NASA’s Parker Solar Probe has uncovered the processes that drive the solar wind – the constant outflow of hot, ionized gas that streams outward from the Sun and fills up the solar system – and how the solar wind couples with solar rotation. The mission has examined the dust of the coronal environment, and spotted particle acceleration events so small that they are undetectable from Earth, which is nearly 93 million miles from the Sun.
During its initial flybys, Parker studied the Sun from a distance of about 15 million miles. That is already closer to the Sun than Mercury, but the spacecraft will get even closer in the future, as it travels at more than 213,000 mph, faster than any previous spacecraft.
The Parker Solar Probe measured a portion of the solar wind coming from a small hole in the Sun’s corona near the equator1. It is the closest look yet at one of the solar wind’s points of origin.
Parts of the solar wind race ahead in high-velocity spikes. “I think of them as rogue waves,” says Justin Kasper, a space scientist at the University of Michigan in Ann Arbor. Within these waves, the speed of the solar wind doubled, and the flow was so strong that it temporarily reversed the wind’s magnetic field.
The probe flew through more than 1,000 of these spikes each time it zipped past the Sun, Kasper says. Scientists don’t yet understand what causes them.
One type of event in particular caught the attention of the science teams – flips in the direction of the magnetic field, which flows out from the Sun, embedded in the solar wind and detected by the FIELDS instrument. These reversals – dubbed “switchbacks” – appear to be a very common phenomenon in the solar wind flow inside the orbit of Mercury, and last anywhere from a few seconds to several minutes as they flow over the spacecraft. Yet they seem not to be present any farther from the Sun, making them undetectable without flying directly through that solar wind the way Parker has.
During a switchback, the magnetic field whips back on itself until it is pointed almost directly back at the Sun. These switchbacks, along with other observations of the solar wind, may provide early clues about what mechanisms heat and accelerate the solar wind. Not only does such information help change our understanding of what causes the solar wind and space weather affecting Earth, it also helps us understand a fundamental process of how stars work and how they release magnetic energy into their environment.
Another surprising finding is how quickly the solar wind rotates around the Sun as the star spins. Models suggest that the wind flows in this direction at a speed of a few kilometers per second — but the Parker Solar Probe measured it moving at around 35 to 50 kilometers a second. “The jury is definitely out on what’s causing this,” says Kasper.
The discovery has major implications. Knowing that the wind is rotating at a different speed than expected could help researchers to improve predictions of when a dangerous solar outburst might reach Earth. The finding also suggests that the solar wind is transporting more energy away from the Sun than previously thought, so the star’s rotation might be slowing down more rapidly than expected. If so, astronomers might need to revise their ideas about how other stars in the Universe age.
The prediction of a supersonic solar wind1 was first confirmed by spacecraft near Earth and later by spacecraft at heliocentric distances as small as 62 solar radii. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second—considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age.
NASA’s Parker Solar Probe mission1 recently plunged through the inner heliosphere of the Sun to its perihelia, about 24 million kilometres from the Sun. Previous studies farther from the Sun (performed mostly at a distance of 1 astronomical unit) indicate that solar energetic particles are accelerated from a few kiloelectronvolts up to near-relativistic energies via at least two processes: ‘impulsive’ events, which are usually associated with magnetic reconnection in solar flares and are typically enriched in electrons, helium-3 and heavier ions2, and ‘gradual’ events which are typically associated with large coronal-mass-ejection-driven shocks and compressions moving through the corona and inner solar wind and are the dominant source of protons with energies between 1 and 10 megaelectronvolts. However, some events show aspects of both processes and the electron–proton ratio is not bimodally distributed, as would be expected if there were only two possible processes. These processes have been very difficult to resolve from prior observations, owing to the various transport effects that affect the energetic particle population en route to more distant spacecraft6. Here we report observations of the near-Sun energetic particle radiation environment over the first two orbits of the probe. We find a variety of energetic particle events accelerated both locally and remotely including by corotating interaction regions, impulsive events driven by acceleration near the Sun, and an event related to a coronal mass ejection. We provide direct observations of the energetic particle radiation environment in the region just above the corona of the Sun and directly explore the physics of particle acceleration and transport.
Remote observations of the solar photospheric light scattered by electrons (the K-corona) and dust (the F-corona or zodiacal light) have been made from the ground during eclipses1 and from space at distances as small as 0.3 astronomical units to the Sun. Previous observations of dust scattering have not confirmed the existence of the theoretically predicted dust-free zone near the Sun. The transient nature of the corona has been well characterized for large events, but questions still remain (for example, about the initiation of the corona and the production of solar energetic particles) and for small events even its structure is uncertain. Here we report imaging of the solar corona during the first two perihelion passes (0.16–0.25 astronomical units) of the Parker Solar Probe spacecraft, each lasting ten days. The view from these distances is qualitatively similar to the historical views from ground and space, but there are some notable differences. At short elongations, we observe a decrease in the intensity of the F-coronal intensity, which is suggestive of the long-sought dust free zone. We also resolve the fine-scale plasma structure of very small eruptions, which are frequently ejected from the Sun. These take two forms: the frequently observed magnetic flux ropes and the predicted, but not yet observed, magnetic islands arising from the tearing-mode instability in the current sheet. Our observations of the coronal streamer evolution confirm the large-scale topology of the solar corona, but also reveal that, as recently predicted, streamers are composed of yet smaller substreamers channelling continual density fluctuations at all visible scales.
During the solar minimum, when the Sun is at its least active, the solar wind is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvénic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain; theories and observations suggest that they may originate at the tips of helmet streamers from interchange reconnection near coronal hole boundaries or within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence heating by reconnection in nanoflares, ion cyclotron wave heating and acceleration by thermal gradients. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe at 36 to 54 solar radii that show evidence of slow Alfvénic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind.
SOURCES- NASA, Nature