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Variation of the spacecraft potential in the magnetosphere
Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
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Variation of the spacecraft potential in the magnetosphere Harri Laakso ∗ ESA, ESTEC Space Science Department, Code SCI-SO, Keplerlaan 1, Postbus 299, 2201 AG Noordwijk, Netherlands
Abstract This paper deals with the surface charging of the Polar satellite, using the spacecraft potential measurements of the electric .eld instrument (EFI) gathered during 48 months in 1996 –1999. For most of the time, the satellite 4oats at a positive potential Vsc in the magnetosphere because of a high 4ux of photoelectrons emitted from the satellite surface; typically Vsc varies between 0 and 50 V so that it is 0 –1 V in the plasmasphere, at the plasmapause it exhibits a steep increase by 3–5 V, and outside the plasmasphere Vsc is usually more than 5 V. On the dayside magnetosphere Vsc is 7–8 V in average, whereas on the nightside it lies in the 10 –20 V range. In the cusp, however, Vsc is only about 3 V, similar to spacecraft potentials observed in the magnetosheath. Highest Vsc ’s occur in the high-altitude (¿ 4RE ) polar cap, where Vsc is usually between 20 and 30 V, and on auroral .eld lines where it is frequently in the 30 –50 V range and occasionally, approximately 1.5% of time, above c 2002 Elsevier Science Ltd. All rights reserved. 50 V. Keywords: Spacecraft potential; Electron density; Plasmapause; Auroral zone; Polar cap
1. Introduction The surface charging of a body (e.g., space vehicle, instrument, astronaut, etc.), immersed in a plasma, is associated with a variety of issues that can sometimes be harmful to the body. On the technical side, the consequences of charging are numerous, including such e=ects as electrostatic .elds, induced currents, optical emissions, and changes in surface, thermal and optical properties (for a more complete list and analysis, see e.g. Hastings and Garrett, 1996). The purpose of this study is to investigate the surface charging of the Polar satellite, using the spacecraft potential measurements of the electric .eld instrument (EFI) gathered during 48 months in 1996 –1999. The electrostatic potential of a satellite can vary over a wide voltage range. In the magnetosphere a conducting spacecraft 4oats for most of the time at a positive potential with respect to the ambient plasma, because the photoelectron 4ux from the body is nearly always higher than the
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ambient electron 4ux to the body. For a positive potential, the surface potential is inversely proportional to the ambient electron density, and their relationship is controlled by the energy distribution of the photoelectrons escaping from the satellite surface (Pedersen, 1995; Laakso and Pedersen, 1998). When moving into the shadow of the Earth, the spacecraft becomes immediately negatively charged (Garrett and Rubin, 1978). For a negative potential, the level of charging is directly proportional to the temperature of the ambient electrons (Laakso et al., 1995), which is usually in the range of kilovolts in the nightside magnetosphere, and then the spacecraft potential can often become several kilovolts negative with respect to the ambient plasma (e.g., Garrett and Rubin, 1978; Mullen et al., 1986). Unfortunately, the negative spacecraft potentials cannot be monitored with the double probe technique. The reason is simple: the satellite potential is measured with respect to a small reference probe that is biased near the potential of the ambient medium. In situations where the satellite turns negative, the biased probe cannot be kept at that potential any more but it becomes even more negative than the spacecraft. The negative surface potentials can be determined from the observations of the electron and ion instruments, because
c 2002 Elsevier Science Ltd. All rights reserved. 1364-6826/02/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 1 2 3 - 2
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the electrons with energies less than the surface potential cannot enter the detector, whereas all ions are accelerated to the detector so that the lowest ion energy equals to the surface potential (Olsen and Purvis, 1983). Thus, the present paper deals with positive charging of the Polar satellite only, that is, normal situations in the magnetosphere where the spacecraft 4oats in the range 0 – 68 V (the signal measured with the Polar EFI experiment saturates when the spacecraft potential is more than approximately +68 V). The contents of the paper is the following: Section 2 provides some information about the double probe technique and the Polar satellite. Section 3 presents the average spacecraft potential of the Polar satellite, using three and half years of observations in 1996 –99; the measurements are binned against the Kp index in order to study the e=ects of geomagnetic activity on the magnitude of the average surface potential. Section 4 displays typical examples of the spacecraft potential variations along the Polar trajectory across the auroral zone, the polar cap region, and the plasmapause. Section 5 summarizes the major .ndings of this study. 2. Double probe technique A double probe antenna consists of two identical conducting electrodes (Maynard, 1998; Pedersen et al., 1998), usually spherical in shape. In order to avoid the e=ects of the charging sheath of a satellite, the sensors are placed far from the satellite; for the magnetospheric satellites, typical distances are 20 –60 m. The double probe antenna monitors two types of variables: (i) the potential di=erence between two probes, which is related to the electric .eld along the antenna, and (ii) the potential di=erence between each probe and the spacecraft, which is approximately the spacecraft potential and which is of our interest in this study. 2.1. Spacecraft potential measurements Fig. 1 presents a sketch of a double probe experiment. The potential di=erence between each probe (V1 and V2 ) and the spacecraft (4oating at Vsc ), that is, GV1s = V1 − Vsc and GV2s = V2 − Vsc , is measured. In a tenuous plasma, when the Debye length is much larger than the size of these bodies (i.e., much larger than 1 m), the sensor and the satellite should 4oat approximately at the same potential. However, a signi.cant potential di=erence exists because of a constant bias current driven from the probes to the satellite; this current is applied for performing accurate electric .eld measurements in tenuous plasmas (Pedersen et al., 1984). The bias current, Ib , is selected at approximately half of the saturation photoelectron current (Iph0 ) of the probe, and is typically in the range 100 –300 nA, and is then meaningful for the probe potential. The bias current is, however, negligible to the satellite potential, because for the
L D
V1
Ib
s/c
Ib
V2
Vs Fig. 1. Con.guration of a double probe experiment, consisting of two spherical probes at the tips of two opposing booms. Both V1 − Vsc and V2 − Vsc are sampled.
