Observations of cosmic-ray helium energy spectra provide important constraints on cosmic ray origin and propagation. However, helium intensities measured at Earth are affected by solar modulation, especially below several GeV/nucleon. Observations of helium intensities over a solar cycle are important for understanding how solar modulation affects galactic cosmic ray intensities and for separating the contributions of anomalous and galactic cosmic rays. The Cosmic Ray Isotope Spectrometer (CRIS) on ACE has been measuring cosmic ray isotopes, including helium, since 1997 with high statistical precision. We present helium elemental intensities between approx. 10 to approx. 100 MeV/nucleon from the Solar Isotope Spectrometer (SIS) and CRIS observations over a solar cycle and compare these results with the observations from other satellite and balloon-borne instruments, and with GCR transport and solar modulation models

Binns, W. R.

Cohen, C. M. S.

Cummings, A. C.

Davis, A. J.

DeNolfo, G. A.

George, J. S.

Hink. P. L.

Israel, M. H.

Lave, K.

Leske, R. A.

Mewaldt, R. A.

Moskalenko, I. V.

Ogliore, R.

Stone, E. C.

Von Rosenvinge, T. T.

Wiedenback, M. E.

Yanasak, N. E.

Observations of cosmic-ray helium energy spectra provide important constraints on cosmic ray origin and propagation. However, helium intensities measured at Earth are affected by solar modulation, especially below several GeV/nucleon. Observations of helium intensities over a solar cycle are important for understanding how solar modulation affects galactic cosmic ray intensities and for separating the contributions of anomalous and galactic cosmic rays. The Cosmic Ray Isotope Spectrometer (CRIS) on ACE has been measuring cosmic ray isotopes, including helium, since 1997 with high statistical precision. We present helium elemental intensities between ∼10 to ∼100 MeV/nucleon from the Solar Isotope Spectrometer (SIS) and CRIS observations over a solar cycle and compare these results with the observations from other satellite and balloon-borne instruments, and with GCR transport and solar modulation models

de Nolfo, G. A.

Hink, P. L.

Ogliore, R. C.

von Rosenvinge, T. T.

Wiedenbeck, M. E.

