C H A N G E O F S O L A R F L A R E P R O T O N T O A L P H A R A T I O S

D U R I N G A N E N E R G E T I C S T O R M P A R T I C L E E V E N T

M. SCHOLER, D. HOVESTADT, and B. H~USLER Max-Planek-Institut fiir Physik und Astrophysik, Institut fiir extraterrestrische Physik

Garching b. Miinehen, Germany

(Received 7 October, 1971)

Abstract. Observations of the temporal behavior of energetic storm protons and alpha particles are presented for the event associated with the storm sudden commencement observed on Earth on March 8, 1970. The data are obtained on board the low altitude polar orbiting satellite GRS-A/AZUR by means of two particle telescopes. Large changes in the proton to alpha ratios for particles of equal energy and for particles of equal energy per nucleon are observed, whereas no significant change in the equal energy per charge ratio is observed. Electric fields, Fermi acceleration and cyclotron reso- nance are discussed as possible modulation mechanisms.

1. Introduction

Large flux enhancements of the 1 to 20 MeV protons have sometimes been observed several hours or days after the onset of a solar flare proton event (see, e.g., Axford and Reid, 19 63; Bryant et al., 1965; Rao et al., 1967; Kahler, 1969). These energetic storm particles (ESP) are usually seen in connection with sudden storm commence- ments (SSC) and only when solar energetic particles from an earlier flare are present in interplanetary space. This led most of the authors to the conclusion that the ESP events are associated with the interplanetary shock waves which produce the SSC's on Earth. In addition, short lasting (10 rain) proton enhancements associated with the shock front itself have been reported by Lanzerotti (1969), Singer (1970), Armstrong et. al. (1970), Ogilvie and Arens (1971) and Palmeira et al. (1971).

Several explanations for the ESP events have been proposed: Axford and Reid (1963) suggest Fermi acceleration of energetic particles by scattering between the shock and the magnetosphere. Van Allen and Ness (1967) have modified the Axford and Reid hypothesis and have proposed Fermi acceleration of energetic particles by scattering between the shock and interplanetary field irregularities far upstream of the shock. Gold (1959) has suggested trapping of solar accelerated particles in a magnetic tongue and Parker (1965) and Rao et al. (1967) propose that the ESP's are accelerated in the shock itself.

Lanzerotti and Robbins (1969) have examined the temporal behavior of the low energy solar alpha to solar proton flux ratios during two interplanetary disturbances. One of the observed changes in the particle ratios is attributed to a source effect. Since the second change is only observed in the equal velocity and rigidity ratios and not in the equal energy per charge ratio they conclude that the modulation is caused by electric fields in the interplanetary disturbance.

The purpose of this paper is to examine the temporal development of the proton

Solar Physics 24 (1972) 475--482. All Rights Reserved Copyright �9 1972 by D. Reidel Publishing Company, Dordrecht-ttolland

476 M. SCHOLER ET AL.

to alpha particle ratios during the ESP event of March 8, 1970 as observed by the polar orbiting satellite GRS-A/AZUR. The ESP's were associated with a strong interplanetary shock wave which caused the SSC at 1417 UT on March 8, 1970. The data show that large changes occur in the proton to alpha ratios for particles of equal rigidities and equal energies per nucleon, whereas no significant change is observed in the equal energy per charge ratio.

2. Satellite and Instrumentation

The satellite GRS-A/AZUR, launched November 1969, is a magnetically stabilized polar orbiting satellite with an apogee of 3145 km, a perigee of 383 km, an inclination of 102.94 ~ and a period of 122 min. Two identical proton alpha particle telescopes (88/1 and 88/2) are oriented one perpendicular (88/1) and one at an angle of 45 ~

TABLE I

Response of the telescopes 88/1 and 88/2 of the satellite AZUR. A to G = Silicon detectors, S =Anti-coincidence scintillator.

