University of Kansas

Space Physics and
Plasma Astrophysics

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Particle Radiation Bursts at Jupiter

The Space Physics group at the University of Kansas (KU) Department of Physics and Astronomy provides scientific research for NASA and has done so for the past 25 years. It is a productive as well as an exciting quest. This is why KU professors and students wish to inform the public and fellow researchers about the discoveries and results that the KU team has made.

The KU Department of Physics and Astronomy has studied particle radiation, with emphasis on particle radiation bursts. Burst events in the neighborhood of Jupiter were studied and documented by Dr. Dennis Haggerty, who, at the time, was a KU graduate student working with an instrument aboard a spacecraft called Ulysses.

First, we must establish some background information on particle radiation and define the Ulysses mission.

Space is radioactive. To many people, this statement may cause alarm, so let's focus some light on particle radiation.

Everyone is continuously exposed to a certain amount of radiation. The radiation comes naturally from the ground, the oceans, the air, radium in drinking water, the human body (which contains a natural potassium radioisotope, among others) and, of course, our subject, outer space. As a matter of fact, in space we find such particles as x-rays, gamma rays, and energetic electrons, protons, ions, and alpha particles. Our galaxy's cosmic rays are a source of highly penetrating ionizing radiation. Gigantic solar flares, which are violent eruptions on the sun, pour out high energy protons into space. Once a flare has begun, energetic particles strike out for a day or more in all directions. People not in a shielded area would not have much time to find shelter. The good news is that on Earth, our air is a shield against deadly high-energy electromagnetic waves and particles. We are protected from space particle radiation by our atmosphere and the force of Earth's magnetic field, which is the region inside of the magnetosphere. Let's examine this in greater detail.

The magnetosphere is the region where a planet's magnetic fields have influence against the solar wind. Solar wind is the Sun's hot coronal gas which expands outward into the solar system and fills interplanetary space. This is where it becomes known as the solar wind. The solar wind flows continuously and contains plasma, which is a gas mixture of electrically charged particles, such as electrons, ions, and protons. These charged particles of the solar wind are repelled by a planet's magnetosphere and they are not able to penetrate it. Therefore, the solar wind exerts a force on the magnetosphere at its outer boundary called the magnetopause. A flowing electric charge (current) is present at the magnetopause.

The solar wind is supersonic (speed greater than the speed of sound), like a supersonic jet. When it encounters an obstacle such as the magnetosphere, a standing shock wave forms. This is called a bow shock, which forms upstream of the magnetopause for the same reason that a shock wave precedes a supersonic jet passage. The plasma downstream slows down and becomes hotter. The downstream plasma is subsonic (speed less than the speed of sound) and is able to flow around the obstacle. The region of space located between the magnetopause and bow shock is called the magnetosheath. Now, let's define the Ulysses mission.

The Space Shuttle Discovery was launched with Ulysses aboard on October 6, 1990. It was a joint effort by NASA and the European Space Agency. Its mission was to fly above the poles of the Sun. Why? Because before Ulysses, scientists could not study two subjects of great importance: the entire set of interactions of the solar wind, magnetic fields, and cosmic rays that takes place at the Sun's poles; and a full understanding of the physics of the Sun. The reason the Sun's poles were not accessible was because spacecraft could not fly above the plane of the ecliptic (the plane in which Earth orbits the Sun). Since launch vehicles could not propel spacecraft up and out of the ecliptic, scientists needed to find an alternative to bend the flight path. Their solution? The immense gravity of the planet Jupiter. By aiming at just the right point near Jupiter to make use of its gravitational field, Ulysses acquired a free ride on a natural launch vehicle. (This is called a gravity assist.) Jupiter's gravity sent the spacecraft plunging back toward the Sun's south pole. This brings us to our subject. The Jupiter flyby provided an excellent opportunity to examine the upstream region, bow shock, magnetosphere, and the unexplored southern region of Jupiter through Dr. Haggerty's contributions to an instrument called HISCALE, which was one of nine instruments aboard Ulysses.

During the Jupiter flyby there were some rather exciting observations of particle radiation bursts. Earth also experiences particle radiation bursts upstream from the bow shock, but these events are more intense at Jupiter. This is why Jupiter was a prime choice for KU's Dr. Haggerty's HISCALE particle radiation research. Let's explore HISCALE's observational findings more closely and also discover why these events are called bursts.