Caltech Authors - Main

30TH INTERNATIONAL COSMIC RAY CONFERENCE
Cosmic-ray helium intensities over the solar cycle from ACE
G. A. DE NOLFO1 , W. R. BINNS2 , C. M. S. COHEN3 ,A. C. CUMMINGS3 , A. J. DAVIS3 , J. S. GEORGE3 ,
P. L. HINK2 , M. H. ISRAEL2 , K. LAVE2 , R. A. LESKE3, R. A. MEWALDT3, I. V. MOSKALENKO4 , R.
C. OGLIORE3 , E. C. STONE3 , T. T. VON ROSENVINGE1 , M. E. WIEDENBECK5 , N. E. YANASAK3 .
1CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD, 20771, USA
2Dept. of Phys. & McDonnell Center for Space Sci., Washington University, St. Louis, MO 63130 USA
3Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 USA
4HEPL & KIPAC, Stanford University, Stanford, CA 94305 USA
5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA
georgia@milkyway.gsfc.nasa.gov
Abstract: Observations of cosmic-ray helium energy spectra provide important constraints on cosmic ray
origin and propagation. However, helium intensities measured at Earth are affected by solar modulation,
especially below several GeV/nucleon. Observations of helium intensities over a solar cycle are important
for understanding how solar modulation affects galactic cosmic ray intensities and for separating the con-
tributions of anomalous and galactic cosmic rays. The Cosmic Ray Isotope Spectrometer (CRIS) on ACE
has been measuring cosmic ray isotopes, including helium, since 1997 with high statistical precision. We
present helium elemental intensities between ∼10 to ∼100 MeV/nucleon from the Solar Isotope Spec-
trometer (SIS) and CRIS observations over a solar cycle and compare these results with the observations
from other satellite and balloon-borne instruments, and with GCR transport and solar modulation models.
Introduction
Helium nuclei in galactic cosmic rays (GCR) are
the second most abundant species in cosmic rays
next to protons and offer important clues to un-
derstanding GCR propagation, particularly since
the isotope 3He is generally believed to be of sec-
ondary origin. In addition, the abundance of he-
lium at 1 AU is strongly influenced by solar mod-
ulation at low energies, and by the addition of the
anomalous helium component during solar mini-
mum conditions (see review [1]).
Comparisons of the time histories of helium inten-
sities observed by spacecraft such as IMP 8 and
Ulysses and the two Voyager spacecraft at different
locations across the heliosphere show interesting
modulation features that help to refine calculations
of solar modulation [2]. Observations of helium
intensities at 1 AU are rather limited, especially
since IMP 8 has been operating with reduced cov-
erage of∼30-50% since 2001 and recently stopped
transmitting data altogether. The Cosmic-Ray Iso-
tope Spectrometer (CRIS) onboard ACE has been
measuring abundances of nuclei with charges be-
tween 2 ≤ Z ≤ 30 and energies 30-500 MeV/nuc
at 1 AU continuously since 1997 with unprece-
dented statistical accuracy. ACE/CRIS observa-
tions are able to provide important coverage of he-
lium intensities at 1 AU concurrent with the lim-
ited observations from IMP 8, with new observa-
tions from HET/LET on STEREO ([3], [4]), and
finally with observations in the distant parts of the
heliosphere from Ulysses and Voyager. We present
the time history of helium GCR intensities derived
from CRIS/ACE starting with the minimum of so-
lar cycle 22 and ending well into solar cycle 23.
Data Analysis
The mass and charge of particles stopping within
CRIS are identified using the dE/dx versus resid-
ual energy method [5]. The methods and selec-
tion criteria used to assign charge and mass to in-
cident light nuclei have been described previously
Proceedingsof the30th International Cosmic Ray Conference
Rogelio Caballero, Juan CarlosD’Olivo, GustavoMedina-Tanco,
LukasNellen, Federico A. Sánchez, JoséF. Valdés-Galicia (eds.)
Universidad Nacional AutónomadeMéxico,
Mexico City, Mexico, 2008
Vol. 1 (SH), pages813–816
ID 663
813
COSMIC RAY HELIUM
Figure 1: Mass histograms for helium in 4 energy
ranges R2: 34-47 MeV/nuc, R4: 64-83 MeV/nuc,
R6: 91-109 MeV/nuc, and R8: 113-131 MeV/nuc
observed during solar minimum.
[6]. Valid events are required to trigger all three
Scintillating Optical Fiber Telescope (SOFT) ho-
doscope planes. The hodoscope is used to ob-
tain a particle trajectory before entering one of 4
stacks of solid-state detectors below SOFT. Shown
in Figure 1 are the resulting mass histograms de-
rived during solar minimum from CRIS. The iso-
topes of He are easily resolved and the number of
events for each isotope is determined by summing
the number of events under each peak. The present
analysis is limited to events whose reconstructed
trajectories have an opening angle of less than 30
degrees. Corrections to the derived intensities are
applied to account for the probability of a particle
surviving fragmentation within the instrument.
The derived charge dependent tracking efficiency
with the SOFT [5] hodoscope is shown in Figure
2. A detailed study of the SOFT hodoscope has re-
sulted in an improved understanding of the system-
atic uncertainties associated with the efficiency for
tracking low Z nuclei. The tracking efficiency for
each of the four CRIS telescopes was determined
separately and the resulting fluxes for the light nu-
clei (He-C) for each telescope were compared for
consistency. Based on these studies, overall sys-
tematic uncertainties were derived for charged par-
ticles from Li to Zn which ranged from 15% to
better than 2%. The SOFT tracking efficiency for
helium is more problematic since the tracking ef-
ficiency drops to ∼10-30% for He in the highest
energy ranges covered by CRIS (Figure 2).
Figure 2: SOFT tracking efficiency derived for
each of the 4 telescopes of CRIS. The symbols are
larger than the statistical uncertainties.
To understand the uncertainties associated with the
SOFT tracking efficiency for He, we compare de-
rived He intensities from CRIS with previous data.
Figures 3, 4, and 5 show the He intensities de-
rived from CRIS in three separate time ranges, a
solar minimum period between Jan. 1 1998 and
Jan. 1 1999, a solar maximum period between Jan.
1 2001 and Jan. 2004, and finally a recent solar
minimum period from Jan. 1 2007 to Apr. 15
2007. Periods of intense solar activity have been
removed using observations from the Solar Isotope
Spectrometer (SIS) also on ACE. The open black
triangles in these figures show the CRIS helium
spectrum derived from the original derivation of