Channel Logic Particle Energy

K1 A B (~ S protons 1.5-2.7 MeV K2 A B 13 g alpha 6-19 MeV K3 B C I) S protons 2.7-5.2 MeV K4 C D E S protons 5.2-10.4 MeV K5 D E ~" S protons 10.4-22 MeV K6 E F G g protons 22-49 MeV K7 F G ~ protons 49-104 MeV

Geometrical factor: 5.80 • 10 -2 cm 2 sr (telescope 88/1 ; 90 ~ 5.95 • 10 -z cm 2 sr (telescope 88/2; 45 ~

(88/2) with respect to the local magnetic field vector. In the Northern Hemisphere telescope 88/2 is pointing upwards. The telescopes are particle range devices consisting of a stack of seven fully depleted silicon detectors. The energy channels are defined by the thickness of the detectors and of the absorbers placed in-between. The stacks are surrounded by a plastic anti-coincidence scintillator and a heavy shielding (only protons with energies > 75 MeV are able to penetrate). Table I shows the energy ranges and the logical condition of the different channels K1 to K7. The geometrical factors of the instruments are 5.80x 10 - a c m z sr (88/1) and 5.95x 10 .2 cm 2 sr respectively. A detailed description of the experiment is given in Achtermann et al.

(1970).

3. Observations

Figure 1 gives the time history of the particle flux as observed in four proton energy channels and one alpha particle energy channel by GRS-A/AZUR. The data plotted in Figure 1 are obtained at an altitude of about 3000 km over the northern polar cap

CHANGE OF SOLAR FLARE PROTON TO ALPHX RATIOS 477

Fig. I.

10 4 --

�9 PROTONS 1.5 - 27 MeV o PROTONS 2.7 - 5.2 MeV �9 PROTONS 5 .2 - lO .4MeV

a PROTONS 10, ' , - 22 MeV i- ALPHAS 6 - 19 MeV

0 7 8 A 9 10 MARCH 70 SSC

Proton fluxes in four energy channels and alpha flux in one energy channel for March 7-10, 1970. Data are averages over the low latitude region of the polar cap.

lO 3 OC

W

~u i0 ~

~E

and are averages over the low latitude region of the polar cap. The low latitude region of the polar cap (at an invariant latitude between 60 ~ and 75 ~ shows in most passes a strong flux enhancement as compared to the central polar cap. Scholer et al. (1971) have shown by comparison of the interplanetary proton fluxes as observed by HEOS A1 with the polar cap proton fluxes that the low latitude region represents the inter- planetary flux more accurately than the central polar cap. This region of flux enhance- ment changes according to gross changes in the configuration of the magnetosphere and has to be determined for each polar pass. All data presented in this publication are averages over the low latitude region of the polar cap.

The low energy protons and alpha particles present in interplanetary space before the ESP event of March 8, 1970 were probably generated by two solar flares: A flare activity on March 6 in McMath region 10595, which was located at N 15 and nine degrees beyond the west limb, and a 2B flare in McMath region 10614 at 10 ~ east solar longitude on March 7, 01 50 UT seem to have emitted energetic particles (see, e.g., McIntosh, 1971; Sakurai, 1971; Achtermann et al., 1971 ; Scholer et al., 1971). The particle data show the temporal characteristics of particles emitted from a flare on the central solar disk: A slow rise to a maximum, which is reached on March 8, 1970 at about 0230 UT, and then an approximately exponential decay over several days. Superimposed on the exponential decay is the appearance of the ESP's. The increase in the 1.5 to 2.7 MeV protons is observed eight hours before the occurrence of the SSC on Earth at 1417 UT on March 8. The increase in the higher energy proton channels and in the alpha particle channel is seen during the orbit following the SSC. Following the ESP event there is a sharp decrease in all energy channels.

The flux ratio of the 1.5 to 5.2 MeV protons to the 6 to t9 MeV alpha particles is shown in Figure 2. This ratio is approximately the ratio for particles of equal energy per nucleon. About eight hours before the March 8 SSC this ratio increases and reaches a sharp maximum during the polar pass immediately after the SSC. The increase

100

corresponds in time to the increase of the 1.5 to 2.7 MeV protons, whereas there is no similar increase of the 6 to 19 MeV alpha particles before the SSC. The ratio drops after the SSC over a time period of more than ten hours to the value previous to the ESP event.

Figure 3 shows the ratio of the 6 to 22 MeV protons to the 6 to 19 MeV alpha

50

20

0

Fig. 2.

- - T ] F ' I I

153 !

PROTONS (1.5, E , 5.2 MeV) RATIO

cr PARTICLES (G - E , 19 MeV )

_ 6 I I , I I I 7 --SSC8 9 10 MARCH 70

A

Proton to alpha ratio for equal energy/nucleon (velocity), March 6-10, 1970.

