| The Beginnings: [In 19591], James A. Van Allen and his colleagues forever changed our popular view of near-Earth space as a bland and empty void. Using instruments onboard the first NASA satellite, Explorer 1, they discovered an unexpected and teeming population of charged particles confined within the Earth's magnetic field. Immediately following this discovery, global models of these trapped particles - the Van Allen radiation belts were developed from their theoretically expected motions in the Earth's field as it was known at that time. This desire to build a global picture based upon the few available measurements in the Earth's space environment has proven prophetic. From these earliest days one of the major quests of space plasma physics has been to develop an accurate global perspective of the magnetosphere and its component parts. |
| Magnetosphere Imager Science Definition Team Interim Report NASA Reference Puhlication 1378 |
Earth is protected from the solar wind, a continuous flow of energetic charged particles from the sun, by Earth's magnetic field. The magnetic field diverts the particles so that most of them flow past Earth without getting very close to the surface. Some of the particles follow magnetic field lines toward the north and south magnetic poles where they cause the aurora as they enter Earth's lower atmosphere. Understanding exactly how these particles behave as they flow outward from the sun toward Earth and what the effect is on Earth's magnetic field and the region around Earth are the goals of the instruments on the IMAGE satellite.
 |
Figure 1. Earth's Magnetosphere
Distances are given in Earth radii. Earth ( * ) is at (0,O). The dashed line is the magnetopause. The sun is to the left. The solar wind distorts Earth's magnetic field: it compresses the day side (toward the sun) and stretches the night side (away from the sun). |
|
Distances in the magnetosphere are often measured in Earth radii (RE), with one Earth radius
amounting to 6371 km or 3960 miles. In these units, the
distance from the Earth's center to the
"nose" of the magnetosphere is about 10.5 RE and to
the flanks abreast of the Earth about 15 RE,
while the radius of the distant tail is 25-30 RE. By way of comparison, the moon!s average
distance is about 60 RE.
These, though, are just averages: the pressure of the solar wind rises and falls, and as it does, the magnetopause shrinks or expands. For instance, when the boundary is hit by a fast flow from a cororal mass ejection the 'nose" is pushed in, occasionally (a few times a year, us@) even past the synchronous orbit At 6.6 RE. |
Exploration of the Earth's Magnetosphere
by: David P. Stern - NASA/GSFC Code 695 Mauricio Peredo - Raytheon STX Corporation |
The region around Earth where Earth's magnetic field is the predominate field is called the magnetosphere. The region of interplanetary space where the solar magnetic field is strongest is called the Interplanetary Magnetic Field, or IMF. As the solar wind flows outward from the sun, the ME is carried along. The boundary between the IMF and the magnetosphere is called the magnetopause. The location of the magnetopause at any given time depends on the balance between the pressure exerted by the solar wind on the magnetosphere pushing in and the pressure of the particles within the magnetosphere pushing out. The pressure exerted by the solar wind depends on the speed of the particles and their density. When an energetic solar event, such as a coronal mass ejection (CME), occurs, the resulting solar wind will have a higher density and higher speed exerting an increased pressure and as a result compressing the day-side of the magnetosphere and decreasing the distance from Earth to the magnetopause.
Determining the location of the magnetopause is important to scientists (and people in general!), because knowing where the magnetopause is and how its location responds to changes in solar wind activity will help us understand the connection between what happens on the sun and the consequences for us here on Earth.
In the past, the only way to determine the location of the magnetopause was to have a satellite carrying a magnetometer (a device for measuring magnetic fields) pass through the boundary. As it passed through the magnetopause, the satellite could determine the location of the magnetopause in that region of space and at that particular time only. This is called an in situ measurement, meaning that the instrument was at the location of the magnetopause when the measurement was taken. If there were a change in solar activity, it would be unlikely that there would be a satellite with the appropriate instruments anywhere near the magnetopause at the right time.
