334 research outputs found
Ionospheric response to the corotating interaction region-driven geomagnetic storm of October 2002
Unlike the geomagnetic storms produced by coronal mass ejections (CMEs), the storms generated by corotating interaction regions (CIRs) are not manifested by dramatic enhancements of the ring current. The CIR-driven storms are however capable of producing other phenomena typical for the magnetic storms such as relativistic particle acceleration, enhanced magnetospheric convection and ionospheric heating. This paper examines ionospheric plasma anomalies produced by a CIR-driven storm in the middle- and high-latitude ionosphere with a specific focus on the polar cap region. The moderate magnetic storm which took place on 14â17 October 2002 has been used as an example of the CIR-driven event. Four-dimensional tomographic reconstructions of the ionospheric plasma density using measurements of the total electron content along ray paths of GPS signals allow us to reveal the large-scale structure of storm-induced ionospheric anomalies. The tomographic reconstructions are compared with the data obtained by digital ionosonde located at Eureka station near the geomagnetic north pole. The morphology and dynamics of the observed ionospheric anomalies is compared qualitatively to the ionospheric anomalies produced by major CME-driven storms. It is demonstrated that the CIR-driven storm of October 2002 was able to produce ionospheric anomalies comparable to those produced by CME-driven storms of much greater Dst magnitude. This study represents an important step in linking the tomographic GPS reconstructions with the data from ground-based network of digital ionosondes
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Remote sensing of thundercloud electric fields
Theoretical and experimental work was performed on emission of photons from the air within and above thunderclouds and within lightning channels. Predictions were made of the telltale emissions from ionized nitrogen molecules and these emissions were recorded. The measurements will be utilized to help to understand the nature of thundercloud-produced airglow
The proton and electron radiation belts at geosynchronous orbit: Statistics and behavior during highâspeed streamâdriven storms
The outer proton radiation belt (OPRB) and outer electron radiation belt (OERB) at geosynchronous orbit are investigated using a reanalysis of the LANL CPA (Charged Particle Analyzer) 8âsatellite 2âsolar cycle energetic particle data set from 1976 to 1995. Statistics of the OPRB and the OERB are calculated, including local time and solar cycle trends. The number density of the OPRB is about 10 times higher than the OERB, but the 1âMeV proton flux is about 1000 times less than the 1âMeV electron flux because the proton energy spectrum is softer than the electron spectrum. Using a collection of 94 highâspeed streamâdriven storms in 1976â1995, the storm time evolutions of the OPRB and OERB are studied via superposed epoch analysis. The evolution of the OERB shows the familiar sequence (1) prestorm decay of density and flux, (2) earlyâstorm dropout of density and flux, (3) sudden recovery of density, and (4) steady storm time heating to high fluxes. The evolution of the OPRB shows a sudden enhancement of density and flux early in the storm. The absence of a proton dropout when there is an electron dropout is noted. The sudden recovery of the density of the OERB and the sudden density enhancement of the OPRB are both associated with the occurrence of a substorm during the early stage of the storm when the superdense plasma sheet produces a âstrong stretching phaseâ of the storm. These storm time substorms are seen to inject electrons to 1âMeV and protons to beyond 1âMeV into geosynchronous orbit, directly producing a suddenly enhanced radiation belt population.Key PointsDuring highâspeed streamâdriven storms, the electron and proton radiation belts are directly enhanced by a single substormThe enhancing substorm occurs during the âstrong stretchingâ phase of the storm caused by the superdense plasma sheetProton and electron injection to 1 MeV is seen for these strong stretching phase substormsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/133567/1/jgra52702.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133567/2/jgra52702_am.pd
An improved empirical model of electron and ion fluxes at geosynchronous orbit based on upstream solar wind conditions
