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A Diagnostic of Coronal Elemental Behavior during the Inverse FIP Effect in Solar Flares  

David Brooks   Submitted: 2018-08-20 20:32

The solar corona shows a distinctive pattern of elemental abundances that is different from that of the photosphere. Low first ionization potential (FIP) elements are enhanced by factors of several. A similar effect is seen in the atmospheres of some solar-like stars, while late-type M stars show an inverse FIP effect. This inverse effect was recently detected on the Sun during solar flares, potentially allowing a very detailed look at the spatial and temporal behavior that is not possible from stellar observations. A key question for interpreting these measurements is whether both effects act solely on low-FIP elements (a true inverse effect predicted by some models), or whether the inverse FIP effect arises because high-FIP elements are enhanced. Here we develop a new diagnostic that can discriminate between the two scenarios, based on modeling of the radiated power loss, and apply the models to a numerical hydrodynamic simulation of coronal loop cooling. We show that when low-/high-FIP elements are depleted/enhanced, there is a significant difference in the cooling lifetime of loops that is greatest at lower temperatures. We apply this diagnostic to a post X1.8 flare loop arcade and inverse FIP region, and show that for this event, low-FIP elements are depleted. We discuss the results in the context of stellar observations, and models of the FIP and inverse FIP effect. We also provide the radiated power-loss functions for the two inverse FIP effect scenarios in machine readable form to facilitate further modeling.

Authors: David H. Brooks
Projects: SDO-AIA

Publication Status: Published in Brooks (2018) ApJ, 863, 140
Last Modified: 2018-08-22 12:07
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Solar Cycle Observations of the Neon Abundance in the Sun-as-a-star  

David Brooks   Submitted: 2018-07-01 20:05

Properties of the Sun's interior can be determined accurately from helioseismological measurements of solar oscillations. These measurements, however, are in conflict with photospheric elemental abundances derived using 3D hydrodynamic models of the solar atmosphere. This divergence of theory and helioseismology is known as the "solar modeling problem." One possible solution is that the photospheric neon abundance, which is deduced indirectly by combining the coronal Ne/O ratio with the photospheric O abundance, is larger than generally accepted. There is some support for this idea from observations of cool stars. The Ne/O abundance ratio has also been found to vary with the solar cycle in the slowest solar wind streams and coronal streamers, and the variation from solar maximum to minimum in streamers (~0.1-0.25) is large enough to potentially bring some of the solar models into agreement with the seismic data. Here we use daily sampled observations from the EUV Variability Experiment on the Solar Dynamics Observatory taken in 2010-2014, to investigate whether the coronal Ne/O abundance ratio shows a variation with the solar cycle when the Sun is viewed as a star. We find only a weak dependence on, and moderate anti-correlation with, the solar cycle with the ratio measured around 0.2-0.3 MK falling from 0.17 at solar minimum to 0.11 at solar maximum. The effect is amplified at higher temperatures (0.3-0.6 MK) with a stronger anti-correlation and the ratio falling from 0.16 at solar minimum to 0.08 at solar maximum. The values we find at solar minimum are too low to solve the solar modeling problem.

Authors: David H. Brooks, Deborah Baker, Lidia van Driel-Gesztelyi, and Harry P. Warren
Projects: SDO-EVE

Publication Status: Published in Brooks et al. (2018) ApJ, 861, 42.
Last Modified: 2018-07-02 10:52
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A Solar cycle correlation of coronal element abundances in Sun-as-a-star observations  

David Brooks   Submitted: 2017-08-30 20:26

The elemental composition in the coronae of low-activity solar-like stars appears to be related to fundamental stellar properties such as rotation, surface gravity, and spectral type. Here we use full-Sun observations from the Solar Dynamics Observatory, to show that when the Sun is observed as a star, the variation of coronal composition is highly correlated with a proxy for solar activity, the F10.7 cm radio flux, and therefore with the solar cycle phase. Similar cyclic variations should therefore be detectable spectroscopically in X-ray observations of solar analogs. The plasma composition in full-disk observations of the Sun is related to the evolution of coronal magnetic field activity. Our observations therefore introduce an uncertainty into the nature of any relationship between coronal composition and fixed stellar properties. The results highlight the importance of systematic full-cycle observations for understanding the elemental composition of solar-like stellar coronae.

