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The design of the instrument is fairly straightforward and takes advantage ofthe fact that the bulk plasma flows are smaller than the speed of the orbitingsatellite. At 800 km (DMSP altitude), the satellite is traveling at about 7km/s while the usual bulk plasma flow is less than 2 km/s in almost all cases.The instrument consists of a Faraday cup with a square opening (about 2.5 cmwide) on the front and a collector divided into four parts in the back (seefigure below, showing a head-on view with the aperture opening and thecollectors behind it). The collector measures the amount of current from thecharged ions which fall onto it and each of the four parts is measuredseparately. The opening has a series of fine wire grids which are used to repelthe negatively charged electrons so that only the positively charged ions enterthe detector for the collectors to see. Since the electrons are so much lighterthan the ions, the negative charge on the top grid easily repels the electronswithout disturbing the path of the ions too much. Subsequent grids below the topgrid are charged positive to cancel out any disturbance to the original flowdirection of the ion.
The instrument is mounted onto the satellite so that the opening faces intothe direction of the satellite's motion (+x or upward in thediagram below). The figure below shows a top view of the instrument in thatorientation. As the satellite moves through the plasma (towards the top of thediagram), the ions flow into the aperture with a speed of Vs(the satellite's velocity, about 7.3 km/s) plus Vd (the speedof the bulk plasma flow parallel to the satellite's velocity vector, usually +/-1 km/s). If the plasma was stationary (relative to the Earth) as the instrumentflew through it, then equal numbers of ions would fall onto the left collectorsas onto the right collectors. Thus the difference in current between the halvesof the collector would be zero. But if the horizontal component of the plasma isto the left (as shown in the figure with the vector Vh) thenthere is more current falling on the left two collectors than on the right two.The faster this horizontal velocity of the plasma is, the greater thisdifference in currents will be. Thus, by knowing the speed of the satellite andthe dimensions of the instrument, we can convert the measured current differenceinto the horizontal velocity for the plasma. The vertical velocity of the plasmais measured in exactly the same manner by using the difference in currentsbetween the top two collectors and the bottom two collectors.
In general the flows measured by the ion drift meter are less than 1 km/sexcept in the auroral region where flows vary from 1 to 2 km/s. The dynamicrange of these instrument is usually about +/- 2.5 km/s.
For more detailed information on the drift meter data for a particularsatellite, click on one of the links below:
(Submitted by Dr.Marc Hairston)
A retarding potential analyzer is a rather generic name usedto describe many devices that utilize a retarding potential to control theenergy of charged particles that have access to a detector. If the energydistribution of the particles is assumed to be a Maxwellian, then the variationin the particle flux as a function of energy can be used to derive thetemperature and kinetic energy with respect to the sensor of the chargedspecies. Additionally if the sensor has a limited acceptance angle then thekinetic energy along the sensor look direction may be used to derive the chargedspecies velocity along the look direction.
In the Earth's atmosphere, the velocity of a vehicle in orbitin the ionosphere is about 7.8km/sec. This velocity imparts an energy of about0.33eV/amu to each particle incident on the sensor collector. In contrast thethermal energy of the ions or electrons with a temperature of 1200 oK is about0.1eV and the equivalent thermal speeds are about 4.4 km/s for H+ ions and 1km/sfor O+ ions.
Limitation of the sensor acceptance angle is not alwaysnecessary especially if the particle thermal speed is much larger than theparticle kinetic energy with respect to the sensor, as is the case with thermalelectrons. A retarding potential analyzer is commonly used to determine theenergy distribution of low energy electrons as well. Also when detecting ions,it may not always be sensible to restrict the acceptance angle of the sensor. Ifthe vehicle is spinning, or the direction of the incident ions is highlyvariable, then a spherical geometry is preferred. Such a geometry almost alwaysallows incoming ions access to the detector.
Planar RPA: A description
The word planar refers to a series of grids which control theenergy of the ions that have access to the detector, and to the aperture planedefining the entrance to the detector itself. In its simplest configuration, aplanar RPA consists of a planar circular entrance aperture followed y a seriesof planar grids that precede the detector itself. The detector is a solid metalconductor at which the current can be measured by a sensitive electrometer. Thearrangement of the entrance aperture and the collector provides restrictedaccess such that only particles with velocities perpendicular to the entranceaperture have access to the collector. The sensor is mounted to viewapproximately parallel to the spacecraft velocity vector, the so calledram-direction, and will have an unrestricted view in thatdirection.
[Under construction.]
Irving Langmuir and H. Mott-Smith, Jr., reported the use of anelectrostatic probe in a laboratory plasma in 1924. This cylindrical probe isnow referred to as the Langmuir Probe (LP), and is extensively used tocharacterize the Earth's ionosphere. This probe can be used to effectivelyperform in-situ measurements.
Electron temperature measurements on board satellites likeDMSP are made using the LP. The probe consists of a small conducting surfacewith cylindrical or sperical geometry. These probes are mounted on short boomsabout 20 cm in length to project beyond the spacecraft sheath. The currentcollection properties of these probes depend in detail on the shape and area ofthe collector. However in the voltage range where the collected electron flux ischanging, the probe geometry is not a factor. This instrument functions byapplying a varying voltage between -5 V and + 5 V. A typical theoretical curveof current vs voltage is shown in the figure below.
All LP characteristic curves have the same regions. The curverepresents the sum of the ions and electron currents Ii and Ie, collected fromthe ionospheric plasma surrounding the sensor. The curve begins in the ionsaturation region where the probe potential is sufficiently negative to repelall the thermal electrons and allow access to the ions. Hence at this point, thecurrent to the probe is primarily due to the ions. The electron retardationregion exists where the ion current is not affected greatly by the potential onthe probe, but some electrons are repelled dependent on their energy. Finally,the electron saturation region exists where the probe is positive with respectto the plasma. Hence all the thermal electrons as attracted to the probe and theprobe potential now repels the ion current. In this region the curve shape isdependent on the probe geometry. The logarithm of the output of this device inthe electron retardation region should be a straight line for which the slope isproportional to the electron temperature. The electron saturation region issimply used to derive electron concentrations. LP measurements are always madewith respect to the spacecraft potential.
[Under construction.]
The Neutral Wind Sensor consists of the Ram Wind Sensor andthe Cross Track WInd Sensor. The Ram Wind Sensor is used to measure the kineticenergy of the neutrals along the sensor look direction. It measures the ionizedfraction of the neutral gas. The RWM, similar to the RPA, measures the ramcomponent of the velocity, Vx. The ram component is mainly dominated by thesatellite velocity. Vx can be derived from the measurement of the kinetic energyof the gas along the sensor look direction, i.e. along the ram direction.
The Cross Track Wind Sensor (CTS) is used to measure thearrival angle of the neutrals with respect to the sensor look direction. CTSmeasures the neutral pressure ratio. Cross track components, vy and Vz arederived by measuring the horizontal and vertical arrival angles.
The figure above shows the Neutral Wind Meter which has theRam Wind Sensor to measure the ram component of velocity and also the CrossTrack Wind Sensor to measure cross track drifts. The Neutral Wind Sensormeasures velocity with respect to the Spacecraft and so the spacecraft velocitymust be removed to obtain the ambient drift.
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Maintained by Sarita VenkatramanCopyright (c) 1999 [Center for Space Sciences]. All rights reserved.Revised: January 22, 2003.