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Observations of Enhanced EUV Continua During An X-class Solar Flare Using SDO/EVE
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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
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Publication Status: ApJL (Accepted)
Last Modified: 2012-01-25 08:42
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Observational Evidence for Gentle Chromospheric Evaporation During the Impulsive Phase of a Solar Flare
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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
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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
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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
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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
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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:
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Publication Status: Published - (2005, MNRAS, 363, 259)
Last Modified: 2006-05-28 03:40
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