675 research outputs found
Candidates for detecting exoplanetary radio emissions generated by magnetosphere-ionosphere coupling
In this paper we consider the magnetosphere-ionosphere (M-I) coupling at
Jupiter-like exoplanets with internal plasma sources such as volcanic moons,
and we have determined the best candidates for detection of these radio
emissions by estimating the maximum spectral flux density expected from planets
orbiting stars within 25 pc using data listed in the NASA/IPAC/NExScI Star and
Exoplanet Database (NStED). In total we identify 91 potential targets, of which
40 already host planets and 51 have stellar X-ray luminosity 100 times the
solar value. In general, we find that stronger planetary field strength,
combined with faster rotation rate, higher stellar XUV luminosity, and lower
stellar wind dynamic pressure results in higher radio power. The top two
targets for each category are Eri and HIP 85523, and CPD-28 332 and
FF And.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Letter
Searching for Star-Planet interactions within the magnetosphere of HD 189733
HD 189733 is a K2 dwarf, orbited by a giant planet at 8.8 stellar radii. In
order to study magnetospheric interactions between the star and the planet, we
explore the large-scale magnetic field and activity of the host star.
We collected spectra using the ESPaDOnS and the NARVAL spectropolarimeters,
installed at the 3.6-m Canada-France-Hawaii telescope and the 2-m Telescope
Bernard Lyot at Pic du Midi, during two monitoring campaigns (June 2007 and
July 2008).
HD 189733 has a mainly toroidal surface magnetic field, having a strength
that reaches up to 40 G. The star is differentially rotating, with latitudinal
angular velocity shear of domega = 0.146 +- 0.049 rad/d, corresponding to
equatorial and polar periods of 11.94 +- 0.16 d and 16.53 +- 2.43 d
respectively. The study of the stellar activity shows that it is modulated
mainly by the stellar rotation (rather than by the orbital period or the beat
period between the stellar rotation and the orbital periods). We report no
clear evidence of magnetospheric interactions between the star and the planet.
We also extrapolated the field in the stellar corona and calculated the
planetary radio emission expected for HD 189733b given the reconstructed field
topology. The radio flux we predict in the framework of this model is time
variable and potentially detectable with LOFAR
Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: implications for detectability of auroral radio emissions
In this paper we provide the first consideration of magnetosphere-ionosphere
coupling at Jupiter-like exoplanets with internal plasma sources such as
volcanic moons. We estimate the radio power emitted by such systems under the
condition of near-rigid corotation throughout the closed magnetosphere, in
order to examine the behaviour of the best candidates for detection with next
generation radio telescopes. We thus estimate for different stellar X-ray-UV
(XUV) luminosity cases the orbital distances within which the ionospheric
Pedersen conductance would be high enough to maintain near-rigid corotation,
and we then consider the magnitudes of the large-scale magnetosphere-ionosphere
currents flowing within the systems, and the resulting radio powers, at such
distances. We also examine the effects of two key system parameters, i.e. the
planetary angular velocity and the plasma mass outflow rate from sources
internal to the magnetosphere. In all XUV luminosity cases studied, a
significant number of parameter combinations within an order of magnitude of
the jovian values are capable of producing emissions observable beyond 1 pc, in
most cases requiring exoplanets orbiting at distances between ~1 and 50 AU, and
for the higher XUV luminosity cases these observable distances can reach beyond
~50 pc for massive, rapidly rotating planets. The implication of these results
is that the best candidates for detection of such internally-generated radio
emissions are rapidly rotating Jupiter-like exoplanets orbiting stars with high
XUV luminosity at orbital distances beyond ~1 AU, and searching for such
emissions may offer a new method of detection of more distant-orbiting
exoplanets.Comment: 15 pages, 9 figures. In press at Mon. Not. R. Astron. So
Numerical simulations of unbounded cyclotron-maser emissions
Numerical simulations have been conducted to study the spatial growth rate and emission topology of the cyclotron-maser instability responsible for stellar/planetary auroral magnetospheric radio emission and intense non-thermal radio emission in other astrophysical contexts. These simulations were carried out in an unconstrained geometry, so that the conditions existing within the source region of some natural electron cyclotron masers could be more closely modelled. The results have significant bearing on the radiation propagation and coupling characteristics within the source region of such non-thermal radio emissions
Emission and propagation of Saturn kilometric radiation: magneto-ionic modes, beaming pattern and polarization state
The Cassini mission crossed the source region of the Saturn kilometric
radiation (SKR) on 17 October 2008. On this occasion, the Radio and Plasma Wave
Science (RPWS) experiment detected both local and distant radio sources, while
plasma parameters were measured in situ by the magnetometer (MAG) and the
Cassini Plasma Spectrometer (CAPS). A goniopolarimetric inversion was applied
to RPWS 3-antenna electric measurements to determine the wave vector k and the
complete state of polarization of detected waves. We identify broadband
extraordinary (X) as well as narrowband ordinary (O) mode SKR at low
frequencies. Within the source region, SKR is emitted just above the X mode
cutoff frequency in a hot plasma, with a typical electron-to-wave energy
conversion efficiency of 1% (2% peak). The knowledge of the k-vector is then
used to derive the locus of SKR sources in the kronian magnetosphere, that
shows X and O components emanating from the same regions. We also compute the
associated beaming angle at the source theta'=(k,-B) either from (i) in situ
measurements or a model of the magnetic field vector or from (ii) polarization