HISCALE was equipped to study interplanetary ions and electrons with a wide range of energies, from high energy particles in the solar wind to particles with extremely high energies which are the Sun's equivalent to cosmic rays. Scientists use HISCALE to understand the mechanisms that release solar flare particles and the dynamic phenomena that are associated with the solar cycle's maximum activity. The instrument uses the flow of high-energy particles from the eruptive processes on the Sun to study structural changes in the corona and in the magnetic field lines. The instrument also provides clues about how the masses of individual particles influence their acceleration due to electromagnetic forces.

In 1979, when the Voyager 1 and Voyager 2 spacecraft flew past Jupiter, unusual increases in radiation intensity began occurring long before the closest approach. These abrupt increases in radiation have come to be recognized as probably being caused by Jupiter. Since Jupiter has very intense radiation, it could be as simple as escaping directly from Jupiter. Another possibility is that the bow shock of Jupiter energizes solar wind to make the bursts. Let's look at some particle events on particular dates.

In 1992 it was reported that burst events observed upstream from Jupiter resembled those seen at Earth. The composition of charged particle radiation bursts provides important information on the origin of the particles. Jupiter's magnetosphere contains ions that are not native to the solar wind. When these ions were detected in Jupiter's upstream charged particle radiation bursts, the conclusion was made that at least some of the particles originated within Jupiter's magnetosphere. No upstream charged particle radiation bursts have been observed at planets without an intrinsic magnetic field.

HISCALE observations of Jupiter's accelerated particles occurred first in the ecliptic plane upstream from the planet's bow shock December 6, 1991. The characteristics of this 1991 event match the characteristics of other upstream particle events and indicate a Jupiter origin. This event was the first of 193 different particle bursts of probable origin from Jupiter. There were many more events observed south of the ecliptic plane than there were in the ecliptic. The average burst magnitudes seem to diminish with distance from Jupiter's bow shock. These events can "turn on" in a matter of seconds.

Ulysses crossed Jupiter's bow shock February 2, 1992. Prior to the bow shock crossing, a series of very intense upstream particle events were detected by HISCALE. On February 18, 1992, a very large charged particle event swept over Ulysses; and March 13, 1992, was a very active time with several bursts convecting past the spacecraft. No evidence was seen that significant changes in the interplanetary magnetic field occurred in relation to the onset of the Jupiter particles.

April 20, 1992, saw multiple events which were remarkable, especially because of the large distance from Jupiter. Oxygen was also present within these particle events, as it was for previous events. The composition of Jupiter events varies from event to event. There is evidence that some events are composed mainly of protons while other events have a significant percentage of heavier ions.

The observations made with Jupiter's bow shock, the transition regions, and especially the magnetosphere itself, qualify Jupiter as a possible source. Jupiter has the necessary energy and plasma to produce strong fluxes of ionized ions and electrons, which could extend to great distances from their source. A comparison of magnetospheric observations to the interplanetary observations provides the best available evidence that Jupiter's effects reach far beyond its interplanetary bow shock.

Many different studies of the composition of Jupiter's magnetosphere show that there is an abundance of not only protons and electrons, but oxygen as well. This leads to the question, why were there so many more observations south of Jupiter than in the ecliptic plane? There were 145 events observed south of the ecliptic plane, a significantly larger number than the 48 observed upstream from Jupiter.

The Voyager spacecraft offered early evidence that, similar to Earth, the interplanetary region outside of Jupiter's bow shock is rich with strongly directional particle radiation events. Ulysses transported electron and ion, solar wind and magnetic field sensors through previously unsampled regions of Jupiter's magnetosphere and the southern region near Jupiter.

KU's Dr. Haggerty's conclusions of HISCALE observation comparisons between interplanetary events and times within Jupiter's bow shock suggest Jupiter as a probable source of the interplanetary particle radiation. Jupiter was found to be a significant source of interplanetary ions and electrons.

Why should the average person need to know about Dr. Haggerty's HISCALE observations regarding particle radiation bursts? Because, in addition to our curious nature, education about particle radiation and the fact that we can monitor it should help allay any fears. It is also desirable to gain knowledge about our surroundings, from our immediate environment, to the universe in which we reside.

Last modified May 26, 2015
T. Hunt-Ward
Writeup by C. Graves