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Modeling Coronal Response in Decaying Active Regions with Magnetic Flux Transport and Steady Heating  

Ignacio Ugarte-Urra   Submitted: 2017-08-16 13:04

We present new measurements of the dependence of the Extreme Ultraviolet radiance on the total magnetic flux in active regions as obtained from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO). Using observations of nine active regions tracked along different stages of evolution, we extend the known radiance - magnetic flux power-law relationship (I∝Φ α ) to the AIA 335 Å passband, and the Fe XVIII 93.93 Å spectral line in the 94 Å passband. We find that the total unsigned magnetic flux divided by the polarity separation (Φ/D) is a better indicator of radiance for the Fe XVIII line with a slope of α =3.22±0.03. We then use these results to test our current understanding of magnetic flux evolution and coronal heating. We use magnetograms from the simulated decay of these active regions produced by the Advective Flux Transport (AFT) model as boundary conditions for potential extrapolations of the magnetic field in the corona. We then model the hydrodynamics of each individual field line with the Enthalpy-based Thermal Evolution of Loops (EBTEL) model with steady heating scaled as the ratio of the average field strength and the length (B/L) and render the Fe XVIII and 335 Å emission. We find that steady heating is able to partially reproduce the magnitudes and slopes of the EUV radiance - magnetic flux relationships and discuss how impulsive heating can help reconcile the discrepancies. This study demonstrates that combined models of magnetic flux transport, magnetic topology and heating can yield realistic estimates for the decay of active region radiances with time.

Authors: Ignacio Ugarte-Urra, Harry P. Warren, Lisa A. Upton, Peter R. Young
Projects: SDO-AIA,SDO-HMI

Publication Status: ApJ (in press)
Last Modified: 2017-08-23 12:50
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Determining heating time scales in solar active region cores from AIA/SDO Fe XVIII images  

Ignacio Ugarte-Urra   Submitted: 2014-01-15 14:23

We present a study of the frequency of transient brightenings in the core of solar active regions as observed in the Fe XVIII line component of AIA/SDO 94 Å filter images. The Fe XVIII emission is isolated using an empirical correction to remove the contribution of ''warm'' emission to this channel. Comparing with simultaneous observations from EIS/Hinode, we find that the variability observed in Fe XVIII is strongly correlated with the emission from lines formed at similar temperatures. We examine the evolution of loops in the cores of active regions at various stages of evolution. Using a newly developed event detection algorithm we characterize the distribution of event frequency, duration, and magnitude in these active regions. These distributions are similar for regions of similar age and show a consistent pattern as the regions age. This suggests that these characteristics are important constraints for models of solar active regions. We find that the typical frequency of the intensity fluctuations is about 1400s for any given line-of-sight, i.e. about 2-3 events per hour. Using the EBTEL 0D hydrodynamic model, however, we show that this only sets a lower limit on the heating frequency along that line-of-sight.

Authors: Ignacio Ugarte-Urra, Harry P. Warren
Projects: Hinode/EIS,SDO-AIA

Publication Status: ApJ (in press)
Last Modified: 2014-01-18 17:12
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Active region transition region loop populations and their relationship to the corona  

Ignacio Ugarte-Urra   Submitted: 2009-01-09 06:52

The relationships among coronal loop structures at different temperatures is not settled. Previous studies have suggested that coronal loops in the core of an active region are not seen cooling through lower temperatures and therefore are steadily heated. If loops were cooling, the transition region would be an ideal temperature regime to look for a signature of their evolution. The Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode provides monochromatic images of the solar transition region and corona at an unprecedented cadence and spatial resolution, making it an ideal instrument to shed light on this issue. Analysis of observations of active region 10978 taken in 2007 December 8 - 19 indicates that there are two dominant loop populations in the active region: core multi-temperature loops that undergo a continuous process of heating and cooling in the full observed temperature range 0.4-2.5 MK and even higher as shown by the X-Ray Telescope (XRT); and peripheral loops which evolve mostly in the temperature range 0.4-1.3 MK. Loops at transition region temperatures can reach heights of 150 Mm in the corona above the limb and develop downflows with velocities in the range of 39-105 km s-1.

