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Solar Flares with an Exponential Growth of the Emission Measure in the Impulsive Phase Derived from X-ray Observations  

Siming Liu   Submitted: 2013-08-06 22:07

The light curves of solar flares in the impulsive phase are complex in general, indicating that multiple physical processes are involved in. With the GOES (Geostationary Operational Environmental Satellite) observations, we find that there are a subset of flares, whose impulsive phases are dominated by a period of exponential growth of the emission measure. The flares occurred from January 1999 to December 2002 are analyzed, and the results from the observations made with both GOES 8 and GEOS 10 satellites are compared to estimate the instrumental uncertainties. Their mean temperatures during this exponential growth phase have a normal distribution. Most flares within the 1σ range of this temperature distribution belong to the GOES class B or C, with the peak fluxes at the GOES low-energy channel following a log-normal distribution. The growth rate and duration of the exponential growth phase also follow a lognormal distribution, in which the duration is distributed in the range from half a minute to about half an hour. As expected, the growth time is correlated with the decay time of the soft X-ray flux. We also find that the growth rate of the emission measure is strongly anti-correlated with the duration of the exponential growth phase, and the mean temperature increases slightly with the increase of the growth rate. The implications of these results on the study of energy release in solar flares are discussed in the end.

Authors: HAN Fei-ran LIU Si-ming
Projects: GOES X-rays

Publication Status: Published in Chinese Astronomy and Astrophysics
Last Modified: 2013-08-07 07:48
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Impulsive Thermal X-ray Emission from a Low-lying Coronal Loop  

Siming Liu   Submitted: 2013-04-23 23:55

Understanding the relationship among different emission components plays an essential role in the study of particle acceleration and energy conversion in solar flares. In flares where gradual and impulsive emission components can be readily identified the impulsive emission has been attributed to non-thermal particles. We carry out detailed analysis of Hα and X-ray observations of a GOES class B microflare loop on the solar disk. The impulsive hard X-ray emission, however, is found to be consistent with a hot, quasi-thermal origin, and there is little evidence of emission from chromospheric footpoints, which challenges conventional models of flares and reveals a class of microflares associated with dense loops. Hα observations indicate that the loop lies very low in the solar corona or even in the chromosphere and both emission and absorption materials evolve during the flare. The enhanced Hα emission may very well originate from the photosphere when the low-lying flare loop heats up the underlying chromosphere and reduces the corresponding Hα opacity. These observations may be compared with detailed modeling of flare loops with the internal kink instability, where the mode remains confined in space without apparent change in the global field shape, to uncover the underlying physical processes and to probe the structure of solar atmosphere.

Authors: Siming Liu, Youping Li, and Lyndsay Fletcher
Projects: RHESSI

Publication Status: Accepted by ApJ
Last Modified: 2013-04-24 12:16
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Elementary Energy Release Events in Solar Flares  

Siming Liu   Submitted: 2009-07-10 11:34

Most theoretical investigations of particle acceleration during solar flares cannot be applied to observations for detailed study of the time evolution. We propose a phenomenological model for turbulence evolution and stochastic particle acceleration that links observations to the energy release and particle acceleration through two coefficients characterizing particle interactions with turbulent electromagnetic fields. In the linear regime the particle distribution does not affect the turbulence energy cascade. It is shown that electron acceleration critically depends on the intensity of small-scale turbulence and an impulsive non-thermal component only appears near the peak of the gradually evolving turbulence intensity. The model naturally reproduces the soft-hard-soft pattern of hard X-ray pulses, and we attribute the observed change in flux and spectral index correlation from the rise to decay phase of some pulses to changes in the background plasma. Detailed modeling of well-observed individual events will probe the energy release processes.

Authors: Siming Liu & Lyndsay Fletcher
Projects: RHESSI

Publication Status: Accept by ApJL
Last Modified: 2009-07-10 12:32
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Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence  

Siming Liu   Submitted: 2009-04-18 04:55

It is well recognized that the presence of magnetic fields will lead to anisotropic energy cascade and dissipation of astrophysical turbulence. With the diffusion approximation and linear dissipation rates, we study the cascade and damping of Alfvén-cyclotron fluctuations in solar plasmas numerically for two diagonal diffusion tensors, one (isotropic) with identical components for the parallel and perpendicular directions (with respect to the magnetic field) and one with different components (non-isotropic). It is found that for the isotropic case the steady-state turbulence spectra are nearly isotropic in the inertial range and can be fitted by a single power-law function with a spectral index of -3/2, similar to the Iroshnikov-Kraichnan phenomenology, while for the non-isotropic case the spectra vary greatly with the direction of propagation. The energy fluxes in both cases are much higher in the perpendicular direction than in the parallel direction due to the angular dependence (or inhomogeneity) of the components. In addition, beyond the MHD regime the kinetic effects make the spectrum softer at higher wavenumbers. In the dissipation range the turbulence spectrum cuts off at the wavenumber, where the damping rate becomes comparable to the cascade rate, and the cutoff wavenumber changes with the wave propagation direction. The angle-averaged turbulence spectrum of the isotropic model resembles a broken power law, which cuts off at the maximum of the cutoff wavenumbers or the 4He cyclotron frequency. Taking into account the Doppler effects, the model naturally reproduces the broken power-law turbulence spectra observed in the solar wind and predicts that a higher break frequency always comes along with a softer dissipation range spectrum that may be caused by the increase of the turbulence intensity, the reciprocal of the plasma beta, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by detailed comparisons with more accurate observations.

