solar euv and uv spectral irradiances and solar indices

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Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 3–15

www.elsevier.com/locate/jastp

Solar EUV and UV spectral irradiances and solar indices

Linton Floyda,�, Jeff Newmarkb, John Cookb, Lynn Herringa, Don McMullinc

aInterferometrics Inc., 14120 Parke Long Court, Chantilly, VA 20151, USAbE.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375, USA

cPraxis Inc., 2200 Mill Rd., Alexandria, VA 22314-4654, USA

Available online 9 September 2004

Abstract

Several experiments have measured solar EUV/UV flux in the last 10–15 years including SUSIM UARS, SOHO

CELIAS SEM, and SOHO EIT and have generated multi-year spectral irradiance time series. Empirical models of these

important sources of radiant energy are often based on solar activity proxies, most often, the solar 10.7 cm radio flux

(F10:7). The short- and long-term correspondence of four solar activity index time series International Sunspot Number,

the He 1083 Equivalent Width, F10:7, and the Mg II core-to-wing ratio are analyzed. All of these show well-correlated

long-term behavior with F10:7 and Mg II showing the greatest long-term agreement among all of the index pairs.

However, during the recent maximum period of solar cycle 23, both the ISN and He 1083 have diverged significantly

from the others. Recent UV and EUV measurements are compared with Mg II and F10:7 to assess their value as solar

activity proxies. In every case, Mg II was found to correlate more strongly than F10:7 with the UV and EUV time series

which correspond to a range of solar atmospheric temperatures of 4000K–2MK. This correspondence indicates that the

mechanisms underlying irradiances changes from upper photospheric chromospheric, transition region, and lower

coronal solar atmospheric layers are closely linked.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Solar indices; UV; EUV; Spectral irradiance

1. Introduction

Solar activity indices have been and are currently used

to model solar spectral irradiances when direct measure-

ments are not available (Hinteregger, 1981; Heath and

Schlesinger, 1986; Cebula et al., 1992). In a series of

papers, Donnelly and his colleagues studied, in great

detail, the relationship of solar activity indices then

available to direct measurements of EUV and UV

irradiances (Donnelly et al., 1982, 1983, 1985, 1986). The

Mg II core-to-wing ratio index was not analyzed because

it was first devised (Heath and Schlesinger, 1986) after

these studies were completed. Since that time, the solar

e front matter r 2004 Elsevier Ltd. All rights reserve

stp.2004.07.013

ing author.

ess: [email protected] (L. Floyd).

UV irradiance (120–400 nm) has been measured con-

tinuously by several experiments using a variety of

techniques. By contrast and after an extended absence,

long-term spectral irradiance measurements in the EUV

(10–120 nm) spectral region only resumed with the

launch of the Solar Heliospheric Observatory (SoHO)

in late 1995.

In this paper, we briefly extend these earlier analyses

to newer UV and EUV irradiance data sets and solar

activity indices. Section 2 introduces four solar indices

and EUV/UV irradiance measurements with an empha-

sis on recent advances and data. The level of correspon-

dence of the variations of four solar activity indices, the

ISN, F10:7, Mg II, and He 1083, is discussed in Section 3.

In Section 4, we display and analyze two interesting

episodes of short-term (weeks to months) differences

d.

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ARTICLE IN PRESSL. Floyd et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 3–154

among solar indices and EUV/UV irradiances in order

to understand these differences. In Section 5, we

compare selected EUV/UV irradiance time series with

that of F10:7 and Mg II solar activity indices.

Table 1

SOHO EIT channel summary

Emission line l (nm) Temp. (K)

He II 30.4 80K

Fe IX/X 17.1 1M

Fe XII 19.5 1.5M

Fe XV 28.4 2M

2. Background

Solar radiation and its variation fundamentally

affects terrestrial atmospheric structure and climate. Of

particular importance are the solar radiant fluxes in the

extreme ultraviolet (EUV, 10–120 nm) and UV

(120–400 nm) spectral regions. Generally, shorter wave-

length radiations originate higher in the solar atmo-

sphere and are absorbed at higher altitudes in the

terrestrial atmosphere. For the most part, EUV radia-

tion emerges from the solar transition region and corona

while UV radiation originates in the transition region,

chromosphere, and upper photosphere. Virtually none

of the solar EUV and UV irradiance below 300 nm

reaches the Earth’s surface. For wavelengths above

300 nm, the incident light is significantly attenuated

through scattering and absorption.

Because of these effects, accurate measurements of

EUV/UV irradiance must be made from above the

terrestrial atmosphere. For more than 20 years, experi-

ments of various designs have measured the solar EUV

and UV spectral irradiance from satellites, balloons, and

rockets (Floyd et al., 2002a; Rottman et al., 2004;

Thuillier et al., 2004). Typically, the responsivities of

these sensitive instruments degrade as a result of intense

UV and EUV exposure. Because monitoring of these

instrumental changes is difficult, the spectral irradiances

often measured with large uncertainties relative to the

corresponding solar variation (Woods et al., 1996;

Cebula et al., 1998). Starting about 1990, several

experiments made significant progress in overcoming

these instrumental effects. In 1991, measurements began

by two solar UV irradiance experiments, SOLSTICE

and SUSIM, each carrying their own means of calibra-

tion (Brueckner et al., 1993; Rottman et al., 1993).

SUSIM utilizes redundant optical elements and mea-

surements of four stable deuterium lamps to account for

changes in its working optical channel. SOLSTICE

measures the irradiance of several stable bright blue

stars for the same purpose. Both of these experiments

continue to make solar UV irradiance measurements

(Rottman, 2000; Floyd et al., 2002a).

