Reconstruction of auroral zone ion outflow during a substorm from VISIONS ENA measurements

Abstract:

Low-altitude observations of ion energization (below 1000 km) provide important constraints on models of ion outflow. As the first step in the chain of energization, this low altitude region is sensitive to a wide range of processes that heat and accelerate ions, including frictional heating, current-driven instabilities, shear-driven instabilities, and ambipolar effects. We have in situ measurements from missions such as Akebono, DE, Freja, and FAST, as well as sounding rockets, focused on the processes that give rise to ion acceleration. In addition, ISRs have measured ion upflows and frictional heating. From these measurements, climatological models of ion outflow have been built up.

The detailed time variation of ion outflows has been difficult to measure, until now. The advent of low energy neutral atom imaging in the last two decades has opened a new view into low-altitude ion acceleration processes, by affording the capability to image ion outflow over a large area. IMAGE, while providing the first global view, was not able to fully resolve the low-altitude region due to its orbit geometry, high orbital velocity, and operating parameters such as integrating period and spin rate.

A low energy neutral atom imager developed at NASA GSFC has flown on two missions, most recently on the VISIONS sounding rocket, launched in February 2013 from Poker Flat, Alaska. VISIONS was launched into an auroral substorm, and combined a short integration time, high sensitivity, and slow rocket velocity to provide the best-resolved images to date of ENAs produced by accelerated ions below 1000 km.

VISIONS has revealed important clues about low-altitude ion acceleration, which were hinted at by previous studies, but are revealed in a new light by ENA imaging. These include: 1) strong association of ion acceleration with regions of intense soft electron precipitation, 2) the fact that upwards ENAs dominate over horizontal ENAs imply either low-altitude wave processes or a “pressure cooker” geometry, which would have significant implications on the total ion outflow rate and its estimation from low-altitude measurements, and 3) the detailed temporal and spatial evolution of ion outflow during a 10 minute period can be used to estimate the ion outflow rate from a single substorm, to a much higher accuracy than previous efforts.