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Disturbance is common in natural ecosystems, but increasingly defines them. While there are many descriptions for the dynamics of an ecosystem's response to disturbance, there are few descriptions for the dynamics of the disturbance itself. I describe a novel application of a model based on the production of amplitude envelopes in acoustics and electronic music synthesis, with varying parameters Attack, Decay, Sustain, and Release (ADSR). I show that varying the parameters of the ADSR model is sufficient to produce and vary the qualitative disturbance regimes described by previous authors, and is capable of producing dynamics not previously considered. I tested the utility of the ADSR model by applying it to a logistic growth model. I found that manipulating the attack and release parameters of the ADSR model changes the population dynamics estimated by these models. This implies that responses to disturbance are determined not only by the resilience and resistance of the ecological system, but also the dynamics of the disturbance itself. My hope is that the ADSR model will prove useful to researchers in either describing disturbances in long‐term ecological data, or in producing disturbances for simulations or experiments. [ABSTRACT FROM AUTHOR]

Naturalistic plantings, such as meadow‐style plantings, can improve the quality of urban green spaces through aesthetic, biodiversity and low maintenance features. Species selection for, and maintenance of naturalistic plantings are key to their success. While herbaceous and grassy meadows can be mowed, naturalistic plantings with woody plants require more intense maintenance to remove biomass and promote resprouting. We aim to understand woody plant responses to diverse disturbance regimes to potentially inform the selection and management of woody species in urban plantings. We conducted a quantitative systematic literature review of 72 papers and investigated what main external (climate, disturbance regime) and internal (buds, life stage, storage reserves) factors influence the resprouting response of woody plants. We found resprouting literature is geographically widespread for woody plants, but studies are skewed towards Temperate climates in USA and Australia, with a focus on high severity and high frequency fire disturbance. Resprouting response was mostly defined as a continuous response to disturbance dependent on disturbance regime, climate and plant traits. Maintenance and management of naturalistic woody plantings, through hard pruning techniques such as coppicing, may be informed by analogous high severity and high frequency disturbance studies. However, the literature on woody plant resprouting has several knowledge gaps for lower severity and lower frequency disturbance regimes and in more arid climates. Future research should evaluate the response of naturalistic woody plantings to disturbance in specific urban contexts. [ABSTRACT FROM AUTHOR]

• The 2019 wildfires raised global concern about wildfires, their causes and effects. • Fire has distinct roles in fire-prone and fire-sensitive ecosystems. • In fire-prone ecosystems, we need to overcome taboos regarding the use of fire. • In fire-sensitive systems, strategies to reduce fires are urgent. • Climate-change and increasing human impact change fire regimes. In the recent past, wildfires have received much attention both by science and by the media, especially in relation to deforestation, degradation and climate change and when affecting fire-sensitive ecosystems. On the other hand, fire is a natural process in many ecosystems, in particular those to be considered fire-dependent; here, maintaining natural fire regimes is important to maintain biodiversity and ecosystem services. In many regions of the world we are still in the need for a better understanding of fire effects on plant populations, communities and ecosystems. For instance, responses of individuals and populations to heat and fire can differ among distinct fire-prone ecosystems, and community level responses often vary depending on historical disturbance regimes or the fire regime as such. Effects on ecosystem processes, e.g. on carbon cycles, can vary greatly in scale and are even more difficult to predict. This adds up to uncertainties regarding appropriate fire management strategies, both in fire-prone and fire-sensitive ecosystems. Here, we introduce a set of opinion papers on fires in Brazil and a forthcoming Virtual Special Issue of Flora – Morphology, Distribution, Functional Ecology of Plants on "Fire and vegetation", with contributions expected from a broad range of topics related to fire, plants, and vegetation. This special issue will contribute to the development not only of more knowledge on fire effects on plants and vegetation, but also to advances in fire management policies in different ecosystems around the world. The main issue, after all, is to go beyond simple classification of fires as 'good' or 'bad', but rather to develop strategies to deal with changing fire regimes and their effects in a dynamic world increasingly modified by human actions. [ABSTRACT FROM AUTHOR]

