Description of the Atmospheric Heating Problem
Our present understanding of atmospheric heating is that mechanical energy created in the convection zone propagates into the outer solar atmosphere where it is dissipated and converted to thermal energy. Questions abound concerning the amount and form of the mechanical energy at the top of the photosphere, whether or not it can propagate into the corona, and once there, how it is dissipated. Observations at the finest spatial scales possible, the goal of VAULT, are a key component to understanding the physics of atmospheric heating.
One category of atmospheric heating theories is based on the input of energy into the magnetized corona by the continual motions of the footpoints in the photosphere (Gold 1964). Heating mechanisms of this sort have been developed by a variety of authors (Sturrock and Uchida 1981; Heyvaerts and Priest 1984; van Ballegooijen 1986; Parker 1988; Longcope and Sudan 1992). These theories suggest that electromechanical energy should be found over a range of scales. The driving motions could exist either in the granular buffeting of magnetic flux tubes or in the supergranular flow patterns that convect the continually emerging intergranular flux to the cell boundaries. These motions then serve to create wave motions or to perform work on the loop footpoints and drive the energy of the atmospheric field configuration above the potential value. Dere (1994) suggested that it would be necessary to recycle the magnetic fields in the supergranular network on a time scale of about 8 hours for these motions to be an effective means of driving energy into the corona. Observations with MDI on SOHO suggest that the supergranular magnetic flux is recycled on a time scale of several days (Title et al. 1997). However, the dissipation of the energy can be expected to occur only on very small spatial scales where it is possible to build up strong velocity gradients and strongly sheared magnetic fields, which can then be dissipated classically through viscous dissipation and joule heating.
Parker (1988) suggests that small-scale tangential discontinuities will develop with an impulsive release of free energy in events that he refers to as nanoflares. Van Ballegooijen (1986) has found that magnetic energy in a coronal arcade should rapidly cascade to small spatial scales where the current density can become large enough to lead to significant heating. Rabin and Moore (1984) have studied the possibility of coronal heating by joule heating resulting from fine-scale currents. Einaudi et al. (1996) have performed a 2D simulation using the reduced MHD equations and shown that a large scale random forcing of a magnetic loop results in intermittent dissipation events in small current sheets. Hendrix and VanHoven (1996) come to similar conclusions using 2D and 3D simulations. These calculations predict that one should see motions and structures at all scales, from forcing scales down to the dissipation scale lengths.
Observational evidence for the possibility of atmospheric heating produced in small-scale transient events can be found in the jets and explosive events seen in HRTS spectrograms (Brueckner and Bartoe 1983; Dere et al. 1991) and in the microflares seen with SMM (Porter et al. 1984). Typical sizes of these events are on the order of 2 arcseconds, but the red- and blue-shifts are often displaced from each other and suggest the presence of structures on smaller scales. Lu and Hamilton (1991) have suggested that the solar corona is in a self-organized critical state. Following this suggestion, we would expect to see increasing numbers of explosive events and microflares as our observational capabilities are extended to smaller spatial scales.
Further indications of sub-resolution structure are predicted by the power spectrum of fluctuations of velocities in the transition region (Dere 1989). Velocities at long wavelength are described by a turbulent (Kolmogorov) power spectrum, P(k)~k-5/3. However, since this power does not reproduce the observed nonthermal line widths, we expect to find additional power at shorter wavelengths (smaller spatial scales) in addition to the power expected from a turbulent cascade in order to explain the nonthermal velocities. The small fill-factor of transition regions structures (Dere et al. 1987) also indicates that the "fine" structures seen by HRTS with sizes less than 3( are composed of filamentary structures with radii no larger than about 100 km.
It is generally believed that magnetic flux at the photosphere is concentrated into flux tubes with a typical dimension of around 0.1 arcsecond and typical field strengths of 1000-2000 Gauss. These fields should spread out fairly rapidly with height above the photosphere. Nevertheless, we know that in the quiet Sun they are associated with fine scale structures such as spicules. When observed at a resolution of 0.33 arcsecond, it should be possible to observe the fine scale evolution of quiet Sun plasmas. Coronal magnetic fields are highly chaotic: small changes in the magnetic field pattern at the photosphere can lead to large changes in the connectivity of magnetic elements, and magnetic reconnection is the means by which these new connections are established. These reconnection processes may also be associated with enhanced plasma heating or mass accelerations in reconnection jets. The diagnosis of magnetic reconnection, as it occurs on the Sun, will be enhanced by higher resolution observations.
Recent Science Results
The first flight of the VAULT payload took place on May 7, 1999 and was a success. The goals were the verification of the design specifications of the instrument and the study of the fine structure of the upper chromosphere. We obtained 16 Lyman-alpha images of an active region and the surrounding quiet Sun. The achieved spatial resolution was 0.33 arcsecond, very close to the design goal of 0.25 arcsecond (Korendyke et al. 2001). The VAULT 1 images are the highest resolution solar images to be obtained from space.
Although the analysis of the flight data is still under way, a first look at the Lyman-alpha images seems to support the theoretical expectations mentioned in the previous section. The VAULT time series show continuous loop motions, in all spatial scales, consisting of flows, brightenings and footpoint shuffling. We have measured flows of about 20-30 km/s along 1- to 20-arcsecond loops of widths ranging from 0.33 arcsecond to 2 arcseconds in both quiet Sun and plage areas. Numerous point-like brightenings, resembling microflares, with lifetimes of the order of 1 minute can be seen throughout. These observations favor magnetic reconnection as an important mechanism for energy release at the base of the corona. We are in the process of compiling a paper with these results. Our initial analysis focused on the collaborative observations that were obtained by TRACE and has yielded two peer-reviewed publications.
First, we analyzed the small-scale structure in moss (Berger et al. 1999) areas using VAULT Lyman-alpha and TRACE 17.1 nm observations (Vourlidas et al. 2001). We found a strong correlation between the fine-scale structures in the two wavelengths but a large discrepancy in their emission measures. The results imply that the moss consists of continuous loops from the lower transition region to the corona. These loops, however, cannot be heated solely by the thermal conduction flux from the corona as it is usually assumed in the classical transition region models (e.g., Wikstøl et al. 1998). If direct mechanical heating is responsible for the Lyman-alpha radiation, then this heating must be positively correlated with the heating of the corona above. This places valuable constraints on the physical mechanism(s) of energy release. Second, the comparison of the TRACE and VAULT Lyman-alpha images has led to an improved method for the removal of the continuum contamination from the TRACE Lyman-alpha images which will enhance the scientific return of the TRACE UV observations (Handy et al. 1999).
The VAULT 2 flight employed a substantially upgraded instrument to observe the Lyman-alpha structures in an active region. The VAULT 2 exposure times were about 2 times shorter with a 5-fold enhancement of the signal-to-noise ratio. This allowed observation of finer scale, low-contrast structures than VAULT 1. Finally, the removal of the focusing and targeting sequence from the flight timeline extended the VAULT science sequence. VAULT 2 obtained an extended 5-minute movie of fine scale structural evolution on the Sun in Lyman-alpha.
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|Very High Resolution Advanced Ultraviolet Telescope|
2003 U.S. Naval Research Laboratory