149 research outputs found
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 L∗ 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 L∗ ranging from 1.0 pA cm−2 at L∗=4.75 to 0.03 pA cm−2 at L∗=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 L∗ ranging from 0.4 pA cm−2 at L∗=4.75 to 0.01 pA cm−2 at L∗=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 cm−2, respectively
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
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
Evidence for acceleration of outer zone electrons to relativistic energies by whistler mode chorus
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
Updated Model of the Solar Energetic Proton Environment in Space
The Solar Accumulated and Peak Proton and Heavy Ion Radiation Environment (SAPPHIRE) model provides environment specification outputs for all aspects of the Solar Energetic Particle (SEP) environment. The model is based upon a thoroughly cleaned and carefully processed data set. Herein the evolution of the solar proton model is discussed with comparisons to other models and data. This paper discusses the construction of the underlying data set, the modelling methodology, optimisation of fitted flux distributions and extrapolation of model outputs to cover a range of proton energies from 0.1 MeV to 1 GeV. The model provides outputs in terms of mission cumulative fluence, maximum event fluence and peak flux for both solar maximum and solar minimum periods. A new method for describing maximum event fluence and peak flux outputs in terms of 1-in-x-year SPEs is also described. SAPPHIRE proton model outputs are compared with previous models including CREME96, ESP-PSYCHIC and the JPL model. Low energy outputs are compared to SEP data from ACE/EPAM whilst high energy outputs are compared to a new model based on GLEs detected by Neutron Monitors (NMs)
Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution
Boron abundances have been derived for seven main sequence B-type stars from
HST STIS spectra around the B III 2066 A line. In two stars, boron appears to
be undepleted with respect to the presumed initial abundance. In one star,
boron is detectable but it is clearly depleted. In the other four stars, boron
is undetectable implying depletions of 1 to 2 dex. Three of these four stars
are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only
rotationally induced mixing predicts that boron depletions are unaccompanied by
nitrogen enrichments. The inferred rate of boron depletion from our
observations is in good agreement with these predictions. Other boron-depleted
nitrogen-normal stars are identified from the literature. Also, several
boron-depleted nitrogen-rich stars are identified, and while all fall on the
boron-nitrogen trend predicted by rotationally-induced mixing, a majority have
nitrogen enrichments that are not uniquely explained by rotation.
The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni)
abundances. The seven B-type stars have near solar iron-group abundances, as
expected for young stars in the solar neighborhood. We have also analysed the
halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence
of the B III line gives the upper limit [B/H]<2.5. These and other published
abundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap
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