Vth - Variable Thermal

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**Relative to coronal abundance for Chianti
**Relative to coronal abundance for Chianti
**Relative to solar abundance for Mewe
**Relative to solar abundance for Mewe
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[[Category:Software]]
===Fitting with Vth===
===Fitting with Vth===
The variable thermal component of OSPEX is used to fit the thermal portion of a RHESSI spectrum. The thermal X-Ray emission of a solar flare is the electromagnetic radiation emitted by electrons in a thermal plasma. The thermal plasma is characterized by frequent exchange of energy among the thermalized electrons by collision until the plasma energies can be characterized by a single temperature.  
The variable thermal component of OSPEX is used to fit the thermal portion of a RHESSI spectrum. The thermal X-Ray emission of a solar flare is the electromagnetic radiation emitted by electrons in a thermal plasma. The thermal plasma is characterized by frequent exchange of energy among the thermalized electrons by collision until the plasma energies can be characterized by a single temperature.  

Revision as of 00:27, 12 September 2009

Contents

Vth

Introduction

Vth fits the thermal part of a spectrum. This usually extends from the ~4keV to ~20keV to ~25keV depending on the attenuator state of the spectrum being fit. Vth fits this portion of the spectrum with a single Emission Measure and Temperature. In cases where the Fe line is present at ~6.7keV Vth can be used to fit this feature as well using either the CHIANTI or MeWe databases for atomic emission lines.

Parameters

Below is the list of parameters available to the user when fitting with vth

Fitting with Vth

The variable thermal component of OSPEX is used to fit the thermal portion of a RHESSI spectrum. The thermal X-Ray emission of a solar flare is the electromagnetic radiation emitted by electrons in a thermal plasma. The thermal plasma is characterized by frequent exchange of energy among the thermalized electrons by collision until the plasma energies can be characterized by a single temperature.

The Emission Measure, a[0], measures the amount of material available to produce the observed flux from a thermal plasma. The temperature (kT), a[1], measures the single temperature that characterizes the plasma in keV. The relative abundance, a[2], measures the abundance of various ions during the flare relative to either the CHIANTI model for coronal abundances or the Mewe model for solar abundances. The most prominent of these lines measured by RHESSI is the Fe line at 6.7keV. Based on models the coronal abundance of Fe relative to hydrogen (H) in the corona is four times the abundance in the photosphere.

An example of a typical fit using vth is shown below.


The vth component of a fit in count flux using a full CHIANTI model(top panel). The same fit is shown in photon flux(bottom panel).


The above fit is a representative use of the vth. vth has been used to model the region of spectrum between 6keV and approximately 20keV. In this case the full CHIANTI model was used. CHIANTI automatically accounts for lines in the spectrum due to atomic interactions. In this case the primary line feature is the Fe line at 6.7keV. If the continuum option is selected with vth the shape of the vth model is determined entirely by the Emission Measure and Temperature, all lines then have to be added individually as Gaussians. The MeWe database also models atomic interactions. It predates CHIANTI and is usually only used in cases where original fits were done using the MeWe database and are being restored.

The thermal component is used to model the data until there is a break in the spectrum. In theory the photon flux falls as an exponential during the thermal portion of the spectrum until it starts to fall as a power law at the break energy during the non-thermal portion of the spectrum. In practice the break in the spectrum is rarely easy to identify. One technique for deciding on good values for the vth parameters is to look at a time series of the values of the parameters over adjacent time intervals. An easy way to do this is using the view fit results GUI. It is expected that the Emission Measure will rise during the impulsive phase of the flare and drop during the gradual phase in a continuous manner, with the temperature doing the same. Both parameters should vary over time in a smooth fashion. If values of the parameters jump up and down over several adjacent time intervals outside of statistical fluctuations its a reasonable to suppose that the fit parameters or the entire model are insufficient to account for the data.

Another good way to check a fit using vth is to look at the values of the parameters. vth gives the temperature, kT, in keV. This can be converted to megakelvin (MK) by multiplying the temperature in vth by 11.6MK/keV. Using this conversion a comparison can be made between the value of the temperature from the fit and a reasonable temperature for the sun. For instance a temperature of 8keV from a fit is unreasonable since it means the sun would have a temperature of 92MK, much larger than anything that has been measured to date. This same test can be applied to other parameters such as the relative Fe abundance. If it becomes large (greater than ~2) this would lead to questioning the fit since the coronal abundance would be eight times the photospheric abundance.

Synopsis

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