6. Perpetual Change

    • The outer atmosphere of the Sun, the solar corona, was first detected during a total solar eclipse, and subsequently observed in broad daylight using coronagraphs aboard satellites.

    • Instead of growing colder at higher regions of its atmosphere, the temperature of the Sun’s corona soars to several million kelvin, hundreds of times hotter than the photosphere just below.

    • The corona’s million-degree temperature was first inferred from forbidden emission lines, which cannot be formed in the terrestrial laboratory where the densities are higher and the temperatures cooler than in the corona.

    • The million-degree temperature of the solar corona has been confirmed by its intense X-ray radiation.

    • Sunlight cannot heat the corona.

    • Magnetic energy can flow from cold to hot regions.

    • One of the earliest explanations of coronal heating involved sound waves generated in the photosphere and turbulent convective zone. The sound (acoustic) waves produce shocks as they travel out and dissipate their energy to heat the chromosphere.

    • Hundreds of thousands of jet-like spicules rise and fall within the chomosphere every five minutes or so.

    • Magnetic fields act like a trap door letting some sound waves through the photosphere, depending on the inclination of the local magnetic field with respect to the propagating waves.

    • The hottest, densest material in the low solar corona, with the most intense X-ray emission, is found where the magnetic fields are strongest.

    • Coronal heating processes are selective in both space and time, depending on the magnetic structure and how magnetically active the Sun is at the time.

    • The tenuous corona is molded and constrained by magnetic fields that are generated down inside the Sun and loop through photosphere sunspots into the corona. Internal motions move these ubiquitous coronal loops in, out and about.

    • The million-degree corona can be seen all across the visible disk of the Sun at X-ray wavelengths. The X-ray images of the quiescent Sun consist of dark regions, called coronal holes, and bright coronal loops.

    • Coronal holes are always present near the north and south poles of the Sun. These holes are regions of open magnetism; with magnetic fields lines that are not connected directly back to the Sun.

    • There are hot, dense coronal loops in active regions, slightly cooler and less dense coronal loops in the quiet Sun outside active regions, and even cooler and more rarefied material in coronal holes.

    • Near the maximum in the 11-year cycle of solar magnetic activity, coronal loops in active regions make up about 80 percent of the total coronal heating energy.

    • Magnetic loops are often coming together in the corona, releasing magnetic energy when they make contact. This magnetic reconnection provides a plausible explanation for heating the million-degree corona.

    • The total X-ray emission from the Sun varies over the 11-year solar magnetic activity cycle, with more intense X-rays at activity maximum.

    • The Skylab, Yohkoh and SOHO spacecraft have demonstrated that the hottest, densest material in the low corona, with the most intense X-ray and extreme-ultraviolet emission, is concentrated within strongly magnetized loops located in solar active regions, and that the active-region coronal loops are constantly varying on all detectable spatial and temporal scales.

    • Observations from TRACE have demonstrated that the corona in active regions is structured on the smallest observable scale, and that it is comprised of numerous long, thin coronal loops with little significant cross-sectional variation or observable twists or braids.

    • Coronal loops of different temperatures co-exist in active regions, with cool loops arching over hot ones.

    • SOHO and TRACE extreme-ultraviolet observations of some coronal loops in active regions are inconsistent with steady, uniformly heated loop models. TRACE measurements of overdense coronal loops can be explained if the loops are intermittently heated by upflows of heated material from the loop footpoints or legs.

    • Bright active region loops are being filled with material heated to coronal temperatures in the chromosphere or transition region, and when the observed loops disappear from view the hot gas is cooling and raining back down to the place it came from.

    • Continued dynamic activity and forced magnetic connections are ubiquitous features throughout the low solar corona. Magnetic concentrations merge together and cancel all the time and all over the Sun.

    • The so-called quiet corona outside active regions, and excluding coronal holes, could be at least partially heated when coronal loops merge and join together into new magnetic configurations. Such magnetic merging and reconnection may occur when newly emerging magnetic fields rise through the photosphere to encounter pre-existing ones in the corona or when existing coronal loops are forced together.

    • Magnetogram observations from SOHO’s Michelson Doppler Imager indicate that the entire magnetic flux in the quiet solar photosphere disappears and is replenished every 15 to 40 hours.

    • Magnetic loops of all sizes rise up into the solar corona from regions of opposite magnetic polarity in the photosphere, forming a magnetic carpet of numerous coronal loops. Most of the loops in the magnetic carpet lie within the chromosphere and transition region, never penetrating the corona.

    • Magnetic energy generated when loops in the magnetic carpet interact can be used to heat the material in their vicinity, and the interacting loops can form shock waves that fling the heated gas into higher places.

    • At distances of more than a few tenths of a solar radius above the solar photosphere, the density of the coronal plasma drops to a point where particle-particle collisions become infrequent. In such an environment, the temperature of the two most numerous kinds of particles, the protons and electrons, can become different from one another. In active regions and the quiet corona, the electrons seem to be hotter; in coronal holes, the protons seem to be hotter.

