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RADIATIONS FROM21 THE SUN By HERBERT FRIEDMAN E. 0. HULBURT CENTER FOR SPACE RESEARCH, NAVAL RESEARCH LABORATORY, .WASHINGTON, D.C. During the IQSY, the minimum of the solar sunspot cycle was observed, and all solar activity phenomena reached their lowest ebb in mid-1964. The measure of an 1 -year solar cycle variation is different for each activity phenomenon. In inte- grated visible light, characterized by a temperature of about 60000 K, no solar variation is vet clearly measurable. P'henomena produced ill temperature regimes of a few tens of thousands of degrees (solar chromosphere) cycle from maximum to minimum as much as 50 per cent. In the few-hundred-thousand-degree range (quiescent corona), the variations are by factors of .3 to 5. In the active corona, temperatures reach several million degrees and the resulting X-ray emission varies by a factor of 7 at long wavelengths (.50 A) to greater than 500 at short wave- lengths (1-S A). The shortest-wavelength X rays are a major controlling influence on the quality of short-wave radio communication. The IQSY provided an opportunity to establish the quiet background level of solar activity. Against this background it was of particular interest to observe the development of individual disturbances in 1965-66 before the sun became so active that multiple events, occurring in overlapping time sequence, confused the indi- vidual analyses. A solar activity center (CA) develops in all area about one-tenth the solar disk. Its development is accompanied by the transient appearance of sunspots, faculae, plages, flares, surges, prominences, coronal condensations, and the emission of radio bursts, X rays, and solar cosmic rays. Some CA's are short-lived; they last only a few weeks. _\Major CA's may live for 200 days, or even longer. The activity phenomena are clearly related to the formation of bipolar magnetic field regions, but we still have no satisfactory understanding of the cyclical behavior of sunspots. The magnetic fields which become visible at the photospheric surface must have existed for centuries in the deeper parts of the sun. This long persistence is a consequence of the very high conductivity of the solar plasma ill the convection zone below the base of the photosphere. The highly conducting gas cannot move, except very slowly, out of the confines of the magnetic field. We still have no physical model of how these magnetic fields originate in the convection zone, except that the magnetic energy must, be derived from the turbulent kinetic energy of the solar plasma. The time necessary for a convection element to rise from the bottom of the solar granulation region to the surface of the photosphere is about 30 days. This time is comparable to the time needed for full development of the active stage of a solar activity center. From observations conducted during the IQSY and 1966, several new concepts of the structure and radiating properties of the solar atmosphere have developed: (1) The solar wind is a primary source of the evolution of active regions in the solar chromosphere and corona. The flow of the wind is so great that the entire corona must be replenished in only a few days. The energy of the solar wind is capable of meeting all the energetic requirements of solar flares. The wind energy is often stored in a typical coronal "helmet" structure that bears a striking resem- 2142 Downloaded by guest on September 30, 2021 VOL. 58, 1967 N. A. S. SYMPOSIUM: H. FRIEDMAN 2143 blance to the earth's magnetospheric tail, which is produced by the pressure of solar wind on the earth's magnetosphere. A feature of the helmet is the sharp spike-shaped coronal streamer (Fig. 1). This form has been clearly revealed by rocket-borne coronagraphs. % T, FIG. 1.-The solar corona photographed by the High Altitude Observatory (G. Newkirk, Jr.)- Eclipse Expedition on November 12, 1966. Characteristic helmet structures with streamer spikes are believed to be produced by the flow of solar wind in the sun's magnetic field. (2) Energetic X-ray emissions are localized in coronal condensations 10 to 100 times as dense as the surrounding corona and covering 1 per cent or less of the solar surface. Important evidence came from the first observation of a solar eclipse from a satellite-the NRL SOLRAD-8-on May 20, 1966. (3) The sources which produce the X-ray emissions are a mixture of thermal and nonthermal processes. Hot plasma condensations exist up to temperatures of 5 or 6 million degrees and are strong X-ray sources. At the same time, very effi- cient acceleration processes appear to be at work almost continuously to produce electrons in the tens of kilo-electron-volt range, which in turn produce X rays. Even cosmic-ray