Bojan Vršnak

FIRST OBSERVATIONS OF A DOME-SHAPED LARGE-SCALE CORONAL EXTREME-ULTRAVIOLET WAVE

We present first observations of a dome-shaped large-scale extreme-ultraviolet coronal wave, recorded by the Extreme Ultraviolet Imager instrument on board STEREO-B on 2010 January 17. The main arguments that the observed structure is the wave dome (and not the coronal mass ejection, CME) are (1) the spherical form and sharpness of the domes outer edge and the erupting CME loops observed inside the dome; (2) the low-coronal wave signatures above the limb perfectly connecting to the on-disk signatures of the wave; (3) the lateral extent of the expanding dome which is much larger than that of the coronal dimming; and (4) the associated high-frequency type II burst indicating shock formation low in the corona. The velocity of the upward expansion of the wave dome (v ~ 650 km s–1) is larger than that of the lateral expansion of the wave (v ~ 280 km s–1), indicating that the upward dome expansion is driven all the time, and thus depends on the CME speed, whereas in the lateral direction it is freely propagating after the CME lateral expansion stops. We also examine the evolution of the perturbation characteristics: first the perturbation profile steepens and the amplitude increases. Thereafter, the amplitude decreases with r –2.5 ± 0.3, the width broadens, and the integral below the perturbation remains constant. Our findings are consistent with the spherical expansion and decay of a weakly shocked fast-mode MHD wave.

Band-splitting of coronal and interplanetary type II bursts. II. Coronal magnetic field and Alfvén velocity

Type II radio bursts recorded in the metric wavelength range are excited by MHD shocks traveling through the solar corona. They often expose the fundamental and harmonic emission band, both frequently being split in two parallel lanes that show a similar frequency drift and intensity behaviour. Our previous paper showed that band-splitting of such characteristics is a consequence of the plasma emission from the upstream and downstream shock regions. Consequently, the split can be used to evaluate the density jump at the shock front and to estimate the shock Mach number, which in combination with the shock speed inferred from the frequency drift provides an estimate of the Alfven velocity and the magnetic field in the ambient plasma. In this paper such a procedure is applied to 18 metric type II bursts with the fundamental band starting frequencies up to 270 MHz. The obtained values show a minimum of the Alfven velocity at the heliocentric distance R 2 amounting tovA 400-500 km s 1 . It then increases achieving a local maximum ofvA 450-700 km s 1 at R 2:5. The implications regarding the process of formation and decay of MHD shocks in the corona are discussed. The coronal magnetic field in the range 1:3< R< 3 decreases as R 3 to R 4 ,o rH 1:5 to H 2 if expressed as a function of the height. The results are compared with other estimates of the coronal magnetic field in the range 1 < R < 10. Combined data show that below H < 0: 3t he magnetic field is dominated by active region fields, whereas above H= 1 it becomes radial, behaving roughly as B= 2 R 2 with a plausible value of B 5n T at 1a .u.

CONNECTING SPEEDS, DIRECTIONS AND ARRIVAL TIMES OF 22 CORONAL MASS EJECTIONS FROM THE SUN TO 1 AU

Forecasting the in situ properties of coronal mass ejections (CMEs) from remote images is expected to strongly enhance predictions of space weather and is of general interest for studying the interaction of CMEs with planetary environments. We study the feasibility of using a single heliospheric imager (HI) instrument, imaging the solar wind density from the Sun to 1 AU, for connecting remote images to in situ observations of CMEs. We compare the predictions of speed and arrival time for 22 CMEs (in 2008-2012) to the corresponding interplanetary coronal mass ejection (ICME) parameters at in situ observatories (STEREO PLASTIC/IMPACT, Wind SWE/MFI). The list consists of front-and backsided, slow and fast CMEs (up to 2700 km s(-1)). We track the CMEs to 34.9 +/- 7.1 deg elongation from the Sun with J maps constructed using the SATPLOT tool, resulting in prediction lead times of - 26.4 +/- 15.3 hr. The geometrical models we use assume different CME front shapes (fixed-Phi, harmonic mean, self-similar expansion) and constant CME speed and direction. We find no significant superiority in the predictive capability of any of the three methods. The absolute difference between predicted and observed ICME arrival times is 8.1 +/- 6.3 hr (rms value of 10.9 hr). Speeds are consistent to within 284 +/- 288 km s(-1) . Empirical corrections to the predictions enhance their performance for the arrival times to 6.1 +/- 5.0 hr (rms value of 7.9 hr), and for the speeds to 53 +/- 50 km s(-1). These results are important for Solar Orbiter and a space weather mission positioned away from the Sun-Earth line.

