ABSTRACT: Anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum generally track galactic cosmic ray (GCR) intensities such
as those measured by neutron monitors, albeit with differences between solar polarity cycles. The unusual cycle 23/24 solar
minimum was long-lasting with very low sunspot numbers and significantly reduced interplanetary magnetic field strength and
solar wind dynamic pressure and turbulence, but also featured a heliospheric current sheet tilt that remained high for an
extended period. Peak ACR intensities did not recover to the maximum values reached during the last two A>0 solar minima and
just barely reached the last A<0 levels. However, GCR intensities in 2009 (neutron monitor rates and also at ∼200MeV/nucleon)
were the highest recorded during the last 50 years, indicating their intensities were not as heavily modulated during their
transport from the outer heliosphere. This unexpected difference in the behavior of ACRs and GCRs remains unexplained, but
suggests that either the ACR source intensity may have weakened since the last A<0 epoch, or perhaps that ACR intensities
at 1 AU in the ecliptic may be more sensitive than GCRs to the higher tilt angle. This seems plausible if the ACR source intensity
is greater at low latitudes during A<0 cycles, while the GCR distribution at the heliospheric boundary is more uniform in
latitude. Shortly after an abrupt increase in the current sheet tilt angle in late 2009, both ACR and GCR intensities showed
dramatic decreases, marking the end of solar minimum modulation conditions for this cycle.
KeywordsCosmic ray modulation–Heliospheric current sheet tilt angle
Space Science Reviews 05/2012; · 3.61 Impact Factor
American Institute of Physics Conference Series; 05/2012
American Institute of Physics Conference Series; 05/2012
ABSTRACT: The solar energetic particle event on 14 December 2006 was observed by several near-Earth spacecraft including the Advanced
Composition Explorer (ACE), STEREO A and B, SOHO and Wind. An interesting feature of this event is a series of unusual fluctuations in the particle intensity that occurred during
the first few hours. These fluctuations were observed inside a magnetic cloud that originated in a solar event on 13 December
and show both similarities and variations at the different spacecraft. Interestingly, the most striking difference is between
observations at the two closely-separated STEREO spacecraft. In particular, large fluctuations in the proton intensity were seen by the High Energy Telescope (HET)
on STEREO A, and to a lesser extent at Wind and ACE, but not by the STEREO B HET. We conclude that the differences in intensity-time profiles were caused by anisotropies
in the particle distribution and the different viewing directions of the individual particle telescopes. The intensity/anisotropy
variations suggest that flux tubes with different particle propagation conditions existed within this magnetic cloud despite
the absence of local magnetic field signatures associated with these regions. The intensity fluctuations are similar to those
occasionally seen in impulsive particle events. There were also spacecraft-to-spacecraft differences during the onset of the
particle event. An initial rapid onset of energetic (> 40MeV) protons was observed by the STEREO A and B spacecraft outside
the magnetic cloud, but not by spacecraft such as SOHO that were already inside the magnetic cloud at this time. The latter
spacecraft observed a slower, lower intensity increase. Evidently, energetic proton propagation from the solar event to the
vicinity of Earth was inhibited within the magnetic cloud compared to outside.
Solar Physics 04/2012; 256(1):443-462. · 2.78 Impact Factor
ABSTRACT: The Low-Energy Telescope (LET) is one of four sensors that make up the Solar Energetic Particle (SEP) instrument of the IMPACT
investigation for NASA’s STEREO mission. The LET is designed to measure the elemental composition, energy spectra, angular
distributions, and arrival times of H to Ni ions over the energy range from∼3 to ∼30MeV/nucleon. It will also identify the
rare isotope 3He and trans-iron nuclei with 30≤Z≤83. The SEP measurements from the two STEREO spacecraft will be combined with data from ACE and other 1-AU spacecraft to
provide multipoint investigations of the energetic particles that result from interplanetary shocks driven by coronal mass
ejections (CMEs) and from solar flare events. The multipoint in situ observations of SEPs and solar-wind plasma will complement
STEREO images of CMEs in order to investigate their role in space weather. Each LET instrument includes a sensor system made
up of an array of 14 solid-state detectors composed of 54 segments that are individually analyzed by custom Pulse Height Analysis
System Integrated Circuits (PHASICs). The signals from four PHASIC chips in each LET are used by a Minimal Instruction Set
Computer (MISC) to provide onboard particle identification of a dozen species in ∼12 energy intervals at event rates of ∼1,000events/sec.
