Quasi-periodic pulsations in solar and stellar flares: re-evaluating their nature in the context of power-law flare Fourier spectra

The nature of quasi-periodic pulsations in solar and stellar flares remains debated. Recent work has shown that power-law-like Fourier power spectra, also referred to as red noise processes, are an intrinsic property of solar and stellar flare signals, a property that many previous studies of this phenomenon have not accounted for. Hence a re-evaluation of the existing interpretations and assumptions regarding QPP is needed. Here we adopt a Bayesian method for investigating this phenomenon, fully considering the Fourier power law properties of flare signals. Using data from the PROBA2/LYRA, Fermi/GBM, Nobeyama Radioheliograph and Yohkoh/HXT instruments, we study a selection of flares from the literature identified as QPP events. Additionally we examine optical data from a recent stellar flare that appears to exhibit oscillatory properties. We find that, for all but one event tested, an explicit oscillation is not required in order to explain the observations. Instead, the flare signals are adequately described as a manifestation of a power law in the Fourier power spectrum, rather than a direct signature of oscillating components or structures. However, for the flare of 1998 May 8, strong evidence for the existence of an explicit oscillation with P ~ 14-16 s is found in the 17 GHz radio data and the 13-23 keV Yohkoh HXT data. We conclude that, most likely, many previously analysed events in the literature may be similarly described in terms of power laws in the flare Fourier power spectrum, without the need to invoke a narrowband, oscillatory component. As a result the prevalence of oscillatory signatures in solar and stellar flares may be less than previously believed. The physical mechanism behind the appearance of the observed power laws is discussed.

Energy release from impacting prominence material following the 2011 June 7 eruption

Solar filaments exhibit a range of eruptive-like dynamic activity, ranging from the full or partial eruption of the filament mass and surrounding magnetic structure as a coronal mass ejection (CME), to a fully confined or failed eruption. On 2011 June 7, a dramatic partial eruption of a filament was observed by multiple instruments on SDO and STEREO. One of the interesting aspects of this event is the response of the solar atmosphere as non-escaping material falls inward under the influence of gravity. The impact sites show clear evidence of brightening in the observed EUV wavelengths due to energy release. Two plausible physical mechanisms explaining the brightening are considered: heating of the plasma due to the kinetic energy of impacting material compressing the plasma, or reconnection between the magnetic field of low-lying loops and the field carried by the impacting material. By analyzing the emission of the brightenings in several SDO/AIA wavelengths, and comparing the kinetic energy of the impacting material (7.6 x 10^26 - 5.8 x 10^27 ergs) to the radiative energy (1.9 x 10^25 - 2.5 x 10^26 ergs) we find the dominant mechanism of energy release involved in the observed brightening is plasma compression.

Hard X-ray and ultraviolet emission during the 2011 June 7 solar flare

The relationship between X-ray and UV emission during flares, particularly in the context of quasi-periodic pulsations, remains unclear. To address this, we study the impulsive X-ray and UV emission during the eruptive flare of 2011 June 7 utilising X-ray imaging from RHESSI and UV 1700A imaging from SDO/AIA. This event is associated with quasi-periodic pulsations in X-ray and possibly UV emission, as well as substantial parallel and perpendicular motion of the hard X-ray footpoints. The motion of the footpoints parallel to the flare ribbons is unusual; it is shown to reverse direction on at least two occasions. However, there is no associated short-timescale motion of the UV bright regions. Additionally, we find that the locations of the brightest X-ray and UV regions are different, particularly during the early portion of the flare impulsive phase, despite their integrated emission being strongly correlated in time. Correlation analysis of measured flare properties, such as the footpoint separation, flare shear, photospheric magnetic field and coronal reconnection rate, reveals that - in the impulsive phase - the 25 - 50 keV hard X-ray flux is only weakly correlated with these properties, in contrast to previous studies. We characterise this event in terms of long-term behaviour, where the X-ray nonthermal, thermal, and UV emission sources appear temporally and spatially consistent, and short-term behaviour, where the emission sources are inconsistent and quasi-periodic pulsations are a dominant feature requiring explanation. We suggest that the short timescale behaviour of hard X-ray footpoints, and the nature of the observed quasi-periodic pulsations, is determined by fundamental, as-yet unobserved properties of the reconnection region and particle acceleration sites. This presents a challenge for current three-dimensional flare reconnection models.

