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Outline of the thesis: CMEs, Radio and X-ray emissions and Space Weather
As was discussed in the previous section, CMEs are observed and studied through corono-graphic images. The basic limitation of the coronograph is that it shows the corona only in the plane of the sky, and blocks by necessity the view on the solar disk. But the ability of CMEs to cause geomagnetic storms (known as geo-effectiveness) depends crucially on the proximity to the Sun-Earth line (halo CMEs are more geo-effective Gopalswamy, Yashiro, and Akiyama [2007]) and the onset and early evolution of CMEs in the low corona are not accessible to coronographic observations from Space.
Radio imaging of the low-coronal manifestations of CMEs is able to show the signatures on the solar disk. Previous studies with the NRH, such as Pick and Vilmer [2008], sug-gest indeed that radio images at metric wavelengths track the early evolution of CMEs well before they become visible in the corona. A characterisation of the radio emission mechanisms as well as the relations with the CME evolution is presented in Chapter 2. The determination of SEP acceleration sites associated with the CME evolution in the corona is illustrated through the study of the eruptive event on 2008 April 26. This event offered an unique opportunity to investigate the physical link between a single well-identified CME, electron acceleration as traced by radio emission, and the produc-tion of SEPs observed in the Space. We conduct a detailed analysis combining radio observations (NRH and Decameter Array, Wind/WAVES spectrograph) with remote-sensing observations of the corona in extreme ultraviolet (EUV) and white light as well as in-situ measurements of energetic particles near 1AU (SoHO and STEREO spacecraft). We demonstrate that is misleading to interpret the multi-spacecraft measurements of SEPs in terms of one acceleration region in the corona. Even though the understanding about how and where particles are accelerated is still an open question, radio emission can provide an important diagnostic of particle acceleration sites as we discuss in next chapters.
We also want to explore if there is a relationship between the polarisation of type IV radio bursts associated with Earth-directed CMEs and the orientation of the interplan-etary magnetic field observed at the ICME arrival. In Chapter 3 we present an initial characterisation of the polarisation of three type IV bursts in order to establish the basis for a future work in this subject. Finally, the other issue related to space weather effects of Earth-directed CMEs is the difficulty to estimate their arrival time because direct coronographic measurements of the propagation speed are not possible from the Sun-Earth line. Thus, various proxies have been devised, based on coronographic measurements to estimate this speed. As an alternative, we explore radiative proxies to estimate this speed based on the signatures on the solar disc. Both observation and theory reveal that the dynamics of a CME in the low corona is closely related to the evolution of the energy release in the associated flare as traced by the soft X-ray and microwave emission. We present in Chapter 4 a reassessment of the statistical relationships between limb-CME velocities and radiative parameters. Then the radiative fluences (SXR and microwave) are used to obtain CME speeds of Earth-directed CMEs.
A description of the CME propagation in the interplanetary space is also presented in Chapter 4 where we use the speed obtained from radiative proxies as an input in one empirical model to predict the arrival time of CMEs at the Earth. The predictions are compared with observed arrival times in situ and with the predictions based on coronographic measurements, as well as with techniques using heliographic imaging and MHD modelling. The main aim of this thesis is to explore complementary diagnostics of CMEs based on radio emission that potentially can be considered in space weather applications.
Interferometry Basics
The interferometry technique is the combination of single elements which work together to form a single telescope. Such arrays are called ’interferometers’ and one of the few so-lar dedicated interferometers is the Nan¸cay Radioheliograph whose north-south antenna array is shown in Figure 2.2. The spatial resolution of an interferometer is determined by the maximum separation between elements. The baseline (B) is the distance between two antennas. If B is considered as the maximum distance of antennas of the array, the spatial resolution is determined by θHPBW = Bλ . To explain the basics of interferometry, we consider a simple two dishes interferometer, as the one shown in Figure 2.3, that observes a point source. The radio signal arrives at different antennae at different times, which means that the signal is observed with a phase difference of φ = ωt = 2πdλ sinθ. This phase difference is one of the principal issues in interferometry, which can be solved by correlating the different signals.
The correlation function (S) of the signals in terms of the delay time t can be described as Z T S=E02 cos(ωt′)cos(ω(t′ + t))dt′, (2.15) 0 where E0 and T are the amplitude of the monochromatic plane wave and the integration period (longer than 2ωπ ) respectively.
Nan¸cay Decametric Array
The Nan¸cay Decametric Array (NDA, Lecacheux [2014]) operates in the 10-80 MHz frequency range and consists in two phased antenna arrays in opposite senses of circular polarisation with 4000 m2 of effective aperture each. The set of receivers of wide band allow to obtain a high resolution and sensitive spectroscopy of Jovian and solar radio emissions with a resolution of 1 sec.
