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Advances in Space Research 37 (2006) 246–252

A flux of EUV emission measured on-board the‘‘CORONAS’’ artificial satellites near minimum and maximum of

the 23rd cycle of solar activity

A.A. Nusinov *, T.V. Kazachevskaya, V.V. Katyushina

Institute of Applied Geophysics, Russian hydremeteorological service, Rostokinskaya st., 9, 129128 Moscow, Russia

Received 29 October 2004; received in revised form 7 March 2005; accepted 9 March 2005

Abstract

The paper presents data on ionizing short-wave UV-emission of the Sun in a wave-range k 6 130 nm measured on-board the

CORONAS-I and CORONAS-F satellites. There were obtained absolute values of solar flux emission in the spectrum range

k 6 130 nm and the band near H La hydrogen line on-board both satellites. Measurements on-board the CORONAS-I satellite coin-

cided with a phase near the minimum of solar activity (F10.7 = 80–100). In the period from March to June of 1994 the solar flux was

equal on the average to 7.5 erg cm�2 s�1 for k 6 130 nm and the intensity of emission in H La was equal to (5.5–6.1) erg cm�2 s�1.

The measurements on-board the CORONAS-F were performed near the maximum of solar activity (F10.7 = 143–279). Emission

intensity in H La line was about 6.8 –8.2 erg cm�2 s�1 and in the wave-range k 6 130 nm it was equal on the average to 11–

13 erg cm�2 s�1. EUV measurements are in agreement with the data obtained on-board UARS satellite and with the results of ion-

ospheric measurement of E-layer critical frequencies. These measurements agree with data of contemporary models as well.

A lot of flares including bright ones were observed. The measurement data of a bright flare of X-ray class X5.3 on 25.05.2001 are

given. The paper presents the results of comparison between the CORONAS-F data and the X-ray fluxes in waveband 0.1–0.8 nm

(GOES).

This comparison demonstrated that X-ray emission measured on-board the GOES spacecraft and measured due to SUVR instru-

ment (with filters in wave-range <12 nm) changed almost synchronously in the ranges 0.1–0.8 and 0.1–12 nm. The data measured

due to VUSS showed that EUV emission appeared a few minutes (1–13) before X-ray emission. Apparently it evidences that at first

a flare begins in the chromosphere and then a heating area is spreading higher. Solar emission increases by �20–30% in the range

k < 130 nm, and only by 8–10% in EUV range. Changes of extreme ultraviolet (EUV) solar flux were registered during the annular

solar eclipse of May 31, 2003, when the CORONAS-F satellite thrice intersected a zone of the solar eclipse.

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

Keywords: Sun; EUV emission; Solar flares; Solar eclipses

1. Introduction

The measurements of the extreme ultraviolet region

of the solar spectrum (the EUV region) are carried out

at rockets and satellites since the middle of the last cen-

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

doi:10.1016/j.asr.2005.03.034

* Corresponding author.

E-mail addresses: musinov@mail.ru, geophys@hydromet.ru (A.A.

Nusinov).

tury, however peculiarities in the instrumentation and

specifics of its calibration create difficulties for the com-parison of measurements conducted on-board different

space vehicles. And the latter is especially important in

studies of the trends of the fluxes of geoeffective radia-

tion. Moreover, there is a problem of a degradation of

the instrument: the ability to take into account a change

of the sensitivity with time becomes principally impor-

tant. Creating the instrumentation and conducting the

ved.

A.A. Nusinov et al. / Advances in Space Research 37 (2006) 246–252 247

measurements of the Sun in the EUV region of the spec-

trum on-board the CORONAS space vehicle we tried to

take into account these complications in measurements.

2. The CORONAS project

The CORONAS project – Complex Orbital Near-

Earth Observations of solar activity is aimed at observa-

tion of the Sun in the wide wave-range (Oraevsky et al.,

2002). The CORONAS-I satellite has been launched to

a circular geocentric orbit on 2 March 1994, the CORO-

NAS-F satellite has been launched on 31 July 2001. The

satellite orbits were almost circular: the maximum (in theapogee) and minimum (in the perigee) distances to the

Earth surface were 539.7 and 498.5 km, respectively.

The orbit plane inclination and the rotation period were

�82.5� and �95 min, respectively. The quasi-synchro-

nous orbit provided the existence of periods up to 20 days

long when the satellite stayed out of the Earth�s shadow.

