arXiv subjects

Scott W. McIntosh

Publications and source records attributed to Scott W. McIntosh.

64 records · Page 4Linked to original sources

arXiv2008

The Inconvenient Truth About Coronal Dimmings

We investigate the occurrence of a CME-driven coronal dimming using unique high resolution spectral images of the corona from the Hinode spacecraft. Over the course of the dimming event we observe the dynamic increase of non-thermal line broadening in the 195.12Angstrom emission line of Fe XII as the corona opens. As the corona begins to close, refill and brighten, we see a reduction of the non-thermal broadening towards the pre-eruption level. We propose that the dynamic evolution of non-thermal broadening is the result of the growth of Alfven wave amplitudes in the magnetically open rarefied dimming region, compared to the dense closed corona prior to the CME. We suggest, based on this proposition, that, as open magnetic regions, coronal dimmings must act just as coronal holes and be sources of the fast solar wind, but only temporarily. Further, we propose that such a rapid transition in the thermodynamics of the corona to a solar wind state may have an impulsive effect on the CME that initiates the observed dimming. This last point, if correct, poses a significant physical challenge to the sophistication of CME modeling and capturing the essence of the source region thermodynamics necessary to correctly ascertain CME propagation speeds, etc.

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arXiv2008

A Coherence-Based Approach for Tracking Waves in the Solar Corona

We consider the problem of automatically (and robustly) isolating and extracting information about waves and oscillations observed in EUV image sequences of the solar corona with a view to near real-time application to data from the Atmospheric Imaging Array (AIA) on the Solar Dynamics Observatory (SDO). We find that a simple coherence / travel-time based approach detects and provides a wealth of information on transverse and longitudinal wave phenomena in the test sequences provided by the Transition Region and Coronal Explorer (TRACE). The results of the search are "pruned" (based on diagnostic errors) to minimize false-detections such that the remainder provides robust measurements of waves in the solar corona, with the calculated propagation speed allowing automated distinction between various wave modes. In this paper we discuss the technique, present results on the TRACE test sequences, and describe how our method can be used to automatically process the enormous flow of data (~1Tb/day) that will be provided by SDO/AIA after launch in late 2008.

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arXiv2008

Reappraising Transition Region Line Widths in light of Recent Alfvén Wave Discoveries

We provide a new interpretation of ultraviolet transition region emission line widths observed by the SUMER instrument on the Solar and Heliospheric Observatory (SOHO). This investigation is prompted by observations of the chromosphere at unprecedented spatial and temporal resolution from the Solar Optical Telescope (SOT) on Hinode revealing that all chromospheric structures above the limb display significant transverse (Alfvenic) perturbations. We demonstrate that the magnitude, network sensitivity and apparent center-to-limb isotropy of the measured line widths (formed below 250,000K) can be explained by an observationally constrained forward-model in which the line width is caused by the line-of-sight superposition of longitudinal and Alfvenic motions on the small-scale (spicular) structures that dominate the chromosphere and low transition region.

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arXiv2007

A Tale Of Two Spicules: The Impact of Spicules on the Magnetic Chromosphere

We use high-resolution observations of the Sun in Ca II H 3968 A from the Solar Optical Telescope on Hinode to show that there are at least two types of spicules that dominate the structure of the magnetic solar chromosphere. Both types are tied to the relentless magnetoconvective driving in the photosphere, but have very different dynamic properties. ``Type-I'' spicules are driven by shock waves that form when global oscillations and convective flows leak into the upper atmosphere along magnetic field lines on 3-7 minute timescales. ``Type-II'' spicules are much more dynamic: they form rapidly (in ~10s), are very thin (<200km wide), have lifetimes of 10-150s (at any one height) and seem to be rapidly heated to (at least) transition region temperatures, sending material through the chromosphere at speeds of order 50-150 km/s. The properties of Type II spicules suggest a formation process that is a consequence of magnetic reconnection, typically in the vicinity of magnetic flux concentrations in plage and network. Both types of spicules are observed to carry Alfven waves with significant amplitudes of order 20 km/s.

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arXiv2007

On The Mass and Energy Loading of EUV Brightpoints

We discuss the appearance of EUV brightpoints (BPs) in the analysis of long-duration observations in the He II 304 Angstrom passband of the Solar and Heliospheric Observatory (SOHO) Extreme-ultraviolet Imaging Telescope (EIT). The signature of the observed 304 Angstrom passband intensity fluctuations around the BPs suggest that the primary source of the mass and energy supplied to the magnetic structure is facilitated by relentless magnetoconvection-driven reconnection, forced by the magnetic evolution of the surrounding supergranules. Further, we observe that if the magnetic conditions in the supergranules surrounding the footpoints of the cool 304 Angstrom BPs are sufficient (large net imbalance with a magnetic field that closes beyond the boundaries of the cell it originates in) the magnetic topology comprising the BP will begin to reconnect with the overlying corona, increasing its visibility to hotter EUV passbands and possibly Soft X-Rays.

