Spectral and Imaging Observations of a White-light Flare in the Mid-Infrared |
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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
Projects: RHESSI,SDO-AIA
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Publication Status: Published ApJ
Last Modified: 2016-09-07 12:14
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Probing the Solar Atmosphere Using Oscillations of Infrared CO Spectral Lines |
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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
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Publication Status: submitted to ApJ
Last Modified: 2010-12-11 12:14
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