Throughout South-East Asia, droughts associated with ENSO (El Niño Southern Oscillation) events have resulted in large-scale fires affecting millions of hectares of rain forest. However, the long-term impacts of these fires on the rain forest faunas are only poorly understood. Our aim was to study the recovery of rain forest butterfly assemblages following the 1997–98 ENSO event, which resulted in the largest-scale fires in the recorded history of the region. A 420-km2 area in the Balikpapan-Samarinda region of East Kalimantan, Indonesian Borneo. Four landscapes were assessed after the 1997–98 ENSO event, including one landscape that was assessed prior to the event. Comparisons of species richness, species composition and guild abundance were made among landscapes and years. The relative importance of environment, geographical distance between sampling sites, and time between sampling years was quantified during the succession phase using a variance partitioning technique. The fires dramatically altered the butterfly community and resulted in a major decline in observed species richness within the landscape surveyed prior to the ENSO event. Following fires in 1998, butterfly assemblages in all landscapes were dominated by large-winged generalist species. During 1999 and 2000, assemblages became increasingly dominated by smaller specialist species. Species endemic to Borneo that were present before fires were absent in 2000, despite intensive sampling over enhanced spatial and environmental scales. Community similarity was significantly dependent upon local environmental variables, geographical distance between sampling sites, and time between sampling years. Together, these explained over 52% of the observed variation in samples. The importance of geographical distance between sampling sites indicates that recovery was dependent upon colonization from proximate habitats. Despite an apparent trend of return to pre-ENSO community structure, low species richness throughout the survey area indicates that full recovery had not taken place by 2000. [ABSTRACT FROM AUTHOR]

Accurately predicting train running times (TRTs) during disturbances is crucial for effective timetable rescheduling. Previous studies on train running prediction have overlooked the detailed textual description of disturbance events. To improve the prediction accuracy, this paper proposes a hybrid neural network model consisting of a transformer encoder and a fully connected neural network (FCNN) to predict the TRTs under disturbances, which is called transformer-FCNN here. In the proposed transformer-FCNN architecture, a transformer encoder is used to process textual disturbance events, while FCNN is used to deal with static features. The performance of transformer-FCNN is validated using data from the Wuhan-Guangzhou and Xiamen-Shenzhen high-speed railways. The results show that the models considering the disturbance category can substantially improve the predictive performance compared with those that do not. Further, based on the same algorithm, models which use textual disturbance event records are shown to reduce the mean absolute error and the root mean squared error, respectively, by over 10.8% and 8.3% on average compared with those that only consider the disturbance category. The proposed model can support potential strategies of train operation control, by providing accurate prediction results. [ABSTRACT FROM AUTHOR]

Moderate severity disturbances, which only kill a subset of canopy trees (e.g., via insects, pathogens, and windthrow), are increasingly widespread in North America, and can alter forest structure and production. Whether the net primary production (NPP) of forest stands differing in pre-disturbance site quality and composition respond similarly to moderate severity disturbance, however, is unknown, but critical to understanding the disturbance response dynamics of patchy landscapes. We experimentally disturbed three, 2-ha stands varying in pre-disturbance primary production and community composition, temporarily reducing live stand basal area by 38% to 66% through the stem girdling of all mature early successional aspen (Populus tremuloides Michx. and Populus grandidentata Michx.) and birch (Betula papyrifera Marshall). Disturbance significantly altered stand-scale physical and biological structure and prompted a similar decade-long pattern of wood NPP decline and recovery. All stands exhibited an initial reduction in wood NPP, followed by a recovery period and eventual return to pre-disturbance levels within eight years, with the most productive stand exhibiting an increase in primary production following recovery. Following wood NPP recovery, more biologically diverse forest canopies with higher leaf area indexes intercepted more light, and, consequently, had higher rates of wood NPP.We conclude that, despite substantial pre-disturbance differences in productivity and community composition, relative wood NPP recovery patterns can be similar, though long-term post-recovery primary production may trend higher in more productive and compositionally diverse stands. We suggest that improved mechanistic understanding of different forest ecosystems' responses to disturbances remains critical to informing management decisions across diverse landscape mosaics. [ABSTRACT FROM AUTHOR]

In a nonlinear control system, hysteresis exists usually as common characteristics. In addition, external output disturbances like modelling error, machine friction and so on also occur frequently. Both of them are considered to cause instability and unsatisfactory performance. In this paper, a practical nonlinear control system design is proposed so as to achieve the simultaneous control of input hysteresis and output disturbance. The system is based on RCF (right coprime factorization theory). Additionally, the proposed design has been applied to a soft robotic finger system and the results of simulations and practical experiments are exhibited, which show the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

