Double Star Tc-2 Neutral Atom Imager On The Global Evolution Of The Magnetic Storm Ring Current Ion Detection And Analysis,

Energetic particles in ring current region are hazardous to spacecrafts. And geo-magnetic disturbance caused by enhanced ring current during magnetic storms can severely mess up high-tech facilities or even destroy them. So studying on the ring current, especially that during magnetic storms, is of great importance. The Double Star Project was the very first space exploring program proposed by Chinese scientists and organized by China supported wide international cooperation. The program includes two satellites, TC-1 on an equator orbit and TC-2 with a polar orbit configuration. TC-2 carried one energetic neutral atom imager (NUADU), dedicated for detecting of the Earth ring current. NUADU, working in a telemetry mode, has the ability to monitor the global distribution of ring current ions and its evolvement dynamics, that offers a powerful artifice for studying the storm-time ring current.The author of this paper took part in development of the NUADU instrument and was responsible for subsequent science data processing when he was working in science operation center of the Double Star Project. This paper intends to study on formation and loss mechanics of the ring current ions, using neutral atom imaging data recorded by NUADU, in combination with a ring current model that proposed by Chinese scientists and energetic atom imaging technique. Following three tasks were carried out:1. Analyzing energetic neutral atom (ENA) images directly, intending to reveal basic law of distribution of ring current ions along local times and its evolvement along with progress of magnetic storms.Direct analyzing shows that during the selected magnetic storm the southward IMF Bz component plays a key role in facilitating ion injection from the magnetotail into the inner magnetosphere with consequent enhancement of the ring current. Ion injection stopped soon after the IMF turned from southward to northward. Also during ion injection episodes, the ion drift paths opened and ring current ions were lost at the noon to dusk magnetopause soon after they were injected at the dusk side region. As a result, the ring current intensified after the occurrence of an open-to-closed transformation of the ion drift paths, rather than at the stage when ion injection was strong. In the main phase of the storm, ion injection took place between about 17:00LT– 22:00LT, leading to an extremely asymmetric ring current. In contrast in the recovery phase of the storm, ion injection region reached the post-midnight sector. We attributed this phenomena to a significant positive IMF By component. A symmetrical ring current was formed after the drift paths changed back to a closed configuration due to a decrease in tail convection. When the storm entered its late recovery phase, the ring current faded away while the Dst still indicated a strong magnetic disturbance. This suggested an important contribution to Dst from the cross tail current. 2. We then studied time variation of ENA fluxes from different energy bands of NUADU, targeting at two major magnetic storms. During the magnetic storms, ENA flux variation shows some important features that may be common for storms. Ignoring other factors that have less effect on ENA flux, the features of ENA variation can give some implications on ring current ion dynamics. The spectrum of ring current ions should have a steep minus slope at energy higher than tens of keV and get harder during main phase of storms possibly due to dramatic increasing of O+ flux. Ions are injected from the magnetotail to the ring current region in a dispersionless mode most probably by enhanced convective electric field that driven by steady southward interplanetary magnetic field (IMF). There's no straight cause and effect relation between sub-storm activities and enhancements of the ring current, although sub-storms may modulate the flux of ring current ions that cause short time variations of ion flux. Before the recovery phase, ion flux increase rapidly for a short time at the dusk side of the Earth. This can be interpreted as a result of piling up of ions due to diminishing of the inner-magnetosphere convective electric field on formation of shielding electric field and subsequent weakening of the escaping loss process at the magnetopause. At the rapid recovery stage of Dst/SYM-H index, the speed of loss process for ring current ions may also takes different stages. This implies that the Dst/SYM-H index can't accurately present intensity of the ring current and the recovery of the ring current may be more complicate than the rapid and slow mode that Dst/SYM-H takes.3. We finally modeled the ring current ion distribution and calculated ENA flux for a virtual NUADU imager. Simulation results were compared with observations. The model gives ENA space distribution rather similar to that of observations, while showing great discrepancy for ion energy spectra. Simulation results show that low-altitude atmospheric ingredients other than H make great contribution to the ENA flux that can be detected from any satellite positions. And stripping of ENAs is also an indispensible factor for ring current ENA simulation. Ions are mainly injected to ring current region in a pattern of adiabatic drift motion driving by enhanced convection electric field during magnetic storms. Those with higher energy are more difficult to access the ring current region. During weak or moderate storms (Kp < 5), for ions with energy higher than 40 keV a flux hole is presented at the dawn-side, indicating the convective electric field is not strong enough to inject such ions to ring current region. The hole shrinks and finally diminishes with the convective electric field becoming stronger. Simulated ENA fluxes show much harder energy spectra than that of observations. Comparision becomes even more apparent at higher Kp values. For the cause of the discrepancy, we are not certain whether there is an energy depending ion loss mechanism that the model does not take into account, or local acceleration has raised the flux of ions with low energy (<80keV).