Authors: David H. Brooks, Deborah Baker, Lidia van Driel-Gesztelyi, Harry P. Warren
Projects: SDO-EVE

Publication Status: Published in Nature Communications, 8, 183 (2017)
Last Modified: 2017-08-31 09:34
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Properties and Modeling of Unresolved Fine Structure Loops Observed in the Solar Transition Region by IRIS  

David Brooks   Submitted: 2016-09-02 00:57

Recent observations from the Interface Region Imaging Spectrograph (IRIS) have discovered a new class of numerous low-lying dynamic loop structures, and it has been argued that they are the long-postulated unresolved fine structures (UFSs) that dominate the emission of the solar transition region. In this letter, we combine IRIS measurements of the properties of a sample of 108 UFSs (intensities, lengths, widths, lifetimes) with one-dimensional non-equilibrium ionization simulations, using the HYDRAD hydrodynamic model to examine whether the UFSs are now truly spatially resolved in the sense of being individual structures rather than being composed of multiple magnetic threads. We find that a simulation of an impulsively heated single strand can reproduce most of the observed properties, suggesting that the UFSs may be resolved, and the distribution of UFS widths implies that they are structured on a spatial scale of 133 km on average. Spatial scales of a few hundred kilometers appear to be typical for a range of chromospheric and coronal structures, and we conjecture that this could be an important clue for understanding the coronal heating process.

Authors: Brooks, David H.; Reep, Jeffrey, W.; Warren, Harry P.
Projects: IRIS

Publication Status: ApJL, 826, L18 (2016)
Last Modified: 2016-09-07 12:13
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Measurements of non-thermal line widths in solar active regions  

David Brooks   Submitted: 2016-09-02 00:54

Spectral line widths are often observed to be larger than can be accounted for by thermal and instrumental broadening alone. This excess broadening is a key observational constraint for both nanoflare and wave dissipation models of coronal heating. Here we present a survey of non-thermal velocities measured in the high temperature loops (1-4 MK) often found in the cores of solar active regions. This survey of Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) observations covers 15 non-flaring active regions that span a wide range of solar conditions. We find relatively small non-thermal velocities, with a mean value of 17.6 ? 5.3 km s-1, and no significant trend with temperature or active region magnetic flux. These measurements appear to be inconsistent with those expected from reconnection jets in the corona, chromospheric evaporation induced by coronal nanoflares, and Alfvén wave turbulence models. Furthermore, because the observed non-thermal widths are generally small, such measurements are difficult and susceptible to systematic effects.

Authors: Brooks, David H.; Warren, Harry P.
Projects: Hinode/EIS

Publication Status: ApJ, 820, 63 (2016)
Last Modified: 2016-09-07 12:14
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Full-Sun observations for identifying the source of the slow solar wind  

David Brooks   Submitted: 2015-05-28 21:16

Fast (>700 km s-1) and slow (~400 km s-1) winds stream from the Sun, permeate the heliosphere and influence the near-Earth environment. While the fast wind is known to emanate primarily from polar coronal holes, the source of the slow wind remains unknown. Here we identify possible sites of origin using a slow solar wind source map of the entire Sun, which we construct from specially designed, full-disk observations from the Hinode satellite, and a magnetic field model. Our map provides a full-Sun observation that combines three key ingredients for identifying the sources: velocity, plasma composition and magnetic topology and shows them as solar wind composition plasma outflowing on open magnetic field lines. The area coverage of the identified sources is large enough that the sum of their mass contributions can explain a significant fraction of the mass loss rate of the solar wind.

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

Publication Status: Nature Communications, 6, 5947
Last Modified: 2015-05-30 04:31
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High Spatial Resolution Observations of Loops in the Solar Corona  

David Brooks   Submitted: 2013-07-16 21:30

Understanding how the solar corona is structured is of fundamental importance to determine how the Sun's upper atmosphere is heated to high temperatures. Recent spectroscopic studies have suggested that an instrument with a spatial resolution of 200 km or better is necessary to resolve coronal loops. The High Resolution Coronal Imager (Hi-C) achieved this performance on a rocket flight in 2012 July. We use Hi-C data to measure the Gaussian widths of 91 loops observed in the solar corona and find a distribution that peaks at about 270 km. We also use Atmospheric Imaging Assembly data for a subset of these loops and find temperature distributions that are generally very narrow. These observations provide further evidence that loops in the solar corona are often structured at a scale of several hundred kilometers, well above the spatial scale of many proposed physical mechanisms.

