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Interferometry And Synthesis In Radio Astronomy

Alfven waves, transverse incompressible magnetic oscillations, have been proposed as a possible mechanism to heat the Sun's corona to millions of degrees by transporting convective energy from the photosphere into the diffuse corona. We report the detection of Alfven waves in intensity, line-of-sight velocity, and linear polarization images of the solar corona taken using the FeXIII 1074.7-nanometer coronal emission line with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. Ubiquitous upward propagating waves were seen, with phase speeds of 1 to 4 megameters per second and trajectories consistent with the direction of the magnetic field inferred from the linear polarization measurements. An estimate of the energy carried by the waves that we spatially resolved indicates that they are too weak to heat the solar corona; however, unresolved Alfven waves may carry sufficient energy.

Alfven Waves in the Solar Corona

Solar eruptive phenomena embrace a variety of eruptions, including flares, solar energetic particles, and radio bursts. Since the vast majority of these are associated with the eruption, development, and evolution of coronal mass ejections (CMEs), we focus on CME observations in this review. CMEs are a key aspect of coronal and interplanetary dynamics. They inject large quantities of mass and magnetic flux into the heliosphere, causing major transient disturbances. CMEs can drive interplanetary shocks, a key source of solar energetic particles and are known to be the major contributor to severe space weather at the Earth. Studies over the past decade using the data sets from (among others) the SOHO, TRACE, Wind, ACE, STEREO, and SDO spacecraft, along with ground-based instruments, have improved our knowledge of the origins and development of CMEs at the Sun and how they contribute to space weather at Earth. SOHO, launched in 1995, has provided us with almost continuous coverage of the solar corona over more than a complete solar cycle, and the heliospheric imagers SMEI (2003–2011) and the HIs (operating since early 2007) have provided us with the capability to image and track CMEs continually across the inner heliosphere. We review some key coronal properties of CMEs, their source regions and their propagation through the solar wind. The LASCO coronagraphs routinely observe CMEs launched along the Sun-Earth line as halo-like brightenings. STEREO also permits observing Earth-directed CMEs from three different viewpoints of increasing azimuthal separation, thereby enabling the estimation of their three-dimensional properties. These are important not only for space weather prediction purposes, but also for understanding the development and internal structure of CMEs since we view their source regions on the solar disk and can measure their in-situ characteristics along their axes. Included in our discussion of the recent developments in CME-related phenomena are the latest developments from the STEREO and LASCO coronagraphs and the SMEI and HI heliospheric imagers.

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Coronal Mass Ejections: Observations

Solar cycle 23 witnessed the most complete set of observations of coronal mass ejections (CMEs) associated with the Ground Level Enhancement (GLE) events. We present an overview of the observed properties of the GLEs and those of the two associated phenom- ena, viz., flares and CMEs, both being potential sources of particle acceleration. Although we do not find a striking correlation between the GLE intensity and the parameters of flares and CMEs, the solar eruptions are very intense involving X-class flares and extreme CME speeds (average ∼2000 km/s). An M7.1 flare and a 1200 km/s CME are the weakest events in the list of 16 GLE events. Most (80 %) of the CMEs are full halos with the three non- halos having widths in the range 167 to 212 degrees. The active regions in which the GLE events originate are generally large: 1290 msh (median 1010 msh) compared to 934 msh (median: 790 msh) for SEP-producing active regions. For accurate estimation of the CME height at the time of metric type II onset and GLE particle release, we estimated the ini- tial acceleration of the CMEs using flare and CME observations. The initial acceleration of GLE-associated CMEs is much larger (by a factor of 2) than that of ordinary CMEs (2.3 km/s 2 vs. 1 km/s 2 ). We confirmed the initial acceleration for two events for which CME measurements are available in the inner corona. The GLE particle release is delayed with respect to the onset of all electromagnetic signatures of the eruptions: type II bursts, low frequency type III bursts, soft X-ray flares and CMEs. The presence of metric type II radio bursts some 17 min (median: 16 min; range: 3 to 48 min) before the GLE onset indicates shock formation well before the particle release. The release of GLE particles occurs when the CMEs reach an average height of ∼3.09 Rs (median: 3.18 Rs; range: 1.71 to 4.01 Rs) for well-connected events (source longitude in the range W20-W90). For poorly connected

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Properties of Ground Level Enhancement Events and the Associated Solar Eruptions During Solar Cycle 23