A new empirical model of the electron fluxes and ion fluxes at geosynchronous orbit (GEO) is introduced, based on observations by Los Alamos National Laboratory (LANL) satellites. The model provides flux predictions in the energy range ~1âeV to ~40âkeV, as a function of local time, energy, and the strength of the solar wind electric field (the negative product of the solar wind speed and the z component of the magnetic field). Given appropriate upstream solar wind measurements, the model provides a forecast of the fluxes at GEO with a ~1âh lead time. Model predictions are tested against inâsample observations from LANL satellites and also against outâofâsample observations from the Compact Environmental Anomaly Sensor II detector on the AMCâ12 satellite. The model does not reproduce all structure seen in the observations. However, for the intervals studied here (quiet and storm times) the normalized rootâmeanâsquare deviationâ<â~0.3. It is intended that the model will improve forecasting of the spacecraft environment at GEO and also provide improved boundary/input conditions for physical models of the magnetosphere.Key PointsNew model of electron and ion fluxes at GEO (driven by âvBz) provides a ~1âh forecast of fluxes in the energy range ~1âeV to ~40âkeVThe main benefit from the new model is the ability to predict the fluxes at GEO in advanceForecasts are a good match to observations during quiet times and storm timesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134149/1/swe20339_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134149/2/swe20339.pd
Detailed Calculation of Test-Mass Charging in the LISA Mission
The electrostatic charging of the LISA test masses due to exposure of the
spacecraft to energetic particles in the space environment has implications in
the design and operation of the gravitational inertial sensors and can affect
the quality of the science data. Robust predictions of charging rates and
associated stochastic fluctuations are therefore required for the exposure
scenarios expected throughout the mission. We report on detailed charging
simulations with the Geant4 toolkit, using comprehensive geometry and physics
models, for Galactic cosmic-ray protons and helium nuclei. These predict
positive charging rates of 50 +e/s (elementary charges per second) for solar
minimum conditions, decreasing by half at solar maximum, and current
fluctuations of up to 30 +e/s/Hz^{1/2}. Charging from sporadic solar events
involving energetic protons was also investigated. Using an event-size
distribution model, we conclude that their impact on the LISA science data is
manageable. Several physical processes hitherto unexplored as potential
charging mechanisms have also been assessed. Significantly, the kinetic
emission of very low-energy secondary electrons due to bombardment of the
inertial sensors by primary cosmic rays and their secondaries can produce
charging currents comparable with the Monte Carlo rates.Comment: 31 pages, 18 figures, 4 tables. to be published in Astroparticle
Physics. Changed due to error found in normalisation of the simulation
result
Perturbed Input Ensemble Modeling With the Space Weather Modeling Framework
To assess the effect of uncertainties in solar wind driving on the predictions from the operational configuration of the Space Weather Modeling Framework, we have developed a nonparametric method for generating multiple possible realizations of the solar wind just upstream of the bow shock, based on observations near the first Lagrangian point. We have applied this method to the solar wind inputs at the upstream boundary of Space Weather Modeling Framework and have simulated the geomagnetic storm of 5 April 2010. We ran a 40âmember ensemble for this event and have used this ensemble to quantify the uncertainty in the predicted SymâH index and ground magnetic disturbances due to the uncertainty in the upstream boundary conditions. Both the ensemble mean and the unperturbed simulation tend to underpredict the magnitude of SymâH in the quiet interval before the storm and overpredict in the storm itself, consistent with previous work. The ensemble mean is a more accurate predictor of SymâH, improving the mean absolute error by nearly 2Â nT for this interval and displaying a smaller bias. We also examine the uncertainty in predicted maxima in ground magnetic disturbances. The confidence intervals are typically narrow during periods where the predicted dBH/dt is low. The confidence intervals are often much wider where the median prediction is for enhanced dBH/dt. The ensemble also allows us to identify intervals of activity that cannot be explained by uncertainty in the solar wind driver, driving further model improvements. This work demonstrates the feasibility and importance of ensemble modeling for space weather applications.Plain Language SummaryForecasts of space weather usually rely on spacecraft measurements of the solar wind from about a million miles away from Earth. Like water flowing toward a rock in a stream, measurements at a single point upstream may not reflect exactly what will hit the Earth. Forecasts that are driven by these measurements have uncertainty due to the uncertainty in the measurements driving the forecast models. We have developed a technique to estimate the uncertainty on space weather predictions