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Borrelly: Magnetic field and plasma observations
Planetary and Space Science 59 (2011) 691–698
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Deep Space 1 at comet 19P/Borrelly: Magnetic field and plasma observations I. Richter a,, C. Koenders a, K.H. Glassmeier a, B.T. Tsurutani a,b, R. Goldstein c a b c
Institut f¨ ur Geophysik und extraterrestrische Physik, Technische Universit¨ at Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, United States Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, United States
a r t i c l e i n f o
abstract
Article history: Received 11 November 2010 Received in revised form 17 January 2011 Accepted 1 February 2011 Available online 19 February 2011
On September 22, 2001 the Deep Space 1 spacecraft performed a flyby at comet 19P/Borrelly at a solar distance of 1.36 AU leading the Earth by 743 in longitude. The spacecraft–comet distance at closest approach was 2171 km. The bow shock had a magnetic compression ratio of 2.5 at a distance of 147 100 km from the nucleus. Deep Space 1 first entered the sheath region essentially from the north polar region. Fluctuations from the cometary ion pickup were present throughout the sheath region and even well upstream of the shock, as expected. The magnetic field pileup region had a peak field strength of 83 nT and was shown to be consistent with a pressure equal to the solar wind ram pressure. The peak field location was offset from the time of closest approach. It is uncertain whether this is a spatial or temporal variation. Draping of magnetic fields around the nucleus was sought, but evidence for this was not apparent in the data. A possible explanation is that the interplanetary solar wind was composed of turbulent short-scale fields, and thus the fields were not symmetric about the point of closest approach. During the flyby phase there were in general few intervals of ACE data where there were large scale Parker spiral fields. With the addition of plasma data, the shock properties are investigated. The characteristics of magnetic draping, pileup and fluctuations are explored. These comet 19P/Borrelly results are contrasted with other cometary flyby results. & 2011 Elsevier Ltd. All rights reserved.
Keywords: 19P/Borrelly Comets Cometary shocks Deep Space 1 Field draping Magnetic field Magnetospheres Pileup Solar wind
1. Introduction Cometary research reveals essential facts about the evolution of our universe. The interaction of comets with the solar wind and the interplanetary plasma (e.g. Coates and Jones, 2009) as well as cometary composition can be studied by investigation of the magnetic field in the vicinity of a comet. A particular opportunity for such an investigation was provided by the flyby of Deep Space 1 (DS1) at comet 19P/Borrelly. Plasma observations made during this flyby (Young et al., 2007) reveal an interesting asymmetry in the bow shock location which is probably due to non-spherical neutral distribution profiles on the comet–solar wind interaction region (e.g. Delamere, 2006; Jia et al., 2008). This makes the DS1 flyby at 19P/Borrelly a very interesting topic for further studies. The DS1 mission (Rayman et al., 2000) was the first mission of NASA’s New Millennium Program. This program was created to test and validate advanced technologies for the future space exploration in the third millennium. DS1 was launched successfully at Cape Canaveral on October 24, 1998. The most remarkable feature of DS1 was the first use of an ion propulsion system in a deep space mission. DS1 was equipped with ion engine diagnostic
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E-mail address: [email protected] (I. Richter). 0032-0633/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2011.02.001
sensors (Brinza et al., 2000) including two ultra small three-axes high-resolution fluxgate magnetometers (FGM) shown in Fig. 1. This instrument, a prototype of the ROSETTA magnetometer (Glassmeier et al., 2007a), was developed at the Institute for Geophysics and Extraterrestrial Physics in Braunschweig. The instrument is able to resolve the magnetic field with a resolution of 0.04 nT within the measurement range of 725 000 nT, and it can operate in a temperature range of 150 3 C to þ 150 3 C. In spite of the high vector sampling rate of 20 Hz, it only consumes 200 mW of power. The primary scientific mission objective was a flyby at the asteroid 9969 Braille on July 29, 1999. Here DS1 revealed the first direct evidence for an asteroidal magnetic field (Richter et al., 2001). In the extended mission, the second scientific objective was the encounter with comet 19P/Borrelly on September 22, 2001 (Rayman and Varghese, 2001). This took place at a distance of 1.36 AU from the Sun and 1.47 AU away from the Earth. DS1 passed 19P/Borrelly with a velocity of 16.58 km/s at a closest approach (C/A) distance of 2171 km at 22:29:33 UTC in the solar wind upstream region. 19P/Borrelly flew through the ecliptic plane from south to north, while DS1 cruised in the ecliptic plane. The angle between the 19P/Borrelly-orbital plane and the DS1 trajectory is 90:83 . The angle between the DS1 trajectory and the comet-Sun line is 90:053 (Figs. 2 and 3). In this paper we provide an analysis of the magnetic field measurements at 19P/Borrelly. The magnetic field data are
