Editing Advanced Composition Explorer (section)

== Science results ==
=== The spectra of particles observed by ACE ===
[[File:ACE O Fluence.png|thumb|upright=1.2|An oxygen fluence observed by ACE (Figure 1)]]

Figure 1 shows the particle fluence (total flux over a given period of time) of oxygen at ACE for a time period just after solar minimum, the part of the 11-year solar cycle when solar activity is lowest.<ref name=mewaldt01>{{cite journal |last=Mewaldt |first=R. A. |title=Long-term fluences of energetic particles in the heliosphere |journal=AIP Conf. Proc. |date=2001 |volume=86 |pages=165–170 |doi=10.1063/1.1433995 |bibcode=2001AIPC..598..165M |display-authors=et al. |url=https://deepblue.lib.umich.edu/bitstream/2027.42/87586/2/165_1.pdf |hdl=2027.42/87586 |hdl-access=free}}</ref> The lowest-energy particles come from the slow and fast solar wind, with speeds from about 300 to about 800&nbsp;km/s. Like the solar wind distribution of all ions, that of oxygen has a suprathermal tail of higher-energy particles; that is, in the frame of the bulk solar wind, the plasma has an energy distribution that is approximately a thermal distribution but has a notable excess above about 5 [[Electronvolt|keV]], as shown in Figure 1. The ACE team has made contributions to understanding the origins of these tails and their role in injecting particles into additional acceleration processes.

At energies higher than those of the solar wind particles, ACE observes particles from regions known as [[corotating interaction region]]s (CIRs). CIRs form because the solar wind is not uniform. Due to solar rotation, high-speed streams collide with preceding slow solar wind, creating shock waves at roughly 2–5 [[astronomical unit]]s (AU, the distance between Earth and the Sun) and forming CIRs. Particles accelerated by these shocks are commonly observed at 1 AU below energies of about 10 MeV per nucleon. ACE measurements confirm that CIRs include a significant fraction of singly charged helium formed when interstellar neutral helium is ionized.<ref name=moebius02>{{cite journal |last=Möbius |first=E. |title=Charge states of energetic (~ 0.5 MeV/n) ions in corotating interaction regions at 1 AU and implications on source populations |journal=Geophys. Res. Lett. |date=2002 |volume=29 |issue=2 |page=1016 |doi=10.1029/2001GL013410 |bibcode=2002GeoRL..29.1016M |s2cid=119651635 |display-authors=et al. |doi-access=free}}</ref>

At yet higher energies, the major contribution to the measured flux of particles is due to solar energetic particles (SEPs) associated with interplanetary (IP) shocks driven by fast coronal mass ejections (CMEs) and solar flares. Enriched abundances of helium-3 and helium ions show that the suprathermal tails are the main seed population for these SEPs.<ref name=desai01>{{cite journal |last=Desai |first=M. I. |title=Acceleration of <sup>3</sup>He nuclei at interplanetary shocks |journal=Astrophysical Journal |date=2001 |volume=553 |issue=1 |pages=L89–L92 |doi=10.1086/320503 |bibcode=2001ApJ...553L..89D |display-authors=et al. |doi-access=free}}</ref> IP shocks traveling at speeds up to about {{cvt|2000|km/s}} accelerate particles from the suprathermal tail to 100 MeV per nucleon and more. IP shocks are particularly important because they can continue to accelerate particles as they pass over ACE and thus allow shock acceleration processes to be studied in situ.

Other high-energy particles observed by ACE are anomalous cosmic rays (ACRs) that originate with neutral interstellar atoms that are ionized in the inner heliosphere to make "pickup" ions and are later accelerated to energies greater than 10 MeV per nucleon in the outer heliosphere. ACE also observes pickup ions directly; they are easily identified because they are singlely charged. Finally, the highest-energy particles observed by ACE are the galactic cosmic rays (GCRs), thought to be accelerated by shock waves from supernova explosions in our galaxy.

=== Other findings from ACE ===
Shortly after launch, the SEP sensors on ACE detected solar events that had unexpected characteristics. Unlike most large, shock-accelerated SEP events, these were highly enriched in iron and helium-3, as are the much smaller, flare-associated impulsive SEP events.<ref name=cohen99>{{cite journal |last=Cohen |first=C. M. S. |title=Inferred charge states of high energy solar particles from the solar isotope spectrometer on ACE |journal=Geophys. Res. Lett. |date=1999 |volume=26 |issue=2 |pages=149–152 |doi=10.1029/1998GL900218 |bibcode=1999GeoRL..26..149C |display-authors=et al. |url=https://authors.library.caltech.edu/48990/1/grl11860.pdf |doi-access=free}}</ref><ref name=mason99>{{cite journal |last=Mason |first=G. M. |title=Particle acceleration and sources in the November 1997 solar energetic particle events |journal=Geophys. Res. Lett. |date=1999 |volume=26 |issue=2 |pages=141–144 |doi=10.1029/1998GL900235 |bibcode=1999GeoRL..26..141M |display-authors=et al. |url=https://authors.library.caltech.edu/48993/1/grl11877.pdf |doi-access=free}}</ref> Within the first year of operations, ACE found many of these "hybrid" events, which led to substantial discussion within the community as to what conditions could generate them.<ref name=cohen12>{{cite journal |last=Cohen |first=C. M. S. |title=Observations of the longitudinal spread of solar energetic particle events in solar cycle 24 |journal=AIP Conf. Proc. |date=2012 |volume=1436 |pages=103–109 |display-authors=et al. |series=AIP Conference Proceedings |url=https://authors.library.caltech.edu/31666/1/APC000103.pdf |doi=10.1063/1.4723596 |bibcode=2012AIPC.1436..103C}}</ref>

