Yolande Leblanc

Planetary radio astronomy observations from Voyager 2 near Saturn

Planetary radio astronomy measurements obtained by Voyager 2 near Saturn have added further evidence that Saturnian kilometric radiation is emitted by a strong dayside source at auroral latitudes in the northern hemisphere and by a weaker source at complementary latitudes in the southern hemisphere. These emissions are variable because of Saturns rotation and, on longer time scales, probably because of influences of the solar wind and Dione. The electrostatic discharge bursts first discovered by Voyager 1 and attributed to emissions from the B ring were again observed with the same broadband spectral properties and an episodic recurrence period of about 10 hours, but their occurrence frequency was only about 30 percent of that detected by Voyager 1. While crossing the ring plane at a distance of 2.88 Saturn radii, the spacecraft detected an intense noise event extending to above 1 megahertz and lasting about 150 seconds. The event is interpreted to be a consequence of the impact, vaporization, and ionization of charged, micrometer-size G ring particles distributed over a vertical thickness of about 1500 kilometers.

Tracing the Electron Density from the Corona to 1 au

We derive the electron density distribution in the ecliptic plane, from the corona to 1 AU, using observations from 13.8 MHz to a few kHz by the radio experiment WAVES aboard the spacecraft Wind. We concentrate on type III bursts whose trajectories intersect the spacecraft, as determined by the presence of burst-associated Langmuir waves, or by energetic electrons observed by the 3-D Plasma experiment. For these bursts we are able to determine the mode of emission, fundamental or harmonic, the electron density at 1 AU, the distance of emission regions along the spiral, and the time spent by the beams as they proceed from the low corona to 1 AU. For all of the bursts considered, the emission mode at burst onset was the fundamental; by contrast, in deriving many previous models, harmonic emission was assumed.By measuring the onset time of the burst at each frequency we are able to derive an electron density model all along the trajectory of the burst. Our density model, after normalizing the density at 1 AU to be ne(215 R0)=7.2 cm−3 (the average value at the minimum of solar activity when our measurements were made), is ne=3.3×105 r−2+4.1×106 r−4+8.0×107 r−6 cm−3, with r in units of R0. For other densities at 1 AU our result implies that the coefficients in the equation need to be multiplied by ne(1 AU)/7.2.We compare this with existing models and those derived from direct, in-situ measurements (normalized to the same density at 1 AU) and find that it agrees very well with in-situ measurements and poorly with ‘radio models’ based on apparent source positions or assumptions of the emission mode. One implication of our results is that isolated type III bursts do not usually propagate in dense regions of the corona and solar wind, as it is still sometimes assumed.

Voyager Planetary Radio Astronomy at Neptune

Detection of very intense short radio bursts from Neptune was possible as early as 30 days before closest approach and at least 22 days after closest approach. The bursts lay at frequencies in the range 100 to 1300 kilohertz, were narrowband and strongly polarized, and presumably originated in southern polar regions ofthe planet. Episodes of smooth emissions in the frequency range from 20 to 865 kilohertz were detected during an interval of at least 10 days around closest approach. The bursts and the smooth emissions can be described in terms of rotation in a period of 16.11 � 0.05 hours. The bursts came at regular intervals throughout the encounter, including episodes both before and after closest approach. The smooth emissions showed a half-cycle phase shift between the five episodes before and after closest approach. This experiment detected the foreshock of Neptunes magnetosphere and the impacts of dust at the times of ring-plane crossings and also near the time of closest approach. Finally, there is no evidence for Neptunian electrostatic discharges.

Tracing shock waves from the corona to 1 AU: Type II radio emission and relationship with CMEs

We report on 10 type II bursts observed with ground-based spectrographs in the meter-decameter range, and with the Radio and Plasma Wave Investigation on the Wind spacecraft from 13.8 to 0.01 MHz. We have selected events with contemporaneous observations of flares and of coronal mass ejections (CMEs) by Large-Angle and Spectrometric Coronagraph (LASCO) telescopes. We trace the history of each event from the time of the impulsive phase of the flare, the CME liftoff time, and the start time of the radio bursts. We derive the speed of the type II shock by using a coronal/solar wind density model, and the height-time progression is compared with that of the CME as observed in the plane of the sky and then converted into the radial direction. For most events a shock at 1 AU was observed in situ. The results show the following: (1) All type II bursts occurred within 2 or 3 min of the impulsive phase of a flare. (2) The speeds of the disturbances from the time of the flares to the time of the shocks at 1 AU were very similar to the speeds of the type II-emitting shocks: they were in the range of 600 to 1300 km s -1 . (3) When the type II burst was observed far out in the solar wind, the progression of the type II source had about the same speed in the solar wind as in the corona. (4) The CME liftoffs were before the flares and the type II bursts by 1-24 min for most of the selected events. As a consequence, in the corona, the type II bursts, being behind the fronts of the CMEs, are usually blast waves. (5) When a shock and CME material are observed at 1 AU, the time of arrival implies a deceleration of the CME in the solar wind, as is observed in the LASCO data. (6) Somewhere in the solar wind the shocks very likely become piston-driven. related to the CME.

