Correlation of Alfvén wave Poynting flux in the plasma sheet at 4–7 RE with ionospheric electron energy flux

Abstract

A comparison of Poynting flux in the plasma sheet at geocentric distances of 4–7 RE to the energy flux of magnetically conjugate precipitating electrons at 100‐km altitude is presented. We have investigated 40 plasma sheet crossings by the Polar satellite, including both cases with large in situ values of Poynting flux (∼1 ergs cm−2 s−1) and cases with low values (≤0.1 ergs cm−2 s−1). The values correspond to ∼125 and ∼12 ergs cm−2 s−1, respectively, when mapped along converging magnetic field lines to 100 km. The north‐south component of the electric field and the east‐west component of the magnetic field were the primary source of the Poynting flux. On the basis of the phase relationship and ratio of E and B, the majority of Poynting flux events were identified as Alfvén waves. The Poynting flux measured at high altitudes by Polar was correlated with the intensity of the conjugate auroral emission in the ultraviolet frequency range, which can be used to estimate energy deposition due to precipitating electron beams. The electron energy flux during times of intense Poynting flux in the plasma sheet exceeded 20 ergs cm−2 s−1. In the absence of strong Poynting flux in the plasma sheet, electron precipitation was small (≤5 ergs cm−2 s−1). The mapped Poynting flux was in almost all events larger by a factor of 1–10 than the ionospheric electron energy flux. These results show that Alfvénic Poynting flux in the midtail region is associated with and capable of powering localized regions of magnetically conjugate auroral emissions. Furthermore, the large Poynting flux events observed at the outer edge of the plasma sheet were conjugate to the poleward border of the active auroral regions, giving further evidence that at least some of the discrete aurora connects to the plasma sheet boundary layer.