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Heliophysics is the exploration of our star, the Sun, and how it influences Earth, space, and planets throughout the Solar System. Heliophysics research helps protect astronauts, spacecraft, and power grids here on Earth.
PUNCH scientists are heliophysicists who research the fundamental physics of the Sun and how it releases charged particles, magnetic fields, and solar storms that affect conditions (space weather) in interplanetary space. These phenomena can be divided into Ambient and Dynamic features of the space between Earth and Sun.
Ambient Solar Wind
What is the solar wind?
The PUNCH mission will make unprecedented global measurements of the faster and slower streams of the solar wind. The solar wind is a million-mile-per-hour stream of charged particles (electrons and ions) emanating from the outer atmosphere of the Sun in all directions.
The solar wind pervades interplanetary space and beyond. Its presence defines the heliosphere – the region of space influenced by the Sun. PUNCH observations are focused on the inner heliosphere between the Sun and Earth orbit. This is the domain of the young solar wind where the sun's outer atmosphere, the corona, transitions to a dynamic flow. The grander heliosphere extends out more than twice as far as the orbit of Pluto until the solar wind is slowed by its encounter with interstellar space at the heliopause.
The solar wind reveals its presence to the naked eye when it interacts with comets, forming long tails as they pass nearer to the Sun. Auroras occur in the polar regions of Earth when the solar wind’s charged particles interact with Earth’s magnetic field and uppermost atmosphere. This interaction provides the atoms in Earth’s atmosphere with extra energy, which is released as bursts of light.
What is solar wind turbulence?
PUNCH will study turbulence in the million mile-per-hour outflow of magnetized plasma from the Sun called the solar wind. Turbulence is a near-universal process that moves energy from large scales to small scales in nearly all liquids and gases. Any change in the speed or direction of a flow results in turbulence.
Everyday examples of turbulent flow are the wake of a motorboat in a river, or the swirls and eddies caused by rocks in a flowing stream, or the tumbling of air over a mountaintop, or the billowing of smoke from a wildfire. Even the blood in our arteries is considered a turbulent flow due to friction that slows the flow of blood near to the artery walls compared to the faster flow at the center. Turbulence can occur for all states of matter that flow, including the plasma of the solar wind.
The time-lapse movie of the solar wind flow (left, below) is a result of the NASA STEREO mission. It reveals the nature of the transition from the Sun’s outer corona to interplanetary space -- where the Sun’s magnetic field has less influence on the plasma’s outward motion. Smoother flow of plasma at the right of the image (nearer to the Sun) takes on a clumpy appearance as the flow proceeds to the left of the image (farther from the Sun). The clumpiness may be due to unstable conditions that cause turbulence and break up the flow, just as the stream from a squirt gun becomes unstable and breaks into blobs of water (right, below) (See a slow-motion music video (YouTube) of this process).
The transition in the structure of the plasma flow occurs in the outermost corona (more than 20x the Sun’s width from the star itself). It is not yet well understood but may be related to the formation of turbulence. PUNCH will image this region with 10x sharper resolution and with 30x more sensitivity than STEREO could, to reveal whether the transition is due to turbulence in the solar wind flow.
What is the Alfvén Zone?
A major objective of the PUNCH mission is to understand the unique physics of the Alfvén Zone – a relatively unexplored region in the outermost atmosphere of the Sun where the solar wind is first released into interplanetary space.
Alfvén waves pervade the solar corona and are important to understanding the physical meaning of the Alfvén Zone.
Alfvén waves in the solar corona allow a disturbance in one region of the corona to be “felt” or “communicated” to the corona as a whole.
The speed of Alfvén waves depends on the strength of the magnetic field and other qualities of plasma environment. These waves are traveling within the coronal plasma that is itself flowing away from Sun at higher and higher speeds as it gets farther and farther from the Sun.
There comes a point when the speed of the outbound flow exceeds the speed of Alfvén waves propagating inward. When this happens, the Alfvén waves are swept outward by the flow before there is time for them to propagate inward, and the outflowing plasma emerges freely into interplanetary space. The released solar plasma (the young solar wind) can no longer affect the corona directly.
The Alfvén Zone in not a fixed “layer” of the solar atmosphere. Computer simulations predict that it occurs over a large and variable range of altitudes where the rapidly outflowing plasma in the solar corona gradually disconnects from the Sun and becomes the young solar wind that is released into interplanetary space.
This transition is not marked by any particular local change in the plasma, but marks a global change in the plasma's relationship to the Sun. Outside the transition, the plasma is fully disconnected from the star and no longer part of the solar corona.
Because it is hard to detect, the Alfven Zone is perhaps the least-studied portion of the Sun’s corona to date. PUNCH will provide the first global maps of this zone and how it moves and changes. These observational data will help to refine the computer models with real data.
Dynamic Solar Wind
What are CMEs?
Coronal Mass Ejections (CMEs) are massive bursts of charged particles and magnetic field from the Sun’s outer atmosphere (the corona) into interplanetary space. PUNCH will be observing near a peak in the Sun's 11-year activity cycle when such solar storms are more likely to occur. Like no spacecraft before it, PUNCH will be able to image and track each visible CME as it travels outward from the solar corona all the way to Earth orbit.
