HAO 2011 Profiles In Science: Dr. Scott McIntosh
Contact:
303-497-1544
mscott@ucar.edu
Dr. Scott McIntosh is a Scientist III Section Head in the High Altitude Observatory of the National Center for Atmospheric Research. He received his PhD in Astrophysics in 1998 from the University of Glasgow, Scotland. His first work with HAO began in 1997 as a graduate student. His primary focus of research is chromospheric dynamics.
Scott's research focuses largely on establishing and understanding the physical connectivity between the Sun's cool surface and its considerably hotter corona.
Professional Website(s): Scott McIntosh
Publications
(1) De Pontieu, McIntosh. 2011: The origins of hot plasma in the solar corona, Science, 331, 55.
Abstract: Combining observations of the Hinode and Solar Dynamics Observatory (SDO) spacecraft we have identified, and started to explore, discrete coronal heating events, for the first time tracking plasma heating from the low chromosphere (~10,000 K) into the extended corona (>2,000,000 K). These episodic heating events appear to be rooted in activity of the smallest (visible) scales of solar magnetism as observed by Hinode spectro-polarimeter, and have been unambiguously shown to manifest as very fine (~100km in diameter), rapidly traveling (~100km/s), and short-lived (10–100s) "Type-II" spicules, so-called to distinguish them from their classical siblings (≤1Mm across, 20-40 km/s, several minutes in duration). Tracking these rapid events through images of the Sun’s chromosphere provided by Hinode has revealed a one-to-one relationship with material that is considerably hotter moving upwards at the same apparent speed into the corona (with SDO) across many temperatures, often exceeding 2 million Kelvin.
These observations, such as shown in Figure 1, have provided a challenge to existing models of coronal heating by demonstrating that the majority of coronal temperature mass is dynamically forced out of the chromosphere pre-heated. The observed process provides strong constraint on the relentless cycling of mass between the Sun’s surface and outer solar atmosphere. Such knowledge will in turn permit the modeling of ultraviolet and extreme-ultraviolet radiation formation in the solar atmosphere that impinges on the Earth’s outer atmosphere.
Observational investigations are planned to isolate the relationship between magnetic field emergence, strength, geometry, and post-emergence evolution as likely “trigger” mechanisms for the launch and heating of the spicule material. It is imperative that the highest quality simulations of the middle atmospheric region are developed in tandem to place the observations in a quantitative context.
Figure 1 caption: Typical dynamic and thermal evolution of “type-II” spicules as observed by Hinode and SDO. These spicules are the result of discretized coronal heating events triggered low in the atmosphere. The panels on the left show (from top to bottom) the temporal evolution of a succession of type-II spicules visible as a dark feature in the top panel (Hα–0.868Å), and associated bright features in He II 304Å intensity and the running time-difference time series for He II 304Å, Fe IX 171Å, and Fe XIV 211Å. The space-time plots (right column) constructed along the axis of the spicule (dotted vertical white lines) show strong brightenings in all images at the bottom of the spicule during its initial stages. These brightenings then move upward with the same apparent speed (of order 70 km/s, see dashed diagonal guideline).
(2) McIntosh, et al. 2011: Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind, Nature, 475, 477.
Abtract: Energy is required to heat the outer solar atmosphere to millions of degrees and to accelerate the solar wind to hundreds of kilometres per second. Alfvén waves (traveling oscillations of ions and magnetic field) have been invoked as a possible mechanism to transport magneto-convective energy upwards along the Sun's magnetic field lines into the corona. Previous observations of Alfvénic waves in the corona revealed amplitudes far too small (0.5km/s) to supply the energy flux (100–200Wm-2) required to drive the fast solar wind or balance the radiative losses of the quiet corona. Here we report observations of the transition region (between the chromosphere and the corona) and of the corona that reveal how Alfvénic motions permeate the dynamic and finely structured outer solar atmosphere. The ubiquitous outward-propagating Alfvénic motions observed have amplitudes of the order of 20km/s and periods of the order of 100–500s throughout the quiescent atmosphere (compatible with recent investigations), and are energetic enough to accelerate the fast solar wind and heat the quiet corona.
