175 research outputs found
Nowcast model for lowâenergy electrons in the inner magnetosphere
We present the nowcast model for lowâenergy (<200 keV) electrons in the inner magnetosphere, which is the version of the Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM) for electrons. Lowâenergy electron fluxes are very important to specify when hazardous satellite surfaceâcharging phenomena are considered. The presented model provides the lowâenergy electron flux at all L shells and at all satellite orbits, when necessary. The model is driven by the realâtime solar wind and interplanetary magnetic field (IMF) parameters with 1 h time shift for propagation to the Earth's magnetopause and by the real time Dst index. Realâtime geostationary GOES 13 or GOES 15 (whenever each is available) data on electron fluxes in three energies, such as 40 keV, 75 keV, and 150 keV, are used for comparison and validation of IMPTAM running online. On average, the model provides quite reasonable agreement with the data; the basic level of the observed fluxes is reproduced. The best agreement between the modeled and the observed fluxes are found for <100 keV electrons. At the same time, not all the peaks and dropouts in the observed electron fluxes are reproduced. For 150 keV electrons, the modeled fluxes are often smaller than the observed ones by an order of magnitude. The normalized rootâmeanâsquare deviation is found to range from 0.015 to 0.0324. Though these metrics are buoyed by large standard deviations, owing to the dynamic nature of the fluxes, they demonstrate that IMPTAM, on average, predicts the observed fluxes satisfactorily. The computed binary event tables for predicting high flux values within each 1 h window reveal reasonable hit rates being 0.660â0.318 for flux thresholds of 5 ·104â2 ·105 cmâ2 sâ1 srâ1 keVâ1 for 40 keV electrons, 0.739â0.367 for flux thresholds of 3 ·104â1 ·105 cmâ2 sâ1 srâ1 keVâ1 for 75 keV electrons, and 0.485â0.438 for flux thresholds of 3 ·103â3.5 ·103 cmâ2 sâ1 srâ1 keVâ1 for 150 keV electrons but rather small Heidke Skill Scores (0.17 and below). This is the first attempt to model lowâenergy electrons in real time at 10 min resolution. The output of this model can serve as an input of electron seed population for realâtime higherâenergy radiation belt modeling.Key PointsNowcast model for lowâenergy electronsOnline nearârealâtime comparison to GOES MAGED dataFirst successful model for lowâenergy electrons in real timePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110719/1/swe20196.pd
A Quantitative Analysis of the Available Multicolor Photometry for Rapidly Pulsating Hot B Subdwarfs
We present a quantitative and homogeneous analysis of the broadband
multicolor photometric data sets gathered so far on rapidly pulsating hot B
subdwarf stars. This concerns seven distinct data sets related to six different
stars. Our analysis is carried out within the theoretical framework developed
by Randall et al., which includes full nonadiabatic effects. The goal of this
analysis is partial mode identification, i.e., the determination of the degree
index l of each of the observed pulsation modes. We assume possible values of l
from 0 to 5 in our calculations. For each target star, we compute a specific
model atmosphere and a specific pulsation model using estimates of the
atmospheric parameters coming from time-averaged optical spectroscopy. For
every assumed value of l, we use a formal chi-squared approach to model the
observed amplitude-wavelength distribution of each mode, and we compute a
quality-of-fit Q probability to quantify the derived fit and to discriminate
objectively between the various solutions. We find that no completely
convincing and unambiguous l identification is possible on the basis of the
available data, although partial mode discrimination has been reached for 25
out of the 41 modes studied. A brief statistical study of these results
suggests that a majority of the modes must have l values of 0, 1, and 2, but
also that modes with l = 4 could very well be present while modes with l = 3
appear to be rarer. This is in line with recent results showing that l = 4
modes in rapidly pulsating B subdwarfs have a higher visibility in the optical
domain than modes with l = 3. Although somewhat disappointing in terms of mode
discrimination, our results still suggest that the full potential of multicolor
photometry for l identification in pulsating subdwarfs is within reach.Comment: 59 pages, 18 figures, accepted for publication in the Astrophysical
Journal Supplement Serie
Extreme internal charging currents in medium Earth orbit: Analysis of SURF plate currents on Giove-A
