164 research outputs found
A spectral method for bipartizing a network and detecting a large anti-community
Relations between discrete quantities such as people, genes, or streets can
be described by networks, which consist of nodes that are connected by edges.
Network analysis aims to identify important nodes in a network and to uncover
structural properties of a network. A network is said to be bipartite if its
nodes can be subdivided into two nonempty sets such that there are no edges
between nodes in the same set. It is a difficult task to determine the closest
bipartite network to a given network. This paper describes how a given network
can be approximated by a bipartite one by solving a sequence of fairly simple
optimization problems. The algorithm also produces a node permutation which
makes the possible bipartite nature of the initial adjacency matrix evident,
and identifies the two sets of nodes. We finally show how the same procedure
can be used to detect the presence of a large anti-community in a network and
to identify it.Comment: 30 page
Two-year observations of the Jupiter polar regions by JIRAM on board Juno
We observed the evolution of Jupiter's polar cyclonic structures over two years between February 2017 and February 2019, using polar observations by the Jovian InfraRed Auroral Mapper, JIRAM, on the Juno mission. Images and spectra were collected by the instrument in the 5âÎŒm wavelength range. The images were used to monitor the development of the cyclonic and anticyclonic structures at latitudes higher than 80° both in the northern and the southern hemispheres. Spectroscopic measurements were then used to monitor the abundances of the minor atmospheric constituents water vapor, ammonia, phosphine and germane in the polar regions, where the atmospheric optical depth is less than 1. Finally, we performed a comparative analysis with oceanic cyclones on Earth in an attempt to explain the spectral characteristics of the cyclonic structures we observe in Jupiter's polar atmosphere
First Observations of CH 4 and Spatially Resolved Emission Layers at Jupiter Equator, as Seen by JIRAM/Juno
In this work, we present the detection of CH4 and urn:x-wiley:21699097:media:jgre22151:jgre22151-math-0003 emissions in the equatorial atmosphere of Jupiter as two well-separated layers located, respectively, at tangent altitudes of about 200 and 500â600 km above the 1-bar level using the observations of the Jovian InfraRed Auroral Mapper (JIRAM), on board Juno. This provides details of the vertical distribution of urn:x-wiley:21699097:media:jgre22151:jgre22151-math-0004 retrieving its Volume Mixing Ratio (VMR), concentration, and temperature. The thermal profile obtained from urn:x-wiley:21699097:media:jgre22151:jgre22151-math-0005 shows a peak of 600â800 K at about 550 km, with lower values than the ones reported in Seiff et al. (1998), https://doi.org/10.1029/98JE01766 above 500 km using VMR and temperature as free parameters and above 650 km when VMR is kept fixed with that model in the retrieval procedure. The observed deviations from the Galileo's profile could potentially point to significant variability in the exospheric temperature with time. We suggest that vertically propagating waves are the most likely explanation for the observed VMR and temperature variations in the JIRAM data. Other possible phenomena could explain the observed evidence, for example, dynamic activity driving chemical species from lower layers toward the upper atmosphere, like the advection-diffusion processes, or precipitation by soft electrons, although better modeling is required to test these hypothesis. The characterization of CH4 and urn:x-wiley:21699097:media:jgre22151:jgre22151-math-0006 species, simultaneously observed by JIRAM, offers the opportunity for better constraining atmospheric models of Jupiter at equatorial latitudes
Turbulence Power Spectra in Regions Surrounding Jupiter's South Polar Cyclones from Juno/JIRAM
We present a power spectral analysis of two narrow annular regions near Jupiter's South Pole derived from data acquired by the Jovian Infrared Auroral Mapper instrument onboard NASA's Juno mission. In particular, our analysis focuses on the data set acquired by the Jovian Infrared Auroral Mapper Mâband imager (hereafter IMGâM) that probes Jupiter's thermal emission in a spectral window centered at 4.8 ÎŒm. We analyze the power spectral densities of circular paths outside and inside of cyclones on images acquired during six Juno perijoves. The typical spatial resolution is around 55 km pixel â»Âč. We limited our analysis to six acquisitions of the South Pole from February 2017 to May 2018. The power spectral densities both outside and inside the circumpolar ring seem to follow two different power laws. The wave numbers follow average power laws of â0.9 ± 0.2 (inside) and â1.2 ± 0.2 (outside) and of â3.2 ± 0.3 (inside) and â3.4 ± 0.2 (outside), respectively, beneath and above the transition in slope located at ~2 Ă 10 â»Âł km â»Âč wave number. This kind of spectral behavior is typical of twoâdimensional turbulence. We interpret the 500 km length scale, corresponding to the transition in slope, as the Rossby deformation radius. It is compatible with the dimensions of a subset of eddy features visible in the regions analyzed, suggesting that a baroclinic instability may exist. If so, it means that the quasiâgeostrophic approximation is valid in this context
Characterization of Mesoscale Waves in the Jupiter NEB by Jupiter InfraRed Auroral Mapper on board Juno
