Loren W. Acton

The Relationship Between X-Ray Radiance and Magnetic Flux

We use soft X-ray and magnetic field observations of the Sun (quiet Sun, X-ray bright points, active regions, and integrated solar disk) and active stars (dwarf and pre-main-sequence) to study the relationship between total unsigned magnetic flux, � , and X-ray spectral radiance, LX. We find thatand LX exhibit a very nearly linear relationship over 12 orders of magnitude, albeit with significant levels of scatter. This suggests a universal relationship between magnetic flux and the power dissipated through coronal heating. If the relationship can be assumed linear, it is consistent with an average volumetric heating rate � Q � �=L, where � B is the average field strength along a closed field line and L is its length between footpoints. The �- LX relationship also indicates that X-rays provide a useful proxy for the magnetic flux on stars when magnetic measurements are unavailable. Subject headings: stars: coronae — stars: magnetic fields — Sun: corona — Sun: magnetic fields — Sun: X-rays, gamma rays

Nature of the heating mechanism for the diffuse solar corona

The temperature of the Suns outer atmosphere (the corona) exceeds that of the solar surface by about two orders of magnitude, but the nature of the coronal heating mechanisms has long been a mystery. The corona is a magnetically dominated environment, consisting of a variety of plasma structures including X-ray bright points, coronal holes and coronal loops. The latter are closed magnetic structures that occur over a range of scales and are anchored at each end in the solar surface. Large-scale regions of diffuse emission are made up of many long coronal loops. Here we present X-ray observations of the diffuse corona from which we deduce its likely heating mechanism. We find that the observed variation in temperature along a loop is highly sensitive to the spatial distribution of the heating. From a comparison of the observations and models we conclude that uniform heating gives the best fit to the loop temperature distribution, enabling us to eliminate previously suggested mechanisms of low-lying heating near the footpoints of a loop. Our findings favour turbulent breaking and reconnection of magnetic field lines as the heating mechanism of the diffuse solar corona.

A Method to Determine the Heating Mechanisms of the Solar Corona

One of the paradigms about coronal heating has been the belief that the mean or summit temperature of a coronal loop is completely insensitive to the nature of the heating mechanisms. However, we point out that the temperature profile along a coronal loop is highly sensitive to the form of the heating. For example, when a steady state heating is balanced by thermal conduction, a uniform heating function makes the heat flux a linear function of distance along the loop, while T7/2 increases quadratically from the coronal footpoints; when the heating is concentrated near the coronal base, the heat flux is small and the T7/2 profile is flat above the base; when the heat is focused near the summit of a loop, the heat flux is constant and T7/2 is a linear function of distance below the summit. It is therefore important to determine how the heat deposition from particular heating mechanisms varies spatially within coronal structures such as loops or arcades and to compare it to high-quality measurements of the temperature profiles. We propose a new two-part approach to try and solve the coronal heating problem, namely, first of all to use observed temperature profiles to deduce the form of the heating, and second to use that heating form to deduce the likely heating mechanism. In particular, we apply this philosophy to a preliminary analysis of Yohkoh observations of the large-scale solar corona. This gives strong evidence against heating concentrated near the loop base for such loops and suggests that heating uniformly distributed along the loop is slightly more likely than heating concentrated at the summit. The implication is that large-scale loops are heated in situ throughout their length, rather than being a steady response to low-lying heating near their feet or at their summits. Unless waves can be shown to produce a heating close enough to uniform, the evidence is therefore at present for these large loops more in favor of turbulent reconnection at many small randomly distributed current sheets, which is likely to be able to do so. In addition, we suggest that the decline in coronal intensity by a factor of 100 from solar maximum to solar minimum is a natural consequence of the observed ratio of magnetic field strength in active regions and the quiet Sun; the altitude of the maximum temperature in coronal holes may represent the dissipation height of Alfven waves by turbulent phase mixing; and the difference in maximum temperature in closed and open regimes may be understood in terms of the roles of the conductive flux there.

