Searching for quasi-periodic oscillations in astrophysical transients using Gaussian processes |
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Andrew Inglis Submitted: 2022-06-02 09:21
Analyses of quasi-periodic oscillations (QPOs) are important to understanding the dynamic behaviour in many astrophysical objects during transient events like gamma-ray bursts, solar flares, magnetar flares and fast radio bursts. Astrophysicists often search for QPOs with frequency-domain methods such as (Lomb-Scargle) periodograms, which generally assume power-law models plus some excess around the QPO frequency. Time-series data can alternatively be investigated directly in the time domain using Gaussian Process (GP) regression. While GP regression is computationally expensive in the general case, the properties of astrophysical data and models allow fast likelihood strategies. Heteroscedasticity and non-stationarity in data have been shown to cause bias in periodogram-based analyses. Gaussian processes can take account of these properties. Using GPs, we model QPOs as a stochastic process on top of a deterministic flare shape. Using Bayesian inference, we demonstrate how to infer GP hyperparameters and assign them physical meaning, such as the QPO frequency. We also perform model selection between QPOs and alternative models such as red noise and show that this can be used to reliably find QPOs. This method is easily applicable to a variety of different astrophysical data sets. We demonstrate the use of this method on a range of short transients: a gamma-ray burst, a magnetar flare, a magnetar giant flare, and simulated solar flare data.
Authors: M. Hübner, D. Huppenkothen, P. D. Lasky, A. R. Inglis, C. Ick, D. W. Hogg
Projects: None
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Publication Status: ApJ (submitted)
Last Modified: 2022-06-03 14:54
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Evaluating Pointing Strategies for Future Solar Flare Missions |
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Andrew Inglis Submitted: 2021-10-27 12:14
Solar flares are events of intense scientific interest. Although certain solar conditions are known to be associated with flare activity, the exact location and timing of an individual flare on the Sun cannot as yet be predicted with certainty. Missions whose science objectives depend on observing solar flares must often make difficult decisions on where to target their observations if they do not observe the full solar disk. Yet, little analysis exists in the literature which might guide these missions' operations to maximize their opportunities to observe flares. In this study we analyze and simulate the performance of different observation strategies using historical flare and active region data from 2011 to 2014. We test a number of different target selection strategies based on active region complexity and recent flare activity, each of which is examined under a range of operational assumptions. In each case we investigate various metrics such as the number of flares observed, the size of flares observed, and operational considerations such as the number of instrument re-points that are required. Overall, target selection methods based on recent flare activity showed the best overall performance, but required more repointings than other methods. The mission responsiveness to new information is identified as a strong factor determining flare observation performance. It is also shown that target selection methods based on active region complexities show a significant pointing bias towards the western solar hemisphere. The number of flares observed grows quickly with field-of-view size until the approximate size of an active region is reached, but further improvements beyond the active region size are much more incremental. These results provide valuable performance estimates for a future mission focused on solar flares, and inform the requirements that would ensure mission success.
Authors: A. R. Inglis, J. Ireland, A. Y. Shih, S. D. Christe
Projects: None
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Publication Status: Accepted for publication in Solar Physics
Last Modified: 2021-10-27 12:31
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Characteristics of ephemeral coronal holes |
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Andrew Inglis Submitted: 2019-06-06 07:52
Small-scale ephemeral coronal holes may be a recurring feature on the solar disk, but have received comparatively little attention. These events are characterized by compact structure and short total lifetimes, substantially less than a solar disk crossing. We present a systematic search for these events, using Atmospheric Imaging Assembly EUV image data from the Solar Dynamics Observatory, covering the time period 2010 - 2015. Following strict criteria, this search yielded four clear examples of the ephemeral coronal hole phenomenon. The properties of each event are characterized, including their total lifetime, growth and decay rates, and areas. The magnetic properties of these events are also determined using Helioseismic and Magnetic Imager data. Based on these four events, ephemeral coronal holes experience rapid initial growth of up to 3000 Mm2/hr, while the decay phases are typically more gradual. Like conventional coronal holes, the mean magnetic field in each ephemeral coronal hole displays a consistent polarity, with mean magnetic flux densities generally < 10 G. No evidence of a corresponding signature is seen in solar wind data at 1 AU. Further study is needed to determine whether ephemeral coronal holes are under-reported events or a truly rare phenomenon.
