Absolute Latitude Research Articles

Latitudinal variation in seagrass herbivory: Global patterns and explanatory mechanisms

AbstractAimThe aim was to quantify latitudinal patterns in seagrass–herbivore interactions in the context of a warming climate.Location: We carried out a global meta‐analysis combined with a field experiment across 1,700 km and 12° of latitude in Western Australia.Time period: 1984–2014.Major taxa studied: Seagrasses.MethodsWe first synthesized the global literature on herbivore exclusion experiments in seagrasses to test whether differences in herbivore impacts are related to latitude and sea surface temperature. We then quantified leaf production and consumption rates in the field at nine meadows of the seagrass Amphibolis antarctica across 1,700 km, from tropical to temperate latitudes. Seagrass biomass and nutritional traits (nitrogen, C:N, phenolics) were also measured.ResultsOur meta‐analysis showed that herbivores had a similar net impact on seagrasses across 37° of absolute latitude, and there was little variation in herbivore exclusion effects at different temperatures. In the field, rates of both production and consumption of seagrass were greatest in the tropics and decreased with latitude. Seagrass nutritional quality was lowest in the tropics, where fish removed c. 30% of primary production. Consumption of the more nutritious temperate seagrasses was lower overall but also highly variable and dominated by invertebrates.Main conclusionsIn tropical latitudes, faster growth rates compensated for greater consumption of A. antarctica by herbivores. This resulted in similar net impacts of herbivores across latitudes, because higher latitude plants grew more slowly but also suffered less herbivory. This match between consumption and production rates might explain the global patterns derived from the literature, which show little latitudinal variation in the effects of consumers on seagrasses. As ocean temperatures continue to rise and overall herbivory levels are expected to increase in temperate regions, the survival of seagrass meadows in higher latitudes will depend on the ability of plants to increase growth at compensatory rates.

Length of cultivation determines native and non-native weed richness in crop fields worldwide

Socio-economic factors often determine the extent to which different global regions have been invaded by non-native plant species, yet few studies examine whether such variables are similarly important for native species richness. In contrast to previous studies that have assembled regional floras for comparison, we examine global patterns of non-native and native plant species richness within a single, globally distributed ecosystem: cultivated wheat fields. Native species richness increased with the historic length of wheat cultivation, total wheat harvested area and arable land area in a country. In contrast, non-native weed species richness declined with increasing length of wheat cultivation, higher fertilizer inputs, and lower absolute latitude. The low percentage of native species in countries with a short history of wheat cultivation suggests that the invasion of crop fields by native plants takes longer than the migration of non-native weed species pre-adapted to crop fields in other parts of the world. Seed contaminants in grain imports are probably one of the main sources of non-native weeds, especially for regions with more recent wheat cultivation histories. To date there has been a tendency for research on agricultural weeds to be dissociated from that of plant invasions, yet we show how cultivation history has shaped the weed flora and why non-native weeds are perceived as such a problem in New World agriculture, as well as confirming at a global scale the role of crop husbandry and agricultural extent on weed floras.

Probing the Southern Fermi Bubble in Ultraviolet Absorption Using Distant AGNs

Abstract The Fermi Bubbles are two giant gamma-ray emitting lobes extending 55° above and below the Galactic center. While the Northern Bubble has been extensively studied in ultraviolet (UV) absorption, little is known about the gas kinematics of the southern Bubble. We use UV absorption-line spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to probe the southern Fermi Bubble using a sample of 17 background AGNs projected behind or near the Bubble. We measure the incidence of high-velocity clouds (HVC), finding that 4 out of 6 sightlines passing through the Bubble show HVC absorption, versus 6 out of 11 passing outside. We find strong evidence that the maximum absolute LSR velocity of the HVC components decreases as a function of galactic latitude within the Bubble, for both blueshifted and redshifted components, as expected for a decelerating outflow. We explore whether the column density ratios Si iv/Si iii, Si iv/Si ii, and Si iii/Si ii correlate with the absolute galactic latitude within the Bubble. These results demonstrate the use of UV absorption-line spectroscopy to characterize the kinematics and ionization conditions of embedded clouds in the Galactic center outflow.

