Statistical properties and possible supply mechanisms of tailward cold O + beams in the lobe/mantle regions

Abstract

We investigate statistical properties of cold O + beams (COBs) streaming tailward at the velocity nearly equal to the major H + component, which were observed by Geotail/low‐energy particle (LEP) instrument in the tail lobe/mantle regions at geocentric distance between 8 and 210 R E (Earth radii) during the solar‐minimum period (October 1993 to March 1995). The average O + density is ∼1.3×10 −3 cm −3 , which corresponds to ∼1.2 % of the proton component. Properties of the flow velocity show that it is not the weakening of the magnetospheric convection but the large parallel velocity which enables the O + ions to remain still in the distant lobe/mantle regions. The occurrence frequency of COBs suggests that O + ions tend to exist in the mantle‐like region rather than tenuous “pure lobe” and that their existence has a clear correlation with the geomagnetic activity. On the basis of the EMF B y dependence of the double‐peaked COB distribution along dawn‐dusk direction, it is shown that COBs exist mostly on loaded quadrants in the north–south and dawn‐dusk asymmetry of sheath plasma entry caused by the IMF B y effect on the dayside reconnection process. The concentration on the loaded quadrants can be seen even in geomagnetic storms. It suggests that frequent COB occurrence at active times is mainly due to the southward orientation of IMF rather than the increase of dynamic pressure itself during the geomagnetic storms. That is, the statistics show that COBs are abundant at geomagnetically active times on loaded quadrants resulting from the dayside reconnection process, where the ions of solar wind origin bear the major component, and their field‐aligned velocity is larger than usual. These COBs should originate in the dayside magnetosphere and/or the polar cap regions. From the COB energy of several keV, which is rather higher than that of cusp/cleft ion outflows, the necessity of extra energization(s) to elevate parallel velocity is suggested. Clear IMF B y dependence, on one hand, provides other possibilities of the COB's source such as the energetic UFI beams and the equatorially trapped ions. The requirements for each candidate so as to be a main contributor to COBs are also discussed.