3. SUVR and VUSS instruments for measurements of

extreme ultraviolet radiation on-board the CORONAS

space vehicle

To conduct regular measurements of the intensity of

ionizing solar radiation in the extreme ultraviolet region

of the spectrum (k < 130 nm) and in the hydrogen line

La (121.6 nm) two types of the instruments were in-stalled on-board the CORONAS-F satellite: solar ultra-

violet radiometer (SUVR) and vacuum ultraviolet solar

spectrometer (VUSS).

The solar ultraviolet radiometer (SUVR) provides

measurements in the wavelength region k < 130 nm

Fig. 1. EUV flux variations with wavelength k < 130 nm (SUVR measureme

August and 16 September 2001. For the same period the data on the flux of so

of 10 are presented.

and also in the hydrogen line La (k = 121.6 nm). For reg-

istration of the solar ultraviolet radiation a thermo-

luminescent method is used. The instrumentation of

the SUVR type has been installed on-board several sat-

ellites and was described by Kazachevskaya et al. (1998).

The vacuum ultraviolet solar spectrometer (VUSS-L)provides measurements in the vicinity of the hydrogen

line La (k = 121.6 nm) and was used at the CORO-

NAS-F satellite. A vacuum photodiode with the cathode

of CuJ and MgF2 entrance window (insensitive to solar

radiation in the visible and near ultraviolet ranges) was

used as a photo-receiver.

4. Measurements results

4.1. The data on the solar flux in the extreme ultraviolet

region of the spectrum

Data on absolute values of solar flux in the wave-

range k < 130 nm and in the hydrogen line La

(k = 121.6 nm) were obtained on-board both CORO-NAS satellites. Observations were performed during dif-

ferent periods of solar activity, therefore the flux values

measured and the flux variations differed.

Measurements on-board CORONAS-I satellite were

performed during a phase near the minimum of solar

activity. During the period from March to June of

1994 (index F10.7 = 80–100) the solar emission flux for

k < 130 nm was on the average 7.5 erg cm�2 s�1 andthe intensity of emission in the La line was equal to

(5.5–6.1) erg cm�2 s�1.

Measurements on-board CORONAS-F satellite were

performed near the maximum of solar activity. Radio

flux-index F10.7 changed from 143 to 279. A lot of flares

nts) and in the F10.7 index. The crosses denote two strong flares: on 25

lar radio emission at a wavelength of 10.7 cm (F10.7) reduced by a factor

Fig. 2. The flare of August 25, 2001: temporal X-ray (0.1-0.8 nm) and

EUV (near 120 nm) changes registered on-board the GOES-8 satellite

and by the VUSS instrument on-board the CORONAS-F satellite.

248 A.A. Nusinov et al. / Advances in Space Research 37 (2006) 246–252

were observed including bright ones – 3b in optic and

X5.3 in X-ray.

According to the SUVR measurements the solar radi-

ation intensity in the hydrogen La line within the obser-

vational period was �6.8–8.2 erg cm�2 s�1. The solar

flux in the EUV region k < 130 nm on the average wasabout 11–13 erg cm�2 s�1. The data obtained agree with

the current ideas on the value of the radiation flux in

these spectral regions (see, for example, Tobiska et al.

(2000)). The estimated relative accuracy of SUVR mea-

surements is about 15%.

4.2. Ultraviolet radiation flux variations with solar

activity

On-board the CORONAS-F the measurements were

conducted in the epoch of a maximum of solar activity

when strong variations of the EUV radiation were ob-

served. Fig. 1 shows the variations in the EUV radiation

in the k < 130 nm region measured by the SUVR device

within the period from 15 August to 11 October 2001

(the data are presented in modified Julian days:52,000). For the sake of a convenience the variations

in EUV measurements are normalized to the date of

the beginning of the measurements: 15 August 2001.

One can see that during this period strong variations

in the fluxes are observed: the amplitude of variations

is up to 80% for F10.7, radio emission fluxes, and the

amplitude is up to 40% for the radiation in the EUV re-

gion of the spectrum.

