No abstract

Context. Twisting motions of different sorts are observed in several layers of the solar atmosphere. Chromospheric sunspot whorls and rotation of sunspots or even higher up in the lower corona sigmoids are examples of the large-scale twisted topology of many solar features. Nevertheless, their occurrence on a large scale in the quiet photosphere has not been investigated yet.Aims. The present study reveals the existence of vortex flows located at the supergranular junctions of the quiet Sun. Methods. We used a 1-h and a 5-h time series of the granulation in blue continuum and G-band images from FG/SOT to derive the photospheric flows. A feature-tracking technique called balltracking was performed to track the granules and reveal the underlying flow fields. Results. In both time series, we identify long lasting vortex flow located at supergranular junctions. The first vortex flow lasts at least 1 h and is ∼20 wide (∼15.5 Mm). The second vortex flow lasts more than 2 h and is ∼27 wide (∼21 Mm).

Context. The atmosphere of the quiet Sun is controlled by photospheric flows sweeping up concentrations of mixed polarity magnetic field. Along supergranule boundaries and junctions, there is a strong correlation between magnetic flux and bright chromospheric and transition region emission. Aims. The aim is to investigate the relationship between photospheric flows and small flare-like brightenings seen in Extreme Ultraviolet images. Methods. We describe observations of small eruptions seen in quiet Sun images taken with the Extreme UltraViolet Imager (EUVI) on STEREO. The photospheric flows during the eruption build-up phase are investigated by tracking granules in high resolution MDI continuum images. Results. Eruptions with characteristics of small coronal mass ejections (CMEs) occur at the junctions of supergranular cells. The eruptions produce brightening at the onset site, dark cloud or small filament ejections, and faint waves moving with plane-of-sky speeds up to 100 km s −1 . In the two examples studied, they appear to be activated by converging and rotating supergranular flows, twisting small concentrations of opposite polarity magnetic field. An estimate of the occurrence rate is about 1400 events per day over the whole Sun. One third of these events seem to be associated with waves. Typically, the waves last for about 30 min and travel a distance of 80 Mm, so at any one time they cover 1/50th of the lower corona.

Abstract. We present a method for tracking solar photospheric flows that is highly efficient, and demonstrate it using high resolution MDI continuum images. The method involves making a surface from the photospheric granulation data, and allowing many small floating tracers or balls to be moved around by the evolving granulation pattern. The results are tested against synthesised granulation with known flow fields and compared to the results produced by Local Correlation tracking (LCT). The results from this new method have similar accuracy to those produced by LCT. We also investigate the maximum spatial and temporal resolution of the velocity field that it is possible to extract, based on the statistical properties of the granulation data. We conclude that both methods produce results that are close to the maximum resolution possible from granulation data. The code runs very significantly faster than our similarly optimised LCT code, making real time applications on large data sets possible. The tracking method is not limited to photospheric flows, and will also work on any velocity field where there are visible moving features of known scale length.

Context. Sun-grazing comets almost never re-emerge, but their sublimative destruction near the sun has only recently been observed directly, while chromospheric impacts have not yet been seen, nor impact theory developed. Aims. We seek simple analytic models of comet destruction processes near the sun, to enable estimation of observable signature dependence on original incident mass M o and perihelion distance q. Methods. Simple analytic solutions are found for M(r) versus q and distance r for insolation sublimation and, for the first time, for impact ablation and explosion. Results. Sun-grazers are found to fall into three (M o , q) regimes: sublimation-, ablation-, and explosion-dominated. Most sun-grazers have M o too small (<10 11 g) or q too large (>1.01 R ) to reach atmospheric densities (n > 2.5 × 10 11 /cm 3 ) where ablation exceeds sublimation. Our analytic results for sublimation are similar to numerical models. For q < 1.01 R , M o > 10 11 g, ablation initially dominates but results are sensitive to nucleus strength P c = 10 6 P 6 dyne/cm 2 and entry angle φ to the vertical. Nuclei with M o 10 10 (P 6 sec φ) 3 g are fully ablated before exploding, though the hot wake itself explodes. For most sun-impactors sec φ 1 (since q ∼ r * ), so for q very close to r * the ablation regime applies to moderate M o ∼ 10 13−16 P 3 6 g impactors unless P 6 0.1. For higher masses, or smaller q, nuclei reach densities n > 2.5 × 10 14 P 6 /cm 3 where ram pressure causes catastrophic explosion. Conclusions. Analytic descriptions define (M o , q) regimes where sublimation, ablation and explosion dominate sun-grazer/-impactor destruction. For q ≺ 1.01 R , M o 10 11 g nuclei are destroyed by ablation or explosion (depending on M o cos 3 φ/P c ) in the chromosphere, producing flare-like events with cometary abundance spectra. For all plausible M o , q and physical parameters, nuclei are destroyed above the photosphere.

We report the controlled injection of near-isolated micron-sized liquid droplets into a low temperature He-Ne steady-state rf plasma at atmospheric pressure. The H 2 O droplet stream is constrained within a 2 mm diameter quartz tube. Imaging at the tube exit indicates a log-normal droplet size distribution with an initial count mean diameter of 15 m falling to 13 m with plasma exposure. The radial velocity profile is approximately parabolic indicating near laminar flow conditions with the majority of droplets travelling at > 75% of the local gas speed and having a plasma transit time of < 100 s. The maximum gas temperature, determined from nitrogen spectral lines, was below 400 K and the observed droplet size reduction implies additional factors beyond standard evaporation, including charge and surface chemistry effects. The successful demonstration of controlled microdroplet streams opens up possibilities for gas-phase microreactors and remote delivery of active species for plasma medicine.

Elongated dust grains exist in astrophysical plasmas. Anisotropic growth of elliptical dust grains, via plasma deposition, occurs if the deposited ions are non-inertial. In reality the extent of such growth depends upon the initial kinetic energy of the ions and the magnitude of the electric field in the sheath. Simulations of the dynamics of the ions in the sheath are reported, showing how elliptical growth is related to the initial eccentricity and size of the seed relative to the sheath length. Consequences for the eventual fate of elliptical dust are then discussed.

A plasma with a volume of 10 −4 m 3 and a very low degree of ionization can be maintained inside a high-Q cavity using up to 1.4 kW of microwave power at 2.45 GHz. Various working gases have been employed, including air, nitrogen and noble gases at pressures up to 2 × 10 5 Pa. The reported results are for nitrogen at atmospheric pressure. The plasma absorbs about 80% of the incident power implying a mean root mean square electric field in the cavity for plasma maintenance of 50 kV m −1 , which is much smaller than that required for breakdown. Estimates of the mean heavy species temperature from the form of the band spectra at 315 and 340 nm give approximately 2200 K. The electron collision frequency and the ratio E/N are 4 × 10 11 s −1 and 1.5 × 10 −20 V m 2 , respectively. Since the electron collision rate is much higher than the microwave frequency, previous results for a dc electric field obtained by Engelhardt et al can be used to infer a characteristic electron energy of 1.1 eV, with a strongly non-Maxwellian distribution function, and an electron density of 7 × 10 16 m −3 . A substantial fraction of the input power is transferred from electrons to the heavy species by inelastic collisions, principally due to the excitation of molecular vibration.