The charge states of energetic iron have been measured directly for the first time in a solar particle event. In the energy interval 0.01 to 0.25 MeV per nucleon, iron is not fully stripped but has a mean ionization state of 11.6. This value is remarkably similar to the mean ionization state of iron in the quiet solar wind and suggests that the charge states were "frozen-in" at a coronal temperature of approximately 1,500,000 K

Fan, C. Y.

Gloeckler, G.

Hovestadt, D.

Sciambi, R. K.

English

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A DIRECT MEASUREMENT OF THE CHARGE STATES OF ENERGETIC IRON
EMITTED BY THE SUN
G. Gloeckler, R. K. Sciambi, C. Y. Fan and
D. Hovestadt
Technical Report # 76-113
Physics Publication # 76-259
May 1976
('N ASA-Lk-146144) A DIPFCT MFASUP,EMENT OF
	 N76- 26133
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UNIVERSITY OF MARYLAND
DEPARTMENT OF PHYSICS AND ASTRONOMY
COLLEGE PARK, MARYLAND
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A PIRECT MEASUREMENT OF THE CHARGE STATES OF ENERGETIC IRON
EMITTED BY THE SUN
G. GLOECKLER AND R. K. SCIAMBI
Department of Physics and Astronomy, University of Maryland
C. Y. PAN
Department of Physics, University of Arizona
AND
D. HOVESTADT
Max-Planck-Institut, Garching, West Germany
Received
ABSTRACT
The charge states of energetic iron have been measured
directly for the first time in a solar particle event of 1974,
May 14-15. We find that in the energy inCerval 0.01 to 0.25
MeV per nucleon, iron . is not fully stripped but has a mean
ionization state of 11.6. This value is remarkably similar to
the mean ionization state of iron in the quiet solar wind and
suggests that the charge states were "frozen-in" at a coronal
temperature of ti 1.5 x 10 6 °K.
Subject headings: energetic solar particles - composition and
energy spectra
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2
I. INTRODUCTION
Among the more characteristic features of the composition of energetic
solar particles below about 10 MeV per nucleon is the systematic enrichment
or overabundance of heavy elements (Price et al. 1971; Fleischer and Hart
1973; Mogro-Compero and Simpson 1972; Hovestadt at al. 1973) which in many
cases increases with decreasing particle velocity (Price at al. 1971;
Lanzerotti, Maclennan, and Graedel 1972; Hovestadt et al. 1973; Crawford
et al. 1975; O'Gallagher et al. 1976). The relative abundances, on the
other hand, have been found to vary not only from one solar event to the
next, but also during a given event (Armstrong and Krimigis 1975; Armstrong
et al. 1976; Van Allen, Venkatarangan, and Venkatesan 1974). In many models
it is assumed that low energy solar particles are not fully ionized, and the
observations are explained by some combination of charge to mass dependent
acceleration, escape and propagation (Cartwright and Mogro-Compero 1972;
Ramudarai 1973; Price et al. 1971; see also Hovestadt (1974) and Gloeckler
(1975) for recent reviews of observations and models). Direct measurements
of the charge states of heavy ions are therefore clearly important. In
this Letter we present the first direct measurement of the ionization states
of energetic iron in a solar flare particle event. Previously, we reported
that carbon and oxygen below ti 1 MeV per nucleon are nearly fully stripped
(Gloeckler, Fan, and Hovestadt 1973; Gloeckler et al. 1975a) and that the
mean charge states of C and 0 observed in nine solar flare particle events
are 5.7 and 6.2 respectively (Sciambi 1975; Sciambi et al. 1976).
II. INSTRUMENTATION
The measurements reported here were made using the ULET sensor and the
Electrostatic Energy vs. Charge Analyzer (EECA) of the University of
Maryland/Max-Planck-Institut experiment on the earth orbiting satellite
[hami
3
IMP 8. ULET is a dE/dx vs. E counter telescope which measures the atomic
number and energy of an incoming particle (Hovestadt and Vollmer 1971).
Because the AE element ib a very thin (N 150 jig/cm 2) flow-through proportional
counter the low energy thresholds for identifying carbon, oxygen and iron
using 2 parameter measurements are about 100 keV per nucleon. The EECA
sensor (Turns et al. 1974), on the other hand, measures directly the
ionization state and kinetic energy of an incoming ion. After deflection in
a known electrostatic field, the energy and amount of deflection determine
the charge	 state of the incoming particle. The energy range of EECA of
ti 35 to 1200 keV per charge is divided into seven discrete energy per charge