One remarkable recent discovery in heliospheric physics has been the ubiquitous presence of suprathermal particles with common spectral shape. This shape unexpectedly occurs in the quiet solar wind; in disturbed conditions downstream from shocks, including CIRs; and elsewhere in the heliosphere. These observations have led Fisk and Gloeckler <ref name=fisk08>{{cite journal |last=Fisk |first=L. A. |title=Acceleration of suprathermal tails in the solar wind |journal=Astrophysical Journal |date=2008 |volume=686 |issue=2 |pages=1466–1473 |doi=10.1086/591543 |bibcode=2008ApJ...686.1466F |display-authors=et al. |doi-access=free}}</ref> to suggest a novel mechanism for the particles' acceleration.

Another discovery has been that the current solar cycle, as measured by sunspots, CMEs, and SEPs, has been much less magnetically active than the previous cycle. McComas et al.<ref name=mccomas08>{{cite journal |last=McComas |first=D. J. |s2cid=14927209 |title=Weaker solar wind from the polar coronal holes and the whole Sun |journal=Geophys. Res. Lett. |date=2008 |volume=35 |issue=18 |page=L18103 |doi=10.1029/2008GL034896 |bibcode=2008GeoRL..3518103M |display-authors=et al. |doi-access=free}}</ref> have shown that the dynamic pressures of the solar wind measured by the Ulysses satellite over all latitudes and by ACE in the ecliptic plane are correlated and were declining in time for about 2 decades. They concluded that the Sun had been undergoing a global change that affected the overall heliosphere. Simultaneously, GCR intensities were increasing and in 2009 were the highest recorded during the past 50 years.<ref name=leske11>{{cite journal |last=Leske |first=R. A. |title=Anomalous and galactic cosmic rays at 1 AU during the cycle 23/24 solar minimum |journal=Space Sci. Rev. |date=2011 |doi=10.1007/s11214-011-9772-1 |display-authors=et al. |bibcode=2013SSRv..176..253L |volume=176 |issue=1–4 |pages=253–263 |s2cid=122973813}}</ref> GCRs have more difficulty reaching Earth when the Sun is more magnetically active, so the high GCR intensity in 2009 is consistent with a globally reduced dynamic pressure of the solar wind.

ACE also measures abundances of cosmic ray [[Isotopes of nickel|nickel-59]] and [[Isotopes of cobalt|cobalt-59]] isotopes; these measurements indicate that a time longer than the half-life of nickel-59 with bound electrons (7.6 × 10<sup>4</sup> years) elapsed between the time nickel-59 was created in a supernova explosion and the time cosmic rays were accelerated.<ref name=wiedenbeck99>{{cite journal |last=Wiedenbeck |first=M. E. |title=Constraints on the time delay between nucleosynthesis and cosmic-ray acceleration from observations of <sup>59</sup>Ni and <sup>59</sup>Co|journal=Astrophysical Journal |date=1999 |volume=523 |issue=1 |pages=L61–L64 |doi=10.1086/312242 |bibcode=1999ApJ...523L..61W |display-authors=et al. |doi-access=free}}</ref> Such long delays indicate that cosmic rays come from the acceleration of old stellar or interstellar material rather than from fresh supernova ejecta. ACE also measures an [[Isotopes of iron|iron-58]]/[[iron-56]] ratio that is enriched over the same ratio in solar system material.<ref name=binns05>{{cite journal |last=Binns |first=W. R. |title=Cosmic-ray neon, Wolf-Rayet stars, and the superbubble origin of galactic cosmic rays|journal=Astrophysical Journal|date=2005|volume=634|issue=1 |pages=351–364 |doi=10.1086/496959 |bibcode=2005ApJ...634..351B |display-authors=et al. |arxiv=astro-ph/0508398 |s2cid=34996423}}</ref> These and other findings have led to a theory of the origin of cosmic rays in galactic [[superbubble]]s, formed in regions where many supernovae explode within a few million years. Recent observations of a cocoon of freshly accelerated cosmic rays in the Cygnus superbubble by the Fermi gamma-ray observatory<ref name=ackermann11>{{cite journal |last=Ackermann |first=M. |s2cid=38789717 |title=A cocoon of freshly accelerated cosmic rays detected by Fermi in the Cygnus superbubble |journal=Science |date=2011 |volume=334 |issue=6059 |pages=1103–1107 |doi=10.1126/science.1210311 |bibcode=2011Sci...334.1103A |display-authors=et al. |pmid=22116880 |url=https://www.openaccessrepository.it/record/141003|url-access=subscription }}{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}}</ref> support this theory.