146 research outputs found
Auroral Cluster: A Space Physics Mission for Multiple, Electronically Tethered Small Satellites
Auroral Cluster is a space physics mission that has been identified by the NASA Space Physics Strategic Implementation Study as a candidate for flight in the next decade. Auroral Cluster will employ multiple spacecraft outfitted with similar complements of science instruments allowing simultaneous multipoint plasma measurements in the Earth\u27s auroral regions. Co-orbiting small satellites (mass \u3c 400 kg each) that are electronically tethered to share distributed spacecraft systems represent an efficient approach for achieving the science goals of the Auroral Cluster mission. Multisatellite missions represent a new trend in gathering space science data and pose many new and difficult challenges for the space systems engineer. The results of an Auroral Cluster feasibility study, which discusses a variety of mission trade-offs, are presented. A discussion of the science background and mission goals is used to identify the technical drivers for the design of the multiple spacecraft system. A mission plan and some considerations for a Auroral Cluster satellite design are presented. Special consideration is given to the spacecraft subsystems that will allow the system to be operated as a network of electronically tethered interdependent small satellites. These subsystems include attitude determination, spatial separation knowledge and control, data storage, and intersatellite communication
On the reheating stage after inflation
We point out that inflaton decay products acquire plasma masses during the
reheating phase following inflation. The plasma masses may render inflaton
decay kinematicaly forbidden, causing the temperature to remain frozen for a
period at a plateau value. We show that the final reheating temperature may be
uniquely determined by the inflaton mass, and may not depend on its coupling.
Our findings have important implications for the thermal production of
dangerous relics during reheating (e.g., gravitinos), for extracting bounds on
particle physics models of inflation from Cosmic Microwave Background
anisotropy data, for the production of massive dark matter candidates during
reheating, and for models of baryogenesis or leptogensis where massive
particles are produced during reheating.Comment: 8 pages, 2 figures. Submitted for publication in Phys. Rev.
Production and dilution of gravitinos by modulus decay
We study the cosmological consequences of generic scalar fields like moduli
which decay only through gravitationally suppressed interactions. We consider a
new production mechanism of gravitinos from moduli decay, which might be more
effective than previously known mechanisms, and calculate the final
gravitino-to-entropy ratio to compare with the constraints imposed by
successful big bang nucleosynthesis (BBN) etc., taking possible hadronic decays
of gravitinos into account. We find the modulus mass smaller than
TeV is excluded. On the other hand, inflation models with high reheating
temperatures GeV can be compatible with BBN thanks
to the late-time entropy production from the moduli decay if model parameters
are appropriately chosen.Comment: 18 pages, 4 figures, to appear in Phys. Rev.
Big Bang nucleosynthesis and cosmic microwave background constraints on the time variation of the Higgs vacuum expectation value
We derive constraints on the time variation of the Higgs vacuum expectation
value through the effects on Big Bang nucleosynthesis (BBN) and the
cosmic microwave background (CMB). In the former case, we include the
(previously-neglected) effect of the change in the deuteron binding energy,
which alters both the He and deuterium abundances significantly. We find
that the current BBN limits on the relative change in \higgs are , where the
exact limits depend on the model we choose for the dependence of the deuteron
binding energy on \higgs.The limits from the current CMB data are much weaker.Comment: 5 pages including 5 figures, accepted for publication in Phys. Rev.
Generalized Global Defect Solutions
We investigate the presence of defect structures in generalized models
described by real scalar field in space-time dimensions. We work with
two distinct generalizations, one in the form of a product of functions of the
field and its derivative, and the other as a sum. We search for static
solutions and study the corresponding linear stability on general grounds. We
illustrate the results with several examples, where we find stable defect
structures of modified profile. In particular, we show how the new defect
solutions may give rise to evolutions not present in the standard scenario in
higher spatial dimensions.Comment: RevTex, 10 pages, 2 figures; version to appear in EPJ
How to optimize nonlinear force-free coronal magnetic field extrapolations from SDO/HMI vector magnetograms?
The SDO/HMI instruments provide photospheric vector magnetograms with a high
spatial and temporal resolution. Our intention is to model the coronal magnetic
field above active regions with the help of a nonlinear force-free
extrapolation code. Our code is based on an optimization principle and has been
tested extensively with semi-analytic and numeric equilibria and been applied
before to vector magnetograms from Hinode and ground based observations.
Recently we implemented a new version which takes measurement errors in
photospheric vector magnetograms into account. Photospheric field measurements
are often due to measurement errors and finite nonmagnetic forces inconsistent
as a boundary for a force-free field in the corona. In order to deal with these
uncertainties, we developed two improvements: 1.) Preprocessing of the surface
measurements in order to make them compatible with a force-free field 2.) The
new code keeps a balance between the force-free constraint and deviation from
the photospheric field measurements. Both methods contain free parameters,
which have to be optimized for use with data from SDO/HMI. Within this work we
describe the corresponding analysis method and evaluate the force-free
equilibria by means of how well force-freeness and solenoidal conditions are
fulfilled, the angle between magnetic field and electric current and by
comparing projections of magnetic field lines with coronal images from SDO/AIA.
