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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 10^4-10^7 K, including transitions from highly ionized iron (>10 MK). Using three density-sensitive Fe XXI ratios, peak electron densities of 10^(11.2)-10^(12.1) 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-class solar flare that occurred on 2011 February 15 at 01:44 UT are presented, obtained using the EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory. The complete EVE spectral range covers the free-bound continua of H I (Lyman continuum), He I, and He II, with recombination edges at 91.2, 50.4, and 22.8 nm, respectively. By fitting the wavelength ranges blue-ward of each recombination edge with an exponential function, lightcurves of each of the integrated continua were generated over the course of the 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. Each free-bound continuum was found to have a rapid rise phase at the flare onset similar to that seen in the 25-50 keV lightcurves from RHESSI, suggesting that they were formed by recombination with free electrons in the chromosphere. However, the free-free emission exhibited a slower rise phase seen also in the soft X-ray emission from GOES, implying a predominantly coronal origin. By integrating over the entire flare the total energy emitted via each process was determined. We find that the flare energy in the EVE spectral range amounts to at most a few per cent of the total flare energy, but EVE gives us a first comprehensive look at these diagnostically important 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. 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. 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) ≈ 5−17 T (MK). Although the hottest emission lines, Fe XXIII and Fe XXIV, exhibited upflows of >200 km/s, 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) ≈ 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 >10^11 ergs/cm^2/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) 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) 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) 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) 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 (>=5x10^9 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 >4x10^10 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 10^10 cm^-3 in the active region core, to 7x10^8 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 2.34x10^6 K in the active region core, and 2.10x10^6 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_tot^(0.81pm0.32). 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
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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MSU Solar Physics.
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