intervals fixed by the locations and widths of 7 rectangular salLd state
detectors. The energy signal from each detector is pulse-height-analyzed.
III. OBSERVATIONS
During the ten day period, 1974 May 7-17, heavy elements in the com-
position of a solar particle event were unusually overabundant, with the
event-averaged iron as abundant as oxygen at 1 MeV per nucleon (Cloeckler
et al. 1975b;Hovestadt et al. 1975). In Figure I we show the coincidence
counting rates for Z > 1 nuclei (predominantly low energy protons and
alphas) and for Z > 2 nuclei respectively as well as the ratio, R, of light
to heavy nuclei. The persistent enhancement of heavy nuclei is evident
from this ratio which drops from its usual value of $ 200 to about 20 on
May 7 and remains at this low value for 10 days. We measured the charge
states of iron during the intensity increase of May 14-15 which was presumably
the result of a west limb 2N flare at 2120 on May 13.
T4
J
fi	 IV. RESULTS
i	 In Figure 2 we show the charge histograms for ions between 130 to 210 	 J
t
	 keV per charge derived from pulse-height analysis of detector P3 energy
signals which were accumulated for a one day period of 3974 May 14 7 h to May 15 5h.
Since the energy per charge window for each of the seven rectangular
detectors is fixed by its location and width as well as by the geometry of
the collimator-deflection system, pulse-height analysi.b of the energy signal
may be directly related to the charge state Q of the incoming ion. In these
histograms the dominant peak (at ro 170 keV) corresponds to Q = 1 particles,
the middle peak (at ti 340 keV) to Q = 2 ions and the smallest peak (at
ti 1 MeV) to ions with charge states in the 5 to 14 range. In particular,
note the absence of Q = 3 and 4 and > 14 charge states.
Except for protons and helium, which are associated with 0 = 1 and
Q = 2 charge states respectively, the EECA sensor does not provide infor-
mation on the atomic number of the particle. It is necessary, therefore,
to know the relative abundance of the dominant heavy elements C, 0 and Fe
before one can establish the extent of ionizatio« of iron.
From the pulse-height analysis data of the ULET we have determined the
f	 relative abundances for He, C, 0 and Fe for the May 14-15 solar particle
event to be approximately 17:0.5:1:1 as shown in Table 1. Using these
observed abundances we can then calculate the expected pulse-height distri-
bution for a given detector under different assumptions regarding the iron
charge states and compare this distribution to the observed charge histogram.
The dashed curve in Figure 2(a) gives the calculated response assuming that
the charge states of iron lie between 9 and 14. The dashed curve in
r	 Figure 2(b) shows the response assuming iron is fully stripped. We cani
i	 i
5
conclude from the excellent fit to the observed histogram in Figure 2(a) that
iron in the iron rich event of 1974, May 14-15 is not fully stripped.
In Figure 3 we show the event-averaged energy spectra for helium,
carbon, oxygen and iron derived from the ULET pulse height data at the
higher energies as well as the helium and iron spectra calculated from the
charge histogram of the various detectors of the EECA sensor at the lower
energies. In the EECA data we have assumed all charge state 2 particles to
be helium and have taken as the iron spectrum the calculated Z > 6 intensities
(corresponding to Q > 5 peaks in the charge histograms) reduced by a factor
of 2.5. We note the following features of these energy spectra.
(1) There is good agreement in the absolute intensities for helium and
iron derived from the ULET (solid symbols) and EECA (open symbols)
data respectively in the common energy intervals. (0.25 to C.6 MeV per nucleon).
(2) There is no significant bendover in the spectra 	 down to the lowest
observable energies (10 keV per nucleon for iron and 40 keV per nucleon
for helium).
(3) There is no pronounced energy dependence in the heavy ion enhancement.
(4) The O/C ratio is anomalously high (ti 7) in the energy interval of ti 1
to 4 MeV per nucleon (see Table 1), reminiscent of the ratio in the
anomalous component of the quiet-time cosmic rays (McDonald et al. 1974;
Klecker et al. 1975, 1976; Gloeckler 1975).
V. IONIZATION STATES OF IRON
To determine the distribution of the ionization states of iron we assume
that the peak corresponding to charge states 5 to 14 in each of the charge
