Polar spacecraft based comparisons of intense electric fields and Poynting flux near and within the plasma sheet‐tail lobe boundary to UVI images: An energy source for the aurora

Abstract

In this paper, we present measurements from two passes of the Polar spacecraft of intense electric and magnetic field structures associated with Alfven waves at and within the outer boundary of the plasma sheet at geocentric distances of 4–6 RE near local midnight. The electric field variations have maximum values exceeding 100 mV/m and are typically polarized approximately normal to the plasma sheet boundary. The electric field structures investigated vary over timescales (in the spacecraft frame) ranging from 1 to 30 s. They are associated with strong magnetic field fluctuations with amplitudes of 10–40 nT which lie predominantly in the plane of the plasma sheet and are perpendicular to the local magnetic field. The Poynting flux associated with the perturbation fields measured at these altitudes is about 1–2 ergs cm−2 s−1 and is directed along the average magnetic field direction toward the ionosphere. If the measured Poynting flux is mapped to ionospheric altitudes along converging magnetic field lines, the resulting energy flux ranges up to 100 ergs cm−2s−1. These strongly enhanced Poynting fluxes appear to occur in layers which are observed when the spacecraft is magnetically conjugate (to within a 1° mapping accuracy) to intense auroral structures as detected by the Polar UV Imager (UVI). The electron energy flux (averaged over a spatial resolution of 0.5° ) deposited in the ionosphere due to auroral electron beams as estimated from the intensity in the UVI Lyman‐Birge‐Hopfield‐long filters is 15–30 ergs cm−2s−1. Thus there is evidence that these electric field structures provide sufficient Poynting flux to power the acceleration of auroral electrons (as well as the energization of upflowing ions and Joule heating of the ionosphere). During some events the phasing and ratio of the transverse electric and magnetic field variations are consistent with earthward propagation of Alfven surface waves with phase velocities of 4000–10000 km/s. During other events the phase shifts between electric and magnetic fields suggest interference between upward and downward propagating Alfven waves. The E/B ratios are about an order of magnitude larger than typical values of c/Σp, where Σp is the height integrated Pedersen conductivity. The contribution to the total energy flux at these altitudes from Poynting flux associated with Alfven waves is comparable to or larger than the contribution from the particle energy flux and 1–2 orders of magnitude larger than that estimated from the large‐scale steady state convection electric field and field‐aligned current system.