An additional control unit, called SEP Central, gathers data from the four SEP sensors, controls the SEP bias supply, and
manages the interfaces to the sensors and the SEP interface to the Instrument Data Processing Unit (IDPU). This article outlines
the scientific objectives that LET will address, describes the design and operation of LET and the SEP Central electronics,
and discusses the data products that will result.
Space Science Reviews 04/2012; 136(1):285-362. · 3.61 Impact Factor
AGU Fall Meeting Abstracts. 12/2011;
AGU Fall Meeting Abstracts. 12/2010;
ABSTRACT: We report measurements of record-setting intensities of cosmic-ray nuclei from C to Fe, made with the Cosmic Ray Isotope Spectrometer carried on the Advanced Composition Explorer in orbit about the inner Sun-Earth Lagrangian point. In the energy interval from ~70 to ~450 MeV nucleon–1, near the peak in the near-Earth cosmic-ray spectrum, the measured intensities of major species from C to Fe were each 20%-26% greater in late 2009 than in the 1997-1998 minimum and previous solar minima of the space age (1957-1997). The elevated intensities reported here and also at neutron monitor energies were undoubtedly due to several unusual aspects of the solar cycle 23/24 minimum, including record-low interplanetary magnetic field (IMF) intensities, an extended period of reduced IMF turbulence, reduced solar-wind dynamic pressure, and extremely low solar activity during an extended solar minimum. The estimated parallel diffusion coefficient for cosmic-ray transport based on measured solar-wind properties was 44% greater in 2009 than in the 1997-1998 solar-minimum period. In addition, the weaker IMF should result in higher cosmic-ray drift velocities. Cosmic-ray intensity variations at 1 AU are found to lag IMF variations by 2-3 solar rotations, indicating that significant solar modulation occurs inside ~20 AU, consistent with earlier galactic cosmic-ray radial-gradient measurements. In 2010, the intensities suddenly decreased to 1997 levels following increases in solar activity and in the inclination of the heliospheric current sheet. We describe the conditions that gave cosmic rays greater access to the inner solar system and discuss some of their implications.
The Astrophysical Journal Letters 10/2010; 723(1):L1. · 5.53 Impact Factor
ABSTRACT: Observations of energetic ions and electrons from STEREO and ACE have been used to investigate the longitudinal extent of particle emissions from <SUP>3</SUP>He-rich solar energetic particle (SEP) events. In the event of 3-4 Nov 2008, ions and electrons were detected 20° ahead and behind the nominal connection from the source region to 1 AU, and electrons were also detected 60° ahead. The results are consistent with those of earlier studies that correlated data from near-Earth spacecraft with Helios data or with observations of source regions on the Sun.
TWELFTH INTERNATIONAL SOLAR WIND CONFERENCE. AIP Conference Proceedings. 01/2010; 1216:621-624 (2010).
38th COSPAR Scientific Assembly; 01/2010
ABSTRACT: The longitudinal position of a spacecraft relative to that of the active region associated with a solar energetic particle (SEP) event is known to have a strong impact on the characteristics of the temporal profile of the observed particle intensities. It is still debated as to whether the composition of the SEP event is similarly impacted. Most compositional studies have been done with single point measurements (typically near L1) and statistical analysis of the active region solar longitudes. With the twin STEREO spacecraft significantly separated from L1-based satellites such as ACE, simultaneous multi-point measurements of SEP events can be made for H-Fe ions from a few hundred keV/n to over 10 MeV/n and for electrons from tens to hundreds of keV. Additionally measurements of associated solar phenomena, e.g., coronal mass ejections, radio bursts, and solar flares can be made from distinctly different vantage points. Here we present particle data from instruments on STEREO and ACE for the recent SEP events of solar cycle 24 and compare their characteristics to the positions of the spacecraft relative to the solar source regions. The solar context for these events will also be discussed as measured by STEREO, SOHO, SDO, and Wind.