PROTONS (6, E , 22 MeV )

10

478 M. SCHOLER ET A L

0 6 I ~ I SSC8 1 g I I0 MARCH 70

Fig. 3. Proton to alpha ratio for energy (rigidity), March 6-10, 1970.

CHANGE OF SOLAR FLARE PROTON TO ALPHA RATIOS 479

particles. This corresponds approximately to the ratio of particles of equal total energy which for protons and alpha particles is the same as the ratio of particles of equal rigidity. The lower limit of the energy channel K4 is 5.2 MeV. In order to correct for the different lower limits of the alpha particle energy channel K2 and the proton channel K4 the following procedure was adopted. The flux of the 5.2 to 6 MeV protons was determined by fitting a power spectrum through the three lowest energy channels and integrating the spectrum between 5.2 and 6 MeV. This contribution was subtracted from the proton channel K4. The relatively high ratio for equal energies until 1200 UT on March 7 is probably due to the first flare, whereas the ratio was lower for the second flare event. Starting hours before the SSC there is a significant

100

50

Fig. 4.

] ' I ' [ ' I '

PROTONS (2.7 ( E < 10.4 MeV ) RATIO

(z-PARTICLES ( 6 ( E , 19 MeV)

I ,~ I SS~8 I 9 I 10MARCH?0 A

Pro ton to a lpha ratio for equal energy/charge, March 6-10, 1970.

decrease in the equal energy (rigidity) ratio, which continues for about four hours after the SSC. The ratio raises again to a mean value of about 3.5 but does not reach the pre-ESP event value.

The ratio for 2.7 to 10.4 MeV protons to the 6 to 19 MeV alpha particles is plotted in Figure 4. This ratio is approximately the ratio for particles of equal energy per charge. Except for a drop at about 1200 UT on March 7 the ratio for particles of equal energy per charge shows no significant change during the ESP event.

4 . D i s c u s s i o n

The data presented here show that during the ESP event the proton to alpha ratios vary greatly in magnitudes according to whether the ratio of equal velocity, equal rigidity or equal energy per charge is considered. The limitations of the data have to be kept in mind: The proton and alpha particle fluxes are observed over the polar cap

4 8 0 M. SCHOLER ET AL.

and do not necessarily reflect the interplanetary fluxes. But it has been shown by Scholer et al. (1971) that low energy particles reach the low latitude region of the polar cap via quasi-trapped trajectories and we expect the ratios in this region to be quite representative of the interplanetary ratios. A second limitation is given by the fact that we can present only data for one alpha particle energy channel.

It is tempting to interpret the lack of change in the equal energy per charge ratio by an electric field dependent modulation mechanism in the interplanetary disturbance as has been done by Lanzerotti and Robbins (1969). In the following we will show that in the case of a power spectrum for both protons and alphas any acceleration mecha- nism which imparts to the alpha particles twice the energy that is imparted to the protons will not change the ratio for equal energy per charge.

Introducing the particle fluxes Ij and Ij and the energies E and Ej. before and after the acceleration process we can write Liouvilles Equation in the following form

Ij (E) /E = Ij (Ej)/E) =const. (1)

The index j designates the two sorts of particles (protons j = p , alphas j = c 0. The energy Ej is given by the energy E before the acceleration plus the energy gain AEj:

E) = E + AEj (2) where

AE~ = 2AE v = 2AE. (3)

Introducing (2) and (3) in (1) we get for the ratio of protons and alpha particles at equal energy per charge after the acceleration:

I~ (E'/2) _ Ip [(E - AE)/2] I" (E') (E - aE) (4)

This means that, if for the initial energy spectra of protons and alpha particles the relation

I, (E/Z) _ const. (5)

holds for the whole energy range, then the ratio for equal energy per charge does not change according to the acceleration process. Power law spectra with the same spectral exponent for both protons and alphas naturally satisfy condition (5).

An electric field is a natural candidate to impart to the doubly charged alphas twice the energy that it imparts to the protons. But if one examines the energy spectra of the protons before and during the ESP event it is as if 1.5 MeV protons have been increased in energy by possibly 600 keV. It can hardly be seen how the solar wind particles with a thermal energy of about 10 eV and a kinetic energy of about 1 keV can generate and maintain electric potentials of such an order of magnitude across the discontinuity.