satellite Iph0 ∼100; 000 nA and Ie0 ∼1000–100; 000 nA, where Ie0 is the ambient electron current at the ambient plasma potential. Thus, the current balance equation for the probe, which yields the probe potential, is written as Ie +Ib −Iph =0, where Ie is the ambient electron current collected by the probe and Iph is the photoelectron current escaping from the surface; the expressions for these current terms can be found, for instance, in Laakso et al. (1995). In tenuous plasmas (i.e., the density less than a few 10 cm−3 ), Ie is often negligible, and then the probe potential is obtained from Ib − Iph = 0. This equation is independent of the ambient plasma parameters, resulting in a potential of few volts positive (Laakso et al., 1995). On the other hand, the current balance equation for the satellite is written Ie − Iph = 0, which yields the spacecraft potential. The investigation of this equation reveals that the satellite potential depends on the density of the ambient electrons and the temperature of the escaping photoelectrons. The photoelectrons are produced by solar EUV radiation that has two intense regimes and that, therefore, create two photoelectron populations at ∼1 eV and at 5 –15 eV, where the 4ux of the .rst population is approximately an order of magnitude higher than that of the second one (Pedersen, 1995). For potentials below about 7–8 V, the current balance happens via escaping low-energy photoelectrons whereas above that potential, all the low-energy photoelectrons return to the surface and the current balance is caused by most energetic photoelectrons escaping from the surface. Instead, the e=ect of the ambient electron temperature on the spacecraft potential variation is less important (Laakso and Pedersen, 1998), but more complicated than that of the electron density. Notice that density and temperature are not interchangeable variables in the equation for the electron collection of the probe (e.g., Laakso et al., 1995). Thus, the ambient plasma density and GV1s are closely related to each other so that for decreasing density, the 4oating potential must increase in order to have a current balance, which happens by collecting more ambient electrons and returning more photoelectrons to the surface. The determination of the bulk density with the double probe technique has a lot of advantages, and therefore,
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
their relationship has been empirically modeled by several authors (Pedersen, 1995; Escoubet et al., 1997; Scudder et al., 2000). In tenuous plasmas, GV1s and GV2s are large and negative because Vsc is large and positive, and V1 and V2 are close to zero because of the bias current driven to the probes. To eliminate the e=ects of the dc electric .eld on the GV1s and GV2s measurements, the variable used is GV12s = (GV1s + GV2s )=2 = (V1 + V2 )=2 − Vsc . The value of (V1 + V2 )=2 is constant and is in the 1–2 V range as long as Vsc is more than about 10 V. For lower Vsc , (V1 + V2 )=2 decreases towards zero, but remains positive in the magnetosphere for most of the time. The accuracy of the electric .eld measurement is approximately 0:1 mV m−1 , and then GV1s is determined at accuracy of 0:01 V. The precision of the spacecraft potential measurements, however, is somewhat poorer because the probe potential with respect to the ambient plasma potential is known at accuracy of 0:5 V. 2.2. Polar spacecraft and EFI experiment
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The Polar satellite was launched on February 24, 1996, into a 90◦ inclination orbit with a 9RE apogee (initially over the northern hemisphere) and a 1:8RE perigee (initially over the southern hemisphere), and an orbital period of about 18 h. The orbital plane rotates about the Earth with respect to the Sun in 12 months so that all local times are covered in a 6-month period. Polar carries a double probe experiment, EFI, which consists of three antennas that are perpendicular to each other (Harvey et al., 1995). Two antennas, using 60 –65 m wire booms, are in the spin plane, and the third solid-boom antenna is along the spin axis where the sensors are only 6 m from the spacecraft; the spin period of the satellite is approximately 5:5 s. To reduce the e=ects of the spacecraft and boom photoelectrons, the sensors are surrounded by a pair of stubs, which are electrically connected to each other, and a pair of guards, also electrically connected to each other (for details, see Harvey et al., 1995). The quality of short-boom measurements is often not as good as that of the other measurements because of spacecraft charging sheath e=ects (Laakso, 2001). The EFI experiment is designed to measure the di=erential potentials between 0 and ∼68 V. The sampling rate of the instrument is normally 10 or 20 Hz; in a burst mode a much higher sampling frequency can be selected, up to 40 kHz (Harvey et al., 1995). Fig. 2 displays a Polar orbit plotted over a sketch of the Earth’s magnetosphere on the noon-midnight meridian; notice that the Polar orbit does not usually lie on this meridian because the orbit rotates about the Earth so that all MLT sectors are sampled in 6 months. The thick line represents the orbit on May 2–3, 1996 (for the spacecraft potential data of this orbit, see Fig. 3); the numbers indicate the UT times of the satellite positions. The plasmasphere, appearing as a dark area in Fig. 2, is the densest magnetospheric region, with a plasma density of more than 100 cm−3 ; there the
Fig. 2. Sketch of the magnetosphere in the noon-midnight meridian. The solid thick line presents a Polar orbit on May 2–3, 1996, and numbers give the UT times of the satellite’s positions.
satellite 4oats near the ambient plasma potential. The largest positive spacecraft potentials, more than 60 V, are observed on auroral .eld lines which map into the plasma sheet. Fig. 3 shows a Vsc variation along the orbit presented in Fig. 2. Notice that the ambient electron density and Vsc are inversely related to each other so that for increasing Vsc density decreases (Laakso and Pedersen, 1998; Scudder et al., 2000). In the beginning of the interval, the satellite is in the plasmasphere where Vsc is less than 1 V (see also Section 4.1). At the outbound crossing of the plasmapause (PP), Vsc increases by several volts. Near 19 UT, Vsc suddenly exhibits 4uctuations when the satellite encounters the cusp and the dayside boundary layers of the magnetosphere. In the cusp, the plasma conditions are rather similar to those in the magnetosheath, and Vsc reduces close to 3 V. Later, in the high-altitude polar cap, Vsc is between 20 and 30 V (see Section 4.3), because of a low electron density. In the nightside the spacecraft passes through auroral .eld lines (AUR); here Vsc becomes 35 V, but sometimes in auroral cavities Vsc exceeds 60 V (see Section 4.2). At the end of the interval, around 9 UT, the satellite crosses .rst the southern auroral zone and the southern polar cap at ∼1RE . In the polar cap Vsc is not as positive as in the high-altitude
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PS - plasmasphere PP - plasmapause DAY - dayside magnetosphere AUR - northern auroral zone PSH - plasma sheet SH - southern polar cap & auroral zone
Polar EFI May 2–3, 1996 18:00 11:04 19.5 4.8 62.8
20:00 09:03 296.7 7 67.8
22:00 03:47 595.1 8.3 69.7
00:00 00:58 257.4 8.9 70.4
02:00 23:23 100.8 8.8 70.3
04:00 22:38 34.5 8 69.3
AUR 06:00 22:33 11.7 6.4 66.7
08:00 23:00 3.8 3.8 59.1
10:00 10:35 2.4 2.3 48.7
Fig. 3. Spacecraft potential variation along the satellite’s orbit on May 2–3, 1996.