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Z
Magnetic Field Data available
143200 km @ 20:00 UTC Bow Shock Borrelly Sun
X
90.05° CA: 2171 km @ 22:29:33 UTC
Fig. 1. The fluxgate magnetometer sensor. Fig. 3. The flyby geometry. The figure shows the xz-projection of the schematic DS1-trajectory in cometo-centered-solar-equatorial (CSEQ)-coordinates, where x is pointing toward the Sun, z is the component of the rotation axis of the Sun which is perpendicular to x, and y completes the right-handed system. The shown bow shock is just a sketch.
Fig. 2. The celestial situation at the flyby time on September 22, 2001. Orbits are displayed in ECLIPJ2000 coordinates. Here x points from the Sun to vernal equinox, y is in the ecliptic plane pointing against the orbital motion of the Earth and z completes the system to be right handed.
contaminated by spacecraft magnetic fields, requiring corrections to remove the interferences. Therefore, we describe the method used to extract the real magnetic background field; namely applying a long-term temperature model of the ion engine fields and a spike detector to get rid of thruster-induced signatures. Furthermore, the unknown DC-level of the background field will be adjusted using the Parker model. With the calibrated data the signature of comet 19P/Borrelly is investigated and compared with the data of different cometary encounters.
2. Data reduction and calibration The two FGMs (inboard: IB, outboard: OB) of DS1 are located outside the spacecraft on a 50-cm-long boom, close to the ion engine beam for diagnostic purpose. The separation distance between both sensors is 46 cm. At this short distance from the spacecraft, the FGM observations are strongly influenced by the ion engine permanent magnets, the field of which is temperature
dependent, the intermittently emitted ion beam, and the magnetically activated hydrazine thrusters used for attitude control of the spacecraft. Due to these interferences, extensive data processing (Richter et al., 2001) is required to extract the scientific signatures from the raw data. First, the instrument raw data is adjusted using the results of the ground calibration (temperaturedependent sensitivity and misalignment of the sensor) and rotated into s/c-coordinates. In the next step, the temperaturedependent influence of the ion engine magnets (permeability is temperature dependent) is eliminated using a linear, long-term model of their magnetic moments. Such a model was developed using quiet phases of the magnetic field over a period of half a year for different temperatures of the ion engine. These temperature dependencies are specified for every sensor location/orientation. The largest gradient ( 2.6 nT/K ) is seen at the IB Z-axis sensor. The ion engine magnetic fields at T ¼ 0 3 C varied from 190 to 6390 nT for the six different sensors. This temperature behavior of the magnets did not change during the mission, therefore a trustable model could be generated. During the flyby interval from 17:00 to 00:00 UTC the engine cooled from 40 to 0 3 C. The temperatures at the FGMs, however, were nearly constant ðT ¼ 2 3 C 7 2 3 CÞ. Spikes originating from the hydrazine thrusters are eliminated by a spike detector. The parameters of this filter are chosen empirically to match the detector with the thruster firings, which occurred rather infrequently at the 19P/Borrelly flyby, and generated spikes of about 15 nT amplitude and 0.3 s duration in the magnetic field data. Due to this large amplitude, the spikes can easily be identified and removed using a short time/long time filter: every data point (short time sample) which deviates more than 8 nT from the moving average (taken over 12 points, long time sample) is removed from the data set. After resampling the data to 1 s means and taking into account the s/c-attitude information (provided by SPICE kernels, see Acton, 1996), the magnetic field data are available in comet centered solar equatorial coordinates (CSEQ), where x is pointing toward the Sun, z is the component of the rotation axis of the Sun which is perpendicular to x, and y completes the right-handed system (y is parallel to the Sun’s equatorial plane). This coordinate system is chosen as the x-axis as the comet-Sun line is the major symmetry axis and the Sun equatorial plane is the major symmetry plane for the solar wind.
