Pergamon Adv. Space Res. Vol. 17, No. 4lS. pp, (4/5)167-(4/5)170,1!396

0273-1177(95)00562-5

THE ENERGY SPECTRA AND OTHER PROPERTIES OF THE GREAT PROTON EVENTS DURING 22-ND SOLAR CYCLE

A. V. Belov and E. A. Broshenko

Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZUIRAN), 142092 Troitsk, Moscow Region, Russia

ABSTRACT

The proton spectra and their variations during the several large solar particle events in 22-nd solar cycle (29.09.89, 19.10.89, 24.10.89, 24.05.90 and 15.06.91) were obtained within the wide energy range. The data of spacecraft measurements and neutron monitors of the world-wide network have been used to analyse the energy and temporal dependencies of proton increases. All peak spectra reveal the similar behaviour in the low energy range and significantly differ at energy > 1 GeV.

INTRODUCTION

The 22-nd cycle contains especially great number of energetic solar proton events. But even on this background we can select several outstanding events which are distinguished by their properties. Five such events revealed like ground level enchancement (GLE) have been examined in this work (see Table 1). The increases on 29.09.89 and 24.10.89 are the largest GLE in this cycle but GLE 42 is the greatest in the last 38 years Ill. During of the GLE 43 (19.10.89) the greatest number of decay protons have been observed 121. On GLE 48 (24.05.90) ground level neutron monitors recorded the greatest number of solar neutrons for the history observation /3,4/. Finally, the hardest gamma-ray (up to 2 GeV) was recorded in GLE 52 (15.06.91) /5/.

TABLE 1. Ground level solar CR enhancements of 22-nd solar cycle, analysed in this paper.

GLE No 42

43

45

48

52

Date 29.09.89

19.10.89

24.10.89

24.05.90

15.06.91

X-Ray max. time

11.33

12.58

18.31

20.51

08.21

Location w105

S27 El0

s30 w57

N33 W78

N33 W69

Max. Increase on Importance Moscow NM, %

X9.81 190

X 13/4B 20

X57/3B 55

X9.3/lB 3

XlU3B 12

Usually proton spectra for solar particle events have been found by the data of any one group of detectors within the limited energy range, for instance, only by spacecraft data or only by stratospheric or ground level observation. But very often we need the total spectrum over the wide energy range. The problem is with unification of all limited parts into complete spectrum. In this paper we tried to obtain and analyse the proton spectra near Earth within the wide energy range (from 10 MeV to 10 GeV for the mentioned above events

DATA AND METHOD

Hourly data of IMP-8 /6/ and five minute of GOES-6,7 data as well as five minute data of about 30 neutron monitors have been used in the method described in /7/. It provided about 50 independent data rows within the different limited energy ranges. In order to unite the local spectra obtained for separate data groups into one total spectrum the regression analysis has been used for each energy range. The values of the fluxes have been taken from different local spectra but only for energies around optimal energy E,, when abs(ln(E) - In(E,))< 1.

(4/5)167

(4/5)168 A. V. Belov and E. A. Eroshenko

DISCUSSION OF THE RESULTS

The energy spectra have been found for each 5-minute interval, and the time dependencies of proton flux were obtain for each energy. In Figure 1 such time profiles of the proton fluxes for a number of different energies are presented for GLE 52 - there are the regular quasidiffusive profiles in the wide energy range.

Since the shape of energy spectrum changes swiftly along with the time, the fluxes of the different energy particles have different profiles. The greater the difference in the energy, the more the distinction in the profile shapes and in the time of maximum flux. In GLE 52 the difference between maxima of 30 MeV and 3 GeV proton profiles is -9 hours, and for 19.10.89 event (GLE43) such a difference reaches -13 hours, The shape of the time-dependence is mainly conditioned by mutual location of the Earth and flare, and also. by conditions of charge particle propagation in the space. For the events under consideration the situation in the space was very different. Some increases occurred on the relatively quiet periods (for example. 19.10.89), others on the Forbush-decrease phase (24.10.89), or on the active recovery (15.06.91). The efficient transport path estimated for the same energy but in the different events are distinguished in several times.

1ci7=

,//-.----- ----____ __ 100 MeV ---_ ~_ _~^-~~_~ -_.-

Fig. 1. Temporal dependence of the differential proton fluxes near Earth for different energies on 15 June 1991.

Having the time profiles of differential fluxes for each energy we can find the spectra in maximum of the increase (peak spectra) which are mostly closed to the emission proton spectra.

IO2

z

4 IO0

i

3 lo-2

‘;;- B IO4 g a

1 o-6

GOES-6, GOES-7

Neutron Monitors

PEAK SPECTRA

near Earth

IO 100 1000 Energy (MeV)

Fig.2. Proton peak spectra, observed near Earth in the enhancements 29.09.89, 19.10.89, 24.10.89, 24.05.90 and 15.06.91.

