Role and origin of the poleward Alfvénic arc

Abstract

AbstractAlfvénic arcs are often found adjacent to the poleward border of the substorm bulge. They are created by precipitating narrow electron beams with a broad energy distribution, mainly below 1 keV, and dominated by Alfvénic structures of small scales and frequencies in the 1 Hz range. They are also associated with transverse ion heating. A balanced field‐aligned current system accompanying the poleward Alfvénic arc reveals its underlying structure being a broad channel of primary energy inflow of several tens of kilometers width. The associated magnetic shear stresses drive a flow along the poleward side of the substorm current wedge. Polar cap convection passes through this channel before entering the substorm bulge. The small‐scale electromagnetic structures are developing from the primary energy inflow by scale breaking and multiple reflections in the ionospheric Alfvén resonator. By their parallel electric field components these structures decouple effectively from the ionosphere. Their energy content is converted into kinetic energy of auroral particles in the topside ionosphere. A selected data set from FAST, DE 2, and Cluster is presented for characterizing the energetic and electromagnetic properties of the Alfvénic arc. Its function in the substorm can be summarized as a preconditioning of the polar cap plasma and magnetic field for entry into the magnetosphere through the poleward arc. This occurs under substantial density depletion.