Modeling of occurrence frequencies of ion conics as a function of altitudeand conic angle

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 degrees to 90 degrees conic angle are used as an input to the model and the occurr ence :frequencies of ion conics with smaller conic angles are numerically c alculated at higher altitudes. The calculated occurrence frequencies are co mpared with the observed ones of ion conics with smaller conic angles. We t ake into account conic angle variation with altitude in both adiabatic and non-adiabatic cases, horizontal extension of ion conics due to E x B drift, and evolution to elevated conics and ion beams in the model. Tn the adiaba tic case, the conic angle decreases with increasing altitude much faster th an was observed. The occurrence frequency of small-angle conics is much lar ger than the observed value without E x 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 x B drift to gyro velocity ratio of 0.08 to 0.6, depending on geomagnetic activities.