Solar Radio Emissions: Investigating Reactivated Prominences
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Published: Fri, 09 Mar 2018
- Madeleine Eve
- Andrew Johnston
Solar Radio Emissions in Investigating Reactivated Prominences Literature Review
Astronomical objects that have a changing magnetic field can produce radio waves, which are the longest waves in the electromagnetic spectrum. By studying the radio waves emitted by the Sun, astronomers can acquire information about its composition, structure and motion. This aim of the present project is to use solar radio emissions produced during the re-activation of prominences in order to investigate possible energy sources for the activation. The purpose of this literature review is to analyse relevant papers on the subject matter that will be covered in this project, and give a summary of the literature in the field, whilst covering the history and importance of the topic, along with what types of instruments can be used to measure radio waves, and how radio waves are useful in studying prominences and their reactivation.
Radio waves are a type of electromagnetic radiation, which is a form of energy produced whenever charged particles are accelerated. They have frequencies from 3kHz to 300GHz, with corresponding wavelengths ranging from just 1mm to 100km. The understanding of solar radio emissions began in 1942, when an English physicist and radio astronomer, James Hey, was tasked to work on radar anti-jamming methods for the military. He had several reports of severe noise jamming of radars signals in the 4-8 meter wavelength range, and after examination, he realised that the direction of maximum interference was coming from the Sun, and concluded that the Sun radiates radio waves (M. Pick, 2008).
The observation of solar radio emissions has proved to be a useful tool in our efforts to understand solar physics., In particular solar radio emissions can be used to study local plasma density and magnetic reconnection, which relates to the release, over periods of a few minutes, of magnetic energy stored in the corona and which accompany solar eruption events like prominences which this project will be focusing on. In addition, radio wave emissions from solar flares offer several unique diagnostic tools which can be used to investigate energy release (A. O. Benz; 2005), plasma heating, particle acceleration, and particle transport in magnetized plasmas. A Solar flare is an observed sudden flash of brightness over the Sun’s surface or the solar limb, powered by magnetic reconnection.
Scientists study the Sun through radio emissions and other electromagnetic emissions and this has an additional advantage in that it provides a better understanding other stars, and the important processes they have to offer, such as nuclear fusion, which is a potential alternative energy source scientists have been trying to recreate on Earth for decades. The study of prominences and other eruptive events is important for providing an insight into the mechanics of the interior of the Sun, and also to assist us in the prediction of ‘space weather,’ which can effect satellites, and the Earth’s atmosphere and magnetic field.
A solar prominence is a large, bright, gaseous feature that is anchored to the surface of the Sun in the photosphere, and extends outwards into the Sun’s corona in a loop shape. Solar prominences are made from plasma that is roughly 100 times cooler and denser than the plasma in the corona and so, when viewed with the sun as a backdrop, they appear dark, and are referred to as ‘filaments.’ They can last for several months, and are held in place above the Sun’s surface by strong magnetic fields. The exact composition of prominences is currently unknown, but it has been proposed that they are made up of roughly 10% helium and 90% hydrogen. Solar prominences, like other erupting projectiles, are useful to observe as they are good indicators of the magnetic field pattern of the sun, since they lie above the magnetic neutral lines.
There are two basic types of prominences: quiescent and active-region prominences. Quiescent prominences are typically larger than active-region prominences, and also extend further into the corona, often reaching up to and over 30 000 kilometres above the Sun’s corona (T. E. Berger, 2012). In addition, quiescent prominences have a magnetic field of roughly 0.5-1mT, allowing them to extend further from the surface of the Sun than active-region prominences, which are much smaller, have much larger magnetic fields of around 2 – 20mT, and mostly do not travel over 30 000km. This project will largely be focusing on Quiescent prominences, as, extending further away from the Sun, they are easier to study using radio waves.
Prominences are always projected from filament channels, which are along polarity inversion lines; where the magnetic field is highly non-potential (J. Chaf, 2005). These channels are the source of all major solar eruptions, such as coronal mass ejections and flares. The temperature of a prominence that hasn’t erupted, is typically , and these often appear as a long horizontal sheet of plasma.
