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Steven Christe   Submitted: 2008-04-16 11:56

We present the first exhaustive list 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 algorithm is described fully in Christe et al. (2008). This paper (along with Hannah (2008)) describes the most recent analysis of this list. This RHESSI Microflare list is not a subset of the official RHESSI flare list (included in SSW and the HESSI GUI). The RHESSI microflare list is now made available to the community in a variety of formats. Click the link to read more about the RHESSI Microflare list and to download the list.

Authors: Steven Christe
Projects: RHESSI

Publication Status: published
Last Modified: 2008-09-23 21:10
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Steven Christe   Submitted: 2008-02-05 19:45

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 1032x1033 erg in 102x103 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.

Authors: Steven Christe
Projects: RHESSI

Publication Status: published
Last Modified: 2008-02-06 05:32
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Steven Christe   Submitted: 2008-02-05 19:44

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 1032x1033 erg in 102x103 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.

Authors: Steven Christe
Projects: RHESSI

Publication Status: published
Last Modified: 2008-02-05 19:44
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Steven Christe   Submitted: 2008-01-15 16:00

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. Using the ratio of photon fluxes between 10-15 keV and 15-20 keV, we find 4< gamma <12, with an average of 7.4. Based on these values, the nonthermal power is calculated. The microflare occurrence frequency varies with the rate of energy release consistent with a power-law with an exponent of -1.7±0.1. We estimate the total energy flux deposited in active regions by microflare-associated accelerated electrons (>10 keV) over the 5 years of observations to be, on average, below 1026 erg s^-1.

Authors: S. Christe, I. G. Hannah, S. Krucker, J. McTiernan, and R. P. Lin
Projects: RHESSI

Publication Status: in press
Last Modified: 2008-01-16 07:41
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