Electron beams and radio waves of solar Type III bursts

Using the radio wave and energetic particle experiments on the Wind spacecraft, we examine how the radio flux density of interplanetary type III bursts depends on the flux and energy of the energetic electrons. We derive the relationship between them, first by giving detailed radio and electron characteristics of one type III burst, and then using the results of similar analyses of 10 bursts. The times of commencement of the radio waves from decametric to kilometric wavelengths, in relation to the onset time of Langmuir waves, demonstrate forcibly that the initial type III radiation is at the fundamental and not the harmonic. Near and after the time of peak flux density the radiation could be either at the fundamental or the harmonic. In our theoretical analysis we examine this point, i.e., how the emissivity of the fundamental and harmonic at the time of peak flux density depends on the beam properties. The data of the 10 events are in good accord (rc ≈ 0.9) with the theoretical relation for fundamental emission, but in disaccord with the theoretical relation for harmonic radiation. For the 10 bursts we find poor correlation between the radio flux density and the electron flux N(E‖) at the energy E‖ estimated to be that of the two-stream instability. However there is excellent correlation when N(E‖) is weighted by E‖ to a high power. From the best fit, we find rc ≈ 0.96 when N(E‖) is replaced by . Finally, we infer the efficiency of energy conversion from the kinetic energy of the electron beam to fundamental emission, and examine the attenuation of the peak emission within the source.

A shock associated (SA) radio event and related phenomena observed from the base of the solar corona to 1 AU

We present for the rst time an almost com- plete frequency coverage of a Shock Associated (SA) radio event and related phenomena observed on May 6, 1996 at 9:27 UT. It is observed from the base of the solar corona up to almost 1 Astronomical Unit (AU) from the Sun by the following radio astronomical instruments: the Ond rejov spectrometer operating between 4.5 GHz and 1 GHz (radi- ation produced near the chromosphere); the Thermopyles Artemis-IV spectrograph operating between 600 MHz and 110 MHz (distance range about 1.1-1.4R from sun center); the Nan cay Decameter Array operating between 75 and 25 MHz (distance range about 1.4-2 R); and the RAD2 and RAD1 radio receivers on the WIND spacecraft covering the range from 14 MHz to about 20 kHz (distance range be- tween 3 R and about 1 AU). Observations at the Nan cay Decameter Array clearly show that the SA event starts from a coronal type II radio burst which traces the progression of a shock wave through the corona above 1.8 R-2 R from the sun center. This SA event has no associated radio emis- sion in the decimetric-metric range, thus there is no evidence for electron injection in the low/middle corona. The SA event enigma: What does SA stand for? Type II and type III solar radio bursts result from the interaction of a disturbing agent {a beam of energetic elec- trons or a shock wave{ with the ambient plasma (Wild and Smerd, 1972). Radiation is produced near the fundamen- tal of the local plasma frequency f p (kHz) =9 n 1 = 2 e (cm 3 ) and/or its second harmonic through various plasma mech- anisms (see e.g. Robinson, 1997). The observed frequency can be converted into an altitude in the corona, assuming a density model and the radiated mode. Dierent frequency drifts reflect dierent velocities along the density gradient in the corona and interplanetary medium, helping us to charac- terize the nature of the exciter: 0.05-0.3c electron beam for

Acceleration of electrons at type II shock fronts and production of shock-accelerated type III bursts

