STEREO/LET Anisotropy Browse Plots:

PURPOSE:

One of the products generated by the STEREO/LET instrument is particle rates
measured in 16 different viewing directions, or sectors, distributed in two
fans each spanning 133 degrees of longitude in the ecliptic and about 30-40
degrees of latitude out of the ecliptic, one pointing toward the Sun centered
along the nominal Parker spiral direction and the other looking in the anti-
Sun nominal field direction; see the LET instrument paper for more details
(Mewaldt et al., Space Sci. Rev. 136, 285, 2008).  These rates are available
at various time resolutions in the "Sectored" directories under 

http://www.srl.caltech.edu/STEREO/Level1/LET_public.html

The browse plots provided here serve as an aid to quickly determine when
interesting periods with anisotropic particle intensities were detected by 
LET, or to help a user decide whether a period that may be interesting for 
other reasons (e.g., a large SEP event) showed detectable anisotropies in 
MeV ions.  The user will need to download the data from the above site to 
do a serious analysis, but these plots should help the user select which of 
these files to download.  

DESCRIPTION/INTERPRETATION:

These browse plots use the 4-6 MeV proton sectored rates, which provide the
best statistical accuracy for LET sectored rates throughout most of the 
STEREO mission (since late 2010 a lower-energy, generally higher-statistics 
1.8-3.6 MeV proton sectored rate is also available).  Each file contains 
data from a single spacecraft, Ahead (A) or Behind (B), for an entire year, 
with 4 days displayed per panel and 8 panels per page.  At each plotted point 
in time, a color bar is shown for each of the 16 sectors, where the color 
represents the ratio of the intensity in that sector divided by the average 
intensity in that time period, as indicated by the color scale on the right 
(for values higher or lower than those shown on the scale, the extreme, 
saturated colors are used).  The vertical position and length of each bar on 
the plot shows the angle to the Sun viewed by that sector and the horizontal 
position the time of the observation.  Data in the bottom half of each panel 
come from the LET fan facing towards the Sun along the Parker spiral (nominally
at 45 degrees on these plots, that is, centered in the lower band), while the 
top half is from the anti-Sun facing fan; the white, blank band in the 
middle is outside the LET field of view.

The 10-minute averages are used, but smoothed over 3 consecutive points.  
If a 10-minute time period does not have at least 3 counts in total 
(distributed over the 16 sectors) it is not plotted at all.  Thus, 
particularly near solar minimum (most obvious in 2009) there are many 
days that are completely blank when intensities were very low, while periods 
with slightly higher (but still quite low) intensities may have only a few 
sporadic lines (e.g., day 7 of 2011 on Ahead).  As the intensities become 
higher, the plot looks "speckled" (e.g., day 13 of 2011 on Ahead) due to 
statistical fluctuations of the very low counts.  In some such cases, 
when obvious patterns in the colors start to stand out from the "noise" 
(e.g., day 63 of 2011 on Ahead), meaningful anisotropies might be obtained 
by integrating over longer time periods (i.e., by using the rates with
lower time resolution).  For still higher intensities, the speckles 
disappear and the colors become smooth (e.g., day 67 of 2011 on Ahead).
Therefore, although the absolute intensity is not shown on these plots
(but can certainly be determined by downloading the data), some indication
of the relative intensity and how it varies with time can be obtained.

Periods of relatively large anisotropies (when intensities in a particular
direction are ~twice or ~half the average values) stand out on these
plots as relatively smooth patches of yellows and reds for high intensities
and deep blues and blacks for low intensities.  The approximate width of
the anisotropies, their flow directions (relative to the Sun or spacecraft,
not relative to the magnetic field), durations, and whether or not the 
flows were bidirectional may all be obtained by inspection.

CAVEATS/CAUTIONS:

1) At very high intensities, the LET instrument starts to disable high
gains in certain detector segments in order to reduce the counting rate
of protons and He to maintain a higher livetime for measuring the heavier
particles.  When in these "dynamic threshold" modes, some of the sectors
are no longer able to measure protons.  In dynamic threshold mode 2, the
outermost sectors of each fan no longer provide proton data, and are 
replaced by black bands in these plots (e.g., day 68 of 2011 on Ahead).
In dynamic threshold mode 3, only the center 2 sectors of each fan still
measure protons, so the black bands at the edges become broader (e.g.,
day 81 of 2011 on Ahead).  See the LET instrument paper (Mewaldt et al., 
Space Sci. Rev. 136, 285, 2008) for more information about dynamic 
thresholds.

2) In general, the LET viewing directions are in the ecliptic.  However, 
there are times when the spacecraft is pointed in atypical directions.
For example, there are periodic SECCHI stepped calibration rolls, and 
there were special observing campaigns for Comet Elenin (on Behind, from
about July 26 to August 12, 2011) and searches for objects near the 
Lagrange points; also STEREO-Behind was "upside down" essentially from 
launch until its final lunar swingby and entry into heliocentric orbit
(from October 26, 2006 until about January 19, 2007), and both spacecraft
were rolled off of the ecliptic by ~20 degrees or more much of the time
they were near Earth (through the first few months of 2007).  So while 
technically the angles shown on these plots are indeed "Angle to Sun" 
as labeled (since all these rolls were about the Sun-Spacecraft line), 
and while usually this means "angle west of the Sun in the ecliptic", 
that isn't always the case.  If you find interesting anisotropies that 
change direction in a peculiar way, please check the spacecraft pointing 
files at

http://www.srl.caltech.edu/STEREO/attorb.html

to be sure that you're seeing something other than an artifact of a
spacecraft roll maneuver.  Plots of the spacecraft roll angle from the
ecliptic (with the same time scales as these anisotropy browse plots)
are also available from 

http://www.srl.caltech.edu/STEREO/Level1/LET_public.html

where you'll also find Quicktime movies of the spacecraft orientation.

3) As described above, the RATIO of the intensity in each sector to
the AVERAGE intensity is being plotted.  Thus, there may often be an
apparent decrease in one direction when there's an increase in another
(e.g. late day 226 to early day 227 of 2011 on Ahead).  This does NOT
necessarily mean that absolute intensities "dropped out" in any direction;
download the data files if you're interested in such a period before 
being led astray.

4) Intensities and directions are shown in the SPACECRAFT frame, not
the solar wind frame.  For serious analysis, correction for the Compton-
Getting effect (i.e., translation to the solar wind frame) should be
done.  For protons of this energy, however, the effect is relatively
small, changing the relative intensities by of order 10%, which might 
not even be detectable on these plots.

5) These plots will (probably) be updated roughly every 2 weeks or so,
and thus will display data that aren't yet available in the public data
releases.  So if you see something interesting here but can't find the
data files (and it's during a time period within the last couple of months),
please be patient and wait for the release of the public data.