northern polar cap because of a strong altitude dependence of the polar cap density (see Section 4.3). In the low-altitude auroral zone, however, the plasma density can be lower than at high altitudes, and then very positive potentials are encountered, although not in this case. 3. Statistical results For a statistical analysis, we have calculated 1-min mean values of Vsc in order to reduce the size of the database. The observations are binned against the 3-h Kp index in order to investigate the e=ects of geomagnetic activity on the spacecraft potential. The observations from April 1996 to December 1999 are used here. Fig. 4 presents the average spacecraft potential which are mapped to the magnetic equator. The top .gure is for active periods when Kp ¿ 3− and the bottom .gure is for quiet periods when Kp = 0 − 0+ . The color scale, shown in the top, is logarithmic in order to separate di=erent plasma regions more clearly. The two black circles represent the L = 4 and 6 distances. Only measurements from the northern hemisphere are used here in order to avoid problems in mapping. Namely, the electron density is implicitly assumed constant along the magnetic .eld lines, which is not valid in general. However, in this case the satellite is always quite close to the magnetic equator, at least in the inner magnetosphere; for instance, the L = 9 .eld line is always crossed at 40◦ or less of magnetic latitude, and so the density is not expected to change very much between those latitudes and the equator. This is because for incompressible plasma, the density is proportional to the cross section of the 4ux tube which does not change much between 0◦ and 40◦ latitudes. The plasmasphere, appearing in dark blue, expands during quiet intervals; for high Kp , the plasmasphere is compressed and asymmetric due to a strong evolution with Kp (for more
details about the structure and dynamics of the plasmasphere, see e.g., Lemaire and Gringauz, 1998). There is also an obvious asymmetry between the dayside and nightside magnetosphere so that on the dayside the average spacecraft voltage is 7–8 V whereas on the nightside it is 10 –20 V. Fig. 5 shows the average satellite potentials in the magnetosphere on the noon-midnight meridian (left panels) and on the dusk–dawn meridian (right panels). The panels from top to bottom are for three di=erent Kp ranges: Kp = 0 − 1− (top panels), Kp = 1+ − 2 (middle panels), and Kp ¿ 3− (bottom panels), corresponding to magnetically quiet, moderately active, and disturbed periods, respectively. The lines in each panel represent the dipole .eld lines of 65◦, 70◦, 75◦, and 80◦ invariant latitude, or 5.6, 8.5, 14.9 and 32:3 L shell. The color scale shown on the top of the .gure is logarithmic and ranges from 1 to 20 V. The selected three Kp ranges have approximately an equal amount of data (the number of data points are approximately 424,000, 535,000, and 539,000 for Kp =0−1− , Kp =1+ −2, and Kp ¿ 3− , respectively). Since the relative number of active days is small, we have binned all of the data gathered for Kp ¿ 3− together. Most bins contain tens of data points and each data point is an average of approximately 200GVps measurements so that the statistics of Fig. 5 can be considered satisfactory. The plasmasphere, a dark blue around the Earth in all panels, evolves strongly with Kp ; particularly strong evolution occurs in the midnight sector, where the plasmasphere becomes quite small and the plasmapause is surprisingly sharp. Remember that the boundaries are usually smoothened in average presentations particularly if the boundary moves over a large spatial range. In the left panels the cusp appears as a region of low spacecraft potentials at 80 –82◦ of invariant latitude near noon, and it tends to move equatorward with increasing Kp . The cusp appears less distinctly during disturbed intervals,
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
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the average Vsc is only 2–3 V (for more details, see Laakso et al., 2002).
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4. Spacecraft potential measurements in speci c magnetospheric regions
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4.1. Plasmapause crossing
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The plasmapause, the outer boundary of the plasmasphere, appears as a sharp density decline so that the number density of ambient electrons is more than 100 in cm3 in the plasmasphere and ¡ 10 in cm3 outside the plasmasphere (Lemaire and Gringauz, 1998). Therefore a plasmapause crossing is also observed as a change of the spacecraft potential, as is seen in Fig. 3. The distance and steepness of the plasmapause depend on many parameters, such as local time and geomagnetic activity (Carpenter and Anderson, 1992). Fig. 6 presents a sequence of the plasmapause crossings near local midnight on April 26 –29, 1998, after a magnetic storm; in each case Vsc is approximately 1 V in the outer plasmasphere, and 5 –8 V in the magnetosphere. On the .rst day the Kp index is above 4, and the plasmasphere is compressed to L = 3:3. On April 27, Kp decreases below 4, and the plasmapause moves to L=4. On the following days, the magnetosphere becomes increasingly quiet, and the plasmapause keeps moving outward. In all cases, the plasmapause transitions look similar, but in the trough Vsc can vary quite di=erently from day to day, because there is never two similar days in the magnetosphere. At the inner edge of the plasmapause the value of Vsc depends on how close to the Earth the plasmapause is located: the farther from the Earth the plasmapause exists, the higher Vsc in the outer plasmasphere is observed. This is because the electron density in the plasmasphere decreases with distance from the Earth (for density models of the plasmasphere, see Carpenter and Anderson, 1992). Also, it seems that the closer to the Earth the plasmapause is located, the steeper Vsc gradient appears.
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4.2. Auroral zone −10
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Fig. 4. Average spacecraft potential in the equatorial plane of the magnetosphere, based on the EFI observations of the Polar satellite in 1996 April–1999 March, for Kp ¡ 1 (bottom panel) and for Kp ¿ 3 (top panel).
likely due to a large motion of the cusp position during disturbed conditions, which signi.cantly spreads out the cusp signatures in the averages. The polar cap region, which maps into the lobes of the magnetotail, appears as a dark red in Fig. 5; in this region the average Vsc is above +20 V. It is obvious forthwith that a signi.cant altitude dependence exists this voltage, because at 1RE altitude (southern polar cap)
On auroral .eld lines, very low densities are frequently observed, during which the spacecraft can be charged very positively, above +50 V. On October 26, 1997, a magnetic storm begins at 15 UT, and half an hour later, the spacecraft crosses the southern auroral oval at an altitude of 1:2RE ; the Vsc observations are shown in the bottom panel of Fig. 7. Two auroral cavities are detected, during which Vsc exceeds 60 V, corresponding to densities of about 0:01 cm−3 (Scudder et al., 2000). One should notice that these are spin-averaged data. An investigation of instantaneous data reveals that the signal becomes occasionally saturated during these events; the saturation happens when the spacecraft potential becomes more than approximately +68 V. Three hours afterwards, the spacecraft is over the northern auroral oval at high altitudes of 5 –6RE . The observations,
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Fig. 5. Average spacecraft potential in two meridional planes: left panels are for the noon-midnight meridian, and right panels are for the dusk–dawn meridian. The panels from top to bottom are for three di=erent Kp levels: Kp = 0 − 1− , Kp = 1+ − 2 and Kp ¿ 3− . The lines in each panel represent dipole magnetic .eld lines at invariant latitudes of 65◦, 70◦, 75◦, and 80◦.
shown in the top panel of Fig. 7, indicate that the auroral cavities are not as tenuous as they are at lower altitudes, and therefore Vsc is close to 40 V only. At high altitudes, the spacecraft velocity is lower and the auroral zone is wider, and therefore the spacecraft remains under the in4uence of auroral environment for a longer time than at low altitudes. Notice that at the end of the interval, near 19:40 UT, the spacecraft moves into the polar cap region which is observed as a steep increase of Vsc . We have studied in more detail the occurrence of large positive potentials on Polar. The two panels of Fig. 8a
shows the probabilities of observing positive potentials of +52 V or more for the northern and southern hemisphere. It turns out that the number of events increases quite rapidly for the potentials less than that potential, and therefore they are ignored here. Notice that only spin-averaged data are used here, and the signal may be partially saturated when the average potential becomes near or more than +50 V. The pro.les for two hemispheres are surprisingly similar. There are approximately 30 events for both hemispheres, and the probability of observing a high-voltage event is 2% for both hemispheres. Fig. 8b shows the
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
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October 26, 1997 MLT = 23:45-00:15 altitude = 5.6-6.4 Re L shell = 16-10
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Fig. 7. Variation of the spacecraft potential across the auroral zone at 1RE altitude (bottom panel) and at 5 –6RE altitude (top panel) on October 26, 1997.