Measurements of the spectrally resolved solar EUV

irradiance began in the 1960s and were made with

increasing sophistication culminating in the long-term

measurements of the Atmosphere Explorer E (AE-E)

whose observations ended in 1981. From that time until

the mid-1990s, only occasional and relatively short-term

solar EUV measurements were made. This time period

which has come to be described as the ‘‘EUV Hole’’ e.g.

(Tobiska, 1996). Models of the EUV based on the AE-E

data and their relationship to solar proxies such as F10:7

have long provided estimates of EUV irradiance in the

absence of measurements. Tobiska (1996), for example,

provides an overview of these models. Solar EUV

measurements of comparable spectral width and resolu-

tion to that of AE-E have begun recently with the Solar

EUV Experiment (SEE) aboard the TIMED in 2002.

Long-term solar EUV measurements at reduced

resolution or cadence began earlier with the launch of

the Solar Heliospheric Observatory (SoHO). SoHO

carries two instruments capable of measuring EUV

irradiance on a daily cadence, the Solar EUV Monitor

(SEM) and the EUV Imaging Telescope (EIT). The

Solar EUV Monitor (SEM), a part of the CELIAS

experiment, observes the EUV in two wavelength ranges

(Hovestadt et al., 1995). Its first-order channel (hereafter

referred to as SEM1) measures an 8 nm portion of the

EUV spectrum roughly centered on the strong He II

30.4 nm transition region emission line. SoHO SEM is

calibrated via periodic rocket flights of a second SEM

that is in turn calibrated both before and after each flight

(Judge et al., 1999). The rocket-borne SEM is calibrated

on the ground both before and after each flight.

EIT images the sun in four EUV wavelength ranges

whose precise wavelength responsivities and equivalent

temperatures are given by Dere et al. (2000). Table 1

presents a summary of these channels that measure solar

irradiance emerging from the solar transition region and

corona. The raw EIT images have been flat-fielded, had

their time- and wavelength-dependent responsivity

calibrated. Although EIT was not originally intended

to produce solar irradiances, these processed images

were summed to produce integrated EUV spectral

irradiances (Newmark et al., 2004). The pixel responsiv-

ity of each of the EIT channels degrades in several

different ways which are difficult to separate (Clette

et al., 2002), but have been empirically modeled and

accounted for. For the most part, the EIT responsivity

degradation is the result of two basic processes:

reduction of the EUV light by surface contaminants

before it reaches the CCD and the reduction of the

CCD’s charge counting efficiency caused by radiation

damage. Radiation damage is the larger of these and

after April 1998, no further degradation caused by

contamination was observed. Periodic bakeouts of the

ARTICLE IN PRESSL. Floyd et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 3–15 5

CCD reverse much of the radiation damage and the

consequent degradation is tracked by the onboard

calibration lamp between bakeouts. By 2001, the overall

degradation exceeded a factor of 10. The combination of

calibration lamp images, solar pointing offset images,

and flights of the EIT calibration rocket are used to

correct the calibration of the individual pixels through

time.

Despite the image calibrations, the resulting irra-

diances still contain systematic errors of �30% resulting

from unresolved responsivity changes. The systematic

trends in the responsivity for each of the four EIT

channels have been accounted for through the use of

correlations with a solar activity index. The index

chosen for this purpose (Clette et al., 2002) was the

Mg II core-to-wing ratio, described in the next section.

The addition of correlation techniques yield a relatively

calibrated EUV irradiance time series in each of the four

EIT bandpasses. EIT irradiances in absolute units have

been produced by combining these relative time series,

comprehensive analyses of the pre-flight instrumental

calibration components (Dere et al., 2000) with a

differential emission measure (DEM) technique (Cook

et al., 1999). Thompson et al. (2002) have intercompared

the absolutely calibrated EIT irradiances with that of

SEM and SOHO CDS, demonstrating the validity of the

calibration with the DEM technique.

The ‘‘EUV hole’’ is not likely to be repeated in the

foreseeable future. The SEM, EIT, and TIMED experi-

ments continue to operate successfully. Future missions

will include the EUV Variability Experiment (EVE)

aboard the Solar Dynamics Observatory (SDO) sched-

uled for launch in 2007 (Woods et al., 2002) and the

GOES-N series of EUV monitors expected begin

operations by the end of 2004. EVE is designed to

measure the EUV irradiance with unprecedented ca-

dence and wavelength resolution. The GOES Solar

Instrument Suite (SIS) has six EUV channels of design

similar to that of SEM1.

3. Solar activity indices

The waxing and waning of the number of sunspots

and sunspot groups on an approximately 11-year cycle

visibly demonstrates that the sun changes through time.

From these observations, Wolf constructed the solar

activity index now known as the international sunspot

number (ISN). In its sanctioned version, ISN observa-

tions extend backward in time to the seventeenth

century. As radiometric solar measurements became

available, it was found that their variations roughly

correlated with the sunspot measure of solar activity.

The full disk solar 10.7 cm radio flux, known as F10:7 has

been measured from observatories on the ground daily

since 1947 (Tapping, 1987). Another gauge of solar

activity, the equivalent width of the He I 1083 nm

infrared absorption line, has been assembled from solar

ground-based images since 1974 (Harvey and Living-

ston, 1994). It has been found to describe chromospheric

radiation more accurately than F10:7 (Donnelly et al.,

1985).

