A generalized approach to model the spectra and radiation dose rate of solar particle events on the surface of Mars

For future human missions to Mars, it is important to study the surface radiation environment during extreme and elevated conditions. In the long term, it is mainly Galactic Cosmic Rays (GCRs) modulated by solar activity that contributes to the radiation on the surface of Mars, but intense solar energetic particle (SEP) events may induce acute health effects. Such events may enhance the radiation level significantly and should be detected as immediately as possible to prevent severe damage to humans and equipment. However, the energetic particle environment on the Martian surface is significantly different from that in deep space due to the influence of the Martian atmosphere. Depending on the intensity and shape of the original solar particle spectra as well as particle types, the surface spectra may induce entirely different radiation effects. In order to give immediate and accurate alerts while avoiding unnecessary ones, it is important to model and well understand the atmospheric effect on the incoming SEPs including both protons and helium ions. In this paper, we have developed a generalized approach to quickly model the surface response of any given incoming proton/helium ion spectra and have applied it to a set of historical large solar events thus providing insights into the possible variety of surface radiation environments that may be induced during SEP events. Based on the statistical study of more than 30 significant solar events, we have obtained an empirical model for estimating the surface dose rate directly from the intensities of a power-law SEP spectra

Teilchendirektionalität und andere Aspekte der Strahlungsumgebung auf dem Mars

Mars as a future target of manned exploration is a place of particular interest in the solar system. Both the past and the present habitability of Mars, and the safety of astronauts working on the Martian surface, depend, among other factors, on the ionizing radiation present on its surface. As part of the Mars Science Laboratory mission, the Radiation Assessment Detector (RAD). One of the scientific goals being addressed by the instrument is to aid in the validation of particle transport models used to simulate the radiation environment on the Martian surface. This thesis aims to address this goal in particular. Global dust storms are a meteorological feature that is unique to Mars. These storms produce a striking difference in the appearance of the planet, obscuring almost all of its surface beneath a dense layer of dust. In order to predict the changes in radiation environment, simulations comparing a global dust storm to normal atmospheric conditions are performed. Additionally, an attempt is made to validate the results of the simulations using a period of enhanced dust activity that is included in the RAD measurement time. The Martian radiation environment is dominated by downward particle fluxes of Galactic Cosmic Rays and the secondary particles produced by them. Near the Martian surface, an upward-directed component composed of secondary particles produced in the Martian soil adds to the total radiation environment. In order to fully characterize the radiation environment, this upward component must be understood as well. A method for discriminating particle directionality in RAD for charged particles is developed through simulations and validated through instrument observations. The RAD instrument is unable to determine directionality for neutrons due to its design. However, since neutrons are important for dosimetry, an instrument design capable of measuring neutron directionality is described and its basic capabilities assessed.Im Sonnensystem ist der Mars als Ziel späterer bemannter Missionen von besonderem Interesse. Die Kenntnis der ionisierenden Strahlung auf dem Mars ist sowohl für die Beurteilung der früheren und heutigen Habitabilität als auch für die Sicherheit zukünftiger Astronauten von großer Bedeutung. Im Rahmen der Mission Mars Science Laboratory mißt das Instrument Radiation Assessment Detector (RAD). Eines der wissenschaftlichen Ziele des Instrumentes ist die Validierung von Teilchentransportmodellen. Dieses Ziel wird in der vorliegenden Arbeit vorrangig verfolgt. Globale Staubstürme sind ein Phänomen, das ausschließlich auf dem Mars vorkommt. Solche Stürme verursachen markante Veränderungen des Planeten, da nahezu die gesamte Oberfläche durch eine dichte Staubschicht verdeckt wird. Um Vorhersagen über diesen Einfluss machen zu können, wurden Simulationen, die einen globalen Staubsturm mit normalen Atmosphärenbedingungen vergleichen, durchgeführt. Zusätzlich wurde ein Versuch unternommen, die Simulationsergebnisse anhand einer innerhalb der RAD-Messungen liegenden Zeit mit erhöhter Staubaktivität zu validieren. Die Strahlungsumgebung auf dem Mars wird durch abwärts gerichtete Teilchenflüsse, Galactic Cosmic Rays und ihre Sekundärteilchen, dominiert. Nahe der Oberfläche existiert zusätzlich eine aufwärts gerichtete Komponente aus innerhalb des Bodens produzierten Sekundärteilchen. Um die Strahlungsumgebung vollständig zu beschreiben, ist es notwendig, diese aufwärts gerichtete Komponente zu verstehen. Hier wurde eine Methode zur Unterscheidung der Teilchenrichtung geladener Teilchen im RAD anhand von Simulationsdaten entwickelt und durch Observationsdaten validiert. Aufgrund seiner Konstruktion ist RAD nicht in der Lage, die Richtung von Neutronen zu messen. Da sie aber für die Dosimetrie relevant sind, wurde ein Instrument, das dazu in der Lage ist, vorgestellt und seine grundlegenden Eigenschaften beurteilt

