Spectral Breaks in Flare HXR Spectra A Test of Thick-Target Nonuniform Ionization as an Explanation Yang Su NASA,CUA,PMO Gordon D. Holman NASA Brian R. Dennis NASA Napa, CA Dec.10.08

 1/2 Nonuniform Ionization  1/3-1/2: Introduction  2/3-1/2: Models  3/3-1/2: RHESSI Observation  2/2 Time evolution and Imaging spectroscopy  Flux of one source from Clean, Pixon  time evolution of spectral breaks  Image Spectroscopy, spectra from footpoints (spectral breaks)

 Solar flare HXR spectra  single / double power-law  time evolution (Dulk et al. 1992; Lin & Schwartz 1987)  break energy: typically between ~50 and 100 keV  Spectral breaks is important  acceleration mechanisms  electron propagation and energy losses  relationships between flare X-ray sources, radio sources, and particles 1/3-1/2 Introduction

 For the count and photon spectra  Instrumental effects, such as pulse pile-up (Smith et al. 2002)  Additional components, such as:  Albedo (Kontar et al. 2006; Kontar & Brown 2006; Zhang & Huang 2004)  emission from thermal plasma 1/3-1/2 Introduction

 For the accelerated electrons  Non-power-law electron distribution from the acceleration process, e.g.  a double power-law electron distribution  a low-energy cutoff (Gan et al. 2002; Sui et al. 2007)  a high-energy cutoff (Holman 2003)  An anisotropic electron pitch-angle distribution (Petrosian 1973; Massone et al. 2004)  Beam-plasma instability (Holman et al. 1982; Melrose 1990)  Return current energy losses (Knight & Sturrock 1977; Zharkova & Gordovskyy 2006)  Nonuniform target ionization (Brown 1973; Brown et al. 1998; Kontar et al. 2002) 1/3-1/2 Introduction

 Aims  Spectrum from nonuniform ionization thick-target with full cross section  Can nonuniform ionization model explain the spectral breaks in observations?  And how many? 1/3-1/2 Introduction

 Nonuniform target ionization  Electron energy losses lower in un-ionized or partially ionized plasma than in fully ionized plasma  Brown et al. 1998, x(N) is the ionization level 2/3-1/2 Model effective column density M

 step-function  Brown 1973, Kontar et al. (2002)  the atmospheric ionization  the Kramers approximation of the cross section, q=1  linear-function  the atmospheric ionization  When N 0 = N 1 =N *, step function  full relativistic cross section of Bethe and Heitler 2/3-1/2 Model

step linear E * = E1= 30 keV E e =60 keV stops here (M 0 ) N N1N1 N0N0 2/3-1/2 Model

δ=4.5 (best fit γ=3) (Brown 1973) 2/3-1/2 Model

Relation between N and E Photon flux from linear-function model F c =10 35 electrons s -1 ; E c = 1 keV (=0 for N 1 =N 0 ) 2/3-1/2 Model

Photon spectra and photon spectral index γ from the four models with δ=4 Arrows: upward knee, downward knee and γ(ε) for fully ionized model (not constant)

Spectra from linear-function model with fixed E 1 and increasing E 0 2/3-1/2 Model

 RHESSI flare sample  2002 February December 31. Non-solar and particle events were excluded.  keV count rate > 300 counts s -1 detector -1. the keV count rate to be at least 3σ above the background count rate. (F 50 )  Radial distance > 927” from disk center (>~ 75 degrees longitude at the solar equator)  This minimizes the impact of albedo on the X-ray spectrum (Kontar et al. 2006)  Detector corrected count rate live times> 90%. This gave a final sample size of 20 flares.  This minimizes the impact of pulse pile-up (Smith et al. 2002; Ka·sparov¶a et al. 2007). 3/3-1/2 RHESSI Observation

 1/3 keV bins from 3 to 15 keV and 1 keV bins above 15 keV  All RHESSI front detectors  no 2 and 7 -- poor energy resolution  no 5 for the 30 Nov 2003 flare -- unusually low livetime  no 8 for some flares -- interference from RHESSI's communication antenna  One spin period, mostly at the HXR peak time  Full RHESSI response matrix, instrumental systematic uncertainty: zero (Sui et al. 2007)  Isothermal + three spectral lines+ nonthermal models  Two steps for fit, first fit above 6 keV, then fix thermal comp. then fit above 15 keV  the ion line complex at ~6.7 keV  the ion/nickel line complex at ~8 keV (Phillips 2004)  and a nonsolar line at ~10.5 keV  CLEAN Images : keV for same time interval 3/3-1/2 RHESSI Observation

Examples for poor fit (left) and good fit (right)

3/3-1/2 RHESSI Observation fit results from: Bpow fit F_ion fit (Kramers) N_ion fit (full cs)

∆γ VS δ ∆γ from bpow fit δ from step-function fit 3/3-1/2 RHESSI Observation

 full cs and Kramers (up to 36% on flux and 6.8% on γ)  step and linear  upper limit on ∆γ of spectra from nonuniform ionization model  In 20 F 50 flares (around peak)  5 with single, 15 with broken  10 out of 15 F 50 flares can not be explained by nonuniform ionization alone  All the 5 that can be explained by non-ion have DF sources -1/2 Summary

 Aims:  spectral breaks VS time  How HXR sources change when the spectra change from single to b-pow  spectrum from each footpoint  relation between spectral breaks for footpoints and total spectrum 2/2 Time evolution and Imaging spectroscopy

Flux from single source of one image: flare id: , 22:32 energy range: keV 2/2 Time evolution and Imaging spectroscopy

pixon D2-D8, , , pixon D2-D8, including background model, Clean D2-D8, different iterations, 300, stop if, Normal, no stop MM=Media Mode 50: : : : : : : : : : : : Clean D2-D8, 4.06s 100: : : : : : /2 Time evolution and Imaging spectroscopy

keV Pixon: center , , circle:9, Flux Area Centroid (X,Y) Peak (X,Y) St Dev (X,Y) Peak Clean D2-D9, different iterations 300, stop if, MM: Normal, MM 50: : : : : : : : : : /2 Time evolution and Imaging spectroscopy

?/? The highest HXR source???

To be continued ?/? Direct observation of reconnection???