- Email: [email protected]
A few remarks about plasma measurements in space
Adv.SpaceRes.Vol. 15,No. 2,Pp.(2)169-(2)170,1995 Cd&O
PIinted
1994cosPAR
in Great Biit&. All .
0273417719 Y s7.GS
A FEW REMARKS ABOUT PLASMA MEASUREMENTS IN SPACE K. Rawer Albert-Ludwigs-Universitdt Germany
FTeibUTg
i.Br., Herrenstr. 43, D-79232 March,
ABSTRACT Measurement techniques applied in space plasma experiments are distinct in on specific! properties of a body many respects, e.g. spatial range, dependence Merits and drawbacks are discussed. moving in a plasma etc.. REMARKS Two classes of techniques can be distinguished in measurements of electron and/or ion total or partial densities in space plasmas according t,o whether particles are extracted from the plasma to be analyzed in a probe or whether one analyzes a global effect in an extended environment of the vehicle. Class I comprises for example Mass-Spectrometers (MS), Langmuir Probes (LPI and on (mostly radio-) wave Retarding Potential Analyzers (RPA) . Class II depends propagation or on plasma oscillations both of which use the dispersion properties of the medium surrounding the vehicle. Esamples are Impedance Probes (IP) and Resonance Cone Probes (RCP) . Since it is intended to assess the parameters\conditions of t.he undisturbed plasma there can be no doubt that it is easier to avoid motion or vehicle effects with class II methods. In the history of space plasma measurements there was in fact a long period during which the results obtained by different instruments showed great discrepancies that could not readily be explained. It was finallv always found that in a competition the class II results were correct. Since the seventies the reliability of class I methods has been Most of the earlier class I measurements, however, should not be improved. taken for granted. Measurement quality depends, of course, on the parameter that is to be determined. The remark made above refers in particular to absolute density determinations. All class I inmstruments suffer in fact from a calibration difficulty. Laboratory calibrations have rather generally failed to give satisfaction. Calibration under space conditions is very difficult if at all possible. These problems are less severe when only relative densities are required. The remaining problems are in general connected with a variable sensitivity depending for example on the particle mass [particularly with MS] or on their energy [LP and RPAI. The very involved environment of a space vehicle causes a lot of problems, particularly to class I measurements. For the ions a satellite is always in supersonic motion: the same holds with rockets except (for vertically launched rockets) in their apogee range. Thus the function of a probe aboard a rocket may become unreliable in the uppermost height range because the flow regime and consequently the calibration is drastically changed. This does not happen with satellites. JASR15:2-L
K. Rawer
0170
Another complication is due to charging of the vehicle which is a normal phenomenon in all space experiments. Normally the potential goes negative until it is so strong that only as many electrons can reach the surface as positive ions arrive. In the absence of electric activities aboard its value depends on the electron temperature, in practise however on such activities, in particular on telemetry and other emissions. These influences can not he predicted a priori. LP and RPA probes allow a determination of the actual potential that varies along the orbit (with the spatial variations of the An ion sheath builds up around the vehicle (a few cm thick, plasma density). normally) so that a strong electric field is present over a short distance perpendicular to the surface. For class I instruments this influences the entry of charged particles into t.he probe, particularly when the extraction procedure is rather compicated as with MS. As a consequence of these vehicle-caused influences absolute density determinations can hardly be expected with class I instruments. These need independent in-flight calibration, either by comparison to reliable groundbased experiments or, better to a class II instrument. Even when only relative i.e.density ratios are intended the permanent changes of the values, environmental conditions along the orbit can cause serious problems. In this respect class II devices are in a much better position. However since the magnetic field of Earth strongly influences the propagation \ dispersion effects that are anisotropic the orientation of the vehicle must rather accurately be known - particularly with RCP. Measuring the same parameter with distinct instruments is, of course, a way for deciding about their reliability. Aboard the AEROS-Bsat.ellite we had two (and their temperature) in a instruments of MS type, one measuring neutrals in an extended small mass range, the other for neutral or ionized particles (including their mass range. There was further an RPA switchable to electrons temperature and potential) or ions. and an IP measuring the total plasma density with high absolute accuracy. When comparing the results it was found that we had certainly published wrong densities if the class II instrument had of the different (positive) ions could of been absent. Relative densities course only be determined with class I instruments. It was found that the RPA was superior to MS insofar as the latter’s sensitivity varied considerably with the ion mass while mass resolution was, of course, superior to that of the RPA. Since we had all these instruments working in parallel we could finally correct for the mass dependence of the MS by calibrating with the RPA results for the main ions and obtain an absolute calibration from the IP. CONCLUSIONS Drawbacks Merits Instrument ______________________---___-----__------_-----___----____--_____------__~_ density calibration, serious temperature, potential LP : environment,al sensit.ivity some environresolution, constant calibration, can RPA : assess temperature, potential mental sensitivity 61 suprathermal populations calibration, serious resolution MS : environmental sensitivity bulk elect.rons only absolute calibration IP : bulk electrons only density and temperature RCP : _________________________________________-_____----___-----_------_------___
PIinted
1994cosPAR
in Great Biit&. All .
