RADIATIONS FROM21 THE 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 ), 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 October 1, 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 -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 October 1, 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 October 1, 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 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 October 1, 2021 2146 N. A. S. SYMPOSIUM: H. FRIEDMAN Ptoc. N. A. S.

eter of the disk. Higher-energy X rays (1-S A) were concentrated in still smaller knots, less than 30 seconds of arc, about 1.5 per cent of the solar diameter. These observations revealed the active region to contain a gradation of temperature and density from a relatively cool, low-density enveloping plasma to a much hotter and denser interior "fireball." Rapid Variations in Solar X-Ray Flux. -SOLRAD measurements give cointinuity over 10 to 15 minutes on a single pass. Under "quiet" conditions, there is rarely any evidence of variations of more than a few per cent in the 44-60-OA band, which emanates primarily from the entire disk. In the shorter-wavelength bands (8-20 A) rapid variations appear, as much as 50 per cent, without any evidence of even a weak flare. At still shorter wavelengths, the flux is always spasmodic. An X-ray spectrum obtained on August 4, 1966. by an N-RI, rocket shows that major spectral variations below 25 A take place in a matter of seconds, without flare activity. Previously quiescent regions brighten momentarily. The changes may be due to (1) temperature, which would need to change from less than a million to several million degrees in a matter of secoiids; and (2) density, which would need to increase by an order of magnitude in a matter of seconds. Such variations sug- gest nonithermal excitation by trapped electrons, precipitating in the active regions. Radio type-Ill noise bursts, which are attributed to energetic electrons that excite plasma oscillations as they speed through the corona at relativistic velocities, are often associated with subflares and appear to be an almost constant feature of solar emission. Their ubiquitous presence and association with minor optical phe- nomena, such as active prominences and filaments, means that there must exist a simple and efficient acceleration mechanism for electrons. A constantt microscale generation of nonthermal electrons must take place which is capable of accelerating electrons to more than 10,000 electron volts and, in turn, exciting X rays. It is also possible that magnetic pinch effects in fine, magnetically trapped threads of plasma could produce rapid temperature and density changes which are reflected in the X-ray emission. Such considerations imply that the lower corona is a jungle of magnetic ropes (Figs. 4-6), tightly looped from one magnetic sunspot to another-a proliferation of auroral precipitation zones coupled by Van Allen belts. Electrons may be transferred from one trapping belt to another and distributed over the disk until they are dumped or escape along a streamer into interplanetary space. Solar Wind, Flares, and Streamers.-The solar wind appears to be the motive force behind the development of solar activity centers. It also propels disturbances toward the earth in the form of streamers and in expanding balloonlike plasmas which produce sudden magnetic storms when they encounter the earth's magneto- sphere. Quiet-sun measurements show that about 1.4 X 1018 kW leaves the lower corona in the kinetic energy of solar wind. Over an active region of typical size, about one thousandth of the disk, this amounts to 1030 ergs per day. Although a sunspot suppresses gas flow through it, ordered flow is improved around the spot periphery. In an active region, the flow may therefore reach 103' ergs per day. The largest solar flares release a total of about 1032 ergs of energy. Only ten days of storage of solar wind could easily supply the energy of a large flare. Appearance of an Active Region as Seen against the Solar Limb. The first evidence of the birth of an active region is the appearance of faculae (bright splotches seen in white light) and plages (mottled orange-peel texture in calcium light). Sunspots Downloaded by guest on October 1, 2021 FIGS. 4-6.-Patterns of threads and loops of magnetic flux tubes in the solar chromosphere and corona. The loops in isolated areas are generally stretched in a particular direction and show bright crowns or arches. In other areas, the loops are twisted chaotically. The tangled threads and loops cover the entire sun like a knitted sweater. The pictures were photographed in hydrogen alpha (red light) with the solar tele- scope at Anacapri. Diameter of sun (1 million miles) on scale of photographs is 1 meter.

.~~~~~~E M> %

FIG.4~~~~~~~

FIG. 5

FIG.6 Downloaded by guest on October 1, 2021 2148 N. A. S. SYMPOSIUM: H. FRIEDMAN PROC. N. A. S.

then begin to form. As the solar wind flow increases, due to ordered magnetic fields, the density of material increases above the spots and is bottled up by the looping magnetic fields that connect sunspot pairs. Within weeks, the active region reaches full development. The spot groupings are complex and a large condensation (4 X 1030 cm3 the volume of about 4000 earths) covers the active region like a mushroom top. As the active region grows in size, it develops sporadic condensations near the tops of magnetic loops. Sporadic condensations are small compared to the large permanent condensation, but are an order of magnitude more dense and twice as hot. In the fully developed phase, bursts of high-energy solar wind surge outward, producing a broad brush appearance above the condensation. After about one month, the spot groups are stretched out and a typical helmet structure develops (Fig. 1). The helmet pattern has the appearance of a Y-type neutral point instability, in the language of plasma physics. The pattern strongly resembles the structure of the earth's geomagnetic tail, which is created by the pressure of solar wind against the earth's magnetosphere. As the instability grows, a time is reached when the magnetic lines may tear near the base of the helmet. A shock wave then carries off a flare surge of high-energy cosmic-ray particles, and electrons and protons are driven dowoi toward the base of the helmet by high electric fields, thus exciting the visible and X-ray flare. Rocket-borne coronagraphs have provided clear pictures of the structure of the streamer spike of the helmet formation extending out as far as 10 solar radii. The rigidity of the streamer forms is clearly evident; yet some photographs show a tendency for a disk-shaped equatorial ring of gas to form, resembling Saturn's rings. It appears that solar rotation eventually draws the magnetically confined gases by centrifugal force into the equatorial plane. In the range of coronagraph pictures (out to 10 solar radii), the solar streamers appear to be narrowly confined spikes, but surrounded by a fine brush of filamentary material. At very large distances, two thirds of the way to the earth, the streamer structure appears to be filamentaryr with a typical diameter of about 2000 miles. Assuming a diameter proportional to distance, this implies that the diameter at the sun is about 60 miles, which is of the order of a diameter of a spicule at the chro- mospheric level. Spicules photographed against the chromospheric limb with a coronagraph are fountainlike jets 50) to 100 miles at the base and about 5000 miles high, which surge and fall in 5 to 10 minutes. At any time, about 100,000 spicules may cover the sun with a much higher concentration in the active plage regions. Spicules may be the routes by which solar wind escapes the photosphere. Downloaded by guest on October 1, 2021