by Steven Christe, Sam Krucker, R. P. Lin
Space Sciences Laboratory, University of California, Berkeley, CA 94720
The Astrophysical Journal, Volume 680, Issue 2, pages 149-152 (ADS link)
Abstract
During a period of 12 minutes on 2002 July 19 14:23-14:35 UT, the WAVES instrument on WIND observed six interplanetary type III radio bursts, one approximately every 2 minutes, and each was accompanied by a weak hard X-ray (HXR) burst (12-15 keV) observed by RHESSI. The radio bursts are observed up to 150 MHz with some up to 600 MHz. Simultaneous observations by TRACE show jetlike eruptions emanating from the region of HXR emission. The observed HXRs are inconsistent with emission from the escaping type III-producing nonthermal electrons. We suggest that the type III acceleration process may be associated with an explosive release of <~5×1026 ergs in the form of a “superhot” (26 MK) thermal plasma in the corona, an energy comparable to that associated with the type III-producing electrons.
by Pascal Saint-Hilaire, Sam Krucker, Steven Christe, and Robert P. Lin
Space Sciences Laboratory, University of California, Berkeley, CA 94720
Received 2008 August 21; accepted 2009 February 2; published 2009 April 20
The Astrophysical Journal, Volume 696, Pages 941–952, 2009 (ADS link)
Abstract
We study the detectability and characterization of electron beams as they leave their acceleration site in the low
corona toward interplanetary space through their nonthermal X-ray bremsstrahlung emission. We demonstrate that
the largest interplanetary electron beams (1035 electrons above 10 keV) can be detected in X-rays with current and future instrumentation, such as RHESSI or the X-Ray Telescope (XRT) onboard Hinode. We make a list of optimal observing conditions and beam characteristics. Amongst others, good imaging (as opposed to mere localization or detection in spatially integrated data) is required for proper characterization, putting the requirement on the number of escaping electrons (above 10 keV) to 3
× 1036 for RHESSI, 3 × 1035 for Hinode/XRT, and 1033
electrons for the FOXSI sounding rocket scheduled to fly in 2011. Moreover, we have found that simple modeling
hints at the possibility that coronal soft X-ray jets could be the result of local heating by propagating electron
beams.
by Steven Christe
U.C. Berkeley Ph.D. Dissertation
file [pdf]
The Sun is the most powerful particle accelerator in the solar system, accelerating ions up to tens of GeV and electrons to hundreds of MeV in solar flares and in coronal mass ejections. Solar flares are the most powerful explosions, releasing up to 1032–1033 erg in 102–103 seconds. How the Sun releases this energy and how it rapidly accelerates electrons and ions with high efficiency, and to such high energies, is still not understood. The process of particle acceleration in magnetized plasmas are thought to occur throughout the universe from Earth’s magnetosphere to active galactic nuclei and supernova shocks. The Sun is a unique laboratory for studying these processes. Its proximity allows us to observe it with unparalleled sensitivity and spatial resolution and energetic particles can be sampled directly at Earth after escaping the Sun. The Sun can provide the key to understanding acceleration processes and energy release occurring on cosmic scales. In this thesis, we consider weak hard X-ray (HXR) bursts. In chapter 1, an introduction to the subject of solar observations is presented. Chapter 2 introduces the theory of Coulomb interactions whose understanding is necessary to the quantitative analysis of HXRs. In Chapter 3, the main instrument used in this study is described, the Reuven Ramaty High Energy Spectroscopic Solar Imager (RHESSI). A statistical analysis of the largest sample of RHESSI microflares is presented in Chapter 4. RHESSI microflares are found to be similar to large flares and not important to coronal heating. In Chapter 5, a series of HXR bursts associated with Type III radio bursts are analyzed. It is found that they are a signature of the acceleration process. In Chapter 6, we introduce HXR focusing optics and a new instrument, FOXSI, short for the Focusing Optics X-ray Solar Imager. With its large sensitivity and dynamic range, FOXSI will directly image energetic electron beams as they are accelerated and travel through the corona. FOXSI will be a pathfinder for the next generation of solar HXR observatories.
