The Atacama Large Millimeter/submillimeter Array (ALMA) will observe a wide variety of phenomena on the Sun:
Significant progress will be made in the following scientific areas:
Flares: Solar flares involve the catastrophic release of energy in the low-solar corona which heats plasma and accelerates ions and electrons to relativistic energies on short timescales. The ALMA will probe emissions from the most energetic electrons, shedding light on the questions of when, where, and by what mechanism are electrons promptly accelerated to high energies.
Filaments: Solar prominences and filaments are, as their name suggests, large filamentary structures composed of relatively cold (~6500K), dense plasma suspended in the hot, 3 million degree solar corona. They occur along magnetic neutral lines in both active and quiet regions of the Sun. Some simply fade away after a lifetime of days to weeks; others are expelled from the Sun in spectacular eruptions. Their birth and death is still mysterious in many ways. The ALMA will span those wavelengths at which filaments become optically thin. Their structure and evolution will be more accessible to the ALMA than any other instrument.
Structure of the low solar atmosphere: One of the great mysteries of the Sun is why it has a solar corona. At the height of the photosphere (the visible surface of the Sun), the temperature is ~5880K. The temperature then decreases with height for several hundred kilometers. But then something amazing occurs: at greater heights, the temperature increases, gradually at first, and then suddenly to ~3 million degrees! The ALMA will problem the "temperature minimum" region of the manifest. By imaging this region of the solar atmosphere at various mm and submm wavelengths, the ALMA will offer a means of characterizing the structure and evolution of the low solar atmosphere and how that structure is maintained. The ALMA will also be able to exploit helioseismology to explore the details of the structure of the low solar atmosphere since the hydromechanical waves cause brightness variations in the mm and submm emission.
New spectral line diagnostics: At wavelengths longward of ~1mm, no spectral lines are available for diagnostic purposes. Pressure and Zeeman broadening (due to the presence of magnetic fields) is so extreme as to render them undetectable. At submm wavelengths, however, it should be possible to detect high-n radio recombination lines of hydrogen and certain ions. These offer the possibility of constraining the temperature, density, magnetic field strength, and mass motions in the low solar atmosphere, layers of the atmosphere that are inaccessible by other means.
The ALMA is designed to operate at millimeter and submillimeter wavelengths, from roughly 350 microns to 9.6 mm. In contrast, the VLA operates at much longer wavelengths, from 1.3 cm to 4 meters; and the HST and groundbased optical telescopes like the VLT or the Subaru operate at much shorter wavelengths (about half a micron). The ALMA therefore fills in the gap between optical/infrared telescopes, and radio telescopes. The reason this is scientifically interesting is the radiation at mm and submm wavelengths is caused by different physical mechanisms than that produced at longer and shorter wavelengths. And it originates in different regions of the solar atmosphere. Mm and submm observations therefore give us a new probe of physical processes on the Sun.