DAYSIDE ISOTROPIC PRECIPITATION OF ENERGETIC PROTONS

Recently it has been shown that isotropic precipitation of energetic p rotons on the nightside is caused by a non-adiabatic effect, namely pi tch-angle scattering of protons in curved magnetic field lines of the tail current sheet. Here we address the origin of isotropic proton pre cipitation on the dayside. Computations of proton scattering regions i n the magnetopheric models T87, T89 and T95 reveal two regions which c ontribute to the isotropic precipitation. The first is the region of w eak magnetic field in the outer cusp which provides the 1-2 degrees wi de isotropic precipitation on closed held lines in a similar to 2-3 ho ur wide MLT sector centered on noon. A second zone is formed by the sc attering on the closed held lines which cross the nightside equatorial region near the magnetopause which provides isotropic precipitation s tarting approximate to 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 alt itude polar spacecraft. We find that isotropic precipitation of >30 to >80 keV protons continues around noon forming the continuous oval-sha ped region of isotropic precipitation. Part of this region lies on ope n held lines in the region of cusp-like or mantle precipitation, its e quatorward part is observed on closed held lines. Near noon it extends similar to 1-2 degrees below the sharp boundary of solar electron flu xes (proxy of the open/closed held line boundary) and equatorward of t he cusp-like auroral precipitation. The observed energy dispersion of its equatorward boundary (isotropic boundary) agrees with model predic tions of expected particle scattering in the regions of weak and highl y curved magnetic held. We also found some disagreement with model com putations. We did not observe the predicted split of the isotropic pre cipitation region into separate nightside and dayside isotropic zones. Also, the oval-like shape of the isotropic boundary has a symmetry li ne in 10-12 MLT sector, which with increasing activity rotates toward dawn while the latitude of isotropic boundary is decreasing. Our concl usion is that for both dayside and nightside the isotropic boundary lo cation is basically controlled by the magnetospheric magnetic field, a nd therefore the isotropic boundaries can be used as a tool to probe t he magnetospheric configuration in different external conditions and a t different activity levels.