SCIENCE SUMMARY
SCIENCE SUMMARY
Previously in this book the reader has been introduced to the idea of a plasma, and to a solar plasma called the solar wind.
Because both the electrons and ions are free to move in a plasma, a wide variety of waves can exist in the solar wind. These waves are called plasma waves. Since the early days of the discovery of the solar wind it has been thought plasma waves play an important role in controlling dynamical processes in the solar wind. Because collisions are extremely rare in the tenuous solar wind, plasma waves play a role similar to collisions in an ordinary gas by scattering particles and providing the dissipation necessary to achieve thermal equilibrium. To detect plasma waves the HELIOS spacecraft included a 32 meter tip-to-tip dipole antenna for the electric field of plasma waves, and a search coil magnetometer for detecting magnetic fields. The signals from the electric antenna were analyzed by three instruments which covered the frequency range from about 10 Hz to 3 MHz. Another instrument analyzed the signals from the search coil magnetometer. In this paper we review results obtained from the HELIOS electric field measurements.
Before discussing the results, it is useful to review the types of waves that can exist in a plasma. Three principal types of plasma waves were detected by the electric field instruments on HELIOS: (1) the free space electromagnetic mode, (2) the electron plasma oscillation mode, and (3) the ion acoustic mode.
The free space electromagnetic mode is the usual electromagnetic mode in free space, of which light and radio waves are common examples.
The electron plasma oscillation is an almost purely oscillatory mode in which the electrons vibrate around their equilibrium position while the ions remain at rest. The resulting charge oscillation produces an electric field but no magnetic field. For this reason these waves are sometimes called electrostatic waves.
The ion acoustic mode is very similar to a sound wave in an ordinary gas, except that the electric field transfers the wave momentum instead of collisions. In contrast to electron plasma oscillations, both the electrons and ions participate in the wave motion. Because of the large inertia of the ions, the propagation speed of the ion acoustic mode is quite slow, normally much less than the solar wind speed, which is typically about 400 km/sec. As in the case of electron plasma oscillations the ion-acoustic mode is electrostatic, with no magnetic field.
Because the electron density decreases with increasing heliocentric radial distance, the laws of physics require that both the electron and ion plasma frequencies decrease with increasing radial distance from the Sun. The resulting radial variation is shown in figure 1. As can be seen the electron plasma frequency, which is the characteristic frequency of electron plasma oscillations and the low frequency limit of the free space electromagnetic mode, increases from about 20 kHz at the Earth's orbit to several hundred MHz at the solar surface. The ion plasma frequency, which is the upper frequency limit of the ion acoustic mode, varies from about 500 Hz at the Earth's orbit to a few MHz at the solar surface.
Type III solar radio bursts are produced by solar flares at the Sun and are characterized by an emission frequency that decreases with increasing time according to a long standing theory, first proposed by Ginzburg and Zheleznyakov (1958), the generation of type III bursts is a two-step process in which (1) electron plasma oscillations are first produced by energetic electrons ejected from a solar flare, and (2) the energy in the plasma oscillation is converted to electromagnetic radiation via a nonlinear coupling process. One of the first notable accomplishments of the HELIOS plasma wave experiment was the confirmation of this basic mechanism.
An example from HELIOS 2 illustrating the simultaneous occurrence of a type III radio burst, electron plasma oscillations, and energetic (20 - 65 keV) electrons from a solar flare is shown in figure 2. This event occurred on November 22, 1977, following a solar flare that started at 09:46 UT.
According to current ideas the electron plamsa oscillations are excited by a beam-plasma instability as the high speed solar wind electrons stream outward from the Sun. These plasma oscillations then produce radio waves at a fundamental frequency and the second harmonic via nonlinear coupling to the free space electromagnetic mode. This process is indicated schematically in figure 1. For a further discussion of the generation of type III bursts, see the accompanying paper by Kellogg.
During the first 10 years of HELIOS observations a total of 238 electron plasma oscillation events were observed in association with type III radio bursts. Most of these events occurred around the time of maximum solar activity, from about 1977 to 1982. The frequency of the plasma oscillation shows a clear tendency to increase with decreasing radial distance from the Sun, as would be expected from figure 1. This trend is illustrated in figure 3 which shows the spectrum of electron plasma oscillation events detected at 0.86, 0.68, and 0.32 AU. The decrease in the electron plasma frequency with increasing radial distance accounts for the decreasing emission frequency of the type III burst as the solar flare electrons move outward from the Sun. The intensity of the electron plasma oscillation also tends to increase with decreasing radial distance from the Sun. The increase in the field strength with decreasing radial distance probably explains why type III radio bursts tend to be more intense at higher frequencies, which are generated closer to the Sun.
Electron plasma oscillations are also observed that are not associated with any detectable type III radio emission. These types of plasma oscillations occur quite frequently, sometimes several times per day. The absence of a detectable radio emission from these events indicates that the plasma oscillations are probably quite localized, and not occurring simultaneously over a large volume of the solar wind, as in the case of the type III related events.
