Physics

1] Ultra-low-frequency (ULF) field line resonances can be used to infer the mass density along magnetospheric magnetic field lines. By specifying how mass density is distributed along the magnetic field (usually a power law as a function of distance from the Earth) and a dipole magnetic field geometry, the MHD standing wave equation can be analytically solved and mass density inferred from observed field line eigenfrequencies. However, the geometry of the Earth's magnetic field can deviate significantly from a dipole, even at relatively low L shells and on the dayside magnetosphere. This study investigates the importance of including a realistic magnetic field geometry when computing plasma mass density from observed field line eigenfrequencies. A generalized version of the toroidal mode MHD standing wave equation is solved using the Tsyganenko (2002a, 2002b) empirical magnetic field model (T01). The results are compared to those found using a dipole. We find that assuming a dipole magnetic field geometry results in an overestimation of mass density. The overestimation is larger for more disturbed levels of geomagnetic activity. Our results have important implications for the inference of heavy ions in the magnetosphere. Namely, an increase in heavy ion concentration as a result of enhanced geomagnetic activity will be exaggerated unless the proper magnetic field geometry is taken into account when calculating mass density from field line eigenfrequencies. Citation: Berube, D., M. B. Moldwin, and M. Ahn (2006), Computing magnetospheric mass density from field line resonances in a realistic magnetic field geometry,

1] Plasmaspheric plumes have ionospheric signatures and are observed as stormenhanced density (SED) in global positioning system (GPS) total electron content (TEC). These ionospheric signatures have been primarily observed over the American sector and in a few limited examples over the European sector. This study examines the longitudinal occurrence frequency of plasmaspheric plumes. We analyzed all images from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) Extreme Ultraviolet Imager (EUV) databases for the first half of 2001 and identified a total of 31 distinct plume intervals observed during different storm events. Out of the total IMAGE EUV plumes that we identified, 12 were projected over North America, 10 over Asia, and the remaining 9 were over Europe and the Atlantic Ocean. Using ground-based GPS TEC from MIT's Madrigal database, we searched for corresponding SED/TEC plumes at different longitudinal sector and found 12 ionospheric SED plume signatures over North America, 4 over Europe, and 2 over Asia. This indicates that the observation probability of an ionospheric SED plume when a plasmaspheric plume is seen is 100% in the American sector, 50% in the European sector, and 20% in the Asian sector. This could be due to the fact that the plumes may be either positioned beyond the limit of the ground-based GPS field of view, which happens mainly when there is less plasmaspheric erosion, or are too weak to be detected by the sparse number of GPS receivers over Asia. The in situ plasma densities from the available coincident defense metrological satellite program (DMSP) satellites were also used to study the characteristics of SED/TEC plume at DMSP orbiting altitude (i.e., $870 km). The TOPographic EXplorer (TOPEX) altimeter TEC also is used to identify the conjugate SED/plume signature over the Southern Hemisphere.

The geosynchronous GOES 5 and GOES 6 satellites frequently observe transient events marked by magnetic field strength increases and bipolar magnetic field signatures lasting several minutes. In this study we report a survey of 87 events observed simultaneously by both GOES spacecraft (for a total of 174 individual observations) from August to December 1984. Events detected in the prenoon sector outnumbered those in the postnoon sector by about a 3 to 1 ratio. The distribution of the events versus local time exhibited a significant prenoon peak like the distribution of magnetic impulse events observed in high-latitude ground magnetometers. A cross-correlation analysis of the two GOES data sets indicated lags that range from 0 to over 2 min, with the majority of the events moving antisunward. The short lags correspond to azimuthal speeds of hundreds of kilometers per second, greater than flow speeds in the magnetosheath, but less than fast mode waves. The short lags may indicate that the events move primarily latitudinally and/or that transient events are seldom localized, but rather occur over extended, if not global, regions. Investigations of event occurrence versus interplanetary magnetic field (IMF) Bz, event motion versus IMF By, and correspondence between upstream plasma data and the events all indicate that pressure pulses are the likely source of many of the events. About 27% of the events with simultaneous solar wind data were preceded by sharp reversals in one or more IMF components, and nearly all of this particular group of events occurred in the dawn sector. This suggests that the pressure pulses may be commonly generated in the foreshock/bow shock region, since the prenoon magnetopause lies generally behind the quasi-parallel bow shock where such pulses are thought to be triggered by IMF discontinuities. Finally, several events in the data set were also observed by the AMPTE/CCE. These are presented as case studies.

