Monday, 7 January 2013
Exhibit Hall 3 (Austin Convention Center)
Energetic heavy ions of solar origin can adversely affect a variety of human operations including but not limited to aircraft, satellites, and spacecraft. In addition, energetic heavy ions can pose a threat to human health, particularly for astronauts beyond Earth's protective atmospheric layer. NOAA has monitored solar energetic particles (protons and alpha particles) since 1975 from geostationary orbit. The Energetic Heavy Ion Sensor (EHIS), which is part of the GOES-R Space Environment In-Situ Suite (SEISS), will augment this longstanding capability. In support of NOAA's goal of a Weather Ready Nation, EHIS will measure elemental ion fluxes from hydrogen (atomic number = 1) to nickel (atomic number = 28) in energy ranges capable of penetrating normal satellite shielding and causing single event effects in electronics. The NESDIS National Geophysical Data Center (NGDC) will derive a Level 2 product from EHIS fluxes in which the observed spectra of flux versus energy are transformed into spectra of flux versus linear energy transfer (LET) or stopping power in silicon. An LET spectrum summed over all energetic ion species is more useful to the spacecraft design and testing communities than energy spectra by particle type. Iron ions are of particular concern because they typically dominate the high end of the LET spectrum up to ~30 MeV cm2 g-1.
In support of the development and testing of the Level 2 LET algorithm, we have developed a proxy data set of energetic heavy ions from measurements by the Solar Isotope Spectrometer (SIS) aboard the Advanced Composition Explorer (ACE) satellite. ACE is located at the L1 Lagrangian point; the heavy ion fluxes measured by GOES-R are expected to be similar to those observed at L1, particularly at higher energies. SIS measures fourteen elements from helium to nickel in eight energy bands that vary by element but generally range from 4 to 150 MeV/nucleon. The upper four SIS energy bands overlap the lower part of the EHIS energy range. The observed energy spectra are fit to a three-parameter expression based on the solution of a differential equation describing energy-dependent diffusion in a solar flare or interplanetary shock plasma (Mazur et al., Ap. J., 401, 398-410, 1992). This physics-based expression fits well most of the observed spectra for all fourteen elemental ions. The resultant fits are used to interpolate to the EHIS channels that overlap with the SIS energy coverage and, when the fit is of sufficient quality, to extrapolate to higher-energy EHIS channels. The proxy data set will be described in detail, and applications of this proxy data set to the EHIS Level 2 product will be shown.
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