Scholarly Work - Physics

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Deciphering Solar Magnetic Activity: On the Relationship between the Sunspot Cycle and the Evolution of Small Magnetic Features
(Institute of Physics Publishing, Inc., 2014) McIntosh, Scott W.; Wang, Xin; Leamon, Robert; Davey, Alisdair; Howe, Rachel; Krista, Larisza; Malanushenko, Anna; Markel, Robert; Cirtain, Jonathan; Gurman, Joseph; Pesnell, William; Thompson, Michael
Sunspots are a canonical marker of the Sun’s internal magnetic field which flips polarity every ∼22 yr. The principal variation of sunspots, an ∼11 yr variation, modulates the amount of the magnetic field that pierces the solar surface and drives significant variations in our star’s radiative, particulate, and eruptive output over that period. This paper presents observations from the Solar and Heliospheric Observatory and Solar Dynamics Observatory indicating that the 11 yr sunspot variation is intrinsically tied to the spatio-temporal overlap of the activity bands belonging to the 22 yr magnetic activity cycle. Using a systematic analysis of ubiquitous coronal brightpoints and the magnetic scale on which they appear to form, we show that the landmarks of sunspot cycle 23 can be explained by considering the evolution and interaction of the overlapping activity bands of the longer-scale variability.
Evidence of nonthermal particles in coronal loops heated impulsively by nanoflares
(2014-10) Testa, Paola; De Pontieu, Bart; Allred, J.; Carlsson, Mats; Reale, F.; Daw, A.; Hansteen, Viggo H.; Martinez-Sykora, J.; Liu, W.; DeLuca, E.E.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Tian, Hui; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; Kleint, Lucia; Kankelborg, Charles; Jaeggli, Sarah
The physical processes causing energy exchange between the Sun’s hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High-resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to 60 seconds) of intensity and velocity on small spatial scales (≲500 kilometers) at the footpoints of hot and dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of nonthermal electrons, which are generated in small impulsive (≲30 seconds) heating events called “coronal nanoflares.” The accelerated electrons deposit a sizable fraction of their energy (≲1025 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona.
Hot Explosions in the Cool Atmoshere of the Sun
(2014-10) Peter, H.; Tian, Hui; Curdt, W.; Schmidt, D.; Innes, D.; De Pontieu, Bart; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; Martinez-Sykora, J.; Kleint, Lucia; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Kankelborg, Charles; Jaeggli, Sarah; Carlsson, Mats; Hansteen, Viggo H.
The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by the Interface Region Imaging Spectrograph (IRIS) reveal that it is difficult to determine what is up and down, even in the cool 6000-kelvin photosphere just above the solar surface: This region hosts pockets of hot plasma transiently heated to almost 100,000 kelvin. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection.
Measurements of EUV Coronal Holes and Open Magnetic Flux
(IOP Publishing, 2014-02) Lowder, Chris; Qiu, Jiong; Leamon, Robert; Liu, Y.
Coronal holes are regions on the Sun’s surface that map the footprints of open magnetic field lines. We have developed an automated routine to detect and track boundaries of long-lived coronal holes using full-disk extremeultraviolet (EUV) images obtained by SOHO/EIT, SDO/AIA, and STEREO/EUVI. We measure coronal hole areas and magnetic flux in these holes, and compare the measurements with calculations by the potential field source surface (PFSS) model. It is shown that, from 1996 through 2010, the total area of coronal holes measured with EIT images varies between 5% and 17% of the total solar surface area, and the total unsigned open flux varies between (2–5)×1022 Mx. The solar cycle dependence of these measurements is similar to the PFSS results, but the model yields larger hole areas and greater open flux than observed by EIT. The AIA/EUVI measurements from 2010–2013 show coronal hole area coverage of 5%–10% of the total surface area, with significant contribution from low latitudes, which is under-represented by EIT. AIA/EUVI have measured much enhanced open magnetic flux in the range of (2–4)×1022 Mx, which is about twice the flux measured by EIT, and matches with the PFSS calculated open flux, with discrepancies in the location and strength of coronal holes. A detailed comparison between the three measurements (by EIT, AIA–EUVI, and PFSS) indicates that coronal holes in low latitudes contribute significantly to the total open magnetic flux. These low-latitude coronal holes are not well measured with either the He I 10830 line in previous studies, or EIT EUV images; neither are they well captured by the static PFSS model. The enhanced observations from AIA/EUVI allow a more accurate measure of these low-latitude coronal holes and their contribution to open magnetic flux.