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In the last step of the raw data processing, the absolute DC level of the external magnetic field has to be estimated. Unfortunately it is not possible to determine the absolute value of the magnetic field at the location of DS1 directly, as the in-flight calibration techniques for spinning satellites (e.g. Anderson et al., 2001) could not be applied, because DS1 is not a spinning threeaxis stabilized spacecraft. Furthermore the encounter did not happen at the Earth where the measurements could have been adjusted with reference data. There was also no possibility to use the IMF data measured by e.g. WIND or ACE, as these spacecrafts were not located at a conjugated field line with respect to the position of DS1 at 19P/Borrelly. Therefore, the only reasonable way to estimate the sensor offsets is to assume a Parker-type solar wind magnetic field in the assigned region and to set the mean residual field components, as measured by the sensors in a quiet period before the encounter, to the theoretically expected values of the Parker field at that location at that time. The Parker field was calculated by means of the spatial Parker field decay law (e.g. Musmann et al., 1977). We checked the results of this adjustment with the Hedgecock-algorithm (Hedgecock, 1975), which is able to determine the unknown offsets under the assumptions that the magnetic field modulus is constant during a certain time interval. As a result the offsets could be evaluated within the order of a few nanotesla. Although the performance of the magnetometer is slightly degraded by the mentioned s/c-generated effects, the overall accuracy of the corrected data is in the order of a few nanotesla and therefore sufficient to reveal the comet’s magnetic signatures discussed below, as the observed magnetic field inside the bow shock will be shown to be much higher. The plasma instrument is described in Young et al. (2007). We use the reduced values of solar wind speed vsw, solar wind mass msw, density n and temperature T to calculate the Alfve´n and magnetosonic speeds. The latter are used in our shock analysis. For this we need to calculate the Alfve´nic and magnetosonic Mach numbers MA ¼ vsw/vA and Mms ¼vsw/(v2A +v2s )1/2 with the Alfve´n speed vA ¼ B=ðm0 nmsw Þ1=2 , the speed of sound vs ¼ ð53 P=ðnmsw ÞÞ1=2 and the total pressure P ¼ nkðTe þTp Þ þ B2 =ð2m0 Þ. In the equations above, the mass and number density are determined over all species. We simplify our calculations by assuming that the plasma consists solely of protons, i.e. msw ¼mp. Due to the lack of measured electron temperatures we looked for an alternative estimate to calculate the plasma pressure. For example, Feldman et al. (1977) assumed Te ¼2Tp; Newbury et al. (1998) found Te ¼141 000 K. We calculated the magnetosonic Mach number with both methods and found out that the results did not differ substantially. Therefore, we used the numerical value by Newbury et al. in this paper for a coarse estimate of the electron temperature. Despite the inconvenient geometry relation between DS1 and ACE, an inspection of the ACE data has been done for a time interval of 10 days before the comet encounter. This coarse overview revealed that the solar wind at that time was highly turbulent, the magnetic field did only for very few time intervals behave as a standard Parker-spiral and changed the polarity (z-component) several times. Thus, neither the ACE plasma nor the ACE magnetic field data were taken into account for in depths comparison with the DS1 data.
3. The magnetic signature of comet 19P/Borrelly Fig. 4 is an overview plot which displays the 1 s magnetic field and 1 min solar wind velocity measurements for the 19P/Borrelly encounter. This interval is about 7.5 h around C/A at 22:29:33 UT (Rayman, 2002) and is denoted by a vertical dashed line. All available magnetic field data are shown.