MLT vs. invariant latitude distribution of the events for the northern and southern hemisphere. Over the southern hemisphere they seem to occur near the typical location of the auroral oval, and thus, many of the events are likely caused by the satellite’s traverse through low-density auroral cavities. Over the northern hemisphere the events appear both in the auroral zone and in the polar cap region.
4.3. Polar cap Fig. 9 shows the monthly averages of the spacecraft potential in the polar cap. The top .gure is for the high-altitude northern polar cap, and the bottom .gure for the low-altitude southern polar cap. The southern observations are made at ∼1RE altitude whereas the northern observations are collected at 4 –8RE altitude. Di=erent geomagnetic conditions
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Fig. 8. (a) Histogram of high positive potentials (more than +50 V with 2 V bins) for the northern (upper panel) and southern (bottom panel) hemispheres. (b) Distribution of the high positive potential events: x- and y-axis represent the magnetic local time and the invariant latitude of the satellite, respectively.
are separated: solid lines are for quiet (Kp = 0 − 1) periods, dotted lines are for weakly disturbed (Kp = 1+ − 2) periods, and dashed lines are for disturbed (Kp ¿ 2+ ) periods. The observations exhibit strong annual variations, caused by corresponding changes of the ambient plasma density in the polar cap region. At 1RE altitude, largest Vsc ’s (5 –7 V) are observed from May to August, corresponding to a winter period over the southern polar region. At that time, the southern polar atmosphere is only weakly ionized, and the polar cap density remains low. On the other hand, from November to February, solar UV radiation continuously ionizes the southern polar atmosphere, causing a high polar cap density which results in low spacecraft potentials. In the high-altitude polar cap, a similar Vsc variation is observed. The voltage range, however, is totally di=erent because of a di=erent density regime. At 4 –8RE altitudes the ambient density is only a fraction of that at 1RE altitude. Also geomagnetic e=ects are signi.cant. The dashed curves show that for high Kp the polar cap density can signi.cantly increase, causing a lower Vsc . This e=ect is particularly clear in winter when the background density is otherwise low, whereas in summer the geomagnetic disturbances do not signi.cantly increase the polar cap density because the density is high all the time. At high altitudes, the e=ects of the geomagnetic activity on Vsc is always less signi.cant. 5. Summary An important objective of the double probe experiment is the monitoring of the spacecraft potential. For that a proper
bias current has to be selected in order to keep the probes near the ambient plasma potential. Unfortunately, only positive spacecraft potentials can be measured with this technique, but fortunately the satellites 4oat normally at positive voltages in the magnetosphere. There are three cases where the satellite can become negatively charged: in the inner plasmasphere, in the shadow of the Earth, and during an intense electron injection. The latter two events can be distinguished in the data as they cause an immediate saturation of the signal, similarly to the occurrence of a low-density plasma region. However, with the EFI data only, one cannot deduce surely the cause of saturation but particle data need to be examined. Fortunately, In this study we have investigated the average spacecraft potential in the magnetosphere, using the observations of the EFI experiment on the Polar satellite in 1996 –1999. For most of the time, however, the satellite 4oats at a positive potential, typically between 0 and 50 V so that in the plasmasphere near the plasmapause Vsc is in the range of 0 –1 V, at the plasmapause, it exhibits a steep increase by 3–5 V, and outside the plasmasphere, Vsc is usually more than 5 V. In the dayside magnetosphere, the average Vsc is usually 7–8 V, whereas on the nightside it is in the 10 –20 V range. In the cusp, however, Vsc is only about 3 V; similar spacecraft potentials are also observed in the magnetosheath. Highest Vsc ’s occur in the high-altitude (¿ 4RE altitude) polar cap, where Vsc is usually between 20 and 30 V, and on auroral .eld lines where it is frequently at 30 –50 V range. A detailed investigation reveals that during approximately 1.5% of the passes the spacecraft observes potentials that are above +50 V. There are also several
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
Kp 0 –1 + 1 –2 + 2 –9
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8 6 4 2 07 1996
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Fig. 9. Monthly averages of the spacecraft potential in the polar cap region at low altitudes (top panel) and at high altitudes (top panel). A clear seasonal variation appears in these plots because of a strong annual density variation of the polar cap density.
events where the signal becomes saturated, which happens when Vsc exceeds approximately +68 V. The payload of the Polar satellite represents the state of the art, and therefore the electron and ion observations along the spacecraft potential measurements should be used in future for the validation and testing of the charging codes for the high-altitude spacecraft. Acknowledgements This work is supported under NASA grant NAG5-3182.
References Carpenter, D.L., Anderson, R.R., 1992. An ISEE=Whistler model of equatorial electron density in the magnetosphere. Journal of Geophysical Research 97, 1097–1108. Escoubet, C.P., Pedersen, A., Schmidt, R., Lindqvist, P.A., 1997. Density in the magnetosphere inferred from ISEE-1 spacecraft potential. Journal of Geophysical Research 102, 17,595–17,609.
Garrett, H.B., Rubin, A.G., 1978. Spacecraft charging at geosynchronous orbit-generalized solution for eclipse passage. Geophysical Research Letters 5, 865–868. Harvey, P., Mozer, F.S., Pankow, D., Wygant, J., Maynard, N.C., Singer, H., Sullivan, W., Anderson, P.B., Pfa=, R., Aggson, T., Pedersen, A., Falthammar, C.-G., Tanskanen, P., 1995. The electric .eld instrument on the Polar satellite. Space Science Reviews 71, 583–596. Hastings, D., Garrett, H., 1996. Spacecraft-Environment Interactions. Cambridge University Press, Cambridge. Laakso, H., Pedersen, A., 1998. Ambient electron density derived from di=erential potential measurements. In: Pfa=, R.F., Borovsky, J.E., Young, D.T. (Eds.), Measurement Techniques in Space Plasmas—Particles, American Geological Union Monograph Series, Vol. 102. American Geological Union, Washington, DC, pp. 49–54. Laakso, H., Aggson, T., Pfa=, R., 1995. Plasma gradient e=ects on double probe measurements in the magnetosphere. Annals of Geophysics 13, 130–146. Laakso, H., Pfa=, R., Janhunen, P., 2002. Polar observations of electron density distribution in the Earth’s magnetosphere, Annales Geophysicae, submitted for publication. Lemaire, J.F., Gringauz, K.I., 1998. The Earth’s Plasmasphere. Cambridge University Press, Cambridge.
