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Scientific Objectives of LASCOThe central scientific questions to be addressed by LASCO are:
To answer these questions, the LASCO investigation will study the transport of mass, momentum, and energy through the corona and into the solar wind, by measuring:
An important feature of the LASCO design is that it will routinely obtain the required synoptic observations simultaneously in the inner (C1), middle (C2), and outer (C3) parts of the field. Consequently, it will be able to trace both the origin and outward extension of coronal structures, and to monitor their temporal evolution.
In addition to quantitative measurements of temperature, density, velocity, and magnetic field direction, C1 will also provide a new and important link between white light outer coronal images (C2, C3), and correlative images of the disk and inner emission-line corona from the SOHO EIT. The SOHO spectrometers (CDS, SUMER) observing on the disk and near the limb will benefit from LASCO imagery depicting the global setting to which their measurements apply. Their off-limb spectral measurements, as well as those by the UVCS of the ultraviolet corona, will have available during scientific analysis, and perhaps for mission operations planning, independent measurements of the electron density by LASCO.
Detailed Scientific Goals
2.1.1 Coronal Heating and Acceleration of the Solar Wind
How is the corona heated? This is perhaps the major unsolved problem in solar coronal physics. Current theories of coronal heating center around either heating by waves guided by the magnetic field, or heating by small-scale reconnection. If wave heating is taking place in the corona, it should be possible to detect it by measuring root mean square (RMS) velocity fluctuations in emission lines formed at coronal temperatures. UV coronal observations show that these velocities should be in the range 20- 30 km/s. We would like to measure the nonthermal velocities of large active region loops, which frequently extend to heights > 10E5 km, with an accuracy of at least 10 km/s. Measurement to this accuracy will provide a critical test of wave heating theories. In addition to being able to measure velocities, it is necessary to image the corona with sufficient spatial resolution to distinguish the major structural elements of the low corona. Images from Skylab, SMM, and Yohkoh in soft X-rays and the XUV show that a large active region loop has a cross-sectional diameter of about 10,000 km. Thus, detailed measurements along a loop requires a spatial resolution in the low corona of roughly 10 arc sec (7200 km). For an isolated large loop, a spatial resolution of 20 arc sec should be adequate for comparing line widths with the typical quiet corona.
If heating by small-scale reconnection is taking place, then theory predicts that the heating rate should drop off rapidly with loop length. Testing this model requires the ability to measure the temperatures, densities, velocities and, hence, energy losses of the coronal plasma as a function of height. To determine the lengths of the loops being observed, it is important to be able to distinguish the large scale structure of the inner corona. As with the wave heating observations, this requires measurements with a spatial resolution in the low corona of 10 to 20 arc sec.
It is not clear that waves alone account for the energy input to the solar wind. Magnetic loops are sometimes ejected during coronal eruptions and flares, and it has been conjectured that many small magnetic loops may be continually emitted outward through coronal holes, heating the gas and perhaps imparting some outward momentum as well. LASCO will measure, in the first few solar radii above the surface of the Sun where the acceleration moves the gas up to supersonic speeds and where most of the thermal energy is consumed, the electron density directly and the electron and ion "temperatures" of the observed outward flow indirectly. It will also measure the direction of the magnetic field. Line intensity ratios utilizing iron lines can provide a measure of the "frozen-in" temperature of the gas, and line widths provide a measure of the unresolved small-scale RMS fluid motions. The variation of temperature and motions with radial distance is related to the energy budget and gas pressure for the expansion. By combining all the observations from LASCO, the equations for conservation of matter, momentum, and energy for steady flow along a smooth magnetic field can be examined term by term.
2.1.2 Coronal Evolution (Coronal Mass Ejections and Magnetic Field)
In addition to the fundamental questions of what heats the corona and what accelerates the wind, there are a host of additional important questions that LASCO will address. These concern primarily the large-scale structure and evolution of the outer corona and its extension into the interplanetary medium. Four such questions are:
What is the effect of emerging magnetic flux on large scale coronal features? Observations of the lowest extent of the corona can be made from the ground. To understand fully the effect of emerging flux, it is necessary to go from observations of emerging flux at the surface, through the innermost corona, to the far outer corona. Thus, LASCO must be able to bridge the gap between the low coronal observations made from the ground and the traditional space borne coronagraphs, by observing the inner and middle corona at the same time. Moreover, it is vital to image the corona outward as far as possible to track the propagation of plasma disturbances which are expected to accompany reconnection processes. It is also important to be able to compare coronagraph observations with simultaneous images obtained at X-ray and XUV wavelengths. Following the effects of emerging flux on large scale coronal features, therefore, requires simultaneous imaging over the solar corona from just above the limb to the far outer corona. The innermost observable distance above the limb should be close enough to the limb to provide some overlap with observations from other SOHO imaging instruments (about 1.1 Rsun), while the outermost observable distance from the limb should be as far out as possible to provide the maximum radial extent to track plasma disturbances (about 30 Rsun).
What are the properties of helmet streamers? Streamers are evolving structures. By comparing observations of individual streamers over a large range of radii as a function of time, we will be able to determine the extent to which the streamer pattern is affected by individual new "condensations" in the lower corona. In addition, by obtaining profiles in the emission lines observable in the inner corona, we will be able to determine the density, temperature, and flow speeds in the legs of helmet streamers. At higher atmospheric levels, changes in the widths and shapes of streamers should reveal the possible existence of magnetic neutral points. Thus, we desire both extended spatial resolution to allow measurements of the shapes of streamers (10 to 20 arc sec in the inner corona, 20 to 30 arc sec in the middle corona), and also the ability to measure line profiles with the same precision outlined in section 2.1.1.
What physical processes are responsible for coronal evolution? The close spatial relation between the global pattern of coronal intensity and the large-scale surface magnetic field clearly shows that coronal evolution reflects evolution of the field. Observations in the inner corona are necessary to determine whether material ejected in a coronal mass ejection (CME) originated in hot coronal condensations over new active regions, or in larger-scale structures which evolved gradually. Coverage of both a large azimuthal and radial extent of the corona is necessary to obtain a complete mass budget of coronal material, and to show the origin of ejected material. To identify the physical mechanism responsible for coronal mass ejections, it is critical to trace the ejected mass back to its source in the low corona.
How does a CME evolve as it moves into the heliosphere? A field-of-view extending from the inner corona near 1.1 Rsun to the far outer corona at about 30 Rsun, will allow us to address key issues about the CME mechanism, and the ultimate assimilation of the CME into the solar wind. For example, it should be possible to determine whether fast CMEs continue to accelerate out to 20-30 Rsun, as radio-scintillation observations have suggested. Increased spatial coverage and increased sensitivity will help to resolve the question of the existence of "forerunners," as well as the occurrence and relative positions of associated shock waves. Moreover, increased spatial coverage makes it possible to determine whether slow CMEs, with speeds less than 400 km/s within 10 Rsun, can accelerate to super fast-mode speeds at greater radial distances.
2.1.3 F Corona and Comets
The white light corona is composed of two components, the Thomson scattered light from free electrons (K corona) and the light diffracted by interplanetary dust particles (F corona). The radial gradients of these two components are such that the K corona dominates inside about 2.5 Rsun, while the F corona dominates beyond. An accurate determination of the F and K coronae, and of the stray light contribution to intensity, is required to derive, in particular, radial profiles of electron density. The separation will be accomplished using a combination of three independent methods: