MESSENGER observations of planetary ion enhancements at Mercury’s northern magnetospheric cusp during Flux Transfer Event Showers

At Mercury, several processes can release ions and neutrals out of the planet’s surface. Here we present enhancements of dayside planetary ions in the solar wind entry layer during flux transfer event (FTE) “showers” near Mercury’s northern magnetospheric cusp. In this entry layer, solar wind ions are accelerated and move downward (i.e. planetward) toward the cusps, which sputter upward-moving planetary ions within 1 minute. The precipitation rate is enhanced by an order of magnitude during FTE showers and the neutral density of the exosphere can vary by >10% due to this FTE-driven sputtering. These in situ observations of enhanced planetary ions in the entry layer likely correspond to an escape channel of Mercury’s planetary ions, and the large-scale variations of the exosphere observed on minute-timescales by ground-based telescopes. Comprehensive, future multi-point measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury’s dynamic exosphere and magnetosphere.

Mapping of the statistical auroral distribution into the magnetosphere

Statistical auroral distributions are used in combination with an empirical model of the Earth's magnetic field in an attempt to determine the large-scale magnetospheric source regions for various types of auroral luminosity. The narrow ring of structured auroral emissions during magnetically quiet intervals appears to be associated with the inner region of the nightside central plasma sheet and the dayside entry layer. Under active conditions these discrete structures expand to fill the entire central plasma sheet. The high-altitude boundary plasma sheet on the other hand is more likely to be related to diffuse auroral emissions poleward of this "oval" and to high-latitude polar auroral arcs. Under this scenario, the region of the magnetosphere bounded by the inner edge of the tail current sheet, the plasmasphere, and the dayside entry layer is the source region for the most equatorward diffuse auroral precipitation.

Daytime aurora observed from Spitsbergen

Daytime (or dayside cleft) aurora is almost a permanent feature of the midday skies over Spitsbergen during the continuous darkness of the polar night It was observed in one or other of its characteristic forms around geomagnetic noon on 58 of 59 clear day sduring the wintersof 1987/1988, 1990/1991, and 1992/1993. The three types of day time aurora were studied by visual, colour photographic, and interference-filter techniques to confirm the precise nature of the observed emissions. Prenoon aurora, which is characterised by diffuse, patchy, green aurora at 557.7 nm, was observed on 42 occasions. It is generated by low-energy electrons of less than 300 eV coming through the entry layer of the dayside cleft. Noontime aurora, which consists largely of pure red emissions at 630.0/636.4 nm, was observed on 50 occasions. It is generated by high-flux, very low-energy electrons of 10–50 eV flowing directly from the solar wind through the polar cusp. Postnoon aurora, which is characterised by discrete, green auroral arcs at 557.7 nm, was also observed on 42 occasions. Like prenoon aurora, it is generated by low-energy electrons of less than 300 eV derived from the entry layer of the cleft Occasionally, some background or diffuse aurora is also observed, generated by high fluxes of low-energy proton precipitation and characterised by the hydrogen lines Hα and Hβ. On the one exceptional day on which daytime aurora was not observed, magnetic activity was exceptionally low.These ground-based observations complement satellite studies of analogous auroral events. In particular, the visual characteristics of the different types of daytime aurora may be explained in terms of the flux rates and energy profiles of the electrons that have been mapped in the different regions of the dayside cleft by satellite-borne detectors.