histograms is produced primarily by the elements C, 0 and Fe. Furthermore,
we take the distribution of ionization states of carbon and oxygen in the
6	 A,
G
May 14-15 event to be the same as in the nine solar particle events studied
by Sciambi et el.(1976) who found the mean charge states to be remarkably
invariant from event to event with an average distribution as given in column
2 of Table 2. We can then subtract from the charge histograms the contribution
of C and 0, whose relative abundances at slightly higher energies are given
in Table 1. The ionization states of iron are obtained by varying systematically
the relative abundances of each charge state iii the range 9 to 14 until the
best X2
 fit to the charge histogram is obtained. The results are shown in
column 3 of Table 2. The most abundant charge states of iron are 11 and 12
and mean ionization state is 11.6.
The charge state distributions of iron measured in the 1974 May 14-15
event as well as those of oxygen in nine other solar particle events are quite
similar to those found in the quiet solar wind (see column 4 of Table 2). On
the other hand, the Pe/0 ratio in the solar wind is ro 0.17 which is considerably
different from our value of m 1 found in this "iron rich" event.
V. DISCUSSION
Our results that energetic iron below 0.28 MeV per nucleon in the
May 14-15 "iron rich" particle event is only partially ionized is consistent
with the conclusions of O'Gallagher et al. (1976) that the mean charge states
of iron in the energy range 0.5 to 5 MeV per nucleon in an ordinary solar
particle event (1974 September 19) is 10 + 5. On the other hand Sullivan
and Price (1973) argue that v 2 MeV per nucleon iron in the solar flare event
of 1974 January 25 is fully stripped. It is not clear at this point whether
this dis^repoacy is due to the difference in the energies of these two
measurements or indicates a variability in the degree of ionization of iron
between flares.
r
i	 -
2	 11,1--1 	 _ .
T,
i
7
fl
When we compare the charge states of energetic ions (columns 2 and 3 of
Table 2) with those found in the quiet solar wind (column 4) and calculated
charge states in the solar corona of temperature 1.5 x 10 6 °K (column 5) the
overall agreement and consistency stands out. The simplest conclusion based
on these comparisons is that the source of heavy ions observed in this solar
particle event is in the lower solar corona where the temperature is between 1-2
x 106 °K , and that the charge states are not changed in subsequent storage
and/or propagation to earth.
We are grateful to E. Tums, J. Cain, P. Laeverenz, E. Kuenneth and
0. Vollmer for designing the University of Maryland/Max-Planck-Institut
experiment and for preparing it for launch and acknowledge the programming
efforts of John Dalton and the calibration of the ULET sensor by B. Klecker.
This work was supported by NASA under contract NAS 5-11063, grant NGR 21-002-
224 and by the German Govetnment.
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8
REFERENCES
Armstrong, T.P., and Krimigis, S.M. 1975, Proc. 14th Inter national Cosmic
Ray Conference, MUnchen, W. Germany, August 15-29, Vol,	 p. 1592.
Armstrong, T.P., Krimigis, S.M., Hovestadt, D., Klecker, B., and Gloeckler, G.
1976, Solar Physics (submitted).
game, S.J. 1972, Solar Wind, ad. C. P. Sonett, P. J. Coleman, Jr., and
J. M. Wilcox (NASA publication SP-30'j, p. 535.
Cartwright, B.G., and Mogro-Compero, A. 1972, 	 177, L43.
Crawford, H.J., Price, P.B., Cartwright, B.G., and Sullivan, J.A. 1975,
Ap.J., 119`5, 213,
Fleischer, R.L., and Hart, H.R. 1973, Phys. Rev. Letters, 30, 31.
Gloeckler, G. 1975, Proc. 14th International Cosmic Ray Conference , MUnchen,
W. Germany, August 15^?9, Vol. 111, p. 3784.
Gloeckler, G., Pan, C,Y „ and Hovestadt, D. 1973, Proc. 13th- -In ternational
Cosmic Ray Conference, University of Denver, August 17.-a0, Vol. 2, p. 1492.
Gloeckler., G., Sciambi, R.K., Fan, C.Y., and Hovostadt, D. 1975a, Proc. 14th
International Cosmic Ray Conference, MUnchen, 11. Germany, August 1 5-29)
Vol. 5, p. 1576.
i
Gloeckler, G., Hovestadt, D., Vollmer, 0., Fan, C.Y. 1975h, Ap.J._ (Letters),
200, L45.
Hovestadt, D. 1974, Solar Wind Three, ad. C.T. Russel, Inst. of Geophys.
and Planetary Phys., University of California,_ Los Angeles.
Hovestadt, D., and Vollmer, 0. 1971, Proc. 12th International Conference on
Cosmic Rays, Hobart, Tasmania, August 16-25, Vol. 4, p. 1608.
1
,
t
F9
Ilovestadt, D., Vollmer, 0., Gloeckler, G., and Fan, C.Y. 1973, Proc. 13th
	
