We have examined radio data from the WAVES experiment on the Wind spacecraft in conjunction with ground-based data in order to investigate the relationship between the shocks responsible for metric type II radio bursts and the shocks in front of coronal mass ejections (CMEs). The bow shocks of fast, large CMEs are strong interplanetary (IP) shocks, and the associated radio emissions often consist of single broad bands starting below approx. 4 MHz; such emissions were previously called IP type II events. In contrast, metric type II bursts are usually narrowbanded and display two harmonically related bands. In addition to displaying complete dynamic spectra for a number of events, we also analyze the 135 WAVES 1 - 14 MHz slow-drift time periods in 2001-2003. We find that most of the periods contain multiple phenomena, which we divide into three groups: metric type II extensions, IP type II events, and blobs and bands. About half of the WAVES listings include probable extensions of metric type II radio bursts, but in more than half of these events, there were also other slow-drift features. In the 3 yr study period, there were 31 IP type II events; these were associated with the very fastest CMEs. The most common form of activity in the WAVES events, blobs and bands in the frequency range between 1 and 8 MHz, fall below an envelope consistent with the early signatures of an IP type II event. However, most of this activity lasts only a few tens of minutes, whereas IP type II events last for many hours. In this study we find many examples in the radio data of two shock-like phenomena with different characteristics that occur simultaneously in the metric and decametric/hectometric bands, and no clear example of a metric type II burst that extends continuously down in frequency to become an IP type II event. The simplest interpretation is that metric type II bursts, unlike IP type II events, are not caused by shocks driven in front of CMEs.

http://iopscience.iop.org/article/10.1086/428820/pdf

Solar type ii radio bursts and ip type ii events

Magnetized giant exoplanets in close orbits around their host star are expected to be strong nonthermal radio emitters. The anticipated radio flux is strong enough to allow its detection on Earth using the next generation of instruments. However, the measured quantity will not be the planetary radio flux but the sum of planetary and stellar emission. We compare the expected stellar and planetary radio signal for stellar systems of different ages. Solar-like stellar wind parameters as well as conditions corresponding to the young solar system (i.e. with increased stellar wind density and velocity) are considered. For young stellar systems, conditions appear to be more favorable than for older stellar systems. It is shown that configurations exist where the separation of the planetary signal from the stellar emission seems feasible.

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The influence of stellar wind conditions on the detectability of planetary radio emissions

Shock waves are associated with multiple powerful coherent radio emissions within the heliosphere and local interstellar medium. The radio emissions definitely driven by shocks include interplanetary type II (solar radio) bursts,“foreshock” emissions from upstream of Earth’s bow shock, and rare emissions from corotating interaction regions (CIRs). Emissions likely driven by shocks, but without definitive observational evidence, include coronal type II bursts, the 2–3 kHz emissions from the outer heliosphere, and drifting pulsating structures from the deep corona. Analogous emissions are also predicted, but not yet observed, for mini-magnetospheres and associated bow shocks on the Moon and for moons like Ganymede, the foreshocks of other planets, particularly Mercury and Jupiter, and supernovae. All these emissions are produced near the electron plasma frequency f pe and/or 2f pe via the so-called “plasma emission” mechanism or linear mode conversion, two of the four coherent radio emission mechanisms observed to date. In each case the theoretical interpretation requires coupling of multiple physical processes from microscales to macroscales. Microscale physics includes the time-varying magnetic overshoots of reforming shocks, electron reflection and acceleration at shocks, growth of Langmuir waves in the upstream foreshock, and the linear or nonlinear conversion of Langmuir energy into radio emission at f pe and/or 2f pe . Intermediate scale physics includes the creation of ripples on the shock on scales of order the decorrelation length of the magnetic field, as well as scattering of the radiation by density irregularities. Macroscale physics includes 3D spatiotemporal variations of the plasma and the shock motion, as well as integration of emission from individual shock ripples over the entire shock. This chapter is a comprehensive review of the field, starting with observations of the emissions definitely and probably driven by shocks. Existing theory is then summarized in some detail, followed by detailed reviews of the observation and theory of type II bursts (both coronal and interplanetary) and the 2–3 kHz emissions from the outer heliosphere, including descriptions of the unresolved issues. The discussion focuses on limitations of the theory and existing observations and ways to address them. The overall conclusions are that the basic theory (electron shock acceleration, development of an electron beam, growth of Langmuir waves, and production of f pe and 2f pe radiation for a macroscopic, rippled, shock) appears to explain the primary observations semiquantitatively, that many observational details and theoretical limitations remain unresolved, and that the next ten years ought to be an exciting time that sees theory and observations brought together quantitatively.

