Hugh Potts

Balltracking: an highly efficient method for tracking flow fields

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.

Control to concentrate drug-coated magnetic particles to deep-tissue tumors for targeted cancer chemotherapy

The control problem for concentrating drug-coated magnetic nano-particles to deep-tissue tumors in a patient is developed and initial control ideas are presented In current implementations of magnetically-controlled drug targeting, stationary magnets (creating static magnetic fields) are held outside the patients body, these magnets attract the nano-particles to their corners, and hence focus them to tumors at or near the skin surface. Deep tissue tumors cannot be targeted in this way. Dynamic control of magnetic fields can allow drug targeting to tumors deep in the body, creating high drug concentrations at the tumor (effective treatment) with low concentrations in the rest of the body (eliminating life-threatening side-effects), and thereby dramatically improving chemotherapy treatment. Here we synthesize the control problem (accurately stating the problem definition is itself a serious challenge), show initial results for focusing magnetic particles to a target in air by feedback control of magnetic fields, and present a first-cut approach for doing the same inside the human blood vasculature network and the surrounding tissue. As such, we are presenting preliminary results in a long-term effort: the development of control algorithms and sensing/actuating technology to concentrate chemotherapy drugs to deep tissue tumors for dramatically improved treatment of cancer.

Elliptical dust growth in astrophysical plasmas

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.

Dynamics of freely-suspended drops

Ferrofluid drops are freely suspended for long periods in air in a novel experiment by using magnetic fields to create an attractive force opposing gravity. The suspended drop is then forced to oscillate by perturbing the supporting magnetic field. In addition to small-amplitude behaviour, the driven drop exhibits high order nonlinear modes of oscillation, and can be driven until it splits. A new simplified analytical expression for the general potential for the drop motion is derived, and numerical simulations of the drop behaviour are presented, showing close agreement with the experimental data.

The optical depth of white-light flare continuum

The white-light continuum emission of a solar flare remains a puzzle as regards its height of formation and its emission mechanism(s). This continuum and its extension into the near-UV contain the bulk of the energy radiated by a flare, and so its explanation is a high priority. We describe a method to determine the optical depth of the emitting layer and apply it to the well-studied flare of 2002 July 15, making use of MDI pseudo-continuum intensity images. We find the optical depth of the visible continuum in all flare images, including an impulsive ribbon to be small, consistent with the observation of Balmer and Paschen edges in other events.

The evolution of electron overdensities in magnetic fields

When a neutral gas impinges on a stationary magnetized plasma an enhancement in the ionization rate occurs when the neutrals exceed a threshold velocity. This is commonly known as the critical ionization velocity effect. This process has two distinct timescales: an ion–neutral collision time and electron acceleration time. We investigate the energization of an ensemble of electrons by their self-electric field in an applied magnetic field. The evolution of the electrons is simulated under different magnetic field and density conditions. It is found that electrons can be accelerated to speeds capable of electron impact ionization for certain conditions. In the magnetically dominated case the energy distribution of the excited electrons shows that typically 1% of the electron population can exceed the initial electrostatic potential associated with the unbalanced ensemble of electrons.

Horizontal supergranule-scale motions inferred from TRACE ultraviolet observations of the chromosphere

Aims. We study horizontal supergranule-scale motions revealed by TRACE observation of the chromospheric emission, and investigate the coupling between the chromosphere and the underlying photosphere. Methods. A highly efficient feature-tracking technique called balltracking has been applied for the first time to the image sequences obtained by TRACE (transition region and coronal explorer) in the passband of white light and the three ultraviolet passbands centered at 1700 A, 1600 A, and 1550 A. The resulting velocity fields have been spatially smoothed and temporally averaged in order to reveal horizontal supergranule-scale motions that may exist at the emission heights of these passbands. Results. We find indeed a high correlation between the horizontal velocities derived in the white-light and ultraviolet passbands. The horizontal velocities derived from the chromospheric and photospheric emission are comparable in magnitude. Conclusions. The horizontal motions derived in the UV passbands might indicate the existence of a supergranule-scale magnetoconvection in the chromosphere, which may shed new light on the study of mass and energy supply to the corona and solar wind at the height of the chromosphere. However, it is also possible that the apparent motions reflect the chromospheric brightness evolution as produced by acoustic shocks which might be modulated by the photospheric granular motions in their excitation process, or advected partly by the supergranule-scale flow towards the network while propagating upward from the photosphere. To reach a firm conclusion, it is necessary to investigate the role of granular motions in the excitation of shocks through numerical modeling, and future high-cadence chromospheric magnetograms must be scrutinized.

Reduction of interpolation errors when using local correlation tracking for motion detection

Local correlation tracking (LCT) is a commonly used motion tracking technique, particularly in solar physics. When used to track motions smaller than one pixel per time sample, interpolation of the original data is required. We demonstrate that it is possible to introduce large systematic errors by using an inappropriate interpolation method, and describe how to avoid these errors. The effect of these errors on the calculated velocity field is demonstrated on simulated solar granulation data.

Large-amplitude ferrofluid surface waves and jets

A non-mechanical technique for investigating fluid surface waves and jets is presented. The method uses a ferrofluid driven by electromagnets, eliminating complicated mechanical wave generating systems. The waves are imaged by a fast digital camera, and associated diagnostics. Experimental results, with accompanying modelling, are presented for cylindrical standing waves, with the maximum amplitude wave being investigated. Exceeding the maximum amplitude results in a dramatic jet, with a maximum acceleration exceeding 70g.

Post-hoc derivation of SOHO Michelson doppler imager flat fields

Context. The SOHO satellite now offers a unique perspective on the Sun as it is the only space-based instrument that can provide large, high-resolution data sets over an entire 11-year solar cycle. This unique property enables detailed studies of long-term variations in the Sun. One significant problem when looking for such changes is determining what component of any variation is due to deterioration of the instrument and what is due to the Sun itself. One of the key parameters that changes over time is the apparent sensitivity of individual pixels in the CCD array. This can change considerably as a result of optics damage, radiation damage, and aging of the sensor itself. In addition to reducing the sensitivity of the telescope over time, this damage significantly changes the uniformity of the flat field of the instrument, a property that is very hard to recalibrate in space. For procedures such as feature tracking and intensity analysis, this can cause significant errors. Aims. We present a method for deriving high-precision flat fields for high-resolution MDI continuum data, using analysis of existing continuum and magnetogram data sets. Methods. A flat field is constructed using a large set (1000−4000 frames) of cospatial magnetogram and continuum data. The magnetogram data is used to identify and mask out magnetically active regions on the continuum data, allowing systematic biases to be avoided. This flat field can then be used to correct individual continuum images from a similar time. Results. This method allows us to reduce the residual flat field error by around a factor 6−30, depending on the area considered, enough to significantly change the results from correlation-tracking analysis. One significant advantage of this method is that it can be done retrospectively using archived data, without requiring any special satellite operations.