Selecting Background for RHESSI Spectra

From RHESSI Wiki

Options for choosing Background when fitting RHESSI Spectra

Introduction

Before a fit is done for a selected time interval using a RHESSI Spectrum file background must be selected. There are two types of background that need to be subtracted from RHESSI spectra. The preflare solar background and the non-solar background. The preflare background is the solar activity measured by RHESSI before a flare begins or after it ends. The non-solar background is primarily due to cosmic-ray interactions with the spacecraft, the spacecraft passing through the South Atlantic Anomaly, or electron precipitation events from the outer radiation belt.

Methods for selecting Background

Using the same background for all energy bands

This is the simplest method for selecting background. The basics are described in OSPEX Documentation. Background is selected by choosing a single time interval and using the spectrum as a function of energy during this interval as the background, The interval selected is usually during nighttime at the start or end of the RHESSI orbit. During these times RHESSI is measuring the non-solar background. The background defined by this time interval is subtracted from all intervals before fitting. Note that this background is constant. It does not change as a function of time. It is assumed that the background remains the same regardless of what happens during the flare.

The next step in complexity when selecting background is to select multiple background intervals. The intervals are selected in the same manner as above. The advantage of selecting multiple intervals is it gives a better idea of how the background changes over the course of the flare if the background intervals are far enough apart in time. For example background intervals can be selected during nighttime at the start of the orbit and nighttime at the end of the orbit. The OSPEX background selection GUI allows the user to select the order of the polynomial used to model background. In cases were a single interval is selected this should be left at the 0Poly option. This means that a zeroth order polynomial is used to fit the background spectrum then subtracted from all intervals fit, otherwise small timescale variations in the background time interval can cause large errors in the background. When selecting more than one time interval a zeroth order polynomial can still be used. In this case the background from each interval is fit with a zeroth order polynomial then the two are averaged to get the overall background. However when the intervals selected are far enough apart in time a first order polynomial (1Poly) can be used to fit the background as well. This fit gives a better idea of how the background changes as a function of time. Examples of both types of background selections are shown below.

A zero order polynomial fit to the background. The two nighttime intervals at the start and end of orbit were selected (top panel). A first order polynomial fit to the background. The same time intervals were selected as in the above panel (lower panel).

Notice that with a 0Poly fit the background does not change as a function of time. The background is an average of the intervals selected. The subtracted background is constant regardless of time during the flare. The 1Poly fit to the background intervals fits a line between the selected intervals. In the case above the interval during nighttime at the end of the orbit has a higher count rate than the the nighttime interval preceding the orbit. The line defining the background has a positive slope as a function of time. This means the background that is subtracted from the data during each time interval increases with time. Using 1Poly with the same background selected for all energy bands results in a linear fit across all selected time intervals. The background will vary linearly with time, increasing or decreasing depending on the count rate during the intervals selected.

There are several other methods for selecting how the background is fit as a function of time. The complete list of options is as follows:

• 0Poly - fits the background intervals selected as a constant in time. Averages all selected intervals to calculate the background. When using a single interval always select this option.
• 1Poly - fits the background as a line. The slope of line depends on the relative count rates of the intervals selected. This should only be used when two or more time intervals are selected for background.
• 2Poly - fits the background as a second degree polynomial. This should only be used in special cases
• 3Poly - fits the background as a third degree polynomial. This should only be used in special cases
• exp - fits the background as an exponential. This should only be used in special cases

Selecting different backgrounds for separate energy bands

In some cases separate backgrounds can be selected for different energy bands of the RHESSI spectrum. At the top left of the Select Background GUI there is a button for selecting separate backgrounds for each energy band. Choosing this option leads to each energy band being listed separately in the GUI, which will now appear as below:

The OSPEX Select Background GUI with the Separate BK for each energy band option selected

Note each energy band is listed separately. The default energy bands are used. The user can change these to any bands using the Change button next to #Bands on the upper right of the GUI. Once energy bands have been selected the intervals used to define the background for each band can be set. This can be done in two ways. The first is by selecting the change button for any defined energy band and selecting the intervals to use for that band. The second is to select the Loop to Set Times button on the upper left of the GUI just above the listing of individual energy bands. This will start with the first energy band, allow the user to set background time intervals, then move on to the next energy band until background has been selected for all energy bands. For each energy band the type of function used to fit the background can be selected as well. The advantages of selecting a separate background for each energy band are different time intervals and fit functions can be chosen for each background. Each interval can set to a time were the user thinks a particular energy band is at the background level, this is useful because all energy bands do not have to be their background levels at the same time. The figure below shows the data along with the background for several energy bands. The energy bands each have two background time intervals defined and are fit with a linear function.

The data minus background along with the background for several energy bands as a function of time

Note the differences in each background. Depending on the energy band the background rises more quickly for some bands than others. For instance the 6.0keV to 11.9keV background is almost constant over time while the 25keV to 50keV background rises fairly sharply.

Using the ratio to the highest energy band to set background for separate energy bands

Another option when selecting different backgrounds for separate energy bands is to use the ratio of any given band to the highest energy band.

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