From in situ measurements in the past it has been determined that the magnetopause on the side toward the sun is located at a distance from Earth that varies from around 90,000 kilometers (14 Earth radii (RE) down to about 32,000 kilometers (5 RE) with 64,000 kilometers (10 RE) as a common value. The exact location at any time depends on the recent activity of the sun and the resultant strength of the solar wind. To determine the location of the magnetopause constantly over a period of time using in situ measurements would require an huge fleet of satellites equipped with magnetometers. Due to the enormous cost, this is not a workable solution. What is needed is a different approach.
| Time variations: Tbe greatest obstacle in all out attempts to synthesize an accurate global picture of the magnetosphere is its time variability. Major regions of the nagnetosphere can alter significantly their shape, composition, and interconnectivity over time scales far shorter than our capability to observe them by in situ measurements. The vast size of the magnetosphere will in all probability, always preclude the establishment of a sufficiently dense network of in situ observations to accurately nieasm the global magnetosphere. |
|
Magnetosphere Imager
Science Definition Team Interim Report NASA Reference Publication 1378 |
Remote sensing is a measurement technique that is an alternative to in situ measurements. In remote sensing, the instrument does not need to be at the location where the measurement is taken. A simple example of a remote sensing instrument is a camera. A camera uses light from the subject to form an image which can then be captured on film. The result is a photograph of an object taken without the camera getting anywhere near the subject. Remote sensing is required to obtain photographic images of distant galaxies and other astronomical bodies.
There is a problem with using remote sensing to measure magnetic fields at locations away from the instruments: at the low magnetic fields found at the magnetopause, it is nearly impossible! There is no easy way to detect or measure the magnetic field itself without being at the location of the measurement. When that is the case, scientists look for something else that can be measured remotely thatmil give the desired information. In the case of the magnetopause there is something else that occurs: at the magnetopause: the charge density of space changes. Just inside the magnetosphere, the charge density (made up mostly of free electrons and hydrogen ions - protons) is about 5 electrons per cubic centimeter or less. Outside the magnetosphere, in the IMF, the density is about the same. At the magnetopause the charge density is much higher - up to 100 electrons per cubic centimeter. Unfortunately, electron charge densities can't easily be remotely sensed either!
This leaves us with the problem of finding something that can be measured remotely to indicate where the magnetopause is located. The solution to our difernma is to use radio frequency electromagnetic waves. These waves travel through space at the speed of light and, like light, their path is changed when they encounter a different medium. For radio waves, thechangein charge density at the magnetopause is a change in medium just like when visible light waves encounter a change in medium such as traveling from from air into glass. When light travels from air to glass, several things happen: the light changes to a slower speed in the glass, the light path changes direction and some of the light is reflected. The last of these behaviors can be used to remotely sense the location of the magnetopause. When radio waves from a satellite within the magnetosphere encounter the increased electron density at the magnetopause, some of the radio wave is reflected. If you can measure the time it takes for the radio wave to travel to the magnetopause and back to the satellite and you know the speed of light, you can easily calculate the distance from the satellite to the magnetopause. This is the principle behind the radio plasma imager.
| The Radio Plasma Imager (RPI) is a transmitter/receiver system that responds to the science requirement for the corwnuous remote sensing of plasma densities, structures and dynamics in the magnetosphere and plasmasphere. The instrument measures the time delay, angle-of- arrival and Doppler shift of magnetospheric echoes over the frequency band from 3 kHz to 3 MHz. This frequency range makes possible remote sensing of plasma densities from 0.1 to 1000 atoms per cubic centimeter. |
The role of the Radio Plasma Imager is to determine the location of the magnetopause. To accomplish this, radio waves are transmitted by IMAGE and the reflections of these waves from the magnetopause are received. To determine the location of the magnetopause, two questions need to be answered: 1. What is the distance from Earth to the magnetopause? and 2. In which direction is that distance measured?
In principle, this determination is made by using the definition of velocity to determine the distance. A radio signal is sent out from IMAGE and the time for the signal to reflect from the magnetopause is measured. This time is called the "echotime". Since you know the speed of light (3.0x10^8 m/s), the distance from IMAGE is found by multiplying half the echo time times the speed. If you know the position of IMAGE relative to Earth, the distance from Earth to the magnetopause can be determined easily. For example, if IMAGE is at apogee (a distance of 8 RE from Earth's center) and the echo time is .086 seconds, calculations show that the magnetopause is located 10 RE from Earth's center (See Example Problem 1 and Example Problem 2). This is a typical location for the magnetopause for normal levels of solar activity.