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Maynard, N.C., 1998. Electric .eld measurements in moderate to high density space plasmas with passive double probes. In: Pfa=, R.F., Borovsky, J.E., Young, D.T. (Eds.), Measurement Techniques in Space Plasmas—Fields, American Geological Union Monograph Series, Vol. 103. American Geological Union, Washington, DC, pp. 13–27. Mullen, E.G., Gussenhoven, M.S., Hardy, D.A., Aggson, T.A., Ledley, B.G., Whipple, E., 1986. SCATHA survey of high-level spacecraft charging in sunlight. Journal of Geophysical Research 91, 1474–1490. Olsen, R.C., Purvis, C.K., 1983. Observations of charging dynamics. Journal of Geophysical Research 88, 5657–5667. Pedersen, A., 1995. Solar wind and magnetosphere plasma diagnostics by spacecraft electrostatic potential measurements. Annals of Geophysics 13, 118–129. Pedersen, A., Cattell, C.A., F˝althammar, C.-G., Formisano, V., Lindqvist, P.-A., Mozer, F., Torbert, R., 1984. Quasistatic
electric .eld measurements with spherical double probes on the GEOS and ISEE satellites. Space Science Reviews 37, 269–312. Pedersen, A., Mozer, F., Gustafsson, G., 1998. Electric .eld measurements in a tenuous plasma with spherical probes. In: Pfa=, R.F., Borovsky, J.E., Young, D.T. (Eds.), Measurement Techniques in Space Plasmas—Fields, American Geological Union Monogram Series, Vol. 103. American Geological Union, Washington, DC, pp. 1–12. Laakso, H., 2001. Polar monitoring of the spacecraft potential in the magnetosphere with a double probe instrument. In Proceedings of the Sixth Spacecraft Charging Technology Conference, CD-rom in ESA SP-476, ESTEC, Noordwijk. Scudder, J.D., Cao, X., Mozer, F.S., 2000. The photoemission current—spacecraft potential relation: key to routine, quantitative low-energy plasma measurements. Journal of Geophysical Research 105, 21,281–21,294.
www.elsevier.com/locate/jastp
Variation of the spacecraft potential in the magnetosphere Harri Laakso ∗ ESA, ESTEC Space Science Department, Code SCI-SO, Keplerlaan 1, Postbus 299, 2201 AG Noordwijk, Netherlands
Abstract This paper deals with the surface charging of the Polar satellite, using the spacecraft potential measurements of the electric .eld instrument (EFI) gathered during 48 months in 1996 –1999. For most of the time, the satellite 4oats at a positive potential Vsc in the magnetosphere because of a high 4ux of photoelectrons emitted from the satellite surface; typically Vsc varies between 0 and 50 V so that it is 0 –1 V in the plasmasphere, at the plasmapause it exhibits a steep increase by 3–5 V, and outside the plasmasphere Vsc is usually more than 5 V. On the dayside magnetosphere Vsc is 7–8 V in average, whereas on the nightside it lies in the 10 –20 V range. In the cusp, however, Vsc is only about 3 V, similar to spacecraft potentials observed in the magnetosheath. Highest Vsc ’s occur in the high-altitude (¿ 4RE ) polar cap, where Vsc is usually between 20 and 30 V, and on auroral .eld lines where it is frequently in the 30 –50 V range and occasionally, approximately 1.5% of time, above c 2002 Elsevier Science Ltd. All rights reserved. 50 V. Keywords: Spacecraft potential; Electron density; Plasmapause; Auroral zone; Polar cap
1. Introduction The surface charging of a body (e.g., space vehicle, instrument, astronaut, etc.), immersed in a plasma, is associated with a variety of issues that can sometimes be harmful to the body. On the technical side, the consequences of charging are numerous, including such e=ects as electrostatic .elds, induced currents, optical emissions, and changes in surface, thermal and optical properties (for a more complete list and analysis, see e.g. Hastings and Garrett, 1996). The purpose of this study is to investigate the surface charging of the Polar satellite, using the spacecraft potential measurements of the electric .eld instrument (EFI) gathered during 48 months in 1996 –1999. The electrostatic potential of a satellite can vary over a wide voltage range. In the magnetosphere a conducting spacecraft 4oats for most of the time at a positive potential with respect to the ambient plasma, because the photoelectron 4ux from the body is nearly always higher than the
∗
Tel.: +31-71-565-3319; fax: +31-71-565-4697. E-mail address: [email protected] (H. Laakso).
ambient electron 4ux to the body. For a positive potential, the surface potential is inversely proportional to the ambient electron density, and their relationship is controlled by the energy distribution of the photoelectrons escaping from the satellite surface (Pedersen, 1995; Laakso and Pedersen, 1998). When moving into the shadow of the Earth, the spacecraft becomes immediately negatively charged (Garrett and Rubin, 1978). For a negative potential, the level of charging is directly proportional to the temperature of the ambient electrons (Laakso et al., 1995), which is usually in the range of kilovolts in the nightside magnetosphere, and then the spacecraft potential can often become several kilovolts negative with respect to the ambient plasma (e.g., Garrett and Rubin, 1978; Mullen et al., 1986). Unfortunately, the negative spacecraft potentials cannot be monitored with the double probe technique. The reason is simple: the satellite potential is measured with respect to a small reference probe that is biased near the potential of the ambient medium. In situations where the satellite turns negative, the biased probe cannot be kept at that potential any more but it becomes even more negative than the spacecraft. The negative surface potentials can be determined from the observations of the electron and ion instruments, because
c 2002 Elsevier Science Ltd. All rights reserved. 1364-6826/02/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 1 2 3 - 2
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the electrons with energies less than the surface potential cannot enter the detector, whereas all ions are accelerated to the detector so that the lowest ion energy equals to the surface potential (Olsen and Purvis, 1983). Thus, the present paper deals with positive charging of the Polar satellite only, that is, normal situations in the magnetosphere where the spacecraft 4oats in the range 0 – 68 V (the signal measured with the Polar EFI experiment saturates when the spacecraft potential is more than approximately +68 V). The contents of the paper is the following: Section 2 provides some information about the double probe technique and the Polar satellite. Section 3 presents the average spacecraft potential of the Polar satellite, using three and half years of observations in 1996 –99; the measurements are binned against the Kp index in order to study the e=ects of geomagnetic activity on the magnitude of the average surface potential. Section 4 displays typical examples of the spacecraft potential variations along the Polar trajectory across the auroral zone, the polar cap region, and the plasmapause. Section 5 summarizes the major .ndings of this study. 2. Double probe technique A double probe antenna consists of two identical conducting electrodes (Maynard, 1998; Pedersen et al., 1998), usually spherical in shape. In order to avoid the e=ects of the charging sheath of a satellite, the sensors are placed far from the satellite; for the magnetospheric satellites, typical distances are 20 –60 m. The double probe antenna monitors two types of variables: (i) the potential di=erence between two probes, which is related to the electric .eld along the antenna, and (ii) the potential di=erence between each probe and the spacecraft, which is approximately the spacecraft potential and which is of our interest in this study. 2.1. Spacecraft potential measurements Fig. 1 presents a sketch of a double probe experiment. The potential di=erence between each probe (V1 and V2 ) and the spacecraft (4oating at Vsc ), that is, GV1s = V1 − Vsc and GV2s = V2 − Vsc , is measured. In a tenuous plasma, when the Debye length is much larger than the size of these bodies (i.e., much larger than 1 m), the sensor and the satellite should 4oat approximately at the same potential. However, a signi.cant potential di=erence exists because of a constant bias current driven from the probes to the satellite; this current is applied for performing accurate electric .eld measurements in tenuous plasmas (Pedersen et al., 1984). The bias current, Ib , is selected at approximately half of the saturation photoelectron current (Iph0 ) of the probe, and is typically in the range 100 –300 nA, and is then meaningful for the probe potential. The bias current is, however, negligible to the satellite potential, because for the
L D
V1
Ib
s/c
Ib
V2
Vs Fig. 1. Con.guration of a double probe experiment, consisting of two spherical probes at the tips of two opposing booms. Both V1 − Vsc and V2 − Vsc are sampled.
