ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2007, Vol. 71, No. 7, pp. 921–923. © Allerton Press, Inc., 2007.Original Russian Text © N.K. Pereyaslova, M.N. Nazarova, I.E. Petrenko, 2007, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2007, Vol. 71, No. 7,pp. 856–958.

921

INTRODUCTION

The results of the investigations of radiative charac-teristics of the solar proton events (SPEs) detected inthe near-Earth space during the 23rd cycle of protonactivity from 1996 to 2003 were reported in [1–3]. Inthis study, we have continued these investigations andestablished a number of features that were observed in2004–2006 in the radiative characteristics of the SPEsdetected in the near-Earth space. The analysis wasbased on the data on the GOES satellite that werereported in Internet. The following radiative character-istics of the SPEs detected in the near-Earth space wereanalyzed: proton fluence

I

(cm

–2

) with

E

p

> 10

MeV,energy flux

W

(MeV cm

–2

), spectral index in the SPEmaximum in the case of power spectral representation,characteristics of the levels of radiative perturbations inthe near-Earth space [3], and perturbation period (indays). The technique described in catalogs [4–6] wasused to identify most of detected SPEs with solar flares(proton sources). The results obtained were used toconstruct a model dependence of the flux and temporalcharacteristics of a mean SPE on the source heliolongi-tudes for the four solar quadrants.

ANALYSIS OF THE RESULTS

Eighteen SPEs with a threshold energy

E

p

> 10 MeVand intensity (in the event maximum)

I

m

≥

1

cm

–2

s

–1

sr

–1

were detected in the near-Earth space in the period from2004 to August 2006. The number

n

SPE

of events wasestimated using the technique reported in SPE catalogs[4–6]. In 2004, eight SPEs, mainly of moderate inten-sity, were detected. The only exception was the SPEthat began on July 22 at

t

0

= 18 UT and reached themaximum value

I

m

(

E

> 10 MeV) = 1000 cm

–2

s

–1

sr

–1

on July 26. The fluence

I

was

1.1

×

10

8

cm

–2

and the

energy flux

W

was

1.4

×

10

9

MeV cm

–2

; the spectrumwas soft:

γ

= 4.2.

The SPEs detected in January 2005 (from January15 to 23) and September (from September 7 to Septem-ber 17) 2005 had long-term time profiles with severalmaxima in the energy range

E

p

> 5–100 MeV, whichwere due to the superposition of flare proton fluxes andacceleration of protons at the shock wave front duringmagnetic storms, which occurred almost continuously.In the period from January 15 to 23, four maxima wereobserved in the SPE time profiles, two of which had thespectral indices

γ

= 1.4 (January 15) and 0.3 (January 20).Figure 1 shows the time dependences of the flux of pro-tons with threshold energies

E

p

≥

10

, 50, and 100 MeV,as well as changes in the 3-hour index

K

p

with indica-tion of the flare coordinates and magnitude. A peak

Characteristics of Solar Proton Events near the Minimumof the 23rd Cycle of Solar Activity

N. K. Pereyaslova, M. N. Nazarova, and I. E. Petrenko

Fedorov Institute of Applied Geophysics, Russian Federal Service on Hydrometeorology and Environmental Monitoring, Rostokinskaya ul. 9, Moscow, 129128 Russia

e-mail: geophys@hydromet.ru

Abstract

—Analysis and systematization of proton activity in the declining phase near the minimum of the 23rdcycle of solar activity in the period 2004–2005 have been performed. Flux, spectral, energy, and some temporalcharacteristics of the solar proton events observed in the near-Earth space during the noted period are investi-gated and reported.

DOI:

10.3103/S1062873807070088

15 16 17 18 19 20 21

10

3

10

2

10

1

10

0

10

–1

10

–2

62

January 2005

J

,

cm

–2

s

–1

sr

–1

K

p

N11E6SFM8.5

N15W262FX3.8

N15W562BX7.1

N14W83BX2.5

Fig. 1.

Time dependences of the fluxes of SCR protons with

E

p

≥

10 (filled circles), 50 (open circles), and (crosses)100 MeV, according to the data of the GOES satellite; flares(SPE sources) with their coordinates, magnitude, X-rayclass, and 3-h

K

p

index are indicated.

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BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES: PHYSICS

Vol. 71

No. 7

2007

PEREYASLOVA et al.

with maximum intensity

I

m

= 3860 cm

–2

s

–1

sr

–1

(

E

>10 MeV) was observed on January 17 at 17 UT. Thetotal fluence over the entire period was

I

= 1.8

×

10

9

cm

–2

.An SPE complex in the energy range

E

p

≥

1–100

MeVwith three peaks of proton fluxes was detected in Sep-tember (from September 7 to 17). The maximum peakwith the intensity

I

m

=

1.7

×

10

3

cm

–2

s

–1

sr

–1

(

E

>10 MeV) was observed on September 11. The smallestvalues of the spectral indices,

γ

= 1.02 and 0.94, wereobserved in this case on September 8 and September 9,respectively. September 2005 turned out to be themonth of the highest solar activity after March 1991.Such rigid spectra were observed in neither of the pre-vious three cycles in the declining phase. One mightsuggest that the rigidities of the spectra recorded onJanuary 20, September 8, and September 9 are due tothe convenient location of the Earth with respect to theSun with respect to the almost simultaneous arrival ofprotons of all energies at the Earth’s orbit. Over year

2005, the total fluence was

3.93

×

10

9

cm

–2

. At the samenumber of events, the total fluence in 2004 was

2.25

×

10

8

, i.e., lower by more than an order of magnitude. Corre-spondingly, the energy flux

W

was

6.0

×

10

10

MeV cm

–2

in2005 and 9.06

×

10

9

MeV cm

–2

in 2004.The events detected in the near-Earth space during

the period close to the minimum of the 23rd cyclecaused enhanced levels of radiation situation. Fourteenperturbations of the radiation situation occurred in2004 and 2005: two strong (>10

3

mrad/day), two mod-erate (10

2

–10

3

mrad/day), and ten weak (12.5 <10

2

mrad/day). The total number of days with perturbedradiation situation in the near-Earth space in 2004 and2005 (the declining phase of proton activity, close to theepoch of minimum) was 18 days.