International Cosmic Ray Conference, University of Denver, A9A
	
17-30,
Vol. ^2, p. 1498.
Ilovestadt, D., Klecker, B., Vollmer, 0., Gloeckler, G., and Fan, C.Y. 1975,
Proc. 14th International Cosmic Ray Conference, Munchen, W. Cermany,
August 15-29, Vol. 2, p. 786.
Jordan, C. 1969, Mon. Not. Roy. Astron. Soc., 142, 501.
Klecker, B., Hovestadt, D., Gloeckler, G., and Fan, C.Y. 1975, Proc. 14th
International Cosmic Ray Conference, Munchen, W. Germany, August 15-291
Vol. 2, p. 786.
Klecker, B., Hovestadt, D., Gloeckler, G., and Pan, C.Y. 1976, Ap. J. (submitted).
Lanzerottl, L.J., Maclennan, C.G., and Graedel, T.E. 1972, Ap. J. (Letters),
173, 1.39.
McDonald, F.B., Teegarden, B.J., Trainor, J.N., and Webber, W.R. 1974,
Ap.J. (Letters), 1887,, L105.
Mogro-Compero, A., and Simpson, J.A. 1972, Ap.J. (Letters), lam, L37.
O'Gallagher, J.J., Ilovestadt, D. Klecker, B., Gloeckler, G., and Fan, C.Y.
1976, Ap.J. (Letters), (submitted).
+	 Price, P.B., ilutcheon, I., Cowsik, R., and Barber, D.J. 1971, Phys. Rnv.
Letters, 216, 916.
i
A
Ramadurai, S. 1973, Astrophys. Letters, 114, 85.
°ri
Sciambi, R.K. 1975, Univ. of Maryland Technical Report 76-092.
Sciambi, R.K., Gloeckler, G., Fan, C.Y., and Ilovestadt, D. 1976, Ap.J. (submitted).'
Sullivan, J.D., and Price, P.B. 1973, Proc. 13th International Cosmic Ray
`	 Conference, University of Denver, August 17-30, Vol. »2.1 p. 1470.
_._. _
10	 1
Tums, E., Gloeckler, G., Fan, C.Y., Cain, J., and Sciambi, R.K. 1974, IEEE
Trans. Nucl. Sci., NS-21 0 No. 1, 210.
Van Allen, J.A., Venkatarangan, P., and Venkatesan, D. 1974, J. Geophy. Res.,
79, 1.M
C. Y. FAN: Department of Physics, University of Arizona, Tucson, AZ 85721
G. GLOECKLER AND R. K. SC IAMBI: Department of Physics and astronomy,
University of Maryland, College Park, MD 20742
D. ROVESTADT: Mai; Planck-Institut fur Physik and Astrophysik, 8046 Garching,
Munchen, W. Germany
k
k
TABLE 1
i'
Event-Averaged Abundances of He, C and Fe Relative to Oxygen
for the 1974 flay 14-15 Solar Particle Event
Chemical
	
