ALBERT

Description: Understanding the direct and indirect impact of the Pacific and Atlantic Oceans on precipitation in the region of Northeast Brazil (NEB) is crucial for monitoring unprecedented drought events. We propose nonlinear methods of phase coherence and generalized event synchronization analysis to understand the underlying mechanism. In particular, the relationships between sea surface temperature (SST) variability and the standard precipitation index are interpreted as direct interactions, while the relationships between surrounding oceans are interpreted as indirect effects on the precipitation. Our results reveal a dominant role of tropical North Atlantic for precipitation deficit and droughts, particularly in recent decades. Meanwhile, the indirect Pacific-North Atlantic phase synchronizations have significant influence on and reinforcement of the droughts in NEB. Furthermore, we find that the instantaneous angular frequencies of precipitation and SST are drastically changed after very strong El Niño and La Niña events, therefore resulting in a higher probability of phase coherence.

Description: Correctly identifying interaction patterns from multivariate time series presents an important step in functional network construction. In this context, the widespread use of bivariate statistical association measures often results in a false identification of links because strong similarity between two time series can also emerge without the presence of a direct interaction due to intermediate mediators or common drivers. In order to properly distinguish such direct and indirect links for the special case of event-like data, we present here a new generalization of event coincidence analysis to a partial version thereof, which is aimed at excluding possible transitive effects of indirect couplings. Using coupled chaotic systems and stochastic processes on two generic coupling topologies (star and chain configuration), we demonstrate that the proposed methodology allows for the correct identification of indirect interactions. Subsequently, we apply our partial event coincidence analysis to multi-channel EEG recordings to investigate possible differences in coordinated alpha band activity among macroscopic brain regions in resting states with eyes open (EO) and closed (EC) conditions. Specifically, we find that direct connections typically correspond to close spatial neighbors while indirect ones often reflect longer-distance connections mediated via other brain regions. In the EC state, connections in the frontal parts of the brain are enhanced as compared to the EO state, while the opposite applies to the posterior regions. In general, our approach leads to a significant reduction in the number of indirect connections and thereby contributes to a better understanding of the alpha band desynchronization phenomenon in the EO state. Functional network representations have recently gained considerable interest in the study of real-world spatially extended dynamical systems like the Earth’s climate or the human brain. In a vast fraction of cases, the existence of network links has been established by resorting to the presence of strong bivariate statistical associations as suggested by symmetric association measures like classical linear (Pearson) correlations. This methodology, however, disregards two relevant aspects: the directionality of dynamical interactions between pairs of actors and the complexity of mutual (synergistic or antagonistic) inter-dependencies that can lead to the spurious identification of connections in case of, for example, common drivers or directed chain-like coupling configurations. In order to address both aspects, various approaches based on nonlinear time series analysis have been developed in the last few years, most of which assume the presence of a continuous temporal variability pattern. However, in the context of event-like data like spikes in neural activity or climate extremes, there still exists a considerable gap in suitable methodologies for unravelling the complex web of directed interactions from multivariate time series. The present work introduces partial event coincidence analysis as a new approach and studies its applicability to different types of model systems as well as real-world EEG data.

Description: Synchronization of coupled oscillators is important for understanding collective dynamics of a variety of natural and artificial systems including neuronal networks, Josephson junctions, and power grids. Despite this ubiquity, it remains unclear how the interaction between oscillator’s dynamics and coupled structure either promotes or inhibits synchrony. Here, we introduce a Lyapunov function of the system such that it can be readily optimized to enhance synchrony of even heterogeneous oscillators on sparse networks. We consider two optimizing problems: frequency allocation and network design. Numerical experiments show that the proposed criterion outperform the promising methods, which is explained by a theoretical framework of the correlation between node degree and frequency magnitude.

Description: Dynamic mesoscale flow channels move across the open field line regions of the polar caps, and then enter the nightside plasma sheet, where they can lead to important space weather disturbances, such as streamers, substorms, and omega bands. We find that the polar cap structures leading to disturbances can have long durations (at least ~1½ to 2 hours), and one flow structure can lead to more than one disturbance as it moves azimuthally within the polar cap. Examples using 630 nm auroral and radar observations indicate that the motion of flow channels within the polar cap may be significantly controlled by the IMF By. This motion appears to possibly be a critical factor in determining when and where a particular disturbance within the nightside auroral oval will be triggered. Also, potentially important is the occasional dramatic azimuthal turning of a flow channel, leading to azimuthal broadening of flow channel contact with the auroral oval and of a subsequent substorm onset. Of additional importance for future understanding of disturbances resulting from polar cap flow channels will be determining conditions along nightside auroral oval field lines (plasma sheet) that interact with an incoming flow polar-cap flow channel to give a particular disturbance. Additionally interesting will be consideration of the generality of geomagnetic disturbances being related to their connections with incoming polar cap flow channels, including the location, time, and type of disturbances, and also whether the duration of the disturbances appears to be related to the duration of an incoming flow channel.

Description: China’s first interplanetary mission to Mars, Tianwen-1, consists of an obiter, a lander and a rover (Zhurong). It was lunched on 23 July 2020, entered Mars obit on 10 February 2021 and landed on Mars on 15 May 2021. This paper will overview the main scientific objectives and the major research advances of Tianwen-1, in particular from the Zhurong rover landed at the southern Utopia Planitia. The Zhurong rover has performed in-situ investigations of the geomorphological features, surface and subsurface geology, magnetic field and environments along its traverse to the south of the landing area. The short-wave infrared spectral data showed the presence of hydrated sulfate/silica materials on the Amazonian terrain at the landing site. These hydrated minerals and associated duricrust suggest that a more active Amazonian hydrosphere for Mars than previously thought. A detailed subsurface image constructed from the rover radar data shows a ~70-m-thick, multi-layered structure below a less than 10-m-thick regolith. The structural layering suggests the occurrence of episodic water-involved resurfacing in Utopia Planitia during the Late Hesperian to Amazonian epochs, although there is no direct evidence for the presence of liquid water today in the upper 80 m. The Zhurong rover also made the first ground magnetic vector measurements at 16 sites along the ~1-km track. This in-situ observation reveals an extremely weak magnetic field, much smaller than that was predicted from the orbit, in contrast to the large crustal magnetic field measured by InSight in Elysium Planitia.