Global importance of large‐diameter trees

AbstractAimTo examine the contribution of large‐diameter trees to biomass, stand structure, and species richness across forest biomes.LocationGlobal.Time periodEarly 21st century.Major taxa studiedWoody plants.MethodsWe examined the contribution of large trees to forest density, richness and biomass using a global network of 48 large (from 2 to 60 ha) forest plots representing 5,601,473 stems across 9,298 species and 210 plant families. This contribution was assessed using three metrics: the largest 1% of trees ≥ 1 cm diameter at breast height (DBH), all trees ≥ 60 cm DBH, and those rank‐ordered largest trees that cumulatively comprise 50% of forest biomass.ResultsAveraged across these 48 forest plots, the largest 1% of trees ≥ 1 cm DBH comprised 50% of aboveground live biomass, with hectare‐scale standard deviation of 26%. Trees ≥ 60 cm DBH comprised 41% of aboveground live tree biomass. The size of the largest trees correlated with total forest biomass (r2 = .62,p < .001). Large‐diameter trees in high biomass forests represented far fewer species relative to overall forest richness (r2 = .45,p < .001). Forests with more diverse large‐diameter tree communities were comprised of smaller trees (r2 = .33,p < .001). Lower large‐diameter richness was associated with large‐diameter trees being individuals of more common species (r2 = .17,p = .002). The concentration of biomass in the largest 1% of trees declined with increasing absolute latitude (r2 = .46,p < .001), as did forest density (r2 = .31,p < .001). Forest structural complexity increased with increasing absolute latitude (r2 = .26,p < .001).Main conclusionsBecause large‐diameter trees constitute roughly half of the mature forest biomass worldwide, their dynamics and sensitivities to environmental change represent potentially large controls on global forest carbon cycling. We recommend managing forests for conservation of existing large‐diameter trees or those that can soon reach large diameters as a simple way to conserve and potentially enhance ecosystem services.

UV Radiation Associates With State Income Through Complex Cognitive Ability in the USA

Abstract. Economic development and national intelligence decline with proximity to the equator. Absolute latitude associates with both, income and IQ, but the nature of their relationship is ambiguous. This study applied structural equation modeling using secondary data pertaining to the 48 contiguous states of the United States of America to test the hypothesis that UV (ultraviolet) radiation associates with income through complex cognitive ability vis-a-vis the hypothesis that UV radiation associates with complex cognitive ability through income. The resulting evidence was consistent with the ability → income pathway and unsupportive of the income → ability model. The findings uphold the cognitive capitalism perspective and may throw light on the evolvement of regional differences in the USA.

Quantifying deforestation and forest degradation with thermal response

Deforestation and forest degradation cause the deterioration of resources and ecosystem services. However, there are still no operational indicators to measure forest status, especially for forest degradation. In the present study, we analysed the thermal response number (TRN, calculated by daily total net radiation divided by daily temperature range) of 163 sites including mature forest, disturbed forest, planted forest, shrubland, grassland, savanna vegetation and cropland. TRN generally increased with latitude, however the regression of TRN against latitude differed among vegetation types. Mature forests are superior as thermal buffers, and had significantly higher TRN than disturbed and planted forests. There was a clear boundary between TRN of forest and non-forest vegetation (i.e. grassland and savanna) with the exception of shrubland, whose TRN overlapped with that of forest vegetation. We propose to use the TRN of local mature forest as the optimal TRN (TRNopt). A forest with lower than 75% of TRNopt was identified as subjected to significant disturbance, and forests with 66% of TRNopt was the threshold for deforestation within the absolute latitude from 30° to 55°. Our results emphasized the irreplaceable thermal buffer capacity of mature forest. TRN can be used for early warning of deforestation and degradation risk. It is therefore a valuable tool in the effort to protect forests and prevent deforestation.