Energy Interval (Mev per nucleon)
Element 0.15-0.3 0.3-0.6 0.6-1.0 1.0-4.0
He 1730 + 225 1620 + 350 800 + 350
C 43 ± 11 55 ± 13 32 ± 14 15 + 9
0 = 100 a 100 100 100
Fe
F
90 + 16 127 + 32 108 + 33 75 + 31
f
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TABLE 2
Ionization State Abundances of Carbon, Oxygen and Irnn
Charge
Average of Nine Solar
Particle Events( a)
1974 May
Solar Particle
14-15
Event
Solar Solar
Stay Wind (b ) Corona(')15 to 600 keV/nuc 8 to 250 keV/nuc
C
+5/C 0.25 + 0.04 0.19
C+6 /C 0.75 + 0.05 0.81
0+6/0 0.72 ± 0.05 0.77 0.76
0+7 /0 0.23 + 0.05 < 0.15 0.23
0+8 /0 0.05 + 0.04 0.01
Pe
+10/Fe 0.14 + 0.04 0.24 0.14
Pe
+11/Fe 0.31 + 0.06 0.21 0.26
Pe
+12/Fe 0.35 + 0.07 0.14 0.32
Pe
+13
"Fe 0.20 ± 0.05 0.16
(a)Sciambi at al.	 (1976)
(b)Bame at al.	 (1972)
(o) Ca":eulated at To Q 1.58 x 106 °K using model of Jordan (1969).
J	
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FIGURE CAPTIONS
Fig. 1--(a) Counting rates of Z > 1 particles (primarily 0.43 - 1.5
MeV protons and 0.25 to 7.0 Mel per nucleon alpha particles) and Z > 2
particles for the 1974 May 5 to 20 time period. (b) The time dependence
of the ratio of Z > 1 to Z > 2 particles. Note that the ratio of light
to heavy ions is small and relatively constant from May 7 to 16 despite
large intensity variations in the individual counting rates. SEV is the
angle between the earth-sun and the earth-satellite vectors.
Results presented in this Letter have been obtained from data recorded
during the one day period of 1974 May 14 7 h to 15 5 h .
Fig. 2=-Charge histograms of the background-corrected pulse height
data for the P3 detector of EECA. The dashed curves respresent the
calculated detector response under two different assumptions: (a) The
charge states of iron are between 9 and 13 and distributed as given in
column 3 of Table 2, (b) the charge states of iron are all 26. On the
basis of comparison between the calculated and measured charge histogram
we conclude that iron in this solar particle event is not ionized beyond
charge 14.
Pig. 3--Differential energy spectra for He, C, 0 and Fe. Obtained
from the EECA sensor (open symbols) and the ULET sensor (solid symbols) of
the University of Maryland/Max-Planck-Institut IMP 8 experiment.
H
t 	
L	
^	 ^	 I
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I	 Mill F- F--FTFM -T—r----Trnrrr 0
LLJ	 0
LLJ 0
	
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	 0--
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(n	 C\j >--
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C:	 0
C3	 C\J--
CL OD	 0
X CL
0')
0 0 0	 Al AC	 (n	 N N0 4-	 -0 CO— N)AlN tA QO--
CO
LLJ4- n0 x 1	 (-0
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" III I f I	 lilt.,	
10 In-U . 	 1	 11111.	 1 1 .	 I	 tilill I 	1	 1111111 1	 1	 1111111 1
N	 0N	 r1r)	 NI	 lo	 lo	 >0	 0	 0	 10	 0	 LLI
cn 0
GNODIIS/SiNnoo	 (.Z< Z)I z z
FIGURE 1
In
r i	 i _i
histogram
histogram
311 iron is
N
1
1
1
(b)
74/05/14:0653
University of Maryland Experiment
	 IMP-8 P3	 to
2	 74/05/15: 0458
	 I
observed histogram
--- calculated histogram
assuming all iron is
Fe 9 to Fe 13
RCarbon
67 8 Oxygen
T T ,
e 10T-1 Iron
i^
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1
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i
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1
l
(a)
10
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N
L
N
N
E
U
t 10^NC
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10 2L0.1	 1.0	 1.0	 10
Total Detected Kinetic Energy , MeV
^^^ 15
FIGURE 2	
rv"Y^^
os
t	 r	
_
10 
102
U
\ 10^
a^
CDin
N 10^
N
U
NN
U
L 10O
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0
c
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0 10 
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104
1,
	 t	 T,	 1	
A
\\ University of Maryland
\	 d\ Max Planck Institut Experiment
\\	 IMP 8
\	 \^\ May 14-15, 1974
\
\
Iron*
	
\	 -
\ 	 ,,.,Helium
rH
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F-	
1
\
\
EECA ULET	 \
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Kinetic Energy (MeV/nuc)
A A+
FIGURE 3
	 r` fit.
A
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A direct measurement of the charge states of energetic iron emitted by the sun

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