Simultaneous observations of the H-alpha and Ca II K floccular lines show that during the first three days of the newly formed flocculi the temperature grows gradually, while the density is practically constant. During the following days an increase in density is observed, The phase of increasing density does not occur in the case of faint flocculi. The K-line analysis of the six newly - formed sunspots indicates that the average cooling upward velocity equals 15 ms”' . An increase in sunspot induction was observed at the time of the flares (46 spots, Fig. 5).
Properties of the line structure of a well developped active region, as visible at the photospheric level, including umbral dots and penumbral filaments of sunspots, facular points, disturbed
granules, are described. Properties of a few cbromospheric features are also descoribed because they are dosely connected to tbe pbotospheric ones. Empirical and theoretical models which can explain the observed properties are discussed.
Present state of X-ray investigations of non-flaring actlve regions is briefly revlewed, It is stressed that it Is very Important to háve simultaneous records of high-quality X-ray spectra and high resolutlon X-ray images. The spectra allow to derive reliahle temperature distrlbutions (differential emission measure) of the coronal plasma. When combined with the X-ray images, it allows to investigate the rate and spatial distrlbution of the energy release (heating of the active region corona), Thls, In tum, provides an important possibiility to test empirically the theories of actlve region heating. These large potential possibillties of active region investigatlon are far from being exhausted and they should be used more efficiently in future Investigations.
Theoretical model calculations of magnetic elemcnts in the photospheric layers of solar active rcgions are cornpared with the results of observations. Emphasis is placed on small scale magnetic elements and an attempt is made to provide an
overview of the contributions that the current state of theory and observation can make to answering the following questions:
What is the field strength and the magnetic field structure of a magnetic element? Are magnetic elements inclined with respcct to
the vertical? Do magnetic canopies exist and how can they be explained? What are the diameters of magnetic elements? What
kinds of mass motions are present in magnetic flux concentrations and what role do they play for their structure and energetics?
How high is the temperature in magnetic elements and what are the underlying heating mechanisms?
Since the 1978-79 Skylab Workshop on Solar Active Regions, significant progress has been made in the diagnostic and modeling of active solar structures. This paper review the status of our knowledge of the physical properties of active regions in the photosphere, chromosphere and transition zone, as inferred from observations. Plages are the main active region which we study. A comparison with quiet Sun and some other solar structures is made in order to in order to emphasize the spedificity of plages properties. To conclude we present some requirements for new observations and theoretical developments to improve our understanding of the physical structure of solar active regions.
Part I of this review considers the outcome of different methods of observation of active regions at mm- and cm-v/aves exploring the source structures and parameters in the chromosphere and corona. Calculations of emission models invoking bremsstrahlung and gyromagnetic radiation are summarlzed applying for a great
variety of physical conditions. Diagnostic capacities of different kinds of microwave burst emission are briefly taken into account.
Part II of the review considers the dm- and m-radio spectral observations as diagnostic tool for the determination of plasma parameters in the coronal parts of solar active regions. The main attention is devoted to the radio diagnoetics of solar flares. The most important types of dm- and m-radio bursts are presented.
the latest theoretical and observational results summarized and their diagnostic aapects briefly discussed.