1,236 research outputs found

    Bootstrapping the Coronal Magnetic Field with STEREO: I. Unipolar Potential Field Modeling

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    We investigate the recently quantified misalignment of Ξ±misβ‰ˆ20βˆ˜βˆ’40∘\alpha_{mis} \approx 20^\circ-40^\circ between the 3-D geometry of stereoscopically triangulated coronal loops observed with STEREO/EUVI (in four active regions) and theoretical (potential or nonlinear force-free) magnetic field models extrapolated from photospheric magnetograms. We develop an efficient method of bootstrapping the coronal magnetic field by forward-fitting a parameterized potential field model to the STEREO-observed loops. The potential field model consists of a number of unipolar magnetic charges that are parameterized by decomposing a photospheric magnetogram from MDI. The forward-fitting method yields a best-fit magnetic field model with a reduced misalignment of Ξ±PFβ‰ˆ13βˆ˜βˆ’20∘\alpha_{PF} \approx 13^\circ-20^\circ. We evaluate also stereoscopic measurement errors and find a contribution of Ξ±SEβ‰ˆ7βˆ˜βˆ’12∘\alpha_{SE}\approx 7^\circ-12^\circ, which constrains the residual misalignment to Ξ±NP=Ξ±PFβˆ’Ξ±SEβ‰ˆ5βˆ˜βˆ’9∘\alpha_{NP}=\alpha_{PF}-\alpha_{SE}\approx 5^\circ -9^\circ, which is likely due to the nonpotentiality of the active regions. The residual misalignment angle Ξ±NP\alpha_{NP} of the potential field due to nonpotentiality is found to correlate with the soft X-ray flux of the active region, which implies a relationship between electric currents and plasma heating.Comment: 12 figures, manuscript submitted to ApJ, 2010 Apr 2

    Coronal Loop Oscillations Observed with AIA - Kink-Mode with Cross-Sectional and Density Oscillations

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    A detailed analysis of a coronal loop oscillation event is presented, using data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) for the first time. The loop oscillation event occurred on 2010 Oct 16, 19:05-19:35 UT, was triggered by an M2.9 GOES-class flare, located inside a highly inclined cone of a narrow-angle CME. This oscillation event had a number of unusual features: (i) Excitation of kink-mode oscillations in vertical polarization (in the loop plane); (ii) Coupled cross-sectional and density oscillations with identical periods; (iii) no detectable kink amplitude damping over the observed duration of four kink-mode periods (P=6.3P=6.3 min); (iv) multi-loop oscillations with slightly (β‰ˆ10\approx 10%) different periods; and (v) a relatively cool loop temperature of Tβ‰ˆ0.5T\approx 0.5 MK. We employ a novel method of deriving the electron density ratio external and internal to the oscillating loop from the ratio of Alfv\'enic speeds deduced from the flare trigger delay and the kink-mode period, i.e., ne/ni=(vA/vAe)2=0.08Β±0.01n_e/n_i=(v_A/v_{Ae})^2=0.08\pm0.01. The coupling of the kink mode and cross-sectional oscillations can be explained as a consequence of the loop length variation in the vertical polarization mode. We determine the exact footpoint locations and loop length with stereoscopic triangulation using STEREO/EUVI-A data. We model the magnetic field in the oscillating loop using HMI/SDO magnetogram data and a potential field model and find agreement with the seismological value of the magnetic field, Bkink=4.0Β±0.7B_{kink}=4.0\pm0.7 G, within a factor of two.Comment: ApJ (in press, accepted May 10, 2011
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