In 2017, the Jupiter InfraRed Auroral Mapper (JIRAM), on board the NASA-ASI Juno mission, observed a wide longitude region (50° Wâ80° E in System III) that was perturbed by a wave pattern centered at 15° N in the Jupiter's North Equatorial Belt (NEB). We analyzed JIRAM data acquired on 2017 July 10 using the M-channel and on 2017 February 2 with the spectrometer. The two observations occurred at different times and at slightly different latitudes. The waves appear as clouds blocking the deeper thermal emission. The wave crests are oriented northâsouth, and the typical wave packet contains 10 crests and 10 troughs. We used Fourier analysis to rigorously determine the wavenumbers associated with the observed patterns at a confidence level of 90%. Wavelet analysis was also used to constrain the spatial localization of the largest energies involved in the process and determine the wavelengths carrying the major contribution. We found wavelengths ranging from 1400 to 1900 km, and generally decreasing toward the west. Where possible, we also computed a vertical location of the cloud pressure levels from the inversion of the spectral radiances measured by the JIRAM spectrometer. The waves were detected at pressure levels consistent with the NH3 as well as NH4SH clouds. Phase velocities could not be determined with sufficient confidence to discriminate whether the alternating crests and troughs are a propagating wave or a manifestation of a fluid dynamical instability
Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) after nine years of operation: a summary
Mars Express, the first European interplanetary mission, carries the Mars Advanced Radar for Subsurface
and Ionosphere Sounding (MARSIS) to search for ice and water in the Martian subsurface. Developed by
an ItalianâUS team, MARSIS transmits low-frequency, wide-band radio pulses penetrating below the
surface and reflected by dielectric discontinuities linked to structural or compositional changes. MARSIS
is also a topside ionosphere sounder,transmitting a burst of short, narrow-band pulses at different
frequencies that are reflected by plasma with varying densities at different altitudes.The radar operates
since July 2005, after the successful deployment of its 40 m antenna, acquiring data at altitudes lower
than 1200 km. Subsurface sounding (SS)data are processed on board by stacking together a batch of
echoes acquired at the same frequency. Onground, SS data are further processed by correlating the
received echo with the transmitted waveform and compensating de-focusing caused by the dispersive
ionosphere. Ground processing of active ionospheric sounding (AIS)data consists in the reconstruction
of the electron density profile as a function of altitude. MARSIS observed the internal structure of Planum
Boreum outlining the Basal Unit, an icy deposit lying beneath the North Polar Layered Deposits thought
to have formed in an epoch in which climate was markedly different from the current one.The total
volume of ice in polar layered deposits could be estimated, and parts of the Southern residual ice cap
were revealed to consist of 10 m of CO2 ice. Radar properties of the Vastitas Borealis Formation point
to the presence of large quantities of ice buried beneath the surface. Observations of the ionosphere
revealed the complex interplay between plasma, crustal magnetic field and solar wind, contributing to
space weather studies at Mars. The presence of three-dimensional plasma structures in the ionosphere
was revealed for the first time. MARSIS could successfully operate at Phobos, becoming the first
instrument of its kind to observe an asteroid-like body. The main goal pursued by MARSIS, the search for
liquid water beneath the surface, remains elusive. However, because of the many factors affecting
detection and of the difficulties in identifying water in radar echoes, a definitive conclusion on its
presence cannot yet be drawn
Turbulence Power Spectra in Regions Surrounding Jupiter's South Polar Cyclones from Juno/JIRAM
We present a power spectral analysis of two narrow annular regions near Jupiter's South Pole derived from data acquired by the Jovian Infrared Auroral Mapper instrument onboard NASA's Juno mission. In particular, our analysis focuses on the data set acquired by the Jovian Infrared Auroral Mapper Mâband imager (hereafter IMGâM) that probes Jupiter's thermal emission in a spectral window centered at 4.8 ÎŒm. We analyze the power spectral densities of circular paths outside and inside of cyclones on images acquired during six Juno perijoves. The typical spatial resolution is around 55 km pixel â»Âč. We limited our analysis to six acquisitions of the South Pole from February 2017 to May 2018. The power spectral densities both outside and inside the circumpolar ring seem to follow two different power laws. The wave numbers follow average power laws of â0.9 ± 0.2 (inside) and â1.2 ± 0.2 (outside) and of â3.2 ± 0.3 (inside) and â3.4 ± 0.2 (outside), respectively, beneath and above the transition in slope located at ~2 Ă 10 â»Âł km â»Âč wave number. This kind of spectral behavior is typical of twoâdimensional turbulence. We interpret the 500 km length scale, corresponding to the transition in slope, as the Rossby deformation radius. It is compatible with the dimensions of a subset of eddy features visible in the regions analyzed, suggesting that a baroclinic instability may exist. If so, it means that the quasiâgeostrophic approximation is valid in this context
TwoâYear Observations of the Jupiter Polar Regions by JIRAM on Board Juno
