Claire Raftery

Multi-wavelength observations and modelling of a canonical solar flare

Aims. We investigate the temporal evolution of temperature, emission measure, energy loss, and velocity in a C-class solar flare from both observational and theoretical perspectives. Methods. The properties of the flare were derived by following the systematic cooling of the plasma through the response functions of a number of instruments – the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI; >5 MK), GOES-12 (5–30 MK), the Transition Region and Coronal Explorer (TRACE 171 A; 1 MK), and the Coronal Diagnostic Spectrometer (CDS; ∼0.03–8 MK). These measurements were studied in combination with simulations from the 0-D enthalpy based thermal evolution of loops (EBTEL) model. Results. At the flare onset, upflows of ∼90 km s −1 and low-level emission were observed in Fe xix, consistent with pre-flare heating and gentle chromospheric evaporation. During the impulsive phase, upflows of ∼80 km s −1 in Fe xix and simultaneous downflows of ∼20 km s −1 in He i and O v were observed, indicating explosive chromospheric evaporation. The plasma was subsequently found to reach a peak temperature of >13 MK in approximately 10 min. Using EBTEL, conduction was found to be the dominant loss mechanism during the initial ∼300 s of the decay phase. It was also found to be responsible for driving gentle chromospheric evaporation during this period. As the temperature fell below ∼ 8M K, and for the next∼4000 s, radiative losses were determined to dominate over conductive losses. The radiative loss phase was accompanied by significant downflows of ≤40 km s −1 in O v. Conclusions. This is the first extensive study of the evolution of a canonical solar flare using both spectroscopic and broad-band instruments in conjunction with a 0-D hydrodynamic model. While our results are in broad agreement with the standard flare model, the simulations suggest that both conductive and non-thermal beam heating play important roles in heating the flare plasma during the impulsive phase of at least this event.

THE 2010 AUGUST 1 TYPE II BURST: A CME-CME INTERACTION AND ITS RADIO AND WHITE-LIGHT MANIFESTATIONS

We present observational results of a type II burst associated with a CME-CME interaction observed in the radio and white-light (WL) wavelength range. We applied radio direction-finding techniques to observations from the STEREO and Wind spacecraft, the results of which were interpreted using WL coronagraphic measurements for context. The results of the multiple radio direction-finding techniques applied were found to be consistent both with each other and with those derived from the WL observations of coronal mass ejections (CMEs). The results suggest that the type II burst radio emission is causally related to the CMEs interaction.

Evidence for internal tether-cutting in a flare/coronal mass ejection observed by Messenger, Rhessi, and Stereo

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.

Investigating the driving mechanisms of coronal mass ejections

Aims. The objective of this study was to examine the kinematics of coronal mass ejections (CMEs) using EUV and coronagraph images, and to make a quantitative comparison with a number of theoretical models. One particular aim was to investigate the acceleration profile of CMEs in the low corona. Methods. We selected two CME events for this study, which occurred on 2006 December 17 (CME06) and 2007 December 31 (CME07). CME06 was observed using the EIT and LASCO instruments on-board SOHO, while CME07 was observed using the SECCHI imaging suite on STEREO. The first step of the analysis was to track the motion of each CME front and derive its velocity and acceleration. We then compared the observational kinematics, along with the information of the associated X-ray emissions from GOES and RHESSI, with the kinematics proposed by three CME models (catastrophe, breakout and toroidal instability). Results. We found that CME06 lasted over eight hours while CME07 released its energy in less than three hours. After the eruption, both CMEs were briefly slowed down before being accelerated again. The peak accelerations during the re-acceleration phase coincided with the peak soft X-ray emissions for both CMEs. Their values were ∼60 m s −2 for CME06 and ∼600 m s −2 for CME07. CME07 reached a maximum speed of over 1000 km s −1 before being slowed down to propagate away at a constant, final speed of ∼700 km s −1 . CME06 did not reach a constant speed but was moving at a small acceleration by the end of the observation. Our comparison with the theories suggested that CME06 can be best described by a hybrid of the catastrophe model and breakout model while the characteristics of CME07 were most consistent with the breakout model. Based on the catastrophe model, we deduced that the reconnection rate in the current sheet for CME06 was intermediate, the onset of its eruption occurred at a height of ∼200 Mm, and the Alfven speed and the magnetic field strength at this height were approximately 130‐250 km s −1 and 7 Gauss, respectively.