X-Ray Network Flares of the Quiet Sun

Temporal variations in the soft X-ray (SXR) emission and the radio emission above the solar magnetic network of the quiet corona are investigated using Yohkoh SXR images with deep exposure and VLA observations in the centimeter radio range. The SXR data show several brightenings, with an extrapolated occurrence probability of one brightening per 3 seconds on the total solar surface. During the roughly 10 minutes of enhanced flux, total radiative losses of the observed plasma are around 1025 ergs per event. These events are more than an order of magnitude smaller than previously reported X-ray bright points or active region transient brightenings. For all of the four SXR events with simultaneous radio observations, a corresponding radio source correlating in space and time can be found. There are several similarities between solar flares and the SXR/radio events presented in this paper. (1) Variations in temperature and emission measure during the SXR enhancements are consistent with evaporation of cooler material from the transition region and the chromosphere. (2) The ratio of the total energies radiated in SXR and radio frequencies is similar to that observed in flares. (3) At least one radio event shows a degree of polarization as high as 35%. (4) In three out of four substructures the centimeter radio emission peaks several tens of seconds earlier than in the SXR emission. (5) The associated radio emission tends to be more structured and to have faster rise times. These events thus appear to be flare-like and are called network flares.

Physical Structure of a Coronal Streamer in the Closed-Field Region as Observed from UVCS/SOHO and SXT/Yohkoh

We analyze a coronal helmet streamer observed on 1996 July 25 using instruments aboard two solar spacecraft, the Ultraviolet Coronagraph Spectrometer (UVCS) on board Solar and Heliospheric Observatory (SOHO) and the Soft X-Ray Telescope (SXT) on board Yohkoh. We derive temperatures and electron densities at 1.15 R☉ from SXT/Yohkoh observations. At this height, the streamer temperature is about log T (K) = 6.28 ± 0.05, and the electron density is about log ne(cm-3) = 8.09 ± 0.26, while at 1.5 R☉ a temperature of log T (K) = 6.2 and a density of log ne(cm-3) = 7.1 are obtained by UVCS/SOHO. Within the measurement uncertainty this suggests a constant temperature from the base of the streamer to 1.5 R☉. Electron density measurements suggest that the gas in the streamer core is close to hydrostatic equilibrium. Comparison with potential field models for the magnetic field suggests a plasma β larger than 1 in the closed-field region in the streamer. In deriving electron densities and temperatures from the SXT/Yohkoh data, we include the effects of abundance anomalies on the SXT filter response. We use the elemental abundances derived from the UVCS/SOHO observations to estimate the first ionization potential and gravitational settling effects. We then give the set of abundances for the solar corona, which agrees with our observations. In addition, we analyzed the SXT data from 6 consecutive days. We found that from 1996 July 22 to July 27, the physical properties of the streamer are nearly constant. We conclude that we may be observing the same loop system over 6 days.

Temperature Tomography of the Soft X-Ray Corona: Measurements of Electron Densities, Tempuratures, and Differential Emission Measure Distributions above the Limb

We analyze long-exposure and off-pointing Yohkoh/SXT data of the solar corona observed on 1992 August 26. We develop a new (temperature) tomography method that is based on a forward-fitting method of a four-parameter model to the observed soft X-ray fluxes F1(h) and F2(h) of two SXT wavelength filters as a function of height h. The model is defined in terms of a differential emission measure (DEM) distribution dEM(h, T)/dT, which includes also a temperature dependence of density scale heights ?n(T) = q??T and allows us to quantify deviations (q? ? 1) from hydrostatic equilibrium (q? = 1). This parametrization facilitates a proper line-of-sight integration and relates the widely used filter ratio temperature TFR to the peak of the DEM distribution. A direct consequence of the multi-scale height atmosphere is that the filter ratio temperature TFR(h) is predicted to increase with height, even if all magnetic field lines are isothermal. Our model fitting reveals that coronal holes and quiet-Sun regions are in perfect hydrostatic equilibrium but that coronal streamers have a scale height that exceeds the hydrostatic scale height by a factor of up to q? 2.3, which underscores the dynamic nature of coronal streamers. Our density measurements in coronal holes are slightly lower than most of the white-light polarized brightness inversions and seem to come closer to the requirements of solar wind models. Our DEM model provides also a physical framework for the semiempirical Baumbach-Allen formula and quantifies the temperature ranges and degree of hydrostaticity of the K, L, and F coronae.

