Retrieval of plasmaspheric He+ density distributions from EUV imaging data

We retrieve the He+ density distribution in the plasmasphere from EUV imaging data, by using a forward modeling technique. We use a parametric model for the density distribution to simulate line-of-sight integrated He+ densities (i.e., EUV images), and then find parameters that give the best fit to real EUV images. The He+ density (nHe+) distribution is described as a function of radial distance (r) and L: nHe+ = n0 (L0/L)^α_L * (r0L/rL0)^α_f, where n0 and L0 are He+ density and L value at the inner boundary of this model (i.e., the topside ionosphere), and α_L and α_f are parameters that represent L and field-aligned dependence of He+ density, respectively. We evaluated how well our forward model can retrieve the He+ density distribution, by performing the following analysis. (1) EUV emission intensities were simulated through the EUV camera response function, given a vantage point of the IMAGE satellite. (2) EUV images were simulated for a large number of (α_L, α_f) pairs: α_L was chosen from 4.0 to 6.0 with 0.1 increment, and α_f was from 0.0 to 2.0 with 0.1 increment. (3) The EUV image corresponding to the (α_L, α_f)=(5.0, 1.0) pair was chosen as our synthetic EUV image. After noise was added to the synthetic image, the forward modeling was applied to all simulated images made in (2). The reduced chi2 (rchi2) was used to determine how well simulated image data fit to the synthetic image. The results of this analysis confirm that the He+ density distributions can be retrieved with good certainty within |40 deg.| MLAT. However, beyond this magnetic latitude it is difficult to determine the L dependence or field-aligned dependence of plasmaspheric He+ density. For further evaluation and better accuracy, we use density distributions provided by physics-based ionosphere/plasmasphere models as our synthetic data. We also apply our forward simulation model to real EUV image data from the EUV imager onboard IMAGE.