Dual source populations of substorm-associated ring current ions

Abstract

Abstract. Sources of low-energy ring current ions in the early morning sector (eastward drifting energy domain of about <5 keV) are examined using both statistical analyses and numerical tracing methods (phase-space mapping and simulation). In about 90% of Cluster perigee traversals at 02∼07 local time, these low-energy ring current ions have dual ion populations: one is wedge-like energy-dispersed ions, and the other is a band-like ions over different latitudes in a narrow energy range at the upper energy threshold of the wedge-like energy-dispersed ions. Both components are most likely created during past substorm activities. Numerical tracing results strongly suggest that these two components have different sources with different temperatures and elapsed times. The band-like part most likely comes from ions with plasma sheet temperature (∼1 keV), and the energy-dispersed part most likely comes from cold ions (temperature <0.1 keV). The source density of the cold component (0.2∼0.5×10 6 /m 3 ) is slightly less than that of the hot component (0.5×10 6 /m 3 ), while Cluster observation shows slightly higher density for the wedge-like part than the low-energy band-like part. The hot source component also explains the observed high-energy (>10 keV) ions drifting westward after adiabatic energization in the nightside under time-varying electric field. The wedge-like part has much shorter elapsed time, i.e., less charge-exchange loss, than the band-like part.

Cluster observation of few-hour-scale evolution of structured plasma in the inner magnetosphere

“…The wedge-like dispersed ions are most often observed in the dawn sector (60 % of traversals), which is consistent with the past results (e.g., . The correlation between the wedge-like dispersed ions and the AL activity is consistent with the previous results Yamauchi et al, 2009b). However, the high occurrence rate of the enhancing cases at all LTs needs quantitative examination (e.g., by numerical simulation in the future).…”

“…During substorms, this demarcation energy increases due to the enhanced electric field (Yamauchi et al, 2009b). In this paper, however, we do not include the change in the ion energy in defining the inbound-outbound differences because we look for the sudden appearance/disappearance of an ion population rather than change in the drift velocity.…”

“…b. Wedge-like energy-latitude dispersed structured ions at sub-keV range (Yamauchi et al, 1996;Ebihara et al, 2001): These ions are found even at low altitude (Sauvaud et al, 1981;Newell and Meng, 1986). The energy of the source population is much lower than that of the first population, and two possible origins have been proposed for this population: a nightside plasma sheet that contains cold plasma of ionospheric origin (Yamauchi et al, 2009b;Engwall et al, 2009) and a direct supply from the ionosphere (Giang et al, 2009). It is not clear whether these ions are the same as sub-keV ions flowing in nearly the field-aligned directions as observed by many equatorial satellites (e.g., Fennel et al, 1981).…”

“…Such a work requires a good classification of the latitudinal ion profiles into several "basic energy-latitude patterns" of ion populations coming from specific source regions via specific transport mechanisms and elapsed times, from 10 min to 10 h (Horwitz and Chappell, 1979;Ejiri et al, 1980;Olsen et al, 1987;Ebihara et al, 2001;Yamauchi et al, 2009b). However, this task is not simple because the observed ion population of < several keV in the inner magnetosphere is a convolution of various ion populations, which are sensitive to changes in the drift direction, location, and magnetospheric activity.…”

Ion drift simulation of sudden appearance of sub-keV structured ions in the inner magnetosphere

“…The third category is found at all local times and can be interpreted as local phenomena with a short timescale of no more than 1 h. On the other hand, the first category (wedge-like dispersed ions) is often observed in the morning-to-noon sectors, and represents the ion population that drifts more than a few hours after they are formed during substorms in the midnight-tomorning sectors (Ebihara et al, 2001;Yamauchi et al, 2009).…”