How climatic variability is linked to the spatial distribution of range sizes: seasonality versus climate change velocity in sphingid moths

AbstractAimTo map the spatial variation of range sizes within sphingid moths, and to test hypotheses on its environmental control. In particular, we investigate effects of climate change velocity since the Pleistocene and the mid‐Holocene, temperature and precipitation seasonality, topography, Pleistocene ice cover, and available land area.LocationOld World and Australasia, excluding smaller islands.MethodsWe used fine‐grained range maps (based on expert‐edited distribution modelling) for all 972 sphingid moth species in the research region and calculated, at a grain size of 100 km, the median of range sizes of all species that co‐occur in a pixel. Climate, topography and Pleistocene ice cover data were taken from publicly available sources. We calculated climate change velocities (CCV) for the last 21 kyr as well as 6 kyr. We compared the effects of seasonality andCCVon median range sizes with spatially explicit models while accounting for effects of elevation range, glaciation history and available land area.ResultsRange sizes show a clear spatial pattern, with highest median values in deserts and arctic regions and lowest values in isolated tropical regions. Range sizes were only weakly related to absolute latitude (predicted by Rapoport's effect), but there was a strong north‐south pattern of range size decline. Temperature seasonality emerged as the strongest environmental correlate of median range size, in univariate as well as multivariate models, whereas effects ofCCVwere weak and unstable for both time periods. These results were robust to variations in the parameters in alternative analyses, among them multivariateCCV.Main conclusionsTemperature seasonality is a strong correlate of spatial range size variation, while effects of longer‐term temperature change, as captured byCCV, received much weaker support.

Insect Development, Thermal Plasticity and Fitness Implications in Changing, Seasonal Environments.

Historical data show that recent climate change has caused advances in seasonal timing (phenology) in many animals and plants, particularly in temperate and higher latitude regions. The population and fitness consequences of these phenological shifts for insects and other ectotherms have been heterogeneous: warming can increase development rates and the number of generations per year (increasing fitness), but can also lead to seasonal mismatches between animals and their resources and increase exposure to environmental variability (decreasing fitness). Insect populations exhibit local adaptation in their developmental responses to temperature, including lower developmental thresholds and the thermal requirements to complete development, but climate change can potentially disrupt seasonal timing of juvenile and adult stages and alter population fitness. We investigate these issues using a global dataset describing how insect developmental responds to temperature via two traits: lower temperature thresholds for development (T0) and the cumulative degree-days required to complete development (G). As suggested by previous analyses, T0 decreases and G increases with increasing (absolute) latitude; however, these traits and the relationship between G and latitude varies significantly among taxonomic orders. The mean number of generations per year (a metric of fitness) increases with both decreasing T0 and G, but the effects of these traits on fitness vary strongly with latitude, with stronger selection on both traits at higher (absolute) latitudes. We then use the traits to predict developmental timing and temperatures for multiple generations within seasons and across years (1970-2010). Seasonality drives developmental temperatures to peak mid-season and for generation lengths to decline across seasons, particularly in temperate regions. We predict that climate warming has advanced phenology and increased the number of generations, particularly at high latitudes. The magnitude of increases in developmental temperature varies little across latitude. Increases in the number of seasonal generations have been greatest for populations experiencing the greatest phenological advancements and warming. Shifts in developmental rate and timing due to climate change will have complex implications for selection and fitness in seasonal environments.

Variation in dawn chorus acoustic complexity across a global latitudinal gradient

The geography of biological sound is a largely unexplored topic that exists at the boundary of ecoacoustics and biogeography. Identification and characterization of patterns of biological sound and soundscape variation across the planet may provide insight into the potential for acoustics as a tool for biodiversity monitoring and conservation. Ecoacoustic indices are audio signal measurements that can be useful for revealing, characterizing, and comparing such soundscape patterns. Some of these indices, such as the Acoustic Complexity Index (ACI), have been shown to correlate with biodiversity and avian vocalization activity in some systems. The goal of this study was to investigate broad-scale trends in the ACI by testing whether the acoustic complexity of dawn chorus soundscapes follows the known latitudinal gradient in avian biodiversity. The ACI was calculated for dawn chorus recordings from 187 sites worldwide, spanning from -45.4° to 68.1° latitude. Acoustic complexity was expected to be highest near the equator and decrease with absolute latitude, tracking the general avian diversity gradient, because higher avian diversity should result in more elaborate dawn chorus soundscapes. However, the ACI did not track the general avian latitudinal diversity gradient. The results and potential explanations for this trend will be discussed.

