Magnetic field observations by the TEMAG experiment on the

TSS-1R satellite for two events of constant stimulated current along the tether are discussed. Previous evidence of a complex, unexpected, electric azimuthal current

circulation in close proximity of the high-voltage spacecraft is further supported and complemented. While previous events cover time intervals shorter than a spin cycle, the two new ones allow a complete coverage for a full spin cycle. The need of deeper theoretical approach as well as of more experimental observations is stressed

Acuña, M.

Candidi, M.

Marcucci, M. F.

Mariani, Franco

Musmann, G.

Ness, N. F.

Orsini, S.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2008-10287-3IL NUOVO CIMENTO Vol. 31 C, N. 2 Marzo-Aprile 2008On the current circulation about a high-voltage s/c: Two morecase-studies by the TSS-1R tethered satellite missionF. Mariani(1)(∗), S. Orsini(2), M. Candidi(2), M. F. Marcucci(2),M. Acun˜a(3), G. Musmann(4) and N. F. Ness(5)(1) Dipartimento di Fisica, Universita` “Tor Vergata” - Rome, Italy(2) INAF-CNR Istituto di Fisica dello Spazio Interplanetario - Rome, Italy(3) NASA/GSFC - Greenbelt, MD 20771, USA(4) Technische Universita¨t Braunschweig - Braunschweig, Germany(5) University of Delaware - Newark, Delaware, USA(ricevuto il 3 Marzo 2008; approvato il 30 Marzo 2008; pubblicato online il 22 Settembre 2008)Summary. — Magnetic field observations by the TEMAG experiment on theTSS-1R satellite for two events of constant stimulated current along the tether arediscussed. Previous evidence of a complex, unexpected, electric azimuthal currentcirculation in close proximity of the high-voltage spacecraft is further supported andcomplemented. While previous events cover time intervals shorter than a spin cycle,the two new ones allow a complete coverage for a full spin cycle. The need of deepertheoretical approach as well as of more experimental observations is stressed.PACS 94.30.Ms – Magnetic pulsations.PACS 07.55.Ge – Magnetometers for magnetic field measurements.1. – IntroductionIn two recent papers ([1], hereafter indicated as paper M; [2]) the pattern of currentcirculation around the TSS-1R satellite was studied during 8 sequences of active pulsedcurrents. The study was TEMAG experiment observation of the small signatures im-pressed on the local magnetic field by the active electric current. The TSS-1R s/c wasflown in February 1996 on a nearly circular orbit at 294 km altitude and 28.5 degree incli-nation. The TEMAG experiment consisted of two tri-axial sensors, mounted on a 87 cmlong boom, as shown in paper M. Peculiar signatures, superimposed to the backgroundgeomagnetic field and consistently related to the boom orientation were seen. The tem-poral duration of each sequence was a fraction of the spin period, while the intervals ofnon-zero current were randomly distributed. This made it possible to build a first-order(∗) E-mail: franco.mariani@roma2.infn.itc© Societa` Italiana di Fisica 135136 F. MARIANI, S. ORSINI, M. CANDIDI, ETC.model of the current flow all around the spinning tethered satellite, close to its equatorialplane. The magnetic field signatures, as seen over different intervals of spin angle alpha(alpha is the angle of the boom with respect to the ram direction, alfa = 0 being the ramdirection) during the 8 events gave consistent evidence of a nearly equatorial sector ofcurrent flowing on the ram hemisphere of the ionospheric environment at a distance ofseveral tens cm ahead of the s/c skin. When the boom was pointing to anti-ram hemi-sphere no significant current flow effect on the magnetic field was detectable, even whenthe stimulated current was on; this fact led us to suggest that the electric circuit closuretoward the s/c skin was via a distributed current diluted enough to become undetectableover the local background magnetic field.This peculiar pattern of current circulation deviates drastically from any simple modelof ionospheric electrons collection, either more or less radial, or along the geomagneticfield lines toward the high voltage s/c. Available model studies describe reasonably wellwhat happens at quite some distance ahead of a spherical (or nearly spherical) s/c in theplasma environment; but they are not adequate to describe the physical situation, at leastin close proximity of non-spherically symmetric s/c. The collected current was actuallysignificantly higher than predicted. More recent simulations and theoretical studies forthe case of spherical charged bodies have been able to explain the higher current, as seenin both TSS-1 and TSS-1R missions. They also show that while the current flows parallelto the magnetic field line at large distance from the satellite it gives way to azimuthaland radial current in the RAM volume ahead of the spherical skin [3].In the specific case of the TSS satellites one has also to expect, and indeed it has beenobserved, some drastic change of the local current collection mechanism, as a consequenceof the presence of the boom (or more generally, of a non-spherical s/c symmetry), as wellas of the deviating anisotropic effects of the geomagnetic field.This implies that on the one