OF JOURNAL GEOPHYSICAL RESEARCH, VOL. 108, NO. A7, 1277, doi:10.1029/2003JA009861, 2003
Proton aurora in the cusp during southward IMF
H. U. Frey and S. B. Mende
Space Sciences Laboratory, University of California, Berkeley, California, USA
S. A. Fuselier
Lockheed Martin ATC, Palo Alto, California, USA
T. J. Immel and N. stgaard
Space Sciences Laboratory, University of California, Berkeley, California, USA Received 24 January 2003; revised 19 March 2003; accepted 29 April 2003; published 8 July 2003.
[1] One of the most distinct aurorae in the high-latitude dayside region occurs at the
footprint of the cusp. The Spectrographic Imager SI-12 channel on the IMAGE spacecraft allows for long-duration observations of the proton aurora in this region and thus enables morphological and quantitative studies of the behavior of the proton aurora under different external solar wind conditions. Here we extend an earlier study of the proton aurora in the cusp during northward interplanetary magnetic field (IMF) conditions [Frey et al., 2002] to southward IMF conditions. We find a very similar dependence of the cusp auroras location in magnetic local time (MLT) on the IMF By component, with locations at prenoon for negative By and postnoon for positive By . We also confirm the previously established dependence of the proton aurora brightness on the solar wind dynamic pressure. However, for similar solar wind pressures the proton aurora is brighter for the southward IMF case than for the northward IMF case. The other major difference between southward and northward IMF conditions is the very strong dependence of the latitude location on the magnitude of the IMF Bz for southward IMF. We discuss both differences in terms of the location of the reconnection site at the magnetopause that INDEX TERMS: 2704 is connected to the ionosphere along cusp magnetic field lines.
Magnetospheric Physics: Auroral phenomena (2407); 2724 Magnetospheric Physics: Magnetopause, cusp, and boundary layers; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; 7835 Space Plasma Physics: Magnetic reconnection; 2455 Ionosphere: Particle precipitation; KEYWORDS: proton aurora, interplanetary magnetic field (IMF), cusp, reconnection, solar wind Citation: Frey, H. U., S. B. Mende, S. A. Fuselier, T. J. Immel, and N. stgaard, Proton aurora in the cusp during southward IMF, J. Geophys. Res., 108(A7), 1277, doi:10.1029/2003JA009861, 2003.
1. Introduction
[2] The objective of the IMAGE mission is to improve the understanding of the interaction between the solar wind, magnetosphere, and ionosphere. One signature of this interaction is the occurrence of aurorae which are observed by the Far Ultraviolet Instrument (FUV) on IMAGE [Mende et al., 2000]. The major advantage of this instrument compared with previously flown ultraviolet imagers is the capability of the Spectrographic Imager SI-12 channel to provide the first global observations of the proton aurora by observing the Doppler-shifted component of Lyman-a radiation from proton precipitation. Precipitating proton contributions to the overall energy deposition into the ionosphere can at certain locations (cusp, dusk region) and times (before substorm onset around midnight) account for a significant portion (20 30%) of the overall energy input [Frey et al., 2001; Hubert et al., 2002].
Copyright 2003 by the American Geophysical Union. 0148-0227/03/2003JA009861$09.00
[3] A very distinct aurora at the high-latitude, dayside region occurs at the footprint of the cusp [Smith and Lockwood, 1996]. In situ electron and ion measurements have shown that the location and spectrum of particle precipitation in the cusp depend on the IMF orientation [Newell et al., 1989] and that they are an indicator of reconnection at the magnetopause [see, e.g., Onsager et al., 1993]. The particle precipitation into the cusp shows differences depending on the external solar wind magnetic field orientation. A velocity (energy)-latitude dispersion can be found, with higher-energy particles precipitating closer to the reconnection site, which is the higher-energy particles at higher latitudes for northward IMF and at lower latitudes for southward IMF [Onsager et al., 1995]. This energy-latitude dispersion is a signature of the underlying reconnection process at high/low latitudes and the transport of reconnected field lines with the less energetic particles to lower/ higher latitudes [Woch and Lundin, 1992; Onsager and Lockwood, 1997]. [4] Precipitating particles create aurora and such auroral forms at the cusp footprint were studied using data from
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FREY ET AL.: CUSP AURORA DURING SOUTHWARD IMF
ground-based meridian scanning photometer and all-sky camera observations [Sandholt et al., 1998]. The rotation of the Earth and the relative motion of the site to different local times allowed for the study of the local time dependence on the IMF east west By component [Sandholt and Farrugia, 1999]. The north south extent of the observations allowed for the investigation of latitudinal motions with respect to the IMF Bz component [Sandholt et al., 1994]. Several studies demonstrated that the cusp fo...
2003JA009861