This paper demonstrates the important interplanetary manifestation of strongly tilted magnetic fields at the flare site. We start with analysis of Big Bear Solar Observatory (BBSO) observations of magnetic structures at sites of two flares responsible for >100 MeV neutron events. Based on these observations, a model of neutron production is considered. This model takes into account the observed large tilt of magnetic field lines at footpoints of flare magnetic loops. Results of the new calculations are compared with both previous calculations and observations. The tilt of magnetic field lines at the flare site is proved to be the most important parameter limiting anisotropy of high-energy secondary emission in solar flares

Kocharov, L. G.

Kovaltsov, G. A.

Tang, F.

Torsti, J.

Usoskin, I. G.

Zirin, H.

English

Caltech Authors - Main

IMPACT OF MAGNETIC ENVIRONMENT ON THE GENERATION
OF HIGH-ENERGY NEUTRONS AT THE SUN
L. G. KOCHAROV and J. TORSTI
Space Research Laboratory, Department of Physics, SF-20014 Turku University, Finland
F. TANG and H. ZIRIN
Big Bear Solar Observatory, Caltech, Pasadena CA 91125, U.S.A.
G. A. KOVALTSOV and I. G. USOSKIN
Ioffe Physical-Technical Institute, St. Petersburg 194021, Russia
(Received 5 July 1996; accepted 11 October 1996)
Abstract. This paper demonstrates the important interplanetary manifestation of strongly tilted
magnetic fields at the flare site. We start with analysis of Big Bear Solar Observatory (BBSO)
observations of magnetic structures at sites of two flares responsible for > 100 MeV neutron events.
Based on these observations, a model of neutron production is considered. This model takes into
account the observed large tilt of magnetic field lines at footpoints of flare magnetic loops. Results
of the new calculations are compared with both previous calculations and observations. The tilt of
magnetic field lines at the flare site is proved to be the most important parameter limiting anisotropy
of high-energy secondary emission in solar flares.
1. Introduction
The acceleration of particles is an important part of the solar flare problem. A
number of acceleration models have been proposed (e.g., Anastasiadis and Vlahos,
1994, and references therein). All the acceleration models face a common problem:
they have no reliable observational spectra of interacting ions to be used as a basic
criterion for the model. One should use -ray and neutron emissions to deduce
spectra of high-energy ions interacting at the Sun (for a review see Mandzhavidze
and Ramaty, 1993). For instance, the 2.22 MeV to the 4–7 MeV -ray line fluence
ratio is a sensitive function of the ion spectrum slope at 30 MeV, but it gives
almost no information about proton spectrum above 100 MeV. For this reason,
both the power law and the Bessel function type spectra of accelerated ions are
used to fit the fluence ratios observed (e.g., Ramaty et al., 1993). Furthermore,
it is known that the 2.22 MeV to the 4–7 MeV fluence ratio may also depend
on the angular distribution of interacting ions (Hua and Lingenfelter, 1987). To
deduce the spectrum of interacting ions at higher energies, >100 MeV, one has to
study >50 MeV neutrons and 0-decay -rays. When studying these emissions,
the angular distribution of interacting particles turns out to be even more important
because of possible anisotropy of any high-energy emission produced. This implies
that ion spectra cannot be accurately deduced from -ray and neutron data without
a study of the angular distribution of interacting particles.
Solar Physics 172: 271–278, 1997.
c 1997 Kluwer Academic Publishers. Printed in Belgium.
GR: 201018810, PIP: 124190 SPACKAP
*127190* sola7009.tex; 17/06/1997; 10:17; v.6; p.1
272 L. KOCHAROV ET AL.
In solar cycle 22, observations of high-energy -ray and neutron flares suggest
more isotropic production of high-energy neutral emissions than was expected earli-
er (Mandzhavidze and Ramaty, 1993, Vilmer et al., 1993). One possible explanation
of these observations is that the high-energy emissions are produced in complex
magnetic structures with varying configurations from flare to flare (Chupp et al.,
1993, Trottet et al., 1993). However, this hypothesis has not been quantitatively
verified. In this paper we study the magnetic environment at sites of two flares
responsible for high-energy neutron and -ray events detected at the Earth’s orbit.
