Recently, efforts to characterize and monitor the state of the magnetosphere have intensified, along with the rising interest in space weather. The latitude of the ion energy flux precipitation maxima (b2i), which almost invariably occurs near the equatorward edge of the nightside main auroral oval, has been suggested as one such parameterization. It has been suggested that b2i corresponds to the ion isotropy boundary (IB), which has been independently researched as a measure of the extent to which the magnetotail is stretched. By comparing simultaneous observations by the Defense Meteorological Satellite Program (DMSP) and NOAA spacecraft, we confirm a close association between b2i and the isotropy boundary of 30 keV protons. Using 2.5 years of simultaneous data from DMSP and GOES spacecraft, we verified that magnetic field inclination (the extent to which the magnetotail is stretched) strongly controls the b2i/IB latitude. Based on use of the b2i latitude, corrected for local time variation, as an index of magnetic stretching in the tail to show a considerable dawn-dusk asymmetry, we find that the magnetic field is more depressed and stretched at dusk than at dawn, and asymmetry increases with increasing magnetotail stretching. This asymmetry is consistent with the rotation of the symmetry line of the b2i(MLT) curve toward premidnight hours and suggests the growth of a so-called partial ring current system with increasing activity. Finally, the utility of the b2i/IB boundary as a characterization of the state of the magnetosphere is shown by demonstrating that the average pressure in the magnetotail is better specified by b2i than by Kp. ¿ 1998 American Geophysical Union