Investigation of Solar Wind Acceleration Near the Sun
FIRST RESULTS FROM THE SOHO ULTRAVIOLET CORONAGRAPH SPECTROMETER
MADISON, WI-- An instrument aboard SOHO designedto artificially eclipse the Sun and reveal itsnormally invisible extended atmosphere hasrecorded surprisingly high temperatures forparticles being spewed into space from coronalholes. It also observed the particlesaccelerating to supersonic velocities very nearthe Sun. The unexpectedly high temperatures maybe the key to understanding how the Sunaccelerates the wind. The results were reported at the summer meeting of the American Astronomical Society on June 11, 1996.
"The observations, the first made in ultravioletlight, are fascinating and spectacular, " saysJohn Kohl, of the Smithsonian AstrophysicalObservatory, and principal investigator for theUltraviolet Coronagraph Spectrometer (UVCS) aboardthe joint US-European Space Agency SOHO satellite.
UVCS/SOHO observations began on January 29, 1996,when the instrument made the first ultravioletimages of those regions in the Sun's corona, orhot, extended atmosphere, where the primaryacceleration of the solar wind occurs. It also made measurements of the temperatures andvelocities of particles in the imaged regions asthey emerged from so-called coronal holes andstreamed away from the Sun to wash over all theplanets of the Solar System, including Earth.
"Although it may be months or even years beforethe full scientific import of these results arerealized," adds Kohl. " We can already see thegeneral behavior of some representative particlesforming the solar wind."
The UVCS/SOHO is a joint project of theSmithsonian Astrophysical Observatory and theUniversity of Florence in collaboration with theUniversity of Padua, the University of Turin, anda consortium of Swiss scientists. The primarysupport is from the National Aeronautics and SpaceAdministration and Italy, with additional funding from Switzerland.
The basic goal of UVCS/SOHO is to understandthe generation of the solar wind and the natureof those regions in the extended solar coronawhere the solar wind originates. Prior to SOHO, there was no detailed description of the Sun'souter atmosphere. Despite tremendous gains inthe past 35 years, observations have beenconcentrated primarily on the base of the solaratmosphere. Information on the extendedatmosphere was limited, primarily, to spatial andtemporal distributions of electron density measured in visible light. The data have notbeen sufficient to explain either how the wind isaccelerated in different regions of the solaratmosphere nor how those processes give rise tothe widely varying properties of the solar winddetected directly at space platforms near theEarth.
Solar Wind from Coronal Holes
Coronal holes, the known source of the highestspeed solar wind streams, are characterized byopen magnetic fields that allow the chargedparticles to escape the Sun's magnetic grasp.Observations of the base of the corona (up toabout 35,000 kilometers above the visible disk)indicate that coronal holes are cooler than otherregions of the solar corona and the gas density islower. The Ulysses satellite, which detected thesolar wind some 300 million kilometers above thesolar poles, inferred that the electrontemperatures reach about 1 million degrees Kelvinat 500,000 kilometers above the visible disk.
UVCS/SOHO has made the first ultraviolet images ofcoronal holes in the extended solar atmosphere(see Figure 1), showing them as diffuse regionswith long, ray-like structures called polarplumes. Perhaps the most interesting UVCS/SOHOresult so far is that the temperatures of protonsand oxygen ions in coronal holes. are much hotterthan the electrons.
Indeed, UVCS/SOHO has determined that electricallycharged oxygen reaches temperatures approaching100 million degrees Kelvin at heights of about500,000 kilometers above the visible solar disk.The hydrogen reaches temperatures of about 6million degrees at the same height. The oxygentemperature is approximately 16 times the hydrogentemperature. This suggests that the particles arebeing heated in proportion to their mass (atomicoxygen being about 16 times more massive thanhydrogen).
The instrument has also provided measurements ofthe outflow velocities for hydrogen and oxygen.For example, oxygen is seen accelerating from lessthan 100 kilometers per second at 250,000kilometers to about 225 kilometers per second at aheight of 1,250,000 kilometers. [For comparison,the radius of the Sun is 690,000 kilometers.] These measurements are the first of their kind.
It is known from other instruments that high-speedsolar wind streams from coronal holes at largedistances from the Sun reach speeds of about 800kilometers per second. The UVCS observationsindicate that the particles are still acceleratingat the highest heights observed by UVCS to date.
What physical processes might cause thisacceleration and the associated heating of theparticles is still a mystery. In fact, the largedifference between the oxygen and hydrogentemperatures may help determine how the wind isaccelerated. Obviously, some processes mustpreferentially heat the more massive particles.One possibility discussed by Ian Axford and JamesMcKenzie of Max Planck Institute for Aeronomy inGermany is a resonance between the frequency ofmagneto- hydrodynamic waves in the corona and thenatural frequency of charged particles rotatingaround coronal magnetic fields in the same waycharged particles move in laboratory particleaccelerators (cyclotrons). However, otherexplanations are also possible, and future UVCSobservations are being designed to test candidatetheories.