In the Anthropocene, intensifying ecological disturbances pose significant challenges to our predictive capabilities for ecosystem responses. Macroecology—which focuses on emergent statistical patterns in ecological systems—unveils consistent regularities in the organization of biodiversity and ecosystems. These regularities appear in terms of abundance, body size, geographical range, species interaction networks, or the flux of matter and energy. This paper argues for moving beyond qualitative resilience metaphors, such as the 'ball and cup', towards a more quantitative macroecological framework. We suggest a conceptual and theoretical basis for ecological resilience that integrates macroecology with a stochastic diffusion approximation constrained by principles of biological symmetry. This approach provides an alternative novel framework for studying ecological resilience in the Anthropocene. We demonstrate how our framework can effectively quantify the impacts of major disturbances and their extensive ecological ramifications. We further show how biological scaling insights can help quantify the consequences of major disturbances, emphasizing their cascading ecological impacts. The nature of these impacts prompts a re-evaluation of our understanding of resilience. Emphasis on regularities of ecological assemblages can help illuminate resilience dynamics and offer a novel basis to predict and manage the impacts of disturbance in the Anthropocene more efficiently. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'. [ABSTRACT FROM AUTHOR]

Since the integration of electronic devices and intelligent electronic devices into the power grid, power quality (PQ) has consistently remained a significant concern for system operators and experts. Maintaining high standards of power quality is crucial to preventing malfunctions and faults in electric assets and connected loads. Recently, PQ studies have shifted their focus to a specific frequency range, previously not considered problematic—the supraharmonic 2 kHz to 150 kHz range. This range is not populated by easily recognizable harmonic components of the 50 Hz to 60 Hz mains fundamental, but by a combination of intentional emissions, switching non-linearities and byproducts, and various types of resonances. This paper aims to provide a detailed analysis of the impact of supraharmonics (SHs) on power network operation and assets, focusing on the most relevant documented negative effects, namely power loss and the heating of grid elements, aging of dielectric materials, failure of medium voltage (MV) cable terminations, and interference with equipment and power line communication (PLC) technology in particular. Under some shareable assumptions, limits are derived and compared to existing ones for harmonic phenomena, providing a clear identification of the primary issues associated with supraharmonics and suggestions for the standardization process. Strictly related is the problem of grid monitoring and assessment of SH distortion, discussing the suitability of normative requirements for instrument transformers (ITs) with a specific focus on their accuracy. [ABSTRACT FROM AUTHOR]

This paper deals with the practical exponential stability of two-dimensional (2-D) nonlinear switched positive systems with impulse, disturbance and all modes unstable. Under the mode-dependent interval dwell time switching, new criteria such that 2-D nonlinear switched positive systems achieve practical exponential stability are derived. Furthermore, the explicit upper bound and convergence rate of the system state are estimated explicitly. Numerical examples indicate the correctness and effectiveness of the obtained results. [ABSTRACT FROM AUTHOR]

Globally, protected areas such as national parks are mainly established to protect the rapidly declining populations of conservation concern wildlife species, as well as ecologically and economically important areas. In addition, more recently, establishing and maintaining protected areas has become an essential part of the global response of governments to combat climate change and its impacts. However, most of these protected areas are facing an unprecedented rate of biodiversity and ecosystem degradation due to human-induced actions. Mitigation of threats and their impacts on biodiversity in these protected areas is very critical. But limited information, needed to develop strategies for mitigation of the ever-growing threats, is available on the biophysical resources and threats to them. This paper presents for the first time, detailed information on the Exceptional Resource Values (ERVs), Principal Ecosystem Components (PECs; or conservation targets), and human-induced threats in the Kafta-Sheraro National Park (KSNP). Standard ecological and social science methods were used to collect and analyze the data. Overall, 14 key ERVs were identified in the park, including several conservation concerns and/or flagship mammal species (e.g. African Elephant (Loxodonta africana)), biome-restricted and migratory bird species (e.g., Demoiselle crane (Anthropoides virgo)), three distinct habitat types, and a hydrological system. Six PECs that were assumed to represent the whole ecosystem components of the park were identified in KSNP: the three habitat types, hydrological system, Elephant, and Roan antelope (Hippotragus equines). The top human-induced threats to these PECs appeared to be fire, cultivation, grazing, settlement, poaching, and mining. In general, KSNP contains outstanding biodiversity and other natural resources that could play a vital role in the socio-economic development of local and regional communities. To ensure the long-term persistence of the ERVs and PECs and the appropriate functioning of the park ecosystems the development and implementation of threat mitigation strategies are required. [ABSTRACT FROM AUTHOR]

