Using observations from the Cosmic Ray Isotope Spectrometer (CRIS) onboard the Advanced Composition Explorer (ACE), we present new measurements of the galactic cosmic ray (GCR) elemental composition and energy spectra for the species B through Ni in the energy range approx. 50-550 MeV/nucleon during the record setting 2009-2010 solar minimum period. These data are compared with our observations from the 1997-1998 solar minimum period, when solar modulation in the heliosphere was somewhat higher. For these species, we find that the intensities during the 2009-2010 solar minimum were approx. 20% higher than those in the previous solar minimum, and in fact were the highest GCR intensities recorded during the space age. Relative abundances for these species during the two solar minimum periods differed by small but statistically significant amounts, which are attributed to the combination of spectral shape differences between primary and secondary GCRs in the interstellar medium and differences between the levels of solar modulation in the two solar minima. We also present the secondary-to-primary ratios B/C and (Sc+Ti+V)/Fe for both solar minimum periods, and demonstrate that these ratios are reasonably well fit by a simple "leaky-box" galactic transport model that is combined with a spherically symmetric solar modulation model

Binns, W. R.

Christian, E. R.

Cummings, A. C.

Davis, A. J.

deNolfo, G. A.

Israel, M. H.

Lave, K. A.

Leske, R. A.

Mewaldt, R. A.

Stone, E. C.

vonRosenvinge, T. T.

Wiedenbeck, M. E.

