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Boulder, CO, United States

The High Altitude Observatory conducts research and provides support and facilities for the solar-terrestrial research community in the areas of solar and heliospheric physics, and the effects of solar variability on the Earth's magnetosphere, ionosphere, and upper atmosphere.HAO is a laboratory of the National Center for Atmospheric Research . NCAR is managed by the University Corporation for Atmospheric Research , and receives substantial funding from the National Science Foundation .HAO's mission is to understand the behavior of the Sun and its impact on the Earth, to support, enhance, and extend the capabilities of the university community and the broader scientific community, nationally and internationally, and to foster the transfer of knowledge and technology. As articulated in its Strategic Plan for 2011-2015, HAO's vision is to: Perform world-leading science to understand fundamentally and with predictive capability the sources and nature of solar and geospace variability; Provide scientific leadership and facilities to serve the wider community in common pursuit of these science objectives, and both support and benefit from the NCAR community; Support the education and training of early-career researchers in solar-terrestrial physics and instrumentation; and Provide advocacy for solar-terrestrial physics, promoting its results, and articulating its societal importance, to the rest of NCAR, the NSF, the university community, and the public.HAO's telescopes are located at its Mauna Loa Solar Observatory, near the summit of that volcano on the big island of Hawaii. NCAR's solar observatory shares space on the campus of NOAA's larger Mauna Loa Observatory. HAO's researchers are based at NCAR headquarters, in Boulder, Colorado.The founding Director of the High Altitude Observatory was Walter Orr Roberts. The current Director is Scott McIntosh. A list of all HAO directors since the founding of the Observatory is given below. Wikipedia.

We present a series of radiative MHD simulations addressing the origin and distribution of the mixed polarity magnetic field in the solar photosphere. To this end, we consider numerical simulations that cover the uppermost 2-6 Mm of the solar convection zone and we explore scales ranging from 2 km to 25 Mm. We study how the strength and distribution of the magnetic field in the photosphere and subsurface layers depend on resolution, domain size, and boundary conditions. We find that 50% of the magnetic energy at the τ = 1 level comes from fields with the less than 500 G strength and that 50% of the energy resides on scales smaller than about 100 km. While the probability distribution functions are essentially independent of resolution, properly describing the spectral energy distribution requires grid spacings of 8 km or smaller. The formation of flux concentrations in the photosphere exceeding 1 kG requires a mean vertical field strength greater than 30-40 G at τ = 1. The filling factor of kG flux concentrations increases with overall domain size as the magnetic field becomes organized by larger, longer-lived flow structures. A solution with a mean vertical field strength of around 85 G at τ = 1 requires a subsurface rms field strength increasing with depth at the same rate as the equipartition field strength. We consider this an upper limit for the quiet Sun field strength, which implies that most of the convection zone is magnetized close to the equipartition. We discuss these findings in view of recent high-resolution spectropolarimetric observations of quiet Sun magnetism. © 2014. The American Astronomical Society. All rights reserved. Source

Rempel M.,High Altitude Observatory
Astrophysical Journal

We analyze in detail the penumbral structure found in a recent radiative magnetohydrodynamic simulation. Near τ = 1, the simulation produces penumbral fine structure consistent with the observationally inferred interlocking comb structure. Fast outflows exceeding 8 km s-1 are present along almost horizontal stretches of the magnetic field; in the outer half of the penumbra, we see opposite polarity flux indicating flux returning beneath the surface. The bulk of the penumbral brightness is maintained by small-scale motions turning over on scales shorter than the length of a typical penumbral filament. The resulting vertical rms velocity at τ = 1 is about half of that found in the quiet Sun. Radial outflows in the sunspot penumbra have two components. In the uppermost few 100km, fast outflows are driven primarily through the horizontal component of the Lorentz force, which is confined to narrow boundary layers beneath τ = 1, while the contribution from horizontal pressure gradients is reduced in comparison to granulation as a consequence of anisotropy. The resulting Evershed flow reaches its peak velocity near τ = 1 and falls off rapidly with height. Outflows present in deeper layers result primarily from a preferred ring-like alignment of convection cells surrounding the sunspot. These flows reach amplitudes of about 50% of the convective rms velocity rather independent of depth. A preference for the outflow results from a combination of Lorentz force and pressure driving. While the Evershed flow dominates by velocity amplitude, most of the mass flux is present in deeper layers and likely related to a large-scale moat flow. © 2011. The American Astronomical Society. All rights reserved. Source

