NOAA Polar Orbiting Environmental Satellites Research Articles

Short‐term variations of the inner radiation belt in the South Atlantic anomaly

AbstractLong‐term variations of South Atlantic anomaly (SAA) are generally derived by fitting a Gaussian‐like function to an averaged distribution of the proton flux at a certain altitude accumulated over time periods for a month or longer. These data do not show the short‐term variation of SAA arising from geomagnetic storm effects whose time scale is less than a month. To investigate the short‐term variations, the features of SAA for the high‐energy protons detected by NOAA Polar Orbiting Environmental Satellites during 1998–2008 have been investigated with a 5 day running average method. It is found that the two SAA parameters for three proton channels reflect the maximal proton flux in SAA and the extension of SAA decreases several percent during geomagnetic storms. Possible reasons for the decreases of the two SAA parameters for high‐energy protons are discussed. Proton losses at the outer boundary of the inner radiation belt can be explained by the field line curvature scattering mechanism, while the decrease of the proton flux near the center of SAA is probably caused by the enhanced neutral atmospheric density during geomagnetic storms. The study of the behavior of high‐energy protons in SAA is useful for understanding of storm time and long‐term variations of the radiation environment near Earth and for constructing dynamic radiation belt models.

A downward revision of a recently reported proton auroral LBH emission efficiency

[1] A significantly higher N2 Lyman-Birge-Hopfield (LBH) emission efficiency for auroral proton precipitation compared to model calculations was reported by Knight et al. (2008) based on a statistical study utilizing coincident far ultraviolet and particle data from the sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5) on board the DMSP satellite F16. Here, the quantity of interest from that study is the median ratio of LBH column emission rates (CERs) from SSUSI and derived from SSJ/5 spectra using monoenergetic emission yields. The median ratio was found to be 2.83 for proton aurora, suggesting the need for significant increases in currently used LBH proton/H-atom impact cross sections. A key step in their analysis was extrapolation of SSJ/5 spectra above 30 keV. Limited testing of this algorithm using NOAA Polar Orbiting Environmental Satellites Total Energy Detector and Medium Energy Proton and Electron Detector (TM) data found no significant bias. This work reports on a more detailed investigation of the algorithm's performance, also using TM data, and has uncovered a bias that reduces the median column emission rates (CER) ratio to 1.75. Within expected uncertainties, including calibration, this still calls for cross section increases but to a lesser extent. The discovered bias becomes apparent with CER thresholding that was overlooked during testing by Knight et al. (2008). Thresholding at 400 Rayleighs (R) is necessary since Knight et al. excluded CERs < 400 R in deriving their median ratio. We show that the algorithm's performance degrades with increasing energy flux of the precipitation. A method is reported for eliminating most of the bias which utilizes auroral Ly α, whose emission strength is closely coupled to spectral hardness.

Elliptical model of cutoff boundaries for the solar energetic particles measured by POES satellites in December 2006

Experimental data from a constellation of five NOAA Polar Orbiting Environmental Satellites (POES), satellites were used for studying the penetration of solar energetic particles (SEP) to high latitudes during long‐lasting SEP events on 5–15 December 2006. We determined cutoff latitudes for electrons with energies &gt;100 keV and &gt;300 keV and for protons with energies from 240 keV to &gt;140 MeV. The large number of satellites allowed us to derive snapshots of the cutoff boundaries with 1‐hour time resolution. The boundaries were fitted well by ellipses. On the basis of the elliptical approach, we developed a model of cutoff latitudes for protons and electrons in the northern and southern hemispheres. The cutoff latitude is represented as a function of rigidity, R, of particles; MLT, geomagnetic indices Dst, Kp, and AE; and dipole tilt angle PS. The model predicts tailward and duskward shifting of the cutoff boundaries in relation to intensification of the cross‐tail current, field‐aligned currents, and symmetrical and asymmetrical parts of the ring current. The model was applied for prediction of polar cap absorption (PCA) effects observed at high latitudes by the Canadian Advanced Digital Ionosonde network of ionosondes. It was found that the PCA effects are related mainly to intense fluxes of &gt;2.5 MeV protons and &gt;100 keV electrons, which contribute mostly to the ionization of ionospheric D‐layer at altitudes of ∼75 to 85 km. This finding was confirmed independently by FORMOSAT‐3/COSMIC observations of the SEP‐associated enhancements of electron content at altitudes of ∼80 km.

Origin of energetic electron precipitation &gt;30 keV into the atmosphere

Energetic electrons are deposited into the atmosphere from Earth's inner magnetosphere, resulting in the production of odd nitrogen (NOx). During polar night, NOx can be transported to low altitudes, where it can destroy ozone, affecting the atmospheric radiation balance. Since the flux of energetic electrons trapped in the magnetosphere is related to solar activity, the precipitation of these electrons into Earth's atmosphere provides a link between solar variability and changes in atmospheric chemistry which may affect Earth's climate. To determine the global distribution of the precipitating flux, we have built a statistical model binned by auroral electrojet (AE) index, magnetic local time (MLT), and L shell of E &gt; 30 keV precipitating electrons from the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA Polar Orbiting Environmental Satellites (POES) low‐altitude satellites NOAA‐15, NOAA‐16, NOAA‐17, and NOAA‐18. We show that the precipitating flux increases with geomagnetic activity, suggesting that the flux is related to substorm activity. The precipitating fluxes maximize during active conditions where they are primarily seen outside of the plasmapause on the dawnside. The global distribution of the precipitating flux of E &gt; 30 keV electrons is well‐correlated with the global distribution of lower‐band chorus waves as observed by the plasma wave experiment onboard the Combined Release and Radiation Effects Satellite (CRRES) satellite. In addition, the electron precipitation occurs where the pitch angle diffusion coefficient due to resonant interaction between electrons and whistler mode chorus waves is high, as calculated using the pitch angle and energy diffusion of ions and electrons (PADIE) code. Our results suggest that lower‐band chorus is very important for scattering &gt;30 keV electrons from Earth's inner magnetosphere into the atmosphere.

