Direct simulations of helical Hall-MHD turbulence and dynamo action

Direct numerical simulations of turbulent Hall dynamos are presented. The evolution of an initially weak and small scale magnetic field in a system maintained in a stationary turbulent regime by a stirring force at a macroscopic scale is studied to explore the conditions for exponential growth of the magnetic energy. Scaling of the dynamo efficiency with the Reynolds numbers is studied, and the resulting total energy spectra are found to be compatible with a Kolmogorov type law. A faster growth of large scale magnetic fields is observed at intermediate intensities of the Hall effect.Comment: 13 pages, 17 figures, ApJ (in press

Comment on ``The linear instability of magnetic Taylor-Couette flow with Hall effect''

In the paper we comment on (R\"udiger & Shalybkov, Phys. Rev. E. 69, 016303 (2004) (RS)), the instability of the Taylor--Couette flow interacting with a homogeneous background field subject to Hall effect is studied. We correct a falsely generalizing interpretation of results presented there which could be taken to disprove the existence of the Hall--drift induced magnetic instability described in Rheinhardt and Geppert, Phys. Rev. Lett. 88, 101103. It is shown that in contrast to what is suggested by RS, no additional shear flow is necessary to enable such an instability with a non--potential magnetic background field, whereas for a curl--free one it is. In the latter case, the instabilities found in RS in situations where neither a hydrodynamic nor a magneto--rotational instability exists are demonstrated to be most likely magnetic instead of magnetohydrodynamic. Further, some minor inaccuracies are clarified.Comment: 3 pages, 1 figure; accepted by Physical Review

Waves, Coriolis force and the dynamo effect

Dynamo activity caused by waves in a rotating magneto-plasma is investigated. In astrophysical environments such as accretion disks and at sufficiently small spatial scales, the Hall effect is likely to play an important role. It is shown that a combination of the Coriolis force and Hall effect can produce a finite $\alpha$-effect by generating net helicity in the small scales. The shear/ion-cyclotron normal mode of the Hall plasma is the dominant contributor to the dynamo action for short scale motions.Comment: 8 pages, 3 figures, ApJ (in press

Experimental study of the surface marine atmospheric boundary layer over Aegean Greece

Micrometeorological measurements within the surface Marine Atmospheric Boundary Layer (MABL) took place at the shoreline of island of Karpathos, at the South East end of the Aegean Sea (Karpathio Pelagos), during summer 2012. Α meteorological mast was installed at a distance less than 30m from the shoreline, instrumented with a sonic anemometer and a fast krypton hygrometer at 14.5m height with a sampling frequency of 10Hz, in order to measure the turbulent transport of mass and energy of the surface MABL. At the same mast slow response sensors (1Hz) were also installed providing vertical profiles of wind speed and direction, temperature and humidity at three levels (3, 8 and 12 m). Satellite sea surface temperature images were also obtained. The spacial and temporal scales of the Aegean Pelagos have not been studied in the past while it is expected to present differences compared with those measured over oceans or open seas. It is worth to mention that this study is a follow-up of a previous study which took place during summer 2011 at Skyros island, northern Aegean provided similar measurements but with different fetch. During Karpathos campaign, more than of 120 hours stationary surface MABL observations of near neutral wind flows were recorded. The vertical profiles of temperature reveal the presence of a coastal internal boundary layer, mostly below 8m height. Eddy correlation analysis is used to study the turbulent fluxes of mass and energy. According to first results on the stable MABL, momentum fluxes depend on stability and give increasing values with the wind speed. The estimated values are higher, almost by a factor of two, compared to typical values measured over the ocean. These increased values could be attributed to a developed sea state since wind stress is greater over young and developing wave fields. Sensible heat flux is found to be increased with stability and rather independent from wind speed while moisture flux seems to not be dependent neither to stability or wind speed. Some part of the records present upward heat fluxes under near neutral conditions and mostly for positive sea-air temperature differences, possibly associated with sea spray or mesoscale heterogeneity of the sea surface temperature. Ongoing investigation of the turbulent exchanges is expected to provide a more thorough insight on the coupling processes between the surface MABL and Aegean sea

Energy transfer in Hall-MHD turbulence: cascades, backscatter, and dynamo action

Scale interactions in Hall MHD are studied using both the mean field theory derivation of transport coefficients, and direct numerical simulations in three space dimensions. In the magnetically dominated regime, the eddy resistivity is found to be negative definite, leading to large scale instabilities. A direct cascade of the total energy is observed, although as the amplitude of the Hall effect is increased, backscatter of magnetic energy to large scales is found, a feature not present in MHD flows. The coupling between the magnetic and velocity fields is different than in the MHD case, and backscatter of energy from small scale magnetic fields to large scale flows is also observed. For the magnetic helicity, a strong quenching of its transfer is found. We also discuss non-helical magnetically forced Hall-MHD simulations where growth of a large scale magnetic field is observed.Comment: 25 pages, 16 figure

