On two methods of statistical image analysis

The computerized brain atlas (CBA) and statistical parametric mapping (SPM) are two procedures for voxel-based statistical evaluation of PET activation studies. Each includes spatial standardization of image volumes, computation of a statistic, and evaluation of its significance. In addition, smoothing and correcting for differences of global means are commonly performed in SPM before statistical analysis. We report a comparison of methods in an analysis of regional cerebral blood flow (rCBF) in 10 human volunteers and 10 simulated activations. For the human studies, CBA or linear SPM standarization methods were followed by smoothing and computation of a statistic with the paired t-test of CBA or general linear model of SPM. No standardization, linear, and nonlinear SPM standardization were applied to the simulations. Significance of the statistic was evaluated using the cluster-size method common to SPM and CBA. SPM employs the theory of Gaussian random fields to estimate the cluster size distributions; simulations described in the Appendix provided empirical distributions derived from t-maps. The quantities evaluated were number and size of functional regions (FRs), maximum statistic, average resting rCBF, and percentage change. For the simulations, the efficiency of signal detection and rate of false positives could be evaluated as well as the distributions of statistics and cluster size in the absense of signal. The similarity of the results yielded by similar methods of analysis for the human studies and the simulated activations substantiates the robustness of the methods for selecting functional regions. However, the analysis of simulated activations demonstrated that quantitative evaluation of significance of a functional region encounters important obstacles at every stage of the analysis. (C) 1999 Wiley-Liss, inc

Dependence of Dayside Electron Densities at Venus on Solar Irradiance

The ionosphere of Venus is a weakly ionized plasma layer embedded in the planet's upper atmosphere. Planetary ionospheres provide an excellent opportunity to study how our variable Sun affects the planets in our solar system. Because ionospheres are reservoirs from which atmospheric species can be lost to space, studying how ionospheres respond to changes in solar activity can help us understand how planetary atmospheres have evolved since their formation. While variations of the main and lower ionospheric peaks of Venus have been well studied, the behavior of the ionosphere above the altitude of the greatest electron density has not been fully constrained. To investigate the behavior of this region, we use electron density profiles obtained by the Venus Radio Science experiment aboard Venus Express. An increase in the response of the electron density to increasing solar irradiance with increasing altitude above the peak is readily apparent in these data. By using a one-dimensional photochemical equilibrium model to investigate the factors that drive the variations of the ionosphere of Venus, we find that changes in the composition of the underlying neutral atmosphere are responsible for the observed increase in ionospheric response with altitude

Sulfuric acid vapor and sulfur dioxide in the atmosphere of Venus as observed by the Venus Express radio science experiment VeRa

The Venus Express radio science experiment VeRa provided more than 900 neutral atmospheric profiles between the years 2006 and 2014. About 800 of these could be used for an analysis of the radio signal absorption at X-Band (wavelength: 3.6 cm), which is mainly caused by sulfuric acid vapor within the Venus atmosphere. The absorptivity profiles were converted into sulfuric acid vapor profiles. The combined measurements from the entire Venus Express mission reveal a distinct latitudinal H2SO4(g) variation. A latitudinal gradient can be observed at the topside of the H2SO4(g) layer, which is located approx. 4 km higher at equatorial latitudes compared to polar latitudes. Regions of enhanced sulfuric acid vapor abundance were found at equatorial and polar latitudes. The highest H2SO4(g) values at equatorial latitudes show mean maximal values of more than 12 ppm at around 47 km altitude. At polar latitudes mean maximal values were found at around 43 km altitude and ranged from 9 to 12 ppm. Both latitudinal regions of increased sulfuric acid vapor abundance are clearly separated by a low abundance region located at mid-latitudes with values of 5 to 7 ppm. A simplified two-dimensional transport model was developed to study the formation processes of sulfuric acid vapor accumulation at equatorial and polar latitudes. It turned out that the H2SO4(g) accumulation observed at high latitudes can be explained by precipitation of H2SO4(l) droplets that evaporate into gaseous sulfuric acid upon entering lower (warmer) altitudes. The influence of wind transport on this formation process was minor. In contrast, the H2SO4(g) accumulation observed at equatorial latitudes could be reproduced in the model by oppositely directed mass transport (upward winds and sedimentation) as well as by simplified evaporation and condensation processes. The low H2SO4(g) abundance observed at mid-latitudes was reproduced by downward winds in the model calculations. The VeRa observations were additionally used to estimate the abundance of SO2 above the cloud bottom. A latitudinal dependence was found with highest values of 90 +/- 60 ppm at equatorial latitudes, compared to 150 +/- 50 ppm and 160 +/- 50 ppm at southern and northern polar latitudes, respectively. Both the equatorial and polar regions displayed show large variability of the H2SO4(g) and SO2 abundances from observation to observation. A weak tidal influence is also visible in the sulfuric acid vapor abundance in the equatorial region. The northern polar H2SO4(g) abundance, as well as the southern and northern SO2 abundances, exhibit distinct long-term variations