Authors: Ignacio Ugarte-Urra, Harry P. Warren, David H. Brooks
Projects: Hinode/EIS

Publication Status: ApJ, in press
Last Modified: 2009-01-09 09:37
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Subject will be restored when possible  

Ignacio Ugarte-Urra   Submitted: 2007-12-10 09:29

Recent observations of coronal hole areas with the XRT and EIS instruments onboard the Hinode satellite have shown with unprecedented detail the launching of fast, hot jets away from the solar surface. In some cases these events coincide with episodes of flux emergence from beneath the photosphere. In this letter we show results of a 3D numerical experiment of flux emergence from the solar interior into a coronal hole and compare them with simultaneous XRT and EIS observations of a jet-launching event that accompanied the appearance of a bipolar region in MDI magnetograms. The magnetic skeleton and topology that result in the experiment bear a strong resemblance to linear force-fee extrapolations of the SOHO/MDI magnetograms. A thin current sheet is formed at the boundary of the emerging plasma. A jet is launched upward along the open reconnected field lines with values of temperature, density and velocity in agreement with the XRT and EIS observations. Below the jet, a split-vault structure results with two chambers: a shrinking one containing the emerged field loops and a growing one with loops produced by the reconnection. The ongoing reconnection leads to a horizontal drift of the vault-and-jet structure. The timescales, velocities, and other plasma properties in the experiment are consistent with recent statistical studies of this type of events made with Hinode data.

Authors: F. Moreno-Insertis, K. Galsgaard, I. Ugarte-Urra
Projects: None

Publication Status: ApJL, in press.
Last Modified: 2007-12-10 11:17
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The Magnetic Topology of Coronal Mass Ejection sources  

Ignacio Ugarte-Urra   Submitted: 2007-03-05 07:36

In an attempt to test current initiation models of coronal mass ejections (CMEs), with an emphasis on the magnetic breakout model, we inspect the magnetic topology of the sources of 26 CME events in the context of their chromospheric and coronal response in an interval of approximately nine hours around the eruption onset. First, we perform current-free (potential) extrapolations of photospheric magnetograms to retrieve the key topological ingredients, such as coronal magnetic null points. Then we compare the reconnection signatures observed in the high cadence and high spatial resolution of the Transition Region And Coronal Explorer (TRACE) images with the location of the relevant topological features. The comparison reveals that only seven events can be interpreted in terms of the breakout model, which requires a multi-polar topology with pre-eruption reconnection at a coronal null. We find, however, that a larger number of events (twelve) can not be interpreted in those terms. No magnetic null is found in six of them. Seven other cases remain difficult to interpret. We also show that there are no systematic differences between the CME speed and flare energies of events under different interpretations.

Authors: I. Ugarte-Urra, H.P. Warren, A.R. Winebarger
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2007-03-05 08:40
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An Investigation into the Variability of Heating in a Solar Active Region  

Ignacio Ugarte-Urra   Submitted: 2006-06-01 14:30

Previous studies have indicated that both steady and impulsive heating mechanisms play a role in active region heating. In this paper, we present a study of 20 hours of soft X-ray and EUV observations of solar active region NOAA AR 8731. We examine the evolution of six representative loop structures that brighten and fade first from X-ray images and subsequently from the EUV images. We determine their lifetime and the delay between their appearance in the different filters. We find that the lifetime in the EUV filters is much longer than expected for a single cooling loop. We also notice that the delay in the loops' appearance in the X-ray and EUV filters is proportional to the loop length. We model one of the loops using a hydrodynamic model with both impulsive and quasi-steady heating functions and find that neither of these simple heating functions can well reproduce the observed loop characteristics in both the X-ray and EUV images. Hence, although this active region is dominated by variable emission and the characteristics of the observed loops are qualitatively consistent with a cooling loop, the timescale of the heating in this active region remains unknown

