On the origins of the upward shift of elevated (bimodal) ion conics in velocity space

Abstract

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.

Solar wind control of Earth's H+ and O+ outflow rates in the 15‐eV to 33‐keV energy range

“…That would seem to allow for greatly varying acceleration rates as well as broad ion pitch angle distributions. In fact, it is a vision that has been prompted by the frequent observations over the last 30 years of transversely accelerated ions, and it is one that is specifically consistent, superficially at least, with published statistics on the altitude dependence of ''conical'' ion velocity distributions [e.g., Peterson et al, 1992;Miyake et al, 1993Miyake et al, , 1996. However, a smoothly progressing form of acceleration alone cannot explain the occasional observation [Lennartsson, 2003] of azimuthally collimated and monoenergetic keV O + ions at 90°pitch angle at high altitude (R $ 6 R E ).…”

The relationship between suprathermal heavy ion outflow and auroral electron energy deposition: Polar/Ultraviolet Imager and Fast Auroral Snapshot/Time‐of‐Flight Energy Angle Mass Spectrometer observations

“…Regardless of what other processes are involved, the escape of heavy ions from Earth's gravity undoubtedly requires either parallel acceleration or perpendicular heating to gain the 10 eV or more of energy needed. In the low-altitude region pertinent to this study (_< 4000 km), conic distributions dominate over beams [Gorney et al, 1981;Miyake et al, 1996], so that perpendicular heating is the most important process. Recent statistical studies using Freja data at 1700 km ] and Fast Auroral Snapshot (FAST) data between 2000 and 4000 km [Lund et al, 2000] suggest that broadband extremely low frequency (BBELF) waves are the dominant heating agent in this region, eclipsing electromagnetic ion cyclotron waves and lower hybrid waves.…”

Modeling of occurrence frequencies of ion conics as a function of altitude and conic angle

“…The energy reaches the maximum when the conic angle is decreased to about 60 . Miyake et al (1996) showed that the ion conics with conic angles near 60 are statistically most energetic. This is explained by the extended energization region along the orbit of conic ions.…”

“…There have been several reports on occurrence frequencies of ion conics based on satellite observations (Gorney et al, 1981;Yau et al, 1984;Kondo et al, 1990;Thelin et al, 1990;Peterson et al, 1992;Miyake et al, 1993Miyake et al, , 1996. Most of them, however, did not examine the quantitative relationship of the observed occurrences between ion conics with di erent conic angles and at di erent altitudes.…”

“…Miyake et al (1993) showed similar changes of conic angle as well as the increase of energy of ion conics with altitudes in Exos-D observations at 10 000 km. It was also revealed that a large fraction of elevated conics can be explained by height-integrated perpendicular energization of ion conics (Temerin, 1986;Miyake et al, 1996).…”