Alan L. Kiplinger

H-alpha spectra of dynamic chromospheric processes in five well-observed X-ray flares

Simultaneous H-alpha and hard X-ray (HXR) spectra were obtained for five solar flares to determine the relationship of H-alpha profiles and the nonthermal part of the flare represented by the hard X-ray burst. All five flares exhibited impulsive-phase redshifted H-alpha in emission, which was temporarily and spatially associated with intense HXR emission and broad impulsive-phase H-alpha wings. A few small regions within two flares showed a blueshifted H-alpha emission which appeared only early in the impulsive phase and was temporally correlated with the HXR emission but not with broad H-alpha wings. Finally, there were both redshifted and blueshifted absorption spectra with properties fully consistent with those known for erupting and untwisting filaments. 31 refs.

The Solar White-Light Flare of 1989 March 7: Simultaneous Multiwavelength Observations at High Time Resolution

Abstract : We present observational data for the 1989 March 7 white-light flare (WLF), including SMM/HXRBS hard X-ray measurements and CCD optical images in 5000 A continuum and at 3 A in the red and blue wings of the H alpha line. The optical data were acquired at a 0.5 s rate under good seeing and were further processed to remove image motion and distortion. The flare kernel shows an impulsive rise in both H alpha and continuum that is well correlated with impulsive hard X-rays. The Hot emission shows a red asymmetry which we interpret as Doppler redshift associated with explosive heating of the chromosphere. These results are consistent with previous observational descriptions which traditionally have been interpreted in terms of heating and ionization of the flare chromosphere by a nonthermal electron beam. However, new observational results which have not been reported previously include (1) the separation of the flare kernel into a bright inner core and a fainter outer region, where the two components display distinctly different temporal behavior and amount of Hot red asymmetry, and (2) a delay, relative to hard X-rays, of approx. 1 s in the impulsive rise of the H alpha wing emission, followed by an additional 1-2 s delay in the 5000 continuum. It is concluded that the observed fluxes and timing of the hard X-ray and WLF optical emissions are consistent with chromospheric heating by nonthermal electrons, with additional visible light continuum possibly being contributed by a backwarmed photosphere irradiated by intense chromospheric recombination continua (principally Balmer continuum) .... Sun: flares, Sun: X-rays, Gamma rays.

SEQUENTIAL CHROMOSPHERIC BRIGHTENINGS BENEATH A TRANSEQUATORIAL HALO CORONAL MASS EJECTION

Analyses of multiwavelength data sets for a solar eruption at ~21:30 UT on 2002 December 19 show evidence for the disappearance of a large-scale, transequatorial coronal loop (TL). In addition, coronal manifestations of the eruption (based on SOHO EIT and LASCO images) include large-scale coronal dimming, flares in each associated active region in the northern and southern hemispheres, and a halo CME. We present detailed observations of the chromospheric aspects of this event based on Hα images obtained with the ISOON telescope. The ISOON images reveal distant flare precursor brightenings, sympathetic flares, and, of most interest herein, four nearly cospatial propagating chromospheric brightenings. The speeds of the propagating disturbances causing these brightenings are 600-800 km s-1. The inferred propagating disturbances have some of the characteristics of Hα and EIT flare waves (e.g., speed, apparent emanation from the flare site, subsequent filament activation). However, they differ from typical Hα chromospheric flare waves (also known as Moreton waves) because of their absence in off-band Hα images, small angular arc of propagation (<30°), and their multiplicity. Three of the four propagating disturbances consist of a series of sequential chromospheric brightenings of network points that suddenly brighten in the area beneath the TL that disappeared earlier. SOHO MDI magnetograms show that the successively brightened points that define the inferred propagating disturbances were exclusively of one polarity, corresponding to the dominant polarity of the affected region. We speculate that the sequential chromospheric brightenings represent footpoints of field lines that extend into the corona, where they are energized in sequence by magnetic reconnection as coronal fields tear away from the chromosphere during the eruption of the transequatorial CME. We report briefly on three other events with similar narrow propagating disturbances that were confined to a single hemisphere.

The relative timing of microwaves and hard X-rays in solar flares

The timing of impulsive microwave and hard X-ray emission in solar flares with sub-second precision was compared. In flares demonstrating time structure on a scale less than approximately 1 s, 10.6 GHz microwaves were delayed with respect to hard X-rays by about 0.2 s. The delay varied from flare to flare and also within flares. No significant correlation was shown between the observed time delays and the peak X-ray or microwave flux or their ratio. Discussion included the electron propagation time from the top to the bottom of the loop, differential light travel time caused by the possible varying locations of the X-ray and microwave sources and the possible contamination of the microwave spikes with approximately 2 x 10 to the 7th K thermal emission.

