Pixon Image Reconstruction Research Articles

Image Reconstruction Techniques in Neutron and Gamma Ray Spectroscopy: Improving Lunar Prospector Data

AbstractWe present improved resolution maps of the Lunar Prospector Neutron Spectrometer thermal, epithermal, and fast neutron data and gamma ray spectrometer Th‐line fluxes via global application of pixon image reconstruction techniques. With the use of mock data sets, we show that the pixon image reconstruction method compares favorably with other methods that have been used in planetary neutron and gamma ray spectroscopy. The improved thermal neutron maps are able to clearly distinguish variations in composition across the lunar surface, including within the lunar basins of Hertzsprung and Schrödinger. The improvement in resolution reveals a correlation between albedo and thermal neutron flux within the basins. The consequent increase in dynamic range confirms that Hertzsprung basin contains one of the most anorthositic parts of the lunar crust, including nearly pure anorthite over a region tens of kilometers in diameter. At Orientale, the reconstructed epithermal neutron data show broad overlap with circular polarization ratio but there remains a mismatch between measures of regolith maturity that sample the surface and those that probe the near subsurface, which is consistent with a complex layering scenario.

Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data

We present a map of the near subsurface hydrogen distribution on Mars, based on epithermal neutron data from the Mars Odyssey Neutron Spectrometer. The map’s spatial resolution is approximately improved two-fold via a new form of the pixon image reconstruction technique. We discover hydrogen-rich mineralogy far from the poles, including ∼10 wt.% water equivalent hydrogen (WEH) on the flanks of the Tharsis Montes and >40 wt.% WEH at the Medusae Fossae Formation (MFF). The high WEH abundance at the MFF implies the presence of bulk water ice. This supports the hypothesis of recent periods of high orbital obliquity during which water ice was stable on the surface. We find the young undivided channel system material in southern Elysium Planitia to be distinct from its surroundings and exceptionally dry; there is no evidence of hydration at the location in Elysium Planitia suggested to contain a buried water ice sea. Finally, we find that the sites of recurring slope lineae (RSL) do not correlate with subsurface hydration. This implies that RSL are not fed by large, near-subsurface aquifers, but are instead the result of either small ( < 120 km diameter) aquifers, deliquescence of perchlorate and chlorate salts or dry, granular flows.

Evidence for explosive silicic volcanism on the Moon from the extended distribution of thorium near the Compton‐Belkovich Volcanic Complex

AbstractWe reconstruct the abundance of thorium near the Compton‐Belkovich Volcanic Complex on the Moon, using data from the Lunar Prospector Gamma Ray Spectrometer. We enhance the resolution via a pixon image reconstruction technique and find that the thorium is distributed over a larger (40km × 75 km) area than the (25km × 35 km) high‐albedo region normally associated with Compton‐Belkovich. Our reconstructions show that inside this region, the thorium concentration is 14–26ppm. We also find additional thorium, spread up to 300km eastward of the complex at ∼2 ppm. The thorium must have been deposited during the formation of the volcanic complex, because subsequent lateral transport mechanisms, such as small impacts, are unable to move sufficient material. The morphology of the feature is consistent with pyroclastic dispersal, and we conclude that the present distribution of thorium was likely created by the explosive eruption of silicic magma.

Expectation maximization for hard X-ray count modulation profiles

This paper is concerned with the image reconstruction problem when the measured data are solar hard X-ray modulation profiles obtained from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)} instrument. Our goal is to demonstrate that a statistical iterative method classically applied to the image deconvolution problem is very effective when utilized for the analysis of count modulation profiles in solar hard X-ray imaging based on Rotating Modulation Collimators. The algorithm described in this paper solves the maximum likelihood problem iteratively and encoding a positivity constraint into the iterative optimization scheme. The result is therefore a classical Expectation Maximization method this time applied not to an image deconvolution problem but to image reconstruction from count modulation profiles. The technical reason that makes our implementation particularly effective in this application is the use of a very reliable stopping rule which is able to regularize the solution providing, at the same time, a very satisfactory Cash-statistic (C-statistic). The method is applied to both reproduce synthetic flaring configurations and reconstruct images from experimental data corresponding to three real events. In this second case, the performance of Expectation Maximization, when compared to Pixon image reconstruction, shows a comparable accuracy and a notably reduced computational burden; when compared to CLEAN, shows a better fidelity with respect to the measurements with a comparable computational effectiveness. If optimally stopped, Expectation Maximization represents a very reliable method for image reconstruction in the RHESSI context when count modulation profiles are used as input data.

Spatial distribution of lunar polar hydrogen deposits after KAGUYA (SELENE)

[1] Although controversial in its physical form, there is mounting evidence of hydrogen enhancements at the lunar poles. The permanently shadowed locales are potential sites for significant concentrations of cold-trapped volatiles, including water ice. We derive maps of the lunar hydrogen distribution near the poles by applying a pixon image reconstruction algorithm to the Lunar Prospector epithermal neutron data coupled with a new map of cold trap locations derived from the KAGUYA (SELENE) altimetry measurements. The results presented in this article require the hydrogen to be concentrated into “cold traps.”

The spatial distribution of polar hydrogen deposits on the Moon

A new analysis of the Lunar Prospector epithermal neutron data is presented, providing an improved map of the distribution of hydrogen near to the lunar poles. This is achieved using a specially developed pixon image reconstruction algorithm to deconvolve the instrumental response of the Lunar Prospector's neutron spectrometer from the observed data, while simultaneously suppressing the statistical noise. The results show that these data alone require the hydrogen to be concentrated into the cold traps at up to 1 wt% water-equivalent hydrogen. This combination of localisation and high concentration suggests that the hydrogen is present either in the form of a volatile compound or as solar wind protons implanted into small regolith grains.

Multi-wavelength spatially resolved analysis of quasi-periodic pulsations in a solar flare

Aims. We aim to perform a spatially resolved analysis of a quasi-periodic pulsation event from 8th May 1998 using microwave data from the Nobeyama Radioheliograph and Radiopolarimeter, and X-ray data from the Yohkoh satellite. Methods. Time spectra of the signals integrated over the emission source are constructed with the use of the Lomb-Scargle periodogram method, revealing the presence of a pronounced 16 s periodicity. The Pixon image reconstruction algorithm and Hanaoka algorithm are used to reconstruct images from the hard X-ray data from Yohkoh/HXT and Nobeyama Radioheliograph respectively. The phase relationship of the microwave emission was analysed with the use of cross-correlation techniques. Results. The flaring loop was resolved in the microwave band. The hard X-ray sources are found to be located near the footpoint and at the loop apex determined by the soft X-ray image. The apex source is much fainter than footpoint one. In microwave, all parts of the loop are seen to oscillate with the same period and almost in phase. It was not possible to determine the spatial structure of the oscillation in the hard X-ray band. The period and the coherent spatial structure of the oscillation are indicative of the presence of either an MHD sausage mode or a periodic regime of magnetic reconnection

A speedy pixon image reconstruction algorithm

A speedy pixon algorithm for image reconstruction is described. Two applications of the method to simulated astronomical data sets are also reported. In one case, galaxy clusters are extracted from multiwavelength microwave sky maps using the spectral dependence of the Sunyaev-Zel'dovich effect to distinguish them from the microwave background fluctuations and the instrumental noise. The second example involves the recovery of a sharply peaked emission profile, such as might be produced by a galaxy cluster observed in X-rays. These simulations show the ability of the technique both to detect sources in low signal-to-noise data and to deconvolve a telescope beam in order to recover the internal structure of a source.