The magnetospheric contribution to the quiet‐time low energy nucleon spectrum in the vicinity of Earth

The upturn in the low energy quiet‐time interplanetary nucleon spectrum has previously been described as of galactic or solar origin. Recent observations have suggested that the magnetosphere may be an important source, at least at the lowest energies. Observations obtained with the JHU/APL experiment on Explorer 47 (IMP‐7) during the March 9–12, 1973 period show that the magnetosphere is the dominant contributor to the quiet time interplanetary proton population in the range 0.29 ≤ Ep ≤ 0.5 MeV, and suggest that it may be an important contributor up to energies of ∼1.5 MeV. The March 9–12 period is the quietest (on a time scale of hours) in a ∼ 2.5 year interval extending from September 72 through February 1975. The quiet‐time energy spectrum may be represented by a power law E −(3.1 ± 0.2) , the H/He ratio at ≲ 2 MeV/nucleon is ∼ 10 and the He/Z ≥ 3 ratio at ∼ 1 MeV/nucleon is ∼ 8. The ratio of anti‐sunward to sunward intensities is ∼ 2.6 and increases to ∼ 8.55 in a frame moving with the solar wind, i.e., the maximum intensity is coming from the direction of the bow shock. The observed intensities are factors of 3 to 10 lower than those reported earlier by the Chicago group, suggesting that the interpretation of the low energy gradient measurement reported by those authors needs to be revised. It is speculated that the low energy (≲ 20 MeV) upturn in the quiet‐time interplanetary proton spectrum may be related to particle emissions from planetary magnetospheres, such as that of Jupiter.