Secondary charging effects due to icy dust particle impacts on rocket payloads

We report measurements of dust currents obtained with a small probe and a larger probe during the flight of the ECOMA-4 rocket through the summer polar mesosphere. The payload included two small dust probes behind a larger dust probe located centrally at the front. For certain phases of the payload rotation, the current registered by one of the small dust probes was up to 2 times the current measured with the larger probe, even though the effective collection area of the larger probe was 4 times that of the small one. We analyze the phase dependence of the currents and their difference with a model based on the assumption that the small probe was hit by charged dust fragments produced in collisions of mesospheric dust with the payload body. Our results confirm earlier findings that secondary charge production in the collision of a noctilucent cloud/Polar Summer Mesospheric Echo (NLC/PMSE) dust particle with the payload body must be several orders of magnitude larger than might be expected from laboratory studies of collisions of pure ice particles with a variety of clean surfaces. An important consequence is that for some payload configurations, one should not assume that the current measured with a detector used to study mesospheric dust is simply proportional to the number density of ambient dust particles. The higher secondary charge production may be due to the NLC/PMSE particles containing multiple meteoric smoke particles

Multi-scale measurements of mesospheric aerosols and electrons during the MAXIDUSTY campaign

We present in situ measurements of small-scale fluctuations in aerosol populations as recorded through a mesospheric cloud system from the Faraday cups DUSTY and MUDD during on the MAXIDUSTY-1 and 1B sounding rocket payloads launched in the summer of 2016. Two mechanically identical DUSTY probes mounted with an inter-spacing of ∼10&thinsp;cm recorded very different currents, with strong spin modulation, in certain regions of the cloud system. A comparison to auxiliary measurement show similar tendencies in the MUDD data. Fluctuations in the electron density are found to be generally anti-correlated to the negative aerosol charge density on all length scales; however, in certain smaller regions the correlation turns positive. We have also compared the spectral properties of the dust fluctuations, as extracted by wavelet analysis, to polar mesospheric summer echo (PMSE) strength. In this analysis, we find a relatively good agreement between the power spectral density (PSD) at the radar Bragg scale inside the cloud system; however the PMSE edge is not well represented by the PSD. A comparison of proxies for PMSE strength, constructed from a combination of derived dusty plasma parameters, shows that no simple proxy can reproduce PMSE strength well throughout the cloud system. Edge effects are especially poorly represented by the proxies addressed here.</p

Gravity compensation in complex plasmas by application of a temperature gradient

Micron sized particles are suspended or even lifted up in a gas by thermophoresis. This allows the study of many processes occurring in strongly coupled complex plasmas at the kinetic level in a relatively stress-free environment. First results are presented. The technique is also of interest for technological applications.Comment: 4 pages, 4 figures, final version to be published in Phys. Rev. Let

Dusty Magnetohydrodynamics in Star Forming Regions

Star formation occurs in dark molecular regions where the number density of hydrogen nuclei, nH, exceeds 10^4 cc and the fractional ionization is 10^-7 or less. Dust grains with sizes ranging up to tenths of microns and perhaps down to tens of nanometers contain just under one percent of the mass. Recombination on grains is important for the removal of gas phase ions, which are produced by cosmic rays penetrating the dark regions. Collisions of neutrals with charged grains contribute significantly to the coupling of the magnetic field to the neutral gas. Consequently, the dynamics of the grains must be included in the magnetohydrodynamic models of large scale collapse, the evolution of waves and the structures of shocks important in star formation.Comment: 17 pages, 3 figures, accepted for publication in the Journal of Plasma Physics special issue in honour of Prof. Padma K. Shukl

A comparison of overshoot modelling with observations of polar mesospheric summer echoes at radar frequencies of 56 and 224 MHz

We have compared radar observations of polar mesospheric summer echoes (PMSEs) modulated by artificial electron heating, at frequencies of 224 MHz (EISCAT VHF) and 56 MHz (MORRO). We have concentrated on 1 day of observation, lasting ∼ 3.8 h. The MORRO radar, with its much wider beam, observes one or more PMSE layers all the time while the VHF radar observes PMSEs in 69% of the time. Statistically there is a clear difference between how the MORRO and the VHF radar backscatter reacts to the heater cycling (48 s heater on and 168 s heater off). While MORRO often reacts by having its backscatter level increased when the heater is switched on, as predicted by Scales and Chen (2008), the VHF radar nearly always sees the "normal" VHF overshoot behaviour with an initial rapid reduction of backscatter. However, in some heater cycles we do see a substantial recovery of the VHF backscatter after its initial reduction to levels several times above that just before the heater was switched on. For the MORRO radar a recovery during the heater-on phase is much more common. The reaction when the heater was switched off was a clear overshoot for nearly all VHF cases but less so for MORRO. A comparison of individual curves for the backscatter values as a function of time shows, at least for this particular day, that in high layers above ∼ 85 km height, both radars see a reduction of the backscatter as the heater is switched on, with little recovery during the heater-on time. These variations are well described by present models. On the other hand, the backscatter in low layers at 81-82 km can be quite different, with modest or no reduction in backscatter as the heater is switched on, followed by a strong recovery for both radars to levels several times above that of the undisturbed PMSEs. This simultaneous, nearly identical behaviour at the two very different radar frequencies is not well described by present modelling

Making judgements about students making work : lecturers’ assessment practices in art and design.

