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The SHRG is a world leader in the study of solar wind composition, heliospheric physics, and solar-planetary interactions.


Fundamental Science
Magnificent CME Erupts on the Sun - August 31
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 the power grid and communication systems on the ground. Our dependence on the Sun raises fundamental, urgent questions that need to be answered: 

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  1. How can we predict solar activity?!
  2. How does the solar radiation and particulate output affects us on Earth?
  3. 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.

Active Research

Solar wind heating and acceleration. 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 measure 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:

  1. How does the solar cycle affects the solar wind?
  2. 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:

  1. What process is responsible for destabilizing prominences?
  2. How is the prominence plasma heated and accelerated
  3. 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 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 their evolution needs to be predicted. In order to develop forecasting capabilities, it is necessary to answer a few fundamental questions:

  1. Why and how is the solar corona heated?
  2. How does the magnetic structures in the solar corona evolve?
  3. 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 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 determine 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 the closed magnetic field in the Sun, which is 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:

  1. What causes the element fractionation in the solar corona?
  2. How is this fractionation tied to magnetic waves and coronal heating?
  3. 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

  1. The first complete measurement of the ion composition of erupting prominences, showing that the plasma is colder than ever thought;
  2. The first measurements of the temperature of erupting prominences close to the Sun, allowing for the determination of their heating during CME onset;
  3. A complete determination of the wind thermodynamic history from the source region to the Heliosphere;
  4. Measurements of the plasma properties of the solar extended corona from visible observations.

Solar Wind / Heliosphere

Fundamental Science

The solar wind is a stream of plasma that flows continually out from the Sun in all directions. It is composed primarily of protons and electrons but there is also a very small amount of helium and other heavier elements such as oxygen, carbon, and iron. By studying the density, velocity, and charge states of solar wind particles, particularly the heavy elements, we are able to gain vital information about temperatures, densities, and physical processes occurring in the “atmosphere” of the Sun without needing to actually send a spacecraft into such an inhospitable environment. Furthermore, both recurrent and transient events in the solar wind, such as boundaries between two different streams (called “Stream Interaction Regions”) or an explosive Coronal Mass Ejection (CME), can directly influence the magnetic fields and local space around the planets with significant, and sometimes disastrous, consequences for satellites and astronauts. 

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Current questions that motivate and drive research are:

  • Can we trace the solar wind back to precise source regions on the surface of the Sun?
  • What physical processes affect the composition of the solar wind and are responsible for accelerating it up to the hundreds of km/s observed near Earth?
  • Can we predict where and when events such as solar flares or CMEs are triggered, what dangers they pose to near-Earth space, and how we might minimize their impacts on society?
Active Research

The Solar and Heliospheric Research Group is a world leader in researching the composition of solar wind. We take a holistic, multi-fronted approach encompassing data, theory, and modeling. In particular, we specialize in analyzing data from the ACE, Ulysses, and Wind spacecraft. Current active topics of research include:

  • Using a fully-dynamic, 3D MHD model to simulate the dynamics of magnetic fields on the Sun that result in the release of the slow solar wind.
  • Surveying the variations in bulk solar wind properties and composition in order to identify different types of solar wind (e.g. “slow” and “fast” types) and then investigate how these types change spatially, at different solar latitudes, and temporally over the 11 year solar activity cycle.
  • Comparing simulation results to in-situ plasma data to test and validate current solar wind origin theories.
  • Studying the compositional signatures of fast/slow stream interaction regions in the solar wind. Using the results of the compositional signatures we then modeling the charge state of the ions in the interface region to explain the unique signatures found.
  • Investigating the abundance variations of the heavy elements and discovering periods of time where the densities of these heavy elements become greatly depleted. We then relate these “heavy ion dropouts” to physical processes occurring in the outer atmosphere of the Sun called the corona and attempt to use these dropouts as tracers of specific source regions.
  • Using auditory analysis techniques to gain new insights into spectral features of the solar wind. Our research has produced the most sensitive diagnostic of the electron temperature in the solar wind source region.

Planetary / Magnetospheric

Fundamental Science

A planetary magnetosphere is defined as the region of space influenced by the planet’s magnetic field. Many planets, such as Earth, have an intrinsic magnetic field that is distorted by the flow of the supersonic solar wind streaming off the Sun. Therefore, planetary magnetospheres are formed by the interaction between the solar wind and the planet’s magnetic field. From these interactions, a magnetosphere has a bullet-like shape with a spherical front followed by a long tail on the end. The shape and specific regions of a typical magnetosphere are illustrated in the figure below.

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A magnetosphere serves to shield the planet from the impinging solar wind and in doing so, responds to changing solar conditions. At Earth, this shield stops the solar wind from stripping away our atmosphere, as well as protects Earth-orbiting spacecraft from constant exposure to the solar wind. This protection is not without a cost, as the magnetosphere also traps high-energy particles to form radiation belts. The effects of solar activity are experienced on Earth nonetheless, in the form of aurora and, under more extreme conditions, disruptions to power grids. The details of these dynamics, such as the mechanisms that transfer energy and momentum from the Sun into the magnetospheres are still not fully understood and remain active research areas.