3.1. Mg II core-to-wing ratio

Heath and Schlesinger (1986) developed the first

version of the Mg II core-to-wing ratio based on solar

UV irradiance from the SBUV experiment aboard

Nimbus-7. They also provided a demonstration of its

use as a proxy or substitute for variations in the solar

UV irradiance. Loosely described, the Mg II index is the

ratio of the chromospheric core to the photospheric

wings of the compound Mg II absorption feature near

280 nm. The core irradiance varies strongly and the

wings vary weakly with solar activity. Because the Mg II

index is a irradiance ratio and because instrumental

responsivity variations with respect to wavelength are

gradual, the Mg II index time series reveals the

underlying solar variations even in the presence of

instrumental trends. Usually, the Mg II index is

constructed to be explicitly unresponsive to responsivity

changes which are linear with respect to wavelength. In

the years since its original formulation, Mg II core-to-

wing ratios have been derived from the solar UV

irradiance measurements from a number of satellite

experiments using different instrumental resolutions and

algorithms (Weber et al., 1998; Floyd et al., 2002b; de

Toma et al., 1997; Viereck et al., 2001; Donnelly and

Puga, 1991; Cebula and Deland, 1998). Generally, the

algorithms are individually optimized for each instru-

ment to minimize trends and noise in the index product.

Although their absolute levels are quite different, owing

largely to instrumental resolution, these various instru-

mental Mg II indices have been found to have very

strong linear relationships to one another. This can be

understood by considering the approximation that only

the core irradiance varies and that the measured

irradiance is strictly proportional to the instrumental

response. (Both of these conditions are usually true to

some level of approximation.) For this idealized model,

all Mg II core-to-wing indices will necessarily be linearly

related to one another.

Given the linear relationship among Mg II indices

derived from different experiments and the need for

proxies for solar activity, composite versions have been

developed, the first of which was that for NOAA-9 and

Nimbus-7 (Donnelly and Puga, 1991). Viereck and Puga

(1999) have extended this to include several later Mg II

time series.

Fig. 1 shows the ISN, F10:7, He I 1083 EW, and

NOAA SEC Mg II Composite time series since 1947

when recorded measurements of F10:7 began. Although

ARTICLE IN PRESS

100

200

300 InternationalSunspot Number

18 19 20 21 22 23

100200300400

SF

UF10.7 cm

Flux

18 19 20 21 22 23

0.27

0.28

0.29

rela

tive

units NOAA SEC

Mg ΙΙ Index

21 22 23

1950 1960 1970 1980 1990 2000Year

5060708090 He 1083

21 22 23

Fig. 1. Four solar activity index time series: International Sunspot Number, F10:7, Mg II core-to-wing ratio, and the He 1083

Equivalent Width. The solid line represents an 81-day Gaussian FWHM filtered version of each.

Table 2

Correlations among solar activity indices (ISN, F10:7, Mg II,

and He 1083), their long-, and short-term components

Index pair Unfiltered Long-term Short-term

ISN F10:7 0.940 0.983 0.804

ISN Mg II 0.913 0.978 0.701

ISN He 1083 0.880 0.954 0.630

F10:7 Mg II 0.956 0.993 0.769

F10:7 He 1083 0.927 0.975 0.674

Mg II He 1083 0.969 0.984 0.856

L. Floyd et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 3–156

different, these time series are very similar in behavior.

The two dominant periodicities are that of the solar

activity cycle (�11 year) and of solar rotation (�27 day).

Both of these periodicities in the solar spectral irradiance

are a result of the behavior of bright regions on the solar

disk. These bright regions include faculae and plages in

the photosphere and chromosphere, respectively, as well

as smaller scale network elements. The number, size, and

intensity of active regions containing sunspots, faculae,

and plages roughly follow the 11-year sunspot cycle. As

viewed from Earth, the sun’s apparent rotation period of

about 27 days modulates the radiation received from

bright regions. Other periodicities have been found for

various time periods, e.g. 150- and 300-day (Lean, 1990;

Pap et al., 1990). Variations on these time scales are

associated with the formation and decay of active

regions. Crane (2001) shows that, for example, for

ISN, the 150-day periods were present in solar cycle 22

but not in solar cycle 23. Thus, at least for this case, the

150-day periodicity is not stationary.

3.2. Comparisons

Linear correlation among different solar indices is a

measure of their mutual correspondence. The first

column of Table 2 displays the linear correlations

between each pair for 5416 days between 7 November

1978 and 22 February 2003. In this comparison, only the

days for which all four of the indices are available

are considered. Doing so eliminates the effects of the

number of data points or the selection effect stemming

from data during specific time periods from influencing

the comparisons. Accordingly, the correlations depend

only on the quality of the measurements and the level of

correspondence between the physical processes respon-

sible for each. All four of the solar activity time series

are well correlated; the lowest correlation factor, r, is

0.880 between ISN and He 1083. The best correlation is

between Mg II and He 1083. This is to be expected since

Donnelly et al. (1986) found that the He 1083 to

correlate well with chromospheric and upper photo-

spheric UV irradiances and the former is derived from

those irradiances. Mg II also correlates better with F10:7

than does ISN.

