[Institute of Space and Astronautical Science (ISAS)] The Forefront of Space Science: "Hinode", the chromosphere, magnetic waves, and coronal heating

Why is the solar corona so hot? The question of why the solar corona is so hot has puzzled solar physicists for many years. In order to solve the mystery, a number of ground-based telescopes and satellites were developed around the world. Despite of such an effort, we have not found the answer to the question. The “coronal heating problem” has been one of the main focuses of missions currently being developed or discussed. This current situation may give you an impression that have not advanced at all in the study of coronal heating. This is not true. The understanding of the mechanism through satellite observations has greatly advanced, and the substantial research content has evolved dramatically. Specifically, observations though the Hinode satellite have found some magnetic waves that seem to be playing an important role in heating the corona. This discovery, together with the data collected by NASA’s solar observation satellite IRIS (Interface Region Imaging Spectrograph), has finally led to the detection of magnetic wave dissipation.

In this article, I will talk about the approaches to the study of Coronal Heating using these solar observation satellites.

Chromospheric activity observed by Hinode

So, why am I talking about the chromosphere when our focus is on the corona? Because we cannot talk about the corona without understanding the chromosphere. The solar atmosphere consists of three layers: the photosphere (whose temperature is about 6,000 degrees), chromosphere (10,000 degrees), and corona (1 million degrees). The photosphere has a thickness of about 500km, while the chromosphere is about 2,000km thick. The corona is the layer above these two layers. Explanation of the solar atmosphere would have been much simpler if these three layers consisting it were parallel to each other. In reality, however, some part of the chromosphere is extending into the layer above it, or the corona, and part of it is even moving through the layer (see Figure 1). This inconsistency is due to the fact that the sun’s atmosphere is divided into these layers based on the temperature. The gas with a temperature of around 10,000 degrees is called “chromosphere”, regardless of how far it is from the Sun’s surface. Likewise, the corona is the gas with a temperature of around 1 million degrees. Due to this, plasma with a temperature of around 10,000 degrees that exists within the corona, such as prominences and spicules, is also defined as chromosphere. I should note that these prominences and spicules which are categorized as chromosphere should not be treated in the same way as the regular, flat chromospheric layer.

When we look at the chromosphere observed through Hinode (Figure 1), we can see that it has a structure consisting of fine thread. A video which captured its motion shows that it keeps moving very actively. This is one of the “surprising” facts that Hinode has discovered, which reaffirmed how complex the study of coronal heating is. Until this discovery, it was believed by solar physicists that the chromosphere is just a layer which lies between the photosphere and the corona and is of no importance in exchange of energy between these layers. Now that we know the middle layer is this active, it is natural to think that understanding how the corona is heated requires the understanding the behavior of the chromosphere as well as the amount of energy accumulated within the layer.

Figure 1. An image captured through Hinode and the parallel model for the sun’s atmosphere [Click for large image] Left: Chromospheric structures on the solar limb captured through Hinode. What is shown on the bottom of the image is the surface of the Sun, while the thick layer shown on the upper half of the image is the corona. The trailing streaky flux is a prominence. This false-color image includes the light emanating from the plasma with a temperature of about 10,000 degrees. Right: the conventional model for the solar atmosphere. In this model, the transition layer, which exists on the boundary between the chromosphere and corona, is omitted and shown as part of the chromosphere. The temperature of the chromosphere rises sharply at this transition layer.

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