R. G. Stone

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.

Quasiperiodic Jovian Radio bursts: observations from the Ulysses Radio and Plasma Wave Experiment

Abstract The Ulysses flyby of Jupiter has permitted the detection of a variety of quasiperiodic magnetospheric phenomena. In this paper, Unified Radio and Plasma Wave Experiment (URAP) observations of quasiperiodic radio bursts are presented. There appear to be two preferred periods of short-term variability in the Jovian magnetosphere, as indicated by two classes of bursts, one with ∼ 40 min periodicity, the other with ∼ 15 min periodicity. The URAP radio direction determination capability provides clear evidence that the 40 min bursts originate near the southern Jovian magnetic pole, whereas the source location of the 15 min bursts remains uncertain. These bursts may be the signatures of quasiperiodic electron acceleration in the Jovian magnetosphere; however, only the 40 min bursts occur in association with observed electron bursts of similar periodicity. Both classes of bursts show some evidence of solar wind control. In particular, the onset of enhanced 40 min burst activity is well correlated with the arrival of high-velocity solar wind streams at Jupiter, thereby providing a remote monitor of solar wind conditions at Jupiter.

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.

Ulysses observations of whistler waves at interplanetary shocks and in the solar wind

This study of whistler wave emission observed by the Ulysses Unified Radio and Plasma Wave (URAP) experiment between 1 and 5 AU is a complement to previous studies of whistler waves observed by the Helios spacecraft between 0.3 and 1 AU. The Helios spacecraft continuously detected a background of whistlers close to the Sun, and this background was found to decrease in intensity with larger heliocentric distance. Ulysses plasma wave observations confirm this trend. Within a heliocentric distance of approximately 2 AU, whistler waves are routinely observed. Beyond about 3 AU, the waves are usually observed only downstream of interplanetary shocks. Moreover, whistler waves are routinely observed within about 2 AU at all heliographic latitudes of the Ulysses trajectory (−80° to +80°). The combined observations from the Helios and Ulysses spacecraft suggest that whistler emission is always present in the solar wind, although at larger heliocentric distances the wave amplitudes are often below the thresholds of the URAP instrument. Observations throughout the first 5 years of the Ulysses mission show a clear correlation of whistler emission intensity with magnetic field strength, or gyrofrequency, such that increases in wave intensities coincide with increases in gyrofrequency. This correlation is especially evident in observations of interplanetary shocks and high-speed streams. A possible cause of this correlation is increased whistler wave growth due to enhanced electron temperature anisotropies in regions of compressed magnetic field. A shift of the background whistler spectrum as a function of gyrofrequency could account for the observed decrease in whistler amplitudes with increasing heliocentric distance.

Multi‐tube model for interplanetary magnetic clouds

Measurements of the polytropic index γ inside a magnetic cloud showed that there are two non-equal tubes inside the cloud [Fainberg et al., 1996; Osherovich et al., 1997]. For both tubes, γ < 1, but each tube has its own polytrope. We test equilibrium solutions which are a superposition of solutions with cylindrical and helical symmetry [Krat and Osherovich, 1978] as a new paradigm for a multi-tube model. Comparison of magnetic and gas pressure profiles for these bounded MHD states with observations suggests that complex magnetic clouds can be viewed as multiple helices embedded in a cylindrically symmetric flux rope.

Ulysses observations of electron and proton components in a magnetic cloud and related wave activity

Fifteen years ago, Burlaga defined magnetic clouds as interplanetary structures with enhanced magnetic field characterized by a smooth rotation of the magnetic field vector and a low proton temperature Tp. Their expansion in the solar wind leads to a depletion of plasma and a cooling of the ion component. Recently, Osherovich and colleagues showed that the electron component in magnetic clouds behaves differently: When the cloud expands, the electron temperature Te anticorrelates with the density and therefore Te increases in the cloud. Since Landau damping is not effective for Te/Tp≫1, they predicted an increase of ion-acoustic wave activity in magnetic clouds. Our paper presents evidence in support of this prediction. For the magnetic cloud observed by Ulysses on June 10–12, 1993 at 4.64 AU at S 32.5 deg, we present observations for both the electron and the proton components and the related plasma waves activity. Our results confirm the anticorrelation between Te and electron density: the data also exh...

Solar wind quasi-invariant as a new index of solar activity

Solar wind parameters, such as magnetic field strength B, plasma density ρ and bulk speed υ correlate with solar cycle measured by sunspot numbers SSN. Using hourly data in the solar wind near the Earth, we show that the energy density ratio QI = (B²/8π)/(ρυ²/2) has a correlation coefficient (cc) with SSN which is appreciably higher than for each of B, ρ and υ taken separately. The proposed quasi-invariant is related to magnetic Mach number MA (QI = MA−2). The yearly median QI for the last 28 years holds a linear relation with SSN with high cc = 0.98, which makes it the best index of solar activity derived from solar wind parameters so far.

Source characteristics of Jovian hectometric radio emissions

Direct confirmation that low-frequency Jovian hectometric (HOM) radio emissions centered near 0° central meridian longitude consist of distinct, oppositely polarized northern and southern beams has been achieved using data from the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses-Jupiter encounter in early February 1992. Distinct northern and southern beams were observed in the frequency range from ∼300 kHz to 1 MHz for at least eight Jovian rotations during the Ulysses inbound pass at distances from 100 to 40 RJ. The radiation from the two magnetic hemispheres was measured from different Jovigraphic longitudes and magnetic (or centrifugal) latitudes. Observed temporal variations in the radio intensities, with time scales on the order of 30 min, may result either from longitudinal variations of the HOM sources or from longitudinal density variations in the Io plasma torus. Using the URAP direction-finding capabilities and assuming a tilted dipole planetary magnetic field model, the three-dimensional HOM source locations, the L shell through these source locations, and the beam opening angles were independently deduced. The HOM sources were found to originate at ∼3 RJ and on low L shells (L ∼ 4 to 6), with beam opening angles ranging from 10° to 50°.