Then we calculate the transport and interaction of accelerated ions, taking into
account the tilted magnetic fields observed.
2. Observations
Two well-known neutron-emitting flares (e.g., Kocharov et al., 1994, 1995) were
observed at Big Bear Solar Observatory on 24 May 1990 and 22 March 1991.
In particular, high-resolution observations in the helium D3 line (5876 Å) are
available for these flares. The D3 images were taken at a 3 per min rate with a
spatial resolution of about 1 arc sec. Vector magnetograms (VMGs) with a spatial
resolution of about 2 arc sec are available for the 22 March 1991 flare only.
It is known that accelerated particle streams and thermal conduction may result
in D3 brightenings in flares (Zirin, 1988). To examine the connection between
D3 brightening and the precipitation of accelerated particles, we compared D3
light curves obtained at BBSO and the intensity-time profiles of the hard X-ray
emission detected on board the GRANAT satellite (Pelaez et al., 1992, Terekhov
et al., 1996). To obtain the D3 light curve, the digitized images of the D3 filtergrams
were obtained first. A small portion of each image that includes only the flare region
is selected such that the background is minimal and the same in all the frames. Then
a flare threshold above the actual background is determined. The light curve is the
number of pixels above this threshold at different times during the flare. The result
is shown in Figure 1. Shown in the same figure is the time profile of100 keV hard
X-ray emission. Coincidence of D3 and hard X-ray brightenings is seen clearly at
the onset of both flares. This supports the idea that the D3 emission was initially
caused by penetration of accelerated particles deep into the solar atmosphere. For
this reason, we use D3 images to ascertain the site where accelerated particles hit
the solar chromosphere.
Figure 2 shows two D3 frames obtained in the beginning and near the maximum
of the 1990 May 24 flare (N33 W78). The flare started with two chains of bright-
enings which later developed into two bright ribbons situated in magnetic field
regions of opposite polarity. Hence, one can regard these chains as footpoints of
the small arcade where accelerated particles precipitated. Late in the flare, at about
21:29 UT, as the temperature cooled, one of the loops was seen in H + 0:6 Å.
Seen in the projection plane, the loop legs were tilted at about 70 and 45 with
sola7009.tex; 17/06/1997; 10:17; v.6; p.2
IMPACT OF MAGNETIC ENVIRONMENT 273
Figure 1. Hard X-ray count rates and HeD3 bright areas for two flares responsible for neutron/-ray
events. t0 is 22:42 UT on 22 March 1991 and 20:45 UT on 24 May 1990.
respect to the solar horizon. Note that in the earlier study (Kocharov et al., 1994)
such relatively small loops of  2  109 cm length were considered to be the
place where the first component of the high-energy ions was confined. And some
much larger loop was suggested for more prolonged confinement of the second
component of accelerated ions.
The D3 image near the maximum of the 1991 March 22 flare (S26 E28) is
shown in Figure 3. Basically four sources of emission,A1, A2, B1, B2, were seen
at opposite sides of the neutral line of the magnetic field, which is also indicated in
the figure. Later in the flare, at about 22:53 UT, several magnetic loops connecting
A1B1 and A2B2 were seen in absorption. From the fact that they were observed in
absorption, we may deduce that their density was below 5 1012 cm 3 (see Zirin,
1988). The distance between the footpoints was (1:5   6)  109 cm. The vector
magnetogram from BSSO was used to calculate the angle between the line of sight
and the solar magnetic field (see Figure 2 by Kocharov et al., 1995, where east is
to the right). The view angle is found to be predominantly about 45 in sources
situated to the west and about 90 to the east of the neutral line. Taking into account
the position of the flare on the solar disk, one can deduce that the dominant tilt of
the magnetic field lines with respect to the solar surface was about 80 and 35
in the west and east sources, respectively. Thus we conclude that, in both flares,
magnetic field lines were essentially tilted at one or both footpoints of the loops
where hard emissions are generated.
3. Model and Results of Calculations
Let us consider the transport and interaction of accelerated particles in a mag-