Twenty-six different plasmaspheric plumes were identified during 2001 from the IMAGE EUV database. Simultaneously the two-dimensional structure of global GPS TEC (total electron content) data has been imaged using several sources: JPL's interpolated Global Ionosphere Maps (GIM) and MIT's Madrigal GPS TEC data. IONEX 2-hour averaged TEC maps were also examined, but they were not helpful in verifying the transient plumes' signatures. Both model and measurement data have observed plumes of greatly elevated GPS TEC, which is associated with the erosion of the outer plasmasphere by strong sub-auroral polarization (SAP) electric fields, even in regions other than North America. We mapped the plasmapause and plume position, extracted from IMAGE EUV data, to the equatorial plane and compared it with GPS TEC maps. GIM data were able to show sharp changes in TEC gradient, but did not have enough resolution to agree with plumes anywhere other than above North America. Generally, we found reasonably good agreements between these independent observations; In particular the comparison with Madrigal GPS TEC was excellent at different longitudinal sectors.

1] The first empirical model of the equatorial mass density of the plasmasphere is constructed using ground-based ULF wave diagnostics. Plasmaspheric mass density between L = 1.7 and L = 3.2 has been determined using over 5200 hours of data from pairs of stations in the MEASURE array of ground magnetometers. The least squares fit to the data as a function of L shows that mass density falls logarithmically with L. Average ion mass as a function of L is also estimated by combining the mass density model with plasmaspheric electron density profiles determined from the IMAGE Radio Plasma Imager (RPI). Additionally, we use the RPI electron density database to examine how the average ion mass changes under different levels of geomagnetic activity. We find that average ion mass is greatest under the most disturbed conditions. This result indicates that heavy ion concentrations (percent by number) are enhanced during large geomagnetic disturbances. We also find that the average ion mass increases with increasing L (below L = 3.2), indicating the presence of a heavy ion torus during disturbed times. Heavy ions must play an important role in storm-time plasmaspheric dynamics. The average ion mass is also used to constrain the concentrations of He + and O + . Estimates of the He + concentration determined this way may be useful for interpreting IMAGE Extreme Ultraviolet Imager (EUV) images. Citation: Berube, D., M. B. Moldwin, S. F. Fung, and J. L. Green (2005), A plasmaspheric mass density model and constraints on its heavy ion concentration,

1] During the early main phase of a geomagnetic storm on 11 April 2001, the Polar satellite was inside the magnetosphere in the prenoon sector ($1000-1100 magnetic local times) and experienced a magnetopause crossing at L % 6 because of the high solar wind dynamic pressure and strong southward interplanetary magnetic field (IMF). Just before the magnetopause crossing, Polar observed cold, dense plasma. That is, the cold, dense plasma was immediately adjacent to the compressed magnetopause. Using simultaneous observations by the IMAGE extreme ultraviolet (EUV) imager, we confirm that the cold, dense plasma observed by Polar is a plasmaspheric drainage plume extending outward from the plasmasphere to the magnetopause during the interval of high geomagnetic activity and strong southward IMF. We compare plasmaspheric mass densities determined from ground magnetometer data at L = 2.3 and 2.9 for a magnetically quiet time interval to mass densities determined during the magnetic storm time interval. We find no significant differences in the mass density between both intervals. These observations suggest that the sunward-convecting plasmaspheric plasma observed at Polar is due to erosion of the outer layers of the plasmasphere beyond L = 2.9.

1] Ultra-low-frequency (ULF) field line resonances can be used to infer the mass density along magnetospheric magnetic field lines. By specifying how mass density is distributed along the magnetic field (usually a power law as a function of distance from the Earth) and a dipole magnetic field geometry, the MHD standing wave equation can be analytically solved and mass density inferred from observed field line eigenfrequencies. However, the geometry of the Earth's magnetic field can deviate significantly from a dipole, even at relatively low L shells and on the dayside magnetosphere. This study investigates the importance of including a realistic magnetic field geometry when computing plasma mass density from observed field line eigenfrequencies. A generalized version of the toroidal mode MHD standing wave equation is solved using the Tsyganenko (2002a, 2002b) empirical magnetic field model (T01). The results are compared to those found using a dipole. We find that assuming a dipole magnetic field geometry results in an overestimation of mass density. The overestimation is larger for more disturbed levels of geomagnetic activity. Our results have important implications for the inference of heavy ions in the magnetosphere. Namely, an increase in heavy ion concentration as a result of enhanced geomagnetic activity will be exaggerated unless the proper magnetic field geometry is taken into account when calculating mass density from field line eigenfrequencies. Citation: Berube, D., M. B. Moldwin, and M. Ahn (2006), Computing magnetospheric mass density from field line resonances in a realistic magnetic field geometry,

Using a two dimensional image of the Earth's plasmasphere taken by the Extreme Ultraviolet Imager (EUV) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft, in-situ electron density measurements from the IM-AGE Radio Plasma Imager (RPI) instrument, measurements of magnetospheric mass density inferred from field line resonant frequencies measured by magnetometers on the Earth's surface, and a model for the density field aligned variation, we construct a computer model for the magnetospheric density on 29 August 2000 at 1519 UT. The purposes of this study are to demonstrate how a density model can be constructed using multiple data sources, to document this particular model, which is being used in studies of ultra low frequency Pi-2 oscillations and plasmaspheric cavity modes, to describe some of the problems involved with EUV density inversion, and to demonstrate some features of the plasmaspheric density, particularly in the region of the dusk plasmatrough and plume.

