Recent IMP-8 GME Science Accomplishments


Continuing energetic particle data from the IMP-8 Goddard Medium Energy Experiment (GME) provide a comprehensive basis for modulation and acceleration studies at 1 AU and a critical, unique and absolutely essential baseline for ongoing Voyager studies of cosmic ray modulation and propagation in the outer.

Data from the GME instrument span 0.5-450 MeV Hydrogen, 2-450 MeV/nuc Helium, ions from Carbon through the Iron group from several to >100 MeV/nuc and relativistic electrons. These data form an extended time base when combined with data from similar instruments flown on IMPs 6 and 7. The quality of the IMP 8 GME data for galactic cosmic ray Hydrogen (2-230 MeV) and Helium (2-450 MeV/nuc) equals to or exceeds that of any other cosmic ray experiment flown since 1971. The GME observations now extend more than a complete heliospheric 22-year cycle from launch in September 1973. These data provide a unique and still essential consistent baseline for understanding NASA’s highly visible deep-space exploration missions as they move outwards towards a historic encounter with the heliopause and heliospheric termination shock and the local interstellar medium beyond.

IMP-8 GME data still offer unique capabilities even in the study of solar energetic particles (SEPs) and interplanetary structures and dynamics at 1 AU. GME H and He SEP spectral measurements are not duplicated by other missions to this day; GME measurements have thus become a critical base for studies of SEP acceleration when combined with heavier-ion and/or lower-energy measurements from Wind, ACE and SAMPEX. The high statistical resolution of GME integrated rate data also continues to be uniquely exploited to study modulation processes and dynamics at 1 AU, particularly in combination with higher-energy neutron monitor observations.


Publication Statistics:

Summary statistics of known GME usage are
47 refereed papers and 29 conference proceedings
from 1996 through 2001 to date !

Refereed Articles
Conference Proceedings
2001 (through May)

Examples: Studies in the Far Heliosphere

Long-term fluxes and spectra from the GME remain a bedrock for ongoing work to define the structure and physics of the outer heliosphere. For example:

McDonald et al (2001d) have combined GME data with data from Voyager 1 and 2 in the distant heliosphere and Ulysses in the inner heliosphere to study cosmic rays over the solar minimum period of cycle 22 and extending beyond 80 AU. The cosmic ray intensity at 1 AU over the 1996-1997 time period exhibits the quasi-plateau like characteristic predicted for qA>0 epochs and the energy spectra of 20-450 MeV/n He and 20-220 MeV H are essentially identical to those observed for 1976-1977 over a similar phase of the heliomagnetic cycle, suggesting that modulation conditions at both times are very similar throughout the heliosphere. This similarity makes it possible to combine the Pioneer 10 data from 1977 with that of Voyager 1 and 2 from 1997. It is found that beyond ~12 AU the radial intensity gradients are very small, which indicates that the intensity levels observed at V-1 for 7-18 MeV/n ACR 0+ and 265 MeV/n He are close to those expected at the termination shock at the V-1 heliolatitude of 34ºN. Measured 1997 intensities of the GCR He and ACR 0+ at 70 AU are significantly less than those observed at the 1987 solar minimum at 42 AU. For the ACR component such changes appear to be qualitatively consistent with Jokipii’s diffusive shock drift acceleration model. For GCR He the difference may be further evidence for modulation in the region of the heliosheath combined with gradient and curvature drift effects over the two different phases of the heliomagnetic cyle.


Fig. 1 (from McDonald et al 2001c). The 1 AU data for GCR He is from the IMP 8 MED, and the ACR O is from IMP-8 (R. Mewaldt) SAMPEX HILT (C. B. Klecker) and WIND EPACT-LEMT (D. Reames) The outer heliosphere GCR He and ACR O intensities are from the P-10 CRT and V-2 CRS. The 1AU ACR O+ time histories graphically illustrate the expected behavior of cosmic rays in the region inside the termination shock over a 22 year heliomagnetic cycle when drift effects are important.

The spatial intensity gradients of 150-380 MeV/n galactic cosmic ray He and 30-60 MeV/n H and He have also been studied over the 1996-2001.3 time period anchored by data from the Goddard MED experiment on IMP 8.

(McDonald et al, 2001b,d). IMP-8 combined with Ulysses and Voyager data provides information on the radial intensity gradients in the inner solar system over the solar minimum period of cycle 22 as well as latitudinal gradients, since Ulysses has just completed its slow latitudinal scan to 80ºS. With the onset of significant solar activity, the intensity gradients of 30-60 MeV/n H and He become very small as their Compton- Getting factor approaches 1 at low energies. However the radial intensity gradient, Gr, of galactic cosmic ray (GCR) He, shows a relatively small variation over the period 1996-2000.5 at a mean radial distance of 3 AU, consistent with the values predicted by Fujii and McDonald over the complete 1974-1996 period using the IMP/ Pioneer/ Voyager data and a radial dependence of the form G0r±. After 2000.5 the heliolatitude of Ulysses begins to change at a more rapid rate and can be used to determine the latitudinal gradient over the slow latitudinal scan. Comparisons of the modulation levels at 1AU and at Ulysses with those in the distant heliosphere out to 80AU indicate much larger changes in the inner heliosphere than observed beyond ~60AU.