netic loop with field lines tilted at one or both footpoints. Neutrons with ener-
gies >100 MeV are mostly produced in (p,p), (p,He) and (He,He) interactions
sola7009.tex; 17/06/1997; 10:17; v.6; p.3
274 L. KOCHAROV ET AL.
Figure 2. The 1990 May 24 flare shown in D3. North is to the left, west is on top.
and are emitted preferentially in the direction of the incident particle. Note that
bremsstrahlung -rays produced by ultra-relativistic electrons are emitted also
preferentially along the direction of motion of the electron, but the cross-section
is more anisotropic. For these reasons, the impact of the magnetic environment
on the production of high-energy neutral emissions may be clearly shown when
analyzing the distribution of primary particles at the instant of interaction. Thus, as
a result of Monte Carlo simulations, we obtain the angular distribution of primary
protons at the instant the secondary neutron is produced,F ('; #). The polar angles
' and # are defined in the frame of the loop (Figure 4(b)). The function F ('; #)
is normalized to unity for an isotropic distribution of the interacting particles. One
can introduce certain simplifications when the only aim is to study the impact of the
tilted field lines on the angular distribution of the high-energy secondary emission.
In particular, the model does not attempt to consider the temporal evolution of the
energy spectrum of accelerated particles. Accelerated particles are proposed to be
injected at the top of the loop and the initial energy of all primary particles is E0,
sola7009.tex; 17/06/1997; 10:17; v.6; p.4
IMPACT OF MAGNETIC ENVIRONMENT 275
Figure 3. The 22 March 1991 flare shown in D3. The black dashed line indicates the magnetic neutral
line. North is to the right, and east is on top.
which is the ‘effective’ energy for the emission generation. For production of neut-
rons detected by ground based neutron monitors, E0 = 600 MeV. In the coronal
part of the loop (of heightR), the magnetic field strength as well as number density
are considered to be constant, Bc and nc, respectively. In the chromosphere, the
magnetic field increases downward with depth,Z  0: B = Bc(1 Z=hm), while
the magnetic field is constant in the photosphere: B = Bph. The density increases
downward with depth: n = n0 exp( Z=hA) (at Z = 0, there is an abrupt density
increase from nc to n0  nc). The mirror ratio,  = Bph=Bc, is considered to be
the same for both footpoints, but the tilt of magnetic field lines, , may be different
(Figure 4(b)). We trace the propagation of accelerated ions inside the loop, taking
into account the pitch-angle scattering on small-scale magnetic inhomogeneities
(MHD turbulence), energy losses and nuclear interactions (similar to Guglenko
et al., 1990). The mean free path describing the pitch-angle scattering, , is con-
sidered to be less than the loop length.
In Figure 4(a) is shown the angular distribution of interacting protons in the
loop plane (' = 180) for two cases: (1) both loop legs are vertical, (2) one
sola7009.tex; 17/06/1997; 10:17; v.6; p.5
276 L. KOCHAROV ET AL.
Figure 4. (a) Angular distribution of protons at the instant of neutron production shown in the loop
plane for two cases: (1) both loop legs are vertical, (2) one leg is tilted by  = 45. (b) The loop
model employed. Parameters of the loop: R = 2 109 cm, nc = 1011cm 3, hm=hA = 2:5,  = 5,
n0 = 1013cm 3, =(R) = 0:1.
leg is tilted by 45 as shown in Figure 4(b). The first case was adopted in many
previous calculations (for a review see Ramaty and Mandzhavidze, 1994). Taking
into account the logarithmic scale applied, the impact of the magnetic tilt is seen
to be very strong. In Figure 5, we show the pattern of two-dimensional distribution
of interacting protons for the case where both loop legs are tilted by 45.
4. Discussion and Conclusions
Results of Monte Carlo simulations are in agreement with the approximated analyt-
ical consideration by Kocharov and Kovaltsov (1995). It was concluded that, in the
direction normal to the magnetic field at the footpoint, the distribution of interact-
ing particles, F?, is rather weakly dependent on model parameters, provided that
the magnetic mirror is thick enough to strongly decelerate trapped particles with
smallest pitch angles and hm=hA  1. In this case, the value of the distribution
function may be estimated as
F?  0:5

