Vth - Variable Thermal

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

• a[0] em_49, emission measure in units of 10⁴⁹cm^-3
• a[1] KT, plasma temperature in keV
• a[2] Relative abundance for Fe, Ni, Si, and Ca. S as well at half the deviation from 1 as Fe.
  • Relative to coronal abundance for Chianti
  • Relative to solar abundance for Mewe

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 the 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 is usually only used in cases where original fits were done using the MeWe database and are being restored.