We observed the evolution of Jupiterâs polar cyclonic structures over two years between February 2017 and February 2019, using polar observations by the Jovian InfraRed Auroral Mapper, JIRAM, on the Juno mission. Images and spectra were collected by the instrument in the 5âÎŒm wavelength range. The images were used to monitor the development of the cyclonic and anticyclonic structures at latitudes higher than 80° both in the northern and the southern hemispheres. Spectroscopic measurements were then used to monitor the abundances of the minor atmospheric constituents water vapor, ammonia, phosphine, and germane in the polar regions, where the atmospheric optical depth is less than 1. Finally, we performed a comparative analysis with oceanic cyclones on Earth in an attempt to explain the spectral characteristics of the cyclonic structures we observe in Jupiterâs polar atmosphere.Plain Language SummaryThe Jovian InfraRed Auroral Mapper (JIRAM) is an instrument onâboard the Juno NASA spacecraft. It consists of an infrared camera, for mapping both Jupiterâs auroras and atmosphere, and a spectrometer. In February 2017, the complex cyclonic structures that characterize the Jupiterâs polar atmospheres were discovered. Here, we report the evolution of those cyclonic structures during the 2 years following the discovery. We use for this purpose infrared maps built by the JIRAM camera images collected at wavelengths around 5 Όm. The cyclones have thick clouds that obstruct most of the view of the deeper atmosphere. However, some areas, near the cyclones, are only covered by thin clouds allowing the spectrometer to see deeper in the atmosphere. In those areas, the instrument was able to detect spectral signatures that permitted estimation of abundances of water vapor, ammonia, phosphine, and germane. Those gases are minor but significant constituents of the atmosphere. Finally, the dynamics of the Jupiterâs polar atmosphere are not well understood and are still under study. Here, to suggest possible mechanisms that governs the polar dynamics, we attempted a comparative analysis with some Earth oceanic cyclones that show similarities with the Jupiter ones.Key PointsThe Jupiterâs polar cyclonic structures did not change much in two years of observations from February 2017 to February 2019Abundances of some atmospheric minor constituents measured in the hottest spots of the polar regions, higher values registered in the southEarth oceanic cyclones analogies suggest a wellâmixed upper boundary layer on Jupiterâs PolesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155949/1/jgre21379_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155949/2/jgre21379.pd
Exospheric Na distributions along the Mercury orbit with the THEMIS telescope
Abstract The Na exosphere of Mercury is characterized by the variability of the emission lines intensity and of its distribution in time scales from less than one hour to seasonal variations. While the faster variations, accounting for about 10â20% of fluctuations are probably linked to the planetary response to solar wind and Interplanetary Magnetic Field variability, the seasonal variations (up to about 80%) should be explained by complex mechanisms involving different surface release processes, loss, source and migrations of the exospheric Na atoms. Eventually, a Na annual cycle can be identified. In the past, ground-based observations and equatorial density from MESSENGER data have been analyzed. In this study, for a more extensive investigation of the exospheric Na features, we have studied the local time and latitudinal distributions of the exospheric Na column density as a function of the True Anomaly Angle (TAA) of Mercury by means of the extended dataset of images, collected from 2009 to 2013, by the THEMIS solar telescope. Our results show that the THEMIS images, in agreement with previous results, registered a strong general increase in sodium abundance at aphelion and a dawn ward emission predominance with respect to dusk ward and subsolar region between 90° and 150° TAA. This behavior can be explained by desorption of a sodium surface reservoir consisting of sodium that is pushed anti-sunward and condenses preferentially in the coldest regions. Our analyses show s a predominance of subsolar line-of-sight column density along the rest of Mercury's orbit. An unexpected relationship between Northward or Southward peak emission and both TAA and local time is also shown by our analysis. This result seems to contradict previous results obtained from different data sets and it is not easily explained, thus it requires further investigations
Two-year observations of the Jupiter polar regions by JIRAM on board Juno
We observed the evolution of Jupiter's polar cyclonic structures over two years between February 2017 and February 2019, using polar observations by the Jovian InfraRed Auroral Mapper, JIRAM, on the Juno mission. Images and spectra were collected by the instrument in the 5âÎŒm wavelength range. The images were used to monitor the development of the cyclonic and anticyclonic structures at latitudes higher than 80° both in the northern and the southern hemispheres. Spectroscopic measurements were then used to monitor the abundances of the minor atmospheric constituents water vapor, ammonia, phosphine and germane in the polar regions, where the atmospheric optical depth is less than 1. Finally, we performed a comparative analysis with oceanic cyclones on Earth in an attempt to explain the spectral characteristics of the cyclonic structures we observe in Jupiter's polar atmosphere
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