Monoenergetic ion drop‐off in the inner magnetosphere

Abstract

Abstract. A monoenergetic drop-off of ions around 10 keY, which we term as "ion drop-off band" (IDB) in this paper, has been observed by Akebono. The IDB is identified as a sharp and deep dip at about 10 keV in ion spectra, which is usually observed at latitudes below the discrete auroral region over several degrees or more. As the ion motion of this energy in this inner part of the magnetosphere is basically described by the adiabatic theory, we have numerically traced the ion drift trajectories. From the results, it is proposed that the lower-energy boundary of the drop-off demarcates the open/closed character of the drift orbits, only below which continuous supply from the magnetotail is present. This model explains the energy, local time, and latitudinal extent of IDB as well as the formation of its poleward edge very well.

Ion multi-nose structures observed by Cluster in the inner Magnetosphere

“…Considering the relatively quiet magnetospheric activity level for this event, as well as the stationarity of the structure between the inbound and the out- bound leg of the Cluster orbit and the typical energy range of the observed nose structure (i.e. 6.45 keV in average), this feature seems to be related to the stationary nose structures described in previous papers (Shirai et al, 1997). The nose structure is also seen in the O + spectrogram.…”

“…Moreover, these structures (that will be referred in the following as "stationary nose structures") appear regardless of the magnetospheric activity level. Using an ion drift trajectory model, Shirai et al (1997) were able to reproduce such structures, and showed that they are the consequence of the open/close character of the particle orbits: at the Akebono location, corotation electric field is sufficient to close the orbit (around the Earth) of low energy particles which are drifting eastward. Also, the magnetic field gradient is strong enough to close the orbits of high energy particles which are drifting westward.…”

“…Double nose structures (referred in the following as "split nose"), as seen in the morning sector by the ION experiment onboard Interball-2, were studied by Buzulukova et al (2003). They are interpreted by the authors as the result of a superposition in the spectrograms of a single stationary nose (Shirai et al, 1997) together with the gap described and analyzed by Lennartsson et al (1979). This gap, which was observed in the morning and pre-morning sector, is created at energies within the nose energy range, leading to a split of the stationary nose into two parts.…”

“…Shirai et al (1997) interpreted them in terms of mono-energetic ion drop off. These gaps, which appear at about 10 keV, were observed over a wide latitudinal range at almost constant energy.…”

“…More recently, Kovrazhkin et al (1999) listed the different types of gaps observed on the ION spectrograms onboard the INTERBALL spacecraft: the one described by Shirai et al (1997) on Akebono data and the one described by Lennartsson et al (1979), corresponding to the very long time of azimuthal drift of particles for a given energy. Double nose structures (referred in the following as "split nose"), as seen in the morning sector by the ION experiment onboard Interball-2, were studied by Buzulukova et al (2003).…”