Abstract
Abstract This is the second of a series of three papers that present a methodology with the aim of creating a set of maps of the coronal density over a period of many years. This paper describes a method for reconstructing the coronal electron density based on spherical harmonics. By assuming a radial structure to the corona at the height of interest, line-of-sight integrations can be made individually on each harmonic basis prior to determining coefficients, i.e., the computationally expensive integrations are calculated only once during initialization. This approach reduces the problem to finding the set of coefficients that best match the observed brightness using a regularized least-squares approach and is very efficient. The method is demonstrated on synthetic data created from both a simple and an intricate coronal density model. The quality of reconstruction is found to be reasonable in the presence of noise and large gaps in the data. The method is applied to both Large Angle and Spectrometric Coronagraph Experiment C2 and Solar Terrestrial Relations Observatory Cor2 coronagraph observations from 2009 March 20, and the results from both spacecraft compared. Future work will apply the method to large data sets.
Highlights
Reliable maps of the coronal density are important for linking various solar wind structures to the low solar atmosphere, for studies of the coronal response to the solar cycle, and for space weather applications, either as an inner boundary conditions for solar wind models, or for direct ballistic extrapolation into interplanetary space
This paper describes a method for reconstructing the coronal electron density based on spherical harmonics
This paper presents a new inversion method based on spherical harmonics for the extended inner solar corona, which is valid for regions where the large-scale structure is close to radial
Summary
Reliable maps of the coronal density are important for linking various solar wind structures to the low solar atmosphere, for studies of the coronal response to the solar cycle, and for space weather applications, either as an inner boundary conditions for solar wind models, or for direct ballistic extrapolation into interplanetary space. Estimates of the coronal electron density can be made through the inversion of coronal visible light observations. This has been achieved using several methods of varying complexity during eclipses, or by coronagraphs, for several decades. This paper presents a new inversion method based on spherical harmonics for the extended inner solar corona, which is valid for regions where the large-scale structure is close to radial. We present an alternative method to calculate the spherical harmonic coefficients based on iteration rather than least squares.
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