149 research outputs found
The Energy of a Plasma in the Classical Limit
When \lambda_{T} << d_{T}, where \lambda_{T} is the de Broglie wavelength and
d_{T}, the distance of closest approach of thermal electrons, a classical
analysis of the energy of a plasma can be made. In all the classical analysis
made until now, it was assumed that the frequency of the fluctuations \omega <<
T (k_{B}=\hbar=1). Using the fluctuation-dissipation theorem, we evaluate the
energy of a plasma, allowing the frequency of the fluctuations to be arbitrary.
We find that the energy density is appreciably larger than previously thought
for many interesting plasmas, such as the plasma of the Universe before the
recombination era.Comment: 10 pages, 2 figures, accepted for publication in Phys.Rev.Let
The Orientation of the Local Interstellar Magnetic Field
The orientation of the local interstellar magnetic field introduces
asymmetries in the heliosphere that affect the location of heliospheric radio
emissions and the streaming direction of ions from the termination shock of the
solar wind. We combine observations of radio emissions and energetic particle
streaming with extensive 3D MHD computer simulations of magnetic field draping
over the heliopause to show that the plane of the local interstellar field is ~
60-90 degrees from the galactic plane. This suggests that the field orientation
in the Local Interstellar Cloud differs from that of a larger scale
interstellar magnetic field thought to parallel the galactic plane
Globally Distributed Energetic Neutral Atom Maps for the "Croissant" Heliosphere
A recent study by Opher et al. (2015) suggested the heliosphere has a
"croissant" shape, where the heliosheath plasma is confined by the toroidal
solar magnetic field. The "croissant" heliosphere is in contrast to the
classically accepted view of a comet-like tail. We investigate the effect of
the "croissant" heliosphere model on energetic neutral atom (ENA) maps.
Regardless of the existence of a split tail, the confinement of the heliosheath
plasma should appear in ENA maps. ENA maps from the Interstellar Boundary
Explorer (IBEX) have shown two high latitude lobes with excess ENA flux at
higher energies in the tail of the heliosphere. These lobes could be a
signature of the confinement of the heliosheath plasma, while some have argued
they are caused by the fast/slow solar wind profile. Here we present ENA maps
of the "croissant" heliosphere, focusing on understanding the effect of the
heliosheath plasma collimation by the solar magnetic field while using a
uniform solar wind. We incorporate pick-up ions (PUIs) into our model based on
Malama et al. (2006) and Zank et al. (2010). We use the neutral solution from
our MHD model to determine the angular variation of the PUIs, and include the
extinction of PUIs in the heliosheath. In the presence of a uniform solar wind,
we find that the collimation in the "croissant" heliosphere does manifest
itself into two high latitude lobes of increased ENA flux in the downwind
direction.Comment: 14 pages, 1 table, 7 figures, Accepted for publication in Ap
M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets
We perform three-dimensional numerical simulations of stellar winds of
early-M dwarf stars. Our simulations incorporate observationally reconstructed
large-scale surface magnetic maps, suggesting that the complexity of the
magnetic field can play an important role in the angular momentum evolution of
the star, possibly explaining the large distribution of periods in field dM
stars, as reported in recent works. In spite of the diversity of the magnetic
field topologies among the stars in our sample, we find that stellar wind
flowing near the (rotational) equatorial plane carries most of the stellar
angular momentum, but there is no preferred colatitude contributing to mass
loss, as the mass flux is maximum at different colatitudes for different stars.
We find that more non-axisymmetric magnetic fields result in more asymmetric
mass fluxes and wind total pressures (defined as the sum of
thermal, magnetic and ram pressures). Because planetary magnetospheric sizes
are set by pressure equilibrium between the planet's magnetic field and , variations of up to a factor of in (as found in the
case of a planet orbiting at several stellar radii away from the star) lead to
variations in magnetospheric radii of about 20 percent along the planetary
orbital path. In analogy to the flux of cosmic rays that impact the Earth,
which is inversely modulated with the non-axisymmetric component of the total
open solar magnetic flux, we conclude that planets orbiting M dwarf stars like
DT~Vir, DS~Leo and GJ~182, which have significant non-axisymmetric field
components, should be the more efficiently shielded from galactic cosmic rays,
even if the planets lack a protective thick atmosphere/large magnetosphere of
their own.Comment: 16 pages, 9 figures, to appear in MNRA
A Summary and Analysis of NASA's Strategic Astrophysics Technology PCOS/COR Investments Since Program Inception
The NASA Strategic Astrophysics Technology (SAT) Program was established in 2009 as a new technology maturation program to fill the gap in the Technology Readiness Level (TRL) range from 3 to 6. Since the inception of the program, 47 tasks have been awarded under the auspices of the NASA Physics of the Cosmos (PCOS) Program in the areas of optics and detectors as well as lasers, electronics, and micro-thruster subsystems. In addition, 31 tasks have been awarded under the auspices of the NASA Cosmic Origins (COR) Program to develop optics, coatings, cooling subsystems, and detectors from the Far-IR to the Far-UV. We present the PCOS/COR portfolio distribution in terms of specific technology areas addressed and show an analysis of the rate and cost of TRL advancements. We present highlights of the infusion success stories that have emerged from the SAT maturation program as it relates to enabling future NASA astrophysics strategic missions. Finally, we present an outlook for future technology priorities for investment by the SAT Program
Magnetic Effects Change Our View of the Heliosheath
There is currently a controversy as to whether Voyager 1 has already crossed
the Termination Shock, the first boundary of the Heliosphere. The region
between the Termination Shock and the Heliopause, the Helisheath, is one of the
most unknown regions theoretically. In the Heliosheath magnetic effects are
crucial, as the solar magnetic field is compressed at the Termination Shock by
the slowing flow. Recently, our simulations showed that the Heliosheath
presents remarkable dynamics, with turbulent flows and the presence of a jet
flow at the current sheet that is unstable due to magnetohydrodynamic
instabilities \cite{opher,opher1}. In this paper we review these recent
results, and present an additional simulation with constant neutral atom
background. In this case the jet is still present but with reduced intensity.