satellite Iph0 ∼100; 000 nA and Ie0 ∼1000–100; 000 nA, where Ie0 is the ambient electron current at the ambient plasma potential. Thus, the current balance equation for the probe, which yields the probe potential, is written as Ie +Ib −Iph =0, where Ie is the ambient electron current collected by the probe and Iph is the photoelectron current escaping from the surface; the expressions for these current terms can be found, for instance, in Laakso et al. (1995). In tenuous plasmas (i.e., the density less than a few 10 cm−3 ), Ie is often negligible, and then the probe potential is obtained from Ib − Iph = 0. This equation is independent of the ambient plasma parameters, resulting in a potential of few volts positive (Laakso et al., 1995). On the other hand, the current balance equation for the satellite is written Ie − Iph = 0, which yields the spacecraft potential. The investigation of this equation reveals that the satellite potential depends on the density of the ambient electrons and the temperature of the escaping photoelectrons. The photoelectrons are produced by solar EUV radiation that has two intense regimes and that, therefore, create two photoelectron populations at ∼1 eV and at 5 –15 eV, where the 4ux of the .rst population is approximately an order of magnitude higher than that of the second one (Pedersen, 1995). For potentials below about 7–8 V, the current balance happens via escaping low-energy photoelectrons whereas above that potential, all the low-energy photoelectrons return to the surface and the current balance is caused by most energetic photoelectrons escaping from the surface. Instead, the e=ect of the ambient electron temperature on the spacecraft potential variation is less important (Laakso and Pedersen, 1998), but more complicated than that of the electron density. Notice that density and temperature are not interchangeable variables in the equation for the electron collection of the probe (e.g., Laakso et al., 1995). Thus, the ambient plasma density and GV1s are closely related to each other so that for decreasing density, the 4oating potential must increase in order to have a current balance, which happens by collecting more ambient electrons and returning more photoelectrons to the surface. The determination of the bulk density with the double probe technique has a lot of advantages, and therefore,
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
their relationship has been empirically modeled by several authors (Pedersen, 1995; Escoubet et al., 1997; Scudder et al., 2000). In tenuous plasmas, GV1s and GV2s are large and negative because Vsc is large and positive, and V1 and V2 are close to zero because of the bias current driven to the probes. To eliminate the e=ects of the dc electric .eld on the GV1s and GV2s measurements, the variable used is GV12s = (GV1s + GV2s )=2 = (V1 + V2 )=2 − Vsc . The value of (V1 + V2 )=2 is constant and is in the 1–2 V range as long as Vsc is more than about 10 V. For lower Vsc , (V1 + V2 )=2 decreases towards zero, but remains positive in the magnetosphere for most of the time. The accuracy of the electric .eld measurement is approximately 0:1 mV m−1 , and then GV1s is determined at accuracy of 0:01 V. The precision of the spacecraft potential measurements, however, is somewhat poorer because the probe potential with respect to the ambient plasma potential is known at accuracy of 0:5 V. 2.2. Polar spacecraft and EFI experiment
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The Polar satellite was launched on February 24, 1996, into a 90◦ inclination orbit with a 9RE apogee (initially over the northern hemisphere) and a 1:8RE perigee (initially over the southern hemisphere), and an orbital period of about 18 h. The orbital plane rotates about the Earth with respect to the Sun in 12 months so that all local times are covered in a 6-month period. Polar carries a double probe experiment, EFI, which consists of three antennas that are perpendicular to each other (Harvey et al., 1995). Two antennas, using 60 –65 m wire booms, are in the spin plane, and the third solid-boom antenna is along the spin axis where the sensors are only 6 m from the spacecraft; the spin period of the satellite is approximately 5:5 s. To reduce the e=ects of the spacecraft and boom photoelectrons, the sensors are surrounded by a pair of stubs, which are electrically connected to each other, and a pair of guards, also electrically connected to each other (for details, see Harvey et al., 1995). The quality of short-boom measurements is often not as good as that of the other measurements because of spacecraft charging sheath e=ects (Laakso, 2001). The EFI experiment is designed to measure the di=erential potentials between 0 and ∼68 V. The sampling rate of the instrument is normally 10 or 20 Hz; in a burst mode a much higher sampling frequency can be selected, up to 40 kHz (Harvey et al., 1995). Fig. 2 displays a Polar orbit plotted over a sketch of the Earth’s magnetosphere on the noon-midnight meridian; notice that the Polar orbit does not usually lie on this meridian because the orbit rotates about the Earth so that all MLT sectors are sampled in 6 months. The thick line represents the orbit on May 2–3, 1996 (for the spacecraft potential data of this orbit, see Fig. 3); the numbers indicate the UT times of the satellite positions. The plasmasphere, appearing as a dark area in Fig. 2, is the densest magnetospheric region, with a plasma density of more than 100 cm−3 ; there the
Fig. 2. Sketch of the magnetosphere in the noon-midnight meridian. The solid thick line presents a Polar orbit on May 2–3, 1996, and numbers give the UT times of the satellite’s positions.
satellite 4oats near the ambient plasma potential. The largest positive spacecraft potentials, more than 60 V, are observed on auroral .eld lines which map into the plasma sheet. Fig. 3 shows a Vsc variation along the orbit presented in Fig. 2. Notice that the ambient electron density and Vsc are inversely related to each other so that for increasing Vsc density decreases (Laakso and Pedersen, 1998; Scudder et al., 2000). In the beginning of the interval, the satellite is in the plasmasphere where Vsc is less than 1 V (see also Section 4.1). At the outbound crossing of the plasmapause (PP), Vsc increases by several volts. Near 19 UT, Vsc suddenly exhibits 4uctuations when the satellite encounters the cusp and the dayside boundary layers of the magnetosphere. In the cusp, the plasma conditions are rather similar to those in the magnetosheath, and Vsc reduces close to 3 V. Later, in the high-altitude polar cap, Vsc is between 20 and 30 V (see Section 4.3), because of a low electron density. In the nightside the spacecraft passes through auroral .eld lines (AUR); here Vsc becomes 35 V, but sometimes in auroral cavities Vsc exceeds 60 V (see Section 4.2). At the end of the interval, around 9 UT, the satellite crosses .rst the southern auroral zone and the southern polar cap at ∼1RE . In the polar cap Vsc is not as positive as in the high-altitude
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PS
PP
DAY
PP
northern polar cap
cusp
PSH
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25 20 15 10 5 0 UT MLT L shell Alt (Re) Inv Lat (˚)
PS - plasmasphere PP - plasmapause DAY - dayside magnetosphere AUR - northern auroral zone PSH - plasma sheet SH - southern polar cap & auroral zone
Polar EFI May 2–3, 1996 18:00 11:04 19.5 4.8 62.8
20:00 09:03 296.7 7 67.8
22:00 03:47 595.1 8.3 69.7
00:00 00:58 257.4 8.9 70.4
02:00 23:23 100.8 8.8 70.3
04:00 22:38 34.5 8 69.3
AUR 06:00 22:33 11.7 6.4 66.7
08:00 23:00 3.8 3.8 59.1
10:00 10:35 2.4 2.3 48.7
Fig. 3. Spacecraft potential variation along the satellite’s orbit on May 2–3, 1996.