Dikpati M.,High Altitude Observatory
Astrophysical Journal

Polar fields in solar cycle 23 were about 50% weaker than those in cycle 22. The only theoretical models which have addressed this puzzle are surface-transport models and flux-transport dynamo models. Comparing polar fields obtained from numerical simulations using surface-flux-transport models and flux-transport dynamo models, we show that both classes of models can explain the polar field features within the scope of the physics included in the respective models. In both models, how polar fields change as a result of changes in meridional circulation depends on the details of meridional circulation profile used. Using physical reasoning and schematics as well as numerical solutions from a flux-transport dynamo model, we demonstrate that polar fields are determined mostly by the strength of a surface poloidal source provided by the decay of tilted, bipolar active regions. The profile of a meridional flow with the latitude and its changes with time have much less effect in flux-transport dynamo models than in surface-transport models. © 2011. The American Astronomical Society. All rights reserved. Source

Rempel M.,High Altitude Observatory
Astrophysical Journal

We present a series of numerical sunspot models addressing the subsurface field and flow structure in up to 16Mm deep domains covering up to two days of temporal evolution. Changes in the photospheric appearance of the sunspots are driven by subsurface flows in several Mm depth. Most of magnetic field is pushed into a downflow vertex of the subsurface convection pattern, while some fraction of the flux separates from the main trunk of the spot. Flux separation in deeper layers is accompanied in the photosphere with light bridge formation in the early stages and formation of pores separating from the spot at later stages. Over a timescale of less than a day we see the development of a large-scale flow pattern surrounding the sunspots, which is dominated by a radial outflow reaching about 50% of the convective rms velocity in amplitude. Several components of the large scale flow are found to be independent from the presence of a penumbra and the associated Evershed flow. While the simulated sunspots lead to blockage of heat flux in the near surface layers, we do not see compelling evidence for a brightness enhancement in their periphery. We further demonstrate that the influence of the bottom boundary condition on the stability and long-term evolution of the sunspot is significantly reduced in a 16 Mm deep domain compared to the shallower domains considered previously. © 2011. The American Astronomical Society. All rights reserved. Source

Using a three-dimensional MHD simulation, we model the quasi-static evolution and the onset of eruption of a coronal flux rope. The simulation begins with a twisted flux rope emerging at the lower boundary and pushing into a pre-existing coronal potential arcade field. At a chosen time the emergence is stopped with the lower boundary taken to be rigid. Then the coronal flux rope settles into a quasi-static rise phase during which an underlying, central sigmoid-shaped current layer forms along the so-called hyperbolic flux tube (HFT), a generalization of the X-line configuration. Reconnections in the dissipating current layer effectively add twisted flux to the flux rope and thus allow it to rise quasi-statically, even though the magnetic energy is decreasing as the system relaxes. We examine the thermal features produced by the current layer formation and the associated "tether-cutting" reconnections as a result of heating and field aligned thermal conduction. It is found that a central hot, low-density channel containing reconnected, twisted flux threading under the flux rope axis forms on top of the central current layer. When viewed in the line of sight roughly aligned with the hot channel (which is roughly along the neutral line), the central current layer appears as a high-density vertical column with upward extensions as a "U"-shaped dense shell enclosing a central hot, low-density void. Such thermal features have been observed within coronal prominence cavities. Our MHD simulation suggests that they are the signatures of the development of the HFT topology and the associated tether-cutting reconnections, and that the central void grows and rises with the reconnections, until the flux rope reaches the critical height for the onset of the torus instability and dynamic eruption ensues. © 2012. The American Astronomical Society. All rights reserved. Source

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