Publication: American Geophysical Union, Fall Meeting 2010, abstract #SH33B-1844. 01/2010;
ABSTRACT: Recent data from ACE show that during the spring and summer of 2009 the
galactic cosmic ray (GCR) intensity at ~100 to ~500 MeV/nuc (near the
peak in the spectrum at 1 AU) reached the highest intensities of the
space era. During mid-2007 the GCR intensities of Be to Ni (4 ≤ Z
≤ 28) had apparently leveled off at intensities comparable to those
measured in the 1976-77 and 1997-1998 solar minima, and the onset of new
solar activity was expected in 2008. Instead, solar-minimum conditions
continued, and the GCR intensity began to increase again in early 2008.
During the second quarter of 2009 the 120-470 MeV/nuc GCR Fe intensity
reached 19% ± 1% above the 1997-1998 solar-minimum level, with
similar increases in other species. Comparisons with earlier spacecraft
and neutron-monitor data show conclusively that the GCR intensities in
the spring and summer of 2009 are the highest of the space era. However,
when viewed in the context of the long-term Be-10 record, the space era
has until now experienced a below-average cosmic-ray intensity. The
record-setting intensity levels are likely due to a combination of
factors that include the weakened interplanetary magnetic field
strength, the reduced solar wind speed and dynamic pressure, and the
extended solar minimum conditions. In addition, during alternate solar
minima, including the present one, the drift pattern of cosmic rays in
the heliosphere is sensitive to the tilt of the interplanetary magnetic
current sheet, which was still inclined at ~20° in mid-2009. If the
current sheet tilt declines further before the onset of new solar
activity, it is likely that the GCR intensity will reach even higher
levels. This paper will compare the 2009 cosmic ray intensities with
those from the past ~50 years and with the long-term archival record,
summarize the role of the various solar-wind parameters in modulating
the near-Earth cosmic ray intensity, and discuss the implications of the
2009 cosmic-ray intensities for estimates of the interplanetary
AGU Fall Meeting Abstracts. 11/2009; -1:08.
ABSTRACT: We report improved measurements of elemental abundances and spectra for galactic cosmic-ray (GCR) nuclei obtained by the Cosmic Ray Isotope Spectrometer on board NASA's Advanced Composition Explorer (ACE) spacecraft during the minimum and maximum phases of solar cycle 23. We discuss results for particles with nuclear charge 5 ≤ Z ≤ 28 and typical energies between 50 and 500 MeV nucleon–1. We demonstrate that a detailed "leaky box" Galactic propagation model combined with a spherically symmetric solar modulation model gives a good (but not perfect) fit to the observed spectra by using a solar modulation parameter of = 325 MV at solar minimum and = 900 MV at solar maximum. Although our results are generally consistent with previous measurements from space-based and balloon-based missions, there are significant differences. The large geometrical acceptance and excellent charge resolution of the instrument result in the most detailed and statistically significant record of GCR composition to date in this energy range. The measurements reported here serve as a high-precision baseline for continued studies of GCR composition, solar modulation over the solar cycle, space radiation hazards, and other applications.
The Astrophysical Journal 06/2009; 698(2):1666. · 6.02 Impact Factor
ABSTRACT: The mean charge states of C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, and Ni ions with ~15-70 MeV nucleon-1 in the two large solar energetic particle (SEP) events of 1992 October 30 and November 2 have been determined using measurements of the invariant latitude of the cosmic-ray cutoffs of these elements from the Mass Spectrometer Telescope on the polar-orbiting SAMPEX satellite. The deduced charge state values are in good agreement with the mean values measured directly in previous SEP events at much lower energies of ~1 MeV nucleon-1. The inferred equilibrium source temperatures are confirmed to be typically 2 × 106 K, which provides additional evidence that SEPs in gradual-type events are accelerated coronal material.