Active Research

In our group, we are currently investigating many aspects of Mercury’s magnetosphere using measurements of ions and magnetic fields from the MESSENGER spacecraft. Mercury orbits the Sun at an average distance of about 0.4 AU, compared to Earth which is located at 1 AU, and therefore experiences a much harsher space environment. Additionally, Mercury’s intrinsic magnetic field is much weaker than Earth’s. These circumstances lead to the creation of a magnetospheric environment with similar regions and processes as Earth but on shorter length and time scales. Understanding Mercury’s magnetosphere will provide clues to the dynamics of Earth’s space environment. Some of these outstanding questions that MESSENGER and our measurements at Mercury can help us to answer include:

  • What mechanisms are responsible for allowing solar wind ions to enter the magnetospheric system?
  • How does the distribution of planetary and solar wind ions inside the magnetosphere affect the system’s dynamics?
  • What affects the rate of magnetic reconnection in planetary magnetospheres?
  • To what extent does a planet’s proximity to the Sun, and therefore its local space environment, affect magnetospheric structure and dynamics?
  • What characteristics dictate how each planetary magnetosphere responds to solar events?In order to understand the way that Mercury’s bow shock deflects solar wind particles around the magnetic obstacle, we are examining the structure of the bow shock and the characteristics of ions being reflected away from this boundary. Also, in order to answer questions regarding the influence of changing upstream conditions on Mercury’s small magnetosphere, we are actively investigating the extent to which magnetic reconnection is responsible for the exchange of plasma and energy between the solar wind and Mercury’s magnetosphere. Inside the magnetosphere, we are studying the distribution and energization of plasma, both of solar and planetary origin, to shed light on the processes that create ions at Mercury and the role that they have in shaping Mercury’s atmosphere.

In order to understand the way that Mercury’s bow shock deflects solar wind particles around the magnetic obstacle, we are examining the structure of the bow shock and the characteristics of ions being reflected away from this boundary. Also, in order to answer questions regarding the influence of changing upstream conditions on Mercury’s small magnetosphere, we are actively investigating the extent to which magnetic reconnection is responsible for the exchange of plasma and energy between the solar wind and Mercury’s magnetosphere. Inside the magnetosphere, we are studying the distribution and energization of plasma, both of solar and planetary origin, to shed light on the processes that create ions at Mercury and the role that they have in shaping Mercury’s atmosphere.

Tracing Plasma in the Heliosphere

Fundamental Science

Using our sensor technologies we can identify particles that come from a variety of sources other than the Sun. Dust and neutral atoms left over from the formation of stars and the solar system, as well as material from comets, planets, and moons, can become ionized when sunlight hits the particles when a charged solar wind ion interacts with the dust and exchanges electrons, or when ions are knocked loose from dust grains by energetic particle impact. This newly charged particle then feels the pull of the solar wind and the heliospheric magnetic field and gets picked up, joining the solar wind on its journey to the far reaches of the heliosphere. By studying the composition and kinematic properties of these pickup ions, we can determine whether the particle originated outside or inside the solar system and how it was produced.

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Important outstanding research questions pertinent to this topic include:

  • What can interstellar pickup ions tell us about the local galactic neighborhood and the motion of the heliosphere through it?
  • What are the sources of pickup ions in the inner heliosphere?
Active Research

Using data from the ACE, Ulysses, and WIND spacecraft, we continue to study pickup ions from a range of sources. For example, from the fragmented Comet 73P, which passed near the Earth in 2006, we traced cometary pickup ions in the form of hydrogen and oxygen ions and are investigating the implications they have for calculating the density and composition of various fragments. We also are able to observe the nature of the energy transfer between the solar wind and the pickup ions as they are accelerated by the solar wind. We observe and characterize pickup ions from interstellar material which enables us to determine what makes up our galactic surroundings. We can trace material in the solar wind that originated in the Venusian atmosphere, as well as to characterize the interplanetary dust that orbits the Sun inside the orbit of Mercury.

Energetic Particles

Fundamental Science

There is a significant population of particles that have energies higher than that of the solar wind. These particles may originate from the Sun or may come from other sources outside the solar system or at the far reaches of the solar system. Particles can be accelerated in the solar corona to become a form of high-energy radiation, or they can be subjected to ongoing acceleration at shocks and inside turbulent regions in the heliosphere. The dynamic properties and composition of these particles reveal mechanisms for acceleration, their sources, and the prime acceleration locations in the heliosphere.

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Current outstanding questions relating to energetic particles include the following:

  • What is the seed population for solar energetic particle events?
  • What mechanism is responsible for accelerating suprathermal and energetic particles?
  • What are the sources of energetic particles in the heliosphere?