Table 1

Flares in EUV measured by the VUSS instrument

Data X-ray class DT(X-EUV) (min) DLa (erg m�2 s�1)

25 August 2001 X5.3 �13 0.158

27 May 2003 X1.3 ��2 0.100

11 June 2003 X1.6 �7 0.115

08 June 2003 M4.0 �2 0.080

26 May 2003 M1.9 0 0.0261

27 May 2003 M1.6 – 0.0109

01 June 2003 C4.6 1 0.015

02 June 2003 C9.0 �6 0.096

5. Observation of solar flares

5.1. The X5.3 flare of August 25, 2001

Measurements on-board CORONAS-F started dur-

ing a period of solar activity maximum. The solar radioflux F10.7 varied within 140–280 s.f.u.

The solar flare of X5.3 class (the optical magnitude

3B), which took place on August 25, 2001, was one of

the most intensive flares registered by SUFR and VUSS

instruments. Fig. 2 gives the VUSS instrument measure-

ments of this flare in the H La line. Besides the data mea-

sured by the VUSS instrument in EUV region the figure

also presents the temporal X-ray changes within the 0.1–0.8 nm wave-range observed on-board the GOES-8 sa-

tellite. A dip in the line near 80 min corresponds to an

entry of the satellite into a zone of twilight (during this

period the orbit was almost solar-synchronous). A break

between the 40th and 50th minutes was caused by a loss

of information while transiting from one communica-

tion session to another. It is evident that moments of

maximum fluxes differ: an X-ray emission achieves max-imum value �13 min later than UV-emission. Besides

that a scale of variations is also different: UV-emission

changes by �2.7% and at the same time X-ray emissionchanges by the dozens of times. It is also important that

duration of the flare in UV region (about several sec-

onds) is considerably shorter than in X-ray region.

These characteristics are typical of the most of the flares

observed and will be generalized below.

5.2. Comparison of flare characteristics in UV and X-ray

wavebands

One of the main characteristics of solar flares is a va-

lue of energy emitted as in the maximum as well as dur-

ing the whole period of a flare.

Table 1 presents data on several flares observed by

the VUSS instrument in 2001–2003; its X-ray classifica-

tion is also given due to measurements on-board the

GOES-8 satellite for each date of observation. Table 1gives: an X-ray class of a flare (column 2), correspond-

ing to data measured by the VUSS instrument, a tempo-

ral difference of a flare�s maximum observed in UV and

X-ray region (column 3). An emission increase DLa line

in the La line is given in the column 4. An unperturbed

Fig. 4. Relation between amplitudes of solar flares in X-ray (0.1–

0.8 nm) and EUV (�120 nm) ranges.

A.A. Nusinov et al. / Advances in Space Research 37 (2006) 246–252 249

value of intensity in La line was taken from the paper

(Nusinov, 2004) and recalculated in accordance with

the measurements performed. It can be seen that an in-

crease of emission in EUV region is equal to several per-

cents even in case of very intensive flares.

A maximum of the most flares observed in EUV re-gion occurs before an appearance of an X-ray maxi-

mum; EUV emission leads X-ray emission by 2–

13 min. According to the data given in Table 1, Fig. 3

presents a dependence of time difference (minutes) of

flares observed in EUV and X-ray regions (an ordinate

axis) on emission intensity in 0.1–0.8 nm wave-range ob-

served on-board the GOES-8 satellite during a maxi-

mum of a flare (an abscissa axis). Fig. 3 shows that atime delay is increasing with an increase of a flare power.

The tendency is evident though for small values of X-ray

magnitudes a spread in data is rather great and a num-

ber of intensive flares is not sufficient for reliable regres-

sive curves. Fig. 4 presents data (on twice logarithmic

scale) on an energy of the flares taken from Table 1.

An emission energy during a flare maximum in the X-

ray range of 0.1–0.8 nm (erg m�2 s�1) is plotted on theabscissa and an increase of emission in EUV region in

La line is plotted on the ordinate. It can be seen that

points observed are good approximated by a straight

line (it is shown in the figure), i.e., there is a power

law dependence between flare fluxes within the range

of two orders of a magnitude of X-ray changes. The

same relation was obtained earlier due to analogous

SUVR measurements of flares on-board the Prognoz-7satellite in the wave range of k < 130 nm (Kazachevs-

kaya et al., 1986) and was also found for soft X-rays

by Nusinov and Chulankin (1997).