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The solar wind speed decreases from 360 km/s at 17:00 UTC to 70 km/s at 22 : 30 UTC. The speed then increases as DS1 departs from the comet. The speed is decreased due to the solar wind pickup of cometary ions. As the solar wind becomes more massive, the speed slows down conserving total momentum. The magnetic field magnitude shows similar features. The magnetic field magnitude is 3 nT at 17:00 UTC and increases to 83 nT in the pileup region near closest approach. A closer look at this figure shows a sharp increase of the magnetic field magnitude at 20 : 00 UTC. This occurs at a distance of about 147 000 km to the comet and about 2:30 h before C/A. The field decrease in the Bx- and By-component starts at about 19:45 UTC, the increase in the Bz begins somewhat later at about 20:00 UTC. This structure may be a cometary shock – the exact determination will be done later in this section using the plasma data. Fluctuations, as many as seen during the ICE encounter with comet 21P/Giacobini–Zinner (Tsurutani and Smith, 1986), and the Giotto encounter with comets 1P/Halley and 26P/Grigg–Skjellerup (e.g. Glassmeier et al., 1989; Glassmeier and Neubauer, 1993), are observed during the entire interval shown from 16:30 to 24:00 UTC. There are small magnetic oscillations upstream of the shock, which are due to instabilities associated with the solar wind pickup of cometary ions. The large fluctuations can be easily seen at and after the shock. There are large variations of about 710 nT in all three components and in the field magnitude. The spectral behavior of this variation not shown in detail, but will be the subject of a future work. It is also noted that there is an asymmetry in the magnetic field magnitude about closest approach, both in the peak and average values. The peak magnetic field of 83 nT is observed at 22 : 35 UTC about 5 min after C/A. The average magnetic field is 18 nT from 21:00 to 22:00 UTC, while from 23:00 to 24:00 UTC the field is higher, 23 nT. Similar features have been noted at other comets. This will be discussed below. Fig. 5 shows a high-resolution interval near the shock from 19:00 UTC until 21:00 UTC. The top three panels show the solar wind speed, density, and proton temperature. The bottom panel contains the derived Alfve´n and magnetosonic Mach numbers (e.g. Tsurutani and Lin, 1985). The bow shock is denoted by a vertical line. There are clear parameter jumps at the shock. The solar wind velocity decreases from 290 to 235 km=s, the density increases from 5 to 9 cm3 , the ion temperature increases from 4:2 104 to 5:8 104 K and the magnetic field magnitude from 6 to 15 nT. The magnetosonic Mach number also decreases from 4 to near 1:5. The post shock Mach number is not below unity possibly because of uncertainties in the plasma and magnetic field parameters. Values of 20% changes will lead to post shock Mach numbers less than 1.0. DS1 traversed the comet 19P/Borrelly system from the north to south (cf. Figs. 2 and 3) and encountered the shock region at r CSEQ ¼ ð4600,41 400,147 100Þ km approaching from the ( y+ z)-quadrant. The comet bow shock can be assumed to have a parabolic shape (e.g. Schmidt and Wegmann, 1982), thus the normal to the shock should have a significant z-component. This is tested by doing a normal calculation using the coplanarity method (Colburn and Sonett, 1966). For this the cross product of the magnetic field vectors immediately upstream of the bow shock at 19:55 UTC and immediate inside the sheath at 20:05 UTC has been calculated. To stabilize the results and minimize the effect of waves, 30 s averages have been used. The calculated shock normal determined from the coplanarity calculation is nCESQ ¼ ð0:67,0:21,0:79Þ. The shock normal therefore has an angle of 553 relative to the comet-Sun direction (xCSEQ) and deviates 23 from the normal of a simple parabolic model bow shock with a subsolar distance of 100 000 km, a 150 000 km distance in the z direction over the comet, and a flaring factor of
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Vsw[km/s]
|B| [nT]
Bz [nT]
By [nT]
Bx [nT]
September 22, 2001: DS1@BORRELLY, CSEQ-Coordinates, 1s data 60 Closest Approach 22:29:33 40 20 0 -20 -40 -60 60 40 20 0 -20 -40 -60 60 40 20 0 -20 -40 -60 90 80 70 60 50 40 30 20 10 0 400 350 300 250 200 150 100 50 0
17:00
18:00
19:00
20:00 21:00 TIME [UTC]
22:00
23:00
00:00
Fig. 4. The upper panels show the magnetic field data in cometo-centered-solar-equatorial (CSEQ) coordinates (1 s averages). The bottom panel visualizes the solar wind speed measured by the PEPE instrument.
