63 Years after Explorer 1, New Discoveries about the Van Allen Belts Continue

By Mara Johnson-Groh
NASA’s Goddard Space Flight Center

On January 31, 1958, the U.S. launched its first satellite: Explorer 1. Among its many achievements, Explorer 1 made the ground-breaking discovery of belts of charged particles encircling Earth.

Visualization of the two concentric donut-shaped Van Allen belts encircling Earth
Visualization of the Van Allen belts based on data from NASA’s SAMPEX mission. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

That discovery is still being studied today. 63 years on, scientists are still learning about these belts – now known as the Van Allen belts – and their effects on Earth and technology in space.

From 2012 to 2019, scientists used NASA’s Van Allen Probes to gather data from the dynamic region discovered by Explorer 1. While the mission is no longer operational, it left a treasure-trove of observations, which are continuing to reveal new things about the belts. In 2020, over 100 scientific papers were published in peer-reviewed international journals using Van Allen Probes data, often leading studies in conjunction with partner missions. Here are three surprising discoveries scientists have recently made about the Van Allen belts.

1) In addition to particles, space is filled with electromagnetic waves called plasma waves, which affect how charged particles in space move. Near Earth, one type of wave, called whistler chorus waves, bounces back and forth following magnetic field lines between Earth’s North and South poles. Observations from the Van Allen Probes and Arase missions recently showed that these waves can leave the equator and reach higher latitudes where they permanently knock particles out of the Van Allen belts – sending the particles out into space never to return.

2) In addition to removing charged particles from the belts, magnetic activity can also add in new particles. Van Allen Probes observations combined with data from a Los Alamos National Lab geosynchronous satellite and one of NASA’s THEMIS satellites showed how hot charged particles can be abruptly transported by magnetic activity across 400,000 miles, from distant regions under the influence of Earth’s magnetic field into the heart of the Van Allen belts.

3) Earth’s magnetic environment and the Van Allen belts are highly influenced by the Sun, particularly when it releases clouds of ionized gas called plasma, which can create hazardous space weather. Some stormy activity from the Sun can create an intense ring of current surrounding Earth. Understanding these currents is critical for predicting their adverse space weather effects on ground-based infrastructure. Using years of Van Allen Probes data, scientists can now accurately model the distribution of the ring current around Earth even during the most intense space weather storms.

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Research Highlights from NASA’s GOLD Mission

By Sarah Frazier
NASA’s Goddard Space Flight Center

A special collection of research in the Journal of Geophysical Research: Space Physics highlights the initial accomplishments of NASA’s GOLD mission. GOLD, short for Global-scale Observations of the Limb and Disk, is an ultraviolet imaging spectrograph that observes Earth from its vantage point on a commercial communications satellite in geostationary orbit.

Since beginning science operations in October 2018, GOLD has kept a constant eye on Earth’s dynamic upper atmosphere, watching changes in the Western Hemisphere, marked by changes in the temperature, composition and density of the gases in this region. 

A few highlights include:

    • Results on one source of airglow seen at night, which relies on electrons on Earth’s day side becoming ionized by sunlight, then being transported along magnetic field lines to the nightside, where they create visible airglow (Solomon, et al)
    • New evidence supporting the idea that the equatorial ionization anomaly appearing in the early morning — a prominent feature in the ionosphere with poorly-understood triggers that can disrupt radio signals — is linked to waves in the lower atmosphere (Laskar, et al)
    • New observations of planet-scale waves in the lower atmosphere that drive change in the ionosphere (Gan, et al & England, et al
    • Multi-instrument measurements of plasma bubbles — “empty” pockets in the ionosphere that can disrupt signals traveling through this region because of the sudden and unpredictable change in density — that suggest they are could be seeded by pressure waves traveling upwards from the lower atmosphere (Aa, et al)
    • Observations showing that plasma bubbles occur frequently at all of the longitudes covered by GOLD with different onset times, providing new information on the influence of the particular configuration of the geomagnetic field at these longitudes (Martinis, et al
    • Measurements of changes in the chemical composition of the thermosphere during the total solar eclipse of July 2, 2019, which give scientists an unprecedented hemisphere-wide look at how the reduction in solar radiation throughout an eclipse affects this part of the atmosphere (Aryal, et al

Read more research from the special collection on the Journal of Geophysical Research: Space Physics website, see an overview of early GOLD results from AGU’s Eos, and see more GOLD mission publications on the mission website.