by S. Christe, I. G. Hannah, S. Krucker, J. McTiernan, and R. P. Lin
Received: 2007 September 19. Accepted: 2007 December 17
The Astrophysical Journal. Volume 677, Issue 2, Page 1385–1394, Apr 2008
file [pdf]
We present the first in-depth statistical survey of all X-ray microflares observed by RHESSI between March 2002 and March 2007, a total of 25,705 events, an order of magnitude larger then previous studies. These microflares were found using a new flare-finding algorithm designed to search the 6-12 keV count-rate when RHESSI’s full sensitivity was available in order to find the smallest events. The peak and total count-rate are automatically obtained along with count spectra at the peak and the microflare centroid position. Our microflare magnitudes are below GOES C Class, on average GOES A Class (background subtracted). They are found to occur only in active regions, not in the “quiet” Sun, and are similar to large flares. The monthly average microflaring rate is found to vary with the solar cycle and ranges from 90 to 5 flares a day during active and quiet times, respectively. Most flares are found to be impulsive (74%), with rise times shorter than decay times. The mean flare duration is ~6 minutes with a 1 minute minimum set by the flare-finding algorithm. The frequency distributions of the peak count-rate in the energy bands, 3-6 keV, 6-12 keV, and 12-25 keV can be represented by power-law distributions with a negative power-law index of 1.50+/-0.03, 1.51+/-0.03, and 1.58+/-0.02, respectively. We find that these power-law indices are constant as a function of time. The X-ray photon spectra for individual events can be approximated with a power-law spectrum, dJ/d(h\nu) ~ (h\nu)-g. Using the ratio of photon fluxes between 10-15 keV and 15-20 keV, we find 410 keV) over the 5 years of observations to be, on average, below 1026 erg s-1.
Säm Krucker, E. Kontar, Steven Christe, R.P. Lin
Astrophysical Journal, 2007, 663, 109
NASA ADS Reference
bibtex reference
file [pdf]
We compare hard X-ray (HXR) photon spectra observed by the RHESSI with the spectra of the electrons in the associated solar impulsive particle events observed near 1 AU by the WIND 3D Plasma and Energetic Particle (3DP) instrument. For prompt events, where the inferred injection time at the Sun coincides with the HXR burst, the HXR photon power-law spectral index γ and the in situ observed electron spectral index δ measured above 50 keV show a good linear fit, δ=γ+0.1(+/-0.1), with correlation coefficient of 0.83, while for delayed events (inferred injection >10 minutes after the HXR burst) only a weak correlation with a coefficient of 0.43 is seen. The observed relationship for prompt events is inconsistent, however, with both the thin target case, where the escaping electrons come from the X-ray-producing electron population, and the thick target case where some of the accelerated source population escapes to 1 AU and the rest produce the HXRs while losing all their energy to collisions. Furthermore, the derived total number of escaping electrons correlates with the number of electrons required to produce observed X-ray flux but is only about ~0.2% of the number of HXR-producing electrons.
Säm Krucker, Steven Christe, R.P. Lin, Gordon J. Hurford, R.A. Schwartz
Solar Physics, 2002, 210, 445
NASA ADS Link
bibtex Reference
file [pdf]
The excellent sensitivity, spectral and spatial resolution, and energy coverage down to 3 keV provided by the Reuven Ramaty High-Energy Solar Spectroscopic Imager mission (RHESSI) allows for the first time the detailed study of the locations and the spectra of solar microflares down to 3 keV. During a one-hour quiet interval (GOES soft X-ray level around B6) on 2 May, 1:40 2:40 UT, at least 7 microflares occurred with the largest peaking at A6 GOES level. The microflares are found to come from 4 different active regions including one behind the west limb. At 7′′ resolution, some events show elongated sources, while others are unresolved point sources. In the impulsive phase of the microflares, the spectra can generally be fitted better with a thermal model plus power law above 6-7 keV than with a thermal only. The decay phase sometimes can be fitted with a thermal only, but in some events, power-law emission is detected late in the event indicating particle acceleration after the thermal peak of the event. The behind-the-limb microflare shows thermal emissions only, suggesting that the non-thermal power law emission originates lower, in footpoints that are occulted. The power-law fits extend to below 7 keV with exponents between -5 and -8, and imply a total non-thermal electron energy content between 1026 1027 erg. Except for the fact that the power-law indices are steeper than what is generally found in regular flares, the investigated microflares show characteristics similar to large flares. Since the total energy in non-thermal electrons is very sensitive to the value of the power law and the energy cutoff, these observations will give us better estimates of the total energy input into the corona.
Seth Kulin, S. Aubin, Steven Christe, Boris Peker, S. L. Rolston, L. A. Orozco
J. Opt. B, Quantum Semiclass. Opt, 3, 353
link
file [pdf]
We present an optical trap for atoms which we have developed for precision spectroscopy measurements. Cold atoms are captured in a dark region of space inside a blue-detuned hollow laser beam formed by an axicon. We analyse the light potential in a ray optics picture and experimentally demonstrate trapping of laser-cooled metastable xenon atoms.