It is necessary to note that the VUSS signal is caused

not only by La fluxes but also by a flux in a wide

(�40 nm) spectral band near 180 nm. Calculation of a

Fig. 3. Dependence of X-ray flare maximum phase time delay

relatively to EUV emission maximum on 0.1–0.8 nm X-ray flux.

detector sensitivity showed that an input of this band

to a total signal was about 70%. Since an emission of

the Sun as a star changes faintly in this range (as in a cy-

cle of activity, as well as during flares) one may assumethat all signal changes during a flare are caused only by

intensity in La line. Then estimations of intensity might

be increased by �3 times and it gives an intensity in-

crease in La line up to �8–10%. Respectively flare fluxes

in La line depicted in Fig. 4 should be increased �thrice.

This value is in a good agreement with estimations of

other investigators discussed above and with the SUVR

data measured during the flares.

5.3. Discussion of flare observations

Simultaneously with our observations there were ob-

tained data on time delay of X-ray maximum phase in

the range of 0.1–0.8 nm relatively to more hard emission

in the range of 50–150 keV, 150–500 keV, 0.5–1.3 MeV,

4–7 MeV measured by SONG instrument on-board theCORONAS-F satellite during the flare on August 25,

2001 (Kuznetsov et al., 2003). A time delay of soft X-

ray maximum comparatively to hard components was

noted from the very beginning of multi-wave observa-

tions (for example, Den and Somov, 1989, and literature

quoted in this paper). The same conclusion concerning

different temporal profiles of intensive flares on March

22, 2000 (class X1.1) is given in Lee et al. (2003), whereX-ray data within the range of 0.1–0.8 nm are compared

with measurements of hard X-ray emission and with

radio emission in the range of 1.6–18.0 GHz. The time

delay is equal to �4 min. It was noted in Qiu et al.

(2004) that short-term increasing of hard X-ray emission

250 A.A. Nusinov et al. / Advances in Space Research 37 (2006) 246–252

within several wave-ranges from 25 to 230 keV regis-

tered on-board the YOHKOH satellite during the flare

of X5.6 class on April 6, 2001 leads X-ray emission in

the range of 0.1–0.8 nm measured on-board the GOES

satellite by �6 min. It is necessary to underline that

EUV changed in the range of Layman continuum regis-tered by the SOHO satellite during the flare on August

25, 2001 is in a good correlation in time with emission

in Ha line (Lemaire et al., 2004).

Further energy release is characterized by that an en-

ergy in EUV (and visible) region decays abruptly while

radiance in soft (�1 keV) X-ray region approaches max-

imum and continues during a time interval exceeding an

impulse phase by an order. In this case energy fluxesdescending down decay rapidly and do not cause addi-

tional radiance of the chromosphere in EUV region.

An existence of a time delay points out that during an

evolution of a flare a heating process of a flare loop be-

gins from its basis at a level of a lower part of transition

layer where the main part of fluxes in the range of

�100 nm is formed and then it extends to a peak of a

loop from where the main part of X-ray is emitted. Atthe present time there is an overwhelming opinion that

solar flares appear in the corona then heating and emis-

sion from the chromosphere take place. But the data gi-

ven above allow to suggest that other mechanisms of

solar flares may exist.

As a rule a general scenario of a flare development

brings to a process of magnetic interchange in corona

and a generation in this field of energy fluxes extendingthen below to the chromosphere and the photosphere

(Den and Somov, 1989; Somov et al., 2002, 2004). In

these fields an energy injected as fluxes of accelerated

particles causes a radiance in UV and visible range

and hydro-dynamical fluxes of plasma extending along

flare loops to the corona. Apparently this scenario refers

only to an impulse phase of a flare. Comparison between

Fig. 3 and Fig. 4 leads to a conclusion that as more en-ergy emitted in a maximum of a flare as longer the time

Fig. 5. Normalized EUV and calculated optic

which is necessary to achieve an X-ray maximum value.

It may correspond to extension of energy from below,

from the field where original energy is released during

impulse phase of a flare. Then it extends to the corona

region where X-ray emission lasts considerably longer.

A difference between energy release times in soft X-rayand in EUV regions is also important. It points out that

mechanisms of its origin are different.

