They are recognized as primary drivers of disturbances in the interplanetary medium. The microscopic and macroscopic processes underlying the mechanism of the energy release are not yet understood. Most recent approaches consider that the coronal heating and the solar activity may result from the superposition of a very large number of incoherent elementary events produced in very small current sheets, the scale of which is too small to be observable.
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The statistical distribution of these events is an important factor and could be derived, in principle, from observations. This statistical approach has led recently to several promising developments: The high temperature of the corona was originally interpreted as arising from the dissipation of waves originating from lower layers. This topic is currently the subject of very intense research. There is a general agreement that the energy distribution of micro events is a power law index 2.
Evidence for fragmented energy release came from radio dynamical spectra Benz Sub-millisecond spikes of narrow band coherent emissions are currently observed in the decimetric-metric domain. They aggregate in clusters that can contain up to thousands of them lasting from few seconds to a few minutes. There are many observational signatures favoring the hypothesis that these spikes coincide with the energy release region. Masuda et al.
Shibata et al. Kundu et al. Radio observations in the dm-m wavelength range see Benz, Chapter 9 also provide information on the acceleration region, which in fact may cover a wide range of heights of the solar corona. The height can, for example, be derived from observations of pairs of type III bursts drifting in opposite directions.
Other information is given by metric spikes, which are most often observed just above the starting frequency of type III bursts as illustrated in Figure 2. Trottet et al. It could involve large scale loop systems as shown by joint soft X-ray and radio studies Manoharan et al. The most direct evidence for energetic particles comes from hard Xray, gamma-ray and radio signatures. With the high spatial and temporal resolution provided by Nobeyama at 17 GHz, it became possible to study the generation and propagation processes of high energy electrons.
The panel on the right illustrates a dynamic radio spectrum, with radio bursts indicated in a diagram of frequency versus time.
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The acceleration site is located in the cusp from where electron beams are accelerated in upward type III and downward direction RS. After thirty years of debate, the general consensus is that temporal and spectral evolution of X-ray and microwave emissions can be explained by a common source of electrons injected impulsively in trap-and-precipitation regions, i.
The developed models assume thick target bremsstrahlung for HXR emission and the gyrosynchrotron mechanism for the radio emission. In conjunction with radio observations, these measurements provide, simultaneously, unique diagnostics on electrons from 10 keV to above 10 MeV and on ions.
The simultaneous peaking of emission in the gamma-ray-line domain and HXR emission indicates that the ion acceleration time scales must be less than 1 s. High resolution imaging spectroscopy from soft X-rays to gamma rays recently became available with the launch of the RHESSI mission in Hurford et al. They propagate to large distances in the heliosphere, often beyond 1 AU. After three decades of research, we still do not know precisely about them; for instance, what is the initiation mechanism?
In that case, the CMEs contain three distinct regions: a bright compression front that surrounds a dark cavity and a bright core inside. The bright core is formed by the material ejected from the cool and dense prominence.
The 30 cm radio flux as a solar proxy for thermosphere density modelling
Both kinds of CMEs can reach similar speeds in the upper corona and can be also decelerated later on. CMEs are associated with on-disk manifestations that have been observed in EUV as large scale dimmings and coronal waves, and in soft X-rays as trans-equatorial loops. Radio observations are sensitive to different aspects of CMEs, depending on the selected observing wavelength domain see Chapter 11 by Vourlidas. An important point is that plots of the speed versus distance are similar for both the leading edge of the CME and the prominence.
There is a clear continuity between the radio depression observed in the low corona and the corresponding CME, when observed together. Metric activity is often observed in association with CMEs e. All the images shown here have been subtracted from a reference image taken before the occurrence of the CME, except the one at UT.
The UT frame is an on-line image with a nearby continuum and is given in order to show the bright streamer adjacent to the CME. Right: Plot of height on log scale against time for different features of the CME, viz. Signatures of these interactions are detected by bursts in the dm-m wavelength domain Maia et al. Moreton waves are interpreted as the chromospheric trace of MHD coronal waves.
