Overview

The EUVSim application takes as input an EUV image and inverts it to return a mapping of the He⁺ density in the SM equatorial plane. The inversion proceeds on two assumptions. The first of these is that the He⁺ density is either constant along a magnetic field line or that its intensity can be represented by a known function. This is a necessary assumption as there is only sufficient information in the data for the inversion to be done spatially in two-dimensions. The second assumption is that the field is dipolar. This will probably be changed in later versions, however, it can be argued that this is not a severe restriction since when the plasmasphere extends outward to high L conditions are quit and when conditions are active the plasmasphere has a restricted range in L.

The entire inversion works with sets of 2D data grids. As such the options often center on various grids. A 2D grid has both an X and Y extent. X is equivalent to the grid columns and Y to the grid rows. There are two basic grid coordinate systems used: the image grid coordinate system and the SM (solar-magnetospheric) coordinate system. The image based grid are used to display and manipulate the measured EUV image. Elevation varies along X and satellite phase along Y. Each column in the grid represents output from a single wedge and strip bin from one of the three EUV heads. The SM based grid is used to map the SM equatorial plane. These grids can either be set up in polar or rectangular coordinates. With polar coordinates the X direction defines geomagnetic longitude and the Y direction the radius.

In the application can be a large number of active grids, one for each data product being output. Dispite this there are only four grids which are used in the inversion algorithm. These are the measured image grid which holds the image as obtained by the EUV instrument less background removal, the solution grid which holds the current He⁺ density map in the inversion, the zero solution grid which contains a mask indicating which cells in solution grid are known to have zero density, and the synthetic image grid which contains a synthetic EUV image build from the density map in the current solution grid. These grids should be noted especially the first three since they can be acted upon by various user controlled algorithms to remove background and to aid in the stability of the inversion.