Authors: Yanwei Jiang, Siming Liu, and Vahé Petrosian
Projects: RHESSI

Publication Status: Accepted by ApJ
Last Modified: 2009-04-18 09:44
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Stochastic Acceleration of 3He and 4He in Solar Flares by Parallel Propagating Plasma Waves: General Results  

Siming Liu   Submitted: 2005-09-17 06:17

We study the acceleration in solar flares of 3He and 4He from a thermal background by parallel propagating plasma waves with a general broken power-law spectrum. The exact dispersion relation for a cold plasma is used to describe the relevant wave modes, and the Coulomb collision loss and escape processes are included. Under the quasi-linear approximation the pitch angle averaged acceleration time of α -particles is at least one order of magnitude longer than that of 3He ions at low energies and starts to approach that of 3He beyond a few tens of keV nucleon-1. Because their loss and escape times are comparable, the acceleration of 4He is suppressed significantly at low energies, and the spectrum of the accelerated α -particles is always softer than that of 3He. Quantitative results depend primarily on the wave generation and damping length scales, on the electron plasma to gyrofrequency ratio, and on the intensity of turbulence. The model gives reasonable account of the observed low-energy 3He and 4He fluxes and spectra in the impulsive solar energetic particle events observed with the {it Advanced Composition Explorer}. Other acceleration processes and/or stochastic acceleration by other wave modes seem to be required to explain the occasionally observed decrease of 3He to 4He ratio at energies beyond a few MeV nucleon-1.

Authors: Siming Liu, Vahé Petrosian, and Glenn M. Mason
Projects: None

Publication Status: ApJ Jan 2006, in press
Last Modified: 2005-09-17 06:17
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Stochastic Acceleration of 3He and 4He by Parallel Propagating Plasma Waves  

Siming Liu   Submitted: 2004-08-13 14:06

Stochastic acceleration of 3He and 4He from a thermal background by parallel propagating turbulent plasma waves with an assumed power-law spectrum of the wavenumber is studied. In the model the acceleration rate is reduced considerably when the interaction is dominated by one of the several resonant wave modes. At low energies, this happens for 4He, but not for 3He. As a result, the acceleration of 4He is suppressed by a barrier below sim 100 keV nucleon-1, and there is a prominent quasi-thermal component in the 4He spectra, while almost all 3He ions are accelerated to high energies. The model accounts for the large enrichment of 3He observed in impulsive solar energetic particle events and fits the spectra of both ions with reasonable plasma parameters. This study also indicates that the acceleration, Coulomb losses and diffusive escape of particles from the acceleration site {it all} play important roles in shaping the ion spectra, which may explain the varied spectral shapes observed recently by the {it Advanced Composition Explorer}. However, for further quantitative analysis and to apply the model to the acceleration of other ions, the generation and evolution of the turbulence need to be addressed.

Authors: Siming Liu, Vahé Petrosian, and Glenn M. Mason
Projects: None

Publication Status: In press in ApJL
Last Modified: 2004-08-13 14:06
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Stochastic Acceleration of Electrons and Protons. I. Acceleration by Parallel Propagating Waves  

Siming Liu   Submitted: 2004-01-27 16:55

Stochastic acceleration of electrons and protons by waves propagating parallel to the large scale magnetic field of magnetized plasmas are studied with emphasis on the feasibility of accelerating particles from a thermal background to relativistic energies and with the aim of determining the relative acceleration of the two species in one source. A general result of this investigation is that stochastic acceleration by these waves results in a distinct quasi-thermal and a hard nonthermal component with the second component being more prominent in hotter plasmas and/or higher turbulence levels. This can explain many of the observed features of solar flares. In regards to proton and electron ratio, we find that in a pure hydrogen plasma, the dominance of the wave-particle interactions by the resonant Alfvén wave reduces the acceleration rate of protons in the intermediate energy range significantly. This is not true for electrons because electron-cyclotron and whistler waves are very efficient in accelerating them from a few keV to relativistic energy. The presence of such an acceleration barrier makes the proton acceleration very inefficient compared with the electron acceleration in solar flares. This difficulty is alleviated when we include the effects of 4He in the dispersion relation and the damping of the turbulent waves by the thermal background plasma. The additional 4He cyclotron branch of the turbulent plasma waves suppresses the proton acceleration barrier significantly and the model gives qualitative explanation for many features associated with solar flare observations. Specifically, we find that the relative acceleration of protons and electrons is very sensitive to a plasma parameter α omega m pe/Omega m e, where omega m pe and Omega m e are the electron plasma and gyrofrequencies, respectively. Protons are preferentially accelerated in weakly magnetized plasmas (large α ). The formalism developed here is applicable to the acceleration of other ion species and to other astrophysical systems.

Authors: Vah'{e} Petrosian and Siming Liu
Projects: RHESSI

Publication Status: Submitted to ApJ
Last Modified: 2004-01-27 16:55
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Abstracts by Author
Solar Flares with an Exponential Growth of the Emission Measure in the Impulsive Phase Derived from X-ray Observations
Impulsive Thermal X-ray Emission from a Low-lying Coronal Loop
Elementary Energy Release Events in Solar Flares
Cascade and Damping of Alfven-Cyclotron Fluctuations: Application to Solar Wind Turbulence
Stochastic Acceleration of 3He and 4He in Solar Flares by Parallel Propagating Plasma Waves: General Results
Stochastic Acceleration of $^3$He and $^4$He by Parallel Propagating Plasma Waves
Stochastic Acceleration of Electrons and Protons. I. Acceleration by Parallel Propagating Waves

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