SDO image of solar eruption.
The Sun is the ultimate driver of everything happening in the Heliosphere: interplanetary space conditions, solar wind properties and propagation, planetary and near-Earth environments and Earth's climate. The Sun is highly variable: on medium- to long-time scales solar activity is subject to a number of cycles with different periods, and experiences long-term variations that affect the Earth's climate - think about the Maunder Minimum and the little Ice Age, the last major solar-driven climate change occurred just 300 years ago. On short timescales, sudden ejections of solar plasma into the Heliosphere (Coronal Mass Ejections - CMEs) and the very fast releases of energy and X-ray radiation occurring in solar flares significantly affect both the Earth's upper atmosphere, man-made satellites, as well as power grid and communication systems on the ground. Our dependence on the Sun raises fundamental, urgent questions that need to be answered:
- How can we predict solar activity?!
- How does the solar radiation and particulate output affects us on Earth?
- How can we minimize the adverse effects of solar activity on human assets on space and the ground?
The SHRG focuses on both measuring, modeling and understanding the evolution and the properties of the solar upper atmosphere hosting the solar activity that influences Earth.
Solar wind heating and acceleration. The solar wind can be studied in two different ways. In-situ measurements of wind compositions provide direct determinations of the solar wind properties in the Heliosphere; remote-sensing (i.e. spectra, images) of the solar wind source regions can be used to measuring the properties of the nascent solar wind in its cradle, as well as of the structures where the wind originates from. We have developed a new diagnostic technique that combines remote-sensing observations and in-situ measurements of the Sun and utilizes them simultaneously to determine the solar wind acceleration, heating, and thermodynamic history throughout the solar wind journey from the Sun to the Heliosphere. We are currently using this new technique to study the evolution of the solar wind and of its source regions across the solar cycle, to answer the questions:
- How does the solar cycle affects the solar wind?
- How do different types of solar wind respond to solar cycle induced variations in their source regions?
CME heating and acceleration. By releasing vast amounts of plasma with speeds of hundreds of miles per second, CMEs have huge impacts on the Heliosphere and on the Earth's upper atmosphere, on communication satellites in space and power grids on the ground. Still, we are far from being able to predict these events, mostly due to our ignorance of how these eruptions are triggered, heated and accelerated at their onset in the inner solar corona. We are using both in-situ measurements (using a new technique we developed) and remote-sensing observations to study the evolution of the plasma dynamics, temperature, density of the coldest - and most interesting - component of a CME: an erupting prominence, whose plasma is suddenly accelerated and heated after having been quiescent for days or weeks. Understanding how erupting prominences are activated is at the core of CME forecasting:
- What process is responsible for destabilizing prominences?
- How is the prominence plasma heated and accelerated
- Why is some prominence plasma so much colder than the rest of the CME when observed in space?
Coronal heating and irradiance. The disk of the Sun visible in the sky with the naked eye consists of the surface layer of the Sun, the photosphere, whose temperature is around 6000 degrees. A few thousand of km above the photosphere (corresponding to roughly 1/100 of the solar radius), the coronal plasma is confined by magnetic fields anchored in the photosphere and is characterized by a temperature between 1 and 2 million degrees. Such a temperature causes the coronal plasma to emit high energy radiation in the UV, EUV and X-ray ranges that interacts with the Earth's upper atmosphere, altering its density and the drag that the latter applies to satellites orbiting our planets. The corona - and its high-energy radiation - are highly variable in time, so that their evolution needs to be predicted. In order to develop forecasting capabilities, it is necessary to answer a few fundamental questions:
- Why and how is the solar corona heated?
- How does the magnetic structures in the solar corona evolve?
- How can we predict the solar radiative output?
The SHRG is actively studying the physical properties of magnetic structures in the solar corona (organized in active regions, small-scale loops, and large-scale streamers) to understand how they are heated and to predict their evolution. Also, some of our team members are helping develop a system that allows the forecasting of the UV, EUV and X-ray radiation of the Sun with up to 7 days in advance.
Composition of the solar atmosphere. The chemical composition of solar plasmas is a fundamental parameter that rules several aspects of the Sun. The composition of the solar interior determines the opacity of the plasma and thus helps determining the structure of the solar interior and the location of the base of the convective zone; the abundance of the elements in the corona determines the rate of radiative emission of the corona itself in the UV to X-ray ranges. The composition of the corona is not necessarily the same as in the photospheres: systematic differences have been found in regions of closed magnetic field in the Sun, which are then propagated into the composition of the solar wind they emit. This fractionation, which depends on the First Ionization Potential of the elements, is thought to be tied to the presence of the same magnetic waves suspected to heat the solar corona. The SHRG group, measuring the composition of solar and wind plasmas using remote-sensing and in-situ measurements, is currently trying to answer the following questions:
- What causes the element fractionation in the solar corona?
- How is this fractionation tied to magnetic waves and coronal heating?
- Can this fractionation be used to determine (and predict) the rate of coronal plasma heating?
Diagnostic techniques of plasmas in the solar corona and solar wind. The SHRG group is very active in developing novel techniques that allow us to measure the properties of coronal and wind plasmas using both existing and new instruments being developed. These new techniques have provided (or will provide) breakthrough measurements, like
- The first complete measurement of the ion composition of erupting prominences, showing that the plasma is colder than ever thought;
- The first measurements of the temperature of erupting prominences close to the Sun, allowing for the determination of their heating during CME onset;
- A complete determination of the wind thermodynamic history from the source region to the Heliosphere;
- Measurements of the plasma properties of the solar extended corona from visible observations.