Several different models have been proposed in order to explain how cool, dense objects like prominences can be supported and thermally isolated from the surrounding hot coronal plasma. It is generally accepted that these models can generally be placed into one of two main categories: dip models, and flux rope models (for example: D. H. Mackay, 2010, D. J. Schmit, 2013, P. F. Chen; 2008). The main similarity between dip models and flux rope models is the suggested existence of concave-upward directed magnetic fields to support the prominence plasma against the downward gravitational force. Following this mechanism, it can be assumed that the plasma in a prominence is frozen to the magnetic field lines. Prominence plasma, however, is actually only partially ionised, and so it is not entirely clear how the non-ionized portion of plasma is supported, and how rapidly the neutral material might drain across the magnetic field lines.
Scientists are still researching how and why prominences are formed, and the cause for their reactivation. The models proposing how prominences are supported are vital in understanding their formation and reactivation.
2 Radio Emissions with Prominences
Measurable coherent radio emissions occur during flares, and are intermittent and in bursts, driven by the magnetic reconnection process, giving them the term ‘radio burst.’ Previous experiments (J. P. Raulin; 2005, J. P. wild; 1956, R. F. Wilson; 1989, G. Swarup; 1959) in measuring radio emissions produced from prominences have found that Type I bursts are predominantly emitted, Type I being characterised by their long lifespan lasting from hours to days, having a frequency of 80-200mHz with corresponding wavelengths of roughly 2m, and being produced by electrons with a charge of several keV within coronal loops. Moving Type IV radio bursts are also associated with prominence eruptions, these last from half an hour to 2 hours, with a frequency of 20-400MHz, and a corresponding wavelength range of 1 to several meters.
As mentioned in the introduction, scientists can use radio waves to gain an insight into how plasmas behave during the prominence eruption process. This can be done through magnetohydrodynamics (MHD), which is the study of the dynamics of electrically conducting fluids. Scientists have previously used MHD equations in investigations to understand the formation and reactivation of prominences (J. A. Linker; 2001, D.J. Schmit;2013, G. P. Zhou;2006, A. K. Srivastava; 2013).
An investigation using SDO/AIA (T. E. Berger; 2012) on the formation of prominences produced a series of images that showed the reactivation of a prominence. The sequence showed that after a prominence has completed its eruptive cycle, it slowly disappears due to drainage and the lateral transport of plasma, and a bright emission cloud forms in the upper regions of the coronal cavity. The cloud descends towards the lower region of the cavity while successively becoming brighter, and a new prominence then forms, rapidly growing in both the vertical and horizontal dimensions. The new prominence is the reactivated old prominence. The coronal cavity core in the image then grows darker as the reactivated prominence continues to grow. The reactivated prominence reaches its maximum size after a number of hours, and the emission cloud in the cavity reduces correspondingly. Using the time sequence of images from this T. E. Bergers paper, an idea of what to search for in data to find reactivated prominences can be formed.
Work has been performed (by C. Chifor; 2006; D. H. Mackay; 2010, D. J. Schmit, 2013) which also investigates how prominences are formed, concluding that reconnection events trigger different phases in prominence eruption. The flux rope model discussed earlier has been found to be a good model in several investigations (S. E. Gibson; 2006, P. F. Chen; 2008, G. P. Zhou, 2006). Helical field lines provide a support for the mass of the prominence, and are capable of storing the magnetic energy needed to propel the prominence. A coronal flux rope can be interpreted as a magnetic structure which consists of field lines that intricately twist around each other a number of times between the two ends that are anchored to the photosphere. Studies mentioned earlier involving MHD have been found to support the flux rope model, making the model a good investigation point for the project.
Further research has been carried out into the cause of reactivated prominences (R. F. Wilson; 1989), producing evidence that suggests that as the initial prominence dissipates, a ‘feed-back’ mechanism occurs, during which interactions of the large scale loops trigger burst activity in lower lying loops.
There are two main types of instruments that can be used to observe objects in the radio wave portion of the electromagnetic spectrum, the type selected for use depending on the strength of the signal and the amount of detail needed. The first type of instrument comprises radio telescopes, which are a form of directional radio antenna. As the range of frequencies in the radio wave portion of the electromagnetic spectrum is very large, there are a variety of different antennae that are used in radio telescopes, differing in their size, design and configuration. When measuring wavelengths of 30-3 meters, the radio telescopes use either directional antenna arrays, or large stationary reflectors with moveable focal points. At shorter wavelengths dish style radio telescopes are more largely used.
