299 research outputs found
Mass Transfer, Transiting Stream and Magnetopause in Close-in Exoplanetary Systems with Applications to WASP-12
We study mass transfer by Roche lobe overflow in close-in exoplanetary
systems. The planet's atmospheric gas passes through the inner Lagrangian point
and flows along a narrow stream, accelerating to 100-200\kms velocity before
forming an accretion disk. We show that the cylinder-shaped accretion stream
can have an area (projected in the plane of the sky) comparable to that of the
planet and a significant optical depth to spectral line absorption. Such a
"transiting cylinder" may produce an earlier ingress of the planet transit, as
suggested by recent HST observations of the WASP-12 system. The asymmetric disk
produced by the accretion stream may also lead to time-dependent obscuration of
the star light and apparent earlier ingress. We also consider the interaction
of the stellar wind with the planetary magnetosphere. Since the wind speed is
subsonic/sub-Alfvenic and comparable to the orbital velocity of the planet, the
head of the magnetopause lies eastward relative to the substellar line (the
line joining the planet and the star). The gas around the magnetopause may, if
sufficiently compressed, give rise to asymmetric ingress/egress during the
planet transit, although more works are needed to evaluate this possibility.Comment: 6 pages with 2 figures. Accepted in ApJ. Small changes (add
discussion on asymmetric disks
Comprehensive Analysis of Coronal Mass Ejection Mass and Energy Properties Over a Full Solar Cycle
The LASCO coronagraphs, in continuous operation since 1995, have observed the
evolution of the solar corona and coronal mass ejections (CMEs) over a full
solar cycle with high quality images and regular cadence. This is the first
time that such a dataset becomes available and constitutes a unique resource
for the study of CMEs. In this paper, we present a comprehensive investigation
of the solar cycle dependence on the CME mass and energy over a full solar
cycle (1996-2009) including the first in-depth discussion of the mass and
energy analysis methods and their associated errors. Our analysis provides
several results worthy of further studies. It demonstrates the possible
existence of two event classes; 'normal' CMEs reaching constant mass for
R_{\sun} and 'pseudo' CMEs which disappear in the C3 FOV. It shows that the
mass and energy properties of CME reach constant levels, and therefore should
be measured, only above \sim 10 R_\sun. The mass density (g/R_\sun^2) of
CMEs varies relatively little ( order of magnitude) suggesting that the
majority of the mass originates from a small range in coronal heights. We find
a sudden reduction in the CME mass in mid-2003 which may be related to a change
in the electron content of the large scale corona and we uncover the presence
of a six-month periodicity in the ejected mass from 2003 onwards.Comment: 42 pages, 16 figures, To appear in Astrophysical Journa
Estimations of changes of the Sun's mass and the gravitation constant from the modern observations of planets and spacecraft
More than 635 000 positional observations (mostly radiotechnical) of planets
and spacecraft (1961-2010), have been used for estimating possible changes of
the gravitation constant, the solar mass, and semi-major axes of planets, as
well as the value of the astronomical unit, related to them. The analysis of
the observations has been performed on the basis of the EPM2010 ephemerides of
IAA RAS in post-newtonian approximation. The obtained results indicate on
decrease in the heliocentric gravitation constant per year at the level The positive secular
changes of semi-major axes have been obtained simultaneously
for the planets Mercury, Venus, Mars, Jupiter, Saturn, as expected if the
geliocentric gravitation constant is decreasing in century wise. The change of
the mass of the Sun due to the solar radiation and the solar wind and
the matter dropping on the Sun (comets, meteors, asteroids and dust) was
estimated. Taking into account the maximal limits of the possible
change, the value falls within the interval in year with the 95% probability. The
astronomical unit (au) is only connected with the geliocentric gravitation
constant by its definition. In the future, the connection between
and au should be fixed at the certain time moment, as it is inconvenient highly
to have the changing value of the astronomical unit.Comment: 20 pages, 4 tables, accepted for publication in Solar System
Research, 2011 (Astronomicheskii vestnik
Observation of time-varying photoconductivity and persistent photoconductivity in porous silicon
Calomplification — the power of generative calorimeter models
Motivated by the high computational costs of classical simulations, machine-learned generative models can be extremely useful in particle physics and elsewhere. They become especially attractive when surrogate models can efficiently learn the underlying distribution, such that a generated sample outperforms a training sample of limited size. This kind of GANplification has been observed for simple Gaussian models. We show the same effect for a physics simulation, specifically photon showers in an electromagnetic calorimeter
Can Streamer Blobs prevent the Buildup of the Interplanetetary Magnetic Field?