The obtained data is a dynamic spectrum: the intensity received is shown as a function of time and frequency (as the dynamic spectra in Fig. 2.10).
Nan¸cay Radioheliograph (NRH)
The Nan¸cay Radioheliograph (NRH, Kerdraon and Delouis [1997]) is an instrument ded-icated to solar observations at long decimetre and metre wavelengths and was designed to observe the total and circularly polarised radiation (complex visibilities in Stokes I and V) from the Sun. The instrument is a T shaped interferometer of 48 antennas spread over two arrays (EW and NS) as is shown in Figure 2.4. In this figure, the position of the antennas in the array are marked by the yellow, green and blue points. Red points are antennas which are not part of the T-shaped array. Observations of the visibilities are done during 7 hours per day in a frequency band of 700 kHz with a selected band between 150 and 450 MHz with sub-second time resolution.
Solar Radio Emission
The radio emission from the Sun at dm-m wavelengths can be classified according to the dynamic spectrum as: Quiet-Sun, Noise Storms and Burst Emission [Kundu, 1965].
Quiet-Sun Emission
This emission results from the thermal bremsstrahlung process in the solar atmosphere and it is distributed over all solar disk. Bremsstrahlung emission is produced as a conse-quence of Coulomb collisions between electrons (test particles) and ions (field particles). Bremsstrahlung is thermal if the test particles have the same thermal distribution that the field particle, while it is called non-thermal Bremsstrahlung when the test particles have a non-thermal distribution. Thermal Bremsstrahlung is observed at soft X-ray (SXR) and microwave and dm-m wavelengths, while Bremsstrahlung produced by non-thermal particles is observable at hard X-ray (HXR) wavelengths [e.g., Aschwanden, 2004].
Table of contents :
Declaration of Authorship
Abstract
Acknowledgements
Contents
List of Figures
Abbreviations
1 Introduction
1.1 Coronal Mass Ejections (CMEs)
1.2 Outline of the thesis: CMEs, Radio and X-ray emissions and Space Weather
2 Radio Diagnostics of the CME Evolution in the low Corona
2.1 Basics of Radio Emission
2.2 Radio Observations
2.2.1 Radio Telescope Basics
2.2.2 Interferometry Basics
2.2.3 Solar Radio Instrumentation at Nan¸cay Station
2.2.3.1 Nan¸cay Decametric Array
2.2.3.2 Nan¸cay Radioheliograph (NRH)
2.3 Solar Radio Emission
2.3.1 Quiet-Sun Emission
2.3.2 Noise Storms
2.3.3 Radio Bursts
2.3.3.1 Microwave Bursts
2.3.3.2 Radio bursts at dm-km wavelengths
2.4 Density model, drift rates and shock parameters from the dynamic spectrum
2.4.1 Density model
2.4.2 Drift rate and exciter speed
2.4.3 Shock parameters
2.5 Study of CME-related particle acceleration regions during a simple eruptive event near solar minimum (paper)
3 Characterisation of Type IV Bursts: Localisation and Polarisation
3.1 Relationship between the CME Propagation and Extension and the Motion and Extension of Radio Sources
3.1.1 Identification of Type IV Radio Burst Sources
3.1.2 Comparison between the Extensions and Locations of CME and the Associated Type IV Radio Sources
3.2 Polarisation of Radio Sources
3.2.1 Polarisation of Electromagnetic Radiation
3.2.2 Stokes Parameters
3.2.3 How is the Polarisation of Radio Sources Related to the Emission Mechanism of Type IV bursts?
3.3 Characterisation of the Polarisation of Type IV Radio Bursts
3.3.1 Event on 2008 April
3.3.2 Event on 2010 April 3
3.3.3 Event on 2012 March 4
3.4 Preliminary Results
4 Radiative Proxies for CME Propagation Speed in ICME Arrival Time Predictions
4.1 CME Radial Propagation Speed
4.2 Propagation of CMEs into the Interplanetary Space
4.2.1 Interplanetary Magnetic Field Configuration
4.2.2 Interplanetary Propagation of CMEs
4.2.2.1 Empirical Interplanetary Propagation Models
4.2.2.2 Numerical MHD-based Propagation Models
4.2.2.3 Analytical Interplanetary propagationModel: Drag-Based Model (DBM)
4.2.3 CME-CME Interaction in the Interplanetary Space
4.3 Soft X-ray and Microwave Emissions and their Relationship with CMEs .
4.3.1 On the statistical relationship between CME speed and soft X-Ray fux and fluence of the associated flare (paper)
4.3.2 Microwave radio emission as a proxy of CME speed in ICME arrival predictions at 1 AU (paper)
4.3.3 Radiative proxies for CME speed in arrival predictions: final remarks
5 Summary and Perspectives
Bibliography