The second type of instrument comprises radio interferometers, which are made up of arrays of telescopes or mirror segments. The main benefit of using a radio interferometer is that the angular resolution is similar to that of a radio telescope with a large aperture, however, radio interferometers do not collect as many photons as radio telescopes, and they cannot detect objects that are too weak. However, an array of telescopes will provide very good resolution as a result of aperture synthesis. Aperture synthesis is an imaging process that mixes signals from the array of telescopes to produce images with an angular resolution equivalent to that of a single instrument with a diameter equal to the overall size of the array of telescopes. This makes it easy to obtain high resolution images of the Sun.
Several different types of data that can be used to review the radio emissions of the Sun in order to extract information on prominences have been researched. The first is SDO/AIA EUV data; SDO being the Solar Dynamics Observatory, which is a NASA mission that has been observing the Sun since 2010. The goal of the SDO is to understand the influence of the sun on the Earth and close space by studying the solar atmosphere over time and space in many wavelengths at the same time. Currently, investigations are focused on how the Suns magnetic field is generated and structured, and how the stored magnetic energy is converted and released into the heliosphere and geospace in the form of solar wind, energetic particles, and variations in solar radiance, which is the measure of the power per unit area on the Earth’s surface.
The SDO uses the Atmosphere Imaging Assembley (AIA), an instrument which provides continuous full-observations of the solar chromosphere and corona in seven extreme ultraviolet channels. The AIA is comprised of four telescopes providing individual light feeds to the instrument. The Extreme Ultraviolet Experiment (EUV) is the instrument that measures the Sun’s extreme ultraviolet irradiance, and incorporates physics based models in order to further understand the relationship between EUV variations and magnetic variation changes in the Sun (N. Labrosse, 2011).
Fig 1. This image is an example of SDO/AIA data, taken from (T. E. Berger; 2012) from a time sequence which investigates the radio emissions from the Sun leading up to the reactivation of a prominence event.
Using the data produced by the two, an image can be created of the Sun that combines physical processes such as prominences, with information on the magnetic field at the time. An example is shown in ‘Fig 1’ above, which shows a reactivated prominence eruption and its corresponding radio emission in the form of a cross-sectional image of the surface of the Sun. Data collected from the AIA has been made public through online databases, providing a ready set of images and films that can be analysed in order to observe prominences and their reactivation for this project.
The second type of data that will be focused on in order to infer radio emissions from the Sun is Nobeyama Radioheliograph data. The Nobeyama Radioheliograph is an array of 84 antennas dedicated for solar observation at the Nobeyama Radio Observatory, located in the Japanese Alps, and was constructed with the purpose of observing the Sun, using non-thermal emissions in particular.
The Nobeyama Radioheliograph is a radio interferometer, and the original data comprises sets of correlation values of all the combination of antennas. The antennas correspond to the spatial Fourier components of the brightness distribution of the solar disk. The Nobeyama Radioheliograph is particularly useful in studying prominences (M. Shimojo, 2005), as due to its large daily observation window, combined with the low time resolution of 1 second, and a spatial resolution of roughly 13”, it can produce highly dynamic images.
Even though the NoRH is ground based, the consequences of the surrounding weather conditions are minimal compared to that of other ground based observations, and observations can take place even in turbulent unclear weather. NoRH has also developed an automatic detection method, the most important factor in using the instrument to detect prominences, as data will be recorded automatically when there is an eruptive projectile. However, due to the limited time resolution and the field of view, NoRH cannot detect vary fast or very slow eruptive events, simultaneous events, and events where the structure has a weak brightness.
Fig 2 This is an image taken by the NoRH (M. Shimojo) which is an example of a prominence eruption, recorded by the automatic limb detection method. The panels are negative images, so the dark region indicates the high temperature.
NoRH uses the radio interferometer to create images of the Sun such as in ‘Fig 2,’ which is an example of use of the automatic limb detection method to record images of prominence eruption. Data recorded from the NoRH automatic limb detector has also been made public through online databases, giving a further set of images that can be analysed in order to extract information on prominences and their reactivation.
The topics covered in the papers that were researched lead to an adequate proposal of how to investigate the reactivation of prominences. Using NoRH and AIA data from SDO, the radio bursts emitted during the collapse and reformation of a prominence, an idea of what causes the reformation can be found. The investigation will centre on the different models, primarily the magnetic flux rope model, and the magnetohydrodynamics behind them that have been proposed for the formation of prominences, and how these models could support the ‘feed-back’ theory.
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