Characterizing the limitations to the coupling between Saturn's ionosphere and middle magnetosphere

Observations of Saturn's ultraviolet and infrared aurora show structures that, when traced along the planetary magnetic field, map to the inner, middle, and outer magnetosphere. From low to high latitudes the structures seen in the UV are the Enceladus footprint, which maps to an equatorial radius of 4 R S (Saturn radii); a diffuse emission that maps to a broad equatorial region from 4-11 RS on the nightside; and a bright ring of emission that maps to ∼15 RS. With the exception of the Enceladus spot, the magnetospheric drivers for these auroral emissions are not yet fully understood. We apply a 1D spatial, 2D velocity space Vlasov solver to flux tubes mapping from equatorial radii of 4, 6, 9, and 13 RS to Saturn's southern atmosphere. The aim is to globally characterize the field-aligned potential structure and plasma density profiles. The ionospheric properties - the field-aligned current densities at the ionospheric boundary, energy intensity profiles and fluxes of the electrons precipitating into the ionosphere - are also determined. We then couple our results to an ionospheric model to calculate the Pedersen conductances at the foot of the relevant flux tubes. We find that for a zero net potential drop between the ionosphere and magnetosphere, there exists a sharp potential drop at ∼1.5 RS along the magnetic field line as measured from the planetary center. The strength of this potential drop is approximately equal to that of the ambipolar potential resulting from the centrifugal confinement of the heavy, cold magnetospheric ion population. We also find that the ionospheric properties respond to changes in the magnetospheric plasma population, which are reflected in the nature of the precipitating electron population. For the flux tube mapping to 9 RS (-70), the incident electron energy flux into the ionosphere resulting from a magnetospheric plasma population with a small fraction of hot electrons is an order of magnitude less than that inferred from observations, implying that significant high-latitude field-aligned potentials (up to 1.5 keV) may exist in the saturnian magnetosphere. Calculated ionospheric Pedersen conductances range from 3.0-18.9 mho, and are thus not expected to limit the currents flowing between the ionosphere and magnetosphere

Current-voltage relation for the Saturnian system

Saturn's magnetosphere is populated by plasma created from neutrals ejected by the moon Enceladus. These neutrals are ionized and picked up by the planetary magnetic field requiring large amounts of angular momentum to be transferred from Saturn's upper atmosphere to the magnetospheric plasma. The resulting upward currents that supply this angular momentum are associated with electrons, which travel toward the planetary atmosphere. At high magnetic latitudes along the flux tube, parallel electric fields may develop to enhance the field-aligned current density flowing between the two regions. We show that, similar to the Jovian system, the current-voltage relation in the Saturnian system must be evaluated at the top of the acceleration region, which occurs at ~1.5 RS along the magnetic field line as measured from the center of the planet. Owing to the large abundance of protons in the Saturnian system, cold electrons carry the majority of the field-aligned current for net potential drops less than 500 V. For the flux tube intersecting the equatorial plane at 4 RS, field-aligned potentials of 50-130 V are consistent with the energy fluxes inferred from the Enceladus emission. In the middle magnetosphere, field-aligned potentials of ∼1.5 kV produce ionospheric electron energy fluxes of 0.3 mW/m2 when hot electrons comprise 0.3% of the magnetospheric electron population. Key Points Current-voltage relation must be evaluated at high magnetic latitudes. Cold electrons contribute strongly to field-aligned current density. Full Knight (1973) current-voltage relation must be applied to Saturnian system

The environment drives microbial trait variability in aquatic habitats

A prerequisite to improve the predictability of microbial community dynamics is to understand the mechanisms of microbial assembly. To study factors that contribute to microbial community assembly, we examined the temporal dynamics of genes in five aquatic metagenome time-series, originating from marine offshore or coastal sites and one lake. With this trait-based approach we expected to find gene-specific patterns of temporal allele variability that depended on the seasonal metacommunity size of carrier-taxa and the variability of the milieu and the substrates to which the resulting proteins were exposed. In more detail, we hypothesized that a larger seasonal metacommunity size would result in increased temporal variability of functional units (i.e., gene alleles), as shown previously for taxonomic units. We further hypothesized that multicopy genes would feature higher temporal variability than single-copy genes, as gene multiplication can result from high variability in substrate quality and quantity. Finally, we hypothesized that direct exposure of proteins to the extracellular environment would result in increased temporal variability of the respective gene compared to intracellular proteins that are less exposed to environmental fluctuations. The first two hypotheses were confirmed in all data sets, while significant effects of the subcellular location of gene products was only seen in three of the five time-series. The gene with the highest allele variability throughout all data sets was an iron transporter, also representing a target for phage infection. Previous work has emphasized the role of phage-prokaryote interactions as a major driver of microbial diversity. Our finding therefore points to a potentially important role of iron transporter-mediated phage infections for the assembly and maintenance of diversity in aquatic prokaryotes

