Mirror Coronagraph for Argentina (MICA).

 Summary

MICA observes the solar corona above the limb (from 1.05 to 2.0 solar radii from Sun center) in various spectral ranges (l5303, l6374, l6563). Emphasis is put on high time resolution observations of transient phenomena and on studies of the structure and evolution of solar prominences and coronal streamers. Furthermore, the extreme sensitivity of the planned instrument with respect to scattered light from the sky will allow to record the southern hemisphere sky "quality" and its changes due to pollution.

 MICA is a ground-based solar telescope placed in El Leoncito, Argentina, as part of a bilateral science program between Argentina and Germany. MICA is an advanced mirror coronagraph that complements the existing and planned solar telescopes in El Leoncito, especially HASTA. MICA is particular useful in producing reference measurements in conjunction with the SOHO mission. Close collaboration with international organizations running both ground-based and satellite-born instrumentation is intended.

l. Scientific objectives
 

 1.1 Coronal physics

High resolution eclipse photographs have revealed that the solar corona is highly structured and variable. In fact, the corona is continually deformed and shaken by the convective motion of its magnetic foot points in the photosphere, and sometimes shattered by explosive releases of energy. The essential questions to be addressed by MICA, in conjunction with several other efforts, both from ground-based and satellite-born telescopes of various kinds, are:

    - How is the corona being heated?
    - Where and how is the solar wind accelerated?
    - What causes coronal transients, and what role do they play in the evolutionary development of large-scale coronal patterns?
 
These are the most important and long-standing problems in all coronal physics, and MICA will produce significant contributions to find their solutions. Therefore, MICA will investigate
 
    - the global distributions of key plasma parameters and their evolution with time,
    - the spatial structure of the corona on both fine and large scale, and their timely evolutions,
    - the processes that occur in coronal transients and the conditions that trigger them, in close association with the measurements by the HASTA instrument.
 
In order to perform measurements relevant to these topics, the MICA telescope shall observe the solar atmosphere as seen above the solar limb, in the distance range from 1.1 to 2 Rs. A new type of coronagraphic system is used that allows to suppress the bright solar disk sufficiently well.
 

The global change of the Earth's atmosphere due to pollution is certainly one of the major issues mankind should be concerned about. Pollution by dust particles, chemicals, and humidity changes the atmosphere's transparency for both the incoming solar spectrum and the reradiated energy. The relative roles of man-made pollution (e.g. from fires, agriculture, chemical industry, cars) and natural pollution (e.g. from volcanoes, meteorite impacts, forest fires) are still far from understood. Careful monitoring of the status of our atmosphere all around the globe is essential for progress in environmental research.

In this context MICA will produce an important contribution. The planned instrumentation allows an extremely sensitive and accurate determination of the sky transparency and the scattered light levels. Amount and spectral distribution of the sunlight scattered in the sky above the observatory are sensitive indicators of amount and type of atmospheric pollution. Therefore, MICA will measure the sky brightness in the visible and near infrared range of the solar spectrum. These data will be generated on a routine basis, every day in all four seasons, for several years. Similar measurements are being planned from observatories in the northern hemisphere. Close collaboration with atmospheric scientists of the various disciplines is intended in order to achieve a global understanding.

2.5. Measurements actually planned for MICA

MICA is equipped with a set of exchangeable color filters. They are narrow-band interference filters mounted in a specifically designed set of wheels. At present, we plan to aim at the spectral ranges listed here:

l. The well-known green coronal line (the emission line of Fe XIV at ~2x10⁶K) is addressed with a 0.9 nm wide interference filter at a wavelength of 530.3 nm. Due to the fully digitized imaging system immediate subtraction of reference images taken with a similar filter in the nearby continuum (e.g., at 526 nm) will reveal structures of the hot corona.

2. Similarly, the coronal red line (from the emission at 637.4 nm of Fe X at ~1x10⁶K) will show structures of the cooler parts of the corona.

3. An narrow-band H-alpha filter (at 656.3 nm) will show the shape and evolution of cold prominences within the MICA field of view, an ideal complement to the HASTA observations of the sun disk in the same spectral range.

MICA profits from an enormous advantage ground observatories usually have compared to space instruments: an almost infinite transmission rate of observational data, the only limitations for MICA being the exposure time, the CCD camera read-out time, and the time it takes the computer to transfer data to the storage medium. We use a 90 MHz PC computer, two 9.0 GB fast hard disks and a sigh speed DAT device. We can look at images on the screen every 2 sec (for exposure times less than 0.5 sec, e.g., in H-alpha). Writing the image data on hard disk takes another 2 sec. A second computer (300 MHz PC computer) process the raw images in almost real time.

That illustrates one of the big advances MICA may achieve: High time resolution observations of fast bright transient phenomena in the lower solar atmosphere. It complements well the HASTA objectives. On the other hand, MICA ideally complements the measurements done by the LASCO telescopes from space.

3. Management section

 3.1. General remarks

The PI of MICA and his colleagues in Lindau are also involved in the "mother project" of MICA, i.e. the LASCO instrumentation on the SOHO spacecraft. The scientific goals of both projects are intimately related with each other. Furthermore, excellent working relations exist to our friends at the Pic du Midi and at Sacramento Peak. Our Bergmodell has been installed on the Pic du Midi Observatory and is being used, in order to further investigate and optimize the capabilities of our system, in direct comparison with the existing coronagraph there, and for first scientific projects. MICA will certainly benefit from the results, technically and scientifically.

The SOHO LASCO space project required several models of the instruments to be built for undergoing the various test programs. That is the reason why we are presently left with at least one complete set of useful mirrors, other optical and mechanical components, electronic subsystems, even a complete telescope box. All that most valuable hardware is to be used in MICA. Especially, the most expensive optical components will be available. Others can be accommodated to the needs of MICA at low cost. The optical system has been developed and tested extremely carefully, such that there remains no risk for MICA. Also, there is a lot of technical experience in assembling, handling, testing and running the telescope, as a byproduct from both LASCO and the Bergmodell campaigns. Certainly MICA will benefit from the achieved quality standards set by a major international space project and from the experience the LASCO team has gained over the years.

Depending on the final MICA time schedule parts of the work can be performed in
our  MPAe workshops, if the overall workload permits.
 

MICA is set out as a bilateral project between Argentina and Germany. Therefore, the workload with respect to both technical and scientific issues should be shared on about equal terms. We agreed on this scheme:

Part of MPAe :

1. fabrication, assembly, test and calibrations of MICA telescope plus thermal canister and driver electronics,
2. installation of control computer plus operation software,
3. purchase of telescope mount and pointing system,
4. purchase of cupola and cupola interface
5. transport of equipment to Argentina,
6. installation and tests of telescope in El Leoncito,
7. participation in measurement campaigns,
8. data management, routine processing and distribution
9. data processing and scientific evaluation

Part of IAFE /OAFA:

1. assistance in telescope integration by guest scientist,
2. organizing local infrastructure in El Leoncito,
3. modifications of observatory building,
4. transport of equipment in Argentina,
5. installation of cupola and telescope mount in observatory,
6. technical assistance in telescope installation,
7. running routine observation programs,
8. shipping data files to other participants,
9. data processing and scientific evaluation.

Scientists from both countries participate in all phases of the project, including data evaluation and publications. Mutual exchange of guest scientists is highly desirable.
 

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