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Lyman Continuum Observations of Solar Flares Using SDO/EVE  

Ryan Milligan   Submitted: 2018-10-25 23:42

The Extreme ultraviolet Variability Experiment was designed to observe the Sun-as-a-star in the extreme ultraviolet; a wavelength range that has remained spectrally unresolved for many years. It has provided a wealth of data on solar flares, perhaps most uniquely, on the Lyman spectrum of hydrogen at high cadence and moderate spectral resolution. In this paper we concentrate on the analysis of Lyman continuum (LyC) observations and its temporal evolution in a sample of six major solar flares. By fitting both the pre-flare and flare excess spectra with a blackbody function we show that the color temperature derived from the slope of LyC reveals temperatures in excess of 104 K in the six events studied; an increase of a few thousand Kelvin above quiet-Sun values (typically ~8000-9500 K). This was found to be as high as 17000 K for the 2017 September 6 X9.3 flare. Using these temperature values, and assuming a flaring area of 1018 cm2, estimates of the departure coefficient of hydrogen, b1, were calculated. It was found that b1 decreased from 102-103 in the quiet-Sun, to around unity during the flares. This implies that LyC is optically thick and formed in local thermodynamic equilibrium during flares. It also emanates from a relatively thin (≲100 km) shell formed at deeper, denser layers than in the quiescent solar atmosphere. We show that in terms of temporal coverage and resolution, EVE gives a more comprehensive picture of the response of the chromosphere to the flare energy input with respect to those of the Skylab/Harvard College Observatory spatially resolved observations of the 1970's.

Authors: Marcos E. Machado, Ryan O. Milligan, Paulo J. A. Simões
Projects: None

Publication Status: Accepted for publication in ApJ.
Last Modified: 2018-10-26 11:49
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Detection of 3-Minute Oscillations in Full-disk Lya Emission During A Solar Flare  

Ryan Milligan   Submitted: 2017-09-27 09:26

In this Letter we report the detection of chromospheric 3-minute oscillations in disk-integrated EUV irradiance observations during a solar flare. A wavelet analysis of detrended Lyman α (from GOES/EUVS) and Lyman continuum (from SDO/EVE) emission from the 2011 February 15 X-class flare (SOL2011-02-15T01:56) revealed a ~3-minute period present during the flare's main phase. The formation temperature of this emission locates this radiation to the flare's chromospheric footpoints, and similar behaviour is found in the SDO/AIA 1600A and 1700A channels, which are dominated by chromospheric continuum. The implication is that the chromosphere responds dynamically at its acoustic cutoff frequency to an impulsive injection of energy. Since the 3-minute period was not found at hard X-ray energies (50-100 keV) in RHESSI data we can state that this 3-minute oscillation does not depend on the rate of energization of non-thermal electrons. However, a second period of 120 s found in both hard X-ray and chromospheric emission is consistent with episodic electron energization on 2-minute timescales. Our finding on the 3-minute oscillation suggests that chromospheric mechanical energy should be included in the flare energy budget, and the fluctuations in the Lyman α line may influence the composition and dynamics of planetary atmospheres during periods of high activity.

Authors: Ryan O. Milligan, Bernhard Fleck, Jack Ireland, Lyndsay Fletcher, Brian R. Dennis
Projects: GOES/EUVS,RHESSI,SDO-AIA,SDO-EVE

Publication Status: Accepted in Astrophysics Journal Letters
Last Modified: 2017-09-27 10:30
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On the Performance of Multi-Instrument Solar Flare Observations During Solar Cycle 24  