To further explore the correspondence among these

solar indices (separately) over solar rotation (�27 day)

and longer time intervals, we filter the time series with a

normalized 81-day FWHM Gaussian function. This

contrasts with the filtering of ISN over approximately

yearly time scales which is used to define the canonical

minima and maxima for each solar cycle. Use of the

Gaussian rather than a ‘‘boxcar’’ of similar length

should produce reduced harmonic distortion of the time

series. Prior to filtering, missing data in Mg II and He

1083 time series were filled by linear interpolation. This

long-term (filtered) component of each solar index is

shown with the bold line in Fig. 1, so, in this case, all

days are represented in the long-term series over the

same time interval. To gauge the long-term correspon-

dence between pairs of solar activity indices, the linear

correlation between the long-term component of each

index are displayed in the second column of Table 2. The

highest long-term correspondence is between F10:7 and

ARTICLE IN PRESS

2000 2001 2002 2003year

0.265

0.270

0.275

0.280

0.285

Mg II

F10.7

ISN

He 1083

Fig. 3. Long-term correspondence among the Mg II, ISN,

F10:7, and He 1083 solar activity indices during the solar cycle

23 maximum. Linear regression was used to adjust the latter

three to the level of Mg II.

L. Floyd et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 3–15 7

Mg II having greater than a 0.99 correlation between

their long-term components. Also among the highest

correlations are (Mg II & He 1083) and (ISN & F10:7).

Fig. 2 displays the long-term Mg II time series and its

residuals when separately fit by each of the other three

indices. (We define the residual as the difference between

the quantity being fitted and the model.) As expected,

the 11-year solar cycle periodicity of the four long-term

indices vary more or less in phase with one another.

Aperiodic variations having characteristic time scales of

between 100 and 400 days are also apparently synchro-

nized, but have inconsistent amplitudes. An example of

these variations occurs during seen in the descending

phase of solar cycle (SC) 21 as displayed in Fig. 1.

Donnelly et al. (1986) associated these intermediate term

variations with the creation and decay of active regions.

When considering the indices together, no discernible or

significant trend in the level of the minima between 1986

and 1996 is observed. Particularly striking is the

divergence among the long-term indices during the

latter stages of the solar cycle 23 maximum.

Fig. 3 displays the long-term behavior of these indices

during the solar cycle 23 maximum. Although the

correspondence between F10:7 and Mg II is roughly

maintained during this period, there is significant

divergence of both ISN and He 1083. Starting in the

last quarter of 2001, all the indices except ISN rise to a

larger, second maximum. Near the end of 2001 when the

period of maximum activity is reached, He 1083

0.265

0.275

0.285 (a) Mg II

-5

0

5 (b) ISN

-5

0

5

resi

dual

s x

10-3

(c) F10.7

1980 1985 1990 1995 2000Year

-5

0

5 (d) He 1083

Fig. 2. Long-term correspondence among the ISN, F10:7, Mg

II, and He 1083 solar activity indices. Panel (a) displays the 81-

day Gaussian filtered Mg II. Panels (b) and (d) show residuals

of fits by similarly filtered ISN, F10:7, and He 1083 time series.

The residuals are defined as the filtered Mg II minus the fitted

value.

continues higher reaching and maintaining levels sig-

nificantly above F10:7 and Mg II. These relative

divergences continue into 2003. The observed divergence

between ISN and F10:7 calculated by the 81-day

Gaussian filtering method is the largest of any time

since the latter measurements began in 1947. It also

causes the time of solar maximum, as officially

calculated by approximately yearly averages, to occur

far earlier in ISN than for the others. In terms of the 81-

day filtered indices given here, the maximum of SC 23 in

ISN occurs in April 2000 while the corresponding

maximum in the other three indices occurs in early

2002, nearly 2 years later.

The corresponding short-term behavior of each solar

activity index is found by subtracting the long-term

version of the index (calculated by filtering as above)

from the index itself. The resulting time series will

contain only variations which occur on time scales of

less than 81 days. This should ensure that variations on

solar rotation time scales are present. The short-term

time series of the four solar activity indices, i.e. on

roughly solar rotation time scales, is always less

correlated with one another than is its long-term

counterpart. The highest short-term correlation is

between Mg II and He 1083 (0.853). This result is

consistent with the results of Donnelly et al. (1985) that

He 1083 is more consistent with the UV irradiance than

is F10:7. The second highest correlation is between ISN

and F10:7 (0.803). A possible explanation for this is a

result of the presence of a significant electron gyroreso-

nance component of the F10:7 flux in addition to that of

ARTICLE IN PRESS

0.2610.262 SUSIM Mg II (V21r2)


thermal emission (Tapping, 1987). The major sites of

this gyroresonance are the strong magnetic fields found

in sunspots.

0.2570.2580.2590.260

1.32

1.34

1.36

1.38

mW

/m2

SUSIM 160-165 nm

42.6

43.0

43.4

43.8

mW

/m2

SUSIM 200-205 nm

NOV DEC JAN FEB MAR APR MAY JUN

Fig. 4. Selected SUSIM solar UV irradiance time series for

1994–1995 displaying unusual 13-day periodicity.

4. Short-term variations in irradiance and indices

Donnelly et al. (1986) cited two principal causes of

short-term (weeks) differences among the UV and EUV

time series. Differences on solar rotation or shorter time

scales can arise due to differences in center-to-limb

variation of bright solar surface features. The depen-

dence of the brightness of a given solar feature as a

function of the cosine of the heliocentric angle, often

denoted as m, is referred to as the center-to-limb

variation. Even non-radiometric time series, such as

ISN, can be understood to have specific center-to-limb

behaviors (Crane, 1998).

