Determination of the source height and anisotropy of solar hard X-rays by measurements with good time resolution

When emitted at the same time, solar hard X-rays reflected by the photosphere arrive at an observer at later times than primary hard X-rays coming directly from the source. This time lag of albedo photons, therefore, should be taken into account in interpreting fine-scale hard X-ray time profiles. If hard X-ray bursts consist of succession of short-lived elementary bursts, under favorable conditions reflected hard X-rays can be resolved from primary hard X-rays with good time resolution. If so, from the time lag and the ratio of the albedo flux to the primary flux, one can determine the source height and anisotropy of solar hard X-rays.     

Iron Kα-fluorescence in solar flares: A probe of the photospheric iron abundance

The X-ray line at 6.4 keV has been observed from solar flares. It is found that K-fluorescence of neutral iron in the photosphere due to thermal (T 10⁷ K) X-rays of the gradual phase is its dominant production mechanism. For a given flux and energy spectrum of incident X-rays, the flux at 1 AU of iron K-photons depends on the photospheric iron abundance, the height of the X-ray source, and the helio-centric angle between the flare and the observer. Therefore, the flux of iron K-photons, when measured simultaneously with the flux and energy spectrum of the X-ray continuum and the flare location, can give us information on the height of the X-ray source and the photospheric iron abundance. Here we present our Monte Carlo calculations of iron K-fluorescence efficiencies, so that they might be useful for interpretations of future measurements of the 6.4 keV line (e.g., by a detector to be flown on the Solar Maximum Mission).     

Periodicities Near 160 Minutes in Flare Occurrence

The 160.01-min periodicity was originally found from line-of-sight velocities of the photosphere, and Kotov and Tsap reported a detection of the same periodicity in flare occurrence times. Intrigued by this, I analyze occurrence times of flares of cycles 19–23 to investigate periodicities in the neighborhood of 160 min, cycle by cycle. The 160.01-min periodicity is not detected from any cycle. However, a 160.69 min periodicity is detected in the spectrum for cycle 19, and a 160.32-min periodicity is detected in the power spectrum for major flares of cycle 21. The 160.32-min periodicity did not influence the occurrence rate of flares with X-ray classes below M3.0. Among major flares, the amplitude of modulation increases with increasing X-ray class.     

High Flare Activity in the Late Declining Phase of Cycle 23

In the declining phase of the current solar cycle (23), a large number of major flares were produced. In this cycle, the monthly sunspot number continuously remained below 100 since October 2002. However, during four epochs since then, flare activity became very high. Compared to this, each of cycles 21 and 22 produced only one epoch of high activity in the declining phase. In the declining phase of cycle 20, similarly to this cycle, there were four epochs of high flare activity. During 2003 and 2004, the distribution of flare sizes measured in GOES classes was much harder (i.e., proportionately more energetic flares) than during the maximum years. Such pronounced hardening of the size distribution was not observed in the previous cycles. It is of theoretical interest to understand why some cycles are very active in the declining phase, and the high level of activity in the declining phase has practical implications for planning solar observations and forecasting space weather.     

Characteristics of gamma-ray line flares as observed in hard X-ray emissions and other phenomena

Observations of gamma-ray lines from solar flares by SMM demonstrated that energetic protons and heavy ions are accelerated during the impulsive phase. In order to understand the acceleration mechanism for gamma-ray producing protons and heavy ions, we have studied the characteristics of the flares from which gamma-ray lines were observed by SMM In order to identify the characteristics unique to the gamma-ray line flares, we have also studied intense hard X-ray flares with no gamma-ray line emissions. We have found the following characteristics: 1) Most of the gamma-ray line flares produced intense radio bursts of types II and IV. 2) For most of the gamma-ray line flares, the time profiles of high-energy (? 300 keV) hard X-rays are delayed by order of several seconds with respect to those of low-energy hard X-rays. The delay times seem to be correlated with the spatial sizes of the flares. 3) In Hα importance, the gamma-ray line flares range from sub-flares to importance-3 flares. 4) The hard X-ray spectra of the gamma-ray line flares are generally flatter (harder) than those of flares with no gamma-ray line emission. From these characteristics, we conclude that the first-order Fermi acceleration operating in a flare loop is likely to be the acceleration mechanism for energetic protons and heavy ions as well as relativistic electrons.