northern polar cap because of a strong altitude dependence of the polar cap density (see Section 4.3). In the low-altitude auroral zone, however, the plasma density can be lower than at high altitudes, and then very positive potentials are encountered, although not in this case. 3. Statistical results For a statistical analysis, we have calculated 1-min mean values of Vsc in order to reduce the size of the database. The observations are binned against the 3-h Kp index in order to investigate the e=ects of geomagnetic activity on the spacecraft potential. The observations from April 1996 to December 1999 are used here. Fig. 4 presents the average spacecraft potential which are mapped to the magnetic equator. The top .gure is for active periods when Kp ¿ 3− and the bottom .gure is for quiet periods when Kp = 0 − 0+ . The color scale, shown in the top, is logarithmic in order to separate di=erent plasma regions more clearly. The two black circles represent the L = 4 and 6 distances. Only measurements from the northern hemisphere are used here in order to avoid problems in mapping. Namely, the electron density is implicitly assumed constant along the magnetic .eld lines, which is not valid in general. However, in this case the satellite is always quite close to the magnetic equator, at least in the inner magnetosphere; for instance, the L = 9 .eld line is always crossed at 40◦ or less of magnetic latitude, and so the density is not expected to change very much between those latitudes and the equator. This is because for incompressible plasma, the density is proportional to the cross section of the 4ux tube which does not change much between 0◦ and 40◦ latitudes. The plasmasphere, appearing in dark blue, expands during quiet intervals; for high Kp , the plasmasphere is compressed and asymmetric due to a strong evolution with Kp (for more
details about the structure and dynamics of the plasmasphere, see e.g., Lemaire and Gringauz, 1998). There is also an obvious asymmetry between the dayside and nightside magnetosphere so that on the dayside the average spacecraft voltage is 7–8 V whereas on the nightside it is 10 –20 V. Fig. 5 shows the average satellite potentials in the magnetosphere on the noon-midnight meridian (left panels) and on the dusk–dawn meridian (right panels). The panels from top to bottom are for three di=erent Kp ranges: Kp = 0 − 1− (top panels), Kp = 1+ − 2 (middle panels), and Kp ¿ 3− (bottom panels), corresponding to magnetically quiet, moderately active, and disturbed periods, respectively. The lines in each panel represent the dipole .eld lines of 65◦, 70◦, 75◦, and 80◦ invariant latitude, or 5.6, 8.5, 14.9 and 32:3 L shell. The color scale shown on the top of the .gure is logarithmic and ranges from 1 to 20 V. The selected three Kp ranges have approximately an equal amount of data (the number of data points are approximately 424,000, 535,000, and 539,000 for Kp =0−1− , Kp =1+ −2, and Kp ¿ 3− , respectively). Since the relative number of active days is small, we have binned all of the data gathered for Kp ¿ 3− together. Most bins contain tens of data points and each data point is an average of approximately 200GVps measurements so that the statistics of Fig. 5 can be considered satisfactory. The plasmasphere, a dark blue around the Earth in all panels, evolves strongly with Kp ; particularly strong evolution occurs in the midnight sector, where the plasmasphere becomes quite small and the plasmapause is surprisingly sharp. Remember that the boundaries are usually smoothened in average presentations particularly if the boundary moves over a large spatial range. In the left panels the cusp appears as a region of low spacecraft potentials at 80 –82◦ of invariant latitude near noon, and it tends to move equatorward with increasing Kp . The cusp appears less distinctly during disturbed intervals,
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
1
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the average Vsc is only 2–3 V (for more details, see Laakso et al., 2002).
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4. Spacecraft potential measurements in speci c magnetospheric regions
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4.1. Plasmapause crossing
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The plasmapause, the outer boundary of the plasmasphere, appears as a sharp density decline so that the number density of ambient electrons is more than 100 in cm3 in the plasmasphere and ¡ 10 in cm3 outside the plasmasphere (Lemaire and Gringauz, 1998). Therefore a plasmapause crossing is also observed as a change of the spacecraft potential, as is seen in Fig. 3. The distance and steepness of the plasmapause depend on many parameters, such as local time and geomagnetic activity (Carpenter and Anderson, 1992). Fig. 6 presents a sequence of the plasmapause crossings near local midnight on April 26 –29, 1998, after a magnetic storm; in each case Vsc is approximately 1 V in the outer plasmasphere, and 5 –8 V in the magnetosphere. On the .rst day the Kp index is above 4, and the plasmasphere is compressed to L = 3:3. On April 27, Kp decreases below 4, and the plasmapause moves to L=4. On the following days, the magnetosphere becomes increasingly quiet, and the plasmapause keeps moving outward. In all cases, the plasmapause transitions look similar, but in the trough Vsc can vary quite di=erently from day to day, because there is never two similar days in the magnetosphere. At the inner edge of the plasmapause the value of Vsc depends on how close to the Earth the plasmapause is located: the farther from the Earth the plasmapause exists, the higher Vsc in the outer plasmasphere is observed. This is because the electron density in the plasmasphere decreases with distance from the Earth (for density models of the plasmasphere, see Carpenter and Anderson, 1992). Also, it seems that the closer to the Earth the plasmapause is located, the steeper Vsc gradient appears.
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4.2. Auroral zone −10
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Fig. 4. Average spacecraft potential in the equatorial plane of the magnetosphere, based on the EFI observations of the Polar satellite in 1996 April–1999 March, for Kp ¡ 1 (bottom panel) and for Kp ¿ 3 (top panel).
likely due to a large motion of the cusp position during disturbed conditions, which signi.cantly spreads out the cusp signatures in the averages. The polar cap region, which maps into the lobes of the magnetotail, appears as a dark red in Fig. 5; in this region the average Vsc is above +20 V. It is obvious forthwith that a signi.cant altitude dependence exists this voltage, because at 1RE altitude (southern polar cap)
On auroral .eld lines, very low densities are frequently observed, during which the spacecraft can be charged very positively, above +50 V. On October 26, 1997, a magnetic storm begins at 15 UT, and half an hour later, the spacecraft crosses the southern auroral oval at an altitude of 1:2RE ; the Vsc observations are shown in the bottom panel of Fig. 7. Two auroral cavities are detected, during which Vsc exceeds 60 V, corresponding to densities of about 0:01 cm−3 (Scudder et al., 2000). One should notice that these are spin-averaged data. An investigation of instantaneous data reveals that the signal becomes occasionally saturated during these events; the saturation happens when the spacecraft potential becomes more than approximately +68 V. Three hours afterwards, the spacecraft is over the northern auroral oval at high altitudes of 5 –6RE . The observations,
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1
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Fig. 5. Average spacecraft potential in two meridional planes: left panels are for the noon-midnight meridian, and right panels are for the dusk–dawn meridian. The panels from top to bottom are for three di=erent Kp levels: Kp = 0 − 1− , Kp = 1+ − 2 and Kp ¿ 3− . The lines in each panel represent dipole magnetic .eld lines at invariant latitudes of 65◦, 70◦, 75◦, and 80◦.