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

Publication Status: Published in ApJ, 722, L19
Last Modified: 2013-07-17 07:02
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Solar Coronal Loops Resolved by Hinode and SDO  

David Brooks   Submitted: 2012-06-06 20:29

Despite decades of studying the Sun, the coronal heating problem remains unsolved. One fundamental issue is that we do not know the spatial scale of the coronal heating mechanism. At a spatial resolution of 1000 km or more it is likely that most observations represent superpositions of multiple unresolved structures. In this letter, we use a combination of spectroscopic data from the Hinode EUV Imaging Spectrometer (EIS) and high resolution images from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory to determine the spatial scales of coronal loops. We use density measurements to construct multi-thread models of the observed loops and confirm these models using the higher spatial resolution imaging data. The results allow us to set constraints on the number of threads needed to reproduce a particular loop structure. We demonstrate that in several cases million degree loops are revealed to be single monolithic structures that are fully spatially resolved by current instruments. The majority of loops, however, must be composed of a number of finer, unresolved threads; but the models suggest that even for these loops the number of threads could be small, implying that they are also close to being resolved. These results challenge heating models of loops based on the reconnection of braided magnetic fields in the corona.

Authors: David H. Brooks, Harry P. Warren, Ignacio Ugarte-Urra
Projects: None

Publication Status: To be published in ApJ Letters
Last Modified: 2012-06-07 09:21
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Establishing a Connection Between Active Region Outflows and the Solar Wind: Abundance Measurements with EIS/Hinode  

David Brooks   Submitted: 2012-04-12 21:50

One of the most interesting discoveries from Hinode is the presence of persistent high-temperature high-speed outflows from the edges of active regions (ARs). EUV imaging spectrometer (EIS) measurements indicate that the outflows reach velocities of 50 km s?1 with spectral line asymmetries approaching 200 km s?1. It has been suggested that these outflows may lie on open field lines that connect to the heliosphere, and that they could potentially be a significant source of the slow speed solar wind. A direct link has been difficult to establish, however. We use EIS measurements of spectral line intensities that are sensitive to changes in the relative abundance of Si and S as a result of the first ionization potential (FIP) effect, to measure the chemical composition in the outflow regions of AR 10978 over a 5 day period in 2007 December. We find that Si is always enhanced over S by a factor of 3-4. This is generally consistent with the enhancement factor of low FIP elements measured in situ in the slow solar wind by non-spectroscopic methods. Plasma with a slow wind-like composition was therefore flowing from the edge of the AR for at least 5 days. Furthermore, on December 10 and 11, when the outflow from the western side was favorably oriented in the Earth direction, the Si/S ratio was found to match the value measured a few days later by the Advanced Composition Explorer/Solar Wind Ion Composition Spectrometer. These results provide strong observational evidence for a direct connection between the solar wind, and the coronal plasma in the outflow regions.

Authors: Brooks, David H., Warren, Harry P.
Projects: Hinode/EIS

Publication Status: Published in ApJ, 727, L13 (2011)
Last Modified: 2012-04-16 09:52
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Flows and Motions in Moss in the Core of a Flaring Active Region:Evidence for Steady Heating  

David Brooks   Submitted: 2011-04-12 00:27

We present new measurements of the time variability of intensity, Doppler, and nonthermal velocities in moss in an active region core observed by the EUV Imaging Spectrometer on Hinode in 2007 June. The measurements are derived from spectral profiles of the Fe XII 195 Å line. Using the 2'' slit, we repeatedly scanned 150'' by 150'' in a few minutes. This is the first time it has been possible to make such velocity measurements in the moss, and the data presented are the highest cadence spatially resolved maps of moss Doppler and nonthermal velocities ever obtained in the corona. The observed region produced numerous C- and M-class flares with several occurring in the core close to the moss. The magnetic field was therefore clearly changing in the active region core, so we ought to be able to detect dynamic signatures in the moss if they exist. Our measurements of moss intensities agree with previous studies in that a less than 15% variability is seen over a period of 16 hr. Our new measurements of Doppler and nonthermal velocities reveal no strong flows or motions in the moss, nor any significant variability in these quantities. The results confirm that moss at the bases of high temperature coronal loops is heated quasi-steadily. They also show that quasi-steady heating can contribute significantly even in the core of a flare productive active region. Such heating may be impulsive at high frequency, but if so it does not give rise to large flows or motions.


Authors: David H. Brooks, Harry P. Warren
Projects: Hinode/EIS

Publication Status: Published in ApJL, 703, L10
Last Modified: 2011-04-12 08:45
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The Role of Transient Brightenings in Heating the Solar Corona  

David Brooks   Submitted: 2008-12-02 21:58

Nanoflare reconnection events have been proposed as a mechanism for heating the corona. Parker's original suggestion was that frequent reconnection events occur in coronal loops due to the braiding of the magnetic field. Many observational studies, however, have focused on the properties of isolated transient brightenings unassociated with loops, but their cause, role, and relevance for coronal heating have not yet been established. Using Hinode SOT magnetograms and high-cadence EIS spectral data we study the relationship between chromospheric, transition region, and coronal emission and the evolution of the magnetic field. We find that hot, relatively steadily emitting coronal loops and isolated transient brightenings are both associated with magnetic flux regions that are highly dynamic. An essential difference, however, is that brightenings are typically found in regions of flux collision and cancellation whereas coronal loops are generally rooted in magnetic field regions that are locally unipolar with unmixed flux. This suggests that the type of heating (transient vs. steady) is related to the structure of the magnetic field, and that the heating in transient events may be fundamentally different than in coronal loops. This implies that they do not play an important role in heating the "quiescent" corona.

