How to observe a
JEE (Jovian Extinction Event)
Based on modeling of
anomalous lightcurve trends found in the IAEP
study, material surrounding Io, Europa, and the Torus of Io should cause a
measurable extinction of the light of an object passing behind these materials
under the right geometries. Using the derived models from this study
predictions have been generated for some JEE events in 2010. These predictions
potentially have at least 10 minutes of error associated with them due to the
undocumented distribution of these extinction materials, thus another reason
for the need for long wing data.
To document a JEE it is
necessary to record as many minutes of data as possible. A minimum of one hour
of data centered on the event time is recommended for conjunction JEEs (CJEE),
stellar JEEs (SJEE), and Torus JEEs (TJEE) involving Europa. Two or more hours
of the data would be very useful for extinction events of Io by Io’s Torus, as
it takes about five hours for Io to enter and exit the tip of the Torus where
the maximum extinction by Torus material occurs.
It is important when
recording any JEE that your field of view be magnified enough to keep the
target and reference moons separated enough from Jupiter that photometric data
can be accurately derived. It is also important to remember that if you have a
manually selectable gain video camera that you adjust the gain so that the
target and reference moons are not over exposed in saturation or underexposed
near the noise floor. If you are using a video camera with no manual gain
control you can defocus the image to spread the light of each moon over a
greater number of pixels to bring the image out of saturation. However care
must be taken when using the defocus method that the light from Jupiter, the
reference moon, or target moons do not overlap into each other. The target moon’s
light can overlap each other, but the other’s must not overlap into the target,
nor Jupiter into the reference moon.
How to record a
CJEE:
1. A JEE by conjunction involves a Jovian moon passing behind Io or Europa close enough line of sight to experience extinction by the occulting body’s atmosphere. One CJEE has already been documented by Terry Redding on 20100106 when Ganymede passed behind Europa. The modeling of the dimming trend fits the derived Europa atmosphere model measuring a projected extinction of about 0.2 magnitude had the entire event been recorded. Weather prevented the recording all the way through conjunction. The modeled data can be found below.
2. When recording a CJEE it is important to keep both target moons in the field of view (FOV) at all times. One other Jovian moon should also be visible in the FOV the entire recording so that the intensity of the target moons can be corrected for changes in viewing conditions.
3. It is desirable for a CJEE to record a minimum of one hour of data centered on the conjunction time.
T. Redding
captured the extinction of Ganymede’s light when it passed behind Europa’s
atmosphere during a conjunction on a 20100106 JEE. (Redding, 2010)
How to record an
SJEE:
1. An SJEE has exactly the same geometry as the CJEE. Use the same recording method as the CJEE.
How to record a
TJEE:
Extinction by material of
the Torus of Io offers the highest number of JEE events visible. The tips of
the Torus present a large amount of Torus material edge on for our line of
sight. Lightcurves in the IA study consistently showed an extra 0.1 magnitude
of extinction when events occurred in the region of the most eastern and western
Torus regions (dubbed the tips). The lightcurve of 20091025 showed two
anomalies outside the occultation data that seem to be associated with a mutual
extinction transition zone in which Io emerged from and Europa entered that
caused a noticeable “notch” in the lightcurve trend (see plot below). Based on
modeling of this and several other lightcurves, as well as Schneider’s Torus of
Io image (also below, Schneider et al., 1991) it is predicted that the most
extinctive part of the tips of Io’s Torus are in an oblong area that extends
outside of Io’s orbit by about 9 Io radii and inside Io’s orbit by 15 Io radii.
Predicted event times are based on this preliminary model and recorded data
will be used to validate or refute these model parameters.
Since a TJEE involves no
other object other than the target moon suffering extinction, TJEE involve the
simplest event to record and reduce. There are no merging intensities to cause
nonlinear camera responses in the lightcurve data.
1. Recording a TJEE only
requires one to keep the target moon and one other moon to be used as a
reference intensity.
2. It must be noted that one
must not use Io as a reference moon when it is at its greatest eastern or
western elongation, as there it will be suffering extinction by its own Torus
rendering Io as no longer being an accurate reference light source.
3. It takes at least five hours
for Io to enter and exit this Torus tip area. More specific information on what
to expect at various places in this five hour journey through the tip region
will be found on the TJEE
predictions page.
Schneider, N. M. et al., “The structure of Io's corona” (1991), ApJ,
368, 298
D.
Clark and S. Degenhardt on
20091025 both captured an anomaly in the atmospheric extinction lightcurve of
Io occulting Europa that could potentially be caused by the Torus wall
surrounding Io. (Degenhardt
et al., 2010)
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