Magnetospheric Imaging
Authors: Burch J. L. 
Published: October 2003

Abstract
The Sun interacts with the Earth to create a region called the magnetosphere, which envelops the atmosphere and shields the immediate human environment from interplanetary space. As the technological age has advanced, the importance of the magnetosphere as a medium that must be monitored and whose behavior must be predicted has rapidly become apparent. The magnetosphere protects the Earth's upper atmosphere from direct exposure to the flow of charged particles from the Sun known as the solar wind. It is also the site of disturbances—geomagnetic storms and magnetospheric substorms—that can adversely affect human activities.
To date, the magnetosphere has been investigated largely on the basis of measurements made by single spacecraft along their orbital tracks. Magnetospheric imaging is needed to provide the missing global context that will allow space researchers, for the first time, to study the Earth's magnetosphere as a coherent global system of interacting components, driven by the highly variable input of mass, momentum, and energy from the solar wind.

In order to understand the physical processes that affect the magnetospheric plasma, requires the nearly instantaneous measurement of its structure is required. This measurement can only be obtained by magnetospheric imaging. In order to image a large, complex region such as the magnetosphere, which is invisible to traditional astronomical techniques, it is necessary to image its component parts. The particular imaging technique to be employed depends to a large degree on the properties of the plasmas that are to be imaged. One technique for magnetospheric imaging is
extreme ultraviolet (EUV) imaging
. This technique makes use of the fact that helium ions on the Sun emit ultraviolet light at 30.4 nm. This light is absorbed and then rapidly re‐emitted by helium ions in the magnetosphere. The greatest concentration of helium ions is in the plasmasphere, which is the high‐altitude extension of the ionosphere. Most of the ions in the plasmasphere are hydrogen, but hydrogen ions produce no light emissions. On the other hand, there is a nearly constant fraction (≈10–15%) of helium in the plasmasphere, which can be used as a tracer for the region. Recently the first images of the plasmasphere taken from outside the region were obtained by an EUV imager on the Japanese Nozomi (Planet‐B) spacecraft while it was in Earth parking orbit prior to its injection into an interplanetary trajectory to Mars (Figure 2). The image in Figure 2 took nearly 21 hours to construct because the spacecraft was not spinning and had to rely on its orbital motion to complete a raster scan of a large portion, but not all, of the plasmasphere (1). For more energetic ion populations,
energetic neutral atom (ENA) imaging
, is used.

For the purposes of this article, magnetospheric imaging will be defined as the collection of techniques that are capable of providing large‐scale images of various magnetospheric charged particle populations on time scales that are relevant to those of magnetospheric disturbances (ie. a few minutes). This definition will include the imaging of precipitating energetic protons. Radio sounding of the magnetosphere a technique employed in NASA's IMAGE (Imager for Magnetopause to Aurora Global Exploration) mission is beyond the scope of the article. However, the radio sounding system on IMAGE has already demonstrated the capability of remotely mapping the plasmasphere, cusp and magnetopause. The current state‐of‐the‐art magnetospheric imaging technology is embodied in the instrumentation developed for the IMAGE mission (launched on March 25, 2000), which is the first mission dedicated to the global imaging of the terrestrial magnetosphere. Thus the description of imaging techniques in this article focuses specifically on the IMAGE instruments. The discussion is ordered according to the particle populations to be imaged.