[en] Fast coronal mass ejections (CMEs) generate standing or bow shocks as they propagate through the corona and solar wind. Although CME shocks have previously been detected indirectly via their emission at radio frequencies, direct imaging has remained elusive due to their low contrast at optical wavelengths. Here we report the first images of a CME-driven shock as it propagates through interplanetary space from 8 R_sun to 120 R_sun (0.5 AU), using observations from the STEREO Heliospheric Imager. The CME was measured to have a velocity of ∼1000 km s^-1 and a Mach number of 4.1 ± 1.2, while the shock front standoff distance (Δ) was found to increase linearly to ∼20 R_sun at 0.5 AU. The normalized standoff distance (Δ/D_O ) showed reasonable agreement with semi-empirical relations, where D_O is the CME radius. However, when normalized using the radius of curvature, Δ/R_O did not agree well with theory, implying that R_O was underestimated by a factor of ∼3-8. This is most likely due to the difficulty in estimating the larger radius of curvature along the CME axis from the observations, which provide only a cross-sectional view of the CME.

[en] The three-dimensional magnetic topology of a solar active region (NOAA 10956) was reconstructed using a linear force-free field extrapolation constrained using the twin perspectives of STEREO. A set of coronal field configurations was initially generated from extrapolations of the photospheric magnetic field observed by the Michelson Doppler Imager on SOHO. Using an EUV intensity-based cost function, the extrapolated field lines that were most consistent with 171 A passband images from the Extreme UltraViolet Imager on STEREO were identified. This facilitated quantitative constraints to be placed on the twist (α) of the extrapolated field lines, where ∇ x B = αB. Using the constrained values of α, the evolution in time of twist, connectivity, and magnetic energy were then studied. A flux emergence event was found to result in significant changes in the magnetic topology and total magnetic energy of the region.

[en] Coronal mass ejections (CMEs) are large-scale ejections of plasma and magnetic field from the solar corona, which propagate through interplanetary space at velocities of ∼100-2500 km s^-1. Although plane-of-sky coronagraph measurements have provided some insight into their kinematics near the Sun (<32 R _sun), it is still unclear what forces govern their evolution during both their early acceleration and later propagation. Here, we use the dual perspectives of the STEREO spacecraft to derive the three-dimensional kinematics of CMEs over a range of heliocentric distances (∼2-250 R _sun). We find evidence for solar wind (SW) drag forces acting in interplanetary space, with a fast CME decelerated and a slow CME accelerated toward typical SW velocities. We also find that the fast CME showed linear (δ = 1) dependence on the velocity difference between the CME and the SW, while the slow CME showed a quadratic (δ = 2) dependence. The differing forms of drag for the two CMEs indicate the forces responsible for their acceleration may be different.

[en] The interaction of open and closed field lines at coronal hole (CH) boundaries is widely accepted to be due to interchange magnetic reconnection. To date, it is unclear how the boundaries vary on short timescales and at what velocity this occurs. Here, we describe an automated boundary tracking method used to determine CH boundary displacements on short timescales. The boundary displacements were found to be isotropic and to have typical expansion/contraction speeds of ≤2 km s^-1, which indicate magnetic reconnection rates of ≤3 x 10^-3. The observed displacements were used in conjunction with the interchange reconnection model to derive typical diffusion coefficients of ≤3 x 10¹³ cm² s^-1. These results are consistent with an interchange reconnection process in the low corona driven by the random granular motions of open and closed fields in the photosphere.

[en] The topical and controversial issue of parameterizing the magnetic structure of solar active regions has vital implications in the understanding of how these structures form, evolve, produce solar flares, and decay. This interdisciplinary and ill-constrained problem of quantifying complexity is addressed by using a two-dimensional wavelet transform modulus maxima (WTMM) method to study the multifractal properties of active region photospheric magnetic fields. The WTMM method provides an adaptive space-scale partition of a fractal distribution, from which one can extract the multifractal spectra. The use of a novel segmentation procedure allows us to remove the quiet Sun component and reliably study the evolution of active region multifractal parameters. It is shown that prior to the onset of solar flares, the magnetic field undergoes restructuring as Dirac-like features (with a Hoelder exponent, h = -1) coalesce to form step functions (where h = 0). The resulting configuration has a higher concentration of gradients along neutral line features. We propose that when sufficient flux is present in an active region for a period of time, it must be structured with a fractal dimension greater than 1.2, and a Hoelder exponent greater than -0.7, in order to produce M- and X-class flares. This result has immediate applications in the study of the underlying physics of active region evolution and space weather forecasting.

[en] The relationship between eruptive flares and coronal mass ejections (CMEs) is a topic of ongoing debate, especially regarding the possibility of a common initiation mechanism. We studied the kinematic and hydrodynamic properties of a well-observed event that occurred on 2007 December 31 using data from MESSENGER, RHESSI, and STEREO in order to gain new physical insight into the evolution of the flare and CME. The initiation mechanism was determined by comparing observations to the internal tether-cutting, breakout, and ideal magnetohydrodynamic (MHD) models. Evidence of pre-eruption reconnection immediately eliminated the ideal MHD model. The timing and location of the soft and hard X-ray sources led to the conclusion that the event was initiated by the internal tether-cutting mechanism. In addition, a thermal source was observed to move in a downward direction during the impulsive phase of the event, followed by upward motion during the decay phase, providing evidence for X- to Y-type magnetic reconnection.