1.5 (Huddleston et al., 1990). The observed bow shock distance is in the same order as simulated by Jia et al. (2008) for different types of neutral distributions. No magnetometer measurements were made on the outbound pass near where a potential shock would have been, because the magnetometer onboard DS1 was classified only as a diagnosis instrument for the ion engine and therefore got low priority for scientific measurements. After crossing the inbound bow shock, the fluctuation level increases dramatically. This behavior has already been recognized during the previous encounters at 1P/Halley, 26P/Grigg–Skjellerup, and 21P/Giacobini–Zinner (Neubauer et al., 1986; Scarf et al., 1986; Smith et al., 1986). The cometary magnetic field directionality has been examined to study the possibility of field draping around the nucleus, as first proposed by Alfve´n (1957). Other cometary encounter data have shown these features (Raeder et al., 1987; Smith et al., 1986; Israelevich et al., 1996). For the data acquisition from the shock to the end of the magnetic data acquisition period (from 20:00 until
24:00 UTC) the direction of the magnetic field in CSEQ coordinates is plotted in (xy)- and (xz)-planes. These are the top and bottom panels of Fig. 6. To the right of the data panel sketches of potential draped magnetic fields are drawn. For the top panel, a red vector indicates Bx 4 0 and By o0 and blue indicates Bx 40 and By 4 0. The other two combination of magnetic field directions are given in the legend. For the bottom panel, blue indicates Bx 4 0 and Bz 40 and red indicates Bx 40 and Bz o 0. In the top panel, the magnetic fields measured just after entering the post shock region are red corresponding to a symbolic field line direction indicated by the red arrow in the upper right figure. For large-scale draped fields, meaning that the field direction changes continuously and smoothly, one would expect the outbound post closest approach field in the direction of the black arrow. Instead it is found that the outbound fields are represented by blue arrows. This direction is not consistent with simple draping as the orientation is opposite to the expected. In the xz-plane shown in the bottom panel the situation is similar. The post shock fields are blue. Therefore, for draped fields,
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PLASMA PARAMETER 350
Mach Number [1]
Temperature Tp[K]
Density n[cm^-3]
Vsw[km/s]
Bow Shock 300 250 200 150 35 30 25 20 15 10 5 0 140000 120000 100000 80000 60000 40000 20000 0 25
Ma Mms
20 15 10 5
0 09-22 09-22 09-22 09-22 09-22 09-22 09-22 09-22 09-22 19:00 19:15 19:30 19:45 20:00 20:15 20:30 20:45 21:00 Date\Time (UTC) Fig. 5. Relevant plasma parameters for the DS1 19P/Borrelly encounter. The velocity, density, and ion temperature have been measured by the PEPE instrument. The Mach numbers are computed using magnetometer and PEPE data.
one would expect that the post closest approach fields would be in the direction of the green arrow. Instead they are represented by red arrows, which means that the measured field direction is more or less opposite to the expected direction. Again, this is inconsistent with large scale draped fields. Some possible explanations will be given in the conclusion section.