the SOFT He tracking efficiency. Comparing the
helium intensities with previous data, particularly
the derived concurrent intensities from IMP 8 and
the high energy data of BESS, suggests that the de-
rived He efficiency for CRIS is over estimated, es-
pecially at high energies (or low values of dE/dx in
Figure 2).
In order to determine an appropriate correction
factor for the SOFT efficiencies, we adjust the
SOFT efficiencies such that the CRIS helium data
matches a modified power-law fit to previous and
concurrent helium data during solar minimum con-
ditions (period 1). The results of this fit are shown
in Figure 3 as the red dot-dashed curve, which
essentially overlies predictions from a transport
model discussed below (solid black curves). The
correction to the SOFT tracking efficiency ranges
from a factor of 1 at the lowest energies to ∼ 12
814
30TH INTERNATIONAL COSMIC RAY CONFERENCE
Figure 3: CRIS helium intensities during solar
minimum conditions (Jan. 1 1998 to Jan. 1
1999). Observations include [7] (open circles),
ACE/SIS (open squares), [8] (upsidedown trian-
gles), [9] (solid diamonds), [10] (asterisk), [11]
(thick asterisk and open diamonds), [12] (solid cir-
cles). Also shown are the GALPROP results with
φ = 500MV.
at the highest energies. This correction introduces
an additional systematic uncertainty, which we are
still in the process of evaluating, and which de-
pends to a large extent on the fit to previous he-
lium data. The correction to the SOFT efficiency
is a one-time correction determined from period 1.
The resulting SOFT efficiency is then applied to
periods 2 and 3 as well.
The adjusted 3He and 4He intensities are shown in
Figures 3, 4, and 5 as the solid black triangles. Dur-
ing solar minimum conditions, the 4He intensity
is strongly affected by anomalous helium, causing
the well known flattening of the helium spectrum
at low energies [1]. A better indication of the spec-
tral shape can be obtained from the 3He intensity,
where there is no contamination from anomalous
helium. The derived CRIS 3He intensities in Fig-
ure 3 are consistent with previous observations at
lower and higher energies.
In addition, solar maximum data should be rela-
tively free of contamination from anomalous he-
lium (see Figure 4). As one can see from Figures
4 and 5, the helium intensities that were adjusted
using data from the 1998 solar minimum condi-
tions, are also in agreement with previous and con-
current data taken during periods when the effects
Figure 4: CRIS helium intensities during solar
maximum conditions (Jan. 1 2001 to Jan. 1 2004)
compared with the results from previous and con-
current experiments and results from GALPROP
with an assumed φ of 1100 & 1200 MV.
of solar modulation were quite different. Finally,
the adjusted helium intensities also agree well with
the predictions of GALPROP (shown as the black
solid lines) [13]. This version of GALPROP is
a diffusive reacceleration model that agrees well
with CRIS/ACE Li, Be, and B observations. De-
tails of the model and the assumptions for the ef-
fects of solar modulation can be found in [14] and
[15]. The levels of solar modulation adopted for
the three time periods used in this study correspond
to a φ of 500 MV, 1200 MV, and 700 MV, respec-
tively.
Observations
Given an improved estimate of the SOFT tracking
efficiency for helium, we can present the time his-
tory of helium intensities over the solar cycle (see
Figure 6). The He time histories (black squares in
top two panels) are compared with IMP 8 in over-
lapping energy ranges. Since the highest CRIS en-
ergy range is from 113-131 MeV/nuc, we fit the
CRIS energy spectrum at each time interval, and
project the value of the CRIS intensity in the en-
ergy range covered by IMP 8 (134-250 MeV/nuc).
The two data sets observe the same overall trends
in solar modulation. The bottom panel in Figure 6
shows the ratio of CRIS to IMP 8 in the two energy
intervals explored.
815
COSMIC RAY HELIUM
Figure 5: CRIS helium intensities during recent
solar minimum conditions (Jan. 1 2007 to April
15 2007) compared with the results from concur-
rent observations of ACE/SIS (open squares), [4]
(asterisks). Also shown are the predictions from
GALPROP with φ = 700MV.
Summary
One-time adjustments to the CRIS He SOFT track-
ing efficiencies have brought good agreement be-
tween ACE and other observations taken at 1 AU.
CRIS has observed He intensities over the en-
tire solar cycle, which will be useful in studying
trends in solar modulation over time, and in deriv-
ing radial and latitudinal gradients with data from
Ulysses and Voyager. We also plan to investigate
the 3He intensity and 3He/4He ratio over the solar
cycle.
Acknowledgements
This work was supported by NASA at GSFC, Cal-
tech, JPL, and Washington University in St. Louis.
References
[1] Gloeckler, G.,, Rev. Geophys. Space
Physics, 17, 569, 1979
[2] Caballero-Lopez, R.A., McDonald, F.B.,
Adv. Space Res. , 32, 627-632, 2003
[3] von Rosenvinge, T.T., et al., submitted to
Space Sci. Rev., 2007
Figure 6: Top two panels: CRIS He intensities ver-
sus time in two separate energy intervals for the en-
tire solar cycle (black squares) compared with the
results from IMP 8 (open diamonds).Bottompanel:
The ratio of CRIS/IMP 8 for both energy intervals.
[4] Mewaldt, R.A., et al., submitted to Space
Sci. Rev., 2007
[5] Stone, E.C., Cohen, C.M.S., Cook, W.R., et
al., Space Sci. Rev., 86, 285-356, 1998
[6] de Nolfo, G.A., et al., Proc. 27th ICRC
(Hamburg), 2, 1667-1670, 2001
[7] Shikaze, Y., et al., submited to Elsevier,
2007
[8] Beatty, J.J., et al., Ap.J., 294, 455-462, 1985
[9] Alcaraz, J., et al., Phys. Letts. B, 494, 193,
2000
[10] Leech, H.W., and O’Gallagher, J.J., Ap.J.,
221, 1110, 1978
[11] Heikkila, B., private communication, 2007
[12] McKibben, B., Private communication,
2007
[13] Strong, A.W., Moskalenko, I.V., Astrophys.
J., 509, 212-228, 1998
[14] de Nolfo, G.A., et al., Adv. in Spac. Sci., 38,
1558, 2006
[15] Moskalenko, I.V., Strong, A.W., Ormes,
J.F., Potgieter, M.S., Astrophys. J., 565,
280-296, 2002
816


Cosmic-ray helium intensities over the solar cycle from ACE

NASA Technical Reports Server

Cosmic Ray Helium Intensities over the Solar Cycle from ACE

http://authors.library.caltech.edu/56617/1/2008-06.pdf

Cosmic Ray Helium Intensities over the Solar Cycle from ACE

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

NASA Technical Reports Server