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

Publication Status: Published in ApJL, 689, L77
Last Modified: 2008-12-03 13:24
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Subject will be restored when possible  

David Brooks   Submitted: 2008-01-30 00:50

Theoretically, magnetic fields are expected to expand as they rise above the photosphere and into the corona, so the apparent uniform cross-sections of active region loops are difficult to understand. There has been some debate as to whether coronal loops really have constant cross-sections, or are actually unresolved and composed of expanding threads within the constant cross-section envelopes. Furthermore, loop expansion is critical to the success or failure of hydrostatic models in reproducing the intensities and morphology of observed emission. We analyze Hinode EIS observations of loops in active region 10953 and detect only moderate apex width expansion over a broad range of temperatures from log Te/K = 5.6 to 6.25. The expansion is less than required by steady-state heating models of coronal emission suggesting that such models will have difficulty reproducing both low and high temperature loop emission simultaneously. At higher temperatures (> log Te/K = 6.3) the apex widths increase substantially, but the emission at these temperatures likely comes from a combination of multiple loops. These observations demonstrate the advantage of EIS over previous instruments. For the first time, active region loops can be examined over a broad temperature range with high temperature fidelity and the same spatial resolution. The results therefore provide further clues to the coronal heating timescale and thus have implications for the direction of future modeling efforts.

Authors: D. H. Brooks, H. P. Warren, I. Ugarte-Urra, K. Matsuzaki, and D. R. Williams
Projects: Hinode/EIS

Publication Status: Published in PASJ Hinode Special Issue
Last Modified: 2008-01-30 09:37
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Subject will be restored when possible  

David Brooks   Submitted: 2008-01-30 00:49

Theoretically, magnetic fields are expected to expand as they rise above the photosphere and into the corona, so the apparent uniform cross-sections of active region loops are difficult to understand. There has been some debate as to whether coronal loops really have constant cross-sections, or are actually unresolved and composed of expanding threads within the constant cross-section envelopes. Furthermore, loop expansion is critical to the success or failure of hydrostatic models in reproducing the intensities and morphology of observed emission. We analyze Hinode EIS observations of loops in active region 10953 and detect only moderate apex width expansion over a broad range of temperatures from log Te/K = 5.6 to 6.25. The expansion is less than required by steady-state heating models of coronal emission suggesting that such models will have difficulty reproducing both low and high temperature loop emission simultaneously. At higher temperatures (> log Te/K = 6.3) the apex widths increase substantially, but the emission at these temperatures likely comes from a combination of multiple loops. These observations demonstrate the advantage of EIS over previous instruments. For the first time, active region loops can be examined over a broad temperature range with high temperature fidelity and the same spatial resolution. The results therefore provide further clues to the coronal heating timescale and thus have implications for the direction of future modeling efforts.

Authors: D. H. Brooks, H. P. Warren, I. Ugarte-Urra, K. Matsuzaki, and D. R. Williams
Projects: Hinode/EIS

Publication Status: Published in PASJ Hinode Special Issue
Last Modified: 2008-01-30 00:49
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Abstracts by Author
A Diagnostic of Coronal Elemental Behavior during the Inverse FIP Effect in Solar Flares
Solar Cycle Observations of the Neon Abundance in the Sun-as-a-star
A Solar cycle correlation of coronal element abundances in Sun-as-a-star observations
Properties and Modeling of Unresolved Fine Structure Loops Observed in the Solar Transition Region by IRIS
Measurements of non-thermal line widths in solar active regions
Full-Sun observations for identifying the source of the slow solar wind
High Spatial Resolution Observations of Loops in the Solar Corona
Solar Coronal Loops Resolved by Hinode and SDO
Establishing a Connection Between Active Region Outflows and the Solar Wind: Abundance Measurements with EIS/Hinode
Flows and Motions in Moss in the Core of a Flaring Active Region: Evidence for Steady Heating
The Role of Transient Brightenings in Heating the Solar Corona
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Subject will be restored when possible

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