Vienna University of Technology

About: Vienna University of Technology is a education organization based out in Vienna, Austria. It is known for research contribution in the topics: Laser & Context (language use). The organization has 16723 authors who have published 49341 publications receiving 1302168 citations.

Widespread seasonal compensation effects of spring warming on northern plant productivity

Abstract: Climate change is shifting the phenological cycles of plants1, thereby altering the functioning of ecosystems, which in turn induces feedbacks to the climate system2. In northern (north of 30° N) ecosystems, warmer springs lead generally to an earlier onset of the growing season3,4 and increased ecosystem productivity early in the season5. In situ6 and regional7–9 studies also provide evidence for lagged effects of spring warmth on plant productivity during the subsequent summer and autumn. However, our current understanding of these lagged effects, including their direction (beneficial or adverse) and geographic distribution, is still very limited. Here we analyse satellite, field-based and modelled data for the period 1982–2011 and show that there are widespread and contrasting lagged productivity responses to spring warmth across northern ecosystems. On the basis of the observational data, we find that roughly 15 per cent of the total study area of about 41 million square kilometres exhibits adverse lagged effects and that roughly 5 per cent of the total study area exhibits beneficial lagged effects. By contrast, current-generation terrestrial carbon-cycle models predict much lower areal fractions of adverse lagged effects (ranging from 1 to 14 per cent) and much higher areal fractions of beneficial lagged effects (ranging from 9 to 54 per cent). We find that elevation and seasonal precipitation patterns largely dictate the geographic pattern and direction of the lagged effects. Inadequate consideration in current models of the effects of the seasonal build-up of water stress on seasonal vegetation growth may therefore be able to explain the differences that we found between our observation-constrained estimates and the model-constrained estimates of lagged effects associated with spring warming. Overall, our results suggest that for many northern ecosystems the benefits of warmer springs on growing-season ecosystem productivity are effectively compensated for by the accumulation of seasonal water deficits, despite the fact that northern ecosystems are thought to be largely temperature- and radiation-limited10.

Automatic network protocol analysis

Abstract: Protocol reverse engineering is the process of extracting application-level specifications for network protocols. Such specifications are very helpful in a number of security-related contexts. For example, they are needed by intrusion detection systems to perform deep packet inspection, and they allow the implementation of black-box fuzzing tools. Unfortunately, manual reverse engineering is a time-consuming and tedious task. To address this problem, researchers have recently proposed systems that help to automate the process. These systems operate by analyzing traces of network traffic. However, there is limited information available at the network-level, and thus, the accuracy of the results is limited. In this paper, we present a novel approach to automatic protocol reverse engineering. Our approach works by dynamically monitoring the execution of the application, analyzing how the program is processing the protocol messages that it receives. This is motivated by the insight that an application encodes the complete protocol and represents the authoritative specification of the inputs that it can accept. In a first step, we extract information about the fields of individual messages. Then, we aggregate this information to determine a more general specification of the message format, which can include optional or alternative fields, and repetitions. We have applied our techniques to a number of real-world protocols and server applications. Our results demonstrate that we are able to extract the format specification for different types of messages. Using these specifications, we then automatically generate appropriate parser code.

Attosecond correlation dynamics

Abstract: Photoemission of an electron is commonly treated as a one-particle phenomenon. With attosecond streaking spectroscopy we observe the breakdown of this single active-electron approximation by recording up to six attoseconds retardation of the dislodged photoelectron due to electronic correlations. We recorded the photon-energy-dependent emission timing of electrons, released from the helium ground state by an extreme-ultraviolet photon, either leaving the ion in its ground state or exciting it into a shake-up state. We identify an optical field-driven d.c. Stark shift of charge-asymmetric ionic states formed after the entangled photoemission as a key contribution to the observed correlation time shift. These findings enable a complete wavepacket reconstruction and are universal for all polarized initial and final states. Sub-attosecond agreement with quantum mechanical ab initio modelling allows us to determine the absolute zero of time in the photoelectric effect to a precision better than 1/25th of the atomic unit of time. Photoemission is not a simple process and it is not instantaneous. Delays of a few attoseconds have now been measured in helium and it seems that they are partly due to electronic correlations.