Ryan Milligan   Submitted: 2017-03-14 04:08

The current fleet of space-based solar observatories offers us a wealth of opportunities to study solar flares over a range of wavelengths. Significant advances in our understanding of flare physics often come from coordinated observations between multiple instruments. Consequently, considerable efforts have been, and continue to be made to coordinate observations among instruments (e.g. through the Max Millennium Program of Solar Flare Research). However, there has been no study to date that quantifies how many flares have been observed by combinations of various instruments. Here we describe a technique that retrospectively searches archival databases for flares jointly observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Solar Dynamics Observatory (SDO)/EUV Variability Experiment (EVE) (Multiple EUV Grating Spectrograph (MEGS)-A and MEGS-B), Hinode/(EUV Imaging Spectrometer, Solar Optical Telescope, and X-Ray Telescope), and Interface Region Imaging Spectrograph (IRIS). Out of the 6953 flares of GOES magnitude C1 or greater that we consider over the 6.5 years after the launch of SDO, 40 have been observed by six or more instruments simultaneously. Using each instrument's individual rate of success in observing flares, we show that the numbers of flares co-observed by three or more instruments are higher than the number expected under the assumption that the instruments operated independently of one another. In particular, the number of flares observed by larger numbers of instruments is much higher than expected. Our study illustrates that these missions often acted in cooperation, or at least had aligned goals. We also provide details on an interactive widget (Solar Flare Finder) now available in SSWIDL that allows a user to search for flaring events that have been observed by a chosen set of instruments. This provides access to a broader range of events in order to answer specific science questions. The difficulty in scheduling coordinated observations for solar-flare research is discussed with respect to instruments projected to begin operations during Solar Cycle 25, such as the Daniel K. Inouye Solar Telescope, Solar Orbiter, and Parker Solar Probe.

Authors: Ryan Milligan, Jack Ireland
Projects: GOES/EUVS,GOES X-rays,Hinode/EIS,Hinode/SOT,Hinode/XRT,IRIS,PROBA2/SWAP,RHESSI,SDO-EVE

Publication Status: 26 pages, 7 figures, 3 tables. Accepted in Solar Physics
Last Modified: 2018-01-09 11:31
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The Anomalous Temporal Behaviour of Broadband Lyman α Emission During Flares From SDO/EVE  

Ryan Milligan   Submitted: 2015-06-05 08:35

Despite being the most prominent emission line in the solar spectrum, there has been a notable lack of studies devoted to variations in Lya emission during solar flares in recent years. However, the few examples that do exist have shown Lya emission to be a substantial radiator of the total energy budget of solar flares (on the order of 10%). It is also a known driver of fluctuations in earth's ionosphere. The EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory now provides broadband, photometric Lya data at 10 s cadence with its Multiple EUV Grating Spectrograph-Photometer (MEGS-P) component, and has observed scores of solar flares in the 5 years since it was launched. However, the MEGS-P time profiles appear to display a rise time of tens of minutes around the time of the flare onset. This is in stark contrast to the rapid, impulsive increase observed in other intrinsically chromospheric features (Ha, Lyb, LyC, C III, etc.). Furthermore, the emission detected by MEGS-P peaks around the time of the peak of thermal soft X-ray emission, rather than during the impulsive phase when energy deposition in the chromosphere - often assumed to be in the form of nonthermal electrons - is greatest. The time derivative of Lya lightcurves also appears to resemble that of the time derivative of soft X-rays, reminiscent of the Neupert Effect. Given that spectrally-resolved Lya observations during flares from SORCE/SOLSTICE peak during the impulsive phase as expected, this suggests that the atypical behaviour of MEGS-P data is a manifestation of the broadband nature of the observations. This could imply that other lines and/or continuum emission that becomes enhanced during flares could be contributing to the passband. Users are hereby urged to exercise caution when interpreting broadband Lya observations of solar flares. Comparisons have also been made with other broadband Lya photometers such as PROBA2/LYRA and GOES/EUVS-E.

Authors: Ryan O. Milligan, Phillip C. Chamberlin
Projects: SDO-EVE

Publication Status: Accepted to A&A Research Notes
Last Modified: 2015-12-11 12:05
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The Temporal Behaviour of Lyman α Emission During Flares From SDO/EVE  