6. Observation of the annular solar eclipse of May 31,

2003 in far ultraviolet spectral region

A rare astronomical phenomenon – an annular solareclipse – was observed on-board CORONAS-F satellite

in far ultraviolet radiation band three times during its

flight. Data on the most impressive eclipse are presented

below. Changes of extreme ultraviolet (EUV) solar flux

were registered during the annular solar eclipse of May

31, 2003 (Espenak and Anderson, 2003), when CORO-

NAS-F satellite intersected thrice a zone of the solar

eclipse. During these periods the solar emission wasmeasured in visible and extreme UV spectral regions

(near La = 121.6 nm by VUSS instrument) and the opti-

cal (visible) channel of EOS. A visible channel is auxil-

iary, its records were not shown. On May 31, 2003, an

annular eclipse of the Sun could be visible from a broad

corridor which traversed the far Northern Hemisphere.

Mutual position for the Sun and the Moon was calcu-

lated as well as a parameter X – a part of the solar discthat was not shaded by the Moon as it had been seen

from the satellite. There exists some peculiarities of

eclipse observations on-board a moving celestial body,

such as an Earth�s satellite. In this case the ‘‘open’’ part

of the Sun, as it could be seen from the satellite, changed

not only its value, but also changed rapidly its position

on the Sun. Observed eclipse effect maxima (ratio of a

minimum signal, when the eclipse was registered, to amaximum signal without an eclipse) were 0.147, 0.091

al signal X for three CORONAS turns.

A.A. Nusinov et al. / Advances in Space Research 37 (2006) 246–252 251

and 0.762 at 03:05:39 UTC, 04:28:20 UTC and 05:53:20

UTC correspondingly. Minimum values of X were

0.201, 0.136 and 0.743 correspondingly. It can be seen

from these data, that the instrument on-board the satel-

lite could not observe a maximum annular eclipse phase:

X = 0.112 for CORONAS-F.Figs. 5(a)–(c) represent changes of normalized

VUSS signal (solid line) together with X (dashed lines)

for three sequential satellite passages of the eclipse

band. In Figs. 5(a)–(c) one can see characteristic pecu-

liarities that were revealed after comparison of these

figures. A relation between an open part of the Sun

and a VUSS signal changed from one turn to another.

At the 1st and 2nd turns minimum VUSS signal valueswere smaller that of X. But when X > 0.7 and at the

3rd turn there is reverse relation. It may be interpreted

as a result of two effects: spatial distribution of differ-

ent emitting elements and a limb darkening. Indeed,

there were three active regions (365, 368, and 373) at

the visible solar disc during the eclipse period. Their

moving relative to the Moon shadow, as seen from

CORONAS-F satellite could cause small, but distinc-tive EUV variations different from those ones in visible

light.

7. Conclusions

1. The data on the solar ultraviolet flux in the spectral

band k < 130 nm (the SUVR device) and in the vicin-ity of the hydrogen Lyman-alpha line with

k = 121.6 nm (the VUSS device) were obtained. The

CORONAS-I measurements were performed during

a phase near the minimum of solar activity (index

F10.7 = 80–100). During the period from March to

June of 1994 the solar emission flux for k < 130 nm

was on the average 7.5 erg cm�2 s�1 and the intensity

of emission in the La line was equal to (5.5–6.1)erg cm�2 s�1. The CORONAS-F measurements were

performed near the maximum of solar activity (index

F10.7 = 143–279). The intensity of emission in the H

La line was about �6.8–8.2 erg cm�2 s�1.

It is demonstrated that the flux values correspond to

the maximum phase of solar activity, the fact con-

firming the point of view on unusual behavior of

the Sun in the 23rd cycle.2. EUV changes registered in the period of solar flares

within the wave-band of �120 nm were analyzed.

Relative values of flares amplitudes in the band of

�120 nm were found. It was confirmed that during

even the most intensive flares an increase of emission

in this band did not exceed several percents.

3. There is a power law correlation between soft X-ray

and EUV emission. It allows to estimate EUV fluxchanges in the periods of flares on the basis of the

X-ray patrol data.

4. Study of the EUV solar flux variations in different

wave-ranges revealed that at the beginning of a flare

and during its maximum phase EUV emission sys-

tematically led soft X-ray emission by 1–13 min. To

all appearance there is a possibility that a flare devel-

opment takes place not in the corona�s loops but inthe upper layers of the solar atmosphere.

Acknowledgments

Authors are grateful to K.V.Kuimov and V.V.Cha-

zov (SAI of the MSU) for the calculations of mutualarrangement for the Sun and the Moon and a parameter

X they performed.

This work was partially supported by the RFBI,

Grant No. 03-02-17109.

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