4. The magnetic pileup region at 19P/Borrelly and comparison to other comets The top panel of Fig. 7 displays the magnetic field magnitude for the 19P/Borrelly encounter. The peak field is 83 nT and occurs at 22 : 34 UT. This peak value occurs 5 min after closest approach (22:29.33 UTC: indicated by a vertical line). It is interesting to mention that the peak fields at the other three comets encountered to date (Neubauer et al., 1986, 1993; Smith et al., 1986) have similar values like 19P/Borrelly and are sometimes offset from closest approach as well. The second panel
shows the 1P/Halley magnetic field, the third the field of 26P/ Grigg–Skjellerup and the bottom panel is the 21P/Giacobini– Zinner field. Vertical lines indicate the closest approach. The peak fields for 1P/Halley are about 56 and 65 nT on the two sides of the magnetic cavity (there are zero fields close to the comet nucleus). This is taken from Neubauer et al. (1986). The closest approach at 1P/Halley was 00:03 UTC. The first peak was 4 min prior to C/A and the second was 2 min after C/A. Thus, there is a small but obvious asymmetry in the fields. The distribution of these peaks is strongly related to the flyby geometry. The peak field at 26P/ Grigg–Skjellerup was 88:7 nT at 15:18:48. The closest approach time was 15:18:43 746 s. Thus, the peak field was within the uncertainty of the C/A time. For 21P/Giacobini–Zinner, the ICE spacecraft went through the tail of the comet. The two peak tail lobes were 58 and 57 nT occurring at 11:01 and 11:04 UT. The time of closest approach was 11:03 (Smith et al., 1986). Smith et al. have said ‘‘The magnetotail crossings are asymmetric with respect to the current sheet, being 160 s inbound and 270 s outbound. Without further analysis, it is uncertain whether to
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-6x104 20: 00
Y(CSEQ) [km]
-4x104
Bx<0, By<0 Bx>0, By<0 Bx>0, By>0 Bx<0, By>0
-2x104
X
20 nT 0 2x104
00: 00 Y
4x104 -6x104 -4x104 -2x104 0 2x104 4x104 X(CSEQ) [km] Borrelly not to scale 2x105
Z(CSEQ) [km]
20: 00
Bx<0, Bz<0 Bx>0, Bz<0 Bx>0, Bz>0 Bx<0, Bz>0
Z
1x105 X 20 nT 0
-1x105 -2x105
00: 00 -1x105
0
X(CSEQ) [km]
1x105
Bow Shock at ~20:00 Closest Approach at 22:29:33
Fig. 6. The flyby geometry of the DS1 encounter with comet 19P/Borrelly and magnetic field directions in the most relevant planes.
attribute such differences to structure or time variation.’’ The same can be said for 19P/Borrelly. There is a definite asymmetry in the peak field/pileup region. It is uncertain at this time whether this is due to actual physical structure or time variations. It is interesting to note that the peak magnetic fields for all comets examined here are similar in magnitude. The 19P/Borrelly, 1P/Halley, 26P/Grigg–Skjellerup and 21P/Giacobini–Zinner fields are 83, 56/65, 88 and 58 nT, respectively. This can be explained by examining the pressure balance between the solar wind ram pressure and the magnetic field pressure. At this time, the solar wind ram pressure was 2.6 nPa. The peak magnetic field at 19P/ Borrelly was 83 nT, giving a magnetic pressure of 2.7 nPa. The two values are reasonably close. Thus, the magnetic field draping effect (Midgley and Davis, 1963; Zwan and Wolf, 1976) has squeezed out the plasma and left primarily a low beta high field region. Another feature obvious from Fig. 7 is the relative strengths of the shocks at the four comets. Earlier in our discussion of the 19P/ Borrelly shock, we noted a magnetic jump of 2:5. All of the other cometary shocks have comparable values. These are the typical values indicated by computer simulation models (Lipatov et al., 2002). An overview of relevant parameters of the mentioned comets is given in Table 1. Parameters of the earlier encounters have been compiled using Glassmeier et al. (1997), Huddleston et al. (1992), Israelevich et al. (1996), Neubauer et al. (1986, 1993), Reinhard (1986), Smith et al. (1986), and von Rosenvinge et al. (1986). The DS1 values come from Boehnhardt et al. (1999), Pittichova´ et al. (2008), Soderblom et al. (2002), Weaver et al. (2003) and Young et al. (2004). The bow shock and the contact surface entities were calculated using the formulae
of Cravens (1986). The bow shock distance of 19P/Borrelly is derived from formulae of Schmidt and Wegmann (1982). The estimates of the perpendicular bow shock distances were calculated with a parabolic flaring factor of 1.5 as used in Huddleston et al. (1992). The modeled and measured shock distances of the comets show a maximum deviation of 20%. Besides the already mentioned feature the table shows that the chance to identify a magnetic cavity was given only at the GIOTTO flyby at 1P/Halley. All the other encounters happened in distances too far away from the contact surface.