side we need to rely on more experimental studies toextend the rather poor statistics (due to the short duration of the TSS missions) and,on the other side, on deeper study of the plasma behavior at small, quasi-microscopic,scale.In this paper we discuss two additional events, when a constant active current wasinjected along the conducting tether for a time interval longer than a full spin period.This complements our previous study [1] in that now the non-zero current intensity duringthe s/c rotation allows uninterrupted observation of the magnetic signatures at TEMAGlocation, whichever the boom (and sensors) orientation, not only in correspondence tothe relatively short time duration of individual sequences of pulsed non-zero current.In sect. 2 we describe and discuss the observational data for the two events, whilesect. 3 summarizes the conclusions.2. – The observational dataFigure 1 shows a sketch of the magnetic field experiment reference system, where Zis pointing vertically downward, X is pointing radial toward the spin axis and Y is thethird axis pointing anti-clockwise. Figure 2 shows the differences Bx − Bxm, By − Bymand Bz − Bzm of the observed X, Y and Z field components of the field observed atthe outboard (upper thick line) and at the inboard sensor (lower thin line) and thosepredicted for the local geomagnetic field by the appropriate IGRF95 model [4, 5] on thes/c orbit, for the two events E1 and E2 (table I).TSS-1R CURRENT CIRCULATION PATTERN 137Fig. 1. – A sketch of the TEMAG instrument and of its geometrical reference system.The units on the plots are the same as those used in paper M, i.e.: the differencesbetween the IGRF95 components and the measured ones. The background, similar andrather large, excursion of the field at the two sensors is indicative of the significant dis-crepancy between the true geomagnetic field and the smoothed IGRF95 model estimatesFig. 2. – Top to bottom panels: observed trends of the magnetic field components at the outboard(thick lines) and inboard (thin lines) sensors, during the events E1 and E2. The plots show thedifference between the IGRF95 values and the measured ones. The bottom plot shows thecurrent intensity I injected along the tether. The top scale of each set of plots shows the spinangle α of the boom with respect to the ram direction.138 F. MARIANI, S. ORSINI, M. CANDIDI, ETC.Table I. – Event list.Day Start time End time I(h:min:s) (h:min:s) (mA)56 234756 235136 20057 005110 005454 400at s/c altitudes. The rather large wave seen on X and Y components may partially bea spin effect due to the modulation by solar radiation on the s/c surface. However, anysuch effect would similarly affect inboard and outboard field components, with no effecton the differences as shown in the figure. Clear field signatures are impressed on thediagram of outboard and inboard components. We cannot simply look at really simul-taneous individual differences among the two sensors components, because of the factthat inboard and outboard sample the field at different times and with different temporalresolution. Some abrupt jumps, simultaneous on both sensors (for example at t = 2900 sof event E1 and t = 3245 s of event E2) are artifacts consequent to sudden not infrequentsmall automatic readjustments of the s/c spin angle necessary to maintain the nominalconstant s/c spin rate.Basically, the pattern of the differences seen in the two figures is quite similar to thatwe have seen in paper M. Even now, with a constant active current flowing along thetether, no significant difference out of the noise background is detectable on the By com-ponents at the two sensors’ locations. As concerns the Bx and Bz components, instead,significant differences are observed, essentially when the boom is spanning the ram sidehemisphere, i.e. when its orientation with respect to the ram direction, is approximatelycontained inside the interval −90 to +90 degrees; the amplitude of the differences isroughly proportional to the current intensity during the two events. The maximum dif-ference on the X and Z outboard and inboard components exceeds 100 nT during E2and is more or less one half of that during E1.Some less evident peculiar features are also seen in fig. 2, in particular the lack ofsignificant jumps of inboard Bx−Bxm and Bz−Bzm at the current switch-on/off duringE1, while, during E2 both Bz −Bzm and Bz −Bzm show a jump of opposite sign. Somesimilar peculiarities were indeed also noticed in a couple of events of paper M; they areclear indications of a small-scale structure of the field.As a whole, we have again observational evidence that also in these two events athoroidal current flow ahead of the s/c on the ram hemisphere is apt to describe the ob-served trends. We must be aware that the model, as outlined in paper M and confirmedhere, is only a first-order, rough, description. Unfortunately, as a matter of fact, ourdata are still the only available direct information we have on the real current circulationin close proximity of a high-voltage s/c. Interpretation of the differences shown in fig. 2is subject, here and in paper M as well, to an intrinsic physical limitation. While