    • Observations from the SOHO’s UltraViolet Coronagraph Spectrometer near activity minimum indicate that heating in polar coronal holes is preferential, extreme and directional. The particles are not in thermal equilibrium, the massive ions are hotter than the less massive ions, and the massive ions have temperatures approaching 200 million kelvin with a higher temperature in the direction perpendicular to the radial direction than parallel to it.

    • A plausible explanation for heating the extended, open-field coronal holes is ion cyclotron heating by Alfvén waves, at least for the heavy ions.

    • The corona is so hot that it is forever expanding into space, extending all the way to the Earth and beyond.

    • The Sun’s hot and stormy atmosphere is forever expanding in all directions, filling the solar system with a ceaseless flow - called the solar wind - that contains electrons, protons and other ions, and magnetic fields.

    • The solar wind was first inferred from comet ion tails, and then sampled using interplanetary spacecraft. The solar wind is the hot corona expanding into space.

    • The Earth is immersed in the solar wind, which engulfs all the planets, so we live inside the expanding outer atmosphere of the Sun.

    • Early spacecraft measurements showed that there are two kinds of wind, a fast one moving at about 750 kilometers per second, and a slow one with about half that speed.

    • Solar-wind protons carry most of the energy of the solar wind. This is because the mass of a proton is 1836 times the mass of an electron, while both electrons and protons move at about the same speed in the solar wind.

    • The charged particles in the solar wind drag the Sun's magnetic field out in the radial direction, and the Sun's rotation bends this radial pattern into an interplanetary spiral shape within the plane of the solar equator.

    • The solar wind has never stopped blowing for more than thirty years of observations with spacecraft, so the solar wind is always being supplied by the Sun.

    • The high-speed wind is a uniform flow, while the slow-speed wind comes in gusts and squalls.

    • The twinkling, or scintillation, of radio sources suggested that a fast wind is streaming out at high solar latitudes near a minimum in the 11-year solar activity cycle, and coordinated X-ray and interplanetary particle and magnetic field data showed that the fast winds originate from polar coronal holes near the activity minimum.

    • Measurements from the Helios 1 and 2 spacecraft indicated that the electrons, protons and helium nuclei in the fast and slow solar winds have different temperatures; in the high-speed wind the more massive particles are hotter.

    • The differing ion composition of the fast and slow solar winds indicates that the wind speed is inversely correlated with the electron temperature at the wind sources, and that the slow wind originates from very different sources than the fast winds.

    • Dark, polar coronal holes, seen in X-ray images of the Sun, contain relatively little hot coronal material; it is flowing out along the open magnetic fields.

    • The Ulysses spacecraft has made measurements all around the Sun, at a distance comparable to that of the Earth. Its first polar orbit occurred near a minimum in the Sun’s 11-year activity cycle, while the second polar orbit was near an activity maximum and the third polar orbit was near the cycle minimum again but with the Sun’s magnetic poles reversed from the first time around.

    • A comparison of Ulysses velocity data and X-ray and white-light coronagraph observations indicated that at activity minimum the uniform, tenuous fast wind rushed out along the open magnetic field lines of polar coronal holes, and that the variable, dense slow wind was confined to low solar latitudes in the vicinity of coronal streamers.

    • Near the maximum in the 11-year solar activity cycle, the large polar coronal holes are replaced by a collection of smaller holes scattered over a wide range of solar latitudes, and the fast wind at this time comes from the interiors of the largest of these “smaller holes.”

    • SOHO observations indicate that the strongest high-speed wind pours out at the edges of a magnetic network in the underlying chromosphere.

    • The speed and composition of the solar wind emerging from a given area have deep roots in the chromosphere and photosphere. Fast winds emerge from regions of open magnetism while slow winds have roots in the closed magnetic regions.

    • The high-speed wind is accelerated very close to the Sun, within just a few solar radii, and the slow component obtains full speed much further away, often spurting out of the stalks of coronal streamers in magnetically driven blobs.

    • Ultraviolet line observations from SOHO indicate that heavy ions in polar coronal holes move faster than light ions in polar coronal holes.

    • The velocity anisotropy and preferential acceleration of heavier ions in coronal holes could be due to waves that resonate with ion cyclotron motion in a magnetic field.

    • Alfvén waves have been observed in the solar wind for decades, leading to speculations that these waves accelerate the fast solar wind to its higher speed.

    • Instruments aboard Ulysses have detected magnetic fluctuations, attributed to Alfvén waves, far above the Sun’s poles; they may block cosmic rays trying to enter these regions.

    • The solar wind creates a teardrop-shaped bubble in interstellar space, known as the heliosphere, which extends out to about 100 times the mean distance between the Earth and the Sun or to about 100 astronomical units. The heliosphere is the region where the solar wind dominates the behavior of charged particles.

    • The Voyager 1 spacecraft crossed the termination shock of the supersonic flow of the solar wind on 16 December 2004, at a distance from the Sun of 94 times the mean distance between the Earth and Sun or 94 astronomical units, becoming the first spacecraft to begin exploring the heliosheath, the outermost layer of the heliosphere.

    • The Voyager 2 spacecraft crossed the termination shock of the solar wind on 30 August 2007 at a distance of 84 astronomical units from the Sun.

Copyright 2010, Professor Kenneth R. Lang, Tufts University