Solar Wind From Helmet Streamers
Helmet streamers are structures in the extendedcorona that are shaped by the Sun's magneticfields. At the current phase of the solaractivity cycle, these structures are found withtheir axis near the solar equator. Directmeasurements of solar wind particles at largedistances from the Sun indicate that helmetstreamers are a source of the "slow," or normalspeed, solar winds. For example, UVCSmeasurements of oxygen outflows show theirvelocity appears to reach about 100 kilometers persecond, out of the tips of streamers, at heightsof 3 million kilometers.
The first image of a helmet streamer inultraviolet light is shown in Figure 2. Thisimage is from light emitted by highly chargedoxygen atoms as they accelerate from the Sun. Figure 3 is an image of the same object, but thistime in light emitted by atomic hydrogen. Noticehow different the object appears when looking atthe hydrogen as opposed to the oxygen.
UVCS/SOHO has measured hydrogen and oxygentemperatures in streamers, as well as the speed ofelectrically charged oxygen as it accelerates fromthe streamers into the solar wind. The hydrogentemperature in streamers is found to vary fromabout 2.1 million degrees Kelvin at lower heightsin the corona to a value of about 1.4 milliondegrees at 2 million kilometers above the visibledisk. In some regions of the streamers, the oxygenand hydrogen temperatures are similar, but inother regions they can be much different.
For example, in the upper part of Figures 2 and 3,the proton and oxygen temperatures are at about 2million degrees Kelvin. By contrast in the lowerstructure that is bright in oxygen but dim inhydrogen, the oxygen temperature is 4 milliondegrees and the hydrogen is only 2 million. Verylittle is known about the physical processes thataccelerate particles out of the streamers. Theymay be similar to the processes acting in coronalholes or another process entirely. UVCS/SOHOobservations are aimed at helping to solve thismystery as well.
Why Study the Solar Wind?
The Sun emits into space a wind of electricallycharged gas consisting of protons, electrons, andhighly charged atoms which reach Earth at speedsof 1.5 to 3 million kilometers per hour. Fortunately, our planet is armed with a naturalshield against this onslaught: the magnetosphere,a distant magnetic and electrically chargedextension of our atmosphere which slows anddeflects the bulk of the stream of particlesemitted by the Sun.
This shield does not provide complete protection,however. Under constant buffeting from the solarwind, the magnetic screen is buckled, distorted,and occasionally torn, causing small holes. Whenthis happens, intense electric currents, magneticstorms and particle accelerations immediatelydevelop. The overall interaction between thesolar wind and the magnetosphere is so violentthat the energy transferred can be as much as 10billion kilowatts - equivalent to worldwide powerconsumption - and the currents induced run tomillions of amps.
The side-effects are well known. The aurorae,which occur within about 20 degrees of the Earth'smagnetic poles, are caused by the precipitation ofcharged particles through the atmosphere at veryhigh altitude. Magnetospheric storms can alsocause power surges in electricity transmissionlines; in March 1989, for instance, there was amassive power failure throughout the province ofQuebec.
These fast moving flows of particles alsopenetrate the Earth's upper atmosphere, where theygenerate such thermal and dynamic effects thatscientists wonder whether they are not having someinfluence on the atmosphere as a whole and,perhaps, even shaping long-term climate patterns.
In orbit, the presence of charged particles canaffect the performance of satellite components andrepresents a threat to astronauts. This "spaceweather" also can wreak havoc with communicationsatellites, and account for the loss of control oftwo communication satellites, Anik E1 and E2, inJanuary 1994. Magnetic storms are powerful enoughto scrub satellite launches. The high energyparticles churned up by the solar wind carryenough energy to damage or even destroy asatellite's sensitive electronic systems or simplyreverse a single bit of information in a computerguidance system.
The storms strike with little warning, andexisting technology gives little time to preparefor them. What is needed is a capability topredict the occurrence of these storms at leastseveral hours to a few days before they happen. Unfortunately, such a capability requires a levelof understanding that doesn't currently exist.
The SOHO satellite carries a group of instrumentscapable of studying the solar wind from deepinside the solar atmosphere, through the solarcorona, and out to the vicinity of the satelliteitself. The instruments are capable of describingsolar wind generation in the detail that is neededto develop an understanding of the physicalprocesses involved in its acceleration.
It is impossible to know how long it will take todevelop a predictive theory. However, the newinformation learned from just the first few monthsof operation of UVCS/SOHO offers a hopeful signthat the solar wind will someday be understoodwell enough to cope with the Earth and spacephenomena that it controls.