using 7Â years of solar wind measurements from two satellites. We have performed computer simulations of the same geomagnetic storm 41 times. In each simulation, the inputs were modified slightly each time to reflect the uncertainty in the measurements. By considering the set of simulations as a whole, we have shown that space weather forecasts can be improved by accounting for the uncertainty in the input data. We have also shown that accounting for uncertainty in the data driving, the model can highlight where incorrect forecasts are due to the uncertainty, as well as where they are due to inadequacies in the model itself. This work shows the importance of ensemble methods and accounting for uncertainties in space weather simulation and forecasting.Key PointsA new nonparametric method for drawing different realizations of solar wind data to drive magnetospheric models is derivedThe new method is used to obtain uncertainties on predicted geophysical indices from the operational Space Weather Modeling FrameworkModel skill can be improved by considering the uncertainty on model inputPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146472/1/swe20747_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146472/2/swe20747.pd
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The magnetospheric trough
The authors review the history of the concepts of the magnetospheric cold-ion trough and hot-electron trough and conclude that the two regions are actually essentially the same. The magnetospheric trough may be viewed as a temporal state in the evolution of convecting flux tubes. These flux tubes are in contact with the earth`s upper atmosphere which acts both as a sink for precipitating hot plasma sheet electrons and as a source for the cold ionospheric plasma leading to progressive depletion of the plasma sheet and refilling with cold plasma. Geosynchronous plasma observations show that the rate of loss of plasma-sheet electron energy density is commensurate with the precipitating electron flux at the low-latitude edge of the diffuse aurora. The rate at which geosynchronous flux tubes fill with cold ionospheric plasma is found to be consistent with previous estimates of early-time refilling. Geosynchronous observations further indicate that both Coulomb collisions and wave-particle effects probably play a role in trapping ionospheric material in the magnetosphere
Adsorption of benzene on Si(100) from first principles
Adsorption of benzene on the Si(100) surface is studied from first
principles. We find that the most stable configuration is a
tetra--bonded structure characterized by one C-C double bond and four
C-Si bonds. A similar structure, obtained by rotating the benzene molecule by
90 degrees, lies slightly higher in energy. However, rather narrow wells on the
potential energy surface characterize these adsorption configurations. A
benzene molecule impinging on the Si surface is most likely to be adsorbed in
one of three different di--bonded, metastable structures, characterized
by two C-Si bonds, and eventually converts into the lowest-energy
configurations. These results are consistent with recent experiments.Comment: 4 pages, RevTex, 2 PostScript gzipped figure
Multistep Dst development and ring current composition changes during the 4â6 June 1991 magnetic storm
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95117/1/jgra16069.pd
The interaction between a sexually transferred steroid hormone and a female protein regulates oogenesis in the malaria mosquito anopheles gambiae
Molecular interactions between male and female factors during mating profoundly affect the reproductive behavior and physiology of female insects. In natural populations of the malaria mosquito Anopheles gambiae, blood-fed females direct nutritional resources towards oogenesis only when inseminated. Here we show that the mating-dependent pathway of egg development in these mosquitoes is regulated by the interaction between the steroid hormone 20-hydroxy-ecdysone (20E) transferred by males during copulation and a female Mating-Induced Stimulator of Oogenesis (MISO) protein. RNAi silencing of MISO abolishes the increase in oogenesis caused by mating in blood-fed females, causes a delay in oocyte development, and impairs the function of male-transferred 20E. Co-immunoprecipitation experiments show that MISO and 20E interact in the female reproductive tract. Moreover MISO expression after mating is induced by 20E via the Ecdysone Receptor, demonstrating a close cooperation between the two factors. Male-transferred 20E therefore acts as a mating signal that females translate into an increased investment in egg development via a MISO-dependent pathway. The identification of this maleâfemale reproductive interaction offers novel opportunities for the control of mosquito populations that transmit malaria
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