Figure 2 caption: Ubiquitous Alfvénic motion above the solar limb. SDO/AIA space-time plots demonstrating the visibility of the ubiquitous transverse waves 34Mm above the solar limb in a coronal hole (a). The intensity images (b) and mean-subtracted intensity images (d) show a number of oscillatory structures that are enhanced using unsharp masking (c, e). The highlighted example oscillation, enclosed in a red rectangle, is compatible with the propagation along the spicule (f), and propagating coronal disturbance (h). A sine wave with a period of 180s and amplitude of 24km/s is drawn on panels f and h as a visual aid to the reader.
(3) McIntosh, et al. 2011: Observing Evolution in the Supergranular Network Length Scale During Periods of Low Solar Activity, ApJL, 730, 1.
Abtract: We present the initial results of an observational study into the variation of the dominant length scale of quiet solar emission: supergranulation. The distribution of magnetic elements in the lanes that from the network affects, and reflects, the radiative energy in the plasma of the upper solar chromosphere and transition region at the magnetic network boundaries forming as a result of the relentless interaction of magnetic fields and convective motions of the Suns' interior. We demonstrate that a net difference of ~0.5 Mm in the supergranular emission length scale occurs when comparing observation cycle 22/23 and cycle 23/24 minima. This variation in scale is reproduced in the data sets of multiple space- and ground-based instruments and using different diagnostic measures. By means of extension, we consider the variation of the supergranular length scale over multiple solar minima by analyzing a subset of the Mount Wilson Solar Observatory Ca II K image record. The observations and analysis presented provide a tantalizing look at solar activity in the absence of large-scale flux emergence, offering insight into times of "extreme" solar minimum and general behavior such as the phasing and cross-dependence of different components of the spectral irradiance. Given that the modulation of the supergranular scale imprints itself in variations of the Suns' spectral irradiance, as well as in the mass and energy transport into the entire outer atmosphere, this preliminary investigation is an important step in understanding the impact of the quiet Sun on the heliospheric system.
Figure 3 caption: From top to bottom, the variation in the supergranular mean radius (<r>) determined from the SOHO/EIT, STEREO A EUVI, STEREO B EUVI, HAO/MLSO PSPT, image sequences and the power-law exponent (δ) of the SOHO/MDI "magnetic range of Influence" (MRoI). The upper four panels show the uncorrected <r> and variation in the solar radius (dot-dashed line) as seen from each observing platform used to correct each timeseries. In the fifth row, we show the adjusted timeseries and compared to the SOHO/VIRGO Total Solar Irradiance timeseries (orange dots). For reference we draw dashed lines for a
(4) McIntosh, et al. 2011: Solar Cycle Variations in the Elemental Abundance of Helium and Fractionation of Iron in the Fast Solar Wind: Indicators of an Evolving Energetic Release of Mass from the Lower Solar Atmosphere, ApJL, 740, 23.
Abstract: We present and discuss the strong correspondence between evolution of the emission length scale in the lower transition region and in situ measurements of the fast solar wind composition during the most recent solar minimum. We combine recent analyses demonstrating the variance in the (supergranular) network emission length scale measured by the Solar and Heliospheric Observatory (and STEREO) with that of the helium abundance (from Wind) and the degree of iron fractionation in the solar wind (from the Advanced Composition Explorer and Ulysses). The net picture developing is one where a decrease in the helium abundance and the degree of iron fractionation (approaching values expected of the photosphere) in the fast wind indicate a significant change in the process loading material into the fast solar wind during the recent solar minimum. This result is compounded by a study of the helium abundance during the space age using the NASA OMNI database, which shows a slowly decaying amount of helium being driven into the heliosphere over the course of several solar cycles.
Figure 4 caption: Combining the analysis of the Wind Helium abundance (AHe) as a function of the solar wind speed in 250-day averages in the top panel with the 28-day running average of the transition region (supergranular) network length scale from SOHO (Fig. 3). The solid black trace in the top panel shows the variation in the smoothed monthly sunspot number over the same time period. The dashed vertical line marks January 1, 2007 as an approximate date when the network scale falls below the mean value of the 1996 solar minimum. The time of minimum scale and Helium abundance occur coincidently close to January 1, 2009.