J. Fainberg

Radio Tracking of Solar Energetic Particles through Interplanetary Space

Energetic particles ejected from the sun generate radio waves as they travel out through the interplanetary medium. Satellite observations of this emission at long radio wavelengths provide a means of investigating properties of the interplanetary medium, including the gross magnetic field configuration over distances of 1 astronomical unit. Results of such observations are illustrated.

A new method for studying remote type II radio emissions from coronal mass ejection‐driven shocks

Some interplanetary shocks associated with coronal mass ejections (CMEs) generate type II radio emissions at the local plasma frequency and/or its harmonic. These type II radio emissions provide a means of remotely studying and tracking CMEs from the solar corona to 1 AU and beyond. New analysis techniques that inherently reveal the dynamics of a CME as it propagates through the interplanetary medium are used for tracking the CME-associated radio emissions. The techniques make use of dynamic spectra of the radio intensity plotted as a function of inverse frequency and time. When in situ measurements are also available, the analyses determine unequivocally whether the type II radio emissions occurred at the fundamental or harmonic of the local plasma frequency in the upstream or downstream regions of the CME-driven shock. These new analysis techniques are applied to three type II radio bursts that were observed by the WAVES radio experiment on the Wind spacecraft on May 13–14, November 4–5, and November 6–7, 1997; each event corresponded to a CME observed by SOHO LASCO (large angle and spectrometric coronagraph), and each event was observed in situ by Wind. We find that the type II radio emissions for each of the three events were generated at both the fundamental and harmonic of the plasma frequency in the upstream region of the CME-driven shock, that the type II emissions appear, in general, to originate in regions along the shock front of higher than normal densities, and that the radio emission sites along the shock front change with time. In one case, additional radio tracking, provided by the direction-finding analysis, was used to locate the sites of the radio emission along the shock front.

Type II solar radio events observed in the interplanetary medium

Fifteen type II solar radio events have been identified in the 2 MHz to 30 kHz frequency range by the radio astronomy experiment on the ISEE-3 satellite over the period from September 1978 to December 1979. These data provide the most comprehensive sample of type II radio bursts hitherto observed at kilometer wavelengths. Dynamic spectra of a number of events are presented. Where possible, the 15 events have been associated with an initiating flare, ground-based radio data, the passage of a shock at the spacecraft and the sudden commencement of a geomagnetic storm. The general characteristics of kilometric type II bursts are discussed.

Plasma diagnosis from thermal noise and limits on dust flux or mass in comet Giacobini-Zinner

Thermal noise spectroscopy was used to measure the density and temperature of the main (cold) electron plasma population during 2 hours (1.5x105 kilometers perpendicular to the tail axis) around the point of closest approach of the International Cometary Explorer (ICE) to Comet Giacobini-Zinner. The time resolution was 18 seconds (370 kilometers) in the plasma tail and 54 seconds (1100 kilometers) elsewhere. Near the tail axis, the maximum plasma density was 670 per cubic centimeter and the temperature slightly above 1 electron volt. Away from the axis, the plasma density dropped to 100 per cubic centimeter (temperature, 2x 104 K) over 2000 kilometers, then decreased to 10 (1.5x 105K) over 15,000 kilometers; outside that region (plasma tail), the density fluctuated between 10 and 30 per cubic centimeter and the temperature between 1x 105 and 4 x105 K. The relative density of the hot population rarely exceeded a few percent. The tail was highly asymmetrical and showed much structure. On the other antenna, shot noise was recorded from the plasma particle impacts on the spacecraft body. No evidence was found of grain impacts on the antennas or spacecraft in the plasma tail. This yields an upper limit for the dust flux or particle mass, indicating either fluxes or masses in the tail smaller than implied by the models or an anomalous grain structure. This seems to support earlier suggestions that these grains are featherlike. Outside the tail, and particularly near 105 kilometers from its axis, impulsive noises indicating plasma turbulence were observed.

TYPE III SOLAR RADIO BURST STORMS OBSERVED AT LOW FREQUENCIES. III. STREAMER DENSITY, INHOMOGENEITIES, AND SOLAR WIND SPEED.

The analysis of a storm of type III solar radio bursts observed in August 1968 between 5 and 0.2 MHz by the RAE-1 satellite has yielded the storm morphology, a possible relation to meter and decameter storms, and an average exciter speed of 0.37 c between 10 and 40 R⊙ (Fainberg and Stone, 1970a, b). A continuation of the analysis, based on the apparent dependence of burst drift rate on heliographic longitude of the associated active region, now provides a distance scale between plasma levels in the streamer, an upper limit to the scale size of coronal streamer density inhomogeneities, and an estimate of the solar wind speed. By fixing one level the distance scale is utilized to determine the electron density distribution along the streamer between 10 and 40 R⊙. The streamer density is found to be 16 times that expected for the solar minimum quiet solar wind. An upper limit to the scale size of streamer density inhomogeneities is estimated to be of the order of 1 or 2 solar radii over the same height range. From the progressive delay of the central meridian passage (CMP) of the lower frequency emission, a streamer curvature is inferred which in turn implies an average solar wind speed of 380 km/sec between 14 and 36 R⊙ within the streamer.