This research study explores the assessment practices in two higher education art and design departments. The key aim of this research was to explore art and design studio assessment practices as lived by and experienced by art and design lecturers. This work draws on two bodies of pre existing research. Firstly this study adopted innovative methodological approaches that have been employed to good effect to explore assessment in text based subjects (think aloud) and moderation mark agreement (observation). Secondly the study builds on existing research into the assessment of creative practice. By applying thinking aloud methodologies into a creative practice assessment context the authors seek to illuminate the ‘in practice’ rather than espoused assessment approaches adopted. The analysis suggests that lecturers in the study employed three macro conceptions of quality to support the judgement process. These were; the demonstration of significant learning over time, the demonstration of effective studentship and the presentation of meaningful art/design work

First modulation of high-frequency polar mesospheric summer echoes by radio heating of the ionosphere

The first high-frequency (HF, 8 MHz) observations of the modulation of polar mesospheric summer echoes (PMSE) by artificial radio heating of the ionosphere are presented and compared to observations at 224 MHz and model predictions. The experiments were performed at the European Incoherent Scatter facility in northern Norway. It is shown that model results are in qualitative and partial quantitative agreement with the observations, supporting the prediction that with certain ranges of ice particle radii and concentration, PMSE at HF radar wavelengths can be enhanced by heating due to the dominance of dust charging over plasma diffusion

A Magnetically Torqued Disk Model for Be Stars

Despite extensive study, the mechanisms by which Be star disks acquire high densities and angular momentum while displaying variability on many time scales are still far from clear. In this paper, we discuss how magnetic torquing may help explain disk formation with the observed quasi-Keplerian (as opposed to expanding) velocity structure and their variability. We focus on the effects of the rapid rotation of Be stars, considering the regime where centrifugal forces provide the dominant radial support of the disk material. Using a kinematic description of the angular velocity, vphi(r), in the disk and a parametric model of an aligned field with a strength B(r) we develop analytic expressions for the disk properties that allow us to estimate the stellar surface field strength necessary to create such a disk for a range of stars on the main-sequence. The model explains why disks are most common for main-sequence stars at about spectral class B2 V. The earlier type stars with very fast and high density winds would require unacceptably strong surface fields (> 10^3 Gauss) to form torqued disks, while the late B stars (with their low mass loss rates) tend to form disks that produce only small fluxes in the dominant Be diagnostics. For stars at B2 V the average surface field required is about 300 Gauss. The predicted disks provide an intrinsic polarization and a flux at Halpha comparable to observations. We also discuss whether the effect on field containment of the time dependent accumulation of matter in the flux tubes/disk can help explain some of the observed variability of Be star disks.Comment: ApJ, in press. 46 pages, 12 figure

A new method of inferring the size, number density, and charge of mesospheric dust from its in situ collection by the DUSTY probe

The linguocultural image of hand in Polish folk languageThe goal of this paper is a reconstruction of the linguocultural image of ręka (hand) in the Polish folk language, as defined by a cognitive model proposed by Bartmiński in the 1980’s. Among the sources considered here are the linguistic system itself (as exemplified by dictionaries of the Polish language, etymological dictionaries, dialect dictionaries) as well as literary texts, including folk songs and prose (e.g. riddles, sayings, fairy tales), and folk traditions and beliefs, recorded in the FOLBAS computer base and in the UMCS Ethnolinguistic Archives. From all these sources there emerges a rich and complex image showing this part of the body mainly from a functional perspective and appreciation of its role in Polish folk culture

A new method of inferring the size, number density, and charge of mesospheric dust from its in situ collection by the DUSTY probe

We present a new method of analyzing measurements of mesospheric dust made with DUSTY rocket-borne Faraday cup probes. It can yield the variation in fundamental dust parameters through a mesospheric cloud with an altitude resolution down to 10&thinsp;cm or less if plasma probes give the plasma density variations with similar height resolution. A DUSTY probe was the first probe that unambiguously detected charged dust and aerosol particles in the Earth's mesosphere. DUSTY excluded the ambient plasma by various biased grids, which however allowed dust particles with radii above a few nanometers to enter, and it measured the flux of charged dust particles. The flux measurements directly yielded the total ambient dust charge density. We extend the analysis of DUSTY data by using the impact currents on its main grid and the bottom plate as before, together with a dust charging model and a secondary charge production model, to allow the determination of fundamental parameters, such as dust radius, charge number, and total dust density. We demonstrate the utility of the new analysis technique by considering observations made with the DUSTY probes during the MAXIDUSTY rocket campaign in June–July 2016 and comparing the results with those of other instruments (lidar and photometer) also used in the campaign. In the present version we have used monodisperse dust size distributions.</p