5. Summary and conclusion Concluding these investigations, it can be stated that in spite of the strong magnetic disturbances of DS1, interesting magnetic field features at comet 19P/Borrelly have been noted. The bow shock had a magnetic compression ratio of 2.5 at a distance of 147 100 km from the nucleus. The sheath region was first entered almost over the north pole at r CSEQ ¼ ð4600,41 400, 147 100Þ km by DS1. Fluctuations from the cometary ion pickup were present throughout the sheath region and even well upstream of the shock, much as expected. The magnetic field pileup region had a peak field strength of 83 nT and was shown to be consistent with a pressure equal to the solar wind ram pressure. The peak field location was offset from the time of closest approach by 5 min. Draping of magnetic fields around the nucleus was sought, but evidence for this was not apparent in the data. A possible explanation is that the interplanetary solar wind was composed of turbulent short-scale fields and thus the fields were not symmetric about the point of
|B| [nT]
|B| [nT]
|B| [nT]
|B| [nT]
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90 80 70 60 50 40 30 20 10 0 17:00
90 80 BORRELLY 70 22. Sep. 2001 60 50 40 30 20 10 0 16:00 18:00 20:00
C/A
22:00
90 80 HALLEY 70 14. March 1986 60 50 40 30 20 10 0 12:00 16:00 20:00
16:00
00:00
C/A
00:00
GRIGG-SKJELLERUP 10. July 1992
16:30
697
15:30
04:00
08:00
12:00
C/A
15:00
14:30
14:00
90 80 GIACOBINI-ZINNER C/A 70 11. Sep. 1985 60 50 40 30 20 10 0 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 TIME (hh:mm)
Fig. 7. The magnetic field magnitudes for 19P/Borrelly, 1P/Halley, 26P/Grigg–Skjellerup and 21P/Giacobini–Zinner. The ordinates for all four panels (field magnitude) are plotted to the same scale and the abscissas (time axes) are different for comparative purposes. The time scale of 26P/Grigg–Skjellerup has been reversed so that the shock is on the left.
Table 1 Relevant cometary parameters. Comet
21P/Giacobini–Zinner
1P/Halley
26P/Grigg–Skjellerup
19P/Borrelly
Mission Flyby date Flyby time Encounter region C/A distance (km) C/A velocity (km/s) Heliocentric distance (AU) Dimensions (radius or axes) (km) Gas production rate (1/s) Solar wind velocity (km/s) Solar wind density (1/cm3) Observed shock distance along trajectory (km) Modeled subsolar bow shock distance (km) Modeled perpendic. bow shock distance (km) Observed peak magnetic field (nT) Time of peak field tC=A 7 Dt (min) Modeled contact surface distance (km) Modeled contact surface field (nT)
ICE 11.09.1985 11:03 Tail 7800 21 1.03 R ¼1.82 2.5 1028 400 6.2 110 000 55 000 83 000 58/57 2/ + 0.5 310 70
GIOTTO 14.03.1986 00:03 Upstream 610 68.4 0.89 15 8 8 8 1029 380 6.0 1 200 000 769 000 1 150 000 57/65 4/ + 2 4440 65
GIOTTO 10.07.1992 15:18:43 Tail 330 14 1.01 R ¼1.3 7 1027 350 8.6 25 000 14 000 21 000 88.7 0 115 71
DS1 22.09.2001 22:29:33 Upstream 2171 16.6 1.36 844 3.5 1028 362 5.2 147 000 95 300 143 000 83 +5 480 57
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closest approach. During the flyby phase there were in general few intervals of large scale Parker spiral fields. Considering all of these pieces, a consistent picture of the magnetic field signature was revealed. Although the FGM was included onboard DS1 as a ’guest instrument’ to serve as ion engine diagnostic tool, it gathered interesting data during the flybys at the asteroid 9969 Braille and the comet 19P/Borrelly. The methods for the science analysis presented here will be enhanced for the ROSETTA project (Glassmeier et al., 2007b). Once ROSETTA will arrive in the comet 67P/Churyumov-Gerasimenko system the instruments of the ROSETTA Plasma Consortium (RPC) will collect unique data and reveal new knowledge about cometary plasma physics.