Relativistic electrons can penetrate spacecraft shielding and can damage satellite components. Spacecraft in medium Earth orbit pass through the heart of the outer radiation belt and may be exposed to large fluxes of relativistic electrons, particularly during extreme space weather events. In this study we perform an extreme value analysis of the daily average internal charging currents at three different shielding depths in medium Earth orbit as a function of Lâ and along the orbit path. We use data from the SURF instrument on board the European Space Agency's Giove-A spacecraft from December 2005 to January 2016. The top, middle, and bottom plates of this instrument respond to electrons with energies >500 keV, >700 keV, and >1.1 MeV, respectively. The 1 in 10 year daily average top plate current decreases with increasing Lâ ranging from 1.0 pA cmâ2 at Lâ=4.75 to 0.03 pA cmâ2 at Lâ=7.0. The 1 in 100 year daily average top plate current is a factor of 1.2 to 1.8 larger than the corresponding 1 in 10 year current. The 1 in 10 year daily average middle and bottom plate currents also decrease with increasing Lâ ranging from 0.4 pA cmâ2 at Lâ=4.75 to 0.01 pA cmâ2 at Lâ=7.0. The 1 in 100 year daily average middle and bottom plate currents are a factor of 1.2 to 2.7 larger than the corresponding 1 in 10 year currents. Averaged along the orbit path the 1 in 10 year daily average top, middle, and bottom plate currents are 0.22, 0.094, and 0.094 pA cmâ2, respectively
A statistical study of the performance of the Hakamada-Akasofu-Fry version 2 numerical model in predicting solar shock arrival times at Earth during different phases of solar cycle 23
The performance of the Hakamada
Akasofu-Fry, version 2 (HAFv.2) numerical model, which provides predictions
of solar shock arrival times at Earth, was subjected to a statistical study
to investigate those solar/interplanetary circumstances under which the
model performed well/poorly during key phases (rise/maximum/decay) of solar
cycle 23. In addition to analyzing elements of the overall data set (584
selected events) associated with particular cycle phases, subsets were
formed such that those events making up a particular sub-set showed common
characteristics. The statistical significance of the results obtained using the
various sets/subsets was generally very low and these results were not significant
as compared with the hit by chance rate (50%). This implies a low level
of confidence in the predictions of the model with no compelling
result encouraging its use. However, the data
suggested that the success rates of HAFv.2 were higher when the background
solar wind speed at the time of shock initiation was relatively fast. Thus,
in scenarios where the background solar wind speed is elevated and the
calculated success rate significantly exceeds the rate by chance, the
forecasts could provide potential value to the customer.
With the composite statistics available for solar cycle 23,
the calculated success rate at high solar wind speed, although clearly above
50%, was indicative rather than conclusive. The RMS error
estimated for shock arrival times for every cycle phase and for the
composite sample was in each case significantly better than would be
expected for a random data set. Also, the parameter "Probability of
Detection, yes" (PODy) which presents the Proportion of Yes observations
that were correctly forecast (i.e. the ratio between the shocks correctly
predicted and all the shocks observed), yielded values for the
rise/maximum/decay phases of the cycle and using the composite sample of
0.85, 0.64, 0.79 and 0.77, respectively. The statistical results obtained
through detailed analysis of the available data provided insights into how
changing circumstances on the Sun and in interplanetary space can affect the
performance of the model. Since shock arrival predictions are widely
utilized in making commercially significant decisions re. protecting space
assets, the present detailed archival studies can be useful in future
operational decision making during solar cycle 24. It would be of added
value in this context to use Briggs-Rupert methodology to estimate the cost
to an operator of acting on an incorrect forecast
SPB stars in the open SMC cluster NGC 371
Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which
depends critically on the metallicity. It has therefore been suggested that
beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower
metallicities than the solar neighborhood. To test this prediction we have
observed the open SMC cluster NGC 371 for 12 nights in order to search for beta
Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in
the upper part of the main sequence, many of which are probably SPB stars. This
result indicates that pulsation is still driven by the kappa mechanism even in
low metallicity environments. All the identified variables have periods longer
than the fundamental radial period which means that they cannot be beta Cep
stars. Within an amplitude detection limit of 5 mmag no stars in the top of the