Solar storms from CMEs are primary features of space weather that can have both dangerous and beautiful consequences when released in the direction of Earth. A CME’s charged particle radiation can harm spacecraft and astronauts in its path. A CME’s impact on our planet’s magnetic “shield” can cause auroral lights to dance more vigorously and glow more brilliantly, even at lower latitudes. Sometimes the impact may be severe enough to induce destructive currents in ground-based power stations and pipelines.
The PUNCH mission will be observing the solar corona during a maximum of the solar activity cycle predicted between 2023 and 2026.
A total solar eclipse of the Sun is visible from the US on 8 April 2024
During totality in 2024, we can watch for a solar storm disrupting the solar corona in the manner that the observers of 1097 and 1860 may have witnessed.
What are CIRs?
PUNCH will provide the first routine imagery of the shape and evolution of the heliopshere’s enigmatic Co-rotating Interaction Regions (CIRs – See Eye Ars). CIRs are long-lasting, large-scale regions of interplanetary space that have enhanced density and magnetic field strength. They are “co-rotating” because, unlike Coronal Mass Ejections (CMEs or solar storms), CIRs sweep around the Sun with the Sun’s rotation (link to YouTube). A CIR typically recurs once every 28 days – the time for the lower latitudes of our star to turn once around. CIRs can cause space weather events near Earth and beyond.
How Do CIRs Form?
The outward motion of the solar wind and the Sun’s rotation combine to create a giant Archimedean spiral pattern, known as the Parker spiral. Both the shape of solar wind structures and the magnetic field in interplanetary space follow this spiral pattern.
The spiral pattern is similar to that made by an old-fashioned rotating sprinkler or by a modern rotary fountain used in water show displays (see the video clip below). The water comes straight out of the nozzles with a particular speed as the nozzles are spun, and so the water stream forms a spiral shape as it flies out. In a similar way, the solar wind flows radially out of the solar corona as the Sun rotates, and individual streams of the solar wind form Archimedean spirals.
The Archimedean spiral is not the same as the logarithmic (or Golden) spiral that also occurs in Nature, for example in galaxies and hurricanes. Both types of spirals are abundant in the natural and human-designed world.
For Archimedean spirals like the solar wind, any ray from the center intercepts successive turnings of the spiral at points with a constant separation distance. For a logarithmic spiral, however, these distances grow larger with each successive turning of the spiral.
Each stream of solar wind leaving the Sun makes a slightly different Archimedean spiral. The pitch of the spiral (how far it moves outward for each rotation) depends on the speed that particular stream is flowing. Fast streams of solar wind travel farther during each rotation, and slow streams of solar wind travel less far. This Parker Spiral animation offers an interactive visualization of how the speed of solar wind outflow determines the shape of the solar wind spiral.
The faster solar wind stream has a more open spiral arms, and the slower stream forms a tighter spiral. When a fast and slower solar wind stream are emerging from the corona next to one another, the different curvature of their Parker spirals can make the neighboring streams collide (or interact) as the faster stream overtakes the slower stream. When this occurs the material from the two streams piles up and makes a spiral whose pitch is intermediate between the original two spirals. This piled-up region is a CIR. The compressed material becomes more dense and hotter than the surrounding solar wind and can even form a persistent shock that sweeps around the Solar System once per month as the Sun rotates.
The diagram above was developed by PUNCH co-investigator Vic Pizzo more than 40 years ago. Since then CIRs have been studied by sampling the piled-up “wall” of material as it sweeps over space probes.
PUNCH will be able to image CIRs better and more routinely than any spacecraft before it, thereby helping to understand this enigmatic source of space weather at Earth.
What are shocks?
PUNCH will be able to see shocks forming and evolving in the inner heliosphere with an unprecedented degree of detail. “Shocks”, in heliophysics, are moving, sudden, violent changes in the solar wind that can be caused by solar storms and other dynamic features such as Co-rotating Interaction Regions (CIRs). The strongest shocks are in front of fast-moving Coronal Mass Ejections (CMEs) which plow through the surrounding solar wind as they cross interplanetary space.
Shocks can happen in any medium that carries wave motion, including air, water, and the solar wind plasma. Normal waves are a smooth, propagating change in the medium – like deep-water ocean waves that can cross thousands of miles and deliver energy across the sea. Shocks modify the medium suddenly, like a crashed “breaker” ocean wave arriving at the shore, dissipating its energy as sound and heat.
Shocks in the solar wind arise from two effects: 1) wave steepening like the ocean breaker that steepens and then crashes as a turbulent shock; and 2) rapid motion like the sonic boom around a supersonic jet.
This simple animation (below, left) illustrates how the V-shaped shock front (the sonic boom) is formed as a rapidly moving object meets and exceeds the sound speed (click to view entire duration of animation). When that shock sweeps over the ground, a person can hear a loud booming sound. The NASA image (below right) captures the first air-to-air images of supersonic shockwave interaction in flight.
Just as the air of Earth’s atmosphere can be shocked and heated by the passage of a rapidly moving jet or an explosion, the plasma of the solar wind can be shocked and heated by a rapidly moving Coronal Mass Ejection (CME). The resolution of PUNCH imagery will allow analysis of the shape of the leading edges of shocks as they form and change in the inner heliosphere.