo r real ti me prediction of solar EUV flux .

Acknowledgements The authors thank Dr L H Brace fo r inventing the

PVO photodiode, Dr TN Woods and Dr G J Rottman for Ly-a data, Drs D L Judge and H S Ogawa fo r SOHO/S EM data, and Dr K H Harvey fo r solar magnetic fie ld data, which were made a\'ail able on website. The authors are also thankful to CSIR fo r awarding the Emeritus Scienti stship to one of the authors (KKM ).

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