This paper will review the input of 65 years of radio observations to our understanding of solar and solar–terrestrial physics. It is focussed on the radio observations of phenomena linked to solar activity in the period going from the first discovery of the radio emissions to present days. We shall present first an overview of solar radio physics focussed on the active Sun and on the premices of solar–terrestrial relationships from the discovery to the 1980s. We shall then discuss the input of radioastronomy both at metric/decimetric wavelengths and at centimetric/millimetric and submillimetric wavelengths to our understanding of flares. We shall also review some of the radio, X-ray and white-light signatures bringing new evidence for reconnection and current sheets in eruptive events. The input of radio images (obtained with a high temporal cadence) to the understanding of the initiation and fast development in the low corona of coronal mass ejections (CMEs) as well as the radio observations of shocks in the corona and in the interplanetary medium will be reviewed. The input of radio observations to our knowledge of the interplanetary magnetic structures (ICMEs) will be summarized; we shall show how radio observations linked to the propagation of electron beams allow to identify small scale structures in the heliosphere and to trace the connection between the Sun and interplanetary structures as far as 4AU. We shall also describe how the radio observations bring useful information on the relationship and connections between the energetic electrons in the corona and the electrons measured in-situ. The input of radio observations on the forecasting of the arrival time of shocks at the Earth as well as on Space Weather studies will be described. In the last section, we shall summarize the key results that have contributed to transform our knowledge of solar activity and its link with the interplanetary medium. In conclusion, we shall indicate the instrumental radio developments at Earth and in space, which are from our point of view, necessary for the future of solar and interplanetary physics.

Sixty-five years of solar radioastronomy: flares, coronal mass ejections and Sun–Earth connection

Forty-one solar type II radio bursts located close to the solar limb (projected radial distance r 0.8 R� )w ere observed at 109 MHz by the radioheliograph at the Gauribidanur observatory near Bangalore during the period 1997–2007. The positions of the bursts were compared with the estimated location of the leading edge (LE) of the associated coronal mass ejections (CMEs) close to the Sun. 38/41 of the type II bursts studied were located either at or above the LE of the associated CME. In the remaining 3/41 cases, the burst was located behind the LE of the associated CME at a distance of <0.5 R� . Our results suggest that nearly all the metric type II bursts are driven by the CMEs.

The location of solar metric type ii radio bursts with respect to the associated coronal mass ejections

We report low-frequency (80 MHz) radio observations of circularly polarized non-thermal type I radio bursts (“noise storms”) in the solar corona whose estimated energy is ∼10 21 erg. These are the weakest energy release events reported to date in the solar atmosphere. The plot of the distribution of the number of bursts (dN) versus their corresponding peak flux density in the range S to S +dS shows a power-law behavior, i.e., dN ∝ S γ dS .T he power-law index γ is in the range −2.2 to −2.7 for the events reported in the present work. The present results provide independent observational evidence for the existence of picoflare category energy releases in the solar atmosphere which are yet to be explored.

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Low-frequency radio observations of picoflare category energy releases in the solar atmosphere

Assuming that metric type II radio bursts from the Sun are due to magnetohydrodynamic shocks driven by coronal mass ejections (CMEs), we estimate the average CME acceleration from its source region up to the position of the type II burst. The acceleration values are in the range {approx}600-1240 m s{sup -2}, which are consistent with values obtained using non-radio methods. We also find that (1) CMEs with comparatively larger acceleration in the low corona are associated with soft X-ray flares of higher energy; the typical acceleration of a CME associated with X1.0 class soft X-ray flare being {approx}1020 m s{sup -2}, and (2) CMEs with comparatively higher speed in the low corona slow down quickly at large distances from the Sun-the deceleration of a CME with a typical speed of 1000 km s{sup -1} being {approx}-15 m s{sup -2} in the distance range of {approx}3-32 R{sub sun}.

Radioheliograph observations of metric type ii bursts and the kinematics of coronal mass ejections

The observations of circularly polarized thermal radio emission from solar coronal streamers at two low frequencies, viz., 77 and 109 MHz, are used to estimate the magnetic field strength (B) at their corresponding radial distances r≈ 1.7 and 1.5 solar radii given by the electron density model of Newkirk. The estimated values of B at the above two distances are ≈5 ± 1 G and 6 ± 2 G, respectively.

Estimation of Magnetic Field in the Solar Coronal Streamers Through Low Frequency Radio Observations

Corona, the outermost region of the Sun, is believed to be present from a heliocentric distance of ~.01 R ʘ (where R ʘ is the radius of the solar photosphere)to more than 1 AU. Being highly tenuous plasma medium, it harbours large scale structures, as multi-frequency observations reveal. One such powerful solar phenomenon is Coronal Mass Ejections (CMEs), which has radio signatures as Type II bursts. Type II solar radio bursts are considered to originate from plasma waves excited by magnetohydrodynamic(MHD) shocks and converted into radio waves at the local plasma frequency and/or its harmonics. They are the direct diagnostic of MHD shocks in the solar atmosphere.

http://iopscience.iop.org/article/10.3847/1538-4357/aa72e7/pdf

C. Kathiravan

Papers

The location of solar metric type ii radio bursts with respect to the associated coronal mass ejections

Low-frequency radio observations of picoflare category energy releases in the solar atmosphere

Radioheliograph observations of metric type ii bursts and the kinematics of coronal mass ejections

Estimation of Magnetic Field in the Solar Coronal Streamers Through Low Frequency Radio Observations

New Evidence for a Coronal Mass Ejection-driven High Frequency Type II Burst near the Sun