359 research outputs found
Particle energization in the inner, nonazimuthally symmetric magnetospheres of neutron stars
The energization process of magnetic pumping, a combination of time dependent magnetic mirror fields with pitch-angle scattering, is applied to trapped charged particles drifting in corotating, azimuthally nonsymmetric neutron star magnetospheres. When particle energization is balanced by synchrotron radiation loss, it is found that protons, rather than electrons, reach considerable kinetic energies and radiate, in the X-ray regime, at rates up to the 10 to the 6th power MeV/proton/sec
Future beam experiments in the magnetosphere with plasma contactors: The electron collection and ion emission routes
Experiments where a highâvoltage electron beam emitted by a spacecraft in the lowâdensity magnetosphere is used to probe the magnetospheric configuration could greatly enhance our understanding of the nearâEarth environment. Their challenge, however, resides in the fact that the background magnetospheric plasma cannot provide a return current that balances the electron beam current without charging the spacecraft to such high potential that in practice prevents beam emission. In order to overcome this problem, a possible solution is based on the emission of a highâdensity contactor plasma by the spacecraft prior to and after the beam. We perform particleâinâcell simulations to investigate the conditions under which a highâvoltage electron beam can be emitted from a magnetospheric spacecraft, comparing two possible routes that rely on the highâdensity contactor plasma. The first is an âelectron collectionâ route, where the contactor has lower current than the electron beam and is used with the goal of connecting to the background plasma and collecting magnetospheric electrons over a much larger area than that allowed by the spacecraft alone. The second is an âion emissionâ route, where the contactor has higher current than the electron beam. Ion emission is then enabled over the large quasiâspherical area of the contactor cloud, thus overcoming the space charge limits typical of ion beam emission. Our results indicate that the ion emission route offers a pathway for performing beam experiments in the lowâdensity magnetosphere, while the electron collection route is not viable because the contactor fails to draw a large neutralizing current from the background.Key PointsThe ion emission route is credible for beam experiments in the magnetosphereThe electron collection route is not viableThe background plasma facilitates beam emissionPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111985/1/jgra51700.pd
Kinetic-scale magnetic turbulence and finite Larmor radius effects at Mercury
We use a nonstationary generalization of the higher-order structure function
technique to investigate statistical properties of the magnetic field
fluctuations recorded by MESSENGER spacecraft during its first flyby
(01/14/2008) through the near Mercury's space environment, with the emphasis on
key boundary regions participating in the solar wind -- magnetosphere
interaction. Our analysis shows, for the first time, that kinetic-scale
fluctuations play a significant role in the Mercury's magnetosphere up to the
largest resolvable time scale ~20 s imposed by the signal nonstationarity,
suggesting that turbulence at this planet is largely controlled by finite
Larmor radius effects. In particular, we report the presence of a highly
turbulent and extended foreshock system filled with packets of ULF
oscillations, broad-band intermittent fluctuations in the magnetosheath,
ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail,
and kinetic-scale fluctuations in the inner current sheet encountered at the
outbound (dawn-side) magnetopause. Overall, our measurements indicate that the
Hermean magnetosphere, as well as the surrounding region, are strongly affected
by non-MHD effects introduced by finite sizes of cyclotron orbits of the
constituting ion species. Physical mechanisms of these effects and their
potentially critical impact on the structure and dynamics of Mercury's magnetic
field remain to be understood.Comment: 46 pages, 5 figures, 2 table
Geoeffectiveness and efficiency of CIR, Sheath and ICME in generation of magnetic storms
We investigate relative role of various types of solar wind streams in
generation of magnetic storms. On the basis of the OMNI data of interplanetary
measurements for the period of 1976-2000 we analyze 798 geomagnetic storms with
Dst < -50 nT and their interplanetary sources: corotating interaction regions
(CIR), interplanetary CME (ICME) including magnetic clouds (MC) and Ejecta and
compression regions Sheath before both types of ICME. For various types of
solar wind we study following relative characteristics: occurrence rate; mass,
momentum, energy and magnetic fluxes; probability of generation of magnetic
storm (geoeffectiveness) and efficiency of process of this generation. Obtained
results show that despite magnetic clouds have lower occurrence rate and lower
efficiency than CIR and Sheath they play an essential role in generation of
magnetic storms due to higher geoeffectiveness of storm generation (i.e higher
probability to contain large and long-term southward IMF Bz component).Comment: 23 pages, 4 figures, 3 tables, submitted to JGR special issue
"Response of Geospace to High-Speed Streams
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
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
Recovery phase of magnetic storms induced by different interplanetary drivers
Statistical analysis of Dst behaviour during recovery phase of magnetic
storms induced by different types of interplanetary drivers is made on the
basis of OMNI data in period 1976-2000. We study storms induced by ICMEs
(including magnetic clouds (MC) and Ejecta) and both types of compressed
regions: corotating interaction regions (CIR) and Sheaths. The shortest,
moderate and longest durations of recovery phase are observed in ICME-, CIR-,
and Sheath-induced storms, respectively. Recovery phases of strong ( nT) magnetic storms are well approximated by hyperbolic functions
with constant times for all types of drivers
while for moderate ( nT) storms profile can not
be approximated by hyperbolic function with constant because
hyperbolic time increases with increasing time of recovery phase.
Relation between duration and value for storms induced by ICME and
Sheath has 2 parts: and duration correlate at small durations while
they anticorrelate at large durations.Comment: 18 pages, 4 figures, 2 tables, submitted to JGR special issue
"Response of Geospace to High-Speed Streams
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
Multiscale modeling of magnetospheric reconnection
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95438/1/jgra18824.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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