[en] Understanding coronal mass ejection (CME) energetics and dynamics has been a long-standing problem, and although previous observational estimates have been made, such studies have been hindered by large uncertainties in CME mass. Here, the two vantage points of the Solar Terrestrial Relations Observatory (STEREO) COR1 and COR2 coronagraphs were used to accurately estimate the mass of the 2008 December 12 CME. Acceleration estimates derived from the position of the CME front in three dimensions were combined with the mass estimates to calculate the magnitude of the kinetic energy and driving force at different stages of the CME evolution. The CME asymptotically approaches a mass of 3.4 ± 1.0 × 10¹⁵ g beyond ∼10 R_☉. The kinetic energy shows an initial rise toward 6.3 ± 3.7 × 10²⁹ erg at ∼3 R_☉, beyond which it rises steadily to 4.2 ± 2.5 × 10³⁰ erg at ∼18 R_☉. The dynamics are described by an early phase of strong acceleration, dominated by a force of peak magnitude of 3.4 ± 2.2 × 10¹⁴ N at ∼3 R_☉, after which a force of 3.8 ± 5.4 × 10¹³ N takes effect between ∼7 and 18 R_☉. These results are consistent with magnetic (Lorentz) forces acting at heliocentric distances of ∼<7 R_☉, while solar wind drag forces dominate at larger distances (∼>7 R_☉).

[en] Solar flare hard X-rays (HXRs) are produced as bremsstrahlung when an accelerated population of electrons interacts with the dense chromospheric plasma. HXR observations presented by Kontar et al. using the Ramaty High-Energy Solar Spectroscopic Imager have shown that HXR source sizes are three to six times more extended in height than those predicted by the standard collisional thick target model (CTTM). Several possible explanations have been put forward including the multi-threaded nature of flare loops, pitch-angle scattering, and magnetic mirroring. However, the nonuniform ionization (NUI) structure along the path of the electron beam has not been fully explored as a solution to this problem. Ionized plasma is known to be less effective at producing nonthermal bremsstrahlung HXRs when compared to neutral plasma. If the peak HXR emission was produced in a locally ionized region within the chromosphere, the intensity of emission will be preferentially reduced around this peak, resulting in a more extended source. Due to this effect, along with the associated density enhancement in the upper chromosphere, injection of a beam of electrons into a partially ionized plasma should result in an HXR source that is substantially more vertically extended relative to that for a neutral target. Here we present the results of a modification to the CTTM, which takes into account both a localized form of chromospheric NUI and an increased target density. We find 50 keV HXR source widths, with and without the inclusion of a locally ionized region, of ∼3 Mm and ∼0.7 Mm, respectively. This helps to provide a theoretical solution to the currently open question of overly extended HXR sources

[en] Coronal bright fronts (CBFs) are large-scale wavefronts that propagate through the solar corona at hundreds of kilometers per second. While their kinematics have been studied in detail, many questions remain regarding the temporal evolution of their amplitude and pulse width. Here, contemporaneous high cadence, multi-thermal observations of the solar corona from the Solar Dynamic Observatory (SDO) and Solar TErrestrial RElations Observatory (STEREO) spacecraft are used to determine the kinematics and expansion rate of a CBF wavefront observed on 2010 August 14. The CBF was found to have a lower initial velocity with weaker deceleration in STEREO observations compared to SDO observations (∼340 km s^–1 and –72 m s^–2 as opposed to ∼410 km s^–1 and –279 m s^–2). The CBF kinematics from SDO were found to be highly passband-dependent, with an initial velocity ranging from 379 ± 12 km s^–1 to 460 ± 28 km s^–1 and acceleration ranging from –128 ± 28 m s^–2 to –431 ± 86 m s^–2 in the 335 Å and 304 Å passbands, respectively. These kinematics were used to estimate a quiet coronal magnetic field strength range of ∼1-2 G. Significant pulse broadening was also observed, with expansion rates of ∼130 km s^–1 (STEREO) and ∼220 km s^–1 (SDO). By treating the CBF as a linear superposition of sinusoidal waves within a Gaussian envelope, the resulting dispersion rate of the pulse was found to be ∼8-13 Mm² s^–1. These results are indicative of a fast-mode magnetoacoustic wave pulse propagating through an inhomogeneous medium.

[en] In this paper, the cooling of 72 M- and X-class flares is examined using GOES/XRS and SDO/EVE. The observed cooling rates are quantified and the observed total cooling times are compared with the predictions of an analytical zero-dimensional hydrodynamic model. We find that the model does not fit the observations well, but does provide a well-defined lower limit on a flare's total cooling time. The discrepancy between observations and the model is then assumed to be primarily due to heating during the decay phase. The decay-phase heating necessary to account for the discrepancy is quantified and found be ∼50% of the total thermally radiated energy, as calculated with GOES. This decay-phase heating is found to scale with the observed peak thermal energy. It is predicted that approximating the total thermal energy from the peak is minimally affected by the decay-phase heating in small flares. However, in the most energetic flares the decay-phase heating inferred from the model can be several times greater than the peak thermal energy.