ln(( cos)=(n0hA))  0:5 ln(hm=hA) + hm=hA   2
 1=2
; (1)
where the contribution of only one magnetic mirror is taken into account,  is the
path length describing both Coulomb and nuclear losses. The typical value of F?
is 0.15. These particles interact while mirroring, and their emission dominates in
sola7009.tex; 17/06/1997; 10:17; v.6; p.6
IMPACT OF MAGNETIC ENVIRONMENT 277
Figure 5. The distribution of protons at the instant of neutron production. Both loop legs are tilted
by 45 in the loop plane (for another parameters see Figure 4).
the direction ' = 180, # = 270 (marked with A in Figure 4(a)). On the other
hand, in the case of the tilted magnetic field, emission in the direction ' = 180,
# = 90 (marked B in Figure 4(a)) is mainly produced by non-mirrored particles.
The intensity of this emission may exceed that produced in the isotropic case.
Provided that the loop legs are not sufficiently tilted, the emission in the upward
direction (marked C in Figure 4(a)) is predominantly generated by particles in the
coronal portion of the loop (C in Figure 4(b)). For this emission, the distribution
of interacting particles is isotropic and depends on trapping time,  , and density at
the top of the loop: Fk  (ncv)=, where v is particle velocity.
It is seen that the tilt of the magnetic field at loop footpoints is the most important
parameter affecting the observed intensity of >100 MeV neutrons and other high-
energy secondary emissions. In the case of the tilted field, neutron emission of
the same order of magnitude may be observed from flares with heliocentric angles
 35 and  80 (e.g., #  35 in Figure 4(a), and #  80, ' = 90 in
Figure 5, respectively). Recently Kocharov et al. (1995) compared high-energy
neutron emissions from the 24 May 1991 and 22 March 1991 flares and concluded
that the relative brightening of the 24 May 1990 flare due to the difference in
flare heliocentric angles did not exceed a factor 3. This result may be easily
explained because, in the case of the 22 March 1991 flare, the magnetic field at
the east footpoints was strongly tilted, as indicated by BBSO magnetograms. Our
calculations reveal strong sensitivity to the magnetic tilt angles. To deduce these
angles, high-resolution VMGs are required.
sola7009.tex; 17/06/1997; 10:17; v.6; p.7
278 L. KOCHAROV ET AL.
Thus we conclude the following: (1) the tilt of magnetic field lines at foot-
points of flare loops is a key parameter for theoretical calculations of the aniso-
tropy of high-energy neutron and -ray emissions. This parameter was not taken
into account in previous calculations. (2) The large magnetic tilt angles actually
observed imply much more isotropic emission than that expected in earlier stud-
ies. (3) High-resolution magnetograms and stereoscopic observations of -ray and
neutron emissions are desirable for a more accurate determination of parameters
of accelerated ions at the Sun.
Acknowledgements
We thank Cl. Barat for useful comments on PHEBUS data. The Academy of Finland
is thanked for the financial support for the University of Turku. L.K. and F.T. were
partially supported by NASA Grant NAG5-2761 (Compton GRO Cycle 4 Guest
Investigator Program). G.K. was supported by Scholarship of the Nordic Council
of Ministers for research in Finland. BBSO is supported by NASA Grant NAGW-
1972, NSF Grant ATM-9525971, and ONR Grant N00014-89-J-1069.
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sola7009.tex; 17/06/1997; 10:17; v.6; p.8


Impact of magnetic environment on the generation of high-energy neutrons at the sun

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Impact of magnetic environment on the generation of high-energy neutrons at the sun

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