Fig. 2 (from McDonald et al 2001d). V2 and IMP 8 helium energy spectra for cycle 22 solar maximum and solar minimum periods


  • Fig. 3 (from McDonald et al 2001d). Time histories (26 day AVG) of 150-380 MeV/n GCR He, 34-50 MeV/n He and 30-69 MeV H from the IMP 8 MED,Ulysses COSPIN KET and the Voyager 1 and 2 CRS experiments.


  • Examples: Energy Spectra of Solar Energetic Particles

    H and He spectral data still uniquely produced by the IMP-8 GME have assumed key importance in recent studies of SEP spectra and acceleration. For example:

    Tylka et al (2000b) examined solar energetic particle (SEP) spectra in two very large "gradual" events (20 April 1998 and 25 August 1998), in which acceleration is caused by fast CME-driven shocks. By combining data from ACE/SIS, Wind/EPACT/LEMT, and IMP8/GME, spectra covered all major species from H to Fe, from ~2 MeV/nuc to the highest energies measured. Spectra during the events reveal significant departures from simple power laws. Of particular note is the behavior at high energies, where the spectra exhibit exponential roll-overs. The fitted e-folding energies reflect both ionic charge states and a complex but orderly temporal evolution. They speculate that this behavior may be related to evolving rigidity-dependence in the near-shock diffusion coefficient, which is of potentially great importance for models of SEP acceleration and transport.

    Fig. 4 (from Tylka et al 2000b). Examples of 8-hour-averaged spectra in the 20 April 1998 event. Each species is color-coded and shown in the same order from top to bottom as the legend at the right. Note the scale factors used in plotting some elements. Data come from IMP-8/GME (n ), Wind/EPACT/LEMT (l ) and ACE/SIS (p ). Curves are exponential fits to the high-rigidity points. Open symbols denote low-rigidity points which were not used in the fits. Datapoints consistent with residual Galactic and/or anomalous cosmic-ray background are not shown.

  • IMP-8 H and He GME data were also part of the Tylka et. al. (2000a) study of thesource population of solar energetic particles in the 2000 Bastille Day event. Fe spectra are strikingly different from those of lighter species. This difference can be explained by shock acceleration from a two-component source population, comprising solar-wind suprathermals and a small (~5%) admixture of remnant flare particles, as previously proposed to explain enhanced 3He/4He in some gradual SEP events. Flare remnants can also account for several previously unexplained features of high-energy solar heavy-ions, as well as important aspects of SEP event-to-event variability. From these results flare activity clearly makes a unique and critical contribution to the source population. But there appears to be no need to invoke any mechanism other than CME-driven shock acceleration in order to understand the unusual spectral, compositional, and charge state characteristics of high-energy solar heavy ions in large, gradual SEP events.


    Coronal Ejecta, Co-Rotating Stuctures and Their Effects at 1AU

    Data from the IMP-8 GME "G" guard detector has been used in multiple studies mapping the effects of interplanetary field and plasma structures on cosmic rays and, in turn, using these cosmic rays as sensitive diagnostics of interplanetary structures. For example:

    Bidirectional Ion Flows (BIFs) have been discussed extensively by Richardson and Cane (1999) combining IMP-8 interplanetary data with worldwide neutron monitor network observations. In a more recent study (Richardson et al 2000) this work was extended to show a close relationship between bidirectional flows of galactic cosmic rays and bidirectional flows of solar/shock-accelerated particles at much lower energies (~ 1 MeV) observed by near Earth s/c Jan-Sept 1982. The bidirectional flows suggest that particles of a wide range of energies are trapped on expanding, looped magnetic field lines, providing that their gyroradii are much smaller than the 0.1 to few AU scale sizes of ejecta.

    Richardson et al (2000) combine GME data (guard rate and 121-154 MeV proton fluxes) with neutron monitor data in studying a 22-year cycle in the amplitude of recurrent, near earth near ecliptic galactic cosmic ray modulations during the solar minimum periods in the mid-1950s to mid-1990s. These modulations are ~50% larger during A>0 epochs than during A<0 epochs, where A is the direction of the solar global magnetic field. A change in the properties of the associated corotating streams does not appear to be responsible. Rather the response of the cosmic rays to solar wind speed enhancements appears to be reduced in A<0 minima. This pattern is inconsistent with a prediction from the drift model of cosmic ray modulation that recurrent modulations should be larger in A<0 epochs. Other factors may contribute, such as an epoch-dependence of the particle diffusion coefficient which may increase the effect of solar wind convection on the local cosmic ray density near earth during A>0 epochs.


    *IMP 8 GME Home page

    Author and Authorizing Official
    Dr. Robert McGuire, (301) 286-7794
    Space Physics Data Facility, Head
    Code 672
    NASA Goddard Space Flight Center
    Greenbelt, MD 20771, USA
    Last Modified: 6/4/2001 REM