# Presentation: 049611

## The Low-Energy Cutoff to the Nonthermal Electron Spectrum Determined From RHESSI Observations

- Linhui Sui (CUA/NASA GSFC), Gordon D. Holman, Brian R. Dennis (NASA GSFC)

## What is the Low-Energy Cutoff?

- Nonthermal Electron Spectrum

**Cutoff Energy**

**Sharp cutoff**

**Flat cutoff**

- Expected Photon Spectrum

**Flattening**

**E****cutoff ****= 25 keV**

## Why Study the Low-Energy Cutoff?

- Identify thermal and nonthermal contribution to the total flare spectra
- Calculate the total energy of  nonthermal electrons
- Constrain models for particle acceleration	(e.g., Holman 1985)

## What is the Difficulty?

- RHESSI Flare Spectrum

**Preheating**

-  High threshold of spectrometers
-  Insufficient spectral resolution
-  Strong thermal bremsstrahlung
-    at low energies

## An Early Impulsive Flare

-  **Hard X-ray flux (> 25 keV) increases before the soft X-ray flux ****	****rises significantly. **

- M1.0 flare on 06/02/2002, Heliocentric Angle: 21o

## Spectral Fitting

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  Cutoff energy: **28 keV**

- Flattening

- EM: 6e46 cm-3  T: 15 MK Index: 5.0

## Spectral Fitting

- Flattening

- EM: 1.6e47 cm-3  T: 15 MK Index: 5.0

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  Cutoff energy: **33 keV**

## Spectral Fitting

- Flattening

- EM: 1.5e47 cm-3  T: 17 MK Index: 4.8

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  Cutoff energy: **38 keV**

## Spectral Fitting

- Flattening

- EM: 8e47 cm-3  T: 14 MK Index: 4.2

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  Cutoff energy: **23 keV**

## Spectral Fitting

- Flattening

- EM: 6e47 cm-3  T: 17 MK Index: 4.3

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  Cutoff energy: **32 keV**

## Spectral Fitting

- EM: 9e47 cm-3  T: 17 MK Index: 3.7

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  ***Upper limit*** of Cutoff energy: **24 keV**

## Time History

**In general, the low-energy cutoff correlates with HXR flux.**
- The cutoff energy suddenly decreased at the second peak.
- The nonthermal energy input correlates with HXR flux.
- E**Nonthermal**** **     **<    **E**Thermal**** **
- 	1029 ergs                4 X1030 ergs

- 25-50 keV

- 50-100 keV

## Photospheric Albedo

- Hard X-rays incident upon the photosphere with energies **> 10 keV** are backscattered due to Compton collisions with electrons. (Tomblin 1972, Bai & Ramaty 1978, Alexander & Brown 2002)
- Albedo effect depends on power-law index, location, source height.
- Observed photon flux:
***       ******I******obs****** = ******I******source****** + ******I******albedo***

- Bai & Ramaty 1978

## Albedo Correction

** ****Albedo effect may contribute the spectral flattening to some extent. **

- No Albedo correction

- Albedo correction (isotropic source)

- Cutoff Energy: 38 keV

- Cutoff Energy: 40 keV

- up

- down

## Preliminary Results of Albedo Correction 

- In many cases, the low-energy cutoff decreases by ~1 to 3 keV with isotropic albedo correction.
- Spectral flattening is unlikely to be fully caused by the albedo effect.

- 25-50 keV

- 50-100 keV

**Albedo**

## Conclusions

- Flattening in the X-ray spectrum around 20 keV has been seen in an **early** impulsive flare.
- Existence of a low-energy cutoff to the nonthermal electron spectrum gives a natural explanation.
- The cutoff energy generally correlates with the HXR flux.
- Albedo effect may contribute to the flattening, but is unlikely to be the only cause.
- Many more similar flares have been seen with RHESSI.

## Spectral Fitting

- EM: 6e47 cm-3  T: 20 MK Index: 4.0

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  ***Upper limit*** of Cutoff energy: **22 keV**

## Spectral Fitting

- EM: 8e47 cm-3  T: 20 MK Index: 4.4

-  Thermal spectra is produced by
-    isothermal  plasma
-  Nonthermal spectra is produced by
-    thick-target bremsstrahlung from
-    nonthermal electrons
-  Two spectral lines: Fe and Fe/Ni
-  ***Upper limit*** of Cutoff energy: **23 keV**

## Flat Cutoff

- Sharp Cutoff

- Flat cutoff

- Sharp Cutoff Energy: 38 keV

- Flat Cutoff Energy: 51 keV

## Albedo Correction

- No Albedo correction

- Albedo correction (anisotropic source)

- Cutoff Energy: 38 keV

- Cutoff Energy: 52 keV

- up

- down

** ****Albedo effect may contribute the spectral flattening to some extent. **

## Albedo Correction

- No Albedo correction

- Cutoff Energy: 38 keV

- Cutoff Energy: 56 keV

- Albedo correction (anisotropic source)

- up

- down

** ****Albedo effect may contribute the spectral flattening to some extent. **