As coral reefs endure increasing levels of disturbance, understanding recovery patterns of reef‐building hard corals is paramount to assessing the sustainability of these ecosystems. At local scales, coral recovery slows down; however, it's unclear how this trend propagates across spatial scales due to the inherent complexity of coral dynamics. In this paper, we aimed to learn about fine scale heterogeneity of coral dynamics and explore implications for assessing coral recovery at larger spatial scales. We developed a spatio‐temporal statistical model to estimate long‐term trajectories of three types of corals and predict their recovery patterns at unobserved locations within a reef. Then, model predictions were used to derive metrics that capture the interplay between coral growth and decline from disturbance(s) across time, space and growth morphology. This model is developed in the context of a substantive case study at Heron Reef using a high spatio‐temporal resolution dataset. Our results revealed that successful coral community recoveries took place in different habitats of Heron Reef and associated with various reasons. Branching corals recovered in the southern slope, due to fast growth in locations that were previously abundant. Plate corals flourished in the northern slope due to fast growth, despite a large decline and low baseline cover. They also recovered in the southern slope but in this case there was both a low decline and baseline cover. At Heron Reef, the recovery of coral communities followed specific conditions that were acting at a fine scale in a complex and heterogeneous way within habitat. This implies that capturing the variability of fine‐scale coral dynamics is an important first step to detect accurate signals of coral recovery at larger spatial scales. The approach proposes here can be further extend to the scale of a reef and beyond enabling assessment of recovery patterns representative at management scales. [ABSTRACT FROM AUTHOR]

Disturbances caused as a result of the misalignment and axial motion of the journal affect the characteristics of the rotor-bearing system. This paper aims to propose an algorithm for the theoretical analysis of a rotor-bearing system that considers these disturbances. A theoretical model for a journal bearing considering disturbances is given. The dynamic equations for a rigid rotor-bearing system are introduced. A detailed algorithm that can simultaneously solve the rotor-dynamic equations and the Reynolds equation is proposed. The static performance, such as the bearing attitude angle and the fluid film pressure, are given, and dynamic characteristics such as the nonlinear dynamic responses and the axial orbits of a rigid rotor-bearing system are presented. The hydrodynamic effect of the bearing is enhanced by the axial disturbance. Disturbances in the circumferential and radial directions lead to variations in the fluid film thickness distribution in the axial direction and the offset of the fluid film pressure distribution in the axial direction. When these disturbances work together, the variation trend is more obvious and affects the capacity and dynamic characteristics of the bearing. When the L/D value of the bearing increases, the clearance between the journal and the bearing decreases rapidly. When the value reaches a certain limit, contact and collision might occur. The theoretical analysis method and the algorithm proposed for a rotor-bearing system considering several disturbances could enhance the design level for a bearing and rotor-bearing system. [ABSTRACT FROM AUTHOR]

Copyright of Romanian Journal of Information Technology & Automatic Control / Revista Română de Informatică și Automatică is the property of National Institute for Research & Development in Informatics - ICI Bucharest and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper is concerned with the attitude tracking problem of quadrotor unmanned aerial vehicles (UAVs) with respect to endogenous uncertainties, exogenous disturbances and actuator failures. Two different control methods are proposed to solve this problem. First, an adaptive extended state observer (AESO)-based control framework is devised to tackle the difficulties caused by model uncertainties and external disturbances. A fault-tolerant control method is proposed to cope with the occurrence of actuator failure, which is modeled as a constant loss of effectiveness. Another method employs AESOs to compensate for lumped disturbances, which include endogenous uncertainties, exogenous disturbances and actuator failures. Then, the error can exponentially converge to a bounded set. Finally, simulations are performed to ensure the feasibility of the designed technique. [ABSTRACT FROM AUTHOR]

This paper investigates an H ∞ cruise control strategy for uncertain high-speed trains (HSTs) with two time-varying delays. The multibody model of the cascading car is established, taking into account the influence of the flexible coupler and the basic resistance (rolling mechanical resistance + aerodynamic resistance) in the actual operation process. There are many uncertainties in the actual operation of the train, such as disturbances caused by gusts and track irregularities and time-varying delays caused by flexible couplers. Based on the above factors, this paper fully considers and supplements the multibody model. A cruise control strategy based on Lyapunov stability theory and linear matrix inequality (LMI) is proposed. The state feedback control rate with input short time delay is designed, and a cruise control method with double time-varying delays is proposed by sampling data. Constructed Lyapunov–Krasovskii function (LKF) with time-delay parameters for time-delay systems to fully introduce the information of time delays. In view of the disturbance effect, the sufficient condition to suppress the disturbance is given. The proposed H ∞ cruise control method controls the fluctuating speed and displacement of the train to quickly converge to the steady state. The HST at equilibrium tracks the desired velocity and ensures a stable relative displacement. The specified H ∞ interference attenuation level is achieved to ensure safe and comfortable operation. Two numerical simulations are carried out to illustrate the effect of state time-varying delay and disturbance on train stationarity, which proves the superiority of the proposed method. [ABSTRACT FROM AUTHOR]