A clear example of this effect is provided by UV

spectral irradiances during 1994–1995 as measured by

SUSIM (Crane et al., 2004). Apparently, bright regions

were concentrated approximately 180 � apart on the

solar surface during this time period. In the 200–205 nm

wavelength range, strong limb darkening (i.e. darkening

as m approaches zero at the limb) is found. The bright

feature distribution causes the irradiance signal to

contain a relatively strong 13-day component. This is

because the strong limb darkening lowers the contribu-

tion of bright regions to the measured irradiance twice

every solar rotation when the bright regions are in the

proximity of both solar limbs. Fig. 4 displays the UV

spectral irradiance time series for 160 nm, 200 nm, and

the Mg II index. The time series exhibit quite different

behavior because there is more center-to-limb darkening

near 200 nm than for either 160 nm or Mg II (Crane

et al., 2004).

Differences in the short-term behavior of solar indices

and spectral irradiances can also be a result of differing

temporal reactions to solar surface changes. The solar

minimum between solar cycles 22 and 23 occurred in

1996. For several months there had been few sunspots

and active regions, typical of solar minimum conditions.

Because decaying active regions continue to be bright

long after sunspots are no longer present, after this

long period with no sunspots, solar spectral irradiances

reached very low levels. Starting on 18 November 1996

(CR1913), newly formed active region AR7999 ap-

peared on the East limb and began to cross the

solar disk (National Geophysical Data Center, 1997).

At this time, only one other identified active region

existed (AR7997). Magnetograms show that only

weak and diffuse magnetic fields were present outside

these two regions. During their movement from East

to West, the magnetic field strength of AR7997 and

AR7999 grew considerably, especially the in latter. On

the next solar rotation, AR7997 (apparently) reappeared

as AR8004 and while AR7999 had no successor other

than a decaying remnant, still having relatively strong

magnetic fields associated with it dispersed over a

wide area.

Several solar irradiance and activity index time series

are available for this time period. Fig. 5 displays a

selection of these for several solar rotation periods

centered on November 1996. The ISN, F10:7, and GOES

X-ray flux (Garcia, 1994), time series reach a maximum

during the end of November during the rotation in

which AR7999 exhibited strong growth. By contrast,

Ly-a, Mg II, 200–205 nm irradiance, He II 30.4 nm, and

He 1083 show larger irradiance in December, on the

rotation where AR8004 (the former AR7997) and

the remnants of AR7999 re-emerge on the east limb.

The indices or irradiances which show an earlier

maximum are, for the most part, those associated with

higher temperature coronal emissions, while those

showing a maximum in the following rotation emerge

from lower levels in the solar atmosphere. This indicates

that brightening occurred earliest at the higher solar

atmospheric levels. This conclusion is essentially the

same as that of Donnelly et al. (1985) who found that

F10:7 and ISN ‘‘tend to rise more steeply and peak earlier

during these episodes than the UV flux and the He I

line’’. The long-term time series shown in Fig. 3 also

support this conclusion.

ARTICLE IN PRESS

0.2560.2570.2580.2590.260 SUSIM Mg II (V21r2)

42.2542.5042.7543.0043.25

mW

/m2

SUSIM 200-205 nm

6.10

6.45

6.80

7.15

mW

/m2

SUSIM Ly-α

40

45

50

55He 10830 EW(NSO/Kitt Peak)

10

11

12

13 SOHO SEM He 304

0

20

40

60

80InternationalSunspot number(WDC)

70

80

90

100

SF

U

F10.7(NGDC)

1

5

10

W/m

2

Xray Background X 10-7

(GOES)

OCT NOV DEC JAN

Fig. 5. Solar spectral irradiance and index time series from late

1996 through early 1997 displaying qualitatively different

behavior.


5. Correspondence of solar spectral irradiance with F10:7

and Mg II

Knowledge of solar EUV and UV irradiances are

required for the understanding and modeling of the

terrestrial atmosphere and climate. The correspondence

among indices and irradiances may also provide insight

into solar mechanisms. As discussed earlier, solar indices

have been widely used to represent solar UV and EUV

irradiances and their variations. Such representations

are useful when actual measurements are either unavail-

able or of insufficient accuracy. The correspondence

between irradiances and indices is established when both

are present as was the case, for example, between F10:7

and the EUV irradiance during the solar cycle 21

maximum (Hinteregger, 1981).

Solar UV irradiance time series from SUSIM and

EUV irradiance data time series from SEM and EIT are

compared with F10:7 and Mg II. The two long-term EUV

irradiance measurement data sets that have become

available since the studies of Donnelly and his colleagues

are those of SEM and EIT which have measurements

since the start of 1996. Comparisons with the ISN and

He 1083 indices were not considered. In the previous

section, we observed that the long-term behavior of Mg

II and F10:7 during the maximum of solar cycle 23 were

similar (Fig. 3) while that of both ISN and He 1083

diverged in opposite directions. The ISN is not derived

from any measurement of radiation, but rather from the

number and distribution of sunspots on the solar disk.

Because the measurements which underlie the He 1083

index are susceptible to weather conditions, He 1083 is

available for approximately 57% of the days since the

time series began in 1974. For these reasons, we consider

only correspondence between UV and EUV irradiance

and the F10:7 and Mg II indices. Linear correlation is

used to establish the level of correspondence between the

irradiances and the Mg II and F10:7 indices.

5.1. Solar UV

The UV spectral irradiances selected for comparison

of this study are those made by SUSIM aboard UARS.

Earlier studies have compared the SUSIM UV spectral

irradiances with those of SOLSTICE and SBUV/2

(Woods et al., 1996; Deland and Cebula, 1998; Floyd

et al., 2003) have shown a reasonable level of

correspondence. The SUSIM UV (V21) irradiances for

four wavelength intervals, representing radiation from

the upper photosphere, chromosphere, and transition

region were compared with Mg II and F10:7 solar

indices. The comparison was made using 3468 daily

values from 12 October 1991 to 29 December 2002.