Coherent Radio Emissions Associated with Solar System Shocks

In order to investigate the formation of radio emitting shock waves above flaring active regions, we combine spectral and imaging observations of type II radio events with X-ray imaging and full-Sun observations and, in one case, with the extrapolated magnetic field configuration in the corona. We confirm and extend earlier findings that type II bursts are emitted above active region loops seen in soft X-ray images. Sources at successively lower frequencies are non-radially displaced from the axis of the active region loops. Two new radio features identified in high resolution spectrograms establish a possible link between the type II emission and the preceding activity in the underlying corona: 1. Groups of fast drift bursts or pulsations with a restricted bandwidth are observed in coronal loops from the impulsive flare phase until the onset of the type II emission. These groups or their high frequency cut-off are found in the spectral range around the backward extrapolated type II lanes. Envelope features of the group (starting frequency and/or cut-off frequency) drift gradually to lower frequencies, at a normalized drift rate similar to the following type II lanes. The sources are located between the sites of H α emission and of the type II emission. We refer to this burst group and its envelope features as a type II precursor . 2. Immediately before the type II emission a short ( ≤ 1 min duration) series of narrow-band bursts occurs at frequencies between the split bands of the type II lanes. As a whole, the burst sequence has an inverted U–shaped spectral envelope. We therefore call it anarc. It has fundamental-harmonic structure as the subsequent type II burst, but no band split. The source is located near or above the summits of the coronal loops where theprecursoremission occurred before, and close to the site where the type II emission starts. The arc feature occurs especially prior to high frequency type II bursts, i.e. type II shocks formed at comparatively low coronal height. It is concluded that the type II burst is related to a plasma jet or a blast wave that originates in closed magnetic structures in the active region, and is first recognized during the early impulsive phase (between the onset and main maximum of the Send offprint requests to : A. Klassen (aklassen@aip.de) hard X-ray emission). This disturbance becomes or launches a shock wave when it attains the summit of these structures. The shock is refracted into overlying field regions, where it becomes visible through the type II emission.

On Shock Wave Formation in the Solar Corona

High-resolution (sub-Debye length grid size and 10 000 particle species per cell), 1.5D particle-in-cell, relativistic, fully electromagnetic simulations are used to model electromagnetic wave emission generation in the context of solar type III radio bursts. The model studies generation of electromagnetic waves by a super-thermal, hot beam of electrons injected into a plasma thread that contains uniform longitudinal magnetic field and a parabolic density gradient. In effect, a single magnetic line connecting Sun to Earth is considered, for which five cases are studied. (i) We find that the physical system without a beam is stable and only low amplitude level electromagnetic drift waves (noise) are excited. (ii) The beam injection direction is controlled by setting either longitudinal or oblique electron initial drift speed, i.e., by setting the beam pitch angle (the angle between the beam velocity vector and the direction of background magnetic field). In the case of zero pitch angle, i.e., when v→b·E→⊥=...

An alternative to the plasma emission model: Particle-in-cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts

We present the results of radio telescope UTR-2 observations of solar Type II radio bursts in the 10–30 MHz frequency range. These events possess a fine structure consisting of fast drift sub-bursts similar to Type III bursts. The frequency drift rate of the Type II bursts at decameter wavelengths is smaller than 0.1 MHz s−1. One of these bursts with herringbone structure has a wave-like backbone that almost does not drift. The features of the observed bursts are discussed.

V. V. Zaharenko

Papers

Observations of Solar Type II bursts at frequencies 10–30 MHz