A global analysis of Islamist violence

This paper examines the relationship between percentage of Muslims in the population and two separate measures of Islamist violence for a large cross-section of countries (n = 168). The first measure of Islamist violence is the number of Islamist attacks 2001–2017 (logged); the second is the number of casualties from Islamist violence 2001–2017 (logged). Percentage Muslim was strongly associated with both measures of Islamist violence (β = .49–50). These associations were not disproportionately driven by co-variation within one or two global regions: positive associations were found within Sub-Saharan Africa (β = .35–36), South & East Asia (β = .49–50), Eurasia (β = .28–37), and the West (β = .35–46). The raw associations within Latin America & Caribbean (β = .16–19) were weak, and those within Middle East & North Africa were negative (β = –.17–20). Yet the results for Middle East & North Africa were attributable to Israel being a major outlier; when Israel was omitted, very weak positive associations emerged (β = .06–.10). In a multiple regression analysis, both associations were robust to controlling for region fixed-effects, land area (logged), absolute latitude, average elevation, terrain roughness, legal origin, GDP per capita (logged), democracy, and ethnic fractionalisation (β = .29–30). Consistent with a previous study, both percentage Muslim (β = .24–58) and indicators of military intervention in the Middle East (β = .21–58) were associated with Islamist violence across Western countries.

Global patterns in the effects of predator declines on sea urchins

Latitudinal gradients in the strength of biotic interactions have long been proposed, but empirical evidence for the expectation of more intense predation, herbivory and competition at low latitudes has been mixed. Here, we use a meta‐analysis to test the prediction that predation pressure on sea urchins, a group of consumers with a particularly strong influence on community structure in the world's oceans, is strongest in the tropics. We then examine which biotic and abiotic factors best correlate with biogeographic and within habitat patterns in sea urchin responses to predation. Consistent with expectations, predator impacts on sea urchins were highest in tropical coral reefs and decreased towards the poles in rocky reef habitats (> 25° absolute latitude). However, latitude and temperature were weakly correlated with effect sizes, and the strongest predictor of predator impacts was sea urchin species. This suggests an important role of prey identity (i.e. traits including behaviour, physical, and chemical defences) rather than large scale abiotic factors in determining variation in interaction strengths. Ecosystem‐shaping sea urchins such as Tripneustes gratilla, Diadema savignyi and Centrostephanus rodgersii were strongly impacted by consumers, indicating a tight coupling between predators of these species and their boom and bust prey. Anthropogenic activities such as over‐fishing, climate change and habitat destruction are causing rapid environmental change, and understanding how predation pressure varies with temperature, across habitats and among prey species, will aid in predicting the likelihood of ecosystem wide effects (via trophic cascades).

Why fly the extra mile? Latitudinal trend in migratory fuel deposition rate as driver of trans-equatorial long-distance migration.

Trans‐equatorial long‐distance migrations of high‐latitude breeding animals have been attributed to narrow ecological niche widths. We suggest an alternative hypothesis postulating that trans‐equatorial migrations result from a possible increase in the rate at which body stores to fuel migration are deposited with absolute latitude; that is, longer, migrations away from the breeding grounds surpassing the equator may actually enhance fueling rates on the nonbreeding grounds and therewith the chance of a successful, speedy and timely migration back to the breeding grounds. To this end, we first sought to confirm the existence of a latitudinal trend in fuel deposition rate in a global data set of free‐living migratory shorebirds and investigated the potential factors causing this trend. We next tested two predictions on how this trend is expected to impact the migratory itineraries on northward migration under the time‐minimization hypothesis, using 56 tracks of high‐latitude breeding shorebirds migrating along the East Asian‐Australasian Flyway. We found a strong positive effect of latitude on fuel deposition rate, which most likely relates to latitudinal variations in primary productivity and available daily foraging time. We next confirmed the resulting predictions that (1) when flying from a stopover site toward the equator, migrants use long jumps that will take them to an equivalent or higher latitude at the opposite hemisphere; and (2) that from here onward, migrants will use small steps, basically fueling only enough to make it to the next suitable staging site. These findings may explain why migrants migrate “the extra mile” across the equator during the nonbreeding season in search of better fueling conditions, ultimately providing secure and fast return migrations to the breeding grounds in the opposite hemisphere.

