M. J. Reiner

The source region of an interplanetary type II radio burst

We present the first observation of the source region of an interplanetary type II radio burst, using instruments on the Wind spacecraft. Type II radio emission tracks the motion of a CME-driven interplanetary (IP) shock which encounters the spacecraft. Upstream of the IP shock backstreaming electrons are observed, first antiparallel to the interplanetary magnetic field (IMF), and then later parallel as well. Langmuir waves are observed concomitant with the shock-accelerated electrons. The electron energy spectrum and Langmuir wave amplitudes are very similar to those observed in the terrestrial electron foreshock. From the connection times to the shock, we infer the existence and characteristic size of large scale structure on the shock front. The type II radio emission seems to be generated in a small bay upstream of the shock, and this may account for some splitting structure observed in the frequency spectrum of many type II bursts.

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.

Constraints on Coronal Mass Ejection Dynamics from Simultaneous Radio and White-Light Observations

Simultaneous radio and white-light observations are used to deduce information on the dynamics of two coronal mass ejection (CME) events that occurred about 2 hr apart on 2001 January 20 and that were associated with eruptions from the same active region on the Sun. The analysis combines both space-based and ground-based data. The radio data were obtained from the WAVES experiment on the Wind spacecraft and from the Culgoora radiospectrograph in Australia. The white-light data were from the LASCO experiment on SOHO and from the Mk4 coronameter at the Mauna Loa Solar Observatory. For these CME events we demonstrate that the frequency drift rate of the type II radio emissions, generated by the shocks driven by the white-light CMEs, are consistent with the plane-of-sky height-time measurements, provided that the propagation direction of the CMEs and their associated radio sources was along a radial line from the Sun at a solar longitude of ~E50°. These results imply that the true CME speeds were estimated to be ~1.4 times higher than the measured plane-of-sky speeds and that the CMEs originated from solar eruptions centered near E50°. This CME origin is consistent with the known active region and flare site associated with these two CME events. Furthermore, we argue that the type II radio emissions generated by these CMEs must have originated in enhanced density regions of the corona. We investigate whether the type II radiation could have originated in one or more dense coronal streamers, whose densities were estimated from the polarization brightness measurements made by LASCO at that time. Finally, we use these radio and white-light observations to speculate about the dynamics and scales involved in the interaction between these two CMEs.

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.

Statistical analysis of coronal shock dynamics implied by radio and white‐light observations

For 19 solar eruptive events we present a statistical comparison of the shock dynamics derived from the measured frequency drift rates of metric and decametric-hectometric (D-H) type II radio bursts with the dynamics of the associated coronal mass ejection (CME). We find that the shock speed parameters derived from the D-H type II radio emissions generated in the high corona (∼2-4 R ○. ; R ○. = 696,000 km) are well correlated with the corresponding CME plane-of-the-sky speeds (correlation coefficient = 0.71). On the other hand, we find no obvious correlation between the shock speed parameters derived from the metric type II radio bursts, generated in the middle corona (1.4-2 R ○. ), and the corresponding CME speeds (correlation coefficient = -0.07). In general, we also find no clear correlation between the shock speed parameters derived from the metric type II bursts and the D-H radio emissions (correlation coefficient = 0.3). However, the metric type II radio bursts sometimes include a second component that is possibly related to the D-H radio emissions. These statistical comparisons of the shock dynamics, implied by the observed metric and D-H type II frequency drift rates, provide further evidence for two distinct coronal shocks. Our statistical analyses are preceded by two specific examples that illustrate the methodology used in this study.

Plasma wave measurements with STEREO S/WAVES: Calibration, potential model, and preliminary results

[1]xa0The S/WAVES experiments on the two STEREO spacecraft measure waves, both in situ plasma waves and remotely generated waves such as Type II and Type III solar bursts. A part of the experiment is aimed at understanding the generation of electromagnetic waves from electrostatic Langmuir waves. For this, rapid measurements of plasma density, sufficiently rapid to be on the time scale of Langmuir wave fluctuations, are deemed necessary. Measurements of the potential of the antennas relative to the spacecraft can supply these rapid measurements. The antennas were not provided with a bias current, and so this unbiased technique has not been used previously. However, the cylindrical antennas of S/WAVES respond to temperature as well as the density of the ambient plasma, giving five quantities, ne, Te, and 3 components of E, to be determined from the three measurements of antenna potential. The work presented here discusses the analysis and interpretation of these measurements from the early part of the mission, when there were frequent observations of foreshock Langmuir waves to use for calibration. A model of the photoemission-plasma equilibrium has been constructed, using these and other measurements. It is shown that the response to one or a few of the five quantities may be negligible, depending on the phenomenon observed, so that useful measurements are obtained of the others. Application to observation and analysis of various plasma wave phenomena will be discussed.

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°.

Radio signatures of the origin and propagation of coronal mass ejections through the solar corona and interplanetary medium

During a 16-day period from February 5 to 20, 2000, a series of decametric-to-kilometric wavelength type II and type III radio events was observed by the WAVES radio experiment on board the Wind spacecraft. These radio events were related to observed coronal mass ejections (CMEs) and their associated flares. Each of the solar eruptive events was initiated by an intense, complex type III radio burst, which occurred within minutes of the liftoff on the CME. Some of the CMEs produced decametric-hectometric (D-H) type II radio emissions, which, when their frequency drift rates were sufficiently well defined, were used to provide a speed estimate. The complex type III and D-H type II radio emissions gave an indication of the presence of a CME well before the CME was first observed in the coronagraph images. This series of CMEs also generated interplanetary (kilometric) type II radio emissions that tracked the CME-associated shock through the interplanetary medium and established the terrestrial connection. Thus the various radio emissions associated with these solar eruptive events provided a global view of each entire Sun-Earth connection event, from the initiation and liftoff of the CME at the Sun, to the propagation of the CME-associated shock through the solar corona and interplanetary medium, to its arrival at 1 AU. Finally, we show that simultaneous Wind/Ulysses observations of the interplanetary type II radio emissions on February 9–10 provide important information on the nature of the type II emission, on the type II source locations, and on the radiation characteristics of the type II emissions. For example, these simultaneous observations clearly indicate that the sporadic nature of the type II radiation was intrinsic to the radio source region.

Type III radio source located by Ulysses/Wind triangulation

Radio triangulation from the widely separated Ulysses and Wind spacecraft is used to reconstruct the trajectory of a type III radio burst in the three-dimensional heliosphere. The derived radio trajectory follows a (Parker) spiral path corresponding to a solar wind speed of ∼200 km/s and progresses to the south of the ecliptic plane. These remote radio observations also measure the interplanetary plasma density along the path of the radio source. The derived average density-distance scale is very similar to the previously derived RAE density scale, which was determined in a different way. The results of the radio triangulation combined with a drift rate analysis give an average electron exciter speed of ∼0.3c. The radio source size and the brightness temperature as viewed from Ulysses and Wind are determined and compared as a function of observing frequency.