Polytropic relationship in interplanetary magnetic clouds

Interplanetary magnetic clouds are expanding MHD configurations characterized by strong magnetic fields, large rotations of the field vector, and low ion temperatures. In this paper we present high time resolution data from the ISEE 3 and IMP 8 spacecraft on the magnetic field and the proton and electron populations in a number of magnetic clouds. Our objective is to study aspects of the thermodynamics of magnetic clouds and the relation between their thermodynamic and magnetic structures. Our analysis suggests the following features of the thermodynamics of magnetic clouds: (1) The electron and ion populations are not in thermal equilibrium with each other, the electron temperature, Te, being in general up to an order of magnitude higher than the proton temperature, Tp. The temperature ratio Te/Tp in these magnetic clouds is larger than typical values of this quantity in the solar wind at comparable heliocentric distances. (2) For the proton component we find that a polytropic law with γp in the range 1.1 < γp < 1.3 is probably adequate to describe the relation between Tp and density. (3) For the electrons (E < 1.18 keV) the energetics are likewise governed by a polytropic law. Unlike the protons the polytrope that describes the electrons has an index that is less than unity, implying anticorrelation between Te and the number density. For the two clouds analyzed where electron data are available, γe ≈ 0.48 ± 0.2. (4) As a corollary of case 3, the variation of Te with density in magnetic clouds is the reverse of that generally found in the inner heliosphere. (5) Electron temperatures are well correlated with the magnetic field strength, the highest values being reached where the field strength maximizes. We interpret these experimental findings along the following lines. While the magnetic field of the cloud expands, the ions are cooled (though not so effectively as in the adiabatic case (γad = 5/3), indicating some exchange of energy with the ambient solar wind). In contrast, since γe < 1, when the density drops as a result of expansion, Te increases and, consequently, a temperature difference develops between the two species. The hot electrons are trapped by the magnetic field in the core of the magnetic cloud. If magnetic clouds originate in the region near the Sun where Tp < Te, the subsequent expansion accentuates this temperature disparity further. Such conditions are favorable for the generation of ion acoustic waves.

On the origin of radio emissions associated with the January 6–11, 1997, CME

Unusual type II radio emissions were generated by an Earth-directed CME that originated at the sun on Jan. 6, 1997. The intensities of the observed radio emissions were significantly higher than for typical type II events, while the source sizes and the overall frequency drift rate were significantly smaller. By introducing a new way of presenting the radio data that inherently reveals the dynamics of the type II radio source, we used these type II radio emissions, observed by the WAVES experiment on Wind, to track this CME through the interplanetary medium (IPM). From an analysis of the observed frequencies, the frequency drift rates and the results of the Wind radio direction finding, we were able to identify specific interplanetary structures such as a CIR that were the probable sources of at least some of the type II radio emissions associated with this event. This is the first time that type II emissions have been traced to specific interplanetary structures.

Source characteristics of Jovian narrow-band kilometric radio emissions

New observations of Jovian narrow-band kilometric (nKOM) radio emissions were made by the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses-Jupiter encounter in early February 1992. These observations have demonstrated the unique capability of the URAP instrument for determining both the direction and polarization of nKOM radio sources. An important result is the discovery that nKOM radio emission originates from a number of distinct sources located at different Jovian longitudes and at the inner and outermost regions of the Io plasma torus. These sources have been tracked for several Jovian rotations, yielding their corotational lags, their spatial and temporal evolution, and their radiation characteristics at both low latitudes far from Jupiter and at high latitudes near the planet. Both right-hand and left-hand circularly polarized nKOM sources were observed. The polarizations observed for sources in the outermost regions of the torus seem to favor extraordinary mode emission.

Satellite observations of type III solar radio bursts at low frequencies

Type III solar radio bursts have been observed from 10 MHz to 10 kHz by satellite experiments above the terrestrial plasmasphere. Solar radio emission in this frequency range results from excitation of the interplanetary plasma by energetic particles propagating outward along open field lines over distances from 5 R⊙ to at least 1 AU from the Sun. This review summarizes the morphology, characteristics and analysis of individual as well as storms of bursts. Substantial evidence is available to show that the radio emission is observed at the second harmonic instead of the fundamental of the plasma frequency. This brings the density scale derived by radio observations into better agreement with direct solar wind density measurements at 1 AU and relaxes the requirement for type III propagation along large density-enhanced regions. This density scale with the measured direction of arrival of the radio burst allows the trajectory of the exciter path to be determined from 10 R⊙ to 1 AU. Thus, for example, the dynamics and gross structure of the interplanetary magnetic field can be investigated by this method. Burst rise times are interpreted in terms of exciter length and dispersion while decay times refer to the radiation damping process. The combination of radio observations at the lower frequencies and in-situ measurements on non-relativistic electrons at 1 AU provide data on the energy range and efficiency of the wave-particle interactions responsible for the radio emission.

Type III solar radio burst storms observed at low frequencies

Storms of type III solar radio bursts observed from 5.4 to 0.2 MHz, indicate the quasicontinuous production of type III events observable for a half solar rotation but persisting in some cases for well over a complete rotation. The characteristics of these storms, including the dependance of occurrence and apparent drift rates on the disc position of the associated active region are discussed. The drift rate dependance is shown to be a consequence of the propagation time of emission from the source to the observer. The occurrence rate of a burst every 10 sec observed near CMP implies that if this level of activity persists, then about a quarter of a million exciter packets are released into the interplanetary plasma during a complete rotation. Storm bursts are less intense than most isolated type IIIs and occur over a more limited frequency range. There appears to be a very close relation between these storms and decametric continuum.