Comparisons between ion distributions retrieved from ENA images of the ring current and contemporaneous, multipoint ion measurements recorded in situ during the major magnetic storm of 15 May 2005

Abstract

The Neutral Atom Detector Unit (NUADU) aboard the TC‐2 spacecraft recorded energetic neutral atom (ENA) image data for >3 h during part of the main and recovery phases of a major magnetic storm on 15 May 2005. A custom designed, constrained linear inversion method was applied to retrieve ring current ion distributions in the ENA records out to L = 6.6. Comparisons were then made between the ion fluxes retrieved from these ENA data (energy ranges 50–81 keV and 81–158 keV) and complementary, contemporaneous particle fluxes measured in situ by Synchronous Orbit Particle Analyzer (SOPA) instruments aboard a series of geosynchronous satellites that were launched to encircle the equatorial plane at L ∼ 6.6 by the Los Alamos National Laboratory (LANL). The ENA data revealed the development of two emission peaks during the main phase of the storm which corresponded to the arrival from the magnetotail of a pair of particle injections. These injections reached, in each case, maximum value before the corresponding AL index amplitude peak was attained. The main phase of the storm concluded (Dst ≈ −256 nT) in the aftermath of this event pair, and the ring current ion fluxes thereafter gradually decayed over several hours (the storm recovery phase). This is the first time that the linear inversion method developed for NUADU data has been globally validated using multipoint in situ measurements made at different magnetic local times. It was found that the higher the derived flux values were on the duskside/nightside, the closer they were to the in situ measurements. The retrieved fluxes obtained on the duskside/nightside tended, however, to be still somewhat underestimated since the ENA measurements concerned were made around the outer edge of the ring current at an altitude where the magnetic field in the inner magnetosphere deviates from that dipole configuration assumed in the inversion procedure to pertain there. The two methods utilized to study ions during a large magnetic storm are complementary in that the retrieved ion fluxes provide high time resolution information concerning the changing, large‐scale structure of ring current events, whereas in situ flux sampling presents “measurement truth” at particular locations within individual ring currents.