Fermi mechanism by scattering of the particles between the shock and inter- planetary field irregularities far upstream of the shock should not lead to any change

CHANGE OF SOLAR FLARE PROTON TO ALPHA RATIOS 481

in the equal energy ratio. The maximum energy gain per encounter with the approach- ing shock is A E = 2 rnvV where m is the particle mass, v the particle velocity and V the shock wave velocity. Under the assumption that the particle makes v/2L round trips per second, where L is the distance from the shock to the mirroring irregularity, the particle increases its energy at the rate

dE/dt = 2E ( V / L ) . (6)

Therefore the energy gain is independent of the particles' mass and charge. The possibility of turbulent acceleration of charged particles by cyclotron resonance

with hydromagnetic waves was discussed by Tverskoi (1968). The acceleration equa-

tion is given by

gF _ 1 v2D (v) - - (7) ~t v 2 ~v

where F(v ) is the distribution function and D (v) the diffusion coefficient in velocity space which depends on the pulsation spectrum. Assuming that the pulsation spectrum has a power law form dh2/dk,,~ k - 2 , where k is the wave number, one finds D (v )~ v. Introducing the differential flux I ( E ) and the energy E into (7) one gets

OI 021 - - = A m l / 2 E 3/2 - - ( 8 ) t}t c~E 2 '

where A is a constant containing the main scale of the pulsation spectrum. Intro-

ducing E* = E l m we finally get

O I 0 2 1 _ = AE*3/2 _ _ ~t ~E . 2 ' (9)

This means that if Ip (E*)/I~ dE*)= const at time t o, it will remain constant during the diffusion process, i.e. the flux ratio of protons and alphas at the same energy per nucleon is independent of time. This is of course a consequence of the assumption of a power law ,-~ k-2 for the pulsation spectrum.

It might well be that a combination of Fermi mechanism at the shock front and cyclotron resonance and scattering in the field irregularities direct upstream of the shock can explain the observed ratio changes. To reach a clear cut decision for a certain modulation mechanism one needs better theoretical models and spectral measurements of protons and alphas during the ratio changes. Observations of the corresponding ratios between protons and heavier nuclei than alpha particles are highly desirable.

Acknowledgements

We are grateful to Dr W. Alpers for helpful discussions. Thanks are due to the technical staff of the Max-Planck-Institut for the development and testing of the instrument and to Dipl.-Math. B. Ebel for the data reduction programming.

482 M. SCHOLER ET AL.

References

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Achtermann, E., H~iusler, B., Hovestadt, D., and Scholer, M. : 1971, Z. Geophys. 37, 211. Armstrong, T. P , Krimigis, S. M., and Behannon, K. W. : 1970, J. Geophys. Res. 75, 5980. Axford, W. I, and Reid, G. C.: 1963, J. Geophys. Res. 68, 1743, Bryant, D. A., Cline, T. L., Desai, U. D., and McDonald, F. B.: 1965, Astrophys. J. 141,478. Gold, T.: 1959, J. Geophys. Res, 64, 1665. Kahler, S. W. : 1969, Solar Phys. 8, 166. Lanzerotti, L. J. and Robbins, M. F. : 1969, Solar Phys. 10, 212. McIntosh, P. S.: 1971, in Report UAG-12, World Data Center A, p. 10. Ogilvie, K. W. and Arens, J. F.: 1971, J. Geophys. Res. 76, 13. Palmeira, R. A. R., Allure, F. R., and Rao, U. R.: 1971, Solar Phys. 21, 204. Parker, E. N. : 1965, Phys. Rev. Letters 14, 55. Rao, U. R., McCracken, K. G., and Bukata, R. P.: 1967, J. Geophys, Res. 72, 4325. Sakurai, K.: 1971, in Report UAG-12, World Data Center, p. 52. Scholer, M., H~iusler, B., and Hovestadt, D. : 1972, Planetary Space Sci. 20, 271. Singer, S.: 1970, in V. Manno and D. E. Page(eds), Intereorrelated Satellite Observations Related

to Solar Events, D. Reidel, Dordrecht, Holland, p. 571. Tverskoi, B. A.: 1968, Soy. Phys. JETP 26, 821. Van Allen, J. A. and Ness, N. F.: 1967, J. Geophys. Res. 72, 935.