Solar energetic particle (SEP) events result from solar flares or from shock waves associated with Coronal Mass Ejections. The particles consist of both ions and electrons that have relativistic energies and reach Earth within minutes to a few hours of the event that initially generated the particles. Because of their very high energies and the speed with which they reach the Earth, these events represent significant space weather hazards. Within Earth's magnetosphere there is often a large difference between the eastward and westward fluxes of the energetic particles. This project will generate a physics-based, quantitative model of the particle fluxes within the magnetosphere at locations where direct observations are unavailable.
The project will utilize global test particle simulations within an empirical magnetosphere mdoel that is driven by the solar wind parameters. The simulations will investigate the electric current systems within the magnetosphere that lead to variations in the geomagnetic cutoffs of the particles. Data from the GOES geosynchronous satellites will be used to determine the magnitude of the east-west asymmetry. It will establish statistical relationships between the state of the solar wind and the relative magnitudes of the eastward and westward particle fluxes.
PIs: Brian T. Kress1 and Juan V. Rodriguez2 1 Department of Physics and Astronomy, Dartmouth College, Hanover, NH. 2 NOAA Space Weather Prediction Center, Boulder CO. Solar and galactic cosmic rays are a space weather hazard posing risks to manned and robotic space flight missions. A solar particle event can expose astronauts, airline crews and passengers to increased levels of harmful radiation [Dyer et al., 2003]. Cosmic rays are also damaging to electronic equipment in space and at high altitudes, causing degradation to microelectronics and single event upsets (SEUs) in electronic systems [Tylka et al., 1996]. The overall goal of the research funded by this grant is to characterize and model solar energetic particle (SEP) access to the near-Earth space environment, to better mitigate their adverse effects on humans and technological systems. In low and mid-latitudes, the Earth’s magnetic field provides some shielding of cosmic rays. Access of cosmic rays to the Earth's magnetosphere is quantified in terms of cutoff rigidity (momentum per unit charge). Cutoff rigidity refers to the threshold rigidity below which the SEP flux is geomagnetically shielded. Numerically computed grids of cutoff rigidities are used to model SEP flux in Earth's upper atmosphere and in low Earth orbit (LEO) [Smart and Shea, 2003]. One example is the Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) model, currently running at the NASA Langley Research Center [Mertens et al., 2010; Kress et al., 2010]. The NAIRAS model is a prototype operational model that provides global, real-time, data-driven predictions of atmospheric ionizing radiation exposure for assessing radiation levels at commercial airline altitudes. In LEO there is little variation in SEP cutoffs with direction of arrival, and vertical cutoff rigidities (referring to arrival from the zenith direction) provide a good approximation for cosmic ray flux incident from all directions [Smart et al., 2006]. At geosynchronous however, due to their large gyroradii, MeV ions incident from the east must penetrate significantly further into the magnetosphere than ions arriving from the west. Cutoff rigidity is higher and observed flux lower for SEPs arriving from the east, while cutoff rigidity is lower and observed flux higher for SEPs arriving from the west. Variations in the east–west SEP flux ratio observed by GOES Energetic Particle Sensors (EPS) have recently been reported by Rodriguez et al. [2010]. In NOAA’s operational processing of EPS count rates into differential fluxes, the differential flux is treated as isotropic over the detector’s field of view (FOV). At geosynchronous, to fully characterize the total incident radiation, the variation in cutoff over direction of arrival must be accounted for. This is illustrated in Figure 1 showing numerically modeled cutoffs over the fields of view of the GOES EPS eastward and westward facing detectors. The work performed for this project generalizes techniques used for modeling SEPs in LEO to include the effect of directional anisotropy needed for modeling SEPs at higher altitudes. Incident SEP flux modeled using geomagnetic cutoffs is directly compared with EPS data by integrating the modeled flux over the detector FOV, folding in the directionally dependent cutoff rigidity and the directional response function of the detector. Selected results from this study are shown in Figure 2. The left panel shows the modeled and observed GOES EPS 8.7-14.5 MeV fluxes. Here cutoffs were modeled using the Tsyganenko and Sitnov [2005] geomagnetic field model. As found in previous studies [Kahler and Ling, 2001; Kress et al., 2010], the numerically modeled cutoffs are in general too high. However, the variations in the modeled fluxes follow the observed variations well during the later half of the simulated period, which is characterized by periodic substorm activity driven by fluctuating interplanetary magnetic field z-component. It is found that ~40-60% reductions of the modeled cutoffs produces the best agreement between modeled and observed fluxes, shown in the right panel of Figure 2. Growing dependence on spacecraft in middle Earth orbit (MEO), and at geosynchronous has increased the need to extend cutoff models throughout the inner magnetosphere. The work performed for this project generalizes techniques used for modeling SEPs in LEO to higher altitudes. Results from this work will directly contribute to the development of models used for monitoring and forecasting space weather hazards, which aid in safeguarding space-based assets such as telecommunication and GPS satellites.
