13,313 research outputs found
Radial distribution of the inner magnetosphere plasma pressure using low-altitude satellite data during geomagnetic storm: the March 1-8, 1982 Event
Plasma pressure distribution in the inner magnetosphere is one of the key
parameters for understanding the main magnetospheric processes including
geomagnetic storms and substorms. However, the pressure profiles obtained from
in-situ particle measurements by the high-altitude satellites do not allow
tracking the pressure variations related to the storms and substorms, because a
time interval needed to do this generally exceeds the characteristic times of
them. On contrary, fast movement of low-altitude satellites makes it possible
to retrieve quasi-instantaneous profiles of plasma pressure along the satellite
trajectory, using the fluxes of precipitating particles. For this study, we
used the Aureol-3 satellite data for plasma pressure estimation, and the IGRF,
Tsyganenko 2001 and Tsyganenko 2004 storm time geomagnetic field models for the
pressure mapping into the equatorial plane. It was found that during quiet
geomagnetic condition the radial pressure profiles obtained coincide with the
profiles, obtained previously from the high-altitude measurements. On the
contrary, it was found that during geomagnetic storm the plasma pressure
profiles became sharper; the position of the maximum of plasma pressure
corresponds to expected one for given Dst minimum; the maximum value of inner
magnetosphere static pressure correlates with the solar wind dynamic pressure.
Increase in the plasma pressure profiles indicates the possibility to consider
the interchange instability as one of important factors for the development of
the main phase of geomagnetic storm.Comment: Accepted in Advances in Space Researc
325 MHz VLA Observations of Ultracool Dwarfs TVLM 513-46546 and 2MASS J0036+1821104
We present 325 MHz (90 cm wavelength) radio observations of ultracool dwarfs
TVLM 513-46546 and 2MASS J0036+1821104 using the Very Large Array (VLA) in June
2007. Ultracool dwarfs are expected to be undetectable at radio frequencies,
yet observations at 8.5 GHz (3.5 cm) and 4.9 GHz (6 cm) of have revealed
sources with > 100 {\mu}Jy quiescent radio flux and > 1 mJy pulses coincident
with stellar rotation. The anomalous emission is likely a combination of
gyrosynchrotron and cyclotron maser processes in a long-duration, large-scale
magnetic field. Since the characteristic frequency for each process scales
directly with the magnetic field magnitude, emission at lower frequencies may
be detectable from regions with weaker field strength. We detect no significant
radio emission at 325 MHz from TVLM 513-46546 or 2MASS J0036+1821104 over
multiple stellar rotations, establishing 2.5{\sigma} total flux limits of 795
{\mu}Jy and 942 {\mu}Jy respectively. Analysis of an archival VLA 1.4 GHz
observation of 2MASS J0036+1821104 from January 2005 also yields a
non-detection at the level of < 130 {\mu}Jy . The combined radio observation
history (0.3 GHz to 8.5 GHz) for these sources suggests a continuum emission
spectrum for ultracool dwarfs which is either flat or inverted below 2-3 GHz.
Further, if the cyclotron maser instability is responsible for the pulsed radio
emission observed on some ultracool dwarfs, our low-frequency non-detections
suggest that the active region responsible for the high-frequency bursts is
confined within 2 stellar radii and driven by electron beams with energies less
than 5 keV.Comment: 11 pages, 5 figures, submitted to A
A Mini-survey of Ultracool Dwarfs at 4.9 GHz
A selection of ultracool dwarfs are known to be radio active, with both
gyrosynchrotron emission and the electron cyclotron maser instability being
given as likely emission mechanisms. To explore whether ultracool dwarfs
previously undetected at 8.5 GHz may be detectable at a lower frequency. We
select a sample of fast rotating ultracool dwarfs with no detectable radio
activity at 8.5 GHz, observing each of them at 4.9 GHz. From the 8 dwarfs in
our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux
level of 286 24 . The light-curve of 2MASS J07464256+2000321, is
dominated towards the end of the observation by a very bright, 100 %
left circularly polarized burst during which the flux reached 2.4 mJy. The
burst was preceded by a raise in the level of activity, with the average flux
being 160 in the first hour of observation rising to
400 in the 40 minutes before the burst. During both periods,
there is significant variability. The detection of 100% circular polarization
in the emission at 4.9 GHz points towards the electron cyclotron maser as the
emission mechanism. However, the observations at 4.9 GHz and 8.5 GHz were not
simultaneous, thus the actual fraction of dwarfs capable of producing radio
emission, as well as the fraction of those that show periodic pulsations is
still unclear, as indeed are the relative roles played by the electron
cyclotron maser instability versus gyrosynchrotron emission, therefore we
cannot assert if the previous non-detection at 8.5 GHz was due to a cut-off in
emission between 4.9 and 8.4 GHz, or due to long term variability
Sporadic Long-term Variability in Radio Activity from a Brown Dwarf
Radio activity has been observed in a large variety of stellar objects,
including in the last few years, ultra-cool dwarfs. To explore the extent of
long-term radio activity in ultra-cool dwarfs, we use data taken over an
extended period of 9 hr from the Very Large Array of the source 2MASS
J05233822-1403022 in September 2006, plus data taken in 2004. The observation
taken in September 2006 failed to detect any radio activity at 8.46 GHz. A
closer inspection of earlier data reveals that the source varied from a null
detection on 3 May 2004, to 95 Jy on 17 May 2004, to 230 Jy