We also compute the available free magnetic energy and discuss the potential
influence of control parameters.Comment: 17 Pages, 6 Figures, Sol. Phys., accepte
Limits on the gravity wave contribution to microwave anisotropies
We present limits on the fraction of large angle microwave anisotropies which
could come from tensor perturbations. We use the COBE results as well as
smaller scale CMB observations, measurements of galaxy correlations, abundances
of galaxy clusters, and Lyman alpha absorption cloud statistics. Our aim is to
provide conservative limits on the tensor-to-scalar ratio for standard
inflationary models. For power-law inflation, for example, we find T/S<0.52 at
95% confidence, with a similar constraint for phi^p potentials. However, for
models with tensor amplitude unrelated to the scalar spectral index it is still
currently possible to have T/S>1.Comment: 23 pages, 7 figures, accepted for publication in Phys. Rev. D.
Calculations extended to blue spectral index, Fig. 6 added, discussion of
results expande
Modelling and Interpreting The Effects of Spatial Resolution on Solar Magnetic Field Maps
Different methods for simulating the effects of spatial resolution on
magnetic field maps are compared, including those commonly used for
inter-instrument comparisons. The investigation first uses synthetic data, and
the results are confirmed with {\it Hinode}/SpectroPolarimeter data. Four
methods are examined, one which manipulates the Stokes spectra to simulate
spatial-resolution degradation, and three "post-facto" methods where the
magnetic field maps are manipulated directly. Throughout, statistical
comparisons of the degraded maps with the originals serve to quantify the
outcomes. Overall, we find that areas with inferred magnetic fill fractions
close to unity may be insensitive to optical spatial resolution; areas of
sub-unity fill fractions are very sensitive. Trends with worsening spatial
resolution can include increased average field strength, lower total flux, and
a field vector oriented closer to the line of sight. Further-derived quantities
such as vertical current density show variations even in areas of high average
magnetic fill-fraction. In short, unresolved maps fail to represent the
distribution of the underlying unresolved fields, and the "post-facto" methods
generally do not reproduce the effects of a smaller telescope aperture. It is
argued that selecting a method in order to reconcile disparate spatial
resolution effects should depend on the goal, as one method may better preserve
the field distribution, while another can reproduce spatial resolution
degradation. The results presented should help direct future inter-instrument
comparisons.Comment: Accepted for publication in Solar Physics. The final publication
(including full-resolution figures) will be available at
http://www.springerlink.co
Cosmic Microwave Background constraint on residual annihilations of relic particles
Energy injected into the Cosmic Microwave Background at redshifts z<10^6 will
distort its spectrum permanently. In this paper we discuss the distortion
caused by annihilations of relic particles. We use the observational bounds on
deviations from a Planck spectrum to constrain a combination of annihilation
cross section, mass, and abundance. For particles with (s-wave) annihilation
cross section, =\sigma_0, the bound is
f[(\sigma_0/6e-27cm^3/s)(\Omega_{X\bar{X}}h^2)^2]/(m_X/MeV)<0.2, where m_X is
the particle mass, \Omega_{X\bar{X}} is the fraction of the critical density
the particle and its antiparticle contribute if they survive to the present
time, h=H_0/(100km/s/Mpc), H_0 is the Hubble constant, and f is the fraction of
the annihilation energy that interacts electromagnetically. We also compute the
less stringent limits for p-wave annihilation. We update other bounds on
residual annihilations and compare them to our CMB bound.Comment: submitted to Phys. Rev.
Properties of high-frequency wave power halos around active regions: an analysis of multi-height data from HMI and AIA onboard SDO
We study properties of waves of frequencies above the photospheric acoustic
cut-off of 5.3 mHz, around four active regions, through spatial maps
of their power estimated using data from Helioseismic and Magnetic Imager (HMI)
and Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory
(SDO). The wavelength channels 1600 {\AA} and 1700 {\AA} from AIA are now known
to capture clear oscillation signals due to helioseismic p modes as well as
waves propagating up through to the chromosphere. Here we study in detail, in
comparison with HMI Doppler data, properties of the power maps, especially the
so called 'acoustic halos' seen around active regions, as a function of wave
frequencies, inclination and strength of magnetic field (derived from the
vector field observations by HMI) and observation height. We infer possible
signatures of (magneto-)acoustic wave refraction from the observation height
dependent changes, and hence due to changing magnetic strength and geometry, in
the dependences of power maps on the photospheric magnetic quantities. We
discuss the implications for theories of p mode absorption and mode conversions
by the magnetic field.Comment: 22 pages, 12 figures, Accepted by journal Solar Physic
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