Abstract
Abstract. The occurrence frequencies of dayside ion conics with various conic angles are obtained as a function of altitude from Exos-D (Akebono) observations. We made a model calculation of ion conic evolution to match the observation results. The observed occurrence frequencies of ion conics with 80° to 90° conic angle are used as an input to the model and the occurrence frequencies of ion conics with smaller conic angles are numerically calculated at higher altitudes. The calculated occurrence frequencies are compared with the observed ones of ion conics with smaller conic angles. We take into account conic angle variation with altitude in both adiabatic and non-adiabatic cases, horizontal extension of ion conics due to E×B drift, and evolution to elevated conics and ion beams in the model. In the adiabatic case, the conic angle decreases with increasing altitude much faster than was observed. The occurrence frequency of small-angle conics is much larger than the observed value without E×B drift and evolution to the other UFIs. An agreement is obtained by assuming non-adiabatic variation of conic angles with altitude and an ion E×B drift to gyro velocity ratio of 0.08 to 0.6, depending on geomagnetic activities.Key words. Ionosphere (particle acceleration) · Magnetospheric physics (auroral phenomena; magnetopause, cusp, and boundary layers).
Highlights
Ion conics have cone-shaped intensity maxima centered on the magnetic ®eld line direction in velocity space and are categorized into two types; standard conics and elevated conics
The purpose of this study is to examine whether the overall feature of observed occurrence frequencies of ion conics can be consistently explained by a model which considers the production of ion conics with a conic angle near 90, successive conic angle variation from adiabatic to non-adiabatic cases as theyow upwards, the eect of E Â B drift, and the possible evolution into elevated conics and ion beams, and is to discuss what are suggested by the parameters which give an agreement to the observation results
Occurrence frequencies of ion conics as a function of altitude and conic angle are obtained from Exos-D observations and are compared with a model of occurrences by taking into account the eect of E Â B drift and the evolution to the other form of UFIs
Summary
Ion conics have cone-shaped intensity maxima centered on the magnetic ®eld line direction in velocity space and are categorized into two types; standard conics and elevated conics. Standard conics have an angular distribution withux maxima at a nearly constant pitch. The simple explanation for their generation is the perpendicular acceleration of ions at low altitudes and the subsequent upward motion of the accelerated ions in the Earth's mirror magnetic ®eld under the conservation of the ®rst adiabatic invariant. Elevated (or bimodal) conics (Klumpar et al, 1984) haveux maxima at a certain pitch angle at high-energy but are more ®eldaligned at low-energy. Various wave-particle interaction processes have been proposed as possible mechanisms for perpendicular energization of ions (see, for a review, Andre and Yau, 1997)
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