Tara Pacific Expedition\u27s atmospheric measurements of marine aerosols across the Atlantic and Pacific Oceans: Overview and preliminary results

Marine aerosols play a significant role in the global radiative budget, in clouds\u27 processes, and in the chemistry of the marine atmosphere. There is a critical need to better understand their production mechanisms, composition, chemical properties, and the contribution of ocean-derived biogenic matter to their mass and number concentration. Here we present an overview of a new dataset of in situ measurements of marine aerosols conducted over the 2.5-yr Tara Pacific Expedition over 110, 000 km across the Atlantic and Pacific Oceans. Preliminary results are presented here to describe the new dataset that will be built using this novel set of measurements. It will characterize marine aerosols properties in detail and will open a new window to study the marine aerosol link to the water properties and environmental conditions

Ocean currents shape the microbiome of Arctic marine sediments

Prokaryote communities were investigated on the seasonally stratified Alaska Beaufort Shelf (ABS). Water and sediment directly underlying water with origin in the Arctic, Pacific or Atlantic oceans were analyzed by pyrosequencing and length heterogeneity-PCR in conjunction with physicochemical and geographic distance data to determine what features structure ABS microbiomes. Distinct bacterial communities were evident in all water masses. Alphaproteobacteria explained similarity in Arctic surface water and Pacific derived water. Deltaproteobacteria were abundant in Atlantic origin water and drove similarity among samples. Most archaeal sequences in water were related to unclassified marine Euryarchaeota. Sediment communities influenced by Pacific and Atlantic water were distinct from each other and pelagic communities. Firmicutes and Chloroflexi were abundant in sediment, although their distribution varied in Atlantic and Pacific influenced sites. Thermoprotei dominated archaea in Pacific influenced sediments and Methanomicrobia dominated in methane-containing Atlantic influenced sediments. Length heterogeneity-PCR data from this study were analyzed with data from methane-containing sediments in other regions. Pacific influenced ABS sediments clustered with Pacific sites from New Zealand and Chilean coastal margins. Atlantic influenced ABS sediments formed another distinct cluster. Density and salinity were significant structuring features on pelagic communities. Porosity co-varied with benthic community structure across sites and methane did not. This study indicates that the origin of water overlying sediments shapes benthic communities locally and globally and that hydrography exerts greater influence on microbial community structure than the availability of methane

Technique: Large‐scale ionospheric conductance estimated from combined satellite and ground‐based electromagnetic data

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95266/1/jgra18651.pd

Using the RDP Classifier to Predict Taxonomic Novelty and Reduce the Search Space for Finding Novel Organisms

BACKGROUND: Currently, the naïve Bayesian classifier provided by the Ribosomal Database Project (RDP) is one of the most widely used tools to classify 16S rRNA sequences, mainly collected from environmental samples. We show that RDP has 97+% assignment accuracy and is fast for 250 bp and longer reads when the read originates from a taxon known to the database. Because most environmental samples will contain organisms from taxa whose 16S rRNA genes have not been previously sequenced, we aim to benchmark how well the RDP classifier and other competing methods can discriminate these novel taxa from known taxa. PRINCIPAL FINDINGS: Because each fragment is assigned a score (containing likelihood or confidence information such as the boostrap score in the RDP classifier), we "train" a threshold to discriminate between novel and known organisms and observe its performance on a test set. The threshold that we determine tends to be conservative (low sensitivity but high specificity) for naïve Bayesian methods. Nonetheless, our method performs better with the RDP classifier than the other methods tested, measured by the f-measure and the area-under-the-curve on the receiver operating characteristic of the test set. By constraining the database to well-represented genera, sensitivity improves 3-15%. Finally, we show that the detector is a good predictor to determine novel abundant taxa (especially for finer levels of taxonomy where novelty is more likely to be present). CONCLUSIONS: We conclude that selecting a read-length appropriate RDP bootstrap score can significantly reduce the search space for identifying novel genera and higher levels in taxonomy. In addition, having a well-represented database significantly improves performance while having genera that are "highly" similar does not make a significant improvement. On a real dataset from an Amazon Terra Preta soil sample, we show that the detector can predict (or correlates to) whether novel sequences will be assigned to new taxa when the RDP database "doubles" in the future