Ryan Milligan   Submitted: 2015-06-05 08:35

Despite being the most prominent emission line in the solar spectrum, there has been a notable lack of studies devoted to variations in Lya emission during solar flares in recent years. The few examples that do exist, however, have shown Lya emission to be a substantial radiator of the total energy budget of solar flares (on the order of 10%). It is also a known driver of fluctuations in earth's ionosphere. The EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory now provides broadband, photometric Lya data at 10 s cadence, and has observed scores of solar flares in the 5 years since it was launched. However, the time profiles appear to display a rise time of tens of minutes around the time of the flare onset. This is in stark contrast to the rapid, impulsive increase observed in other intrinsically chromospheric features (Ha, Lyb, LyC, C III, etc.). Furthermore, the Lya emission peaks around the time of the peak of thermal soft X-ray emission, rather than during the impulsive phase when energy deposition in the chromosphere - often assumed to be in the form of nonthermal electrons - is greatest. The time derivative of Lya lightcurves also closely resembles that of the time derivative of soft X-rays, rather reminiscent of the Neupert Effect. To establish whether this atypical behaviour is a characteristic of flare heating in the lower solar atmosphere during explosive events, or a manifestation of the broadband nature of the EVE observations, comparisons have been made with spectrally-resolved Lya measurements during flares from SORCE/SOLSTICE, and other broadband photometers such as PROBA2/LYRA and GOES/EUVS-E.

Authors: Ryan O. Milligan, Phillip C. Chamberlin
Projects: SDO-EVE

Publication Status: Submitted to A&A Research Notes
Last Modified: 2015-06-10 13:22
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Extreme Ultra-Violet Spectroscopy of the Flaring Solar Chromosphere  

Ryan Milligan   Submitted: 2015-01-12 10:47

The extreme ultraviolet portion of the solar spectrum contains a wealth of diagnostic tools for probing the lower solar atmosphere in response to an injection of energy, particularly during the impulsive phase of solar flares. These include temperature and density sensitive line ratios, Doppler shifted emission lines and nonthermal broadening, abundance measurements, Differential Emission Measure profiles, and continuum temperatures and energetics, among others. In this paper I shall review some of the advances made in recent years using these techniques, focusing primarily on studies that have utilized data from Hinode/EIS and SDO/EVE, while also providing some historical background and a summary of future spectroscopic instrumentation.

Authors: Ryan O. Milligan
Projects: Hinode/EIS,SDO-EVE,SoHO-CDS,SoHO-SUMER

Publication Status: Submitted to Solar Physics for Topical Issue on Solar and Stellar Flares
Last Modified: 2015-01-12 11:58
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The Radiated Energy Budget Of Chromospheric Plasma In A Major Solar Flare Deduced From Multi-Wavelength Observations  

Ryan Milligan   Submitted: 2014-06-23 02:34

This paper presents measurements of the energy radiated by the lowersolar atmosphere, at optical, UV, and EUV wavelengths, during anX-class solar flare (SOL2011-02-15T01:56) in response to an injectionof energy assumed to be in the form of nonthermal electrons. HardX-ray observations from RHESSI were used to track the evolution of theparameters of the nonthermal electron distribution to reveal the totalpower contained in flare accelerated electrons. By integrating overthe duration of the impulsive phase, the total energy contained in thenonthermal electrons was found to be >2x1031 erg. The response of thelower solar atmosphere was measured in the free-bound EUV continua ofH I (Lyman), He I, and He II, plus the emission lines of He II at 304Aand H I (Lya) at1216A by SDO/EVE, the UV continua at 1600A and 1700A by SDO/AIA, andthe WL continuum at 4504A, 5550A, and 6684A, along with the Ca II Hline at 3968A using Hinode/SOT. The summed energy detected by theseinstruments amounted to ~3x1030 erg; about 15% of the totalnonthermal energy. The Lya line was found to dominate the measuredradiative losses. Parameters of both the driving electron distributionand the resulting chromospheric response are presented in detail toencourage the numerical modelling of flare heating for this event, todetermine the depth of the solar atmosphere at which these line andcontinuum processes originate, and the mechanism(s) responsible fortheir generation.

Authors: Ryan O. Milligan, Graham S. Kerr, Brian R. Dennis, Hugh S. Hudson, Lyndsay Fletcher, Joel C. Allred, Phillip C. Chamberlin, Jack Ireland, Mihalis Mathioudakis, & Francis P. Kennan
Projects: Hinode/SOT,RHESSI,SDO-AIA,SDO-EVE

Publication Status: ApJ, Accepted
Last Modified: 2014-06-23 12:07
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Continuum Contributions to the SDO/AIA Passbands During Solar Flares  