NASA Technical Reports Server

33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013
THE ASTROPARTICLE PHYSICS CONFERENCE
Elemental GCR Observations During the 2009–2010 Solar Minimum Period
K. A. LAVE1, M. H. ISRAEL1, W. R. BINNS1, E. R. CHRISTIAN2, A. C. CUMMINGS3, A. J. DAVIS3,
G. A. DE NOLFO2, R. A. LESKE3, R. A. MEWALDT3, E. C. STONE3, T. T. VON ROSENVINGE2,
M. E. WIEDENBECK4
1 Washington University, St. Louis, MO 63130, USA
2 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
3 California Institute of Technology, Pasadena, CA 91125, USA
4 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
klave@physics.wustl.edu
Abstract: Using observations from the Cosmic Ray Isotope Spectrometer (CRIS) onboard the Advanced Com-
position Explorer (ACE), we present new measurements of the galactic cosmic ray (GCR) elemental composition
and energy spectra for the species B through Ni in the energy range ∼50–550 MeV/nucleon during the record
setting 2009–2010 solar minimum period. These data are compared with our observations from the 1997–1998
solar minimum period, when solar modulation in the heliosphere was somewhat higher. For these species, we
find that the intensities during the 2009–2010 solar minimum were ∼20% higher than those in the previous so-
lar minimum, and in fact were the highest GCR intensities recorded during the space age. Relative abundances
for these species during the two solar minimum periods differed by small but statistically significant amounts,
which are attributed to the combination of spectral shape differences between primary and secondary GCRs in
the interstellar medium and differences between the levels of solar modulation in the two solar minima. We also
present the secondary-to-primary ratios B/C and (Sc+Ti+V)/Fe for both solar minimum periods, and demonstrate
that these ratios are reasonably well fit by a simple “leaky-box” galactic transport model that is combined with a
spherically symmetric solar modulation model.
Keywords: cosmic rays, Galaxy: abundances, Sun: activity
1 Introduction
Solar modulation in the heliosphere distorts the interstel-
lar composition and energy spectra of inwardly diffusing
galactic cosmic ray (GCR) nuclei. Those nuclei with inter-
stellar energies less than a few GeV/nucleon suffer signifi-
cant energy loss due to adiabatic deceleration. During solar
minimum periods of the∼11 year solar cycle, these losses
are minimized.
In the solar system, observations during solar minima
give us the best opportunity to study GCR nuclei close
to interstellar medium (ISM) conditions. This informa-
tion can provide insight to interstellar transport processes,
which then yield better estimates for the GCR source com-
position. The most recent solar minimum period, which
occurred between 2007–2010, exhibited very low levels
of solar activity. Observations made during this time are
therefore the closest we have come near Earth to observing
GCRs in ISM conditions.
The Cosmic Ray Isotope Spectrometer (CRIS, [1]) on
board NASA’s Advanced Composition Explorer (ACE) has
been continuously observing GCR nuclei arriving near
Earth since its launch on 25 August 1997. Using high statis-
tics observations from CRIS, we present the elemental en-
ergy spectra and composition of boron through nickel (5≤
Z ≤ 28) in the energy range ∼50–550 MeV/nucleon. Data
were obtained from a 297-day period from 2009 March 23
to 2010 January 13, which is characterized as having the
highest GCR intensities of the space era [2]. These results
are compared with observations from the 1997–1998 solar
minimum period [3], covering data from 1997 December 5
to 1998 April 19, which exhibited somewhat higher levels
of solar modulation. Additionally, we present the B/C and
(Sc+Ti+V)/Fe ratios for both solar minimum periods, and
demonstrate that a “leaky-box” interstellar transport model
combined with a spherically symmetric solar modulation
model is able to give reasonably good fits to the observed
ratios.
2 CRIS Instrument
The CRIS instrument consists of four silicon solid-state-
detector (SSD) stacks; in each stack, fifteen circular SSDs
are grouped into nine individual detectors (E1–E9; see Fig-
ure 1). A scintillating optical fiber trajectory (SOFT) ho-
doscope positioned above the stacks is used to determine
the trajectories of incident particles. The hodoscope is com-
posed of three x–y tracking layers, with an additional set
of layers serving as a trigger. For a particle stopping in the
silicon detectors, the charge, mass, and incident energy at
the top of the instrument are determined using the total en-
ergy deposited (E’ in MeV) in the detector in which the
particle stopped and multiple measurements of the rate of
energy loss (dE/dx in MeV g−1 cm2) in the other detectors
through which it passed. For a more detailed description of
the CRIS instrument, see [1] and [4].
Events used in this analysis were those produced by nu-
clei that have incident angles ≤30◦ from the normal that
stopped in the active areas of detectors E2–E8. The bottom
detector (E9) in each stack is used to eliminate those parti-
cles that do not stop in the preceeding detectors or that in-