Fig. 6 displays UV irradiances integrated over suitable

intervals to improve their noise quality along with

residuals of each fit. Generally, the quality of the fits are

higher for shorter wavelengths. This is consistent with

the view that longer-term trends in the difference

between indices and irradiances are the result of

instrumental changes that remain unaccounted for in

the in-flight calibration process. Given no other in-

formation, such unwanted effects could be present in

either the index or the irradiance measurement or both.

ARTICLE IN PRESS

0.260.270.28

ratio

Mg ΙΙ(a)

-101

Ly-α r = 0.972(b)

-10010

-0.1 0.0 0.1

160-165 nm r = 0.961(c)

-505

-101

200-205 nm r = 0.952(d)

resi

dual

s

perc

ent

-202

1992 1994 1996 1998 2000 2002Year

-4-2024 235-240 nm r = 0.895(e)

-101

100

200

300

SF

U

F10.7(f)

-101

Ly-α r = 0.932(g)

-10010

-0.1 0.0 0.1

160-165 nm r = 0.919(h)

-505

-101

200-205 nm r = 0.895(i)

resi

dual

s

perc

ent

-202

1992 1994 1996 1998 2000 2002Year

-4-2024 235-240 nm r = 0.846(j)

-101

Fig. 6. Solar UV integrated spectral irradiance time series

separately fitted by Mg II and F10:7. Residuals are plotted below

each time series. The correlation coefficients (r) are also given

for each case.


However, we note that the solar indices have been

selected (and often explicitly designed) for their stability

and accuracy, Accordingly, most of the long-term trends

found in the differences are caused by instrumental

effects in the measured irradiances (e.g. Deland and

Cebula, 1998).

Solar UV irradiance variations are well known to

generally increase with decreasing wavelength, (e.g.

Floyd et al. (2002b)). Uncalibrated instrumental re-

sponsivity changes also often increase with decreasing

wavelength especially below 140 nm, but typically not as

strongly. Accordingly, trends in the residuals of index

fits to irradiances will be relatively larger for longer

wavelengths, corresponding to what is observed. Alter-

natively, if the instrumental trends were instead present

in the measurement or computation of the solar indices,

then the level of the residuals would be simply

proportional to the level of solar variation, which is

not observed. The observed wavelength dependence of

long-term trends in the residuals indicates that they are

unlikely to be a result of a true difference in what is

being measured, but rather a result of systematic errors

in the measurements. By contrast, instrumental changes

are normally smaller in the short-term because of their

monotonic exposure or (sometimes) time dependence.

Accordingly, short-term residuals often reflect real solar

differences in measures of solar activity or spectral

irradiance examples of which were shown above.

Using statistical correlation as a gauge of the

correspondence quality, we find that, in every case, Mg

II is a better solar UV irradiance proxy than is F10:7.

Although, the Mg II index data now extend back to late

1978, we recall that the index was devised only in 1986

which is why it was not considered in the earlier studies.

Given the correspondence noted earlier between Mg II

and He 1083, the better correspondence with Mg II is

consistent with the earlier finding that the He 1083 better

describes UV irradiances Donnelly et al. (1985). Con-

sidering that F10:7 and Mg II originate in the corona and

chromosphere, respectively, it is to be expected that

solar UV emissions from the upper photosphere,

chromosphere, and lower transition region would be

better described by Mg II. Our earlier result showing the

excellent long-term correspondence between F10:7 and

Mg II indices indicates that most of the improvement as

a UV proxy by Mg II may be found in its short-term

variations.

5.2. Solar EUV

The two long-term EUV irradiance measurement data

sets that have become available since the studies of

Donnelly and his colleagues are that of SEM and EIT.

These now extend from solar minimum through max-

imum of SC 23. Both of these experiments measure the

strong He II EUV emission line at 30.4 nm, although the

bandpass of EIT not quite identical to that of SEM first-

order channel (SEM1). SEM1 has undergone moderate

degradation (o40%) which has been corrected through

the use of a parameterized degradation model and

coincident measurements by a second SEM flown

aboard rockets (Judge et al., 1999, 2002). Although

it is sometimes better to find the correspondence of

measurement signals with solar indices rather than the

processed irradiances (e.g. Floyd et al. (2002a), the

degradation in the SEM1 is easily sufficient to disturb


the correlation with the solar indices. Fig. 7 shows the

correspondence of SEM (26–34 nm) integrated irra-

diances with Mg II and F10:7 solar activity indices.

Panels b and c of Fig. 7 display the residuals of separate

linear fits of SEM with Mg II and F10:7. As was the case

for the UV irradiances, Mg II describes the 26–34 nm

irradiance more effectively than does F10:7 as has been

shown in earlier studies (Viereck et al., 2001; Floyd and

Herring, 2000) Repeating the pattern established for

solar UV irradiances, Donnelly et al. (1986) showed that

He II 30.4 nm was better described by He 1083 than by

F10:7. Examination of the residuals shows that, in

particular, the short-term variations are better described

by Mg II. Nevertheless, there are periods such as the

latter third of 1997 and the 3 months beginning

December 2002, where the residuals (and not the

irradiances themselves) of the Mg II fit are dominated

by 13.5-day periodicities. A likely cause is that the

center-to-limb variation of Mg II is not the same as for

the 26–34 nm irradiance.

As explained earlier, the instrumental calibrations of

the EIT image data were incompletely effective, so

that trends in overall responsivity remained in the

irradiances during periods between CCD bakeouts.