Ryan Milligan   Submitted: 2013-08-16 08:40

Data from the Multiple EUV Grating Spectrograph (MEGS-A) component of the Extreme Ultraviolet Experiment (EVE) onboard the Solar Dynamics Observatory (SDO) were used to quantify the contribution of continuum emission to each of the EUV channels of the Atmospheric Imaging Assembly (AIA), also on SDO, during an X-class solar flare that occurred on 2011 February 15. Both the pre-flare subtracted EVE spectra and fits to the associated free-free continuum were convolved with the AIA response functions of the seven EUV passbands at 10 s cadence throughout the course of the flare. It was found that 10-25% of the total emission in the 94A, 131A, 193A, and 335A passbands throughout the main phase of the flare was due to free-free emission. Reliable measurements could not be made for the 171A channel, while the continuum contribution to the 304A channel was negligible due to the presence of the strong He II emission line. Up to 50% of the emission in the 211A channel was found to be due to free-free emission around the peak of the flare, while an additional 20% was due to the recombination continuum of He II. The analysis was extended to a number of M- and X-class flares and it was found that the level of free-free emission contributing to the 171A and 211A passbands increased with increasing GOES class. These results suggest that the amount of continuum emission that contributes to AIA observations during flares is more significant than that stated in previous studies which used synthetic, rather than observed, spectra. These findings highlight the importance of spectroscopic observations carried out in conjunction with those from imaging instruments so that the data are interpreted correctly.

Authors: Ryan O. Milligan & Sarah A. McElroy
Projects: SDO-AIA,SDO-EVE

Publication Status: ApJ (Accepted)
Last Modified: 2013-08-19 09:04
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Time-Dependent Density Diagnostics of Solar Flare Plasmas Using SDO/EVE  

Ryan Milligan   Submitted: 2012-07-05 10:02

Temporally-resolved electron density measurements of solar flare plasmas are presented using data from the EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory (SDO). The EVE spectral range contains emission lines formed between 104-107 K, including transitions from highly ionized iron (>10 MK). Using three density-sensitive Fe XXI ratios, peak electron densities of 1011.2-1012.1 cm-3 were found during four X-class flares. While previous measurements of densities at such high temperatures were made at only one point during a flaring event, EVE now allows the temporal evolution of these high-temperature densities to be determined at 10 s cadence. A comparison with GOES data revealed that the peak of the density time profiles for each line ratio correlated well with that of the emission measure time profile for each of the events studied.

Authors: Ryan O. Milligan, Michael B. Kennedy, Mihalis Mathioudakis, Francis P. Keenan
Projects: SDO-EVE

Publication Status: ApJL, accepted
Last Modified: 2012-07-07 14:50
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Observations of Enhanced EUV Continua During An X-class Solar Flare Using SDO/EVE  

Ryan Milligan   Submitted: 2012-01-25 03:19

Observations of extreme-ultraviolet (EUV) emission from an X-classsolar flare that occurred on2011 February 15 at 01:44 UT are presented, obtained using the EUVVariability Experiment (EVE)onboard the Solar Dynamics Observatory. The complete EVE spectralrange covers the free-boundcontinua of H I (Lyman continuum), He I, and He II, with recombinationedges at 91.2, 50.4, and22.8 nm, respectively. By fitting the wavelength ranges blue-ward ofeach recombination edge with anexponential function, lightcurves of each of the integrated continuawere generated over the course ofthe flare, as well as emission from the free-free continuum(6.5-3 7nm). The He II 30.4 nm and Lyman- α 121.6 nm lines, and soft X-ray (0.1-0.8 nm) emission from GOES are also included for comparison. Eachfree-bound continuum was found to have a rapid rise phase at the flareonset similar to that seenin the 25-50 keV lightcurves from RHESSI, suggesting that they wereformed by recombination withfree electrons in the chromosphere. However, the free-free emissionexhibited a slower rise phase seenalso in the soft X-ray emission from GOES, implying a predominantlycoronal origin. By integratingover the entire flare the total energy emitted via each process wasdetermined. We find that the flareenergy in the EVE spectral range amounts to at most a few per cent ofthe total flare energy, butEVE gives us a first comprehensive look at these diagnosticallyimportant continuum components.