teract and produce one or more long-range fragments. Ad-
ditionally, particles must not have a calculated stop depth
https://ntrs.nasa.gov/search.jsp?R=20140010418 2019-08-31T19:55:37+00:00Z
Lave et al. 2009–2010 CRIS Measurements
33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013
4RIGGER ,AYERS
3CINTILLATING /PTICAL &IBER 4RAJECTORY (ODOSCOPE
3ILICON 3OLID 3TATE $ETECTORS
#OSMIC 2AY )SOTOPE 3PECTROMETER #2)3	
Figure 1: CRIS instrument cross section (two of the four
detector stacks shown). The arrow represents an incident
particle stopping in the bottom of detector E7.
within 160 μm of the top or bottom face of the detector to
avoid complications in interpreting energy losses for parti-
cles that stop near surface “dead layers” in the detector.
3 Elemental Spectra
Elemental intensities for boron through nickel (5≤ Z≤ 28)
are determined for seven energy bins corresponding to
GCRs stopping in detectors E2–E8 of the CRIS instru-
ment. Figure 2 shows the energy spectra for the 2009–2010
solar minimum period, where we have applied energy-
independent scale factors to the data to allow for an easy
comparison between the spectral shapes. The spectra have
been corrected for livetime, geometrical acceptance of the
instrument, energy intervals, fragmentation in the instru-
ment, and hodoscope efficiencies. Plotted uncertainties are
the quadratic sum of the statistical and systematic contribu-
tions. The vertical dotted line at 160 MeV/nucleon shows
that there is a common energy at which CRIS is sensitive
to all the species considered here.
Figure 3 compares selected CRIS primary element spec-
tra (carbon, oxygen, silicon, and iron) from the 1997–1998
[3] and 2009–2010 solar minimum periods. For all four
species, the data are plotted at the mid-point energies of
each energy interval. We see that at each energy, the 2009–
2010 intensities are ∼20% higher than those from the
1997–1998 solar minimum. In Figure 4, we show that most
of the elements between boron (Z = 5) and nickel (Z = 28)
have an∼20% increase in intensity for 2009–2010 as com-
pared to 1997–1998.
4 Composition
Relative elemental abundances are calculated by first fit-
ting parabolas in log(Intensity) versus log(Energy/nucleon)
to the energy spectra, as shown in Figure 2. We then take ra-
tios of the curves at the common energy, 160 MeV/nucleon,
to determine the relative abundances.
Table 1 gives the 1997–1998 and 2009–2010 CRIS so-
lar minimum relative abundances. The values have been
Figure 2: CRIS elemental spectra for the 2009–2010 solar
minimum. Energy-independent scale factors have been ap-
plied to the data to allow for easy comparison of the spec-
tral shapes. The solid lines are quadratic fits to the data,
which are used to determine the relative abundances at 160
MeV/nucleon (indicated by the vertical dotted line).
Figure 3: Comparison of the CRIS primary element spec-
tra for the 1997–1998 and 2009–2010 solar minima. Only
statistical uncertainties are shown.
Lave et al. 2009–2010 CRIS Measurements
33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013
Figure 4: Ratio of the 2009–2010 solar minimum intensi-
ties at 160 MeV/nucleon relative to the 1997–1998 inten-
sities. Species that are mostly primary are given as filled
red squares, while species that are mostly secondary are
given as filled blue triangles; species that have signifi-
cant primary and secondary components are given as open
black circles. Only statistical uncertainties are shown. The
weighted averages of the ratios for the primary (solid red
horizontal line) and secondary (solid blue horizontal line)
species are shown; the dotted lines give the 1σ uncertain-
ties on each average.
normalized to Si≡1000, and only statistical uncertainties
are given. The differences between the relative abundances
for the two solar minima are small, but nevertheless statis-
tically significant, as illustrated in Figure 4.
Solar modulation in 2009–2010 was lower than in 1997–
1998, so the GCRs CRIS observed in 2009–2010 had lower
interstellar energies. At CRIS energies, lower-energy par-
ticles traverse less material in the Galaxy, on average, re-
sulting in the production of fewer secondary species (those
produced by fragmentation in the ISM). This is consistent
with the results in Figure 4. Species that are considered to
be mostly primary material from the source are shown as
red squares, while species that are mostly secondary mate-
rial are given as blue triangles. The horizontal solid lines
show the average ratios of the intensities for the primary
(red) and secondary (blue) species, weighted by the statis-
tical uncertainties of the ratios. The horizontal dotted lines
are the 1σ uncertainties on the weighted averages. We note
that the species which have significant primary and sec-
ondary components (open black circles) are not used when
computing the averages. Although the 2009–2010 inten-
sities are higher than those in 1997–1998, the secondary
species do not show as large of an increase as the primary
species in 2009–2010, compared with 1997–1998.
5 Secondary-To-Primary Ratios
The secondary-to-primary ratios B/C and (Sc+Ti+V)/Fe
are commonly studied to test interstellar transport models.