measurements. Obtained results, similar for both modes, suggest
quasi-perpendicular emission for local sources, whereas the beaming pattern of
distant sources appears as a hollow cone with a frequency-dependent constant
aperture angle: theta'=75{\deg}+/-15{\deg} below 300kHz, decreasing at higher
frequencies to reach theta'(1000kHz)=50{\deg}+/-25{\deg}. Finally, we
investigate quantitatively the SKR polarization state, observed to be strongly
elliptical at the source, and quasi-purely circular for sources located beyond
approximately 2 kronian radii. We show that conditions of weak mode coupling
are achieved along the ray path, under which the magneto-ionic theory
satisfactorily describes the evolution of the observed polarization
Overview of Saturn lightning observations
The lightning activity in Saturn's atmosphere has been monitored by Cassini
for more than six years. The continuous observations of the radio signatures
called SEDs (Saturn Electrostatic Discharges) combine favorably with imaging
observations of related cloud features as well as direct observations of
flash-illuminated cloud tops. The Cassini RPWS (Radio and Plasma Wave Science)
instrument and ISS (Imaging Science Subsystem) in orbit around Saturn also
received ground-based support: The intense SED radio waves were also detected
by the giant UTR-2 radio telescope, and committed amateurs observed SED-related
white spots with their backyard optical telescopes. Furthermore, the Cassini
VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared
Spectrometer) instruments have provided some information on chemical
constituents possibly created by the lightning discharges and transported
upward to Saturn's upper atmosphere by vertical convection. In this paper we
summarize the main results on Saturn lightning provided by this
multi-instrumental approach and compare Saturn lightning to lightning on
Jupiter and Earth.Comment: 10 pages, 6 figures, 2 tables; Proc. PRE VII conference Graz Sept.
201
Surveying the Dynamic Radio Sky with the Long Wavelength Demonstrator Array
This paper presents a search for radio transients at a frequency of 73.8 MHz
(4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength
Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope,
located on the site of the Very Large Array in New Mexico. The field of view of
the individual dipoles was essentially the entire sky, and the number of
dipoles was sufficiently small that a simple software correlator could be used
to make all-sky images. From 2006 October to 2007 February, we conducted an
all-sky transient search program, acquiring a total of 106 hr of data; the time
sampling varied, being 5 minutes at the start of the program and improving to 2
minutes by the end of the program. We were able to detect solar flares, and in
a special-purpose mode, radio reflections from ionized meteor trails during the
2006 Leonid meteor shower. We detected no transients originating outside of the
solar system above a flux density limit of 500 Jy, equivalent to a limit of no
more than about 10^{-2} events/yr/deg^2, having a pulse energy density >~ 1.5 x
10^{-20} J/m^2/Hz at 73.8 MHz for pulse widths of about 300 s. This event rate
is comparable to that determined from previous all-sky transient searches, but
at a lower frequency than most previous all-sky searches. We believe that the
LWDA illustrates how an all-sky imaging mode could be a useful operational
model for low-frequency instruments such as the Low Frequency Array, the Long
Wavelength Array station, the low-frequency component of the Square Kilometre
Array, and potentially the Lunar Radio Array.Comment: 20 pages; accepted for publication in A
The LOFAR Transients Pipeline
Current and future astronomical survey facilities provide a remarkably rich
opportunity for transient astronomy, combining unprecedented fields of view
with high sensitivity and the ability to access previously unexplored
wavelength regimes. This is particularly true of LOFAR, a
recently-commissioned, low-frequency radio interferometer, based in the
Netherlands and with stations across Europe. The identification of and response
to transients is one of LOFAR's key science goals. However, the large data
volumes which LOFAR produces, combined with the scientific requirement for
rapid response, make automation essential. To support this, we have developed
the LOFAR Transients Pipeline, or TraP. The TraP ingests multi-frequency image
data from LOFAR or other instruments and searches it for transients and
variables, providing automatic alerts of significant detections and populating
a lightcurve database for further analysis by astronomers. Here, we discuss the
scientific goals of the TraP and how it has been designed to meet them. We
describe its implementation, including both the algorithms adopted to maximize
performance as well as the development methodology used to ensure it is robust
and reliable, particularly in the presence of artefacts typical of radio
astronomy imaging. Finally, we report on a series of tests of the pipeline
carried out using simulated LOFAR observations with a known population of
transients.Comment: 30 pages, 11 figures; Accepted for publication in Astronomy &
Computing; Code at https://github.com/transientskp/tk
Properties of Saturn Kilometric Radiation measured within its source region
On 17 October 2008, the Cassini spacecraft crossed the southern sources of
Saturn kilometric radiation (SKR), while flying along high-latitude nightside
magnetic field lines. In situ measurements allowed us to characterize for the
first time the source region of an extra-terrestrial auroral radio emission.
Using radio, magnetic field and particle observations, we show that SKR sources
are surrounded by a hot tenuous plasma, in a region of upward field-aligned
currents. Magnetic field lines supporting radio sources map a continuous,
high-latitude and spiral-shaped auroral oval observed on the dawnside,
consistent with enhanced auroral activity. Investigating the Cyclotron Maser
Instability (CMI) as a mechanism responsible for SKR generation, we find that
observed cutoff frequencies are consistent with radio waves amplified
perpendicular to the magnetic field by hot (6 to 9 keV) resonant electrons,
measured locally
- âŠ