The table contains the main parameters determiningthe level of radiation situation in the near-Earth space:proton fluence, energy flux transferred by protons to thenear-Earth space, spectral index, and the number ofSPEs. It follows from the tabular data that in the 23rdcycle of proton activity, in comparison with the previ-ous cycles, the radiation parameters in the period underconsideration (in the declining phase two years beforethe minimum of this cycle) have the largest values.

Thus, September 2005 was found to be the monthwith the highest solar activity after March 1991 (thesecond maximum of the 22nd solar cycle) [6].

APPLICATION OF THE RESULTS OF ANALYSISTo predict the time

t

0

from the flare onset to theonset of SPE arrival at the near-Earth space and thetime

t

m

of detection of the maximum proton flux on thebasis of the 170 SPEs identified with the source anddetected in the declining phase of cycles 20, 21, 22, and23, a model was developed for the dependence of thetimes

t

0

and

t

m

and the fluence on the heliolongitudes ofthe proton flare for the four solar quadrants. This modelis presented in Fig. 2.

The time distributions of

t

0

and

t

m

are approximatedby the quadratic dependences

North

:

t

0

= 7.12 – 0.07

λ

+ 0.0005

λ

2

,

t

m = 17.2 – 0.171λ + 0.0008λ2,

South: t0 = 6.5 – 0.13λ + 0.0008λ2, tm = 16.6 – 0.195λ + 0.001λ2.

Here, λ is the heliolongitude counted from the centralmeridian (+ and – for West and East, respectively). Itcan be seen that the fastest arrival of protons from thedetected SPEs is observed for the flares in the heliolon-gitudes 60°–90° of the western hemisphere of the Sun.

The highest mean SPE fluences are observed fromthe flares close to the central meridian in the range ofheliolongitudes from 0° to 30° in the eastern hemi-sphere of the Sun. Approaching the eastern limb, thefluence decreases approximately by two orders of mag-nitude. The proton fluences from flares in the western

Radiation parameters in the declining phase close tothe minima of solar cycles

Cycle Years ΣI, cm–2 ΣW,MeV cm–2 γav nSPE

20 1974–1975 9.2 × 108 8.8 × 109 2.7 11

21 1984–1985 2.1 × 109 3.5 × 1010 2.5 17

22 1994–1995 1.9 × 109 2.3 × 1010 3.1 3

23 2004–2005 4.3 × 109 6.3 × 1010 3.0 17

1010109108107106107108109

1010

60° 0° 60° WE

40

60° 0° 60° WE

302010

10203040

S

N (a) (b)I, cm–2

t0, tm, h

Fig. 2. Dependences of the radiation characteristics of amean SPE with Ep ≥ 10 MeV on the flare heliolongitude for thedeclining phase of solar cycles: (a) proton fluence I, cm–2 and(b) time parameters, (open circles) t0 and (filled circles) tm;the dashed curve is an approximation of the heliolongitudedependence; a mean-square deviation is shown in the 30°heliolongitude interval.

BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES: PHYSICS Vol. 71 No. 7 2007

CHARACTERISTICS OF SOLAR PROTON EVENTS 923

hemisphere of the Sun in the range of longitudes from0° to 60°, both for the northern and southern hemi-spheres, are within mean deviations and almost do notchange. The fluences from flares in the range from 60°to 90° in the eastern hemisphere are somewhat smaller.

CONCLUSIONS

Thus, in the declining phase, near the minimum ofthe 23rd cycle (from January to September 2005), theproton fluence with Ep > 10 MeV increases by a factorof 2–5 in comparison with the analogous periods of theprevious cycles.

High proton fluxes Im are observed in the maxima ofSPEs, as well as an increase in the number of periods ofperturbed radiative situation in the near-Earth space. Inthe declining phase of the 23rd cycle of solar activity,September 2005 turned out to be the month with thehighest solar activity after March 1991 (the secondmaximum of the 22nd solar cycle) [6].

REFERENCES

1. Pereyaslova, N.K. and Nazarova, M.N., Izv. Akad. Nauk,Ser. Fiz., 2005, vol. 69, no. 6, p. 783.

2. Pereyaslova, N.K., Nazarova, M.N., and Petrenko, I.E.,Geomagn. Aeron., 2005, vol. 45, no. 3, p. 1 [Geomagn.Aeron. (Engl. Transl.), vol. 45, no. 3, p. 308].

3. Nazarova, M.N., Pereyaslova, N.K., and Petrenko, I.E.,Trudy FTI, Dergachev, V.A., Ed., St. Petersburg: Izd-voFTI, 1992, p. 77.

4. Katalog solnechnykh protonnykh sobytii 1970–1979 gg.(Catalogue of Solar Proton Events 1970–1979),Logachev, Yu.I., Ed., Moscow: IZMIRAN, 1983.

5. Katalog solnechnykh protonnykh sobytii 1980–1986 gg.(Catalogue of Solar Proton Events 1980–1986),Logachev, Yu.I., Ed., Moscow: MK AN SSSR. MTsDB2, 1990.

6. Catalogue of Solar Proton Events 1987–1996,Logachev, Yu.I., Ed., Moscow: Izd-vo MGU, 1998.