A study of the ultraviolet evolution of U Geminorum between outbursts

This paper examines the ultraviolet evolution of the dwarf nova U Gem from one outburst to the next. A series of seven sets of IUE observations that monitor the quiescent period of 22-103 days after outbursts is presented. An eighth set of observations obtained 75 days after the second outburst is included for comparison. In agreement with earlier studies, analyses of Ly-alpha profiles and 1200-2000-A continuum spectra during 103 days of quiescence are consistent with the presence of a 30,000 K white dwarf. Direct evidence is provided that the UV flux levels continue to decrease throughout optical quiescence. These changes in flux appear consistent with the added presence of a hot component (perhaps the inner part of the disk or boundary layer) in addition to the white dwarf. Analyses of the absorption line spectra are also inconsistent with a single photospheric origin. 17 refs.

Coordinated spectral and temporal H-alpha observations of a solar flare

We report simultaneous observations of a flare (1991 January 11 18:25 UT) with a combination of spectra and high time resolution images in H-alpha. The Mees Solar Observatory CCD Imaging Spectrograph obtains spectra and spectroheliograms with a repetition rate of 14 s. These data permit the identification of sites of nonthermal electron precipitation or of high coronal pressure. We observe extremely strong electron precipitation in this flare; we find reasonable agreement between the observed profiles and theoretical precipitation spectra. We discuss the possibility that the spectra classed as precipitation signatures are in fact the result of a projection effect, and we indicate limitations of the one-dimensional theoretical models.

A consistent picture of coronal and chromospheric processes in a well-observed solar flare

We have analyzed the solar flare of 15:22 UT 1980 June 24 using simultaneous observations in hard X-rays, soft X-rays, and H..cap alpha.. line profiles. The X-ray observations were made with instruments aboard the Solar Maximum Mission satellite, and the H..cap alpha.. profiles were taken with a CCD detector at Sacramento Peak Observatory. We used the 1984 theoretical profiles of Canfield, Gunkler, and Ricchiazzi to analyze the H..cap alpha.. data. We studied various flare phenomena, including heating of the chromosphere by nonthermal electrons, enhanced coronal pressure, enhanced thermal conduction, chromospheric evaporation and mass motion. We find that we can make a consistent picture of the flare in terms of coronal and chromospheric processes.

The spatial, spectral, and temporal character of the hard X-ray flare of 1982 February 3

This paper presents spatially resolved, hard and soft X-ray, 17 GHz microwave and H-alpha observations of an X1.1 flare which occurred on February 3, 1982. The bulk of the 25-50 keV hard X-ray emission during impulsive peaks is produced by two sources of differing brightness which are separated by about 50 arcsec. Soft X-ray images reveal a single component that resides between the two hard sources. On the decay of the event, X-ray spectra provide direct evidence for a hot component that dominates the less than 50 keV flux. Hard X-ray images obtained at these times show a single source that is nearly coincident with the soft X-ray source. Interferometric microwave observations obtained during impulsive peaks support the idea that microwave emission is produced in the vicinity of high magnetic fields near the sunspot. The influence of an asymmetric loop or system of asymmetric loops is essential to interpreting the observations. 43 references.

Hard x-ray spectroscopy for proton flare prediction

High energy interplanetary proton events can jeopardize vital military and civilian spacecraft by disrupting logical circuits and by actually damaging spacecraft electronic components. Studies of solar hard x-rays indicate that high-energy proton events observed near Earth are highly associated with an uncommon type of solar flare exhibiting temporal progressively hardening hard x-ray spectra. A hard x-ray spectrometer is being developed by the Czech Astronomical Institute to provide a test bed for evaluating this phenomenon as a possible proton-storm prediction method. The instrument is designed to measure hard x-ray spectra in a high fluence, high-energy particle background environment such as that found at geosynchronous altitude. This experiment has been selected for space flight by the DoD Space Test Program and will fly aboard the Department of Energy satellite, Multi-spectral thermal Imager, scheduled for a three year mission, beginning in late 1999. The timing of this mission, fortuitously, coincides with the experiment are: 1) to evaluate the efficacy of this type of solar instrument in predicting interplanetary proton storms; 2) to study the high-energy physics of solar flares in concert with the premier flight of the NOAA soft x-ray imaging telescope, SXI, on the GOES 12 weather satellite and other solar mission. If the first goal is demonstrated by this mission, continuous monitoring of the Sun for proton events could become operational from geo-synchronous orbit during solar cycle 24.