Momentum balance of dayside E region neutral winds during geomagnetically quiet summer days

Abstract

An intent of this paper is to study dayside quiet time E region neutral momentum balance in the auroral ionosphere. Neutral wind vectors were obtained by using the European Incoherent Scatter (EISCAT) Common Program 2 (CP‐2) data between 98 and 119 km height from 1100 to 1300 UT (from 1200 to 1400 LT and from 1400 to 1600 MLT) on June 15 and 16, 1993. Individual forcing terms in the neutral momentum equation were determined from the derived neutral winds with the MSISE‐90 model. In order to calculate the velocity change with time including advection, designated as the time‐dependent term, and the viscous force in the neutral momentum equation, a polynomial fitting procedure was applied to perform the vertical and time differentiation of the derived neutral winds. Below 106 km height, the time‐dependent term was greater than the pressure gradient force except for the meridional component at 98 km height on June 15. Between 110 and 116 km height, the pressure gradient force was in the same order of magnitude as that of the Coriolis force and the time‐dependent term was smaller than these two. The neutral wind flow was nearly perpendicular to the pressure gradient force. At 119 km height, the viscous drag was in the same order of magnitude as those of the pressure gradient and Coriolis forces and greater than the Pedersen and Hall drags. The magnitude of ion drag was ∼7% of the Coriolis force at this height level.