Corrigendum: Using river discharge to model and deconstruct the latitudinal diversity gradient for fishes of the Western Hemisphere

An error in the discharge (Q) data used in ‘Using river discharge to model and deconstruct the latitudinal diversity gradient for fishes of the Western Hemisphere’ (McGarvey & Terra, 2016) was recently brought to our attention: Q values from a subset of river gauging stations were not converted to m3 s−1 units. (Thanks to David Griffiths for the notification.) When all Q data errors were corrected (see Table S1 in Corrigendum Supporting Information), modest changes occurred in the results, as outlined below. Analysis with the corrected data changed SDR coefficients (c and z values) slightly. Importantly, each of the mean annual Q, annual low flow Q, and annual high flow Q models remained highly significant (P < 0.001) with comparable overall model fits (r2 and AICc values; see Corrigendum Fig. S2.1 and Table S2.1). In the corrected analyses, residuals from the hemispheric-scale SDR models remained highly correlated with absolute latitude. However, the corrected relationship displayed a unimodal rather than linear trend (Corrigendum Fig. S2.2a). The optimal tropical–subtropical boundary remained at 23.5° absolute latitude when moving window ANOVA tests were repeated for mean annual Q, low flow Q and high flow Q residuals, but the optimal subtropical–temperate boundary shifted from 40.0° to 32.5° absolute latitude (Corrigendum Fig. S2.2b). All deconstructed SDRs remained significant at P ≤ 0.05 with differing changes in model coefficients and fits (see Corrigendum Fig. S2.3 and Table S2.2). The largest changes were observed for spatial autoregressive models of temperate rivers, with dramatically increased model fits (e.g. r2 = 0.99; see Corrigendum Fig. S2.3c). Structural equation modelling (SEM) results based on the corrected data were similar to those in the original analysis, with one notable change. The direct paths from annual low flow Q to native fish richness (Corrigendum Fig. S3.1b) and from hydrology to native fish richness (Corrigendum Fig. S3.1c) became insignificant (P > 0.05). This change provided new insight to the SDR: the strength of the empirical correlation between Q and fish richness is likely driven by the fact that Q is an integrative measure of climate and physical habitat effects, rather than a direct or mechanistic link between Q and richness per se. Complete details on all of the adjusted results, including the corrected raw data and an expanded discussion of the SEM results, are provided in the online Supporting Information. We encourage readers to consult this updated information and we apologize for our mistake. Table S1 Corrected raw data for the 107 Western Hemisphere rivers included in this study. Appendix S2 Corrected results for all SDR and deconstruction analyses. Appendix S3 Corrected SEM analyses and expanded discussion. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Molecular evolutionary rates are not correlated with temperature and latitude in Squamata: an exception to the metabolic theory of ecology?

BackgroundThe metabolic theory of ecology stipulates that molecular evolutionary rates should correlate with temperature and latitude in ectothermic organisms. Previous studies have shown that most groups of vertebrates, such as amphibians, turtles and even endothermic mammals, have higher molecular evolutionary rates in regions where temperature is high. However, the association between molecular evolutionary rates and temperature or latitude has never been tested in Squamata.ResultsWe used a large dataset including the spatial distributions and environmental variables for 1,651 species of Squamata and compared the contrast of the rates of molecular evolution with the contrast of temperature and latitude between sister species. Using major axis regressions and a new algorithm to choose independent sister species pairs, we found that temperature and absolute latitude were not associated with molecular evolutionary rates.ConclusionsThis absence of association in such a diverse ectothermic group questions the mechanisms explaining current pattern of species diversity in Squamata and challenges the presupposed universality of the metabolic theory of ecology.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0666-4) contains supplementary material, which is available to authorized users.