The Martian Surface Radiation Environment- A Comparison of Models and MSL/RAD Measurements

Context: The Radiation Assessment Detector (RAD) on the Mars Science Laboratory (MSL) has been measuring the radiation environment on the surface of Mars since August 6th 2012. MSL-RAD is the first instrument to provide detailed information about charged and neutral particle spectra and dose rates on the Martian surface, and one of the primary objectives of the RAD investigation is to help improve and validate current radiation transport models. Aims: Applying different numerical transport models with boundary conditions derived from the MSL-RAD environment the goal of this work was to both provide predictions for the particle spectra and the radiation exposure on the Martian surface complementing the RAD sensitive range and, at the same time, validate the results with the experimental data, where applicable. Such validated models can be used to predict dose rates for future manned missions as well as for performing shield optimization studies. Methods: Several particle transport models (GEANT4, PHITS, HZETRN/OLTARIS) were used to predict the particle flux and the corresponding radiation environment caused by galactic cosmic radiation on Mars. From the calculated particle spectra the dose rates on the surface are estimated. Results: Calculations of particle spectra and dose rates induced by galactic cosmic radiation on the Martian surface are presented. Although good agreement is found in many cases for the different transport codes, GEANT4, PHITS, and HZETRN/OLTARIS, some models still show large, sometimes order of magnitude discrepancies in certain particle spectra. We have found that RAD data is helping to make better choices of input parameters and physical models. Elements of these validated models can be applied to more detailed studies on how the radiation environment is influenced by solar modulation, Martian atmosphere and soil, and changes due to the Martian seasonal pressure cycle. By extending the range of the calculated particle spectra with respect to the experimental data additional information about the radiation environment is gained, and the contribution of different particle species to the dose is estimated

Modeling the variations of Dose Rate measured by RAD during the first MSL Martian year: 2012-2014

The Radiation Assessment Detector (RAD), on board Mars Science Laboratory's (MSL) rover Curiosity, measures the {energy spectra} of both energetic charged and neutral particles along with the radiation dose rate at the surface of Mars. With these first-ever measurements on the Martian surface, RAD observed several effects influencing the galactic cosmic ray (GCR) induced surface radiation dose concurrently: [a] short-term diurnal variations of the Martian atmospheric pressure caused by daily thermal tides, [b] long-term seasonal pressure changes in the Martian atmosphere, and [c] the modulation of the primary GCR flux by the heliospheric magnetic field, which correlates with long-term solar activity and the rotation of the Sun. The RAD surface dose measurements, along with the surface pressure data and the solar modulation factor, are analysed and fitted to empirical models which quantitatively demonstrate} how the long-term influences ([b] and [c]) are related to the measured dose rates. {Correspondingly we can estimate dose rate and dose equivalents under different solar modulations and different atmospheric conditions, thus allowing empirical predictions of the Martian surface radiation environment

Insufficient insulin administration to diabetic rats increases substrate utilization and maintains lactate production in the kidney

Good glycemic control is crucial to prevent the onset and progression of late diabetic complications, but insulin treatment often fails to achieve normalization of glycemic control to the level seen in healthy controls. In fact, recent experimental studies indicate that insufficient treatment with insulin, resulting in poor glycemic control, has an additional effect on progression of late diabetic complications, than poor glycemic control on its own. We therefore compared renal metabolic alterations during conditions of poor glycemic control with and without suboptimal insulin administration, which did not restore glycemic control, to streptozotocin (STZ)‐diabetic rats using noninvasive hyperpolarized (13)C‐pyruvate magnetic resonance imaging (MRI) and blood oxygenation level–dependent (BOLD) (1)H‐MRI to determine renal metabolic flux and oxygen availability, respectively. Suboptimal insulin administration increased pyruvate utilization and metabolic flux via both anaerobic and aerobic pathways in diabetic rats even though insulin did not affect kidney oxygen availability, HbA(1c), or oxidative stress. These results imply direct effects of insulin in the regulation of cellular substrate utilization and metabolic fluxes during conditions of poor glycemic control. The study demonstrates that poor glycemic control in combination with suboptimal insulin administration accelerates metabolic alterations by increasing both anaerobic and aerobic metabolism resulting in increased utilization of energy substrates. The results demonstrate the importance of tight glycemic control in insulinopenic diabetes, and that insulin, when administered insufficiently, adds an additional burden on top of poor glycemic control