Abstract. 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. , 1983]. It was shown to be the low-altitude signature of the boundary between regions of adiabatic and chaotic motion of protons in the equatorial magnetosphere. The latter •is a region of isotropic ion precipitation. As the IB is controlled solely by the magnetic field in the current sheet, the IB position reflects changes in the equatorial magnetic field in the near tail and can be used to monitor these changes. Direct determination of this boundary is not possible from the current generation DMSP spacecraft because of the lack of pitch angle coverage (only precipitating fluxes are measured). In this paper we rigorously test the relationship of b2i to the IB and test the ability of b2i to characterize the state of the magnetotail. 4739

Abstract.Recently it has been shown that isotropic precipitation of energetic protons on the nightside is caused by a non-adiabatic eect, namely pitch-angle scattering of protons in curved magnetic ®eld lines of the tail current sheet. Here we address the origin of isotropic proton precipitation on the dayside. Computations of proton scattering regions in the magnetopheric models T87, T89 and T95 reveal two regions which contribute to the isotropic precipitation. The ®rst is the region of weak magnetic ®eld in the outer cusp which provides the 1±2°wide isotropic precipitation on closed ®eld lines in a $2±3 hour wide MLT sector centered on noon. A second zone is formed by the scattering on the closed ®eld lines which cross the nightside equatorial region near the magnetopause which provides isotropic precipitation starting %1.5±2 h MLT from noon and which joins smoothly the precipitation coming from the tail current sheet. We also analyzed the isotropic proton precipitation using observations of NOAA low altitude polar spacecraft. We ®nd that isotropic precipitation of >30 to >80 keV protons continues around noon forming the continuous oval-shaped region of isotropic precipitation. Part of this region lies on open ®eld lines in the region of cusp-like or mantle precipitation, its equatorward part is observed on closed ®eld lines. Near noon it extends $1±2°below the sharp boundary of solar electron¯uxes (proxy of the open/closed ®eld line boundary) and equatorward of the cusp-like auroral precipitation. The observed energy dispersion of its equatorward boundary (isotropic boundary) agrees with model predictions of expected particle scattering in the regions of weak and highly curved magnetic ®eld. We also found some disagreement with model computations. We did not observe the predicted split of the isotropic precipitation region into separate nightside and dayside isotropic zones. Also, the oval-like shape of the isotropic boundary has a symmetry line in 10±12 MLT sector, which with increasing activity rotates toward dawn while the latitude of isotropic boundary is decreasing. Our conclusion is that for both dayside and nightside the isotropic boundary location is basically controlled by the magnetospheric magnetic ®eld, and therefore the isotropic boundaries can be used as a tool to probe the magnetospheric con®guration in dierent external conditions and at dierent activity levels.

Abstract.Recently it has been shown that isotropic precipitation of energetic protons on the nightside is caused by a non-adiabatic eect, namely pitch-angle scattering of protons in curved magnetic ®eld lines of the tail current sheet. Here we address the origin of isotropic proton precipitation on the dayside. Computations of proton scattering regions in the magnetopheric models T87, T89 and T95 reveal two regions which contribute to the isotropic precipitation. The ®rst is the region of weak magnetic ®eld in the outer cusp which provides the 1±2°wide isotropic precipitation on closed ®eld lines in a $2±3 hour wide MLT sector centered on noon. A second zone is formed by the scattering on the closed ®eld lines which cross the nightside equatorial region near the magnetopause which provides isotropic precipitation starting %1.5±2 h MLT from noon and which joins smoothly the precipitation coming from the tail current sheet. We also analyzed the isotropic proton precipitation using observations of NOAA low altitude polar spacecraft. We ®nd that isotropic precipitation of >30 to >80 keV protons continues around noon forming the continuous oval-shaped region of isotropic precipitation. Part of this region lies on open ®eld lines in the region of cusp-like or mantle precipitation, its equatorward part is observed on closed ®eld lines. Near noon it extends $1±2°below the sharp boundary of solar electron¯uxes (proxy of the open/closed ®eld line boundary) and equatorward of the cusp-like auroral precipitation. The observed energy dispersion of its equatorward boundary (isotropic boundary) agrees with model predictions of expected particle scattering in the regions of weak and highly curved magnetic ®eld. We also found some disagreement with model computations. We did not observe the predicted split of the isotropic precipitation region into separate nightside and dayside isotropic zones. Also, the oval-like shape of the isotropic boundary has a symmetry line in 10±12 MLT sector, which with increasing activity rotates toward dawn while the latitude of isotropic boundary is decreasing. Our conclusion is that for both dayside and nightside the isotropic boundary location is basically controlled by the magnetospheric magnetic ®eld, and therefore the isotropic boundaries can be used as a tool to probe the magnetospheric con®guration in dierent external conditions and at dierent activity levels.