Authors: A. R. Inglis, R. E. O'Connor, W. D. Pesnell, M. S. Kirk, N. Karna
Projects: SDO-AIA,SDO-HMI,Wind
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Publication Status: Accepted for publication in ApJ
Last Modified: 2019-06-07 14:16
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Coronal hard X-ray sources revisited |
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Andrew Inglis Submitted: 2018-09-21 10:39
This paper reports on the re-analysis of solar flares in which the hard X-rays (HXRs) come predominantly from the corona rather than from the more usual chromospheric footpoints. All of the 26 previously analyzed event time
intervals, over 13 flares, are re-examined for consistency with a flare model in which electrons are accelerated near the top of a magnetic loop that has a sufficiently high density to stop most of the electrons by Coulomb collisions before they can reach the footpoints. Of particular importance in the previous analysis was the finding that the length of the coronal HXR source increased with energy in the 20 - 30 keV range. However, after allowing for the possibility that footpoint emission at the higher energies affects the inferred length of the coronal HXR source, and using analysis techniques that suppress the possible influence of such footpoint emission, we conclude that there is no longer evidence that the length of the HXR coronal sources increase with increasing energy. In fact, for the 6 flares and 12 time intervals that satisfied our selection criteria, the loop lengths decreased on average by 1.0
± 0.2 arcsec between 20 and 30 keV, with a standard deviation of 3.5 arcsec. We find strong evidence that the peak of the coronal HXR source increases in altitude with increasing energy. For the thermal component of the emission, this is consistent with the standard CHSKP flare model in which magnetic reconnection in a coronal current sheet results in new hot loops being formed at progressively higher altitudes. The explanation for the nonthermal emission is not so clear.
Authors: B. R. Dennis, M. A. Duval-Poo, M. Piana, A. R. Inglis, A. G. Emslie, J. Guo, Y. Xu
Projects: RHESSI
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Publication Status: ApJ, accepted
Last Modified: 2018-09-21 15:31
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Energy release in the solar atmosphere from a stream of infalling prominence debris |
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Andrew Inglis Submitted: 2017-09-18 11:12
Recent high-resolution and high-cadence EUV imaging has revealed a new phenomenon, impacting prominence debris, where prominence material from failed or partial eruptions can impact the lower atmosphere, releasing energy. We report a clear example of energy release and EUV brightening due to infalling prominence debris that occurred on 2011 September 7-8. The initial eruption of material was associated with an X1.8-class flare from AR11283, occurring at 22:30 UT on 2011 September 7. Subsequently, a semi-continuous stream of this material returned to the solar surface with a velocity v > 150 km s-1, impacting a region remote from the original active region between 00:20 - 00:40 UT on 2011 September 8. Using SDO/AIA, the differential emission measure of the plasma was estimated throughout this brightening event. We found that the radiated energy of the impacted plasma was Lrad ~1027 ergs, while the thermal energy peaked at ~1028 ergs. From this we were able to determine the mass content of the debris to be in the range 2x1014 < m < 2x1015 g. Given typical promimence masses, the likely debris mass is towards the lower end of this range. This clear example of a prominence debris event shows that significant energy release takes place during these events, and that such impacts may be used as a novel diagnostic tool for investigating prominence material properties.
