# Solar Orbiter -- Payload Working Group

## Action Completion Form

***[Action ID Number:]{.underline}*** 1.3 (VIM)

***[Responsible Working Group Member:]{.underline}*** Valentín Martínez
Pillet

***[Date of Completion:]{.underline}*** 18 September 2002

***[Action:]{.underline}*** Demonstrate that the limited telemetry can
be used to provide the science return expected from the Visible-light
Imaging Magnetograph

***[Report:]{.underline}***

The nominal detector for VIM is a 2K x 2K pixels, 12 bits system that
can receive light from either a high resolution or a low resolution
channel, each one providing different spatial samplings and field of
views (FOV). For this action item, both channels are treated the same
and the use of one or the other will depend on the science operation
mode of the spacecraft.

VIM will detect intensity images in different positions within a
selected spectral line and in different polarization modes (probably
including an unpolarized mode). For calibration purposes, sometimes,
these intensity frames (or the Stokes parameters easily deduced from
them) will be stored. But these data will represent a small fraction of
the total and will not compromise the telemetry rates. Here we consider
only the cadences and telemetry rates needed for different observing
modes that should constitute the fundamental science operation modes of
the instrument. The use of these modes will depend on the science
targets selected for each orbit based on the science plans of the
spacecraft. In any of these modes, VIM will provide a combination of the
following physical magnitudes:

-   Ic or continuum intensity images. A temperature indicator that
    provides the photospheric context. 8 bits compressed to 4 bits per
    pixel.

-   V~los~ the line-of-sight (LOS) velocity frames. They provide the
    Doppler signals needed for local helioseismology. 10 bits compressed
    to 5, some applications may use only 4.

-   B~los~ the LOS component of the magnetic field. They are basically
    maps of circular polarization over the observed area. 10 bits
    compressed to 5, some applications may use only 4.

-   B~trans~ the transverse to the LOS component of the magnetic field.
    They represent maps of linear polarization. 8 bits compressed to 4.

-   φ the azimuth of the transverse component in a plane perpendicular
    to the LOS. Also obtained from linear polarization measurements. 8
    bits compressed to 4.

The final 4/5 bits per pixels estimates provided here, assume a lossless
compression scheme with an efficiency of a factor 2. Note that from the
original 12 bits we have first thrown out the 2 to 4 less significant
ones. Thus the total reduction factors are between 2 to 3. These
compressed estimates have been used in the following description of
example observing modes that could produce the desired scientific
results from VIM.

[Mode 1. Low resolution, high cadence mode:]{.underline}

On-chip binning to 512 x 512 pixels of 1 physical magnitude at a cadence
of 1 per minute require a telemetry rate of 18-22 kbs. This mode can be
used for storing V~los~ over the whole FOV at a high cadence for local
helioseismology.

This mode uses the nominal telemetry rate of 20 kbs.

[Mode 2. Medium resolution, medium cadence mode:]{.underline}

Binning to 1024 x 1024 pixels of 1 physical magnitude at a cadence of 1
every two minutes require a telemetry rate of 35-44 kbs. This mode can
be used for sending Ic, V~los~ or B~los~ for general purposes (e.g.,
magnetic field evolution).

This mode uses a telemetry rate 2 times larger than the nominal value of
20 kbs.

[Mode 3. High resolution, high/medium cadence mode:]{.underline}

This mode is similar to the two previous ones but instead of binning
pixels, a selection of a subframe (512 or 1024) is done, thus
prioritizing spatial resolution at the expenses of FOV and keeping a
reasonable cadence.

This mode uses a telemetry rate up to 2 times larger than the nominal
value of 20 kbs.

[Mode 4. Photospheric context:]{.underline}

In this mode three quantities (B~los~, B~trans~ and φ for vector
magnetometry or B~los,~ V~los~, Ic for dynamical studies) can be sent
over the full 2K frame every 5 minutes at a rate of 160 kbs (4 bits per
magnitude). The vector magnetometry case enables to follow the evolution
of the magnetic field in the photosphere and higher at a sufficiently
high cadence for most of the coronal phenomena.

This mode uses a telemetry rate 8 times larger than the nominal value of
20 kbs.

One expects the use of mode 4 (or similar) frequently during perihelion
phases. This would strongly indicate the need to investigate higher
telemetry rates (at a spacecraft level) and/or the use of lossy schemes
whenever these modes are used.

Peak telemetry rates of 3 physical magnitudes over the full frame every
minute of 800 kbs (40 times the nominal value) must also be considered.

As it is evident VIM will require often larger rates than the nominal
one. The way in which this could be achieved is:

-   By using those orbits where the telemetry rates of the spacecraft
    are larger than the nominal 75 kbs, by factors between 2 to 8 (see
    Assessment Study) . Thus careful planning of the scientific
    objectives of each orbit should be made beforehand.

-   By increasing the resources of the spacecraft (in realistic ways as
    increased on-board memory, a second downlink ground station).
    Nominal telemetry rates 8 times higher than the current values allow
    near optimum scientific return at all orbits.

-   By studying lossy approaches that do not compromise the science
    output of some experiments. This point should be of interest to all
    instruments and experience from other spacecrafts under development
    is available. We propose to study this point in a coordinated way
    between all science teams in the future.

-   By pre-programming flexible data acquisition rates in parallel with
    the rest of the instruments. Modes that demand high telemetry rates
    should have low duty cycles.

***[Conclusion:]{.underline}***

*As it is shown, even with the nominal telemetry rates of 20 kbs VIM
will be able to provide at all orbits magnetic and velocity context data
to help understanding the upper atmospheric processes studied by the
rest of the instruments.*

*The science goal of local helioseismology puts a high demand on
observing cadence but not so strong on spatial resolution. Modes 1 and 2
have telemetry rates that can be accommodated in different orbits to
achieve this science goal. The same is true for observing modes with a
small field of view but high spatial resolution. However, modes with
full field of view and high spatial resolution (and/or high temporal
cadence) indicate a clear [need for higher telemetry rates]{.underline}
(up to a factor eight) than the current nominal values. These modes can
also achieve better telemetry performance through the use of lossy
compression approaches (with compression factors higher than 3) but
their impact on the scientific quality of the data must be
investigated.*

***[References:]{.underline}***

ESA-SCI(2000)6 Solar Orbiter Assessment Study Report, July 2000, ESA