Authors: Ryan O. Milligan, Phillip C. Chamberlin, Hugh S. Hudson, Thomas N. Woods, Mihalis Mathioudakis, Lyndsay Fletcher, Adam F. Kowalski, Francis P. Keenan
Projects: RHESSI,SDO-EVE

Publication Status: ApJL (Accepted)
Last Modified: 2012-01-25 08:42
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Spatially-Resolved Nonthermal Line Broadening During the Impulsive Phase of a Solar Flare  

Ryan Milligan   Submitted: 2011-07-21 10:56

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Ryan O. Milligan
Projects: Hinode/EIS

Publication Status: Accepted to The Astrophysics Journal
Last Modified: 2012-01-24 13:32
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Evidence of a Plasmoid-Looptop Interaction and Magnetic Inflows During a Solar Flare/CME Eruptive Event  

Ryan Milligan   Submitted: 2010-03-02 09:03

Observational evidence is presented for the merging of a downward-propagating plasmoid with a looptop kernel during an occulted limb event on 2007 January 25. RHESSI lightcurves in the 9?18 keV energy range, as well as that of the 245 MHz channel of the Learmonth Solar Observatory, show enhanced nonthermal emission in the corona at the time of the merging suggesting that additional particle acceleration took place. This was attributed to a secondary episode of reconnection in the current sheet that formed between the two merging sources. RHESSI images were used to establish a mean downward velocity of the plasmoid of 12 km s-1. Complementary observations from the SECCHI suite of instruments onboard STEREO-Behind showed that this process occurred during the acceleration phase of the associated CME. From wavelet-enhanced EUVI, images evidence of inflowing magnetic field lines prior to the CME eruption is also presented. The derived inflow velocity was found to be 1.5 km s-1. This combination of observations supports a recent numerical simulation of plasmoid formation, propagation and subsequent particle acceleration due to the tearing mode instability during current sheet formation.

Authors: Ryan O. Milligan, R. T. James McAteer, Brian R. Dennis, C. Alex Young
Projects: RHESSI,STEREO

Publication Status: ApJ (Accepted)
Last Modified: 2010-03-02 09:25
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Evidence of a Plasmoid-Looptop Interaction and Magnetic Inflows During a Solar Flare/CME Eruptive Event  

Ryan Milligan   Submitted: 2010-03-02 09:03

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Ryan O. Milligan, R. T. James McAteer, Brian R. Dennis, C. Alex Young
Projects: RHESSI,STEREO

Publication Status: ApJ (Accepted)
Last Modified: 2012-01-24 13:31
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Velocity Characteristics of Evaporated Plasma Using Hinode/EIS  

Ryan Milligan   Submitted: 2009-02-25 09:58

This paper presents a detailed study of chromospheric evaporation using the EUV Imaging Spectrometer (EIS) onboard Hinode in conjunction with HXR observations from RHESSI. The advanced capabilities of EIS were used to measure Doppler shifts in 15 emission lines covering the temperature range T = 0.05-16MK during the impulsive phase of a C-class flare on 2007 December 14. Blueshifts indicative of the evaporated material were observed in six emission lines from Fe XIV-XXIV (2-16 MK). Upflow velocity was found to scale with temperature as v_up (km s-1) ≈ 5-17 T (MK). Although the hottest emission lines, Fe XXIII and Fe XXIV, exhibited upflows of >200 km s-1, their line profiles were found to be dominated by a stationary component in stark contrast to the predictions of the standard flare model. Emission from O VI-Fe XIII lines (0.5-1.5 MK) was found to be redshifted by v_down (km s-1) ≈ 60-17 T (MK) and was interpreted as the downward-moving ''plug'' characteristic of explosive evaporation. These downflows occur at temperatures significantly higher than previously expected. Both upflows and downflows were spatially and temporally correlated with HXR emission observed by RHESSI that provided the properties of the electron beam deemed to be the driver of the evaporation. The energy contained in the electron beam was found to be >1011 ergs/cm2/s consistent with the value required to drive explosive chromospheric evaporation from hydrodynamic simulations.

