Hα Sources Research Articles

Structure and dynamics of cool flare loops

MSDP observations of the 16 May, 1981 two-ribbon flare are used to study the physical structure and the dynamical behaviour of cool flare loops. The loops have been detected in the Hα line just after the flare maximum and they appeared in absorption against the disk. Using the first-order differential cloud model (DCM1) technique, we derive empirically some basic plasma parameters at 15 points along one loop leg. The flow velocities and the true heights have been reconstructed with respect to a geometrical projection. Subsequently, detailed non-LTE models of cool loops have been constructed in order to fit Hα source function values previously derived from DCM1 analysis. It is demonstrated that this source function is rather sensitive to the radial component of the flow velocity (the so-called Doppler brightening) and to enhanced irradiation of the loops from the underlying flare ribbons. In this way, we have been able to estimate quantitatively all plasma parameters which determine the physical structure of cool loops (i.e., the temperature, pressure, density), as well as the momentum-balance condition within the loops. For these dark loops we have arrived at relatively low gas pressures of the order of 0.1–0.5 dyne cm-2 with corresponding electron densities around 1011 cm-3. Pressure-gradient forces have been found to be of small importance in the momentum-balance equation, and thus they cannot explain departures from a free-fall motion found in our MSDP data analysis. We propose three possible solutions to this problem.

R Aquarii - H-alpha map of subarcsecond inner structure

A 45 mas resolution image of R Aqr, observed near minimum brightness with speckle interferometry in a 1.8 nm optical bandpass centered on Hα, was obtained with the NOAO 4 m Mayall telescope at Kitt Peak. Three Hα sources were resolved: 1) a bright central Hα source which has a 2 cm counterpart previously designated C1 and probably arising from a hot companion and its accretion disk as they orbit a long-period variable; 2) a ∼3.3 mag fainter Hα source lying ∼0.53″ along p.a. ∼46°30 relative to C1, corresponding to C2 in 2 cm continuum; the flux ratio of C1 to C2 in Hα is 18:1 compared to 6:1 in 2 cm continuum; and 3) a ∼2.4 mag fainter Hα source lying ∼0.124″ along p.a. ∼226°.40 relative to C1; this source has no radio counterpart

Studies of solar flares using optical, X-ray and radio data

I have studied a number of flares for which good X-ray and optical data were available. An average lag of 5.5 s between hard X-ray (HXR) start and Hα start, and HXR peak and Ha peak was found for 41 flares for which determination was possible. Allowing for time constants the time lag is zero. The peak Hα lasts until 5–6 keV soft X-ray (SXR) peak. The level of Hα intensity is determined by the SXR flux. Multiple spikes in HXR appear to correspond to different occurrences in the flare development. Flares with HXR always have a fast Hα rise. Several flares were observed in the λ 3835 band; such emission appears when the 5.1–6.6 keV flux exceeds 5 × 104 ph cm-2 s-1 at the Earth. Smaller flares produce no λ3835 emission; we conclude that coronal back conduction cannot produce the bright chromospheric network of that wavelength. The nearly simultaneous growth of Hα emission at distant points means an agent travelling faster than 5 × 103 km s-1 is responsible, presumably electrons. In all cases near the limb an elevated Ha source is seen with the same time duration as HXR flux; it is concluded that this Hα source is almost always an elevated cloud which is excited by the fast electrons. A rough calculation is given. Another calculation of Hα emission from compressed coronal material shows it to be inadequate. In several cases homologous flares occur within hours with the same X-ray properties. Radio models fit, more or less, with field strengths on the order of 100G. A number of flares are discussed in detail.

Radio Observations of Hii Regions in M33

The Westerbork Synthesis Radio Telescope was used to map the continuum radiation of M33 at 1415 MHz. Of 67 radio sources with fluxes S> 1.8 mJy (3σ), 60% coincide with Hα sources. These are all intrinsically stronger than 4 × the Orion nebula, i.e., they are giant Hiiregions. The two strongest sources, NGC 604 (58 mJy) and NGC 595 (20 mJy), are similar to W51.