271,375 research outputs found
Magnetic Field Strength in the Upper Solar Corona Using White-light Shock Structures Surrounding Coronal Mass Ejections
To measure the magnetic field strength in the solar corona, we examined 10
fast (> 1000 km/s) limb CMEs which show clear shock structures in SOHO/LASCO
images. By applying piston-shock relationship to the observed CME's standoff
distance and electron density compression ratio, we estimated the Mach number,
Alfven speed, and magnetic field strength in the height range 3 to 15 solar
radii (Rs). Main results from this study are: (1) the standoff distance
observed in solar corona is consistent with those from a magnetohydrodynamic
(MHD) model and near-Earth observations; (2) the Mach number as a shock
strength is in the range 1.49 to 3.43 from the standoff distance ratio, but
when we use the density compression ratio, the Mach number is in the range 1.47
to 1.90, implying that the measured density compression ratio is likely to be
underestimated due to observational limits; (3) the Alfven speed ranges from
259 to 982 km/s and the magnetic field strength is in the range 6 to 105 mG
when the standoff distance is used; (4) if we multiply the density compression
ratio by a factor of 2, the Alfven speeds and the magnetic field strengths are
consistent in both methods; (5) the magnetic field strengths derived from the
shock parameters are similar to those of empirical models and previous
estimates.Comment: Accepted for publication in ApJ, 11 Figures, 1 Tabl
Magnetic Field Strength in the Upper Solar Corona Using White-light Shock Structures Surrounding Coronal Mass Ejections
To measure the magnetic field strength in the solar corona, we examined 10
fast (> 1000 km/s) limb CMEs which show clear shock structures in SOHO/LASCO
images. By applying piston-shock relationship to the observed CME's standoff
distance and electron density compression ratio, we estimated the Mach number,
Alfven speed, and magnetic field strength in the height range 3 to 15 solar
radii (Rs). Main results from this study are: (1) the standoff distance
observed in solar corona is consistent with those from a magnetohydrodynamic
(MHD) model and near-Earth observations; (2) the Mach number as a shock
strength is in the range 1.49 to 3.43 from the standoff distance ratio, but
when we use the density compression ratio, the Mach number is in the range 1.47
to 1.90, implying that the measured density compression ratio is likely to be
underestimated due to observational limits; (3) the Alfven speed ranges from
259 to 982 km/s and the magnetic field strength is in the range 6 to 105 mG
when the standoff distance is used; (4) if we multiply the density compression
ratio by a factor of 2, the Alfven speeds and the magnetic field strengths are
consistent in both methods; (5) the magnetic field strengths derived from the
shock parameters are similar to those of empirical models and previous
estimates.Comment: Accepted for publication in ApJ, 11 Figures, 1 Tabl
Tides or dark matter sub-halos: Which ones are more attractive?
Young tidal dwarf galaxies (TDGs) are observed in the tidal debris of
gas-rich interacting galaxies. In contrast to what is generally assumed to be
the case for isolated dwarf galaxies, TDGs are not embedded in their own dark
matter (DM) sub-halo. Hence, they are more sensitive to stellar feedback and
could be disrupted on a short time-scale. Detailed numerical and observational
studies demonstrate that isolated DM-dominated dwarf galaxies can have
lifetimes of more than 10 Gyr. For TDGs that evolve in a tidal field with
compressing accelerations equal to the gravitational acceleration within a DM
sub-halo typical of an isolated dwarf galaxy, a similar survival time is
expected. The tidal acceleration profile depends on the virial mass of the host
galaxy and the distance between the TDG and its host. We analytically compare
the tidal compression to the gravitational acceleration due to either cuspy or
cored DM sub-halos of various virial masses. For example, the tidal field at a
distance of 100 kpc to a host halo of 10^13 Msol can be as stabilizing as a
10^9 Msol DM sub-halo. By linking the tidal field to the equivalent
gravitational field of a DM sub-halo, we can use existing models of isolated
dwarfs to estimate the survivability of TDGs. We show that part of the
unexpectedly high dynamical masses inferred from observations of some TDGs can
be explained by tidal compression and hence TDGs require to contain less
unobservable matter to understand their rotation curves.Comment: 11 pages, 7 figures, accepted for publication in MNRA
The Underestimation Of Egocentric Distance: Evidence From Frontal Matching Tasks
There is controversy over the existence, nature, and cause of error in egocentric distance judgments. One proposal is that the systematic biases often found in explicit judgments of egocentric distance along the ground may be related to recently observed biases in the perceived declination of gaze (Durgin & Li, Attention, Perception, & Psychophysics, in press), To measure perceived egocentric distance nonverbally, observers in a field were asked to position themselves so that their distance from one of two experimenters was equal to the frontal distance between the experimenters. Observers placed themselves too far away, consistent with egocentric distance underestimation. A similar experiment was conducted with vertical frontal extents. Both experiments were replicated in panoramic virtual reality. Perceived egocentric distance was quantitatively consistent with angular bias in perceived gaze declination (1.5 gain). Finally, an exocentric distance-matching task was contrasted with a variant of the egocentric matching task. The egocentric matching data approximate a constant compression of perceived egocentric distance with a power function exponent of nearly 1; exocentric matches had an exponent of about 0.67. The divergent pattern between egocentric and exocentric matches suggests that they depend on different visual cues
Coronal Magnetic Field Measurement from EUV Images made by the Solar Dynamics Observatory
By measuring the geometrical properties of the coronal mass ejection (CME)
flux rope and the leading shock observed on 2010 June 13 by the Solar Dynamics
Observatory (SDO) mission's Atmospheric Imaging Assembly (AIA) we determine the
Alfv\'en speed and the magnetic field strength in the inner corona at a
heliocentric distance of ~ 1.4 Rs. The basic measurements are the shock
standoff distance (deltaR) ahead of the CME flux rope, the radius of curvature
of the flux rope (Rc), and the shock speed. We first derive the Alfv\'enic Mach
number (M) using the relationship, deltaR/Rc = 0.81[(gamma-1) M^2 +
2]/[(gamma+1)(M^2-1)], where gamma is the only parameter that needed to be
assumed. For gamma =4/3, the Mach number declined from 3.7 to 1.5 indicating
shock weakening within the field of view of the imager. The shock formation
coincided with the appearance of a type II radio burst at a frequency of ~300
MHz (harmonic component), providing an independent confirmation of the shock.
The shock compression ratio derived from the radio dynamic spectrum was found
to be consistent with that derived from the theory of fast mode MHD shocks.
From the measured shock speed and the derived Mach number, we found the
Alfv\'en speed to increase from ~140 km/s to 460 km/s over the distance range
1.2 to 1.5 Rs. By deriving the upstream plasma density from the emission
frequency of the associated type II radio burst, we determined the coronal
magnetic field to be in the range 1.3 to 1.5 G. The derived magnetic field
values are consistent with other estimates in a similar distance range. This
work demonstrates that the EUV imagers, in the presence of radio dynamic
spectra, can be used as coronal magnetometers.Comment: 25 pages, 6 figures, 2 table
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