This paper schemes a new nonlinear method for controlling and suppressing of chaos instability in the PMSM. First, a nonlinear model of PMSM is considered that is able to describe the dynamic behaviour of the PMSM in different functional conditions, including the occurrence of chaos. As an integral part of most physical systems, disturbance with an unknown upper limit is considered in the design of the control scheme. Parametric uncertainty is also included in the PMSM system model and a new control technique is presented based on robust adaptive and sliding mode approaches to prevent chaos. In the proposed method, adaptive technique is used to estimate the upper bound of uncertain terms, disturbances and nonlinear terms, and the robust sliding mode scheme is designed to stabilize the closed-loop PMSM system in contradiction of chaos. Using comparisons, the efficiency of the proposed method is shown in different working conditions. [ABSTRACT FROM AUTHOR]

This paper considers a construction procedure of a satellite reference angular motion in the vicinity of an unstable gravitational equilibrium position. The satellite is stabilized on the reference trajectory by the magnetic coils. The problem is solved in several stages. An optimization problem to be solved by the particle swarm optimization method is formulated at each stage. Cost functions are based on the linearized model analysis. The main stage is the construction of a special reference motion, which provides the minimum control torque projection on the geomagnetic induction vector. Optimal geomagnetic field dipole approximation for a given time interval is considered to reduce tracking errors. The paper compares combinations of different cost functions in terms of the terminal attitude accuracy in the presence of perturbations. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Al-Anbar University for Islamic Sciences is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Fuel economy (FE) improvements for hybrid electric vehicles using a predictive Optimal Energy Management Strategy (Optimal EMS) is an active subject of research. Recent developments have focused on real-time prediction-based control strategies despite the lack of research demonstrating the aspects of prediction that are most important for FE improvements. In this paper, driving-derived nonstochastic prediction errors are applied to a globally optimal control strategy implemented on a validated model of a 2010 Toyota Prius, and the FE results are reported for each type of prediction error. This paper first outlines the real-world drive cycle development, then the baseline model development that simulates a 2010 Toyota Prius, followed by an implementation of dynamic programming (DP) to derive the globally optimal control, and finally the use of the DP solution to evaluate prediction errors. FE comparisons are reported for perfect prediction, prediction errors from 14 alternate drive cycles, and prediction errors from 6 alternate vehicle parameters. The results show that FE improvements from the Optimal EMS are maintained under mispredicted stops, traffic, and vehicle parameters, while route changes and compounded drive cycle mispredictions may result in FE improvements being lost. Taken together, these results demonstrate that implementation of an Optimal EMS can result in a reliable FE improvement. [ABSTRACT FROM AUTHOR]

Copyright of EMERG: Energy. Environment. Efficiency. Resources. Globalization is the property of Romanian National Committee of World Energy Council and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Applied Ecology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Extreme events are of interest worldwide given their potential for substantial impacts on social, ecological, and technical systems. Many climate‐related extreme events are increasing in frequency and/or magnitude due to anthropogenic climate change, and there is increased potential for impacts due to the location of urbanization and the expansion of urban centers and infrastructures. Many disciplines are engaged in research and management of these events. However, a lack of coherence exists in what constitutes and defines an extreme event across these fields, which impedes our ability to holistically understand and manage these events. Here, we review 10 years of academic literature and use text analysis to elucidate how six major disciplines—climatology, earth sciences, ecology, engineering, hydrology, and social sciences—define and communicate extreme events. Our results highlight critical disciplinary differences in the language used to communicate extreme events. Additionally, we found a wide range in definitions and thresholds, with more than half of examined papers not providing an explicit definition, and disagreement over whether impacts are included in the definition. We urge distinction between extreme events and their impacts, so that we can better assess when responses to extreme events have actually enhanced resilience. Additionally, we suggest that all researchers and managers of extreme events be more explicit in their definition of such events as well as be more cognizant of how they are communicating extreme events. We believe clearer and more consistent definitions and communication can support transdisciplinary understanding and management of extreme events. [ABSTRACT FROM AUTHOR]