1] During the early main phase of a geomagnetic storm on 11 April 2001, the Polar satellite was inside the magnetosphere in the prenoon sector ($1000-1100 magnetic local times) and experienced a magnetopause crossing at L % 6 because of the high solar wind dynamic pressure and strong southward interplanetary magnetic field (IMF). Just before the magnetopause crossing, Polar observed cold, dense plasma. That is, the cold, dense plasma was immediately adjacent to the compressed magnetopause. Using simultaneous observations by the IMAGE extreme ultraviolet (EUV) imager, we confirm that the cold, dense plasma observed by Polar is a plasmaspheric drainage plume extending outward from the plasmasphere to the magnetopause during the interval of high geomagnetic activity and strong southward IMF. We compare plasmaspheric mass densities determined from ground magnetometer data at L = 2.3 and 2.9 for a magnetically quiet time interval to mass densities determined during the magnetic storm time interval. We find no significant differences in the mass density between both intervals. These observations suggest that the sunward-convecting plasmaspheric plasma observed at Polar is due to erosion of the outer layers of the plasmasphere beyond L = 2.9.

1] The first empirical model of the equatorial mass density of the plasmasphere is constructed using ground-based ULF wave diagnostics. Plasmaspheric mass density between L = 1.7 and L = 3.2 has been determined using over 5200 hours of data from pairs of stations in the MEASURE array of ground magnetometers. The least squares fit to the data as a function of L shows that mass density falls logarithmically with L. Average ion mass as a function of L is also estimated by combining the mass density model with plasmaspheric electron density profiles determined from the IMAGE Radio Plasma Imager (RPI). Additionally, we use the RPI electron density database to examine how the average ion mass changes under different levels of geomagnetic activity. We find that average ion mass is greatest under the most disturbed conditions. This result indicates that heavy ion concentrations (percent by number) are enhanced during large geomagnetic disturbances. We also find that the average ion mass increases with increasing L (below L = 3.2), indicating the presence of a heavy ion torus during disturbed times. Heavy ions must play an important role in storm-time plasmaspheric dynamics. The average ion mass is also used to constrain the concentrations of He + and O + . Estimates of the He + concentration determined this way may be useful for interpreting IMAGE Extreme Ultraviolet Imager (EUV) images. Citation: Berube, D., M. B. Moldwin, S. F. Fung, and J. L. Green (2005), A plasmaspheric mass density model and constraints on its heavy ion concentration,

1] Plasmaspheric plumes have ionospheric signatures and are observed as stormenhanced density (SED) in global positioning system (GPS) total electron content (TEC). These ionospheric signatures have been primarily observed over the American sector and in a few limited examples over the European sector. This study examines the longitudinal occurrence frequency of plasmaspheric plumes. We analyzed all images from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) Extreme Ultraviolet Imager (EUV) databases for the first half of 2001 and identified a total of 31 distinct plume intervals observed during different storm events. Out of the total IMAGE EUV plumes that we identified, 12 were projected over North America, 10 over Asia, and the remaining 9 were over Europe and the Atlantic Ocean. Using ground-based GPS TEC from MIT's Madrigal database, we searched for corresponding SED/TEC plumes at different longitudinal sector and found 12 ionospheric SED plume signatures over North America, 4 over Europe, and 2 over Asia. This indicates that the observation probability of an ionospheric SED plume when a plasmaspheric plume is seen is 100% in the American sector, 50% in the European sector, and 20% in the Asian sector. This could be due to the fact that the plumes may be either positioned beyond the limit of the ground-based GPS field of view, which happens mainly when there is less plasmaspheric erosion, or are too weak to be detected by the sparse number of GPS receivers over Asia. The in situ plasma densities from the available coincident defense metrological satellite program (DMSP) satellites were also used to study the characteristics of SED/TEC plume at DMSP orbiting altitude (i.e., $870 km). The TOPographic EXplorer (TOPEX) altimeter TEC also is used to identify the conjugate SED/plume signature over the Southern Hemisphere.

