The active Sun – Prominences
Our blog this month looks at one of the more visually spectacular features of the Sun. Prominences are eruptive features seen within the chromosphere and corona, the lower and outer atmosphere of the Sun, and are typically plume shaped. Their appearance readily reveals that they are associated with magnetic activity and the form and structure of the classical loop prominence can be seen to follow magnetic field lines of force.
Our blog this month looks at one of the more visually spectacular features of the Sun. Prominences are eruptive features seen within the chromosphere and corona, the lower and outer atmosphere of the Sun, and are typically plume shaped. Their appearance readily reveals that they are associated with magnetic activity and the form and structure of the classical loop prominence can be seen to follow magnetic field lines of force.
Ce mois nous considérons une des caractéristiques visuellement spectaculaires du Soleil: les protubérances. Les protubérances sont des caractéristiques éruptives observées dans la chromosphère et la couronne, et sont généralement en forme de plume. Leur apparence montre qu’elles sont liées à l’activité magnétique et on peut voir que la forme et la structure de la protubérance (en forme de boucle) suit les lignes de force du champ magnétique.
Eruptive loop prominence, March 2011
Courtesy of NASA (Solar Dynamics Observatory)
Courtesy of NASA (Solar Dynamics Observatory)
There are two fundamental types of prominence, quiescent and eruptive (or active). The distinction reflects energy and longevity. For example, formation velocities are typically 5 km/s for quiescent; but up to 1000km/s for the ‘fast-ejection’ eruptive forms. Both types are associated with magnetic fields (typically 5 & 100 Gauss for quiescent and eruptive respectively) and both are, in human-terms, highly energetic and hot (5000 to 20000 Kelvin).
Il existe deux types de protubérance solaire, quiescente et éruptive. Les deux types sont liées à des champs magnétiques (respectivement 5 et 100 gauss pour les protubérances quiescentes et éruptives) et les deux sont très énergiques et très chaudes (5000 à 20000 kelvin).
Prominences are dynamic features of the Sun and a quiescent prominence can evolve into an eruptive prominence. This process is known as a disparition brusque and was first observed by the French astronomer Henri Deslandres (b.1853 d.1948) in 1889 from Meudon. Disparition brusque are also associated with coronal mass ejections (CME) which we will look at in a future blog.
Les protubérances sont des caractéristiques dynamiques du Soleil et une protubérance quiescente peut se transformer en protubérance éruptive. Ce processus est connu sous le nom de ‘disparition brusque’ d’abord observé par l’astronome français Henri Deslandres (1853-1948) en 1889 à Meudon. Une disparition brusque est aussi liée à des éjections de masse coronales (EMC) que nous étuderions dans un prochain article
Prominences are classified by their morphology (shape) and characteristics and our book on the Sun discusses in detail the differences between the various forms of prominences.
In 1877 the Italian astronomer Angelo Secchi (b.1818 d.1878) provided the base classification of quiescent and eruptive. This was extended and an extensive classification of six major classes was proposed by the American Edison Pettit (b.1889 d.1962) in 1943. Alternative systems have been proposed more recently. The most commonly seen type of prominence are:
1) Loop – The archetypal prominence, where an arch is formed, generally between two active regions. Material flows along the flux lines of magnetic field and the plasma returns the photosphere along each leg of the loop. Loop prominences are closely associated with solar flares and are sometimes referred to as post-flare loops
2) Spray – This is where a loop becomes disconnected/open-ended and material is ‘sprayed’ into the corona. These prominences are associated with magnetic reconnection events and are the most energetically active of prominences.
In 1877 the Italian astronomer Angelo Secchi (b.1818 d.1878) provided the base classification of quiescent and eruptive. This was extended and an extensive classification of six major classes was proposed by the American Edison Pettit (b.1889 d.1962) in 1943. Alternative systems have been proposed more recently. The most commonly seen type of prominence are:
1) Loop – The archetypal prominence, where an arch is formed, generally between two active regions. Material flows along the flux lines of magnetic field and the plasma returns the photosphere along each leg of the loop. Loop prominences are closely associated with solar flares and are sometimes referred to as post-flare loops
2) Spray – This is where a loop becomes disconnected/open-ended and material is ‘sprayed’ into the corona. These prominences are associated with magnetic reconnection events and are the most energetically active of prominences.
Spray prominence, March 2011
Courtesy of NASA (Solar Dynamics Observatory)
Courtesy of NASA (Solar Dynamics Observatory)
3) Surge – where plasma rises rapidly in a near vertical column and generally falls back to the photosphere from near/close to its origin.
4) Fast Ejection – These are similar to surge prominences, but here a ‘blob’ of plasma is ejected into the corona. No loop is seen to form in this class.
5) Coronal Rain – This is the collapse of loop prominences along and between both legs of the arch.
6) Coronal Cloud – These are irregular masses of plasma suspended within the corona. The plasma returns to the photosphere only very slowly (i.e. a few weeks).
4) Fast Ejection – These are similar to surge prominences, but here a ‘blob’ of plasma is ejected into the corona. No loop is seen to form in this class.
5) Coronal Rain – This is the collapse of loop prominences along and between both legs of the arch.
6) Coronal Cloud – These are irregular masses of plasma suspended within the corona. The plasma returns to the photosphere only very slowly (i.e. a few weeks).
Coronal cloud, November 2012
Courtesy of NASA (Solar Dynamics Observatory)
Courtesy of NASA (Solar Dynamics Observatory)
The frequency of prominences seen closely follows that of the solar cycle; there are more observed at times of higher sunspot activity than at solar minimum. Their origin has been, and continues to be, studied extensively and this is an active area of research for both theoreticians and observational astronomers. Their formation is associated with instabilities within magnetic fields (flux tubes) caused by shearing motion (rotational and convection) within the photosphere and chromosphere.
Filaments are prominences which are seen against the disc of the Sun. They appear dark due to absorption within the higher temperature corona. They are most often seen (and persistent) in specific latitudes (near the poles and at mid-latitude in both hemispheres) and in neutral magnetic regions between two opposite polarity areas.
Filaments are prominences which are seen against the disc of the Sun. They appear dark due to absorption within the higher temperature corona. They are most often seen (and persistent) in specific latitudes (near the poles and at mid-latitude in both hemispheres) and in neutral magnetic regions between two opposite polarity areas.
Solar filament, November 2010
Courtesy of NASA (Solar Dynamics Observatory)
Courtesy of NASA (Solar Dynamics Observatory)
The image above shows the clear association between a prominence and the filament on the right of the image. Another fainter filament can also be seen on the lower left.
Although highly energetic, these are relatively weak features when we compare their energies to solar flares and Coronal mass ejections. Whilst prominences generally have little effect on the ‘space-weather’ near the Earth, flares and CMEs can be significant hazards both for space probes and manned flights; but also the Earth. We will begin to look at these latter threats in next month’s blog.
Although highly energetic, these are relatively weak features when we compare their energies to solar flares and Coronal mass ejections. Whilst prominences generally have little effect on the ‘space-weather’ near the Earth, flares and CMEs can be significant hazards both for space probes and manned flights; but also the Earth. We will begin to look at these latter threats in next month’s blog.
