arXiv subjects

Amy R. Winebarger

Publications and source records attributed to Amy R. Winebarger.

41 records · Page 3Linked to original sources

arXiv2010

Evidence for Steady Heating: Observations of an Active Region Core with Hinode and TRACE

Previous observations have not been able to exclude the possibility that high temperature active region loops are actually composed of many small scale threads that are in various stages of heating and cooling and only appear to be in equilibrium. With new observations from the EUV Imaging Spectrometer (EIS) and X-ray Telescope (XRT) on \textit{Hinode} we have the ability to investigate the properties of high temperature coronal plasma in extraordinary detail. We examine the emission in the core of an active region and find three independent lines of evidence for steady heating. We find that the emission observed in XRT is generally steady for hours, with a fluctuation level of approximately 15% in an individual pixel. Short-lived impulsive heating events are observed, but they appear to be unrelated to the steady emission that dominates the active region. Furthermore, we find no evidence for warm emission that is spatially correlated with the hot emission, as would be expected if the high temperature loops are the result of impulsive heating. Finally, we also find that intensities in the "moss", the footpoints of high temperature loops, are consistent with steady heating models provided that we account for the local expansion of the loop from the base of the transition region to the corona. In combination, these results provide strong evidence that the heating in the core of an active region is effectively steady, that is, the time between heating events is short relative to the relevant radiative and conductive cooling times.

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arXiv2007

Observation and Modeling of Coronal "Moss" With the EUV Imaging Spectrometer on Hinode

Observations of transition region emission in solar active regions represent a powerful tool for determining the properties of hot coronal loops. In this Letter we present the analysis of new observations of active region moss taken with the Extreme Ultraviolet Imaging Spectrometer (EIS) on the \textit{Hinode} mission. We find that the intensities predicted by steady, uniformly heated loop models are too intense relative to the observations, consistent with previous work. To bring the model into agreement with the observations a filling factor of about 16% is required. Furthermore, our analysis indicates that the filling factor in the moss is nonuniform and varies inversely with the loop pressure.

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arXiv2006

Static and Dynamic Modeling of a Solar Active Region. I: Soft X-Ray Emission

Recent simulations of solar active regions have shown that it is possible to reproduce both the total intensity and the general morphology of the high temperature emission observed at soft X-ray wavelengths using static heating models. There is ample observational evidence, however, that the solar corona is highly variable, indicating a significant role for dynamical processes in coronal heating. Because they are computationally demanding, full hydrodynamic simulations of solar active regions have not been considered previously. In this paper we make first application of an impulsive heating model to the simulation of an entire active region, AR8156 observed on 1998 February 16. We model this region by coupling potential field extrapolations to full solutions of the time-dependent hydrodynamic loop equations. To make the problem more tractable we begin with a static heating model that reproduces the emission observed in 4 different \textit{Yohkoh} Soft X-Ray Telescope (SXT) filters and consider dynamical heating scenarios that yield time-averaged SXT intensities that are consistent with the static case. We find that it is possible to reproduce the total observed soft X-ray emission in all of the SXT filters with a dynamical heating model, indicating that nanoflare heating is consistent with the observational properties of the high temperature solar corona.

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arXiv2006

Hydrostatic Modeling of the Integrated Soft X-Ray and EUV Emission in Solar Active Regions

Many studies of the solar corona have shown that the observed X-ray luminosity is well correlated with the total unsigned magnetic flux. In this paper we present results from the extensive numerical modeling of active regions observed with the \textit{Solar and Heliospheric Observatory} (\textit{SOHO}) Extreme Ultraviolet Telescope (EIT), the \textit{Yohkoh} Soft X-ray Telescope (SXT), and the \textit{SOHO} Michelson Doppler Imager (MDI). We use potential field extrapolations to compute magnetic field lines and populate these field lines with solutions to the hydrostatic loop equations assuming steady, uniform heating. Our volumetric heating rates are of the form $ε_H\sim \bar{B}^α/L^β$, where $\bar{B}$ is the magnetic field strength averaged along a field line and $L$ is the loop length. Comparisons between the observed and simulated emission for 26 active regions suggest that coronal heating models that scale as $ε_H\sim \bar{B}/L$ are in the closest argreement the observed emission at high temperatures. The field-braiding reconnection model of Parker, for example, is consistent with our results. We find, however, that the integrated intensities alone are insufficent to uniquely determine the parameterization of the volumetric heating rate. Visualizations of the emission are also needed. We also find that there are significant discrepancies between our simulation results and the lower temperature emission observed in the EIT channels.

astro-ph↗

arXiv2005

Cooling Active Region Loops Observed With SXT and TRACE

An Impulsive Heating Multiple Strand (IHMS) Model is able to reproduce the observational characteristics of EUV (~ 1 MK) active region loops. This model implies that some of the loops must reach temperatures where X-ray filters are sensitive (> 2.5 MK) before they cool to EUV temperatures. Hence, some bright EUV loops must be preceded by bright X-ray loops. Previous analysis of X-ray and EUV active region observations, however, have concluded that EUV loops are not preceded by X-ray loops. In this paper, we examine two active regions observed in both X-ray and EUV filters and analyze the evolution of five loops over several hours. These loops first appear bright in the X-ray images and later appear bright in the EUV images. The delay between the appearance of the loops in the X-ray and EUV filters is as little as 1 hour and as much as 3 hours. All five loops appear as single ``monolithic'' structures in the X-ray images, but are resolved into many smaller structures in the (higher resolution) EUV images. The positions of the loops appear to shift during cooling implying that the magnetic field is changing as the loops evolve. There is no correlation between the brightness of the loop in the X-ray and EUV filters meaning a bright X-ray loop does not necessarily cool to a bright EUV loop and vice versa. The progression of the loops from X-ray images to EUV images and the observed substructure is qualitatively consistent with the IMHS model.

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