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ENA Imaging: How is it done?

Global Imaging of the Ring Current

The ring current is composed of energetic ions that are confined by the Earth's magnetic field. Ring current ions spiral about the magnetic field lines, as depicted above. By studying the time-dependent global distribution the ring current ions, we can learn a lot about the physics of the inner magnetosphere. We can fly spacecraft through the ring current, but that only gets us a finite number of individual single-point measurements. It's like trying to understand the weather by driving a car across the countryside while holding a thermometer through the window of our car. By the time we drive a significant distance, the global weather pattern has changed. Nowadays, we can get an instantaneous picture of large weather patterns using weather satellites, showing hurricanes, large precipitation fronts, etc. It would be nice to have such a global imaging capability to study the Earth's ring current. The problem is, it's tough to get a global picture of the ring current. Because the ring current ions are confined by the Earth's magnetic field, only local measurements of the ring current can yield direct information about the ion population. However, an indirect technique called ENA imaging has been developed which can provide a global view of the ring current ions.

Charge Exchange

ENA imaging relies on a process called charge exchange, in which energetic ions `steal' electrons from cold neutral atoms in the Earth's geocorona. Initially, before the electron transfer, the energetic ion is bound to the Earth's magnetic field, forced to spiral around magnetic field lines. However, by taking an electron from a nearby neutral atom, the energetic ion becomes an energetic neutral, and is suddenly freed from the magnetic field. The newly-created energetic neutral atom (ENA) flies off in whatever direction it happened to be traveling at the time of the electron transfer. Thus, by placing an ENA detector at a remote location, we can capture these escaping ENAs and get information about the ring current ion population.

ENA Image Inversion

Because ENAs are created via an interaction between the neutral geocorona and the ring current, ENA images are by their nature a convolution of both these different populations. In addition, each pixel of an ENA image is an integration of all the ENAs captured from a particular angle of observation. In effect, this collapses the 3D population into a 2D image. To provide quantitative information about the ring current, it is therefore necessary to do two things: (1) deconvolve the ion population from the geocorona, and (2) try to reconstruct the 3D ion population from the 2D image. This process (1)-(2) is known as image inversion. One of the deficiencies of ENA image inversion is that it is an underconstrained mathematical problem; there are more variables than there are constraints. The variables include the 3D ion pressure (or density) distribution and the ion pitch-angle distribution. The constraints are just the ENA pixels, and there are not enough pixels to provide complete information about all the variables. Therefore, we must make assumptions about some of the variables. Usually, we assume the pitch-angle distribution is isotropic (uniform), and invert the ENA image to produce the pressure distribution. Even though we know that the ring current pitch-angle distribution is not isotropic, without more information, this seems to be the best we can do.

Stereo Imaging

The TWINS mission provides stereo ENA imaging. By observing the ring current from two different vantage points, we provide additional constraints to the inversion process, and thus can obtain higher-quality, more realistic ion pressure and pitch-angle distributions.

 

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