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SOLAR NEUTRON MEASUREMENTS ON THE SOLAR PROBE

K. Kudela,* S. N. Kuznetsov** and A. AntalovB*

* Institute of Experimental Physics, SAS, KoSice, Slovakia ** Skobeltgx Institute of Nuclear Research, Moscow State University, Moscow, Russia

ABSTRACT

We outline briefly possibilities for measurement of solar neutrons on the Solar Probe and their relevance for understanding acceleration processes at the solar surface, based on the current status of the solar

neutron investigation and some of the devices used until now.

INTRODUCTION.

Gamma rays and neutrons produced by nuclear interactions of highly energetic ions with the ambient solar atmosphere are not affected by magnetic and electric fields from the place of their production until the detector, providing thus a direct channel of information on the spectra of accelerated protons at the Sun. Solar Probe (SP) will enable us to study different kinds of solar radiation, including neutral, from close proximity to the Sun. We briefly summarize the current status of the knowledge of the solar neutron (SN) flux near the Earth, and stress potential possibilities for SN measurements on SP which can contribute to better understanding of the acceleration processes.

CURRENT STATUS

Although the idea that ions accelerated to GeV energies in solar flares (SF) can produce a SN flux observable from the Earth is more than four decades old /l/? for a long time only upper limits of the SN flux were obtained in experiments on balloons and satellites /2/. First observations of SN were reported from the SF on 2 1 June, 1980 on SMM /3/. Relativistic neutrons were observed by SMM also on 3 June, 1982 /4/. From that flare the high energy neutrons were detected for the first time also by tbrce ground- based neutron monitors (NM) in the proper local time sector /j-7/. Combination of the SMM and NM measurements lead to the estimation of the SN emissivity spectrum, not observed as proton GLE at the Earth, in the energy range 100 - 2000 MeV / 8/. The search for responses of NM to possible SN events motivated better temporal resolution of existing NMs and building new equipment having better sensitivity and directional properties to the SN /9, lo/. Other “candidates” of SN, usually expected as short-time spikes in NM records in time coincidence with satellite gamma ray line observations were described / 1 l- 131 and possible anisotropy of SN production at large heliocentric angles, was reported /13/. During the solar cycle 22 four SN events with relativistic neutrons were observed by NMs /14-16/. While for the first SN events the observations were consistent with SN production just after the impulsive flare. new data from GAMMA-l spacecraft and by COMPTEL telescope on GRO show that time-extended production is also observed in SF /17,18/. Another source of information on SN, at lower energies than from NM. is the measurement of SN decay products, protons observed in interplanetary space / 19/. or electrons 1201.

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(WI0 K. Kudelo er al.

Almost all SN events were observed from Earth orbit. This limits the possibilities of further progress. An exhaustive review of the problematics can be found in 12 11 and the theoretical background - production of SN in the conditions of the solar surface in 122.231

POTENTIAL POSSIBILITIES ON SOLAR PROBE.

The efficiency of observing photons with a given device when the SP will approach the Sun, is increasing

as l/r2, where r is distance to the Sun. For neutron detection, however, the closer position gives more advance, depending on energy. Having a device at 1 AU with efficiency 1. the yield Y(E,r) at distance r will be

Y(r,E) = Pr(E)/(Pl(E).r2) = rm2. exp (( 1-r).(l-l3 )- 2 l/2 .(l3.~.r)-~) (1)

where p = v/c, r is lifetime of neutron, and P,(E) is the survival probablitity of a neutron with kinetic

energy E at distance r in AU. For the Earth’s orbit. the probability is Pi(E). Fig 1 shows the yield

function for the positions r=O.3, 0.7 and 0.033 AU.

10000000

1000000

100000

G 10000

; z 1000

100

10

1

0.1 1 10 100 1000

Energy of neutron GleW

A SP approaching the Sun to I 0.3 AU, provides the opportunity to measure directly SN at low energies, 1 - 10 MeV, impossible from Earth’s orbit. Assuming the spectra for 3 June, 1982. the total emissivity

Q=7.4.1031.E-2.4 for 100 - 2000 MeV /8/ and extrapolating spectra to lower energies, relatively high fluxes of SN can be observed with existing techniques applied in space for the purpose of neutron albedo

studies (e.g. /22/). Using He3 or BF3 proportional counters surrounded by the moderators with different

thicknesses will enable us to measure neutrons in different energy intervals within ~10 MeV. If the

efficiency is 10 cm2, then for 3 June. 1982 SF we estimate 2. lo5 SN counts at r=O.3 AU and at i=O.7 still 800 counts in 1 - 10 MeV region. At 7 solar radii it is possible to register neutrons with energy > 175 keV

with a set of He3 counters with different shielding with a device similar to that described in /22/.

Solar Neutron Measurements on Solar probe (B)Si

For IO-100 MeV the COMPTEL instrument could give well measurable fluxes at SP. Assuming its sensitivity (Fig. 2 in /25/) and spectra in / 17/, the integrated number of counts for 15 June 15,199l flare

would be 6000, two orders more than near the Earth. Thus better temporal resolution for the time extended acceleration processes could be achieved.

The SF on 24 May, 199 1 had a SN flux (at energies detected by NMs) about 70 times higher than that on 3 June, 1982 /26/ and was clearly detected at several American NMs. NM are threshold detectors of nucleons, not distinguishing between p and n.Nevertheless. in the high energy region, NMs will probably remain the only source of SN measurements. Recently it was found that abilities of NM network to detect SN is higher than expected earlier /27/. Measurements on SP by devices similar to COMPTEL could provide also intercallibration in this energy interval 60 - 200 MeV.

SUMMARY

Solar Probe mission could help significantly in the correct description of the acceleration of protons at the Sun, if SN detection would be included in its scientific programme in connection with gamma ray line and

continuum measurements. Independently of the “state” of the interplanetary medium leading to scattering of

the protons. independently of the position of SF with respect to the field line connecting it to Earth’s orbit, and independently of coronal transport of protons, the flux and energy spectra of accelerated protons could

be deduced from SN measurements over the wide energy range of hundreds of keV to few GeV. A larger number of SF could be examined than only those studied from protons near the Earth. One approach is to fly one device measuring low energies SN (< 1 OMeV) possibly using a proportional counter system, a second in the range 10 - 100 MeV using a similar technique as COMPTEL, and coordinate these measurements with the NM world network plus new SN ground based devices under development to cover the energy range above 100-200 MeV. The comparison with proton and electron measurements on the SP would allow us to deduce the profiles of n decay products. This approach would be aided by correlated measurements by similar equipment near the Earth’s orbit.

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