The Astrophysical Journal 01/2009; 452(2):L149. · 6.02 Impact Factor
ABSTRACT: Using advanced instrumentation on the ACE spacecraft, we have conducted a survey of solar energetic particle spectra in 3He-rich events over a broad energy range ~80 keV nucleon-1 to 15 MeV nucleon-1 during the period 1997 September-2001 March. The spectra of 4He and heavy ions (C, N, O, Ne, Mg, Si, S, Ca, Fe) were generally similar over this range but often hardened below ~1 MeV nucleon-1. In most of the events there was even stronger hardening of the 3He spectrum below ~1 MeV nucleon-1, leading to an energy-dependent 3He : 4He ratio. These observations point to unique and distinct properties of 3He in these events and place new constraints on models that seek to explain enhancements of 3He and heavy ions using the same mechanisms. In addition to the events with spectra in the form of power laws or double power laws, there is a second class of event in which the low-energy 3He and Fe spectra are rounded, while the 4He remains a power law. In these cases 3He and Fe spectra can be fitted at low energies by a stochastic acceleration model, but this model does not explain the higher energy portions of these spectra, nor the power-law spectral forms of the 4He. These observations appear to require an additional mechanism, such as acceleration by cascading MHD turbulence. The 3He enrichment pattern that we observe suggests that all these different spectral features might be due to processes with a common origin but then followed by different acceleration histories.
The Astrophysical Journal 12/2008; 574(2):1039. · 6.02 Impact Factor
ABSTRACT: Using the Solar Isotope Spectrometer on the Advanced Composition Explorer (ACE), we have measured the 11.0-21.8 MeV nucleon-1 heavy (Z ≥ 6) element abundances of 39 small solar energetic particle (SEP) events that occurred between 1998 April 3 and 2002 February 26. Using He isotope data from the Ultra-Low-Energy Isotope Spectrometer on ACE, we have classified the events according to their 0.5-2.0 MeV nucleon-1 3He/4He ratios. We find that their average heavy-element composition is similar to that of either large gradual events or 3He-rich events, depending on their 3He/4He ratio. As seen in recent studies of small SEP events, we find that the heavy-element intensities relative to C increase with the 3He/4He ratios. The dependence of the heavy-element abundances on first ionization potential (FIP) has been derived, using a model consisting of a power law in Q/M (and Z) times a step function in FIP. We report the magnitude of the FIP factor in each event and find that it varies from ~2 to 7 with no clear dependence on the 3He/4He ratio.
The Astrophysical Journal 12/2008; 594(1):592. · 6.02 Impact Factor
ABSTRACT: We report the abundances of neon isotopes in the Galactic cosmic rays (GCRs) using data from the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE). These abundances have been measured for seven energy intervals over the energy range of 84 ≤ E/M ≤ 273 MeV nucleon-1. We have derived the 22Ne/20Ne ratio at the cosmic-ray source using the measured 21Ne, 19F, and 17O abundances as "tracers" of secondary production of the neon isotopes. Using this approach, the 22Ne/20Ne abundance ratio that we obtain for the cosmic-ray source is 0.387 ± 0.007(statistical) ± 0.022(systematic). This corresponds to an enhancement by a factor of 5.3 ± 0.3 over the 22Ne/20Ne ratio in the solar wind. This cosmic-ray source 22Ne/20Ne ratio is also significantly larger than that found in anomalous cosmic rays, solar energetic particles, most meteoritic samples of matter, and interplanetary dust particles. We compare our ACE CRIS data for neon and refractory isotope ratios, and data from other experiments, with recent results from two-component Wolf-Rayet (W-R) models. The three largest deviations of GCR isotope ratios from solar system ratios predicted by these models, 12C/16O, 22Ne/20Ne, and 58Fe/56Fe, are indeed present in the GCRs. In fact, all of the isotope ratios that we have measured are consistent with a GCR source consisting of about 80% material with solar system composition and about 20% W-R material. Since W-R stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of these data with W-R models suggests that superbubbles are the likely source of at least a substantial fraction of GCRs.