Further study, e.g., including neutrals and the tilt of the solar rotation from
the magnetic axis, is required before we can definitively address how the
Heliosheath behaves. Already we can say that this region presents remarkable
dynamics, with turbulent flows, indicating that the Heliosheath might be very
different from what we previously thought.Comment: 6 pages, 5 figures, to appear in IGPP 3rd Annual International
Astrophysics Conference, "PHYSICS OF THE OUTER HELIOSPHERE
Magnetized jets driven by the sun: the structure of the heliosphere revisited
The classic accepted view of the heliosphere is a quiescent, comet-like shape
aligned in the direction of the Sun's travel through the interstellar medium
(ISM) extending for 1000's of AUs (AU: astronomical unit). Here we show, based
on magnetohydrodynamic (MHD) simulations, that the tension (hoop) force of the
twisted magnetic field of the sun confines the solar wind plasma beyond the
termination shock and drives jets to the North and South very much like
astrophysical jets. These jets are deflected into the tail region by the motion
of the Sun through the ISM similar to bent galactic jets moving through the
intergalactic medium. The interstellar wind blows the two jets into the tail
but is not strong enough to force the lobes into a single comet-like tail, as
happens to some astrophysical jets (Morsony et al. 2013). Instead, the
interstellar wind flows around the heliosphere and into equatorial region
between the two jets. As in some astrophysical jets that are kink unstable
(Porth et al. 2014) we show here that the heliospheric jets are turbulent (due
to large-scale MHD instabilities and reconnection) and strongly mix the solar
wind with the ISM beyond 400 AU. The resulting turbulence has important
implications for particle acceleration in the heliosphere. The two-lobe
structure is consistent with the energetic neutral atoms (ENAs) images of the
heliotail from IBEX (McComas et al. 2013) where two lobes are visible in the
North and South and the suggestion from the CASSINI (Krimigis et al. 2009,
Dialynas et al. 2013) ENAs that the heliosphere is "tailless".Comment: 19 pages, 5 figures; Astrophysical Journal Letters; in pres
Modeling highway runoff pollutant levels using a data driven model
Pollutants accumulated on road pavement during dry periods are washed off the surface with runoff water during rainfall events, presenting a potentially hazardous non-point source of pollution. Estimation of pollutant loads in these runoff waters is required for developin
MAGNETIC FLUX CONSERVATION IN THE HELIOSHEATH INCLUDING SOLAR CYCLE VARIATIONS OF MAGNETIC FIELD INTENSITY
In the heliosheath (HS), Voyager 2 has observed a flow with constant radial velocity and magnetic flux conservation. Voyager 1, however, has observed a decrease in the flow's radial velocity and an order of magnitude decrease in magnetic flux. We investigate the role of the 11 yr solar cycle variation of the magnetic field strength on the magnetic flux within the HS using a global 3D magnetohydrodynamic model of the heliosphere. We use time and latitude-dependent solar wind velocity and density inferred from Solar and Heliospheric Observatory/SWAN and interplanetary scintillations data and implemented solar cycle variations of the magnetic field derived from 27 day averages of the field magnitude average of the magnetic field at 1 AU from the OMNI database. With the inclusion of the solar cycle time-dependent magnetic field intensity, the model matches the observed intensity of the magnetic field in the HS along both Voyager 1 and 2. This is a significant improvement from the same model without magnetic field solar cycle variations, which was over a factor of two larger. The model accurately predicts the radial velocity observed by Voyager 2; however, the model predicts a flow speed ~100 km s[superscript −1] larger than that derived from LECP measurements at Voyager 1. In the model, magnetic flux is conserved along both Voyager trajectories, contrary to observations. This implies that the solar cycle variations in solar wind magnetic field observed at 1 AU does not cause the order of magnitude decrease in magnetic flux observed in the Voyager 1 data.United States. National Aeronautics and Space Administration (Earth and Space Science Fellowship Program Grant NNX14AO14H
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