These trends were removed, for each bakeout period,

by solving for a linear trend in the overall responsivity

while using a solar index to remove any ambiguity

caused by solar irradiance changes. Application of the

corrected responsivity to the measured irradiances

produces the corrected irradiances in relative units that

are considered here.

1.0

2.0

3.0

4.0 (a) 26 - 34 nm SEM

X1010 ph/cm2/s

-0.5

0.0

0.5(b) Mg II fit residuals X1010

r= 0.981 STD= 1.290245e+09

1996 1998 2000 2002 2004

-0.5

0.0

0.5(c) F10.7 fit residuals X1010

r= 0.953 STD= 2.032950e+09

Fig. 7. SEM 26–34 nm integrated irradiance time series (panel

a) and the residuals of fits with Mg II and F10:7 in panels (b) and

(c), respectively.

Although the EIT irradiance time series are given in

relative units, their correspondence with solar activity

indices can nevertheless be analyzed by a similar process

as was done for the SEM and SUSIM irradiances

earlier. However, such a comparison is more compli-

cated for EIT than for SEM or SUSIM because one

must avoid the circular reasoning stemming from the use

of a solar index in the final instrumental calibration. For

the sanctioned version of the EIT irradiances, the Mg II

index was used to reduce the data. To aid in the

assessment of the bias that is introduced to the solar

index analysis by the calibration processing technique,

the EIT irradiances were also reduced in a parallel

stream using F10:7 instead. Each of the four EIT

channels (30.4, 17.1, 19.5, and 28.4 nm) were passed

through each of these two reduction schemes altogether

yielding a total of eight time series.

Fig. 8 displays the EIT irradiances as derived by the

two methods and the corresponding linear fits with Mg

II and F10:7. For each EIT channel, 2106 daily

irradiances extending from 2 February 1996 to 21

October 2002 were considered. Perhaps surprisingly, in

all cases, the EUV irradiances are described better by

Mg II than by F10:7, even in those cases where the EIT

irradiances were produced using F10:7. As before,

examination of the residuals indicates that the short-

term behavior of Mg II better matches that of the EIT

EUV irradiances. To understand this we note that the

long-term behaviors of these two indices were found

earlier to be very similar. Since instrumental calibration

changes generally take place over these same long-term

time scales, the calibration process which uses either Mg

II or F10:7 as the solar activity index is not sensitive

enough to this choice to overcome the correspondence of

EIT irradiance with Mg II.

The four EIT channels and their corresponding

irradiances emerge from solar atmospheric layers having

temperatures ranging from 80000K to 2MK. Donnelly

et al. (1986) suggested that He 1083 was better for

estimating daily values of chromospheric EUV fluxes,

but that F10:7 was better estimating daily values of

coronal fluxes, such as Fe XV and Fe XVI as measured

by AE-E. The result we report here derived from EIT

measurements differs somewhat from that earlier study.

The Mg II index, not available for the analysis of AE-E

irradiance data, correlates with the coronal Fe XV better

than does F10:7. If this new result is confirmed by

measurements by TIMED/SEE or by SDO/EVE, then

this establishes the transition point for irradiance time

series behavior above the chromospheric–coronal tran-

sition region into the million degree corona.

As a practical matter, several solar EUV irradiance

models which utilize F10:7 are not based on simple linear

relationships with the index (e.g. Hinteregger, 1981;

Tobiska et al., 2000). Rather, solar EUV irradiances are

represented by a linear combination of F10:7 and its

ARTICLE IN PRESS

1.01.52.02.5

MgII red.F10.7 red.

(a) EIT 30.4 nm

(T ~ 80000 K)

-20

0

20 (b) Cal: MgII Fit: MgII r=0.980

-20

0

20 (c) Cal: MgII Fit: F10.7 r=0.942

-20

0

20 (d) Cal: F10.7 Fit: MgII r=0.962

1997 1998 1999 2000 2001 2002

-20

0

20 (e) Cal: F10.7 Fit: F10.7 r=0.958

0.51.0

1.5

2.02.5

MgII red.F10.7 red.

(f) EIT 17.1 nm

(T ~ 1 MK)

-20

0

20(g) Cal: MgII Fit: MgII r=0.961

-20

0

20(h) Cal: MgII Fit: F10.7 r=0.924

-20

0

20(i) Cal: F10.7 Fit: MgII r=0.939

1997 1998 1999 2000 2001 2002

-20

0

20(j) Cal: F10.7 Fit: F10.7 r=0.934

1.0

2.0

3.0

4.0

MgII red.F10.7 red.

(k) EIT 19.5 nm

(T ~ 1.5 MK)

-20

0

20 (l) Cal: MgII Fit: MgII r=0.978

-20

0

20 (m) Cal: MgII Fit: F10.7 r=0.935

-20

0

20 (n) Cal: F10.7 Fit: MgII r=0.958

1997 1998 1999 2000 2001 2002

-20

0

20 (o) Cal: F10.7 Fit: F10.7 r=0.948

2468

10

MgII red.F10.7 red.

(p) EIT 28.4 nm

(T ~ 2 MK)

-200

20(q) Cal: MgII Fit: MgII r=0.985

-200

20(r) Cal: MgII Fit: F10.7 r=0.948

-200

20(s) Cal: F10.7 Fit: MgII r=0.967

1997 1998 1999 2000 2001 2002

-200

20(t) Cal: MgII Fit: F10.7 r=0.964

Fig. 8. Relatively calibrated EIT irradiance time series constructed using Mg II (see text) for 30.4, 17.1, 19.5, and 28.4 nm are displayed

in panels (a), (f), (k), (p). The solid and dashed lines display the Mg II model and the dashed. Note that the corresponding residuals of

linear fits with Mg II and F10:7 are shown in panels (b), (g), (l), (q) and (c), (h), (m), (r), respectively. Similar EIT series, constructed

instead using F 10:7, are similarly fit with Mg II and F10:7. Their residuals are displayed in panels (d), (i), (n), (s) and (e), (j), (o), (t),

respectively.