A global review of freshwater crayfish temperature tolerance, preference, and optimal growth

Conservation efforts, environmental planning, and management must account for ongoing ecosystem alteration due to a changing climate, introduced species, and shifting land use. This type of management can be facilitated by an understanding of the thermal ecology of aquatic organisms. However, information on thermal ecology for entire taxonomic groups is rarely compiled or summarized, and reviews of the science can facilitate its advancement. Crayfish are one of the most globally threatened taxa, and ongoing declines and extirpation could have serious consequences on aquatic ecosystem function due to their significant biomass and ecosystem roles. Our goal was to review the literature on thermal ecology for freshwater crayfish worldwide, with emphasis on studies that estimated temperature tolerance, temperature preference, or optimal growth. We also explored relationships between temperature metrics and species distributions. We located 56 studies containing information for at least one of those three metrics, which covered approximately 6 % of extant crayfish species worldwide. Information on one or more metrics existed for all 3 genera of Astacidae, 4 of the 12 genera of Cambaridae, and 3 of the 15 genera of Parastacidae. Investigations employed numerous methodological approaches for estimating these parameters, which restricts comparisons among and within species. The only statistically significant relationship we observed between a temperature metric and species range was a negative linear relationship between absolute latitude and optimal growth temperature. We recommend expansion of studies examining the thermal ecology of freshwater crayfish and identify and discuss methodological approaches that can improve standardization and comparability among studies.

Optimum tilt and azimuth angles for fixed solar collectors in South Africa using measured data

The solar resource in South Africa is among the best in the world. South Africa has seen an increase in local and international investment in solar power. The optimum angles at which a solar collector should be fixed are often debated. Similar publications on this topic often rely on mathematical models of solar resource rather than measured data. Measured data is preferred since the effects of pollution and weather are included. In this work, recently measured data from nine measuring stations in the Southern African Universities Radiometric Network (SAURAN) equipped with pyranometers and pyrheliometers are considered. The data is used to calculate the annual solar insolation on fixed collectors at all possible installation angles. SolTrace, a ray-tracing software, is used for validation. Relative to horizontally-fixed collectors, the annual solar insolation is 10% more for optimally-fixed collectors and 45% more for solar-tracking collectors. The effects of tilt and azimuth angles are presented on contour plots, which are convenient for cost analysis and the determination of the annual insolation on building surfaces. The optimum azimuth angle is a function of the longitude angle minus the absolute latitude angle, which suggests that frontal and convective precipitation effects influence the optimum azimuth angle.

Climate and the Emergence of Global Income Differences

The latitude gradient in comparative development is a striking fact: as one moves away from the equator, economic activity rises. While this regularity is well known, it is not well understood. In the present paper we take a step towards unpacking this gradient. Perhaps the strongest correlate with (absolute) latitude is the intensity of ultraviolet radiation (UV-R), which epidemiological research has shown to be a cause of a wide range of diseases. We establish that UV-R is strongly and negatively correlated with economic activity, both across and within countries. We propose, and test, a mechanism that links UV-R to current income differences via the impact of disease ecology on the timing of the take-off to sustained growth.