Verification of the Visible Infrared Imager Radiometer Suite (VIIRS) End-to-End (E2E) sensor calibration is highly recommended before launch, to identify any anomalies and to improve our understanding of the sensor on-orbit calibration performance. E2E testing of the Reflective Solar Bands (RSB) calibration cycle was performed pre-launch for the VIIRS Fight 1 (F1) sensor at the Ball Aerospace facility in Boulder CO in March 2010. VIIRS reflective band calibration cycle is very similar to heritage sensor MODIS in that solar illumination, via a diffuser, is used to correct for temporal variations in the instrument responsivity. Monochromatic light from the NIST T-SIRCUS was used to illuminate both the Earth View (EV), via an integrating sphere, and the Solar Diffuser (SD) view, through a collimator. The collimator illumination was cycled through a series of angles intended to simulate the range of possible angles for which solar radiation will be incident on the solar attenuation screen on-orbit. Ideally, the measured instrument responsivity (defined here as the ratio of the detector response to the at-sensor radiance) should be the same whether the EV or SD view is illuminated. The ratio of the measured responsivities was determined at each collimator angle and wavelength. In addition, the Solar Diffuser Stability Monitor (SDSM), a ratioing radiometer designed to track the temporal variation in the SD BRF by direct comparison to solar radiation, was illuminated by the collimator. The measured SDSM ratio was compared to the predicted ratio. An uncertainty analysis was also performed on both the SD and SDSM calibrations.

We have monitored FG SgeÏs spectroscopic changes since the time just prior to its dramatic fading in 1992 August. The most signiÐcant qualitative changes in the spectrum include large variations in the strength of the molecular bands and the gradual appearance of broad blueshifted high-velocity (D200 C 2 km s~1 relative to the photosphere) absorption components of the Na I D lines. During the deep minima of 1994 May and 1996 June, an emission-line spectrum temporarily appeared superimposed on a weak continuum ; in addition to the previously reported nebular emission lines, the spectra displayed the C 2 and rare earth element lines in emission. Much of the behavior exhibited by FG Sge since 1992 resembles that seen in R CrB stars, including the photometric behavior, the evolution of the Na I D line proÐles, variations of the band strengths, and the appearance of narrow emission lines. C 2

The C I, Na I D, and Halpha lines of the post-AGB binary HR 4049 have been studied. Na I D variability results from a photospheric absorption component ([Na/H]=-1.6+/-0.2) which follows the velocity of the primary and a stationary, non-photospheric component. An emission component is attributed to the circumbinary disc, and an absorption component to mass-loss from the system with

We have used moderately high-resolution, high S/N spectra to study the chemical composition of 10 field ab-type RR Lyrae stars. Variables having accurate photometric and radial velocity data were selected, in order to derive a precise estimate of the atmospheric parameters independently of excitation and ionization equilibria. A new temperature scale was determined from literature Infrared Flux Method measures of subdwarfs and the Kurucz (1992) model atmospheres, and used to calibrate colors for both dwarfs and RR Lyraes. Photometric reddening estimates for the program stars were carefully examined, and compared with other determinations. The applicability of Kurucz (1992) model atmospheres in the analysis of RR Lyraes at minimum light was analyzed: we found that they are able to reproduce colors, excitation and ionization equilibria as well as the wings of H α . The comparison solar abundances were carefully determined. From a new analysis of weak Fe I lines with accurate gf s (Bard & Kock 1994) we derived log ǫ(F e) ⊙ = 7.52, in agreement with the Fe abundances determined from meteorites and Fe II lines.

High-resolution optical spectra of thirteen blue horizontal-branch (BHB) stars in the globular cluster M13 show enormous deviations in element abundances from the expected cluster metallicity. In the hotter stars (T eff > 12000 K), helium is depleted by factors of 10 to 100 below solar, while iron is enhanced to three times the solar abundance, two orders of magnitude above the canonical metallicity [Fe/H] ≃ −1.5 dex for this globular cluster. Nitrogen, phosphorus, and chromium exhibit even more pronounced enhancements, and other metals are also mildly overabundant, with the exception of magnesium, which stays very near the expected cluster metallicity. These photospheric anomalies are most likely due to diffusion -gravitational settling of helium, and radiative levitation of the other elements -in the stable radiative atmospheres of these hot stars. The effects of these mechanisms may have some impact on the photometric morphology of the cluster's horizontal branch and on estimates of its age and distance.

High-quality Keck HIRES echelle spectra of two A-supergiants in M31, designated 41[3654 (spectral type A2 Ia-O) and 41[3712 (A3 Ia-O), at 10.2 kpc galactocentric distance are analyzed by means of NLTE uniÐed model atmospheres to determine the properties of their stellar winds, in particular the wind momenta. We obtain mass-loss rates of 1.9 ] 10~6 and 1.1 ] 10~6 yr~1 for 41[3654 and M _ 41[3712, respectively, with an accuracy of 15% from the Ðt of the Ha proÐles. The terminal velocities are 200^25 km s~1.