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تسجيل مستخدم جديدCME Acceleration as a Probe of the Coronal Magnetic Field
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By 2050, we expect that CME models will accurately describe, and ideally predict, observed solar eruptions and the propagation of the CMEs through the corona. We describe some of the present known unknowns in observations and models that would need to be addressed in order to reach this goal. We also describe how we might prepare for some of the unknown unknowns that will surely become challenges.
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We determine the coronal magnetic field strength in the heliocentric distance range 6 to 23 solar radii (Rs) by measuring the shock standoff distance and the radius of curvature of the flux rope during the 2008 March 25 coronal mass ejection (CME) im
aged by white-light coronagraphs. Assuming the adiabatic index, we determine the Alfven Mach number, and hence the Alfven speed in the ambient medium using the measured shock speed. By measuring the upstream plasma density using polarization brightness images, we finally get the magnetic field strength upstream of the shock. The estimated magnetic field decreases from ~48 mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field can be described by a power law in agreement with other estimates at similar heliocentric distances.
We use high time cadence images acquired by the STEREO EUVI and COR instruments to study the evolution of coronal mass ejections (CMEs), from their initiation, through the impulsive acceleration to the propagation phase. For a set of 95 CMEs we deriv
ed detailed height, velocity and acceleration profiles and statistically analysed characteristic CME parameters: peak acceleration, peak velocity, acceleration duration, initiation height, height at peak velocity, height at peak acceleration and size of the CME source region. The CME peak accelerations derived range from 20 to 6800 m s^2 and are inversely correlated to the acceleration duration and to the height at peak acceleration. 74% of the events reach their peak acceleration at heights below 0.5 Rsun. CMEs which originate from compact sources low in the corona are more impulsive and reach higher peak accelerations at smaller heights. These findings can be explained by the Lorentz force, which drives the CME accelerations and decreases with height and CME size.
With a test-particle simulation, we investigate the effect of large-scale coronal magnetic fields on electron acceleration at an outward-propagating coronal shock with a circular front. The coronal field is approximated by an analytical solution with
a streamer-like magnetic field featured by partially open magnetic field and a current sheet at the equator atop the closed region. We show that the large-scale shock-field configuration, especially the relative curvature of the shock and the magnetic field line across which the shock is sweeping, plays an important role in the efficiency of electron acceleration. At low shock altitudes, when the shock curvature is larger than that of magnetic field lines, the electrons are mainly accelerated at the shock flanks; at higher altitudes, when the shock curvature is smaller, the electrons are mainly accelerated at the shock nose around the top of closed field lines. The above process reveals the shift of efficient electron acceleration region along the shock front during its propagation. It is also found that in general the electron acceleration at the shock flank is not so efficient as that at the top of closed field since at the top a collapsing magnetic trap can be formed. In addition, we find that the energy spectra of electrons is power-law like, first hardening then softening with the spectral index varying in a range of -3 to -6. Physical interpretations of the results and implications on the study of solar radio bursts are discussed.
Solar flares and coronal mass ejections (CMEs) are closely coupled through magnetic reconnection. CMEs are usually accelerated impulsively within the low solar corona, synchronized with the impulsive flare energy release. We investigate the dynamic e
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Coronal mass ejections (CMEs) erupt and expand in a magnetically structured solar corona. Various indirect observational pieces of evidence have shown that the magnetic field of CMEs reconnects with surrounding magnetic fields, forming, e.g., dimming
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