on 18 June 2004. The lack of detection in September 2006 suggests at least a
factor of ten flux variability at 8.46 GHz. Three short photometric runs did
not reveal any optical variability. In addition to the observed pulsing nature
of the radio flux from another ultra-cool source, the present observations
suggests that ultra-cool dwarfs may not just be pulsing but can also display
long-term sporadic variability in their levels of quiescent radio emission. The
lack of optical photometric variability suggests an absence of large-scale
spots at the time of the latest VLA observations, although small very high
latitude spots combined with a low inclination could cause very low amplitude
rotational modulation which may not be measurable. We discuss this large
variability in the radio emission within the context of both gyrosynchrotron
emission and the electron-cyclotron maser, favoring the latter mechanism.Comment: 7 pages, 2 figures, 1 table, accepted for publication in A&A Letter
Detection of Radio Emission from the Hyperactive L Dwarf 2MASS J13153094-2649513AB
We report the detection of radio emission from the unusually active L5e + T7
binary 2MASS J13153094-2649513AB made with the Australian Telescope Compact
Array. Observations at 5.5 GHz reveal an unresolved source with a continuum
flux of 370+/-50 microJy, corresponding to a radio luminosity of L_rad = nuL_nu
= (9+/-3)x10^23 erg/s and log10(L_rad/L_bol) = -5.44+/-0.22. No detection is
made at 9.0 GHz to a 5 sigma limit of 290 microJy, consistent with a power law
spectrum S_nu ~ nu^-a with a > 0.5. The emission is quiescent, with no evidence
of variability or bursts over 3 hr of observation, and no measurable
polarization (V/I < 34%). 2MASS J1315-2649AB is one of the most radio-luminous
ultracool dwarfs detected in quiescent emission to date, comparable in strength
to other cool sources detected in outburst. Its detection indicates no decline
in radio flux through the mid-L dwarfs. It is unique among L dwarfs in having
strong and persistent Halpha and radio emission, indicating the coexistence of
a cool, neutral photosphere (low electron density) and a highly active
chromosphere (high electron density and active heating). These traits, coupled
with the system's mature age and substellar secondary, makes 2MASS J1315-2649AB
an important test for proposed radio emission mechanisms in ultracool dwarfs.Comment: 5 pages, 4 figures, accepted for publication in ApJ Letter
Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
[Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical
observation of the L dwarf binary 2MASSW J0746+20. We detect strong radio
emission, dominated by short-duration periodic pulses at 4.86 GHz with
P=124.32+/-0.11 min. The stability of the pulse profiles and arrival times
demonstrates that they are due to the rotational modulation of a B~1.7 kG
magnetic field. A quiescent non-variable component is also detected, likely due
to emission from a uniform large-scale field. The H-alpha emission exhibits
identical periodicity, but unlike the radio pulses it varies sinusoidally and
is offset by exactly 1/4 of a phase. The sinusoidal variations require
chromospheric emission from a large-scale field structure, with the radio
pulses likely emanating from the magnetic poles. While both light curves can be
explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out
a symmetric dipole topology since it would result in a phase lag of 1/2
(poloidal field) or zero (toroidal field). We therefore conclude that either
(i) the field is dominated by a quadrupole configuration, which can naturally
explain the 1/4 phase lag; or (ii) the H-alpha and/or radio emission regions
are not trivially aligned with the field. Regardless of the field topology, we
use the measured period along with the known rotation velocity (vsini=27 km/s),
and the binary orbital inclination (i=142 deg), to derive a radius for the
primary star of 0.078+/-0.010 R_sun. This is the first measurement of the
radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides
a constraint on the mass-radius relation below 0.1 M_sun. We find that the
radius is about 30% smaller than expected from theoretical models, even for an
age of a few Gyr.Comment: Submitted to Ap
Power and multistakeholderism in internet global governance. Towards a synergetic theoretical framework
With the advancement of multistakeholder collaboration as a governance principle in theglobal Internet Governance, how to investigate the political process in a ‘shared power’environment emerges as a challenging methodological issue. In this paper, a synergetic theoretical approach is proposed to the study of Internet governance political process, which focuses on the concept of power, and crosses the boundaries of three academic fields, namely, Political Philosophy, Political Science and International Relations, and Organization Studies. This approach aggregates, in a descending analytical manner, concepts intrinsically linked to the contemporary shifting governance paradigm (i.e. governmentality, global governance, global public-policy networks, shared power, multistakeholder collaboration). In addition, such an approach brings the collaborative process into focus (rather than the decisions it leads to) by accentuating the productive potential of a collaboration based on the ‘shared power’ formula. Each of those theoretical reflections on shifting power relations provides building elements for a synergetic theoretical framework that can be, and has been, applied to the investigation of the emergent Internet governance regime. As a result, stakeholder alliances can be mapped, instances of power dynamics can be discerned, and some longitudinal tangible and intangible outcomes of the multistakeholder collaboration can be envisioned
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