theobserved magnetic perturbation is shown to be the effect of an azimuthal current flowingahead of the s/c on its equatorial plane, the actual current density cannot be determined.The magnetic perturbation at TEMAG is an integral effect of the current density distri-bution with radial distance; no inference can be made from this integral measurement,on the actual distribution of current in the plasma. In this paper, similarly to what wedid in paper M, we assume an equatorial concentrated current with the same intensityTSS-1R CURRENT CIRCULATION PATTERN 139Table II. – Angular intervals.Event E1 Event E2ψ1 (deg) −120 −90ψ2 100 60r0(cm) 200 200as the current flowing along the tether, as shown in the bottom panels of fig. 2. As anecessary consequence, full qualitative comparison with the predictions from simulationsis not possible; it is however important to remark that observations and predicted ex-istence of azimuthal current flow on the RAM side of the s/c environment qualitativelyagree.With these limitations in mind, we tried to test a best match approach using a simpleset of simple parameters, as shown in table II. The accompanying figure 3, adapted frompaper M, illustrates the geometry.Figure 4 shows the differences among the computed Bx, By and Bz field componentsat the two sensors using the values shown in the table. The trends seen in the figurematch reasonably well those inferred from fig. 2. Any variation of the values of table II,in the order of 10 degrees or so, while obviously leads to somewhat different results, doesnot however significantly affect the shape of the physical features.So, as a whole, the phenomenology observed during the two new events is substantiallysimilar to that observed in the 8 events of active, pulsed current discussed in M. Theexistence of a thoroidal current flow ahead of the s/c when the boom points on the ramside hemisphere is also applicable to these two new events, and this adds confidence tothe current pattern model we propose. Also in this case the lack of detectable signatureson the observed field in the anti-ram hemisphere is an obstacle to get some more hinton the diluted current pattern in the s/c wake and its final precipitation toward the s/cskin before flowing away along the conducting tether.r2 10AZIMUTHAL    CURRENT RAMEQUATORIAL PLANESpinFig. 3. – The equatorial projection of a clockwise azimuthal current, flowing between azimuthalangles ψ2 and ψ1 at radial distance r0.140 F. MARIANI, S. ORSINI, M. CANDIDI, ETC.Fig. 4. – Top to bottom panels: differences between outboard and inboard field components,as computed for the best match values of angles ψ1 and ψ2 and distance r0. The bottom plotsshow the current intensity I. The top scale of each set of plots shows the spin angle α of theboom with respect to the ram direction.3. – ConclusionsThe results discussed in this paper complement and confirm the conclusions outlinedin paper M. The relatively simple models describing current collection in the ionosphereby nearly spherical high-voltage s/c are valid beyond some distance well ahead of thes/c, of the order of its geometrical size. But in its close proximity a very complex currentcirculation pattern builds up, strongly affected by the geometrical shape of the s/c system,in our specific case of TSS-1R mainly by the boom.Any mapping and description, or practical application, of electric currents captured bya high-voltage s/c and channeled toward a conducting tether cannot rely on a macroscopicscale. It must rather be based on a still missing quasi-microscopic scale theoretical studyof the plasma, in close proximity to the actual conducting s/c surface. On the other hand,the limited available statistics (due to the short duration of the TSS missions) requiresfurther experimental observations to better describe and understand the ionosphericelectron collection and circulation under different physical conditions and geometries.∗ ∗ ∗The TSS missions were a joint Italy-USA venture. Support by Italian Space AgencyASI and by NASA, as well as by German DLR is gratefully acknowledged. We like tothank once again all the TSS team and the many colleagues who contributed during manyyears of hard work to the success of the TSS innovative missions. We thank S. Zampierifor manuscript editing and camera-ready preparation.TSS-1R CURRENT CIRCULATION PATTERN 141REFERENCES[1] Mariani F., Orsini S., Candidi M., Marcucci M. F., Acun˜a M., Musmann G. andNess N. F., J. Geophys. Res., 105 (2000) 18537.[2] Mariani F., Candidi M., Orsini S., Terenzi R., Agresti R., Musmann G., Rahm M.,Acun˜a M., Panetta P., Ness N. F. and Neubauer F., Geophys. Res. Lett., 25 (1998)425.[3] Singh N., Leung W. C. and Singh G. M., J. Geophys. Res., 105 (2000) 20935.[4] Langel R. A., J. Geomagn. Geoelectr., 43 (1991) 1007.[5] Barton C. E., EOS Trans., 77 (1996) 16.

On the current circulation about a high-voltage s/c: Two more case-studies by the TSS-1R tethered satellite mission

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On the current circulation about a high-voltage s/c: Two more case-studies by the TSS-1R tethered satellite mission

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