0273417719 Y s7.GS
A FEW REMARKS ABOUT PLASMA MEASUREMENTS IN SPACE K. Rawer Albert-Ludwigs-Universitdt Germany
FTeibUTg
i.Br., Herrenstr. 43, D-79232 March,
ABSTRACT Measurement techniques applied in space plasma experiments are distinct in on specific! properties of a body many respects, e.g. spatial range, dependence Merits and drawbacks are discussed. moving in a plasma etc.. REMARKS Two classes of techniques can be distinguished in measurements of electron and/or ion total or partial densities in space plasmas according t,o whether particles are extracted from the plasma to be analyzed in a probe or whether one analyzes a global effect in an extended environment of the vehicle. Class I comprises for example Mass-Spectrometers (MS), Langmuir Probes (LPI and on (mostly radio-) wave Retarding Potential Analyzers (RPA) . Class II depends propagation or on plasma oscillations both of which use the dispersion properties of the medium surrounding the vehicle. Esamples are Impedance Probes (IP) and Resonance Cone Probes (RCP) . Since it is intended to assess the parameters\conditions of t.he undisturbed plasma there can be no doubt that it is easier to avoid motion or vehicle effects with class II methods. In the history of space plasma measurements there was in fact a long period during which the results obtained by different instruments showed great discrepancies that could not readily be explained. It was finallv always found that in a competition the class II results were correct. Since the seventies the reliability of class I methods has been Most of the earlier class I measurements, however, should not be improved. taken for granted. Measurement quality depends, of course, on the parameter that is to be determined. The remark made above refers in particular to absolute density determinations. All class I inmstruments suffer in fact from a calibration difficulty. Laboratory calibrations have rather generally failed to give satisfaction. Calibration under space conditions is very difficult if at all possible. These problems are less severe when only relative densities are required. The remaining problems are in general connected with a variable sensitivity depending for example on the particle mass [particularly with MS] or on their energy [LP and RPAI. The very involved environment of a space vehicle causes a lot of problems, particularly to class I measurements. For the ions a satellite is always in supersonic motion: the same holds with rockets except (for vertically launched rockets) in their apogee range. Thus the function of a probe aboard a rocket may become unreliable in the uppermost height range because the flow regime and consequently the calibration is drastically changed. This does not happen with satellites. JASR15:2-L
K. Rawer
0170
Another complication is due to charging of the vehicle which is a normal phenomenon in all space experiments. Normally the potential goes negative until it is so strong that only as many electrons can reach the surface as positive ions arrive. In the absence of electric activities aboard its value depends on the electron temperature, in practise however on such activities, in particular on telemetry and other emissions. These influences can not he predicted a priori. LP and RPA probes allow a determination of the actual potential that varies along the orbit (with the spatial variations of the An ion sheath builds up around the vehicle (a few cm thick, plasma density). normally) so that a strong electric field is present over a short distance perpendicular to the surface. For class I instruments this influences the entry of charged particles into t.he probe, particularly when the extraction procedure is rather compicated as with MS. As a consequence of these vehicle-caused influences absolute density determinations can hardly be expected with class I instruments. These need independent in-flight calibration, either by comparison to reliable groundbased experiments or, better to a class II instrument. Even when only relative i.e.density ratios are intended the permanent changes of the values, environmental conditions along the orbit can cause serious problems. In this respect class II devices are in a much better position. However since the magnetic field of Earth strongly influences the propagation \ dispersion effects that are anisotropic the orientation of the vehicle must rather accurately be known - particularly with RCP. Measuring the same parameter with distinct instruments is, of course, a way for deciding about their reliability. Aboard the AEROS-Bsat.ellite we had two (and their temperature) in a instruments of MS type, one measuring neutrals in an extended small mass range, the other for neutral or ionized particles (including their mass range. There was further an RPA switchable to electrons temperature and potential) or ions. and an IP measuring the total plasma density with high absolute accuracy. When comparing the results it was found that we had certainly published wrong densities if the class II instrument had of the different (positive) ions could of been absent. Relative densities course only be determined with class I instruments. It was found that the RPA was superior to MS insofar as the latter’s sensitivity varied considerably with the ion mass while mass resolution was, of course, superior to that of the RPA. Since we had all these instruments working in parallel we could finally correct for the mass dependence of the MS by calibrating with the RPA results for the main ions and obtain an absolute calibration from the IP. CONCLUSIONS Drawbacks Merits Instrument ______________________---___-----__------_-----___----____--_____------__~_ density calibration, serious temperature, potential LP : environment,al sensit.ivity some environresolution, constant calibration, can RPA : assess temperature, potential mental sensitivity 61 suprathermal populations calibration, serious resolution MS : environmental sensitivity bulk elect.rons only absolute calibration IP : bulk electrons only density and temperature RCP : _________________________________________-_____----___-----_------_------___