&M Roton EVW& DO+ 22-od Solar Cycle (4isflfi9

In Figure 2 the proton peak spectra within the wide energy range are shown for the five events. Although all these events are GLEs (and this fact already put the limit on the possible variations of the spectra), nevertheless, all spectra differ by the rn~~de and slope. These ~~inctions are especially great at the high energies. From the other side all spectra are similar since they sotten with the energy increase. In some cases the softening is gradual but in other it occurs with a sharp break, mainly under energies around 1 GeV. One can see from Figure 2 that events 29.09.89, 19.10.89 and 24.17.89 have a similar maximum fluxes within the low energy range and differ strongly (more than by an order) under higher (>i GeV) energies. On the contrary, proton spectra for 24.05.90 and 15.06.9 1 almost coincide at the high energies and differ strongly at the low one. Index y of the power-Iaw rigidity tkction depends strongly on the energy in some cases. Thus, for 19.10.89 y changes from -2.8 for E=30 MeV to -6.8 for E=6 GeV. In 29.09.89 event the spectrum slope changes weaker and magnitudes y are -2.5 and -3.8 for low and high energies respectively. Analysis of all spectra shows that behaviour of proton fluxes within the wide energy range may be approximated, as a rule, by power-law rigidity Rmction with linear dependence of index from In(R) (R - rigidity).

One can notice that solar proton spectra near Earth (and consequence, the emission spectra) within the low energy range are the same as unmodulated galactic CR spectrum (~2.5-3 .O). It seems, the solar accelerator reproduces its properties for low energy particles (up to 300 MeV) every time in mighty proton solar flare. The mechanism of the high energy particles acceleration may be switched in or not, but if it starts, its properties may be different.

Besides of the peak spectra, we can obtain spectra for any fixed moment. As a rule, the shape of the energy spectra is changing with the time in the regular way and we see the gradual softening of the observed spectra (Figure 3).

10 100 1000 Energy (MeVl

IO2

loo

10“

W4

10d ~ 10

Fig. 3. The differential proton spectra observed near Earth at different times on 15.06.91 arid 19.10.89 and peak spectrum for 15.06.91 enuancement.

The temporal dependencies of the intensity may be simulated by means of different assumptions of particle generation and propagation. Thus. for isotropic increase on 15 June 199 1 the energy spectrum and duration of the proton ejection were obtained in the frame of simple diffision model /8/. Within the 80-350 MeV energy range the model of long emission matches to experimental data better than the model of ithe nstantaneous emission. Analysis of the obtained results /81 shows that the protons of relatively low energy (<SO MeV) have been ejected in the impulse phase of the flare. From the other side, high energy protons

(3150 MeV) have been generated alter the end of impulse phase, and onset of their ejection approximately coincided with magnetic field reconstruction and with the second phase microwave burst onset 181.

We had an unique chance to compare proton and solar neutron spectra into the same flare - during 24.05.90 (GLE 48) event. As is well known, this event is the only one when a great number of solar protons and neutrons /3,4/ was observed on the Earth. The proton and neutron spectra obtained respectively in 171 and /9l are plotted in Figure 4. One can say, these spectra have the similar slope at low energies and differ greatly at the energy increase. Under efficient energy of the neutron monitors (2-3 GeV) these spectra are intersected and namely by this reason the maximum magnitudes both of these increases (proton and neutron intensity) are close in neutron monitor observations.

A. V. B&v and E. A. l-?sosbcako

lo2 24 May ‘I990

I I L I 10 100 1000 Energy (MeV)

Fig. 4. The differential peak spectra of the solar neutrons and protons near the Earth 24 May 1990.

Those fact, that in the maximum of increase the number of neutrons at the high energies was greater than of the proton number (Figure 4), must not be surprised since we consider not the generation spectrum but the spectrum near Earth. The events exist when the protons do not rich the Earth while the neutrons are recorded (for instance, 03.06.82, 22.03.91). But what can cause the distinction in the shape of neutron spectrum from generated them proton spectrum? One of the possible reason may be the difference of angle distribution of different energy neutrons escaped by the Sun.

Acknowledgements. This work was supported by the federal program “Agronomy”, grant 94-02-03096. We are gratefil to all researchers who provided us by the neutron monitors data in this work. We thank also M. Shea, D. Smart and L.C. Gentile for their help in receiving these data.

1. Smart D.F.and Shea M.A. A comparison of the magnitude of the 29 September 1989 high energy event with solar cycle 17, 18, and 19 events. Proc. 22 ZCRC,v. 3, 101-104, (1991)

2. Shea M.A. and Smart D.F. The evolution of the anisotropy of solar neutron decay protons during the 19 October 1989 Solar Cosmic Ray Event. Proc. 22 ICRC,v. 3,41-44., (1991).

3. Shea M.A. and Smart D.F. Solar Neutron Event on 24 May 1990. Frac. of &he First S0LT.P Sympos. 2, 217-221, (1992).

4. Pyle K.R., Shea M.A. and Smart D.F. Solar Flare Generated Neutrons Observed by NMs on 24 May 1990, Proc. 22-ndICR(7,3, 57-60, (1991).

5. Akimov V.V., Afanassyev V.G., Belaousov A.S. et.aI., Observation of High Energy Gamma-Rays From the Sun with the Gamma-l Telescope (E>30 MeV). Proc. 22-nd ZCRC, 3, 73-76, (199 1)

6. Armstrong T. National Space Science Data Centre (NASA/GSFC), Data Set 73-078A-08G-IMP8.

7. Belov A.V. and Eroshe~o E.A., The proton spectra of the four remarkable GLE in 22 solar cycle. Nuclear tacks and radiaticm measurements, (I 994), in press.

8. Belov A.V., Eroshenko E.A. and Livshits M.A. The Energy Spectra of the Accelerated Particles Near the Earth and in the Source in 15 June 199 1 Enhancement. Proceeding of Eight International Sumposium on Solar Terrestrial Physics, part 1,26, (1994).

9. Belov A.V. and L&hits M.A., The neutron flare 24 May 1990, Pisma v Astron. Jurnal, 20, 1 I (1994).