Coronal Mass Ejections continuously drag closed magnetic field lines away
from the Sun, adding new flux to the interplanetary magnetic field (IMF). We
propose that the outward-moving blobs that have been observed in helmet
streamers are evidence of ongoing, small-scale reconnection in streamer current
sheets, which may play an important role in the prevention of an indefinite
buildup of the IMF. Reconnection between two open field lines from both sides
of a streamer current sheet creates a new closed field line, which becomes part
of the helmet, and a disconnected field line, which moves outward. The blobs
are formed by plasma from the streamer that is swept up in the trough of the
outward moving field line. We show that this mechanism is supported by
observations from SOHO/LASCO. Additionally, we propose a thorough statistical
study to quantify the contribution of blob formation to the reduction of the
IMF, and indicate how this mechanism may be verified by observations with
SOHO/UVCS and the proposed NASA STEREO and ESA Polar Orbiter missions.Comment: 7 pages, 2 figures; accepted by The Astrophysical Journal Letters;
uses AASTe
Structure and Dynamics of the Sun's Open Magnetic Field
The solar magnetic field is the primary agent that drives solar activity and
couples the Sun to the Heliosphere. Although the details of this coupling
depend on the quantitative properties of the field, many important aspects of
the corona - solar wind connection can be understood by considering only the
general topological properties of those regions on the Sun where the field
extends from the photosphere out to interplanetary space, the so-called open
field regions that are usually observed as coronal holes. From the simple
assumptions that underlie the standard quasi-steady corona-wind theoretical
models, and that are likely to hold for the Sun, as well, we derive two
conjectures on the possible structure and dynamics of coronal holes: (1)
Coronal holes are unique in that every unipolar region on the photosphere can
contain at most one coronal hole. (2) Coronal holes of nested polarity regions
must themselves be nested. Magnetic reconnection plays the central role in
enforcing these constraints on the field topology. From these conjectures we
derive additional properties for the topology of open field regions, and
propose several observational predictions for both the slowly varying and
transient corona/solar wind.Comment: 26 pages, 6 figure
Reconnectionless CME eruption: putting the Aly-Sturrock conjecture to rest
We demonstrate that magnetic reconnection is not necessary to initiate fast
CMEs. The Aly-Sturrock conjecture states that the magnetic energy of a given
force free boundary field is maximized when the field is open. This is
problematic for CME initiation because it leaves little or no magnetic energy
to drive the eruption, unless reconnection is present to allow some of the
field to escape without opening. Thus, it has been thought that reconnection
must be present to initiate CMEs. This theory has not been subject to rigorous
numerical testing because conventional MHD numerical models contain numerical
diffusion, which introduces uncontrolled numerical reconnection. We use a
quasi-Lagrangian simulation technique to run the first controlled experiments
of CME initiation in the complete lack of reconnection. We find that a flux
rope confined by an arcade, when twisted beyond a critical amount, can escape
to an open state, allowing some of the surrounding arcade to shrink and
releasing magnetic energy from the global field. This mechanism includes a true
ideal MHD instability. We conclude that reconnection is not a necessary trigger
for fast CME eruptions.Comment: 16 pages, 5 figure
How Many CMEs Have Flux Ropes? Deciphering the Signatures of Shocks, Flux Ropes, and Prominences in Coronagraph Observations of CMEs
We intend to provide a comprehensive answer to the question on whether all
Coronal Mass Ejections (CMEs) have flux rope structure. To achieve this, we
present a synthesis of the LASCO CME observations over the last sixteen years,
assisted by 3D MHD simulations of the breakout model, EUV and coronagraphic
observations from STEREO and SDO, and statistics from a revised LASCO CME
database. We argue that the bright loop often seen as the CME leading edge is
the result of pileup at the boundary of the erupting flux rope irrespective of
whether a cavity or, more generally, a 3-part CME can be identified. Based on
our previous work on white light shock detection and supported by the MHD
simulations, we identify a new type of morphology, the `two-front' morphology.
It consists of a faint front followed by diffuse emission and the bright
loop-like CME leading edge. We show that the faint front is caused by density
compression at a wave (or possibly shock) front driven by the CME. We also
present high-detailed multi-wavelength EUV observations that clarify the
relative positioning of the prominence at the bottom of a coronal cavity with
clear flux rope structure. Finally, we visually check the full LASCO CME
database for flux rope structures. In the process, we classify the events into
two clear flux rope classes (`3-part', `Loop'), jets and outflows (no clear
structure). We find that at least 40% of the observed CMEs have clear flux rope
structures. We propose a new definition for flux rope CMEs (FR-CMEs) as a
coherent magnetic, twist-carrying coronal structure with angular width of at
least 40 deg and able to reach beyond 10 Rsun which erupts on a time scale of a
few minutes to several hours. We conclude that flux ropes are a common
occurrence in CMEs and pose a challenge for future studies to identify CMEs
that are clearly not FR-CMEs.Comment: 26 pages, 9 figs, to be published in Solar Physics Topical Issue
"Flux Rope Structure of CMEs
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