Long-term observations of the trapped high-energy proton population ( L < 4) by the NOAA Polar Orbiting Environmental Satellites (POES)

The Space Environment Monitor (SEM) onboard the NOAA POES satellites has been measuring the near-Earth charged particle environment since 1978, providing an extensive database that can be used for studying the long-term behavior of this population of trapped particles. POES stands for Polar Orbiting Environmental Satellite. These satellites orbit at ∼840 km altitude and at an inclination of 98°. The SEM-1 instrument was flown on the POES satellites beginning in 1978 with TIROS-N and NOAA-6 in 1979 and continuing to NOAA-14 launched in 1995 with the exception of NOAA-9 and NOAA-11 (NOAA-13 failed shortly after launch). Its replacement, SEM-2, has flown on the POES NOAA-15, -16, and -17 satellites (from 1998). Here we present the results of a study on the statistical variations of the high-energy trapped proton environment. Among the detectors in SEM, the four SEM-2 omni-directional proton detectors for energies >16 MeV, >36 MeV, >70 MeV, and >140 MeV provide the data most relevant to this study. For this study, the SEM-2 omni-directional detector data from each satellite (NOAA-15, -16, and -17) were accumulated over 8-s intervals and arranged/collected on a 1° latitude by 2° longitude grid. The data set covers the period between June 1, 1998 and June 30, 2005. This amounts to about 15 satellite-years of data encompassing much of a solar cycle. The spatial grid covers sub-satellite locations at latitude center values from 65.5° north to 72.5° south and longitude center values from 1° east to 359° east. Typically there are 1700–2100 individual data entries per detector within each longitude–latitude bin. For each spatial bin, we also computed for detailed analysis various geomagnetic field parameters (e.g., McIlwain’s L value, local field strength, field strength at the equator along the field line, etc.) for the field line threading the center of the bin. For this purpose, we used the International Geomagnetic Reference Field (IGRF) Model, Epoch 2003.5. Each data sample within the spatial bin also contains the information on geomagnetic activity and an index indicating whether a Solar Energetic Particle (SEP) event was in progress at the time the data were taken and the magnitude of that event. The final data set thus obtained forms an extensive source of information useful for studying the long-term near-Earth trapped proton environment as a function of solar cycle, magnetic activity and the role of SEP events.

Global 30–240 keV proton precipitation in the 17–18 April 2002 geomagnetic storms: 3. Impact on the ionosphere and thermosphere

NOAA Polar Orbiting Environmental Satellites 30–240 keV proton precipitation measurements in the 17–18 April 2002 geomagnetic storms are used with a Monte Carlo ion transport model to obtain ionization and heating rates that are subsequently fed into the Global Ionosphere Thermosphere Model to investigate the proton impact on the ionosphere and thermosphere. Simulation results show that after the addition of proton precipitation in a moderate geomagnetic storm (specifically, the one during mid‐April 2002), there are places at low altitudes (100–120 km) on the nightside undergoing significant increases in electron and nitric oxide (NO) densities. The enhancement can be as large as several factors or even by an order of magnitude. Moreover, the temporal profiles of the enhancement in ionospheric electron densities demonstrate a direct correlation with proton precipitation imposed on the topside boundary, and there is no integral effect. This is in contrast with a continuous buildup process illustrated in the time variation of the thermospheric NO density enhancement because NO at these altitudes has a long lifetime. In addition, by including high‐energy precipitating protons in a global ionosphere thermosphere coupled model, significant changes take place in the ion convection (locally around ±20%) and in the neutral winds (locally around ±40%). This study represents the first attempt to understand the global influence of proton precipitation on the ionosphere and thermosphere using in situ observational data.

Clouds as Seen by Satellite Sounders (3I) and Imagers (ISCCP). Part III: Spatial Heterogeneity and Radiative Effects

Abstract Their relatively good spectral resolution makes infrared sounders very useful for the determination of cloud properties (day and night), and their coarse spatial resolution has less effect on clouds with large spatial extents like cirrus clouds. The Improved Initialization Inversion (3I) algorithms convert TIROS-N Operational Vertical Sounder observations from the NOAA Polar Orbiting Environmental Satellites into atmospheric temperature and humidity profiles and into cloud and surface properties. On the other hand, the relatively high spatial resolution of the imagers from the geostationary and polar orbiting satellites used in the International Satellite Cloud Climatology Project (ISCCP) is important for the determination of properties of clouds with smaller spatial extents like boundary layer clouds. By combining these quite different datasets some insight into the behavior of retrieved cloud properties with spatial heterogeneity is gained. The effective cloud amount as determined by 3I and ISC...