Authors: Ryan O. Milligan, Brian R. Dennis
Projects: Hinode/EIS,RHESSI

Publication Status: ApJ (accepted)
Last Modified: 2009-04-29 04:11
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Velocity Characteristics of Evaporated Plasma Using Hinode/EIS  

Ryan Milligan   Submitted: 2009-02-25 09:58

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Ryan O. Milligan, Brian R. Dennis
Projects: Hinode/EIS,RHESSI

Publication Status: ApJ (accepted)
Last Modified: 2012-01-24 13:33
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A Hot Microflare Observed with RHESSI and Hinode  

Ryan Milligan   Submitted: 2008-05-05 09:23

RHESSI and Hinode observations of a GOES B-class flare are combined to investigate the origin of 15 MK plasma. The absence of any detectable hard X-ray emission coupled with weak blueshifted emission lines (indicating upward velocities averaging only 14 km s-1) suggests that this was a result of direct heating in the corona, as opposed to nonthermal electron precipitation causing chromospheric evaporation. These findings are in agreement with a recent hydrodynamical simulation of microflare plasmas which found that higher temperatures can be attained when less energy is used to accelerate electrons out of the thermal distribution. In addition, unusual redshifts in the 2 MK Fe XV line (indicating downward velocities of ∼14 km s-1) were observed cospatial with one of the flare ribbons during the event. Downflows of such high temperature plasma are not predicted by any common flare model.

Authors: Ryan O. Milligan
Projects: Hinode/EIS,Hinode/SOT,Hinode/XRT,RHESSI

Publication Status: ApJL, accepted
Last Modified: 2008-09-23 21:02
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Subject will be restored when possible  

Ryan Milligan   Submitted: 2008-05-05 09:22

RHESSI and Hinode observations of a GOES B-class flare are combined to investigate the origin of 15 MK plasma. The absence of any detectable hard X-ray emission coupled with weak blueshifted emission lines (indicating upward velocities averaging only 14 km s-1) suggests that this was a result of direct heating in the corona, as opposed to nonthermal electron precipitation causing chromospheric evaporation. These findings are in agreement with a recent hydrodynamical simulation of microflare plasmas which found that higher temperatures can be attained when less energy is used to accelerate electrons out of the thermal distribution. In addition, unusual redshifts in the 2 MK Fe XV line (indicating downward velocities of ∼14 km s-1) were observed cospatial with one of the flare ribbons during the event. Downflows of such high temperature plasma are not predicted by any common flare model.

Authors: Ryan O. Milligan
Projects: Hinode/EIS,Hinode/SOT,Hinode/XRT,RHESSI

Publication Status: ApJL, accepted
Last Modified: 2008-09-23 21:02
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Observational Evidence for Gentle Chromospheric Evaporation During the Impulsive Phase of a Solar Flare  

Ryan Milligan   Submitted: 2006-03-23 05:52

Observational evidence for gentle chromospheric evaporation during the impulsive phase of a C9.1 solar flare is presented using data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory. Until now, evidence for gentle evaporation has only been reported during the decay phase of a solar flare, where thermal conduction is thought to be the driving mechanism. Here we show that the chromospheric response to a low flux of nonthermal electrons (>=5x109 ergs cm-2 s-1) results in plasma upflows of 13±16, 16±18, and 110±58 km s-1 in the cool He I and O V emission lines and the 8 MK Fe XIX line. These findings, in conjunction with other recently reported work, now confirm that the dynamic response of the solar atmosphere is sensitively dependent on the flux of incident electrons.

Authors: Ryan O. Milligan, Peter T. Gallagher, Mihalis Mathioudakis, Francis P. Keenan
Projects: RHESSI

Publication Status: Published (2006, ApJL, 642, 169)
Last Modified: 2006-05-28 03:38
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RHESSI and SOHO/CDS Observations of Explosive Chromospheric Evaporation  

Ryan Milligan   Submitted: 2005-09-22 02:36

Simultaneous observations of explosive chromospheric evaporation are presented using data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) onboard SOHO. For the first time, co-spatial imaging and spectroscopy have been used to observe explosive evaporation within a hard X-ray emitting region. RHESSI X-ray images and spectra were used to determine the flux of non-thermal electrons accelerated during the impulsive phase of an M2.2 flare. Assuming a thick-target model, the injected electron spectrum was found to have a spectral index of 7.3, a low energy cut-off of 20 keV, and a resulting flux of >4x1010 ergs cm-2 s-1. The dynamic response of the atmosphere was determined using CDS spectra, finding a mean upflow velocity of 230±38 km s-1 in Fe XIX (592.23A), and associated downflows of 36±16 km s-1 and 43±22 km s-1 at chromospheric and transition region temperatures, respectively, relative to an averaged quiet-Sun spectra. The errors represent a 1 sigma dispersion. The properties of the accelerated electron spectrum and the corresponding evaporative velocities were found to be consistent with the predictions of theory.

