7 research outputs found
Search for evidence of low energy protons in solar flares
We searched for linear polarization in the H alpha line using the Stokes Polarimeter at Mees Solar Observatory and present observations of a flare from NOAA active region 6659 which began at 01:30 UT on 14 Jun. 1991. Our dataset also includes H alpha spectra from the Mees charge coupled device (MCCD) imaging spectrograph as well as hard x ray observations from the Burst and Transient Source Experiment (BATSE) instrument on board the Gamma Ray Observatory (GRO). The polarimeter scanned a 40 x 40 inch field of view using 16 raster points in a 4 x 4 grid. Each scan took about 30 seconds with 2 seconds at each raster point. The polarimeter stopped 8.5 inches between raster points and each point covered a 6 inch region. This sparse sampling increased the total field of view without reducing the temporal cadence. At each raster point, an H alpha spectrum with 20 mA spectral sampling is obtained covering 2.6 A centered on H alpha line center. The preliminary conclusions from the research are presented
First 3D Reconstructions of Coronal Loops with the STEREO A+B Spacecraft: IV. Magnetic Modeling with Twisted Force-Free Fields
The three-dimensional (3D) coordinates of stereoscopically triangulated loops
provide strong constraints for magnetic field models of active regions in the
solar corona. Here we use STEREO/A and B data from some 500 stereoscopically
triangulated loops observed in four active regions (2007 Apr 30, May 9, May 19,
Dec 11), together with SOHO/MDI line-of-sight magnetograms. We measure the
average misalignment angle between the stereoscopic loops and theoretical
magnetic field models, finding a mismatch of for a
potential field model, which is reduced to for a
non-potential field model parameterized by twist parameters. The residual error
is commensurable with stereoscopic measurement errors (). We developed a potential field code that deconvolves a
line-of-sight magnetogram into three magnetic field components , as well as a non-potential field forward-fitting code that determines
the full length of twisted loops ( Mm), the number of twist
turns (median ), the nonlinear force-free -parameter
(median cm), and the current density
(median Mx cm s). All twisted loops are found
to be far below the critical value for kink instability, and Joule dissipation
of their currents is found be be far below the coronal heating requirement. The
algorithm developed here, based on an analytical solution of nonlinear
force-free fields that is accurate to second order (in the force-free parameter
), represents the first code that enables fast forward-fitting to
photospheric magnetograms and stereoscopically triangulated loops in the solar
corona.Comment: The Astrophysical Journal (in press), 37 pages, 14 Figure
Multi-component decomposition of astronomical spectra by compressed sensing
Funding: STFC Ernest Rutherford Fellowship (grant agreement No. ST/R004285/1) (PA).The signal measured by an astronomical spectrometer may be due to radiation from a multi-component mixture of plasmas with a range of physical properties (e.g., temperature, Doppler velocity). Confusion between multiple components may be exacerbated if the spectrometer sensor is illuminated by overlapping spectra dispersed from different slits, with each slit being exposed to radiation from a different portion of an extended astrophysical object. We use a compressed sensing method to robustly retrieve the different components. This method can be adopted for a variety of spectrometer configurations, including single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer mission), and slot spectrometers (which produce overlappograms).Publisher PDFPeer reviewe
Recent STEREO Observations of Coronal Mass Ejections
Over 400 CMEs have been observed by STEREO SECCHI COR1 during the mission's three year duration (2006-2009). Many of the solar activity indicators have been at minimal values over this period, and the Carrington rotation-averaged CME rate has been comparable to that measured during the minima between Cycle 21-22 (SMM C/P) and Cycle 22-23 (SOHO LASCO). That rate is about 0.5 CMEs/day. During the current solar minimum (leading to Cycle 24), there have been entire Carrington rotations where no sunspots were detected and the daily values of the 2800 MHz solar flux remained below 70 sfu. CMEs continued to be detected during these exceptionally quiet periods, indicating that active regions are not necessary to the generation of at least a portion of the CME population. In the past, researchers were limited to a single view of the Sun and could conclude that activity on the unseen portion of the disk might be associated with CMEs. But as the STEREO mission has progressed we have been able to observe an increasing fraction of the Sun's corona with STEREO SECCHI EUVI and were able to eliminate this possibility. Here we report on the nature of CMEs detected during these exceptionally quiet periods, and we speculate on how the corona remains dynamic during such conditions