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What Spring is like on Uranus and Neptune

By Miles Hatfield
NASA’s Goddard Space Flight Center

A new NASA study finds that our distant planetary neighbors, Uranus and Neptune, may have magnetic “seasons:” A time of the year when aurora glow brighter and atmospheric escape may quicken.

Study authors Dan Gershman and Gina DiBraccio, of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, published the results in Geophysical Research Letters. Though these seasonal changes haven’t been directly observed, the results show that a combination of strong solar activity and Uranus’ and Neptune’s unusually tilted magnetic fields is likely to trigger them.

From Mercury to Neptune, every planet in our solar system feels the unceasing stream of the solar wind. This barrage of solar particles, traveling hundreds of miles a second, drags the Sun’s magnetic field out to space, inevitably colliding with planetary magnetic fields.

But each planet responds differently. For planets closer to the Sun, like Mercury and Earth, the solar wind can really shake things up. Strong blasts of solar wind create our northern lights – at their worst, they can even cause electrical surges that lead to blackouts. (Mercury is hit so hard that it can’t even sustain an atmosphere.)

On Jupiter and Saturn, the solar wind’s blast has little effect. This is not because they’re farther away from the Sun – the most important factor is their magnetic fields, which are optimally positioned to protect them. These planets have strong magnetic fields aligned almost perfectly vertically, like a spinning top.  As the solar wind blows past Saturn, for instance, it hits its equator, meeting its magnetic shield where it is strongest.

An animation of Saturn’s magnetosphere as measured during the Voyager 1 flyby.
Credits: NASA/Scientific Visualization Studio/Tom Bridgman

Uranus and Neptune are even farther away from that strong solar wind source, but their magnetic axes make them vulnerable. Uranus’ magnetic axis is tilted by a full 60 degrees. This means that for a portion of its 84-year-long trip around the Sun, the Sun shines almost directly into the planet’s magnetic north pole, where the planet is least protected. Neptune’s axis is similarly tilted – though only by 47 degrees.

Animated GIF showing Uranus’ magnetic field. The yellow arrow points to the Sun, the light blue arrow marks Uranus’ magnetic axis, and the dark blue arrow marks Uranus’ rotation axis.
Credits: NASA/Scientific Visualization Studio/Tom Bridgman

With that background knowledge, Gershman and DiBraccio set out to study how the solar wind would affect the ice giants. Using historical data from the Helios, Pioneer and Voyager spacecrafts, Gershman and DiBraccio measured the Sun’s magnetic field throughout the solar system.

The results showed that during intense conditions, the solar wind can be as impactful near Uranus and Neptune as it normally is near Mercury, some 1.5 billion miles closer to the Sun.

Such intense conditions aren’t even a rarity. The enhanced solar activity Gershman and DiBraccio studied occurs regularly, as part of the 11-year solar cycle. The solar cycle refers to the periodic flipping of the Sun’s magnetic field, in which activity rises and falls.  At the high point, known as solar maximum, the Sun’s magnetic field throughout space can double in strength.

If the Sun enters solar maximum when Uranus or Neptune is at the appropriate angle, the effects, Gershman and DiBraccio argue, could be extreme. These planets so far from the Sun could suddenly be driven by it. Though the seasonal effects have not yet been directly observed, the physics suggests that aurora should brighten and spread further across the planet. Globs of particles known to escape the Uranian atmosphere may do so at a quickened pace. But only a few Earth-years later, it all goes away and the planets enter a new magnetic season.

The only close-up measurements we have of the planets are from the single flyby of Voyager 2 in 1986 and 1989, respectively. But a future NASA mission to the Ice Giants may well change that, giving us the first glimpse of their other-worldly magnetic seasons.

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