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arXiv2007

Does High[Plasma]-Beta Dynamics "Load" Active Regions?

Using long-duration observations in the He II 304 Angstrom passband of the Solar and Heliospheric Observatory (SOHO) Extreme-ultraviolet Imaging Telescope (EIT) we investigate the spatial and temporal appearance of impulsive intensity fluctuations in the pixel light curves. These passband intensity fluctuations come from plasma emitting in the chromosphere, transition region and lowest portions of the corona. We see that they are spatially tied to the supergranular scale and that their rate of occurrence is tied to the unsigned imbalance of the magnetic field in which they are observed. The signature of the fluctuations (in space and time) is consistent with their creation by magnetoconvection forced reconnection that is driven by the flow field in the high-beta plasma. The signature of the intensity fluctuations around an active region suggest that the bulk of the mass and energy supplied into the active region complex observed in the hotter coronal plasma is supplied by this process, dynamically forcing the looped structure from beneath.

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arXiv2007

Empirical Solar Wind Forecasting from the Chromosphere

Recently, we correlated the inferred structure of the solar chromospheric plasma topography with solar wind velocity and composition data measured at 1AU. We now offer a physical justification of these relationships and present initial results of a empirical prediction model based on them. While still limited by the fundamentally complex physics behind the origins of the solar wind and how its structure develops in the magnetic photosphere and expands into the heliosphere, our model provides a near continuous range of solar wind speeds and composition quantities that are simply estimated from the inferred structure of the chromosphere. We suggest that the derived quantities may provide input to other, more sophisticated, prediction tools or models such as those to study Coronal Mass Ejections (CME) propagation and Solar Energetic Particle (SEP) generation.

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arXiv2007

The Post-Eruptive Evolution of a Coronal Dimming

We discuss the post-eruptive evolution of a "coronal dimming" based on observations of the EUV corona from the Solar and Heliospheric Observatory and the Transition Region and Coronal Explorer. This discussion highlights the roles played by magnetoconvection-driven magnetic reconnection and the global magnetic environment of the plasma in the "filling" and apparent motion of the region following the eruption of a coronal mass ejection (CME). A crucial element in our understanding of the dimming region evolution is developed by monitoring the disappearance and reappearance of bright TRACE "moss" around the active region giving rise to the CME. We interpret the change in the TRACE moss as a proxy of the changing coronal magnetic field topology behind the CME front. We infer that the change in global magnetic topology also results in a shift of energy balance in the process responsible for the production of the moss emission while the coronal magnetic topology evolves from closed, to open and back to closed again because, following the eruption, the moss reforms around the active region in almost exactly its pre-event configuration. As a result of the moss evolution, combining our discussion with recent spectroscopic results of an equatorial coronal hole, we suggest that the interchangeable use of the term "transient coronal hole" to describe a coronal dimming is more than just a simple coincidence.

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arXiv2006

Observations Supporting the Role of Magnetoconvection in Energy Supply to the Quiescent Solar Atmosphere

Identifying the two physical mechanisms behind the production and sustenance of the quiescent solar corona and solar wind poses two of the outstanding problems in solar physics today. We present analysis of spectroscopic observations from the Solar and Heliospheric Observatory that are consistent with a single physical mechanism being responsible for a significant portion of the heat supplied to the lower solar corona and the initial acceleration of the solar wind; the ubiquitous action of magnetoconvection-driven reprocessing and exchange reconnection of the Sun's magnetic field on the supergranular scale. We deduce that while the net magnetic flux on the scale of a supergranule controls the injection rate of mass and energy into the transition region plasma it is the global magnetic topology of the plasma that dictates whether the released ejecta provides thermal input to the quiet solar corona or becomes a tributary that feeds the solar wind.

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arXiv2006

Simple Magnetic Flux Balance as an Indicator of Neon VIII Doppler Velocity Partitioning in an Equatorial Coronal Hole

We present a novel investigation into the relationship between simple estimates of magnetic flux balance and the Ne VIII Doppler velocity partitioning of a large equatorial coronal hole observed by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer (SUMER) on the Solar and Heliospheric Observatory (SOHO) in November 1999. We demonstrate that a considerable fraction of the large scale Doppler velocity pattern in the coronal hole can be qualitatively described by simple measures of the local magnetic field conditions, i.e., the relative unbalance of magnetic polarities and the radial distance required to balance local flux concentrations with those of opposite polarity.

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