In the first few months of operation of HELIOS 1, enhanced electric field intensities were discovered in the solar wind at frequencies between the ion and electron plasma frequencies Typically , two or three periods of enhanced electric field intensities occur during each solar rotation, separated by periods of relatively low intensity. The periods of enhanced activity usually last from a few hours to several days. Subsequent investigations showed that the frequency spectrum of the electric field fluctuation depend on the radial distance from the Sun, generally increasing in frequency, intensity and occurrence with decreasing distance from the Sun. A representative set of electric field spectra taken at 1.73, 0.98, and 0.47 AU, is shown in figure 4. The spectrum at 1.73 AU is from VOYAGER 1. The trend toward increasing intensity and frequency with decreasing heliocentric radial distance is clearly evident.
Over 10 years since the discovery of this noise, a fairly convincing case has been made that these waves are ion acoustic waves. At first glance it does not appear that the spectrum is consistent with an ion acoustic wave interpretation, since the peak in the spectrum occurs well above the ion plasma frequency, whereas the ion acoustic mode can only propagate at frequencies below the ion plasma frequency. This difficulty was resolved by Gurnett and Anderson. Because the solar wind velocity is much greater than the ion acoustic speed, they concluded that the frequency should be almost entirely determined by the Doppler shift caused by the motion of the solar wind. The Doppler shift depends on wavelength and is given by f = V / lambda, where lambda is the wavelength of the waves.
The minimum wavelength varies from about 60 meters at the orbit of Earth to 20 meters at the HELIOS perihelion. The corresponding maximum frequencies, assuming a nominal solar wind velocity of 400 km/sec, are 6.6 and 20 kHz. These maximum frequencies are seen to be in excellent agreement with the upper cutoff frequency of the observed electric field spectrum.
Even after ten years of study the origin of the solar wind ion acoustic waves is not yet clearly established. Two factors, the electron to ion temperature ratio, Te / Ti, and the electron head flux, Qe, seem to control the intensity of the ion acoustic waves.
These dependencies support a theory, first by Forslund (1970), in which the ion acoustic waves are excited by the electron heat flux in the solar wind. If the electron heat flux is the origin of enhanced acoustic wave intensities, it is possible that these waves may play an important role in regulating the thermal conduction of heat away from the Sun by the solar wind!
Solar flares often produce shock waves that propagate through the solar wind out to the orbit of the Earth. These shock waves are called interplanetary shocks and are almost always accompanied by enhanced plasma emissions. During the early part of the HELIOS mission interplanetary shocks were quite rare. However, later, around solar maximum, from about 1977 to 1982, many shocks were observed. The plasma wave signatures associated with these shocks are highly variable and depend on the detailed structure of the shock. A representative example is shown in figure 5. This shock was detected by HELIOS 2 on March 30, 1976. The plasma wave signature in this case is quite straightforward and s\consists of a burst of electron plasma oscillations upstream of the shock and an abrupt broadband burst of electric field noise at the shock crossing, which was at 17:44:00.5 UT +/- 0.5 sec. The broadband burst of noise gradually decays downstream of the shock over a period of half an hour or more.
Electron plasma oscillations are frequently observed upstream of the Earth's bow shock and are known to be caused by a beam of electrons streaming into the solar wind from the shock. The mechanism of exciting the plasma oscillations is essentially the same as the oscillations associated with type III radio bursts, except that the electrons originate from the shock instead of the solar flare. In the region close to the Sun, electron plasma oscillations of this type are believed to cause type II and type IV solar radio bursts via a nonlinear coupling process very similar to the generation of type III radio bursts. Interestingly enough, shock-associated electron plasma oscillations are quite rare in the HELIOS data. The March 30, 1976, event is one of the few shocks with upstream electron plasma oscillations. The reasons for the relatively low occurrence of upstream electron plasma oscillations ahead of interplanetary shocks is not completely understood, but is probably related to the lower Mach number of interplanetary shocks.
The intense broadband burst of electric field noise at the shock is observed on essentially every interplanetary shock detected by HELIOS. The shape of the spectrum of this noise is very similar to the spectrum of the ion acoustic noise described previously, but is usually more intense, sometimes reaching peak broadband field strengths of several mV/m. Because of the similarity to the ion acoustic wave spectrum, it is generally believed that this noise is caused by ion acoustic waves generated in the shock. Studies of similar turbulence in the Earth's bow shock indicate that this noise probably plays an important role in heating the plasma at the shock. Because particle collisions in the tenuous solar wind are extremely rare, some type of turbulent process must be present to provide dissipation and to head the plasma at the shock.
This summary of results from the HELIOS plasma wave experiment demonstrates that this investigation has produced many important new results over the 10 year period since HELIOS 1 was launched. This investigation confirmed a basic theory for the generation of type III radio bursts that was first proposed over 20 years ago, and it revealed the existence of enhanced levels of ion acoustic wave turbulence in the solar wind. The long duration of the observations and the extended radial distance coverage provided a vast quantity of data on the temporal and radial variation of these and other plasma wave phenomena over almost an entire solar cycle. The results obtained show that the plasma processes occurring in the solar wind are very complicated and many important questions still remain to be answered. Hopefully, with the continued operation of HELIOS 1 and further study of the existing data some of these questions can be answered.