particles of million-volt energies appear to be almost continuously generated by some still mysterious process. (4) Energetic X-ray emissions fluctuate rapidly in intensity-often 50 per cent in the span of a few seconds. Such behavior implies the existence of energetic trapped electrons (analogous to Van Allen belt particles) precipitating deep in the solar atmosphere (analogous to auroral zone precipitation). Alternatively, hot plasma may be pinched in magnetic "ropes" of high density (which occupy perhaps Downloaded by guest on September 30, 2021 2144 N. A. S. SYMPOSIUM: H. FRIEDMAN PROC. N. A. S. 1 per cent of the volume of a large coronal condensation). A variety of optical evidence exists for such fine filamentary detail in coronal loops and prominences. (Figs. 2 and 3). The clearest eclipse photographs of coronal streamers show a fine ............ A_ _ ; .a~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~........ FIGS. 2 and 3.-Loops of glowing gas (prominences) photographed on the limb of the sun with the Sacramento Peak coronagraph. Downloaded by guest on September 30, 2021 VOL. 58, 1967 N. A. S. SYMPOSIUM: H. FRIEDMAN 2145 combed detail. A single rope or thread may stretch 20,000 to 100,000 miles from sunspot to sunspot but can generally be observed for durations of only ten minutes to an hour. The appearance of the entire bundle of such threads which make up the condensation does not change much in the course of a day. 1lagnetic pinch effects may heat the contained plasma, and mass movements of large tubes of plasma may introduce rapid plasma compression or expansion, or electrical dis- charges. (5) With all the available observational data, it is still not possible to predict the eruption of a major solar flare with high confidence to within better than about five days. Greatly improved satellite instruments for solar studies and better- coordinated ground-based observations may lead to much superior prediction criteria in the next few years. Solar Cycle Variations in Ultraviolet and X rays.-What fraction of the variation of X-ray emission is associated with active regions, or plages? Essentially, all of the emission is associated with ions, which are formed at equilibrium temperatures greater than 1.5 million degrees Kelvin. The solar cycle variation follows the growth of active plage regions. Below 20 A (radiations which affect the lower E and D regions of the ionosphere), a detectable X-ray background began to appear in late 1965. The first large spot to develop (March 1966) increased the 8-20-A flux by 50 times; yet this spot occupied less than one thousandth of the disk area. We conclude that the corona immediately over this spot had an X-ray brightness 5000 times as great as the surrounding corona. A factor of 2 in the enhanced emission may be attributed to increased temperature. The remaining increase of 2500 must be due to the greater density of the condensed corona over the active region a 50-fold increase in density since X-ray brightness varies with the square of the density. In one day the X-ray flux from this spot was observed to decrease to only 10 per cent of its initial value. No obvious clue to the change in temperature or density was evident from observations of the chromosphere or photosphere in white light, in calcium light (Ca II K, X 3934), or in the hydrogen red line(Ha, X6566). Some of the complex small spot configuration surrounding the main large spot appeared to have vanished. The activity must have been concentrated near the tops of magnetic loops overlying the spot. An expansion of such loops could quickly reduce the X-ray flux. At what height is the source of X-ray emission located? The approach of the active region from behind the sun was detected two days before the spot appeared at the edge of the disk. Some of the X-ray emission must therefore originate as high as 100,000 miles above the solar surface. The Solar Eclipse of May 1966.-The NRJL SOLRAD satellites monitor solar X-ray emission and transmit the information continuously. Some 15- observatories around the world receive these data in addition to the U.S. network of DOD stations and NASA \iLnitrack stations. The intersection of the path of the satellite and the eclipse shadow of M\ay 20, 1966, occurred almost directly over the Arcetri Observa- tory in Florence, Italy. As the moon eclipsed active centers on the disk, the shut- ting off of X-ray emission proceeded very abruptly, which means that the active centers were very small. M\Iedium-energy X rays (8-20 A) were concentrated in coronal knots measuring less than 50 seconds of arc, about 3 per cent of the diam- Downloaded by guest on September 30, 2021 2146 N. A. S. SYMPOSIUM: H. FRIEDMAN Ptoc.

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