A real-time agro-food authentication and supervision system on a novel code for improving traceability credibility

Counterfeiting products and abusing labels lead to less credibility for traceability system in China recently. Authentication and supervision agencies driven by government departments play an important role for ensuring the quality safety in the case of lacking the willingness and credit of enterprises. A complete authentication and supervision flow framework was constructed based on an identification code (IdC) for authenticated origin base, which linked two actors of the agencies and the enterprises, and three subsystems of On-line Authentication Subsystem (OAS), Safety Production Management Client (SMC) and Mobile Supervision Application (MSA). IdC consisted of longitude and latitude of origin base as position code, production code and authentication type code. With a relative position partition method on 6 zones every 27° for China map and a coordination transformation algorithm, an absolute longitude and latitude value was converted into a relative position value and a zone mark value. IdC and packaging date code formed initial traceability code (TC). 8 digits packaging date code was reconstructed into 3 digits relative time value and 1 digit period mark according to a relative time period partition method with a period of 999d as time intervals and four periods form a cycle. Validation code was generated integrating the zone mark value, period mark value and authentication type code. Therefore, transformed 20 digits TC with the characters of shorter code length and stronger encryption was formed with IdC, relative time value and validation code. Three subsystems for different actors which provide the main function such as origin base registration, agency authentication, QR code generation, data uploading and product verification, were developed. The system has been used in Tianjin city from 2012. 213 enterprises were audited through OAS and used SMC. Through investigating 8 supervision agency staffs, 30 origin base owners, and 50 customers, it is shown that the positive effects are approved by most of the investigators and two negative effects for enhancing the costs and doubting the authentication reliability are laid by 17 enterprises and 12 customers. Furthermore, 4 typical cases for counterfeiting and abusing the labels were exampled and can be solved to a certain extent with the system. However, except for the technology itself, a management measures fitting the supervision flow and system need to draft in order to improve the system application well in the future.

Saturn’s variable thermosphere from Cassini/UVIS occultations

We retrieved the density and temperature profiles in Saturn’s thermosphere from 26 stellar occultations observed by the Cassini/UVIS instrument. These results expand upon and complement the previous analysis of 15 Cassini/UVIS solar occultations by Saturn’s upper thermosphere. We find that the exospheric temperatures based on the stellar occultations agree with the solar occultations and range from 380K to 590K. These temperatures are also consistent with the recent re-analysis of the Voyager/UVS occultations. The retrieved density profiles support our earlier inference that the shape of the atmosphere at low pressures is consistent with a meridional trend of increasing temperatures with absolute latitude. This implies a high-latitude heat source, such as auroral heating, although the existing circulation models that include auroral heating still underestimate the equatorial temperatures by overestimating the meridional temperature gradient. This suggests either that the circulation models are somehow incomplete or there is some other heat source at low to mid latitudes that is relatively less efficient than high-latitude heating. We also find evidence for the expansion of the exobase by about 500km between 2006 and 2011 near the equator, followed by possible contraction after 2011. The expansion appears to be caused by significant warming of the lower thermosphere that anti-correlates with solar activity and may be connected to changes in global circulation. Lastly, we note that our density profiles are in good general agreement with the Voyager/UVS data. In particular, the Voyager density profiles are most consistent with the Cassini/UVIS stellar occultations from late 2008 and early 2009 that roughly coincide in season with the Voyager flybys.

Main field and secular variation candidate models for the 12th IGRF generation after 10 months of Swarm measurements

We describe the main field and secular variation candidate models for the 12th generation of the International Geomagnetic Reference Field model. These two models are derived from the same parent model, in which the main field is extrapolated to epoch 2015.0 using its associated secular variation. The parent model is exclusively based on measurements acquired by the European Space Agency Swarm mission between its launch on 11/22/2013 and 09/18/2014. It is computed up to spherical harmonic degree and order 25 for the main field, 13 for the secular variation, and 2 for the external field. A selection on local time rather than on true illumination of the spacecraft was chosen in order to keep more measurements. Data selection based on geomagnetic indices was used to minimize the external field contributions. Measurements were screened and outliers were carefully removed. The model uses magnetic field intensity measurements at all latitudes and magnetic field vector measurements equatorward of 50° absolute quasi-dipole magnetic latitude. A second model using only the vertical component of the measured magnetic field and the total intensity was computed. This companion model offers a slightly better fit to the measurements. These two models are compared and discussed.We discuss in particular the quality of the model which does not use the full vector measurements and underline that this approach may be used when only partial directional information is known. The candidate models and their associated companion models are retrospectively compared to the adopted IGRF which allows us to criticize our own choices.