DMSP/F2 electron observations of equatorward auroral boundaries and their relationship to magnetospheric electric fields

Abstract

Energetic electron measurements from a particle detector on the DMSP/F2 satellite have been used to determine the corrected geomagnetic latitude ΛCGM of the equatorward boundary of the auroral oval. The satellite was launched into a nearly sun‐synchronous, polar orbit centered on the 0700–1900 MLT meridian. Because of the wobble of the dipole and a very slow precessional motion of the orbit more than 6000 boundary crossings could be studied in the 1600–2300 and 0400–1000 MLT sectors. The detectors, which have large geometric factors, look radially away from the earth and detect precipitating electrons with energies between 50 eV and 20 keV. In the evening sector the equatorward boundaries are precisely determined from the rise in the total electron flux. The morningside boundary cannot always be clearly delineated. Data were divided into 1‐hour magnetic local time bins with ∼400 samples per bin. In each of the MLT bins, ΛCGM was found to be linearly correlated to the KP index. Regression values of ΛCGM were projected to the magnetic equatorial plane by using the Mead‐Fairfield magnetic field model. The projected boundaries are not in good agreement with the injection boundaries of Mauk and McIlwain (1974). They are best fit to boundaries derived by using Volland‐Stern type electric fields with the axis of symmetry rotated into the evening sector for KP <2 and into the afternoon sector for KP ≥2.

The inner edge of the plasma sheet and the diffuse aurora

“…At low altitudes extensive measurements have been made of a sharp low latitude boundary to auroral electrons which can be observed both optically (e.g., Sheehan and Carovillano, 1978) and in precipitating electrons (e.g.. Gussenhoven et al,1981). Existing data (Meng et al, 1979;Slater et al, 1980;Horwitz et al, 1982) support the commonly made assumption that this low altitude boundary corresponds to the inner edge of the plasma sheet.…”

“…During more quiet times the inner edge is further out and separated from the inner region of high background counting rates. Gussenhoven et al (1981) have noted a similar problem with penetrating radiation in their low altitude data. Vasyliunas (1968b) did not find inner edges in this region, however we find clear inner edges during more rare quiet conditions.…”

Plasma sheet access to geosynchronous orbit

“…The latitude of the equatorward edge of the diffuse aurora has been determined from DMSP measurements of precipitating plasma sheet electrons and has been shown to be well correlated with Kp [Gussenhoven et al, 1981[Gussenhoven et al, , 1983. From a large body of DMSP measurements, Gussenhoven et al found the empirical relationship Am = 67.8 -2.07 Kp.…”

Modeling the electromagnetic ion cyclotron wave‐induced formation of detached subauroral proton arcs

“…To investigate this further we simulated ring current dynamics during this event using an eastward offset of 2 hours in the convection electric field. Previous studies [e.g., Gussenhoven et al, 1981;Matsui et al, 2004] have indicated that the symmetry axis of the plasma convection pattern may be rotated by a few hours from the dawn-dusk direction during disturbed periods, and better agreement between measured and observed ion fluxes can be obtained if an offset is applied [e.g., Kistler et al, 1989;Jordanova et al, 1999]. A rotation of the axis of symmetry and further distortion of the electric field may be due to disruptions of electric field shielding and fieldaligned currents, as well as changes in the near-terminator conductivity gradient or in the magnetic configuration [Wolf et al, 2005, and references therein].…”

“…At higher time resolution, the equatorward boundary of the auroral electron precipitation observed by the DMSP satellite and reported as the Auroral Boundary Index (ABI) [Gussenhoven et al, 1981[Gussenhoven et al, , 1983] provides a similarly good monitor of the strength of the convection [Thomsen, 2004]. For the simulations described here, we use the tabulated value of ABI to control the strength of the convection by first converting it to an effective Kp through the relationship derived by Gussenhoven et al, namely Kp eff = (67.8 À ABI)/2.07.…”