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Description of the Electron Spin-Angle Distribution Plots

Description of the Electron Spin-Angle Distribution Plots

John Steinberg Mar 6, 2002

General

In each panel, the relative electron flux at a given energy is represented using a color scale, the highest flux being red. The electron flux is plotted as a function of spacecraft spin angle (vertical axis) and time (horizontal axis). Genesis is oriented with the spin axis vector lying in the ecliptic and generally pointed sunward. The spin angle (vertical axis) value of 0 , or equivalently 360 , corresponds to the position where the GEM aperture look direction is northward. Strictly speaking, it is not the electron flux that is plotted, but rather a reduced distribution function value. But you need not understand what is meant by "reduced distribution function" in order to use these plots to make qualitative conclusions about the solar wind electrons. The dedicated reader will find the reduced distribution function explained at the bottom of this page.

Coronal hole and interstream solar wind

Measured in the vicinity of Earth, solar wind electrons at energies above about 100 eV can generally be considered to be supra-thermal electrons. The supra-thermal electrons typically do not have the same flux in all directions. Rather, these electrons usually have a flux peaked along a direction parallel (or anti-parallel) to the local interplanetary magnetic field direction. As the Genesis spacecraft rotates, the field of view for GEM sweeps across the interplanetary magnetic field direction. GEM then detects a peak flux centered at the spin angle into which the field direction projects. For ordinary stationary solar wind flows ( i.e. not coronal mass ejection transient ) the supra-thermal electrons stream away from the sun along magnetic field lines having only one foot in the solar corona. The electron flux is uni-directional, and GEM detects a peak flux only once per spin.

Here is an example of a well-defined uni-directional electron flux from the Genesis Browse plots

The time interval shown is 1 day. The single red "stripe" is the uni-directional supra-thermal electron flux. Note that the peak angle is not steady, but has some jitter. This is because the interplanetary magnetic field, which is guiding the electrons, is not steady but fluctuates.

 

Coronal mass ejection solar wind

At times, the supra-thermal electrons exhibit peak flux both parallel and anti-parallel to the interplanetary magnetic field. This phenomenon is often refered to as "counter-streaming electrons" or "bi-directional electron streaming" (BDES). The interplanetary magnetic field within many coronal mass ejection (CME) transient flows is believed to form loops that return to the solar corona along both directions. Observation of BDES is interpreted to be a probable indicator of those closed loops. However, BDES does not occur exclusively within CMEs, but can occur for other reasons as well. Thus, the Genesis regime selection algorithm does not select the CME regime based on BDES alone. An additional CME indicator is required. When the electron flux is bi-directional, GEM will detect a peak flux twice per spin, and the two peaks will be 180 apart.

Here are several one-day examples of good coronal mass ejection-related bi-directional electron streaming:

Above is a "typical best example" day.

Early on this day, passage of an interplanetary shock is seen by the high flux filling in at all angles. Post-shock, we see two distinct beams emerge. Note that the plot "wraps" in angle: the peak flux at 0 and at 360 are from the same beam!

Above is a one-day example exhibiting a distinct onset of BDEs.

Solar energetic particle events

One a few occasions, GEM has been hampered by the effects of solar energetic particle events. The penetrating particle radiation results in a high background count rate for GEM which overwhelms the solar wind electron count rate. This is an infrequent occurrence. However, you may notice the plots going "all red" when SEP events happen. (Please not that "all red" does not necessarily mean the instrument saturated; it may simply be that the color scale for plotting reached saturation, but not the instrument.)

Here is what the one-day plots look like when solar energetic particles produce a high background count rate:

 

And the kitchen sink

Above we have shown examples of clear uni-directional electrons, and clear bi-directional electrons. But, as you may have guessed, the signatures in the browse data plots are not always so clear or persistent. The example below shows a variety of features all on one day. The electron signiture is initially indeterminate, follow by intervals which are uni-directional, and intermittantly bi-directional. Finally at the end of the day the signatures are indeterminate again. We also show the bottom panel from the Genesis Browse Solar Wind summary plot for the same time interval. Compare for yourself the results obtained from the autonomous onboard BDE detection algorithm to the Electron Spin-Angle Distribution plot.

Here is a day of Genesis observations with some uni-directional electrons, some bi-directional electrons, and some intervals which are indeterminate.

 

As you use the Browse data plots, you will probably find it useful to swtich back and forth between the Solar Wind summary plot and the Electron Spin Angle Distribution plot, and compare the autonomous BDE detection algorithm results to the color plots.

What we really plot: the reduced distribution function

From the counts accumulated by the Genesis Electron Monitor, we obtain the electron distribution function

which has units of

( electrons) / ( (cm-3)(cm/s)-3)

The variables , and are defined as follows:

Particle speed (cm/s)

Polar angle relative to the spacecraft spin axis vector direction

Azimuth angle or spacecraft spin angle

The distribution function is defined so that the electron number density n (cm-3) is obtained when the distribution function is integrated over all velocity space as follows:

The quanitity whose magnitude is represented by color intensity in the Genesis "spin angle plots" is the distribution function where we have integrated over the polar angle.

which still has units of

( electrons) / ( (cm-3)(cm/s)-3)

How to use the color scale bar

The scale bar on the right hand side of each plot indicates the range of values of distribution function covered by the color bar within each plot. The color scale is DIFFERENT FOR EACH PANEL! The color red does not indicate the same flux in different panels. The table below indicates how to interpret the color scale within each panel of the plot fragment shown above. The thick line segments by each panel indicate how the range displayed for a given panel compares to the total sensitivity range of GEM.

Range of distribution function value for each panel

PANEL

BLUE

minimum value of

(cm-3)(cm/s)-3

RED

maximum value of

(cm-3)(cm/s)-3

TOP

1.7x10-32

32 x10-32

5

5.6 x10-32

120 x10-32

4

1.9 x10-31

46 x10-31

3

6.3 x10-31

172 x10-31

2

2.1 x10-30

66 x10-30

Bottom

7.3 x10-30

251 x10-30

Questions?

 


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