All-sky imager observations at South Pole Station: Recent advances and future challenges

第3回極域科学シンポジウム/第36回極域宙空圏シンポジウム 11月26日（月）、27日（火） 国立極地研究所 2階ラウン

Intermittency on Simultaneous Observations of Riometer at Several Antarctic Locations

It is well known that auroral radio wave absorption, as measured by riometers, consists of periods of relative quiescence which are interrupted by short bursts of activity. Such patterns in activity are observed in systems ranging from the stock market to turbulence, i.e. they exhibit intermittency. In the case of the auroral absorption it has also been found that intermittency strongly depends on the magnetic local time, being largest in the night-time sector. This can be interpreted as indicating that the precipitating particles responsible of the absorption exhibit intermittency, especially near the substorm eye, where the level of turbulence increases. Here, we analyse simultaneous observations of riometer absorption at seven Antarctic locations, to determine whether they exhibit intermittency. We determine the Probability Distribution Functions of the fluctuations of riometer absorption for absorption events larger than 0.1 dB, as well as those for the time-intervals between absorption events. Observations are for locations within the austral auroral absorption zone and on the polar cap. It is found that the parameters of a power law used to describe the calculated PDFs are consistent with the formation of coherent structures being more frequent within the auroral zone, as a manifestation of intermittency

25 Years of Self-organized Criticality: Concepts and Controversies

Introduced by the late Per Bak and his colleagues, self-organized criticality (SOC) has been one of the most stimulating concepts to come out of statistical mechanics and condensed matter theory in the last few decades, and has played a significant role in the development of complexity science. SOC, and more generally fractals and power laws, have attracted much comment, ranging from the very positive to the polemical. The other papers (Aschwanden et al. in Space Sci. Rev., 2014, this issue; McAteer et al. in Space Sci. Rev., 2015, this issue; Sharma et al. in Space Sci. Rev. 2015, in preparation) in this special issue showcase the considerable body of observations in solar, magnetospheric and fusion plasma inspired by the SOC idea, and expose the fertile role the new paradigm has played in approaches to modeling and understanding multiscale plasma instabilities. This very broad impact, and the necessary process of adapting a scientific hypothesis to the conditions of a given physical system, has meant that SOC as studied in these fields has sometimes differed significantly from the definition originally given by its creators. In Bak’s own field of theoretical physics there are significant observational and theoretical open questions, even 25 years on (Pruessner 2012). One aim of the present review is to address the dichotomy between the great reception SOC has received in some areas, and its shortcomings, as they became manifest in the controversies it triggered. Our article tries to clear up what we think are misunderstandings of SOC in fields more remote from its origins in statistical mechanics, condensed matter and dynamical systems by revisiting Bak, Tang and Wiesenfeld’s original papers

Conjugate observations of traveling convection vortices: the field-aligned current system

Analysis of a single traveling convection vortex (TCV) event that combines magnetometer, auroral imager, and riometer observations from conjugate hemispheres provides a complete view of the field-aligned current (FAC) system associated with the TCV. Conjugate observations clearly show that the TCV is a pair of FACs that flow alternatively out of and then into both ionospheres. Imaging observations at 427.8 nm indicate that the upward FAC region is associated with a narrow arc 50-100 km wide and at least 600 km long. Mapping the imaging observations made in the Southern Hemisphere to the Northern Hemisphere shows excellent collocation of the precipitation with the flow vortex associated with the upward FAC as inferred from a large two-dimensional network of magnetometers. Also consistent with the flows inferred from the magnetometers, the arc is found to be skewed with respect to the direction of motion of the transient. Low-altitude spacecraft passes near the time of the event suggest that the FAC is coincident with, and possibly confined to, a relatively narrow region (less than 3 in latitude) in which 1-10 keV electrons characteristic of the central plasma sheet are present