The relationship between hard X-ray pulse timings and the locations of footpoint sources during solar flares

The cause of quasi-periodic pulsations (QPP) in solar flares remains the subject of debate. Recently, Nakariakov & Zimovets (2011) proposed a new model suggesting that, in two-ribbon flares, such pulsations could be explained by propagating slow waves. These waves may travel obliquely to the magnetic field, reflect in the chromosphere and constructively interfere at a spatially separate site in the corona, leading to quasi-periodic reconnection events progressing along the flaring arcade. Such a slow wave regime would have certain observational characteristics. We search for evidence of this phenomenon during a selection of two-ribbon flares observed by RHESSI, SOHO and TRACE; the flares of 2002 November 9, 2005 January 19 and 2005 August 22. We were not able to observe a clear correlation between hard X-ray footpoint separations and pulse timings during these events. Also, the motion of hard X-ray footpoints is shown to be continuous within the observational error, whereas a discontinuous motion might be anticipated in the slow wave model. Finally, we find that for a preferential slow wave propagation angle of 25-28 degrees that is expected for the fastest waves, the velocities of the hard X-ray footpoints lead to estimated pulse periods and ribbon lengths significantly larger than the measured values. Hence, for the three events studied, we conclude that the observational characteristics cannot be easily explained via the Nakariakov & Zimovets (2011) propagating slow wave model when only angles of 25-28 degrees are considered. We provide suggested flare parameters to optimise future studies of this kind.

Instrumental oscillations in RHESSI count rates during solar flares

Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft motion by studying persistent instrumental oscillations found in the lightcurves measured by RHESSIs X-ray detectors in the 6-12 keV and 12-25 keV energy range during the decay phase of the flares of 2004 November 4 and 6. Methods: The various motions of the RHESSI spacecraft which may contribute to the manifestation of oscillations are studied. The response of each detector in turn is also investigated. Results: We find that on 2004 November 6 the observed oscillations correspond to the nutation period of the RHESSI instrument. These oscillations are also of greatest amplitude for detector 5, while in the lightcurves of many other detectors the oscillations are small or undetectable. We also find that the variation in detector pointing is much larger during this flare than the counterexample of 2004 November 4. Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead to clearly observable oscillations in count rates, posing a significant hazard for data analysis. This issue is particularly problematic for detector 5 due to its design characteristics. Dynamic correction of the RHESSI counts, accounting for the livetime, data gaps, and the transmission of the bi-grid collimator of each detector, is required to overcome this issue. These corrections should be applied to all future oscillation studies.

Automated identification of neurons and their locations

Individual locations of many neuronal cell bodies (>10^4) are needed to enable statistically significant measurements of spatial organization within the brain such as nearest-neighbor and microcolumnarity measurements. In this paper, we introduce an Automated Neuron Recognition Algorithm (ANRA) which obtains the (x,y) location of individual neurons within digitized images of Nissl-stained, 30 micron thick, frozen sections of the cerebral cortex of the Rhesus monkey. Identification of neurons within such Nissl-stained sections is inherently difficult due to the variability in neuron staining, the overlap of neurons, the presence of partial or damaged neurons at tissue surfaces, and the presence of non-neuron objects, such as glial cells, blood vessels, and random artifacts. To overcome these challenges and identify neurons, ANRA applies a combination of image segmentation and machine learning. The steps involve active contour segmentation to find outlines of potential neuron cell bodies followed by artificial neural network training using the segmentation properties (size, optical density, gyration, etc.) to distinguish between neuron and non-neuron segmentations. ANRA positively identifies 86[5]% neurons with 15[8]% error (mean[st.dev.]) on a wide range of Nissl-stained images, whereas semi-automatic methods obtain 80[7]%/17[12]%. A further advantage of ANRA is that it affords an unlimited increase in speed from semi-automatic methods, and is computationally efficient, with the ability to recognize ~100 neurons per minute using a standard personal computer. ANRA is amenable to analysis of huge photo-montages of Nissl-stained tissue, thereby opening the door to fast, efficient and quantitative analysis of vast stores of archival material that exist in laboratories and research collections around the world.