Simulations of phase space distributions of storm time proton ring current

Abstract

We use results of guiding-center simulations of ion transport to map phase space densities of the stormtime proton ring current. We model a storm as a sequence of substorm-associated enhancements in the convection electric field. Our pre-storm phase space distribution is an analytical solution to a steady-state transport model in which quiet-time radial diffusion balances charge exchange. This pre-storm phase space spectra at L~2-4 reproduce many of the features found in observed quiet-time spectra. Using results from simulations of ion transport during model storms having main phases of 3, 6, and 12 hr, we map phase space distributions from the pre-storm distribution in accordance with Liouville's theorem. We find stormtime enhancements in the phase space densities at energies E~30-160 keV for L~2.5-4. These enhancements agree well with the observed stormtime ring current. For storms with shorter main phases (~3 hr), the enhancements are caused mainly by the trapping of ions injected from open night side trajectories, and diffusive transport of higher-energy (;• 160 keV) ions contributes little to the stormtime ring current. However, the stormtime ring current is augmented also by the diffusive transport of higher-energy ions (E >_ 160 keV) during storms having longer main phases (>_ 6 hr). In order to account for the increase in Dst associated with the formation of the stormtime ring current, we estimate the enhancement in particle-energy content that results from stormtime ion transport in the equatorial magnetosphere. We find that transport alone cannot account for the entire increase in IDstl typical of a major storm. However, we can account for the entire increase in IDstl by realistically increasing the stormtime outer boundary value of the phase space density relative to the quiet-time value. We compute the magnetic field produced by the ring current itself and find that radial profiles of the magnetic field depression resemble those obtained from observational data. 1. Introduction The ring current consists of geomagnetically trapped ions and electrons in the 10-200 keV energy range [e.g., Frank, 1967; Williams, 1981a]. The intensity of the ring current is commonly measured by the geomagnetic index Dst [e.g., Mayaud, 1980, pp. 115-129]. The quiet-time ring current is believed [e.g., Hamilton et al., 1988] to contribute about 10-20 nT to -Dst, but this is mostly offset by magnetopause currents during the geomagnetically quiet intervals which define the baseline (Dst = 0) for the index. Thus the increase in IDstl to -200 nT typically observed during the main phase of a major geomagnetic storm must be associated [cf. Dessler and Parker, 1959; Sckopke, 1966; Carovillano and Maguire, 1966] with about a 10 to 20-fold increase in the energy content of the trapped-particle population.Indeed, large increases in trapped-been observed in connection with major geomagnetic storms. Such particle flux increases extend from L ~ 7 to as low as L-2 and span energies from 1 keV to several hundred keV. However, the main con...

Relative contribution of electrons to the stormtime total ring current energy content

“…We consider the proton loss due to charge exchange (see Chen et al [1994] for details). For electrons, we consider losses due to wave-particle interactions (see Liu et al [2003] for details).…”

“…The LANL/ MPA data were provided by H. Korth, and explained by Korth et al [1999]. The electron [Albert, 1994] and proton [Chen et al, 1994] initial conditions are taken from theoretical solutions of steady state transport equations, and normalized with quiet time boundary conditions. We use averaged LANL/MPA data for Kp = 1 as the quiet time boundary conditions to normalize our initial conditions, and for Kp = 3 as the stormtime boundary conditions.…”

From the Sun’s atmosphere to the Earth’s atmosphere: an overview of scientific models available for space weather developments

“…At least four groups are working at present along this path. They are located at the University of Michigan, the Aerospace Corporation, NASA-Goddard Space Flight Centre, and Rice University (see Chen et al, 1994 for a review of these works). Differences between their models are mainly related to a different selection of aspects of the problem that are treated at a state-of-the-art level or neglected.…”

Space Storms and Space Weather Hazards

“…Simulations not only complement the observational studies, but are in fact essential to the understanding of ring current and storm dynamics. Among the efforts regarding the "substorm-induced electric field" question, we should mention Chen et al (1994), who used spike-like enhancements of the convection electric field to simulate the effect of individual substorms, and Fok et al (1996), who used an iductive localised electric field, tied to successive cycles of stretching and diplarisation of the Tsyganenko model magnetic field. The results of both efforts suggested that the substorm contribution was subtle, and possibly negative to the development of a ring current.…”

“…The first enhancement in the ring current may be due to the magnetospheric convection driven by the southward IMF (e.g., McPherron, 1997), while the second ring current enhancement may be due to the substorm-associated accumulation of a new O + population. This second growth of the ring current must be driven by "highly fluctuating" electric fields (e.g., Chen et al, 1994), resulting from substorm expansions. It is well known that toward the end of the main phase of magnetic storms, the occurrence of intense substorms is very frequent and that the O + energy density in the inner magnetosphere is strongly correlated with these substorm activities (Daglis et al, 1994).…”