These ratios are less sensitive to the source spectrum and
the solar modulation model than the energy spectra, and
CRIS Relative Elemental Abundances
At 160 MeV/nucleon
Element 1997–1998 2009–2010
B 1788.6± 30.1 1725.7± 19.4
C 7227.0± 73.3 7235.4± 45.0
N 1705.2± 20.9 1678.9± 12.3
O 7067.2± 70.9 7137.0± 42.7
F 99.4± 3.5 97.3± 2.1
Ne 1005.5± 14.1 998.9± 8.4
Na 190.3± 4.9 185.2± 2.9
Mg 1374.3± 17.3 1375.3± 10.3
Al 199.4± 4.7 203.2± 2.8
Si 1000.0± 13.0 1000.0± 7.8
P 26.7± 1.5 26.7± 0.9
S 155.7± 3.7 157.0± 2.2
Cl 26.0± 1.7 24.8± 0.8
Ar 58.4± 2.1 55.3± 1.2
K 39.9± 1.7 40.1± 1.0
Ca 126.5± 3.3 119.5± 1.9
Sc 26.4± 1.1 25.3± 0.9
Ti 102.3± 3.1 100.5± 1.8
V 46.0± 2.1 48.1± 1.3
Cr 100.2± 3.3 98.8± 1.9
Mn 63.3± 2.7 61.9± 1.6
Fe 673.7± 10.9 671.4± 6.5
Co 4.4± 0.3 3.7± 0.4
Ni 31.6± 2.2 29.9± 1.3
Table 1: Values are normalized to Si≡1000. Only the statis-
tical uncertainties are given. The absolute intensities (with
combined statistical and systematic uncertainties) for sil-
icon at 160 MeV/nuc are (108.2± 3.4)× 10−9 (cm2 s
sr MeV/nuc)−1 for 1997–1998 and (132.5± 4.1)× 10−9
(cm2 s sr MeV/nuc)−1 for 2009–2010.
they probe the amount of material that GCRs traverse in
the ISM before escaping from the Galaxy.
In Figure 5 we show the CRIS B/C and (Sc+Ti+V)/Fe
ratios for the 1997–1998 and 2009–2010 solar minima
(red circles and blue diamonds, respectively). Additional
measurements from other experiments are shown at en-
ergies above 400 MeV/nucleon (HEAO: [5], TRACER:
[6], CRN: [7], ATIC-2: [8], AMS-01: [9], CREAM: [10]).
Also shown are the ratios calculated from our transport
model [4, 3], which uses a steady-state “leaky-box” in-
terstellar model combined with the spherically-symmetric
Fisk model for solar modulation [11]. We have character-
ized the levels of solar modulation using the “modulation
potential” φ , with values of 325 MV and 250 MV for the
1997–1998 and 2009–2010 solar minima, respectively.
Characteristic maxima in both the data and the models
are seen near∼1 GeV/nucleon for both ratios. These ratios
correspond to the distribution of path lengths that GCRs
observed near Earth traversed in the Galaxy. At higher en-
ergies the ratios decrease with increasing energy because
higher-energy GCRs escape more easily from the Galaxy.
At lower energies the ratios increase with increasing en-
ergy; this indicates there is a depletion of path lengths at
low energies [12, 13, 14, 15].
Above∼1 GeV/nucleon, where the effects of solar mod-
Lave et al. 2009–2010 CRIS Measurements
33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013
Figure 5: CRIS B/C and (Sc+Ti+V)/Fe ratios for 1997–
1998 and 2009–2010 (red circles and blue diamonds, re-
spectively). For references to the other data, see Section 5.
The solid and dashed curves are the result of our transport
model using modulation values of 325 MV (1997–1998)
and 250 MV (2009–2010), respectively.
ulation become negligible, the B/C ratio is well fit by the
model. At CRIS energies, the models for both solar min-
imum periods have stronger energy dependences than the
data, which are observed to have relatively flat shapes. The
differences between the models and the data are, on aver-
age, 5% and 6% for the 1997–1998 and 2009–2010 solar
minima, respectively.
Both models have approximately the right shape for the
(Sc+Ti+V)/Fe ratios, though the observed ratios at CRIS
energies are slightly underestimated. For the 1997–1998
solar minimum the model is ∼5% lower than the data,
while it is ∼7% lower than the observed 2009–2010 ratio.
Above∼5 GeV/nucleon there are similar difficulties fitting
the data, which are somewhat overestimated by the model.
While small adjustments to the modulation values (±25
MV) can help us better fit the low-energy B/C ratios, the
(Sc+Ti+V)/Fe ratio is quite insensitive to small changes
in the modulation value [3]. Additional measurements of
nuclear fragmentation cross sections are needed to further
improve our model results.
6 Conclusions
We have presented the elemental energy spectra and rel-
ative abundances for the GCR species boron to nickel
during the recent 2009–2010 solar minimum period us-
ing the CRIS instrument. Compared with CRIS observa-
tions from the 1997–1998 solar minimum, we see that
the 2009–2010 GCR intensities increased by ∼20%. The
relative elemental abundances are similar in both time
periods, but do exhibit small differences associated with
the differing solar modulation levels in conjunction with
differences in spectral shape between primary and sec-
ondary species. The secondary-to-primary ratios B/C and
(Sc+Ti+V)/Fe are also shown for both time periods, and
we have demonstrated that a “leaky-box” interstellar trans-
port model combined with a spherically symmetric solar
modulation model gives a reasonable fit to both ratios.
Acknowledgements
This research was supported by NASA at Caltech,
Washington University, JPL, and GSFC under grants
NNX08AI11G and NNX10AE45G.
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Elemental GCR Observations during the 2009-2010 Solar Minimum Period

Elemental GCR Observations during the 2009-2010 Solar Minimum Period

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