On magnetic activity band overlap, interaction, and the formation of complex solar active regions
(Institute of Physics Publishing Ltd., 2014-11) McIntosh, Scott W.; Leamon, Robert
Recent work has revealed a phenomenological picture of the how the ∼11 yr sunspot cycle of the Sun arises. The production and destruction of sunspots is a consequence of the latitudinal–temporal overlap and interaction of the toroidal magnetic flux systems that belong to the 22 yr magnetic activity cycle and are rooted deep in the Sun’s convective interior. We present a conceptually simple extension of this work, presenting a hypothesis on how complex active regions can form as a direct consequence of the intra- and extra-hemispheric interaction taking place in the solar interior. Furthermore, during specific portions of the sunspot cycle, we anticipate that those complex active regions may be particularly susceptible to profoundly catastrophic breakdown, producing flares and coronal mass ejections of the most severe magnitude.
On the prevalence of small-scale twist in the solar chromosphere and transition region
(De Pontieu, B., L. Rouppe van der Voort, S. W. McIntosh, T. M. D. Pereira, M. Carlsson, V. Hansteen, H. Skogsrud, et al. “On the Prevalence of Small-Scale Twist in the Solar Chromosphere and Transition Region.” Science 346, no. 6207 (October 16, 2014): 1255732–1255732. doi:10.1126/science.1255732., 2014-10) De Pontieu, Bart; Rouppe van der Voort, L.; McIntosh, Scott W.; Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo H.; Skogsrud, H.; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; DeLuca, E.E.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Kankelborg, Charles; Jaeggli, Sarah; Kleint, Lucia; Martinez-Sykora, J.
The solar chromosphere and transition region (TR) form an interface between the Sun’s surface and its hot outer atmosphere. There, most of the nonthermal energy that powers the solar atmosphere is transformed into heat, although the detailed mechanism remains elusive. High-resolution (0.33–arc second) observations with NASA’s Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere and TR that are replete with twist or torsional motions on sub–arc second scales, occurring in active regions, quiet Sun regions, and coronal holes alike. We coordinated observations with the Swedish 1-meter Solar Telescope (SST) to quantify these twisting motions and their association with rapid heating to at least TR temperatures. This view of the interface region provides insight into what heats the low solar atmosphere.
Prevalence of small-scale jets from the networks of the solar transition region and chromosphere
(2014-10) Tian, Hui; DeLuca, E.E.; Cranmer, S.R.; De Pontieu, Bart; Peter, H.; Martinez-Sykora, J.; Golub, Leon; McKillop, Sean; Reeves, Kathy K.; Miralles, M.P.; McCauley, P.; Saar, Steven; Testa, Paola; Weber, Mark A.; Murphy, N.; Lemen, James; Title, Alan M.; Boerner, P.; Hurlburt, Neal E.; Tarbell, Ted D.; Wuelser, Jean-Pierre; Kleint, Lucia; Kankelborg, Charles; Jaeggli, Sarah; Carlsson, Mats; Hansteen, Viggo H.; McIntosh, Scott W.
As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind.
Why I-Love-Q: Explaining why universality emerges in compact objects
(American Physical Society, 2014) Yagi, Kent; Stein, Leo C.; Pappas, George; Yunes, Nicolás; Apostolatos, Theocharis A.
Black holes are said to have no hair because all of their multipole moments can be expressed in terms of just their mass, charge and spin angular momentum. The recent discovery of approximately equation-of-state-independent relations among certain multipole moments in neutron stars suggests that they are also approximately bald. We here explore the yet unknown origin for this universality. First, we investigate which region of the neutron star’s interior and of the equation of state is most responsible for the universality. We find that the universal relation between the moment of inertia and the quadrupole moment is dominated by the star’s outer core, a shell of width 50%–95% of the total radius, which corresponds to the density range 1014–1015 g/cm3. In this range, realistic neutron star equations of state are not sufficiently similar to each other to explain the universality observed. Second, we study the impact on the universality of approximating stellar isodensity contours as self-similar ellipsoids. An analytical calculation in the nonrelativistic limit reveals that the shape of the ellipsoids per se does not affect the universal relations much, but relaxing the self-similarity assumption can completely destroy it. Third, we investigate the eccentricity profiles of rotating relativistic stars and find that the stellar eccentricity is roughly constant, with variations of roughly 20%–30% in the region that matters to the universal relations. Fourth, we repeat the above analysis for differentially rotating, noncompact, regular stars and find that this time the eccentricity is not constant, with variations that easily exceed 100%, and moreover universality is lost. These findings suggest that universality arises as an emergent approximate symmetry: as one flows in the stellar-structure phase space from noncompact star region to the relativistic star region, the eccentricity variation inside stars decreases, leading to approximate self-similarity in their isodensity contours, which then leads to the universal behavior observed in their exterior multipole moments.