They are often the seat of coronal electron acceleration processes revealed by the presence of nonthermal radio continua stationary type IV bursts lasting for several hours and of gradual hard X-ray emissions. Dimmings seen in EUV were indeed found to be essentially restricted to the region traced by the transit of the Moreton waves. In this sense, the shock is simply a means of promoting reconnection far from the original instability.
These conclusions however cannot be generalized. Indeed, for other events, the triggering was initiated by reconnection at a magnetic coronal null point, followed by secondary reconnections in a large coronal volume.
These results call for understanding the relationships among EUV coronal waves discovered by Thompson et al. The exact nature of these waves is not yet fully understood. A small number of EUV waves that have sharp bright fronts, like the event shown in Figure 2. Detection of radio emission from CME loops offers a number of important diagnostics of CMEs during their early phase, such as constraints on the thermal plasma density.
In conclusion, these results obtained from data analysis of a few individual events illustrate the wide variety of areas where radio observations can bring important and unique contributions to the general understanding of these phe- Overview of Solar Radio Physics and Interplanetary Disturbances 35 Figure 2.
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Example of an EIT wave from September Arrows indicate the EIT-wave front s. The absence of joint spectral and imaging observations simultaneously covering the microwave, decimetric and metric wavelength domains drastically limit such investigations. This approach provides important information such as the location of the null points, the current sheets, the separatrix layers and the bald patches. Most of the studies were based on the temporal relationship between these events, which required an assumption on the model of density to derive the shock speed parameters.
Upper panel: Progression of the type II like emission seen at three frequencies from Maia et al. CME bow shock picture. Although the two-shock scenario seems to be the favored view, there are studies supporting a different scenario. First, it was shown that metric type II bursts, which are much more frequent than interplanetary type II bursts, do not generally extend up to hectometric wavelengths Gopalswamy et al. No similar correlation was found for the metric type II bursts Reiner et al. However, recent radio imaging observations showed that some CME-driven shocks can take place at a rather low altitude in the corona.
Weak radio type IIlike bursts, most often not detected on radio spectrograph data, were observed by the NRH, in close temporal and spatial association with the leading edge of CMEs Maia et al. One example is displayed in Figure 2. The corresponding spectrum shows evidence for both fundamental and harmonic emission.
In that case, the CME-driven shock was probably formed at an altitude of the order of 0. One notes that the detection of such events is very rare, due to the simultaneous presence of strong metric emitting sources; this detection is mostly limited to the cases of partly occulted events associated with eruptive regions located behind the solar limb.
In conclusion, these results can suggest the following scenario: in the vicinity of the place where they are launched, fast CMEs are completely wrapped up by a shock wave; in front of the CME nose, energy will be given by the plasma cloud located below which acts as a piston.
Solar and Space Weather Radiophysics : Current Status and Future Developments
Interplanetary Coronal Mass Ejections Observations from Ulysses and SOHO missions have provided new insights on dynamical processes in the heliosphere and on their association with solar activity. Interplanetary CME ICME is the term used to describe the CME-like disturbances in the solar wind which, most often, when sampled by in-situ observations, include an IP shock preceding a turbulent plasma sheath and an ejecta. The propagation and evolution of ICMEs depend strongly on the ambient solar wind and on its structure.
Interplanetary disturbances are distorted by the slow and fast solar winds. Burlaga et al. Gopalswamy et al. The thin dashed feature is a type II burst. A bright emission is detected between Their transit time from the sun to the earth range between 2 and 5 days. Earth directed CMEs, called front-side halos, can reach the earth and generate severe geoeffects. To establish the solar origin of interplanetary disturbances, on-disk observations are of crucial importance. Radio techniques are the only means that allow the sampling of a large range of altitudes and to track precisely the propagation of these disturbances.
In the interplanetary medium, the location of the type II emissions associated with ICMEs can be obtained by triangulation. Another method consists in the measurement of the interplanetary scintillation. Interplanetary scintillation IPS arises when the radiation from a distant compact radio source is scattered by electron density irregularities in the so- Overview of Solar Radio Physics and Interplanetary Disturbances 39 lar wind, producing a random diffraction pattern on the ground. An increase in scintillation is observed when the line of sight passes through an ICME.
By observing a large number of radio sources, distributed around the Sun, IP disturbances can be detected.
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