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A Very Narrow RHESSI X-ray Flare on 25 September 2011  

Brian Dennis   Submitted: 2019-10-30 09:15

The unusually narrow X-ray source imaged with RHESSI during an impulsive spike lasting for ~10 s during the GOES C7.9 flare on 25 September 2011 (SOL2011-09-25T03:32) was only ~2~arcsec wide and ~10~arcsec long. Comparison with HMI magnetograms and AIA images at 1700 Å shows that the X-ray emission was primarily from a long ribbon in the region of positive polarity with little if any emission from the negative polarity ribbon. However, a thermal plasma source density of ~1012}~cm^{-3 estimated from the RHESSI-derived emission measure and source area showed that this could best be interpreted as a coronal hard X-ray source in which the accelerated electrons with energies less than ~50 keV were stopped by Coulomb collisions in the corona, thus explaining the lack of the more usual bright X-ray footpoints. Analysis of RHESSI spectra shows greater consistency with a multi-temperature distribution and a low energy cutoff to the accelerated electron spectrum of 22 keV compared to 12 keV if a single temperature distribution is assumed. This leads to a change in the lower limit on the total energy in electrons by an order of magnitude given the steepness of the best-fit electron spectrum with a power-law index of ~6.

Authors: Brian R. Dennis, Anne K. Tolbert

Publication Status: Accepted for publication in the Astrophysical Journal
Last Modified: 2019-10-30 13:06
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Always a Farm Boy  

Brian Dennis   Submitted: 2018-05-12 14:16

This is the memoir of Brian Dennis - his life story to date, and his involvement with high energy solar physics.

Authors: Brian R. Dennis
Projects: RHESSI,Solar Max Misson

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2018-05-14 10:46
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Detection and Interpretation Of Long-Lived X-Ray Quasi-Periodic Pulsations in the X-Class Solar Flare On 2013 May 14  

Brian Dennis   Submitted: 2017-06-13 16:25

Quasi-periodic pulsations (QPP) seen in the time derivative of the GOES soft X-ray light curves are analyzed for the near-limb X3.2 event on 14 May 2013. The pulsations are apparent for a total of at least two hours from the impulsive phase to well into the decay phase, with a total of 163 distinct pulses evident to the naked eye. A wavelet analysis shows that the characteristic time scale of these pulsations increases systematically from ~25 s at 01:10 UT, the time of the GOES peak, to ~100 s at 02:00 UT. A second ridge in the wavelet power spectrum, most likely associated with flaring emission from a different active region, shows an increase from ~40 s at 01:40 UT to ~100 s at 03:10 UT. We assume that the QPP that produced the first ridge result from vertical kink-mode oscillations of the newly formed loops following magnetic reconnection in the coronal current sheet. This allows us to estimate the magnetic field strength as a function of altitude given the density, loop length, and QPP time scale as functions of time determined from the GOES light curves and RHESSI images. The calculated magnetic field strength of the newly formed loops ranges from about ~500 G at an altitude of 24 Mm to a low value of ~10 G at 60 Mm, in general agreement with the expected values at these altitudes. Fast sausage mode oscillations are also discussed and cannot be ruled out as an alternate mechanism for producing the QPP.

Authors: Brian R. Dennis, Anne K. Tolbert, Andrew Inglis, Jack Ireland, Tongjiang Wang, Gordon D. Holman, Laura A. Hayes, and Peter T. Gallagher

Publication Status: Published in ApJ, 834:84, 2017
Last Modified: 2017-06-14 10:01
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Hard X-ray Flare Source Sizes Measured with RHESSI  

Brian Dennis   Submitted: 2009-04-23 08:55

RHESSI observations of 18 double hard X-ray sources seen at energies above 25 keV are analyzed to determine the spatial extent of the most compact structures evident in each case. The following four image reconstruction algorithms were used: Clean, Pixon, and two routines using visibilities - Maximum Entropy and Forward Fit (VFF). All have been adapted for this study to optimize their ability to provide reliable estimates of the sizes of the more compact sources. The source fluxes, sizes, and morphologies obtained with each method are cross-correlated and the similarities and disagreements are discussed. The FWHM of the major axes of the sources with assumed elliptical Gaussian shapes are generally well correlated between the four image reconstruction routines and vary between the RHESSI resolution limit of ~2'' up to ~20'' with most below 10''. The FWHM of the minor axes are generally at or just above the RHESSI limit and hence should be considered as unresolved in most cases. The orientation angles of the elliptical sources are also well correlated. These results suggest that the elongated sources are generally aligned along a flare ribbon with the minor axis perpendicular to the ribbon. This is verified for the one flare in our list with coincident TRACE images. There is evidence for significant extra flux in many of the flares in addition to the two identified compact sources, thus rendering the VFF assumption of just two Gaussians inadequate. A more realistic approximation in many cases would be of two line sources with unresolved widths. Recommendations are given for optimizing the RHESSI imaging reconstruction process to ensure that the finest possible details of the source morphology become evident and that reliable estimates can be made of the source dimensions.

Authors: Brian R. Dennis and Rick L. Pernak
Projects: RHESSI

Publication Status: Accepted by ApJ
Last Modified: 2009-04-23 09:12
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Review of Selected RHESSI Solar Results  

Brian Dennis   Submitted: 2005-03-27 13:23

We review selected science results from RHESSI solar observations made since launch on 5 February 2002. A brief summary of the instrumentation is given followed by a sampling of the major science results obtained from the soft X-ray, hard X-ray, and gamma-ray energy domains. The thermal continuum measurements and detection of Fe-line features are discussed as they relate to parameters of the thermal flare plasma for several events, including microflares. Observations of X-ray looptop, and rising above-the-loop sources are discussed as they relate to standard models of eruptive events and the existence of a current sheet between the two. Hard X-ray spectra and images of footpoints and coronal sources are presented, showing how they can be used to separate thermal and nonthermal sources and determine the magnetic reconnection rate. Gamma-ray line images and spectra are presented as they relate to determining the location, spectra, and angular distribution of the accelerated ions and the temperature of the chromospheric target material. Finally, we discuss the overall energy budget for two of the larger events seen with RHESSI.

Authors: Brian R. Dennis, Hugh S. Hudson, and Sam Krucker
Projects: RHESSI

Publication Status: Proceedings of CESRA Workshop 2004 The high energy solar corona: waves, eruptions, particles held on the Isle of Skye, Scotland, June 2004. Published in Lecture Notes in Physics, Berlin Springer Verlag, K.- L. Klein and A. L. MacKinnon, editors, vol. 725, page 33, 2007.
Last Modified: 2009-04-23 08:11
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Abstracts by Author
A Very Narrow RHESSI X-ray Flare on 25 September 2011
Always a Farm Boy
Detection and Interpretation Of Long-Lived X-Ray Quasi-Periodic Pulsations in the X-Class Solar Flare On 2013 May 14
Hard X-ray Flare Source Sizes Measured with RHESSI
Review of Selected RHESSI Solar Results

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