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Energy dependence of electron trapping in a solar flare  

David Alexander   Submitted: 2002-09-20 16:34

Observations of an energy-dependent asymmetry in footpoint hard X-ray emission by RHESSI for the M4.0 solar flare of 2002 March 17 allows us to probe the dynamics of particle transport with energy and time. The presence of such an asymmetry is most readily explained by the effects of a converging magnetic field with different rates of convergence at the different footpoints, as would be expected from realistic surface field distributions. Such a geometry has been discussed in the context of a trap-plus-precipitation model where the transport of energetic particles in the flare is governed by the precipitation out of the coronal trap via collisions, wave-particle interactions or some other scattering process, into the high density chromosphere. Comparison of RHESSI observations with a trap-plus-precipitation model allows us to use the energy-dependence of the asymmetry and the observed ratio of footpoint to coronal emission at the different energies to assess the role of the trapping in the transport of energetic electrons and to probe the nature of the particle precipitation process inside the loss-cone.

Authors: David Alexander and Thomas R. Metcalf

Publication Status: accepted
Last Modified: 2002-09-20 16:34
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CME acceleration in the low solar corona  

David Alexander   Submitted: 2001-07-03 14:39

We report on a unique observation of the early and rapid acceleration of a fast CME in the low solar corona. The coronal disturbance associated with a LASCO CME and concurrent X1.2 flare on 1998 April 23 was well-observed by the Yohkoh Soft X-ray Telescope. The X-ray observations clearly show an accelerating structure structure, reaching 750-900 km s-1 in ~500s at a height of only 1.3 solar radii, indicating an acceleration well in excess of 1600 m s-2. The soft X-ray observations are coincident with the onset of the CME, which ultimately attained a velocity of 1390 km s-1 and generated a number of interplanetary radio signatures, frequently associated with fast events. Assuming a constant acceleration, a0, throughout the duration of the X-ray observations, we determine a best-fit value of a0 = 1650 m s-2, while application of a fluxrope model yields a variable acceleration which reaches a peak of ~2560 m s-2 within the first 210 Mm of the solar corona.

Authors: D. Alexander, T. R. Metcalf and N. V. Nitta

Publication Status: Geophys. Res. Lett. submitted
Last Modified: 2001-07-03 14:39
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Evidence for the flare trigger site and 3-D reconnection in multi-wavelength observations of a solar flare  

David Alexander   Submitted: 2001-02-06 09:39

Based on a multi-wavelength data set and a topological model for the magnetic field, we argue that a M1.9 flare which occurred on 3-May-99 shows evidence of 3-d coronal reconnection in a spine-fan configuration. Images from the Transition Region and Coronal Explorer allow the detailed examination of the structures involved in the flare and pre-flare in the 171A (1MK) EUV passband, and the Lyman α (10,000-20,000K) passband. Yohkoh Hard X-ray Telescope maps the position of non-thermal electron precipitation and the Soft X-ray Telescope reveals pre-flare and flare heating on large and small-scales. While the flare appears to be driven by changes in small scale field close to the photosphere, near the interface between strong opposite magnetic polarities, the result is the disruption of large-scale field. We demonstrate how this observed activity on large and small scales, along with many other aspects of the flare, find a qualitative explanation in the three-dimensional reconfiguration of coronal magnetic field, following a small-scale flux cancelation at the photosphere.

Authors: L. Fletcher, T. R. Metcalf, D. Alexander, L. A. Ryder and D. S. Brown

Publication Status: ApJ, 554, 451, 2001
Last Modified: 2001-07-10 17:38
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Chromospheric Heating in the Late Phase of Two-ribbon Flares  

David Alexander   Submitted: 2001-02-05 14:00

Fast upflows observed in the late gradual phase of an M6.8 two-ribbon flare by SOHO/CDS have provided evidence for the presence of chromospheric evaporation more than an hour after the impulsive phase of the flare. The chromospheric heating necessary to generate these upflows requires the continued injection and deposition of energy, which we presume to be provided by magnetic reconnection in the flaring corona. We investigate the nature of the transport of this energy from the reconnection site to the chromosphere by comparing the observed upflow velocities with those expected from different chromospheric heating models. A non-thermal beam of energetic electrons (>15 keV) that is capable of generating the observed velocities would also generate significant hard X-ray emission which is not observed at this stage of the flare. We conclude, therefore, that the most likely energy transport mechanism is thermal conduction.

Authors: A. Czaykowska, D. Alexander and B. De Pontieu

Publication Status: ApJ, Accepted
Last Modified: 2001-02-05 14:04
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Heating the atmosphere above sunspots  

David Alexander   Submitted: 1999-11-04 19:03

We present our results of a hybrid model of sunspots and their overlying corona. The two-layer model considers both the nonlinear, compressible magnetoconvection beneath the photosphere and potential, or linear force-free, models of the coronal fields. Heating of the plasma along the field lines is then treated using quasi-static and steady-state models with the heating rate being specified by the dynamics of the magnetoconvection. Two distinct magnetoconvection scenarios are considered. The first describes magnetoconvection in a 2D axisymmetric geometry and considers the time development of the overlying coronal field. The second describes a 3D cylindrical geometry with a static coronal field configuration. Both scenarios diverge from the standard practice of assuming constant temperature and vertical magnetic field conditions at the top surface. Instead, a radiative linear force-free field condition is adopted. Extrapolation of the top surface boundary conditions results in a coronal field configuration which is assumed to be filled with plasma heated to coronal temperatures. The heating rate and thermodynamic behaviour of the plasma is related to the sub-surface model by assuming that individual fluxtubes are heated uniformly with the necessary energy being generated from the dissipation of the Poynting flux entering the coronal volume. Radiation and conductive losses are included. The combination of a sunspot model, whereby the surface field is completely specified, with a coronal heating model, in which the plasma parameters are specified for a given energy input, allows us to explore a broad class of heating paradigms.

Authors: Alexander, D., Hurlburt, N. E. and Rucklidge, A. M.

Publication Status: Proceedings of SOHO 8 (in press)
Last Modified: 1999-11-04 19:03
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High-resolution Observations of Plasma Jets in the Solar Corona  

David Alexander   Submitted: 1999-11-04 19:02

We present recent observations of coronal jets, made by TRACE and {it Yohkoh}/SXT on 1998 May 28 and 1998 August 19. The high spatial resolution of TRACE enables us to see in detail the process of material ejection; in the line of Iron IX (one million degrees) we see both bright emitting material and dark absorbing/scattering material being ejected, i.e. both hot and cold material, highly collimated and apparently ejected along the direction of the overlying field lines. Bright ejecta are seen simultaneously in Lyman α for one event and Yohkoh/SXT in the other. The jets on the two days are different in that the August 19 jet displays the morphology typical of a one-sided anemone jet while the May 28 jet exhibits a two-sided jet morphology. The August 19 jet shows evidence for rotation and an interesting bifurcation at large distances from the energy release site. We study the physical properties and energetics of these jetting events, and conclude that existing theoretical models capture the essential physics of the jet phenomena.

Authors: Alexander, D. and Fletcher, L.

Publication Status: Solar Physics (accepted)
Last Modified: 1999-11-04 19:02
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Abstracts by Author
Energy dependence of electron trapping in a solar flare
CME acceleration in the low solar corona
Evidence for the flare trigger site and 3-D reconnection in multi-wavelength observations of a solar flare
Chromospheric Heating in the Late Phase of Two-ribbon Flares
Heating the atmosphere above sunspots
High-resolution Observations of Plasma Jets in the Solar Corona

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