Authors: Ignacio Ugarte-Urra, Amy R. Winebarger, and Harry P. Warren
Projects: None

Publication Status: Astrophysical Journal, 643:1245-1257, 2006
Last Modified: 2006-06-01 20:42
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CDS wide slit time-series of EUV coronal bright points  

Ignacio Ugarte-Urra   Submitted: 2004-06-16 17:30

Wide slit (90''x240'') movies of four Extreme Ultraviolet coronal bright points (BPs) obtained with the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SoHO) have been inspected. The wavelet analysis of the He I 584.34~AA, O V 629.73~AA and Mg VII/IX 368~AA time-series confirms the oscillating nature of the BPs, with periods ranging between 600 and 1100 seconds. In one case we detect periods as short as 236 seconds. We suggest that these oscillations are the same as those seen in the chromospheric network and that a fraction of the network bright points are most likely the cool footpoints of the loops comprising coronal bright points. These oscillations are interpreted in terms of global acoustic modes of the closed magnetic structures associated with BPs.

Authors: I. Ugarte-Urra, J.G. Doyle, V.M. Nakariakov and C.R. Foley
Projects: None

Publication Status: Astronomy and Astrophysics, accepted (2004)
Last Modified: 2004-06-16 17:30
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Signature of oscillations in coronal bright points  

Ignacio Ugarte-Urra   Submitted: 2004-06-16 17:25

A detailed study of two consecutive bright points observed simultaneously with the Coronal Diagnostic Spectrometer (CDS), the Extreme ultraviolet Imaging Telescope (EIT) and the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO) is presented. The analysis of the evolution of the photospheric magnetic features and their coronal counterpart shows that there is a linear dependence between the EIT Fe XII 195~AA flux and the total magnetic flux of the photospheric bipolarity. The appearance of the coronal emission is associated with the emergence of new magnetic flux and the disappearance of coronal emission is associated with the cancellation of one of the polarities. In one of the cases the disappearance takes place ~3-4 hours before the full cancellation of the weakest polarity. The spectral data obtained with CDS show that one of the bright points experienced short time variations in the flux on a time scale of 420-650 seconds, correlated in the transition region lines (O V 629.73~AA and O III 599.60~AA) and also the He I 584.34 Å line. The coronal line (Mg IX 368.07~AA) undergoes changes as well, but on a longer scale. The wavelet analysis of the temporal series reveals that many of these events appear in a random fashion and sometimes after periods of quietness. However, we have found two cases of an oscillatory behaviour. A sub-section of the O V temporal series of the second bright point shows a damped oscillation of five cycles peaking in the wavelet spectrum at 546 seconds, but showing in the latter few cycles a lengthening of that period. The period compares well with that detected in the S VI 933.40 Å oscillations seen in another bright point observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer, which has a period of 491 seconds. The derived electron density in the transition region was 3x1010 cm-3 with some small variability, while the coronal electron density was 5x108 cm-3.

Authors: I. Ugarte-Urra, J. G. Doyle, M. S. Madjarska and E. O'Shea
Projects: None

Publication Status: Astronomy and Astrophysics, v.418, p.313-324 (2004)
Last Modified: 2004-06-16 17:25
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Abstracts by Author
Modeling Coronal Response in Decaying Active Regions with Magnetic Flux Transport and Steady Heating
Determining heating time scales in solar active region cores from AIA/SDO Fe XVIII images
Active region transition region loop populations and their relationship to the corona
Subject will be restored when possible
The Magnetic Topology of Coronal Mass Ejection sources
An Investigation into the Variability of Heating in a Solar Active Region
CDS wide slit time-series of EUV coronal bright points
Signature of oscillations in coronal bright points

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