We present evidence of electron acceleration by type II-burst-emitting shocks in the corona. Some of the electrons travel outward along open magnetic field lines and produce “shock-accelerated type III bursts” (or SA type III bursts) along their paths. The SA type III bursts are evident in dynamic spectra that cover part or all of the range from metric to kilometric wavelengths. The unique feature of our observations is the complete or near-complete frequency coverage from about 2 GHz to < 0.1 MHz, that is, ≲ 1.01 Ro to 1 AU. A sample of eight events is presented. All would be classified as “shock accelerated events” at hectometric wavelengths, as first defined by Cane et al. [1981]. Our complete spectra frequently show several to many type III-like bursts emanating from near the type II burst toward low frequencies, with no trace of emission at frequencies higher than that of the type II burst. The drift rates of these SA type III bursts are similar to those of normal type III bursts, and the exciting electrons have speeds of order 0.1c to 0.2c, or energies of 3–10 keV and higher. Their intensity at hectometer wavelengths is similar to that of normal type III bursts. They often persist to the lowest frequencies observable, near the local plasma frequency at 1 AU. In most of the events examined, there were no microwave bursts from the low corona whose intensity profiles were similar to the hectometric profiles. We therefore conclude that these SA type III bursts originate in type II shocks and are caused by energetic electrons accelerated at the shocks. Of the eight events analyzed, three contain only SA type III bursts. For the remainder, normal type III bursts predominate initially, followed by SA type III bursts later in the event. We emphasize the need for spectra with near-continuous coverage, especially from decametric to kilometric wavelengths, to identify SA type III bursts unambiguously and to distinguish between the contributions of normal and SA type III bursts.

Type II flare continuum in the corona and solar wind

A solar radio type II/type IV event with exceptionally low frequency flare continuum radiation was observed on May 2, 1998 with the Wind spacecraft. This flare continuum, associated with the type II burst (FCII), descended to 7.5 MHz (2.5–3 solar radii), the lowest frequency ever observed for this type of emission. It lasted for >2 hours at 13.8 MHz. Simultaneous observations were made with ground-based radiospectrographs, and with the Extreme Ultraviolet Imaging Telescope (EIT) and Large Angle and Spectrometric Coronagraph (LASCO) telescopes. The radio event consists of a group of intense type III bursts observed from 1000 MHz down to 0.03 MHz, the plasma frequency at 1 AU. The type II burst was recorded from 45 MHz down to 0.4 MHz, and an interplanetary shock was observed at 1 AU on May 4 at 0500 UT. The type II shock commenced within a few minutes of the flash phase of the flare and of the liftoff time of a coronal mass ejection (CME) observed by EIT and LASCO. The derived speeds of the type II shock, the CME in the plane of the sky, and the shock from the Sun to 1 AU are all ≈ 1000 km s−1. After estimating the liftoff time and radial speed of the CME front, we find that the type II shock and flare continuum were in the wake of the CME. This event shows evidence of acceleration of electrons in the corona out to 3 AS for ≳2 hours. Theoretical implications on the generation of the flare continuum radiation and its relation to the observed brightness temperature are considered. The source model of type II-flare continuum of Robinson [1985], in which electrons are accelerated by the shock wave traversing CME expanding loops, is discussed in view of these observations.

Type II shock and CME from the corona to 1 AU

We explore the relationship among solar flares, type II-associated shock waves in the corona and solar wind, coronal mass ejections (CMEs), and shock waves observed at 1 AU. A clear example is that of 1997 Nov 4, where radio waves were observed continuously from 1800 MHz to 30 kHz. The CME was observed from 2.3 to 30 R o . A shock was observed at 1 AU, followed by CME material. In addition a 2B, X2.1 flare initiated a wave in the low corona, and streams of fast electrons produced intense type III bursts along their path, and Langmuir waves at 1 AU. The liftoff time of the CME coincided to within a few minutes with the time of the impulsive flare and the initiation of the type II shock wave. The speed of the CME in the plane of the sky was similar to the derived speed of the type II shock from 1.26 to 78 R o and the average speed of the shock detected at 1 AU, 646 km/s. Therefore, the observations of this event demonstrate that type II shock waves, CMEs and flares can be closely interrelated, that coronal and interplanetary type II shock waves can be inseparable, and that the ensemble can commence at < 1.3 R o and continue outward to produce a shock and geophysically important effects at Earth.

Flux and images of Jupiter at 13, 22 and 36 cm before, during and after SL‐9 impacts

Two radio telescopes in Australia, the Australia Telescope and the Molonglo Observatory Synthesis Telescope were used to observe Jupiter before, during and after the impacts of comet Shoemaker-Levy 9 in July 1994. The telescopes observed at wavelengths of 13, 22 and 36 cm and produced 2-D images of Jupiter–s synchrotron radiation belts, showing that the brightness of a portion of these belts was strongly affected by the comet impacts. Starting about one day after the first impact and reaching a peak soon after the time of the last impact the integrated flux density increased by approximately 25–30% at 13 and 22 cm and 40% at 36 cm.