[NOTE: The following analysis is given in 2 dimensions only (the x-y plane). IMAGE must do the computation in three dimensions. This involves the use of the third antenna - the z-antenna. This antenna is shorter than the x- and y-antennas, so the voltages induced on it by a given signal echo will be smaller. The z-voltage is amplified by IMAGE before the computation is made to allow for the difference in antenna lengths. The method of determining the distance and direction to the magnetopause is similar to the 2-dimensional analysis.]
| RPI will have two crossed 500-m tip-to-tip thin wire dipole antennas in the spin plane, and a 20-m tip-to-tip tubular dipole antenna along the spin axis. All three antennas will be used for reception to determine the angles of arrival of the echoes [Calvert et al., 1995]. |
| RPI Technical Description |
Radio waves reflect according to the Law of Reflection just like light does. In order for radio waves to be reflected back to their point of origin (the IMAGE spacecraft), the waves have to strike the boundary between media - the magnetopause - perpendicular to that boundary. The waves must travel fight down the normal to the surface in order to be reflected back to @GE. If the inner surface of the magnetosphere is smooth, then the reflection will come from exactly one point. If the inner surface of the magnetosphere is not smooth, then reflection could be returned to IMAGE from several locations on the magnetosphere. The following figures illustrate two possibilities.
Figure 2. Ray diagrams reflection from the magnetopause.
| (a) Reflection from a smooth curved surface. Only Ray 2 will reflect back to IMAGE. | (b) Reflection from a more complex swface. IMAGE will receive reflections from all four rays shown. |
Figure 3. Reflected Radio Wave Returns to the IMAGE Antenna As the reflected wave returns, one antenna wire will register the voltage due to one component (the y-component) while other wire will register the voltage induced by the x-component.
Radio waves, like all electromagnetic waves, are transverse waves. The E-field oscillates at right angles to the direction of travel of the wave. When the returned signal encounters the crossed antennas, only the component of the signal parrallel to the antenna wire will induce a voltage in that wire. The voltage induced will be proportional to the length of the wire and the E-field component in that direction. From the measured voltages, the direction of the returned signal can be determined.
In Figure 3, the angle that the E-field makes with the antenna is 45', so the x-component and the y-component will be equal. But if the x- and y-components are equal, the reflected radio wave could have come from any of four possible directions: 45, 135, 225 or 315 degrees, since a wave reflected from any of these directions would have equal components.
IMAGE must distinguish among these possibilities. The way this is done is by determining the phase between the components. Figure 5 illustrates the difference in the phase of the components between waves returning from 45 and 135 degrees.
Figure 5. Phase relationship between E-field components. To consider phase, select one direction for the E-field vector and compare the components: For the top vector (arriving from 45 degrees), the x-component is negative and the y- component is positive. These components are said to be out of phase. For the bottom vector (arriving from 135 degrees), the x-component is positive and the y-component is also positive. These components are said to be in phase.
Notice that if the returning radio wave is coming from a direction of 225 degrees, the phase relation of the components will be the same as if the echo were returning from 45 degrees. This ambiguity can be resolved in two ways. First, it is known that the magnetopause is located generally away from Earth as viewed from IMAGE. Second, if IMAGE is traveling toward the magnetopause then the reflected wave will be Doppler shifted to a higher frequency and so will its components. The components which show the expected Doppler shift can be used to determine the direction to the magnetopause.
Once the general direction to the magnetopause is known, analysis of the phase relationship between the components of the E-field vector can then determine which of the two remaining possible directions is correct.
For example, if the general direction to the magnetopause is between O' and 180', the x- component antenna registers a .29 microvolt induced voltage, the y-component antenna registers a .50 microvolt induced voltage and the voltages are in phase, then the signal is reflecting from a direction of 120 degrees. (See Example 3a) If the voltages were out of phase, the direction would be 59.9'. (See Example 3b.)
All of the computations are done automatically by IMAGE as it rotates in space and moves along its orbit. A complete set of measurements and computations is done every two minutes which is the rotation period of IMAGE. As the measurements are taken, IMAGE keeps track of its orientation in space and the location of Earth so that the results can be given in Earth-centered coordinates.
The above example was given in 2 dimensions only ( the x-y plane). IMAGE must do the computation in three dimensions. This involves the use of a third antenna, the z-antenna. This antenna is shorter than the x and y antennas, so the voltages induced on it by a given signal will be smaller. The z-voltage is amplified by IMAGE before the computation is made to allow for the difference in antenna lengths.