Analysis of the indoor air quality in Greek primary schools

The exposure of children to indoor air pollutants in school classrooms might cause them adverse health effects. In order to confront this issue, the in-depth study and evaluation of the indoor air quality in classrooms is necessary. The aims of this study are to characterize the environmental factors that affect indoor air quality. Several indoor air pollutants such as the concentrations of the particulate matter (PM) of several different size ranges, carbon dioxide (CO2), carbon monoxide (CO) and VOCs, were simultaneously measured in classrooms as well as the outdoor environment in nine primary schools in Athens, Greece during April 2013. Measurements were performed for more than 7 hours per day, for a period of one to five days in a classroom, per school. The first results indicate extreme PM10 concentrations in many cases with varying fluctuations throughout the day, mainly attributed to the presence of students, inadequate level of ventilation and chalk dust while the ultrafine particles (UFP) remained in rather low levels. In most of the cases the indoor concentrations exceeded the outdoor ones by more than ten times. Carbon dioxide concentrations in many cases exceeded the recommended limit value indicating inadequate levels of ventilation

Flux measurements in the surface marine Atmospheric Boundary Layer over the Aegean Sea, Greece

Micro-meteorological measurements within the surface Marine Atmospheric Boundary Layer took place at the shoreline of two islands at northern and south-eastern Aegean Sea of Greece. The primary goal of these experimental campaigns was to study the momentum, heat and humidity fluxes over this part of the north-eastern Mediterranean Sea, characterized by limited spatial and temporal scales which could affect these exchanges at the air–sea interface. The great majority of the obtained records from both sites gave higher values up to factor of two, compared with the estimations from the most widely used parametric formulas that came mostly from measurements over open seas and oceans. Friction velocity values from both campaigns varied within the same range and presented strong correlation with the wind speed at 10 m height while the calculated drag coefficient values at the same height for both sites were found to be constant in relation with the wind speed. Using eddy correlation analysis, the heat flux values were calculated (virtual heat fluxes varied from −60 to 40 W/m2) and it was found that they are affected by the limited spatial and temporal scales of the responding air–sea interactionmechanism. Similarly, the humidity fluxes appeared to be strongly influenced by the observed intense spatial heterogeneity of the sea surface temperature

A study on the thermal environment in Greek primary schools based on questionnaires and concurrent measurements

The present study investigates the indoor thermal comfort perceived by students through a questionnaire survey conducted during spring 2013 in naturally ventilated primary schools in Athens. Thermal environment parameters such as air temperature, relative humidity, air velocity and mean radiant temperature were simultaneously measured. Then, Fanger’s indices of Predicted Mean Vote (PMV) and Percentage of People Dissatisfied (PPD) were calculated by using clothing and metabolic rates. The main purpose of this work is the evaluation of the ability of the answers from students to be sufficient to assess the thermal environment of classrooms. The possible associations between subjective thermal sensation votes and objective measurements are examined by comparing students’ answers based on the seven point thermal sensation scale and the results taken by the calculated indices of PMV and PPD

Experimental study of the turbulent structure of the surface marine Atmospheric Boundary Layer over the Aegean Pelagos under etesian winds

In order to study the physical processes of the turbulent transportation of mass and energy within the surface Marine Atmospheric Boundary Layer over the Aegean Pelagos, field measurements were conducted on the island of Skyros, mainly under the etesian winds, during summer 2011. Α meteorological mast was installed close to the shoreline, instrumented with fast anemometer (sonic) and hydrometer measuring the three components of the wind, the virtual temperature and water vapor at 10m height with a sampling frequency of 20Hz. At the same mast slow response sensors were measuring wind speed and direction, temperature and humidity at three levels (2, 6 and 10 m). Weak stable to near neutral flows were recorded during the experimental period. The eddy correlation analysis re-vealed the momentum and heat fluxes values which are presented and discussed. The estimated values are related both with stability and wind speed variations

On the link between Indian summer monsoon and the Etesian pattern over the Aegean Sea

The Indian summer monsoon (ISM) is a key factor influencing the eastern Mediterranean climate. During early summer period, the thermal low over Pakistan and northwestern India extends towards the Mediterranean through inner Iraq and Turkey. On the other hand, the Azores high expands eastwards and meets the Balkan high pressure system, forming one common system. Greece lies at the transitional zone between these two pressure systems, where persistent northeasterly wind currents form especially over the Aegean, known as the Etesians. The objective of this study is to investigate the interrelation between the ISM and the Etesian winds over the Aegean Sea during summer for an extended period of 44 years (1958-2001) with the aid of composite and correlation analyses and ERA 40 datasets. More specifically, the study will focus on the ISM activity during and prior to or after ‘Etesian episodes’ in order to determine the extent to which one system may lead another. The present analysis reveals that during enhanced monsoon years two upper level ridges prevail over the greater area of interest, one over western Europe and northern Africa, which is part of the Azores high and a second one over Pakistan region, Persian Gulf and Middle Asia, which extends to the west and connecting to the first one forms a slight trough over Eastern Mediterranean. At lower levels, an intensification of the thermal Asian low as well as of the high pressure system prevailing over western and central Europe is apparent. Concurring with this pattern in upper and lower troposphere, anomalously strong northerlies appear over the Aegean during periods with enhanced monsoon activity, as horizontal surface wind anomalies depict. In agreement with previous studies, mid-level subsidence during excess monsoon rainfall periods is found to be more intense over the Eastern Mediterranean, Iraq, Saudi Arabia and east of the Caspian Sea