435 research outputs found
Interpretation of the prominence differential emissions measure for 3 geometries
Researchers have used prominence extreme ultraviolet line intensities observed from Skylab to derive the differential emission measure Q(T) in the prominence-corona (PC) interface from 3 x 10,000 to 3 times 1 million K, including the effects of Lyman Continuum absorption. Using lines both shortward and longward of the Lyman limit, researchers have estimated the importance of absorption as function of temperature. The magnitude of the absorption, as well as its rate of increase as a function of temperature, place limits on the thread scales and the character of the interfilar medium. Researchers have calculated models based on three assumed geometries: (1) threads with hot sheaths and cool cores; (2) isothermal threads; and (3) threads with longitudinal temperature gradients along the magnetic field. Comparison of the absorption computed from these models with the observed absorption in prominences shows that none of the geometries is totally satisfactory
Meterwave observations of a coronal hole
Meter-wave maps are presented showing a coronal hole at 30.9, 50.0, and 73.8 MHz using the Clark Lake Radioheliograph in October 1984. The coronal hole seen against the disk at all three frequencies shows interesting similarities to, and significant differences from its optical signatures in HeI lambda10830 spectroheliograms. The 73.8 MHz coronal hole, when seen near disk center, appears to coincide with the HeI footprint of the hole. At the lower frequencies, the emission comes from higher levels of the corona, and the hole appears to be displaced, probably due to the non-radial structure of the coronal hole. The contrast of the hole relative to the quiet Sun is much greater than reported previously for a coronal hole observed at 80 MHz. The higher contrast is certainly real, due to the superior dynamic range, sensitivity, and calibration of the Clark Lake instrument. Using a coronal hole model, the electron density is derived from radio observations of the brightness temperature. A very large discrepancy is found between the derived density and that determined from Skylab EUV observations of coronal holes. This discrepancy suggests that much of the physics of coronal holes has yet to be elucidated
RHESSI Observations of a Simple Large X-ray Flare on 11-03-2003
We present data analysis and interpretation of a simple X-class flare
observed with RHESSI on November 3, 2003. In contrast to other X-class flares
observed previously, this flare shows a very simple morphology with well
defined looptop (LT) and footpoint (FP) sources. The almost monotonic upward
motion of the LT source and increase in separation of the two FP sources are
consistent with magnetic reconnection models proposed for solar flares. In
addition, we find that the source motions are relatively slower during the more
active phases of hard X-ray emission; the emission centroid of the LT source
shifts toward higher altitudes with the increase of energy; the separation
between the LT emission centroids at two different photon energies is
anti-correlated with the FP flux. Non-uniformity of the reconnecting magnetic
fields could be a possible explanation of these features.Comment: To appear in the Astrophysical Journal Letters (12 pages, 4 figures
Fast Time Structure During Transient Microwave Brightenings: Evidence for Nonthermal Processes
Transient microwave brightenings (TMBs) are small-scale energy releases from
the periphery of sunspot umbrae, with a flux density two orders of magnitude
smaller than that from a typical flare. Gopalswamy et al (1994) first reported
the detection of the TMBs and it was pointed out that the radio emission
implied a region of very high magnetic field so that the emission mechanism has
to be gyroresonance or nonthermal gyrosynchrotron, but not free-free emission.
It was not possible to decide between gyroresonance and gyrosynchrotron
processes because of the low time resolution (30 s) used in the data analysis.
We have since performed a detailed analysis of the Very Large Array data with
full time resolution (3.3 s) at two wavelengths (2 and 3.6 cm) and we can now
adequately address the question of the emission mechanism of the TMBs. We find
that nonthermal processes indeed take place during the TMBs. We present
evidence for nonthermal emission in the form of temporal and spatial structure
of the TMBs. The fast time structure cannot be explained by a thermodynamic
cooling time and therefore requires a nonthermal process. Using the physical
parameters obtained from X-ray and radio observations, we determine the
magnetic field parameters of the loop and estimate the energy released during
the TMBs. The impulsive components of TMBs imply an energy release rate of 1.3
x 10^22 erg/s so that the thermal energy content of the TMBs could be less than
10^24 erg.Comment: 15 pages (Latex), 4 figures (eps). ApJ Letters in press (1997
Quantitative characterization of the x-ray imaging capability of rotating modulation collimators with laser light
We developed a method for making quantitative characterizations of bi-grid rotating modulation collimators (RMC ’s) that are used in a Fourier transform x-ray imager. With appropriate choices of the collimator spacings, this technique can be implemented with a beam-expanded He -Ne laser to simulate the plane wave produced by a point source at infinity even though the RMC ’s are diffraction limited at the He -Ne wavelength of 632.8 nm. The expanded beam passes through the grid pairs at a small angle with respect to their axis of rotation, and the modulated transmission through the grids as the RMC ’s rotate is detected with a photomultiplier tube. In addition to providing a quantitative characterization of the RMC ’s, the method also produces a measured point response function and provides an end-to-end check of the imaging system. We applied our method to the RMC ’s on the high-energy imaging device (HEIDI) balloon payload in its preflight configuration. We computed the harmonic ratios of the modulation time profile from the laser measurements and compared them with theoretical calculations, including the diffraction effects on irregular grids. Our results indicate the 25-in. (64-cm) x-ray imaging optics on HEIDI are capable of achieving images near the theoretical limit and are not seriously compromised by imperfections in the grids
Analytic Solutions of the Vector Burgers Equation
The well-known analytical solution of Burgers' equation is extended to curvilinear coordinate systems in three dimensions by a method that is much simpler and more suitable to practical applications than that previously used. The results obtained are applied to incompressible flow with cylindrical symmetry, and also to the decay of an initially linearly increasing wind
Millimeter wavelength observations of solar flares for Max 1991
The Hat Creek millimeter-wave interferometer (to be known as the Berkeley-Illinois-Maryland Array, BIMA) is being upgraded. The improved array will become available during the coming solar maximum, and will have guaranteed time for solar observing. The Hat Creek millimeter-wave interferometer is described along with the improvements. The scientific objectives are briefly discussed
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