Authors: A. R. Inglis, H. R. Gilbert and L. Ofman
Projects: SDO-AIA
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Publication Status: ApJL, accepted
Last Modified: 2017-09-20 09:05
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A large-scale search for evidence of quasi-periodic pulsations in solar flares |
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Andrew Inglis Submitted: 2016-10-24 19:30
The nature of quasi-periodic pulsations in solar flares is poorly constrained, and critically the general prevalence of such signals in solar flares is unknown. Therefore, we perform a large-scale search for evidence of signals consistent with quasi-periodic pulsations in solar flares, focusing on the 1 - 300s timescale. We analyse 675 M- and X-class flares observed by GOES in 1-8A soft X-rays between 2011 February 1 and 2015 December 31. Additionally, over the same era we analyse Fermi/GBM 15-25 keV X-ray data for each of these flares that was associated with a Fermi/GBM solar flare trigger, a total of 261 events. Using a model comparison method, we determine whether there is evidence for a substantial enhancement in the Fourier power spectrum that may be consistent with a QPP signature, based on three tested models; a power-law plus a constant, a broken power-law plus constant, and a power-law-plus-constant with an additional QPP signature component. From this, we determine that ~30% of GOES events and ~8% of Fermi/GBM events show strong signatures consistent with classical interpretations of QPP. For the remaining events either two or more tested models cannot be strongly distinguished from each other, or the events are well-described by single power-law or broken power-law Fourier power spectra. For both instruments, a preferred characteristic timescale of ~5-30 s was found in the QPP-like events, with no dependence on flare magnitude in either GOES or GBM data. We also show that individual events in the sample show similar characteristic timescales in both GBM and GOES datasets. We discuss the implications of these results for our understanding of solar flares and possible QPP mechanisms.
Authors: A. R. Inglis, J. Ireland, B. R. Dennis, L. A. Hayes, P. T. Gallagher
Projects: Fermi/GBM,GOES X-rays
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Publication Status: Accepted for publication in ApJ
Last Modified: 2016-10-25 12:27
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Quasi-Periodic Fluctuations and Chromospheric Evaporation in a Solar Flare Ribbon Observed by Hinode/EIS, IRIS, and RHESSI |
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Andrew Inglis Submitted: 2016-10-17 11:44
The Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS) obtained rapid cadence (11.2 s) EUV stare spectra of an M7.3 flare ribbon in AR 12036 on 2014 April 18. Quasi-periodic (P ≈ 75.6 ? 9.2 s) intensity fluctuations occurred in emission lines of O iv, Mg VI, Mg vii, Si vii, Fe xiv, and Fe xvi during the flare's impulsive rise, and ended when the maximum intensity in Fe xxiii was reached. The profiles of the O iv?Fe xvi lines reveal that they were all redshifted during most of the interval of quasi-periodic intensity fluctuations, while the Fe xxiii profile revealed multiple components including one or two highly blueshifted ones. This indicates that the flare underwent explosive chromospheric evaporation during its impulsive rise. Fluctuations in the relative Doppler velocities were seen, but their amplitudes were too subtle to extract significant quasi-periodicities. RHESSI detected 25x100 keV hard-X-ray sources in the ribbon near the EIS slit's pointing position during the peaks in the EIS intensity fluctuations. The observations are consistent with a series of energy injections into the chromosphere by nonthermal particle beams. Electron densities derived with Fe xiv (4.6 ? 1010 cm-3) and Mg vii (7.8 ? 109 cm-3) average line intensity ratios during the interval of quasi-periodic intensity fluctuations, combined with the radiative loss function of an optically thin plasma, yield radiative cooling times of 32 s at 2.0 ? 106 K, and 46 s at 6.3 ? 105 K (about half the quasi-period); assuming Fe xiv's density for Fe xxiii yields a radiative cooling time of 103 s (13 times the quasi-period) at 1.4 ? 107 K.
