Yang Su NASA,CUA,PMO young.su@yahoo Gordon D. Holman NASA Brian R. Dennis NASA

Spectral Breaks in Flare HXR Spectra

A Test of Thick-Target Nonuniform Ionization as an Explanation

Yang Su NASA,CUA,[email protected]

Gordon D. Holman NASABrian R. Dennis NASA

Napa, CA Dec.10.08

1/2 Nonuniform Ionization1/3-1/2: Introduction 2/3-1/2: Models3/3-1/2: RHESSI Observation

2/2 Time evolution and Imaging spectroscopyFlux of one source from Clean, Pixontime evolution of spectral breaksImage Spectroscopy, spectra from footpoin

ts (spectral breaks)

Solar flare HXR spectrasingle / double power-lawtime evolution (Dulk et al. 1992; Lin &

Schwartz 1987)break energy: typically between ~50 and 100

keV

Spectral breaks is importantacceleration mechanismselectron propagation and energy lossesrelationships between flare X-ray sources,

radio sources, and particles

1/3-1/2 Introduction

For the count and photon spectraInstrumental 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


For the accelerated electronsNon-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)


AimsSpectrum from nonuniform ionization

thick-target with full cross sectionCan nonuniform ionization model exp

lain the spectral breaks in observations?

And how many?


Nonuniform target ionizationElectron 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-functionBrown 1973, Kontar

et al. (2002)the atmospheric

ionization

the Kramers approximation of the cross section, q=1

linear-functionthe atmospheric

ionization

When N0 = N1=N*, step function

full relativistic cross section of Bethe and Heitler

2/3-1/2 Model

step linear

1

0

0

1

0

E*=E1=30 keV

Ee=60 keV stops here (M0)

N

N1

N0

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

Fc=1035 electrons s-1; Ec= 1 keV

(=0 for N1=N0)

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 E1 and increasing E0

2/3-1/2 Model

RHESSI flare sample2002 February 12 - 2004 December 31. Non-solar

and particle events were excluded.12-25 keV count rate > 300 counts s-1 detector-1.

the 50-100 keV count rate to be at least 3σ above the background count rate. (F50)

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 mo

dels 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 : 40-60 keV for same time interval


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


fit results from:Bpow fitF_ion fit (Kramers)N_ion fit (full cs)

∆γ VS δ∆γ from bpow fitδ from step-function fit


full cs and Kramers (up to 36% on flux and 6.8% on γ)

step and linear upper limit on ∆γ of spectra from non

uniform ionization model In 20 F50 flares (around peak)

5 with single , 15 with broken 10 out of 15 F50 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 f

ootpoints and total spectrum

2/2 Time evolution and Imaging spectroscopy

Flux from single source of one image:flare id: 4010604, 22:32energy range: 40-60 keV


pixon D2-D8, -973.855, 75.125, 9 32.147 pixon D2-D8, including background model , 32.106

Clean D2-D8, different iterations, 300, stop if, 46.290

Normal, no stop MM=Media Mode50: 51.356 50: 30.223100: 43.952 100: 33.154300: 37.070 300: 34.189 500: 35.290 500: 34.456 700: 34.434 700: 34.5291000: 33.751 1000: 34.667

Clean D2-D8, 4.06s100: 43.996 100: 33.311300: 37.001 300: 34.1961000: 33.477 1000: 34.514


150-250 keVPixon: center -972.855, 73.125, circle:9, 2.4521 Flux Area Centroid (X,Y) Peak (X,Y) St Dev (X,Y) Peak2.4521 290.00 -973.92 72.75 -973.28 73.15 3.94 3.95

0.026954

Clean D2-D9, different iterations300, stop if, 6.5105 MM: 4.0746Normal, MM50: 5.3177 50: 4.2477100: 3.9565 100: 4.4367 300: 1.8182 300: 4.5518 500: 1.1578 500: 4.5647 700: 0.9261 700: 4.5654


?/? The highest HXR source???

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

Yang Su NASA,CUA,PMO young.su@yahoo Gordon D. Holman NASA Brian R. Dennis NASA

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