The Utrecht solar hard X-ray spectrometer S-100 on board the ESRO TD-IA satellite covers the energy range above 25 keV with 12 logarithmically spaced channels. Continuous sun-pointing is combined with high time resolution: 1.2 s for the four low energy channels (25-90 keV) and 4.8 s for the others. It is emphasized that the instrument design and calibration yield data virtually free of pile-up and other instrumental defects.A complete set of observations is presented for all well-observed flares during the period March 12, 1972 to October 1, 1973, including four from the highly active period August 1-8, 1972. Photon spectra are computed every 1.2 s for each event by deconvolution through the instrument response, rather than by fitting techniques. Using these actual photon spectra, the index 3' for the best fitting single power law and the minimum (thick target) injection rate of electrons above 25 keV, F25, are calculated.Results for 3' and E25 at 1.2 s intervals are presented for each event. Examination of all these results tentatively suggests a real distinction between events of a purely impulsive nature and prolonged events.Techniques of time series analysis are applied to the burst time profiles. Specifically: (l) The fluctuations present in the series are shown to be compatible with Poisson noise in the count rate.(2) It is emphasized that, without spatial resolution, the X-ray source must be characterized by the e-folding time scale r of the total count rate; examination of individual r's through the event shows very few statistically real r's as short as 1.2 s, confirming (1).(3) For all events, the series are Fourier analysed; no small events showed statistically significant periodicities, but the large event of August 4, 1972 exhibited real periods of 30, 60 and 120 s in both the flux and the spectral index.(4) Statistically real, small timing differences (-0.2 s) are shown to exist between spike peaks at different photon energies.A search is made for correlations between instantaneous values of inferred parameters (e.g. F25, 3" and the time scales). Most results are negative, but in the August 4 and 7, 1972 events a very well defined path was followed through the (F25, 3")-plane, giving insight into the electron acceleration process.Finally some general conclusions are drawn concerning the implications of our analysis for the physics of particle acceleration, including the possibility of two classes of event. Specifically, the severe problems posed by the large electron fluxes (equivalent current ~10 t7 A) demanded by the data are discussed in relation to flare theories. Some possibilities for getting around these problems, such as by reacceleration in a confinement region, are briefly considered.
Our intent is to provide a simple and quantitative understanding of the variability of the axial dipole component of the geomagnetic field on both short and long time scales. To this end we study the statistical properties of a prototype nonlinear mean field model. An azimuthal average is employed, so that (1) we address only the axisymmetric component of the field, and (2) the dynamo parameters have a random component that fluctuates on the (fast) eddy turnover time scale. Numerical solutions with a rapidly fluctuating a reproduce several features of the geomagnetic field (1) a variable, dominantly dipolar field with additional h e structure due to excited overtones, and sudden reversals during which the field becomes almost quadrupolar, (2) aborted reversals and excursions, (3) intervals between reversals having a Poisson distribution. These properties are robust, and appear regardless of the type of nonlinearity and the model parameters. A technique is presented for analysing the statistical properties of dynamo models of this type. The Fokker-Planck equation for the amplitude a of the fundamental dipole mode shows that a behaves as the position of a heavily damped particle in a bistable potential =(I -d)', subject to random forcing. The dipole amplitude oscillates near the bottom of one well and makes occasional jumps to the other. These reversals are induced solely by the overtones. Theoretical expressions are derived for the statistical distribution of the dipole amplitude, the variance ofthe dipole amplitude between reversals, and the mean reversal rate. The model explains why the reversal rate increases with increasing secular variation, as observed. Moreover, the present reversal rate of the geodynamo, once per (2-3) x lo5 year, is shown to imply a secular variation of the axial dipole moment of .-+ 15% (about the current value). The theoretical dipole amplitude distribution agrees well with the Sint-800 data. 263Downloaded by [University of Leeds] at 08:26 04 October 2015 264 P. HOYNG et nl.
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