Authors: J. W. Brosius, A. N. Daw and A. R. Inglis
Projects: Hinode/EIS,IRIS,RHESSI
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Publication Status: The Astrophysical Journal, 830, 101, 2016
Last Modified: 2016-10-19 13:08
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SunPy - Python for Solar Physics |
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Andrew Inglis Submitted: 2015-05-18 15:37
This paper presents SunPy (version 0.5), a community-developed Python package for solar physics. Python, a free, cross-platform, general-purpose, high-level programming language, has seen widespread adoption among the scientific community, resulting in the availability of a large number of software packages, from numerical computation (NumPy, SciPy) and machine learning (scikit-learn) to visualisation and plotting (matplotlib). SunPy is a data-analysis environment specialising in providing the software necessary to analyse solar and heliospheric data in Python. SunPy is open-source software (BSD licence) and has an open and transparent development workflow that anyone can contribute to. SunPy provides access to solar data through integration with the Virtual Solar Observatory (VSO), the Heliophysics Event Knowledgebase (HEK), and the HELiophysics Integrated Observatory (HELIO) webservices. It currently supports image data from major solar missions (e.g., SDO, SOHO, STEREO, and IRIS), time-series data from missions such as GOES, SDO/EVE, and PROBA2/LYRA, and radio spectra from e-Callisto and STEREO/SWAVES. We describe SunPy's functionality, provide examples of solar data analysis in SunPy, and show how Python-based solar data-analysis can leverage the many existing tools already available in Python. We discuss the future goals of the project and encourage interested users to become involved in the planning and development of SunPy.
Authors: The SunPy Community, S. J. Mumford; S. Christe; D. Pérez-Suárez; J. Ireland; A. Y. Shih; A. R. Inglis; S. Liedtke; R. J. Hewett; F. Mayer; K. Hughitt; N. Freij; T. Meszaros; S. M. Bennett; M. Malocha; J. Evans; A. Agrawal; A. J. Leonard; T. P. Robitaille; B. Mampaey; J. Iván Campos-Rozo; M. S. Kirk
Projects: None
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Publication Status: Computational Science & Discovery, accepted
Last Modified: 2015-05-19 08:51
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Quasi-periodic pulsations in solar and stellar flares: re-evaluating their nature in the context of power-law flare Fourier spectra |
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Andrew Inglis Submitted: 2014-10-31 08:31
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.
Authors: A. R. Inglis, J. Ireland, M. Dominique
Projects: None
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Publication Status: ApJ, accepted
Last Modified: 2014-10-31 09:40
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Investigating the differential emission measure and energetics of microflares with combined SDO/AIA and RHESSI observations |
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Andrew Inglis Submitted: 2014-05-22 08:09
An important question in solar physics is whether solar microflares, the smallest currently observable flare events in X-rays, possess the same energetic properties as large flares. Recent surveys have suggested that microflares may be less efficient particle accelerators than large flares, and hence contribute less nonthermal energy, which may have implications for coronal heating mechanisms. We therefore explore the energetic properties of microflares by combining Extreme Ultraviolet (EUV) and X-ray measurements.
We present forward-fitting differential emission measure (DEM) analysis of 10 microflares. The fitting is constrained by combining, for the first time, high temperature RHESSI observations and flux data from SDO/AIA. Two fitting models are tested for the DEM; a Gaussian distribution and a uniform DEM profile. A Gaussian fit proved unable to explain the observations for any of the studied microflares. However, 8 of 10 events studied were reasonably fit by a uniform DEM profile. Hence microflare plasma can be considered to be significantly multi-thermal, and may not be significantly peaked or contain resolvable fine structure, within the uncertainties of the observational instruments.
The thermal and non-thermal energy is estimated for each microflare, comparing the energy budget with an isothermal plasma assumption. From the multithermal fits the minimum non-thermal energy content was found to average approximately 30% of the estimated thermal energy. By comparison, under an isothermal model the non-thermal and thermal energy estimates were generally comparable. Hence, multi-thermal plasma is an important consideration for solar microflares that substantially alters their thermal and non-thermal energy content.
Authors: A. R. Inglis, S. Christe
Projects: RHESSI,SDO-AIA
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Publication Status: ApJ, accepted
Last Modified: 2014-05-26 13:03
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Energy release from impacting prominence material following the 2011 June 7 eruption |
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Andrew Inglis Submitted: 2013-09-09 19:50
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 1026 - 5.8 x 1027 ergs) to the radiative energy (1.9 x 1025 - 2.5 x 1026 ergs) we find the dominant mechanism of energy release involved in the observed brightening is plasma compression.