81-day running mean. In the models, the coefficients are

selected to optimize the time series correspondence with

the EUV. To evaluate this for the EIT irradiance, we

separately find fits of linear combinations of each index

and its 81-day running mean each with the two sets of

differently calibrated EIT time series described above.

ARTICLE IN PRESS

Table 3

Multiple regression correlation coefficients for fits of EIT

irradiances

M reduction F reduction

EIT l (nm) M& �M F& �F M& �M F& �F

30.4 0.961 0.942 0.939 0.945

17.1 0.981 0.963 0.959 0.965

19.5 0.990 0.976 0.969 0.982

28.4 0.981 0.960 0.962 0.968


More precisely, we use a total of 8 multiple regression

fits using each of the following equations

I ¼ a0 þ a1F þ a2 �F

I ¼ b0 þ b1M þ b2 �M

For each day, I represents is an EIT-measured

irradiance, F and M are F10:7 and Mg II, �M and �F are

their centered running means, The determined coeffi-

cients are a0, a1, a2, b0, b1, and b2 for each fit. Table 3

displays the multiple correlation coefficients of the two

fits for each of the four EIT channels. In contrast to the

one index fit, in every case, the EIT data are fit better by

the index and 81-day average that was used to construct

the irradiance time series, although correlations in the

case of calibration and fitting by Mg II are higher than

the corresponding case for F10:7. Accordingly, these

results can lead to no firm conclusions on the relative

merits of Mg II and F10:7 in such models.

6. Discussion and conclusions

The daily time series of four well-known solar activity

indices: ISN, F10:7, He 1083, and Mg II, have been

analyzed and compared. These indices are often used as

proxies of the components of irradiance from solar

bright regions, including faculae, plages, and network.

The four analyzed time series each consist of 5416 data

points extending from November 1978 to February

2003. To avoid biasing effects, only those daily data that

are common to all four were considered. Overall, each of

these indices exhibits similar behavior, the lowest

correlation among them is between He 1083 and ISN

ðr ¼ 0:880Þ. The two highest correlations are between

Mg II and He 1083 ðr ¼ 0:969Þ and between ISN and

F10:7 (r=0.940). Using time-domain filtering techniques,

we have separated the long- and short-term components

and again compared the time series. In terms of

statistical correlation, the highest long-term correlation

was found for Mg II and F10:7. In particular, the largest

long-term divergence between ISN and F10:7, since 1947

when the latter series began, was experienced in

2001–2002 during the recent solar cycle 23 maximum.

During approximately the same time period, the He

1083 index significantly diverges from F10:7, in the

opposite direction, by an amount (roughly) equalled

only once before. Finally, we find that much larger

differences exist among short-term components of these

solar indices as indicated by much lower correlations.

Accordingly, caution should be exercised when inferring

long-term solar variations from short-term variations in

measurements and activity indices.

Using solar UV and EUV spectral irradiances from

SUSIM, SEM, and EIT, we have shown that the Mg II

index time series describes that of solar irradiances more

effectively than does F10:7. Given the long-term corre-

spondence of Mg II and F10:7, we conclude that this

difference arises from short-term variations. Short-term

differences can arise because of (1) non-thermal

contributions to the indices, (2) differing center-to-limb

variations, and (3) differences among atmospheric layers

in the onset times of episodes of variable solar activity.

The irradiances of the four EIT channels are formed

in both chromospheric and lower transition region

layers of the solar atmosphere. Earlier studies have

indicated that F10:7 better represents variations in

coronal flux and He 1083 better represents chromo-

spheric or transition region flux (Donnelly et al., 1986).

The results based on the Mg II index and irradiances

derived from EIT images presented here indicate that

if there exists a solar atmospheric temperature for

which F10:7 is a better proxy, then it is for coronal

radiation above 2MK. That the irradiance from

these diverse atmospheric layers are well correlated with

the same solar activity index indicates that, in some way,

the corresponding heating mechanisms are closely

coupled.

More research is needed to further understand the

physical processes that provide the basis for solar

activity indices and spectral irradiances. The upcoming

STEREO mission (Howard et al., 2000) will provide

simultaneous EUV images from two vantage points

thus providing the first direct measure of center-to-

limb variation at these wavelengths. Images at other

wavelengths (such as the UV which originates lower

in the solar atmosphere) and from different directions

would be needed to provide further insights. Solar

irradiance models have been constructed based on

the empirical correlations with solar activity indices.

Given the unusual and currently unexplained behavior

of the solar activity indices during the solar cycle 23

maximum, continued and more detailed solar irradi-

ance measurements and imaging are needed. With

improved measurements, a better understanding of

solar mechanisms should result allowing our models

of solar radiant behavior to operate more reliably in the

future.

ARTICLE IN PRESSL. Floyd et al. / Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 3–1514

Acknowledgements

We gratefully acknowledge that the NSO/Kitt Peak

data used here are produced cooperatively by NSF/

NOAO, NASA/GSFC, and NOAA/SEL. Sunspot, 10.7

radio flux, and GOES X-ray data were obtained from

the National Geophysical Data Center. Rodney Viereck

kindly provided the NOAA SEC Mg II index.

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