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Spectral and Imaging Observations of a White-light Flare in the Mid-Infrared  

Matthew Penn   Submitted: 2016-08-31 15:03

We report high-resolution observations at mid-infrared wavelengths of a minor solar flare, SOL2014-09-24T17T17:50 (C7.0), using Quantum Well Infrared Photodetector (QWIP) cameras at an auxiliary of the McMath-Pierce telescope. The flare emissions, the first simultaneous observations in two mid-infrared bands at 5 μm and 8 μm with white-light and hard X-ray coverage, revealed impulsive time variability with increases on time scales of ∼4 s followed by exponential decay at ∼10 s in two bright regions separated by about 13". The brightest source is compact, unresolved spatially at the diffraction limit (1.3" at 5 μm). We identify the IR sources as flare ribbons also seen in white-light emission at 6173 Å observed by SDO/HMI, with twin hard X-ray sources observed by RHESSI, and with EUV sources (e.g., 94 Å) observed by SDO/AIA. The two infrared points have closely the same flux density (fν, W/m2Hz) and extrapolate to a level about an order of magnitude below that observed in the visible band by HMI, but with a flux more than two orders of magnitude above the free-free continuum from the hot (∼15 MK) coronal flare loop observed in the X-ray range. The observations suggest that the IR emission is optically thin, this constraint and others suggest major contributions from a density less than about 3x1013~cm-3. We tentatively interpret this emission mechanism as predominantly free-free emission in a highly ionized but cool and rather dense chromospheric region.

Authors: M Penn, S Krucker, H Hudson, M Jhabvala, D Jennings, A Lunsford, P Kaufmann

Publication Status: Published ApJ
Last Modified: 2016-09-07 12:14
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Infrared Solar Physics  

Matthew Penn   Submitted: 2014-05-06 15:46

The infrared solar spectrum contains a wealth of physical data about our Sun, and is explored using modern detectors and technology with new ground-based solar telescopes. The scientific motivation behind exploring these wavelengths is presented, along with a brief look at the rich history of observations here. Several avenues of solar physics research exploiting and benefiting from observations at infrared wavelengths from roughly 1000nm to 12400nm are discussed, and the instrument and detector technology driving this research is briefly summarized. Finally, goals for future work at infrared wavelengths are presented in conjunction with ground and space-based observations.

Authors: M Penn
Projects: None

Publication Status: accepted at Living Reviews in Solar Physics
Last Modified: 2014-05-07 06:01
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Probing the Solar Atmosphere Using Oscillations of Infrared CO Spectral Lines  

Matthew Penn   Submitted: 2010-12-10 13:34

Oscillations were observed across the whole solar disk using the Doppler shift and line depth of spectral lines from the CO molecule near 4666~nm with the National Solar Observatory's McMath/Pierce solar telescope. Power, coherence, and phase spectra were examined, and diagnostic diagrams reveal power ridges at the solar global mode frequencies to show that these oscillations are solar p-modes. The phase was used to determine the height of formation of the CO lines by comparison with the IR continuum intensity phase shifts as measured in Kopp et al., 1992; we find the CO line formation height varies from 425 < z < 560 km as we move from disk center towards the solar limb 1.0 > mu > 0.5. The velocity power spectra show that while the sum of the background and p-mode power increases with height in the solar atmosphere as seen in previous work, the power in the p-modes only (background subtracted) decreases with height, consistent with evanescent waves. The CO line depth weakens in regions of stronger magnetic fields, as does the p-mode oscillation power. Across most of the solar surface the phase shift is larger than the expected value of 90 degrees for an adiabatic atmosphere. We fit the phase spectra at different disk positions with a simple atmospheric model to determine that the acoustic cutoff frequency is about 4.5 mHz with only small variations, but that the thermal relaxation frequency drops significantly from 2.7 to 0 mHz at these heights in the solar atmosphere.

Authors: M Penn, T Schad, E Cox
Projects: None

Publication Status: submitted to ApJ
Last Modified: 2010-12-11 12:14
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Long-term Evolution of Sunspot Magnetic Fields  

Matthew Penn   Submitted: 2010-09-03 17:11

Independent of the normal solar cycle, a decrease in the sunspot magnetic field strength has been observed using the Zeeman-split 1564.8nm Fe I spectral line at the NSO Kitt Peak McMath-Pierce telescope. Corresponding changes in sunspot brightness and the strength of molecular absorption lines were also seen. This trend was seen to continue in observations of the first sunspots of the new solar Cycle 24, and extrapolating a linear fit to this trend would lead to only half the number of spots in Cycle 24 compared to Cycle 23, and imply virtually no sunspots in Cycle 25. We examined synoptic observations from the NSO Kitt Peak Vacuum Telescope and initially (with 4000 spots) found a change in sunspot brightness which roughly agreed with the infrared observations. A more detailed examination (with 13,000 spots) of both spot brightness and line-of-sight magnetic flux reveals that the relationship of the sunspot magnetic fields with spot brightness and size remain constant during the solar cycle. There are only small temporal variations in the spot brightness, size, and line-of-sight flux seen in this larger sample. Because of the apparent disagreement between the two data sets, we discuss how the infrared spectral line provides a uniquely direct measurement of the magnetic fields in sunspots.

Authors: Matthew J Penn and William Livingston
Projects: National Solar Observatory (Sac Peak)

Publication Status: To appear in IAU Symposium 273
Last Modified: 2010-09-03 20:23
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Abstracts by Author
Spectral and Imaging Observations of a White-light Flare in the Mid-Infrared
Infrared Solar Physics
Probing the Solar Atmosphere Using Oscillations of Infrared CO Spectral Lines
Long-term Evolution of Sunspot Magnetic Fields

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