Using data from the low-altitude polar orbiting satellite NOAA-15 the proton precipitation rate into the upper atmosphere can be monitored with a 100-min time resolution. From the total power of precipitating protons in the midnight/evening local time sector, a parameter Q(t) is used as a proxy for estimating the energy injection rate into the ring current (RC) due to energetic protons. The injection rate Q(t) is not based upon solar wind parameters but directly on the observed proton precipitation rate. This is done under the assumption that the loss cone acts as a splash catcher for the protons injected into the RC. The protons in the loss cone thus do not represent a loss from the RC but are, in fact, a measure of the proton injection rate into the RC. Using the Burton equation and Q(t) as a measure for the true energy injection rate, a value proportional to the energy content in the RC due to energetic protons can be calculated. Assuming a decay constant for the ions in the RC and that the magnetic field depression is proportional to the RC energy content, a RC index can be determined. When Dst is corrected for magnetopause and tail currents, the linear correlation coefficient between the corrected Dst* and the RC index is between 0.8 and 0.9; thus ~70--80% of the variance in this corrected Dst* can be accounted for. The high correlation strongly indicates that Q(t) is a measure of the true energy injection rate and that Dst* is a measure of the energy content in the RC. Observations of proton precipitation measured by low-altitude polar-orbiting satellites thus have the potential for deriving a space-based Dst index in near real time that is not influenced by magnetic fields generated by magnetopause, field-aligned, and tail currents.