Authors: H. R. Gilbert, A. R. Inglis, M. L. Mays, L. Ofman, B. J. Thompson, C. A. Young
Projects: SDO-AIA,STEREO
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Publication Status: ApJ Letters, accepted
Last Modified: 2013-09-10 13:20
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Hard X-ray and ultraviolet emission during the 2011 June 7 solar flare |
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Andrew Inglis Submitted: 2013-09-04 08:01
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.
Authors: A. R. Inglis, H. R. Gilbert
Projects: RHESSI,SDO-AIA,SDO-HMI
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Publication Status: ApJ, accepted
Last Modified: 2013-09-04 11:42
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The relationship between hard X-ray pulse timings and the locations of footpoint sources during solar flares |
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Andrew Inglis Submitted: 2012-01-27 10:56
The cause of quasi-periodic pulsations (QPP) in solar flares remainsthe subject of debate.Recently, Nakariakov & Zimovets (2011) proposed a new model suggestingthat, in two-ribbonflares, such pulsations could be explained by propagating slow waves.These waves may travelobliquely to the magnetic field, reflect in the chromosphere andconstructively interfere at aspatially separate site in the corona, leading to quasi-periodicreconnection events progressingalong the flaring arcade. Such a slow wave regime would have certainobservational characteristics.We search for evidence of this phenomenon during a selection oftwo-ribbon flares observed byRHESSI, SOHO and TRACE; the flares of 2002 November 9, 2005 January 19and 2005 August22. We were not able to observe a clear correlation between hard X-rayfootpoint separationsand pulse timings during these events. Also, the motion of hard X-rayfootpoints is shown to becontinuous within the observational error, whereas a discontinuousmotion might be anticipatedin the slow wave model. Finally, we find that for a preferential slowwave propagation angle of25-28 degrees that is expected for the fastest waves, the velocitiesof the hard X-ray footpointslead to estimated pulse periods and ribbon lengths significantlylarger than the measured values.Hence, for the three events studied, we conclude that theobservational characteristics cannot beeasily explained via the Nakariakov & Zimovets (2011) propagating slowwave model when onlyangles of 25-28 degrees are considered. We provide suggested flareparameters to optimise futurestudies of this kind.
Authors: A. R. Inglis & B. R. Dennis
Projects: RHESSI,SoHO-EIT,TRACE
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Publication Status: ApJ (accepted)
Last Modified: 2012-01-27 22:34
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A multi-periodic oscillatory event in a solar flare |
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Andrew Inglis Submitted: 2008-11-17 08:25
Aims: Ratios of different significant periods found in the light curves of a solar flare exhibiting quasi-periodic pulsations (QPP) are used to distinguish between the possible physical mechanisms responsible for such periodic behaviour.
Methods: Time series data of the flaring event of 2002 July 3, observed via the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters, and the RHESSI satellite, are investigated with the use of the Lomb-Scargle periodogram technique. Images of the event are also recovered via the RHESSI, SOHO, and Nobeyama Radioheliograph instruments.
Results: Statistical analysis of the Lomb-Scargle periodogram results indicates three distinct periods above the 99% confidence level in Nobeyama Radioheliograph and Radiopolarimeter data, at 28 s, 18 s, and 12 s, respectively. The two longest of these periods were also observed in the RHESSI data at the same confidence level. Wavelet analysis demonstrated that multiple periods occurred simultaneously without any significant frequency shift over time. Reconstructed images of the event reveal a very compact flare structure unsuitable for spatially resolved analysis. Consideration of the period ratios leads to the conclusion that the cause of this multi-periodic event is likely to be a kink mode periodically
triggering magnetic reconnection.
Authors: A. R. Inglis and V. M. Nakariakov
Projects: RHESSI,SoHO-EIT,SoHO-MDI
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Publication Status: A&A (accepted)
Last Modified: 2008-11-17 17:33
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