HR-diagram show variability with periods shorter than the fundamental radial
period. So if beta Cep stars are present in the cluster they oscillate with
amplitudes below 5 mmag, which is significantly lower than the mean amplitude
of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is
more common in the upper part of the main sequence than in the lower. We have
also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the
cluster field.Comment: 8 pages, 11 figures. Accepted for publication in MNRA
Self-trapped electrons and holes in PbBr crystals
We have directly observed self-trapped electrons and holes in PbBr
crystals with electron-spin-resonance (ESR) technique. The self-trapped states
are induced below 8 K by two-photon interband excitation with pulsed
120-fs-width laser light at 3.10 eV. Spin-Hamiltonian analyses of the ESR
signals have revealed that the self-trapping electron centers are the dimer
molecules of Pb along the crystallographic a axis and the
self-trapping hole centers are those of Br with two possible
configurations in the unit cell of the crystal. Thermal stability of the
self-trapped electrons and holes suggests that both of them are related to the
blue-green luminescence band at 2.55 eV coming from recombination of spatially
separated electron-hole pairs.Comment: 8 pages (7 figures, 2 tables), ReVTEX; revised the text and figures
1, 4, and
Proton irradiation of a swept charge device at cryogenic temperature and the subsequent annealing
A number of studies have demonstrated that a room temperature proton irradiation may not be sufficient to provide an accurate estimation of the impact of the space radiation environment on detector performance. This is a result of the relationship between defect mobility and temperature, causing the performance to vary subject to the temperature history of the device from the point at which it was irradiated. Results measured using Charge Coupled Devices (CCD) irradiated at room temperature therefore tend to differ from those taken when the device was irradiated at a cryogenic temperature, more appropriate considering the operating conditions in space, impacting the prediction of in-flight performance. This paper describes the cryogenic irradiation, and subsequent annealing of an e2v technologies Swept Charge Device (SCD) CCD236 irradiated at â35.4°C with a 10 MeV equivalent proton fluence of 5.0 Ă 108 protons centerdot cmâ2. The CCD236 is a large area (4.4 cm2) X-ray detector that will be flown on-board the Chandrayaan-2 and Hard X-ray Modulation Telescope spacecraft, in the Chandrayaan-2 Large Area Soft X-ray Spectrometer and the Soft X-ray Detector respectively. The SCD is readout continually in order to benefit from intrinsic dither mode clocking, leading to suppression of the surface component of the dark current and allowing the detector to be operated at warmer temperatures than a conventional CCD. The SCD is therefore an excellent choice to test and demonstrate the variation in the impact of irradiation at cryogenic temperatures in comparison to a more typical room temperature irradiation
Photometric studies of three multiperiodic Beta Cephei stars: Beta CMa, 15 CMa and KZ Mus
We have carried out single and multi-site photometry of the three Beta Cephei
stars Beta and 15 CMa as well as KZ Mus. For the two stars in CMa, we obtained
270 h of measurement in the Stromgren uvy and Johnson V filters, while 150 h of
time-resolved Stromgren uvy photometry was acquired for KZ Mus. All three stars
are multi-periodic variables, with three (Beta CMa) and four (15 CMa, KZ Mus)
independent pulsation modes. Two of the mode frequencies of 15 CMa are new
discoveries and one of the known modes showed amplitude variations over the
last 33 years. Taken together, this explains the star's diverse behaviour
reported in the literature fully.
Mode identification by means of the amplitude ratios in the different
passbands suggests one radial mode for each star. In addition, Beta CMa has a
dominant l=2 mode while its third mode is nonradial with unknown l. The
nonradial modes of 15 CMa, which are l <= 3, form an almost equally split
triplet that, if physical, would imply that we see the star under an
inclination angle larger than 55 degrees. The strongest nonradial mode of KZ
Mus is l=2, followed by the radial mode and a dipole mode. Its weakest known
mode is nonradial with unknown l, confirming previous mode identifications for
the star's pulsations.
The phased light curve for the strongest mode of 15 CMa has a descending
branch steeper than the rising branch. A stillstand phenomenon during the rise
to maximum light is indicated. Given the low photometric amplitude of this
nonradial mode this is at first sight surprising, but it can be explained by
the mode's aspect angle.Comment: 12 pages, 11 figure
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