Rapid sunspot motion during a major solar flare

A major solar flare on 15 November, 1991 produced a striking perturbation in the position and shape of the sunspot related most closely to the flare. We have studied these perturbations by use of the aspect-sensor images from the Soft X-ray Telescope on board YOHKOH, and with ground-based data from the Mees Solar Observatory. The perturbation occurred during the impulsive phase of the flare, with a total displacement on the order of 1 arc sec. The apparent velocity of approximately 2 km s−1 exceeds that typically reported for sunspot proper motions even in flare events. We estimate that the magnetic energy involved in displacing the sunspot amounted to less than 4 × 1030 ergs, comparable to the radiant energy from the perturbed region. Examination of the Mees Observatory data shows that the spot continued moving at lower speed for a half-hour after the impulsive phase. The spot perturbation appears to have been a result of the coronal restructuring and flare energy release, rather than its cause.

Temperature and density structure of the 1991 November 2 flare observed by the Yohkoh soft X-ray telescope and hard X-ray telescope

We measure temperature and density structure for a soft X-ray solar flare observed by the Soft X-Ray Telescope (SXT) on board the Yohkoh satellite. For a flare observed by the SXT on 1991 November 2, we have calculated temperature and emission measure as functions of space and time. The plasma density is calculated from the emission measure using simple geometrical assumptions (for example, cylindrical or spherical symmetry). The energy in the soft X-ray plasma also is calculated. Initial results from this analysis show the following: (1) The flare plasmas range in temperature from several million K up to 25 million K; (2) the flare has impulsive soft X-ray emission from a loop footpoint, which is also the location of the impulsive hard X-ray burst observed by the Yohkoh HXT; and (3) the plasma density, pressure, and energy all increase during the flare

Jovian X‐ray emission from solar X‐ray scattering

Soft x-ray emissions with brightnesses of about 0.01-0.2 Rayleighs have been observed from both the equatorial and auroral regions of Jupiter. It has been proposed that the equatorial emission, like the auroral emission, may be largely due to precipitation of energetic heavy ions into the atmosphere [Waite et al., 1997]. In this paper we model two alternative mechanisms for low-latitude x-ray emission: (1) elastic scattering of solar x-rays by atmospheric neutrals, (2) fluorescent scattering of carbon K-shell x-rays from methane molecules located below the jovian homopause. Our modeled brightnesses agree, up to a factor of two, with the bulk of low-latitude ROSAT measurements. This suggests that solar photon scattering (approximately 90 % elastic scattering) may act in conjunction with energetic heavy ion precipitation to generate jovian equatorial x-ray emission.

A Stable Filament Cavity with a Hot Core

We present observations of a long-lived solar filament cavity with soft X-ray sources along its axis. This structure appeared above the southern polar crown polarity-inversion line for approximately three rotations during 1997 June-August, centered at a west-limb passage on approximately July 3. At the limb, the Yohkoh soft X-ray data showed a bright region situated above and around the projected filament location but near the axis of the cavity. We describe measurements of the geometry of the cavity, which we interpret as a flux rope that is partially embedded in the photosphere, and use the Yohkoh data to describe the physical parameters of the structure. We find that the core consists of an unresolved mass of filamentary substructures, with a volume filling factor significantly less than unity for the soft X-ray telescope (SXT) resolution. The core has a higher temperature than the cavity surrounding it, ruling out explanations in terms of a transition region supported by thermal conduction. Transient activity occurred in the polar crown region, but no detectable destabilization or eruption of the cavity structure resulted from it. We suggest that the bright structure at the core of the cavity corresponds to higher altitude coronal segments of the field lines that support the filament material.