Abstract. The present paper focuses on the altitude dependence of oxygen ion conics in the dayside cusp/cleft region. Here, combining oxygen data from the Akebono, Interball-2 and Cluster satellites allows, for the first time, one to follow the global development of energetic (up to ∼10 keV) ion outflow over a continuous and broad altitude range up to about 5.5 Earth radii (R E ). According to earlier statistical studies, the results are consistent with a height-integrated energization of ions at altitudes up to 3.5 R E . Higher up, the results inferred from Cluster observations put forward evidence of a saturation of both a transverse energization rate and ion gyroradii. We suggest that such results may be interpreted as finite perpendicular wavelength effects (a few tens of km) in the wave-particle interactions. To substantiate the suggestion, we carry out two-dimensional, Monte Carlo simulations of ion conic production that incorporate such effects and limited residence times due to the finite latitudinal extent of the heating region.

We present a statistical study of elevated (bimodal) ion conics observed by the Exos D (Akebono) satellite from 0900 to 1500 magnetic local time (MLT). We especially focus on a comparative analysis of elevated conics with standard conics and ion beams. Elevated conics are observed mainly above 6,000 km, while standard conics begin to appear at lower altitudes. The energy of elevated conics is usually higher than that of standard ion conics. This is consistent with the idea that the elevated conics evolve from standard conics through some acceleration process while they flow up to high altitudes. Since the elevated conics studied here received additional energization as they evolve from standard conics, the velocity filter mechanism previously invoked to explain elevated conics is inconsistent with our results. Our observation suggests that two kinds of acceleration processes are responsible for the upward shift in the velocity distribution of elevated conics. The elevated conics with a large cone angle and small upward shift in velocity space are most likely to be caused by the perpendicular energization extended along the field line. The MLT occurrence of this subset of elevated conics is similar to that of standard conics. The ion conics with a cone angle near 60° are found to be the most energetic, which strongly suggests the extended energization. On the other hand, the elevated conics with a small cone angle and large upward shift are mainly due to the parallel acceleration acting in tandem with perpendicular energization. This subset of elevated conics is found more often in the afternoon sector than are standard conics, which evokes rather some similarity to ion beams.

The altitude dependence of ion conics is investigated by using EXOS D observations on the dayside below 10,000 km altitude. The cone angle of ion conics tends to decrease with increasing altitude, but not so much as expected from a simple adiabatic model. The conic temperature, on the other hand, tends to increase with increasing altitude. The occurrence frequency of ion conics increases with altitude below 6000 km but is approximately constant above 6000 km. The appearance of newly born conics and the extinction of old conics in the statistics at any altitude could make some contribution, if the appearance and the extinction are large enough, to the observation results: less significant change in cone angle and increasing temperature with altitude, but this effect alone hardly provides a full explanation for the differences in the conic characteristics between the observations and the simple adiabatic model. The results rather seem to reflect the real evolution of an ion conic as ions flow up along the field line, suggesting nonconservation of the adiabatic invariant and the height-integrated transverse acceleration of ion conics over a wide range of altitude. 11,127

Intact lunar lava tubes offer a pristine environment to conduct scientific examination of the Moon's composition and potentially serve as secure shelters for humans and instruments. We investigated the SELENE Lunar Radar Sounder (LRS) data at locations close to the Marius Hills Hole (MHH), a skylight potentially leading to an intact lava tube, and found a distinctive echo pattern exhibiting a precipitous decrease in echo power, subsequently followed by a large second echo peak that may be evidence for the existence of a lava tube. The search area was further expanded to 13.00–15.00°N, 301.85–304.01°E around the MHH, and similar LRS echo patterns were observed at several locations. Most of the locations are in regions of underground mass deficit suggested by GRAIL gravity data analysis. Some of the observed echo patterns are along rille A, where the MHH was discovered, or on the southwest underground extension of the rille.

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