ACCELERATION IN FAST HALO CMEs AND SYNCHRONIZED FLARE HXR BURSTS

We study two well-observed, fast halo CMEs, covering the full CME kinematics including the initiation and impulsive acceleration phase, and their associated flares. We find a close synchronization between the CME acceleration profile and the flare energy release as indicated by the RHESSI hard X-ray flux onsets, as well as peaks occur simultaneously within 5 minutes. These findings indicate a close physical connection between both phenomena and are interpreted in terms of a feedback relationship between the CME dynamics and the reconnection process in the current sheet beneath the CME.

CHARACTERISTICS OF KINEMATICS OF A CORONAL MASS EJECTION DURING THE 2010 AUGUST 1 CME–CME INTERACTION EVENT

We study the interaction of two successive coronal mass ejections (CMEs) during the 2010 August 1 events using STEREO/SECCHI COR and heliospheric imager (HI) data. We obtain the direction of motion for both CMEs by applying several independent reconstruction methods and find that the CMEs head in similar directions. This provides evidence that a full interaction takes place between the two CMEs that can be observed in the HI1 field of view. The full de-projected kinematics of the faster CME from Sun to Earth is derived by combining remote observations with in situ measurements of the CME at 1 AU. The speed profile of the faster CME (CME2; similar to 1200 km s(-1)) shows a strong deceleration over the distance range at which it reaches the slower, preceding CME (CME1; similar to 700 km s(-1)). By applying a drag-based model we are able to reproduce the kinematical profile of CME2, suggesting that CME1 represents a magnetohydrodynamic obstacle for CME2 and that, after the interaction, the merged entity propagates as a single structure in an ambient flow of speed and density typical for quiet solar wind conditions. Observational facts show that magnetic forces may contribute to the enhanced deceleration of CME2. We speculate that the increase in magnetic tension and pressure, when CME2 bends and compresses the magnetic field lines of CME1, increases the efficiency of drag.

Stability of prominences exposing helical-like patterns

The internal structure of prominences appearing as twisted tubes was studied. The sample embraced 15 stable and 13 eruptive prominences, exposing patterns which possibly reflect a helical configuration. The equivalent pitch angles (ϑ) of twisted fine structure features were measured. In some cases the evolution of the internal structure was followed and 49 independent measurements of the parameter ϑ were performed in total. The results are presented in the plane relating the parameter ϑ and the normalized prominence height. The eruptive prominences occupy the region characterized by ϑ > 50° and h > 0.8d, where h and d are the prominence height and the footpoint half-separation, respectively. All prominences characterized by h < 0.6d or by ϑ < 35° were stable. Such a result is in good agreement with an order of magnitude treatment of the forces acting in a curved magnetic tube, anchored at both ends in the photosphere.

Band-splitting of coronal and interplanetary type II bursts - I. Basic properties

Patterns analogous to the band-splitting of metric type II bursts are found in a number of type II bursts observed in the dekameter-kilometer wavelength range. A similarity of morphological and frequency-time characteristics of two emission components are indicative of a common source. Relative frequency splits span in the range

The CME-flare relationship: Are there really two types of CMEs?

\Delta f/f=0.05{-}0.6

The role of aerodynamic drag in propagation of interplanetary coronal mass ejections

. At radial distances between 2 and 4

Dynamics of plasmoids formed by the current sheet tearing

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