Authors: R. O. Milligan, P. T. Gallagher, M. Mathioudakis, D. S. Bloomfield, F. P. Keenan, R. A. Schwartz
Projects: RHESSI

Publication Status: Published (2006, ApJL, 638, 117)
Last Modified: 2006-05-28 03:41
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Plasma Diagnostics of Active Region Evolution and Implications for Coronal Heating  

Ryan Milligan   Submitted: 2005-09-02 03:38

A detailed study is presented of the decaying solar active region NOAA 10103 observed with the Coronal Diagnostic Spectrometer (CDS), the Michelson Doppler Imager (MDI) and the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). Electron density maps formed using Si X (356.03A/347.41A) show that the density varies from 1010 cm-3 in the active region core, to 7x108 cm-3 at the region boundaries. Over the five days of observations, the average electron density fell by 30%. Temperature maps formed using Fe XVI (335.41A)/Fe XIV (334.18A) show electron temperatures of 2.34x106 K in the active region core, and 2.10x106 K at the region boundaries. Similarly to the electron density, there was a small decrease in the average electron temperature over the five day period. The radiative, conductive, and mass flow losses were calculated and used to determine the resultant heating rate (P_H). Radiative losses were found to dominate the active region cooling process. As the region decayed, the heating rate decreased by almost a factor of five between the first and last day of observations. The heating rate was then compared to the total unsigned magnetic (Phi_tot) flux, yielding a power-law of the form P_H ~ Phi_tot0.81pm0.32. This result suggests that waves rather than nanoflares may be the dominant heating mechanism in this active region.

Authors: R. O. Milligan, P. T. Gallagher, M. Mathioudakis, F. P. Keenan, D. S. Bloomfield
Projects:

Publication Status: Published - (2005, MNRAS, 363, 259)
Last Modified: 2006-05-28 03:40
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Abstracts by Author
Lyman Continuum Observations of Solar Flares Using SDO/EVE
Detection of 3-Minute Oscillations in Full-disk Lya Emission During A Solar Flare
On the Performance of Multi-Instrument Solar Flare Observations During Solar Cycle 24
The Anomalous Temporal Behaviour of Broadband Lyman-alpha Emission During Flares From SDO/EVE
The Temporal Behaviour of Lyman-alpha Emission During Flares From SDO/EVE
Extreme Ultra-Violet Spectroscopy of the Flaring Solar Chromosphere
The Radiated Energy Budget Of Chromospheric Plasma In A Major Solar Flare Deduced From Multi-Wavelength Observations
Continuum Contributions to the SDO/AIA Passbands During Solar Flares
Time-Dependent Density Diagnostics of Solar Flare Plasmas Using SDO/EVE
Observations of Enhanced EUV Continua During An X-class Solar Flare Using SDO/EVE
Spatially-Resolved Nonthermal Line Broadening During the Impulsive Phase of a Solar Flare
Evidence of a Plasmoid-Looptop Interaction and Magnetic Inflows During a Solar Flare/CME Eruptive Event
Evidence of a Plasmoid-Looptop Interaction and Magnetic Inflows During a Solar Flare/CME Eruptive Event
Velocity Characteristics of Evaporated Plasma Using Hinode/EIS
Velocity Characteristics of Evaporated Plasma Using Hinode/EIS
A Hot Microflare Observed with RHESSI and Hinode
Subject will be restored when possible
Observational Evidence for Gentle Chromospheric Evaporation During the Impulsive Phase of a Solar Flare
RHESSI and SOHO/CDS Observations of Explosive Chromospheric Evaporation
Plasma Diagnostics of Active Region Evolution and Implications for Coronal Heating

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Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

Archive Maintainer
Alisdair Davey



© 2003 Solar Physics Group - Montana State University