The Astrophysical Journal 12/2008; 634(1):351. · 6.02 Impact Factor
ABSTRACT: We report on abundance measurements of 10Be, 26Al, 36Cl, and 54Mn in the Galactic cosmic rays (GCRs) using the Cosmic-Ray Isotope Spectrometer (CRIS) instrument aboard the Advanced Composition Explorer spacecraft at energies from ~70 to ~400 MeV nucleon-1. We also report an upper limit on the abundance of GCR 14C. The high statistical significance of these measurements allows the energy dependence of their relative abundances to be studied. A steady-state, leaky-box propagation model, incorporating observations of the local interstellar medium (ISM) composition and density and recent partial fragmentation cross section measurements, is used to interpret these abundances. Using this model, the individual galactic confinement times derived using data for each species are consistent with a unique confinement time value of τesc = 15.0 ± 1.6 Myr. The CRIS abundance measurements are consistent with propagation through an average ISM hydrogen number density nH = 0.34 ± 0.04 H atoms cm-3. The surviving fractions, f, for each radioactive species have been calculated. From predictions of the diffusion models of Ptuskin & Soutoul, the values of f indicate an interstellar cosmic-ray diffusion coefficient of D = (3.5 ± 2.0) × 1028 cm2 s-1.
The Astrophysical Journal 12/2008; 563(2):768. · 6.02 Impact Factor
ABSTRACT: Measurements of the abundances of cosmic-ray 59Ni and 59Co are reported from the Cosmic-Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer. These nuclides form a parent-daughter pair in a radioactive decay which can occur only by electron capture. This decay cannot occur once the nuclei are accelerated to high energies and stripped of their electrons. The CRIS data indicate that the decay of 59Ni to 59Co has occurred, leading to the conclusion that a time longer than the 7.6 × 104 yr half-life of 59Ni elapsed before the particles were accelerated. Such long delays indicate the acceleration of old, stellar or interstellar material rather than fresh supernova ejecta. For cosmic-ray source material to have the composition of supernova ejecta would require that these ejecta not undergo significant mixing with normal interstellar gas before ~105 yr has elapsed.
The Astrophysical Journal 12/2008; 523(1):L61. · 6.02 Impact Factor
ABSTRACT: We report the discovery of energetic neutral atoms (ENAs) emitted during
the X9 solar event of 5 December 2006. Beginning ~1 hour following the
onset of this flare, located at ~E70, the Low Energy Telescopes (LETs)
on both STEREO A and B spacecraft observed a sudden increase in the
count rate of 1.6 to 15 MeV protons, with a typical velocity-dispersive
profile. At this time both STEREO spacecraft were located near Earth
and were oriented such that both LETs viewed the Sun directly, and, in
combination, covered 360° in heliographic longitude. Particle
arrival directions are measured to 7° in longitude. It was found
that more than 60% of the 1.6 - 15 MeV protons observed within 3 hours
of the flare onset (before the bulk of the solar energetic particles
began arriving at Earth) arrived from a longitude within 10° of the
Sun. When the arrival time of each of ~100 individual proton events is
corrected back to the Sun using the measured kinetic energy, the
resulting emission profile is very similar to the GOES soft x-ray
profile, and continues for ~1 hour or more. At higher energies, the 13
to 100 MeV proton count rates in the STEREO High Energy Telescopes
(HETs), which do not view the Sun directly, showed only a small
increase. While the time profile observed by the LETs is consistent with
the possibility of neutron-decay protons, both the arrival directions
and energy spectrum argue strongly that the majority of the proton
events were due to energetic neutral hydrogen atoms that arrived from
the solar direction and were stripped in passing through the windows of
the LET telescopes. To our knowledge, this is the first reported
observation of ENA emission from a solar flare/coronal mass ejection.
This talk will present the observations and discuss possible origins for
the production of ENAs in a large solar event.
AGU Fall Meeting Abstracts. 11/2008; -1:02.