4,670 research outputs found
Teaching the Doppler Effect in Astrophysics
The Doppler effect is a shift in the frequency of waves emitted from an
object moving relative to the observer. By observing and analysing the Doppler
shift in electromagnetic waves from astronomical objects, astronomers gain
greater insight into the structure and operation of our universe. In this
paper, a simple technique is described for teaching the basics of the Doppler
effect to undergraduate astrophysics students using acoustic waves. An
advantage of the technique is that it produces a visual representation of the
acoustic Doppler shift. The equipment comprises a 40 kHz acoustic transmitter
and a microphone. The sound is bounced off a computer fan and the signal
collected by a DrDAQ ADC and processed by a spectrum analyser. Widening of the
spectrum is observed as the fan power supply potential is increased from 4 to
12 V.Comment: 9 pages, 5 figures, published in Eur. J. Phy
The interaction between transpolar arcs and cusp spots
Transpolar arcs and cusp spots are both auroral phenomena which occur when
the interplanetary magnetic field is northward. Transpolar arcs are associated
with magnetic reconnection in the magnetotail, which closes magnetic flux and
results in a "wedge" of closed flux which remains trapped, embedded in the
magnetotail lobe. The cusp spot is an indicator of lobe reconnection at the
high-latitude magnetopause; in its simplest case, lobe reconnection
redistributes open flux without resulting in any net change in the open flux
content of the magnetosphere. We present observations of the two phenomena
interacting--i.e., a transpolar arc intersecting a cusp spot during part of its
lifetime. The significance of this observation is that lobe reconnection can
have the effect of opening closed magnetotail flux. We argue that such events
should not be rare
Modem design for a MOBILESAT terminal
The implementation is described of a programmable digital signal processor based system, designed for use as a test bed in the development of a digital modem, codec, and channel simulator. Code was written to configure the system as a 5600 bps or 6600 bps QPSK modem. The test bed is currently being used in an experiment to evaluate the performance of digital speech over shadowed channels in the Australian mobile satellite (MOBILESAT) project
Temperature Evolution of the Quantum Gap in CsNiCl3
Neutron scattering measurements on the one-dimensional gapped S=1
antiferromagnet, CsNiCl3, have shown that the excitation corresponding to the
Haldane mass gap Delta at low temperatures persists as a resonant feature to
high temperatures. We find that the strong upward renormalisation of the gap
excitation, by a factor of three between 5 and 70K, is more than enough to
overcome its decreasing lifetime. We find that the gap lifetime is
substantially shorter than that predicted by the scaling theory of Damle and
Sachdev in its low temperature range of validity. The upward gap
renormalisation agrees with the non-linear sigma model at low temperatures and
even up to T of order 2Delta provided an upper mass cutoff is included.Comment: Latex, 3 figures, accepted by Pysical Review
Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of magnetosphere-ionosphere decoupling by field-aligned auroral voltages
We consider the effect of field-aligned voltages on the magnetosphere-ionosphere coupling current system associated with the breakdown of rigid corotation of equatorial plasma in Jupiter's middle magnetosphere. Previous analyses have assumed perfect mapping of the electric field and flow along equipotential field lines between the equatorial plane and the ionosphere, whereas it has been shown that substantial field-aligned voltages must exist to drive the field-aligned currents associated with the main auroral oval. The effect of these field-aligned voltages is to decouple the flow of the equatorial and ionospheric plasma, such that their angular velocities are in general different from each other. In this paper we self-consistently include the field-aligned voltages in computing the plasma flows and currents in the system. A third order differential equation is derived for the ionospheric plasma angular velocity, and a power series solution obtained which reduces to previous solutions in the limit that the field-aligned voltage is small. Results are obtained to second order in the power series, and are compared to the original zeroth order results with no parallel voltage. We find that for system parameters appropriate to Jupiter the effect of the field-aligned voltages on the solutions is small, thus validating the results of previously-published analyses
Two and Three Dimensional Incommensurate Modulation in Optimally-Doped BiSrCaCuO
X-ray scattering measurements on optimally-doped single crystal samples of
the high temperature superconductor BiSrCaCuO reveal
the presence of three distinct incommensurate charge modulations, each
involving a roughly fivefold increase in the unit cell dimension along the {\bf
b}-direction. The strongest scattering comes from the well known (H, K
0.21, L) modulation and its harmonics. However, we also observe broad
diffraction which peak up at the L values complementary to those which
characterize the known modulated structure. These diffraction features
correspond to correlation lengths of roughly a unit cell dimension,
20 in the {\bf c} direction, and of 185
parallel to the incommensurate wavevector. We interpret these features as
arising from three dimensional incommensurate domains and the interfaces
between them, respectively. In addition we investigate the recently discovered
incommensuate modulations which peak up at (1/2, K 0.21, L) and related
wavevectors. Here we explicitly study the L-dependence of this scattering and
see that these charge modulations are two dimensional in nature with weak
correlations on the scale of a bilayer thickness, and that they correspond to
short range, isotropic correlation lengths within the basal plane. We relate
these new incommensurate modulations to the electronic nanostructure observed
in BiSrCaCuO using STM topography.Comment: 8 pages, 8 figure
A note on the ring current in Saturn's magnetosphere: Comparison of magnetic data obtained during the Pioneer-11 and Voyager-1 and -2 fly-bys
International audienceWe examine the residual (measured minus internal) magnetic field vectors observed in Saturn's magnetosphere during the Pioneer-11 fly-by in 1979, and compare them with those observed during the Voyager-1 and -2 fly-bys in 1980 and 1981. We show for the first time that a ring current system was present within the magnetosphere during the Pioneer-11 encounter, which was qualitatively similar to those present during the Voyager fly-bys. The analysis also shows, however, that the ring current was located closer to the planet during the Pioneer-11 encounter than during the comparable Voyager-1 fly-by, reflecting the more com-pressed nature of the magnetosphere at the time. The residual field vectors have been fit using an adaptation of the current system proposed for Jupiter by Connerney et al. (1981a). A model that provides a reasonably good fit to the Pioneer-11 Saturn data extends radially between 6.5 and 12.5 RS (compared with a noon-sector magnetopause distance of 17 RS), has a north-south extent of 4 RS, and carries a total current of 9.6 MA. A corresponding model that provides a qualitatively similar fit to the Voyager data, determined previously by Connerney et al. (1983), extends radially between 8 and 15.5 RS (compared with a noon-sector magnetopause distance for Voyager-1 of 23?24 RS), has a north-south extent of 6 RS, and carries a total current of 11.5 MA
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