Ion composition of substorms during storm-time and non-storm-time periods

摘要

The oxygen to hydrogen ratio (O~+/H~+) is investigated for storm-time substorms (D_(st) = -195 nT) and for substorms in absence of storms (non-storm-time substorms) (D_(st) ≥ - 30 nT). For this study we use CRRES particle and electric field data as well as indicators for storm (D_(st)) and substorm (AE) activity and the IMF B_z-component from IMP-8. Injection of particles into the inner magnetosphere during storms and substorms is consistent with enhanced convection caused by large-scale storm-time or dipolarization dawn-dusk electric fields in the near-Earth magnetotail. The strength of this convection is related to the magnitude and duration of the negative IMF B_z, which are both larger for storm-time substorms than for non-storm-time substorms. Non-storm-time substorms lead to only minor ring current enhancements at large L-values (L = 5.0-7.0)_, while storm-time substorms can contribute the main ring current. During storms the ring current maximum can move in as far as L = 3 from its quiet time position at L = 4.5. Storm-time substorms have O~+/H~+ ratios of several 100%, whereas non-storm-time substorms show an O~+/H~+ ratio between 15% and 65%. The obvious composition difference between storm-time and non-storm-time substorms indicates that during storms one or a combination of the following factors play a role: (1) oxygen ions are preferentially accelerated/transported in the plasma sheet, (2) pre-storm conditions lead to a large reservoir of oxygen ions tailward of the oxygen trapping boundary, or (3) polar ionospheric outflow is enhanced leading to an increased convection source of oxygen ions.