This paper devotes to the design of all feasible output feedback controllers for the robust output tracking of Boolean control networks with disturbances (DBCNs). First, the output feedback–based robust control invariant set (RCIS) is defined, and a calculation method to find all output feedback–based RCISs of a given set is derived via the truth matrix technique. Second, a class of complete family of robust reachable sets (CFRRSs) is developed; the solvability of robust output tracking problem by output feedback control is verified. Furthermore, a constructive algorithm of all feasible output feedback controllers is presented. Finally, the study of a network about Escherichia coli lactose operon shows the effectiveness of the theoretical results. [ABSTRACT FROM AUTHOR]

This paper presents a novel leader-follower consensus control for a particular class of nonlinear multi-agent mechanical systems in the presence of control input constraints and external disturbances which includes robot system dynamics with a wide range of potential applications in industry. In this case, one of the agents is selected as the leader to direct the other agents in such a way that the whole system can reach consensus within certain prescribed performance transient bounds. Due to the presence of disturbances in most practical systems, the effect of limited disturbance in the consensus control method has been investigated, and actuator saturation is included in the design process. A terminal sliding mode control method has been adapted to ensure the stability of the overall system and fast finite-time leader-follower consensus control. The simulation results of the multi-agent nonlinear robot system in MATLAB environment, in different scenarios with simultaneous consideration of actuator saturation and external disturbance, will show the efficiency of the proposed control method. [ABSTRACT FROM AUTHOR]

In this paper, a LuGre model–based robust adaptive control (RAC) approach is presented for a pump-controlled hydraulic actuator. We first decompose the LuGre friction model into its steady-state model and a lumped dynamic part applying the mean value theorem, which are compensated by a feedforward term and a robust adaptive term, respectively. The robust adaptive term also plays a part in mismatched disturbance attenuation. In addition, parametric uncertainties and matched disturbances are handled by σ -modified adaptation laws and a robust control law, respectively. The stability of the closed-loop system is proved via the Lyapunov analysis. The efficacy and robustness of the proposed approach are validated by comparative experiments. Compared with common adaptive friction compensation methods, the proposed method has a simpler structure, less computational burden, better control performance, and stronger robustness. Moreover, since the available information is separated from the LuGre model and acts as a model-based compensation term, the design conservativeness of RAC is effectively reduced. [ABSTRACT FROM AUTHOR]

The thermospheric density and its variations are crucial to aerospace activities as well as space weather research and operation. However, due to the difficulties in observing the thermosphere, there has been a lack of effective descriptions for the general characteristics of thermospheric density. In this paper, the Two‐Line‐Element data sets (TLEs) from multi‐target low Earth orbit satellites are used to derive a proxy of the daily average atmospheric density in the thermospheric shell located in the vicinity of LEOs' orbital altitude. It captures the overall characteristics of the thermosphere and exhibits good correlations (∼0.9) with modeled and observed thermospheric density. By applying the spectral whitening method to this proxy, a new index JsT ${J}_{s}^{T}$ is derived to describe non‐periodic perturbation of the density where the specific satellite passed by. The fact that the JsT ${J}_{s}^{T}$ obtained from different satellites within the same thermospheric shell presents significant consistency to each other means that the new index is a good indicator for the overall feature of the variations of thermospheric density, and it is possible to define a unified regional index JrT ${J}_{r}^{T}$ to describe density disturbances for the thermospheric shell where these satellites fly through. Moreover, the JrT ${J}_{r}^{T}$ at different altitudes also present good consistency suggesting the possibility of defining a global index JpT ${J}_{p}^{T}$, capable of describing the density variation of the entire thermosphere. Plain Language Summary: Due to limited in situ observation, the thermospheric density is not easy to obtain. Herein, we have extracted the density information of the thermosphere based on the Two‐Line‐Element data sets and developed a new index using the spectral whitening method, which describes aperiodic disturbances of the density. The construction of this index series can address the limitations of existing quantitative or graded descriptions of space weather disturbances and the problem of mismatched or inconsistent parameters/indices. Ultimately, this will contribute to the monitoring and warning of space weather causal chains. Key Points: A proxy of daily average thermospheric mass density was extracted from Two‐Line‐Element data setsBy applying the spectral whitening method to this proxy, a new index is derived to describe non‐periodic perturbations in the thermosphereA unified regional index was defined to describe density disturbances for the thermospheric shell where these satellites fly through [ABSTRACT FROM AUTHOR]