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High-temperature differential emission measure and altitude variations in the temperature and density of solar flare coronal X-ray sources  

Natasha Jeffrey   Submitted: 2015-10-29 03:10

The detailed knowledge of plasma heating and acceleration region properties presents a major observational challenge in solar flare physics. Using the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the high temperature differential emission measure, DEM(T), and the energy-dependent spatial structure of solar flare coronal sources are studied quantitatively. The altitude of the coronal X-ray source is observed to increase with energy by ~+0.2 arcsec/keV between 10 and 25 keV. Although an isothermal model can fit the thermal X-ray spectrum observed by RHESSI, such a model cannot account for the changes in altitude, and multi-thermal coronal sources are required where the temperature increases with altitude. For the first time, we show how RHESSI imaging information can be used to constrain the DEM(T) of a flaring plasma. We develop a thermal bremsstrahlung X-ray emission model with inhomogeneous temperature and density distributions to simultaneously reproduce: i) DEM(T), ii) altitude as a function of energy, and iii) vertical extent of the flaring coronal source versus energy. We find that the temperature-altitude gradient in the region is ~+0.08 keV/arcsec (~1.3 MK/Mm). Similar altitude-energy trends in other flares suggest that the majority of coronal X-ray sources are multi-thermal and have strong vertical temperature and density gradients with a broad DEM(T).

Authors: Natasha Jeffrey, Eduard Kontar & Brian Dennis
Projects: RHESSI

Publication Status: A & A (accepted)
Last Modified: 2015-11-02 10:52
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On the variation of solar flare coronal X-ray source sizes with energy  

Natasha Jeffrey   Submitted: 2014-04-14 04:54

Observations with RHESSI have enabled the detailed study of the structure of dense hard X-ray coronal sources in solar flares. The variation of source extent with electron energy has been discussed in the context of streaming of non-thermal particles in a one-dimensional cold-target model, and the results used to constrain both the physical extent of, and density within, the electron acceleration region. Here we extend this investigation to a more physically realistic model of electron transport that takes into account the finite temperature of the ambient plasma, the initial pitch-angle distribution of the accelerated electrons, and the effects of collisional pitch-angle scattering. The finite temperature results in the thermal diffusion of electrons, that leads to the observationally-inferred value of the acceleration region volume being an overestimate of its true value. The different directions of the electron trajectories, a consequence of both the non-zero injection pitch-angle and scattering within the target, cause the projected propagation distance parallel to the guiding magnetic field to be reduced, so that a one-dimensional interpretation can overestimate the actual density by a factor of up to ~6. The implications of these results for the determination of acceleration region properties (specific acceleration rate, filling factor, etc.) are discussed.

Authors: Jeffrey, Natasha L. S., Kontar, Eduard P., Bian, Nicolas H., Emslie, A. Gordon
Projects: RHESSI

Publication Status: Accepted for publication in the Astrophysical Journal
Last Modified: 2014-04-14 12:50
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Temporal variations of X-ray solar flare loops: length, corpulence, position, temperature, plasma pressure and spectra  

Natasha Jeffrey   Submitted: 2013-02-14 02:17

The spatial and spectral properties of three solar flare coronal X-ray loops are studied before, during and after the peak X-ray emission. Using observations from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we deduce the temporal changes in emitting X-ray length, corpulence, volume, position, number density and thermal pressure. We observe a decrease in the loop length, width and volume before the X-ray peak, and an increasing number density and thermal pressure. After the X-ray peak, volume increases and loop corpulence grows due to an increasing width. The volume variations are more pronounced than the position variations, often known as magnetic line contraction. We believe this is the first dedicated study of the temporal evolution of X-ray loop lengths and widths. Collectively, the observations also show for the first time three temporal phases given by peaks in temperature, X-ray emission and thermal pressure, with minimum volume coinciding with the X-ray peak. Although the volume of the flaring plasma decreases before the peak in X-ray emission, the relationship between temperature and volume does not support simple compressive heating in a collapsing magnetic trap model. Within a low beta plasma, shrinking loop widths perpendicular to the guiding field can be explained by squeezing the magnetic field threading the region. Plasma heating leads to chromospheric evaporation and growing number density, producing increasing thermal pressure and decreasing loop lengths as electrons interact at shorter distances and we believe after the X-ray peak, the increasing loop corpulence.

Authors: Jeffrey, N. L. S. , Kontar, E. P.
Projects: RHESSI

Publication Status: ApJ (published)
Last Modified: 2013-03-12 16:36
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Spatially resolved hard X-ray polarization in solar flares: effects of Compton scattering and bremsstrahlung  

Natasha Jeffrey   Submitted: 2011-10-26 13:07


Aims: We study the polarization of hard X-ray (HXR) sources in the solar atmosphere, including Compton backscattering of photons in the photosphere (the albedo effect) and the spatial distribution of polarization across the source.
Methods: HXR photon polarization and spectra produced via electron-ion bremsstrahlung emission are calculated from various electron distributions typical for solar flares. Compton scattering and photoelectric absorption are then modelled using Monte Carlo simulations of photon transport in the photosphere to study the observed (primary and albedo) sources. Polarization maps across HXR sources (primary and albedo components) for each of the modelled electron distributions are calculated at various source locations from the solar centre to the limb.
Results: We show that Compton scattering produces a distinct polarization variation across the albedo patch at peak albedo energies of 20-50 keV for all anisotropies modelled. The results show that there are distinct spatial polarization changes in both the radial and perpendicular to radial directions across the extent of the HXR source at a given disk location. In the radial direction, the polarization magnitude and direction at specific positions along the HXR source will either increase or decrease with increased photon distribution directivity towards the photosphere. We also show how high electron cutoff energies influence the direction of polarization at above ~100 keV.
Conclusions: Spatially resolved HXR polarization measurements can provide important information about the directivity and energetics of the electron distribution. Our results indicate the preferred angular resolution of polarization measurements required to distinguish between the scattered and primary components. We also show how spatially resolved polarization measurements could be used to probe the emission pattern of an HXR source, using both the magnitude and the direction of the polarization.

Authors: Jeffrey, N. L. S., Kontar, E. P.
Projects: None

Publication Status: A&A (submitted)
Last Modified: 2011-10-27 10:07
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Abstracts by Author
High-temperature differential emission measure and altitude variations in the temperature and density of solar flare coronal X-ray sources
On the variation of solar flare coronal X-ray source sizes with energy
Temporal variations of X-ray solar flare loops: length, corpulence, position, temperature, plasma pressure and spectra
Spatially resolved hard X-ray polarization in solar flares: effects of Compton scattering and bremsstrahlung

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

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