The Extreme-ultraviolet Variability Experiment (EVE) on board the Solar Dynamics Observatory (SDO) maps the radiation environment as it climbs to its mission orbit.
Please read the paragraphs below in association with the notes on the video.
1) The Earth's Magnetic Field
The Earth's magnetic field is quite complicated, especially due to interaction with the Sun. This magnetic field traps charged particles and forms the Van Allen Belts.
In this video, we ignore all the complications of the field and treat it as an off-center tilted bar magnet (as a dipole). We define a coordinate system which is centered on the dipole and aligned with its axis. We arbitrarily select one meridian plane as a reference, like the Prime Meridian. It doesn't matter which plane we use, because the dipole model of the magnetic is symmetrical about the dipole axis. We presume that the radiation belts share this symmetry.
2) SDO and EVE in geosynchronous transfer orbit
The SDO spacecraft was launched into a geostationary transfer orbit (GTO) on 11 Feb 2010. This orbit has a low point well inside the inner radiation belt and a high point near the outer edge of the outer belt, and so is a good sampling orbit.
On board SDO, the EVE instrument electronics were activated on 18 Feb 2010 at 15:59UTC. Most of the instrument is still awaiting activation, but one science channel, the MEGS-Photometer, was activated with the instrument electronics. While this channel is an ultraviolet instrument, not a radiation instrument, it is affected by radiation. We can use it as a simple radiation couner.
3) Mapping the radiation belts
EVE sends down one measurement every 10 seconds. Down here at the EVE Science Operations Center, we use that data to construct a 1-minute average measurement. We can then plot that measurement as color, as can be seen here.
We presume that the radiation is symmetrical in a ring around the magnetic field axis, so every point we measure on the orbit is actually a measurement of a whole ring. We represent that by coloring the whole ring the same color as our measurement on the orbit.
4) Radiation in the dipole frame
In order to see the belts more clearly, we can look from the dipole frame. We pick an arbitrary meridian plane in that frame to plot against. Then, for every instant, we look at the distance that the spacecraft is away from the axis and above the equatorial plane, and plot a point on the meridian plane. In other words, we color the spot on the plane where each measurement ring intersects the plane, the same color as the ring.
The display in the lower right corner is the same meridian plane, just frozen in space so that we can see it all the time.
Early in the SDO mission, the GTO period is near 12 hours, almost exactly twice around each day. Since the magnetic field is tilted relative to the Earth's rotation, we see two loops in the plot, as the magnetic field tilts back and forth.
As the orbit continues to be raised, different parts of the belts are mapped out.
The inner radiation belt can be seen quite clearly as the red, yellow, and green areas close to the Earth. The outer belt is visible also as a blue and purple area. As seen by the EVE instrument, the inner belt is a couple of hundred times more intense than the outer belt. This is due more to the design of the EVE instrument than any actual difference in intensity. The two radiation belts have different kinds of radiation, and EVE is much more sensitive to the kind in the inner belt.
5) Visualizing the belts in 3D
We can visualize the belts in 3D as well. Remember that each point on the meridian plane represents a ring around the magnetic axis. We can draw and color all of these rings simultaneously to see the Van Allen belts. Note that the belts appear to wobble back and forth as they follow the magnetic field.
The SDO mission is managed by NASA Goddard Space Flight Center. The EVE instrument was built, is controlled, and data analyzed, by the University of Colorado's Laboratory for Atmospheric and Space Physics. Tom Woods is the principal investigator for the EVE instrument. Visualization by Chris Jeppesen.