shown in the top panel of Fig. 7, indicate that the auroral cavities are not as tenuous as they are at lower altitudes, and therefore Vsc is close to 40 V only. At high altitudes, the spacecraft velocity is lower and the auroral zone is wider, and therefore the spacecraft remains under the in4uence of auroral environment for a longer time than at low altitudes. Notice that at the end of the interval, near 19:40 UT, the spacecraft moves into the polar cap region which is observed as a steep increase of Vsc . We have studied in more detail the occurrence of large positive potentials on Polar. The two panels of Fig. 8a
shows the probabilities of observing positive potentials of +52 V or more for the northern and southern hemisphere. It turns out that the number of events increases quite rapidly for the potentials less than that potential, and therefore they are ignored here. Notice that only spin-averaged data are used here, and the signal may be partially saturated when the average potential becomes near or more than +50 V. The pro.les for two hemispheres are surprisingly similar. There are approximately 30 events for both hemispheres, and the probability of observing a high-voltage event is 2% for both hemispheres. Fig. 8b shows the
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
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April 26, 1998, ~15 UT April 27, 1998, ~9 UT April 28, 1998, ~3 UT April 28, 1998, ~21 UT April 29, 1998, ~15 UT
6 4 2 0 3.5
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October 26, 1997 MLT = 23:45-00:15 altitude = 5.6-6.4 Re L shell = 16-10
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Fig. 7. Variation of the spacecraft potential across the auroral zone at 1RE altitude (bottom panel) and at 5 –6RE altitude (top panel) on October 26, 1997.
MLT vs. invariant latitude distribution of the events for the northern and southern hemisphere. Over the southern hemisphere they seem to occur near the typical location of the auroral oval, and thus, many of the events are likely caused by the satellite’s traverse through low-density auroral cavities. Over the northern hemisphere the events appear both in the auroral zone and in the polar cap region.
4.3. Polar cap Fig. 9 shows the monthly averages of the spacecraft potential in the polar cap. The top .gure is for the high-altitude northern polar cap, and the bottom .gure for the low-altitude southern polar cap. The southern observations are made at ∼1RE altitude whereas the northern observations are collected at 4 –8RE altitude. Di=erent geomagnetic conditions
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MLT
Fig. 8. (a) Histogram of high positive potentials (more than +50 V with 2 V bins) for the northern (upper panel) and southern (bottom panel) hemispheres. (b) Distribution of the high positive potential events: x- and y-axis represent the magnetic local time and the invariant latitude of the satellite, respectively.
are separated: solid lines are for quiet (Kp = 0 − 1) periods, dotted lines are for weakly disturbed (Kp = 1+ − 2) periods, and dashed lines are for disturbed (Kp ¿ 2+ ) periods. The observations exhibit strong annual variations, caused by corresponding changes of the ambient plasma density in the polar cap region. At 1RE altitude, largest Vsc ’s (5 –7 V) are observed from May to August, corresponding to a winter period over the southern polar region. At that time, the southern polar atmosphere is only weakly ionized, and the polar cap density remains low. On the other hand, from November to February, solar UV radiation continuously ionizes the southern polar atmosphere, causing a high polar cap density which results in low spacecraft potentials. In the high-altitude polar cap, a similar Vsc variation is observed. The voltage range, however, is totally di=erent because of a di=erent density regime. At 4 –8RE altitudes the ambient density is only a fraction of that at 1RE altitude. Also geomagnetic e=ects are signi.cant. The dashed curves show that for high Kp the polar cap density can signi.cantly increase, causing a lower Vsc . This e=ect is particularly clear in winter when the background density is otherwise low, whereas in summer the geomagnetic disturbances do not signi.cantly increase the polar cap density because the density is high all the time. At high altitudes, the e=ects of the geomagnetic activity on Vsc is always less signi.cant. 5. Summary An important objective of the double probe experiment is the monitoring of the spacecraft potential. For that a proper
bias current has to be selected in order to keep the probes near the ambient plasma potential. Unfortunately, only positive spacecraft potentials can be measured with this technique, but fortunately the satellites 4oat normally at positive voltages in the magnetosphere. There are three cases where the satellite can become negatively charged: in the inner plasmasphere, in the shadow of the Earth, and during an intense electron injection. The latter two events can be distinguished in the data as they cause an immediate saturation of the signal, similarly to the occurrence of a low-density plasma region. However, with the EFI data only, one cannot deduce surely the cause of saturation but particle data need to be examined. Fortunately, In this study we have investigated the average spacecraft potential in the magnetosphere, using the observations of the EFI experiment on the Polar satellite in 1996 –1999. For most of the time, however, the satellite 4oats at a positive potential, typically between 0 and 50 V so that in the plasmasphere near the plasmapause Vsc is in the range of 0 –1 V, at the plasmapause, it exhibits a steep increase by 3–5 V, and outside the plasmasphere, Vsc is usually more than 5 V. In the dayside magnetosphere, the average Vsc is usually 7–8 V, whereas on the nightside it is in the 10 –20 V range. In the cusp, however, Vsc is only about 3 V; similar spacecraft potentials are also observed in the magnetosheath. Highest Vsc ’s occur in the high-altitude (¿ 4RE altitude) polar cap, where Vsc is usually between 20 and 30 V, and on auroral .eld lines where it is frequently at 30 –50 V range. A detailed investigation reveals that during approximately 1.5% of the passes the spacecraft observes potentials that are above +50 V. There are also several
H. Laakso / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1735 – 1744
Kp 0 –1 + 1 –2 + 2 –9
35
Vs (volts)
30
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northern polar cap 4-8 RE altitude
25 20 15 10 10
southern polar cap 1 RE altitude
Vs (volts)
8 6 4 2 07 1996
01
07 1997
01
07 1998
01
07 1999
Fig. 9. Monthly averages of the spacecraft potential in the polar cap region at low altitudes (top panel) and at high altitudes (top panel). A clear seasonal variation appears in these plots because of a strong annual density variation of the polar cap density.
events where the signal becomes saturated, which happens when Vsc exceeds approximately +68 V. The payload of the Polar satellite represents the state of the art, and therefore the electron and ion observations along the spacecraft potential measurements should be used in future for the validation and testing of the charging codes for the high-altitude spacecraft. Acknowledgements This work is supported under NASA grant NAG5-3182.
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