394 research outputs found
An empirical study of the “prototype walkthrough”: a studio-based activity for HCI education
For over a century, studio-based instruction has served as an effective pedagogical model in architecture and fine arts education. Because of its design orientation, human-computer interaction (HCI) education is an excellent venue for studio-based instruction. In an HCI course, we have been exploring a studio-based learning activity called the prototype walkthrough, in which a student project team simulates its evolving user interface prototype while a student audience member acts as a test user. The audience is encouraged to ask questions and provide feedback. We have observed that prototype walkthroughs create excellent conditions for learning about user interface design. In order to better understand the educational value of the activity, we performed a content analysis of a video corpus of 16 prototype walkthroughs held in two HCI courses. We found that the prototype walkthrough discussions were dominated by relevant design issues. Moreover, mirroring the justification behavior of the expert instructor, students justified over 80 percent of their design statements and critiques, with nearly one-quarter of those justifications having a theoretical or empirical basis. Our findings suggest that PWs provide valuable opportunities for students to actively learn HCI design by participating in authentic practice, and provide insight into how such opportunities can be best promoted
The quasi-free-standing nature of graphene on H-saturated SiC(0001)
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001)
which was prepared by intercalation of hydrogen under the buffer layer. Using
infrared absorption spectroscopy we prove that the SiC(0001) surface is
saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer
layer into graphene which exhibits a slight tensile strain and short range
defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier
mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the
buffer layer a strongly reduced temperature dependence of the mobility is
observed for graphene on H-terminated SiC(0001)which justifies the term
"quasi-free-standing".Comment: 3 pages, 3 figures, accepted for publication in Applied Physics
Letter
A statistical study of the post-impulsive-phase acceleration of flare-associated coronal mass ejections
It is now generally accepted that the impulsive acceleration of a coronal
mass ejection (CME) in the inner corona is closely correlated in time with the
main energy release of the associated solar flare. In this paper, we examine in
detail the post-impulsive-phase acceleration of a CME in the outer corona,
which is the phase of evolution immediately following the main impulsive
acceleration of the CME; this phase is believed to correspond to the decay
phase of the associated flare. This observational study is based on a
statistical sample of 247 CMEs that are associated with M- and X-class GOES
soft X-ray flares from 1996 to 2006. We find that, from many examples of
events, the CMEs associated with flares with long-decay time (or so-called
long-duration flares) tend to have positive post-impulsive-phase acceleration,
even though some of them have already obtained a high speed at the end of the
impulsive acceleration but do not show a deceleration expected from the
aerodynamic dragging of the background solar wind. On the other hand, the CMEs
associated with flares of short-decay time tend to have significant
deceleration. In the scattering plot of all events, there is a weak correlation
between CME post-impulsive-phase acceleration and flare decay time. The CMEs
deviated from the general trend are mostly slow or weak ones associated with
flares of short-decay time; the deviation is caused by the relatively stronger
solar wind dragging force for these events. The implications of our results on
CME dynamics and CME-flare relations are discussed.Comment: 32 pages, 9 figures, accepted for publication in Ap
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
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
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
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
Study the build-up, initiation and acceleration of 2008 April 26 coronal mass ejection observed by STEREO
In this paper, we analyze the full evolution, from a few days prior to the
eruption to the initiation, and the final acceleration and propagation, of the
CME that occurred on 2008 April 26 using the unprecedented high cadence and
multi-wavelength observations by STEREO. There existed frequent filament
activities and EUV jets prior to the CME eruption for a few days. These
activities were probably caused by the magnetic reconnection in the lower
atmosphere driven by photospheric convergence motions, which were evident in
the sequence of magnetogram images from MDI (Michelson Doppler Imager) onboard
SOHO. The slow low-layer magnetic reconnection may be responsible for the
storage of magnetic free energy in the corona and the formation of a sigmoidal
core field or a flux rope leading to the eventual eruption. The occurrence of
EUV brightenings in the sigmoidal core field prior to the rise of the flux rope
implies that the eruption was triggered by the inner tether-cutting
reconnection, but not the external breakout reconnection. During the period of
impulsive acceleration, the time profile of the CME acceleration in the inner
corona is found to be consistent with the time profile of the reconnection
electric field inferred from the footpoint separation and the RHESSI 15-25 keV
HXR flux curve of the associated flare. The full evolution of this CME can be
described in four distinct phases: the build-up phase, initiation phase, main
acceleration phase, and propagation phase. The physical properties and the
transition between these phases are discussed, in an attempt to provide a
global picture of CME dynamic evolution.Comment: 28 pages, 8 figures, accepted for publication in Ap
Electron and proton heating by solar wind turbulence
Previous formulations of heating and transport associated with strong
magnetohydrodynamic (MHD) turbulence are generalized to incorporate separate
internal energy equations for electrons and protons. Electron heat conduction
is included. Energy is supplied by turbulent heating that affects both
electrons and protons, and is exchanged between them via collisions. Comparison
to available Ulysses data shows that a reasonable accounting for the data is
provided when (i) the energy exchange timescale is very long and (ii) the
deposition of heat due to turbulence is divided, with 60% going to proton
heating and 40% into electron heating. Heat conduction, determined here by an
empirical fit, plays a major role in describing the electron data
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