Acknowledgments The work of K.-H. Glassmeier, C. Koenders, and I. Richter was ¨ financially supported by the German Bundesministerium fur ¨ LuftWirtschaft und Technologie and the Deutsches Zentrum fur und Raumfahrt under the contracts 50 OO 99037 for DS1 and 50 QP 1001 for ROSETTA. The work of B.T. Tsurutani at the Jet Propulsion Laboratory, California Institute of Technology was done under a contract with the National Aeronautics and Space Administration. Also the work by R. Goldstein was supported through a contract to the Southwest Research Institute by the National Aeronautics and Space Administration. All computations concerning the s/c position and orientation have been calculated with the use of the SPICE software developed by NASA’s NAIF team. Special thanks go to Boris Semenov who provided the SPICE-kernels on very short notice. References Acton, C.H., 1996. Ancillary data services of NASA’s navigation and ancillary information facility. Planet. Space Sci. 44, 65–70. Alfve´n, H., 1957. On the theory of comet tails. Tellus IX. ˜ a, M.H., Russell, C.T., Anderson, B.J., Zanetti, L.J., Lohr, D.H., Hayes, J.R., Acun Mulligan, T., 2001. In-flight calibration of the NEAR magnetometer. IEEE Trans. Geosci. Remote Sensing 39, 907–917. Boehnhardt, H., Rainer, N., Birkle, K., Schwehm, G., 1999. The nuclei of comets 26P/ Grigg–Skjellerup and 73P/Schwassmann-Wachmann. Astron. Astrophys. 341, 912–917. Brinza, D.E., et al., 2000. Ion propulsion subsystem environmental effects on Deep Space 1: initial results from the IPS Diagnostic Subsystem. DS1 Technology Validation Report. JPL Pub. 00-10(4b). Download: /http://nmp-techval-re ports.jpl.nasa.gov/DS1/IPS_Integrated_Report.pdfS. Coates, A.J., Jones, G.H., 2009. Plasma environment of Jupiter family comets. Planet. Space Sci. 57, 1175–1191. Colburn, D.S., Sonett, C.P., 1966. Discontinuities in the solar wind. Space Sci. Rev. 5, 439–506. Cravens, T.E., 1986. The physics of cometary contact surface. ESA Spec. Publ. SP250, pp. 241–246. Delamere, P.A., 2006. Hybrid code simulations of the solar wind with comet 19P/ Borrelly. J. Geophys. Res. 111, A12217. doi:10.1029/2006JA011859. Feldman, W., Asbridge, J., Bame, S., Gosling, J., 1977. Plasma and magnetic fields from the Sun. In: White, O.R. (Ed.), The Solar Output and its Variation. Colorado University Press, Boulder, pp. 351–382. ˜ a, M.H., Goldstein, M.L., Johnstone, A.D., Glassmeier, K.-H., Coates, A.J., Acun Neubauer, F.M., Re´me, H., 1989. Spectral characteristics of low-frequency plasma turbulence upstream of comet 1P/Halley. J. Geophys. Res. 94, 37–48. Glassmeier, K.-H., Tsurutani, B.T., Neubauer, F.M., 1997. Adventures in parameter space a comparison of low-frequency plasma waves at comets. In: Hada, T., Matsumoto, H. (Eds.), Nonlinear Waves and Chaos in Space Plasmas. Terra Scientific Publishing Company, Tokyo, pp. 77–119. Glassmeier, K.-H., Richter, I., Diedrich, A., Musmann, G., Auster, U., Motschmann, U., Balogh, A., Carr, C., Cupido, E., Coates, A., Rother, M., Schwingenschuh, K., ¨ K., Tsurutani, B., 2007a. RPC-MAG: the fluxgate magnetometer in the Szego, ROSETTA plasma consortium. Space Sci. Rev. 128, 649–670. ¨ Glassmeier, K.-H., Boehnhardt, H., Koschny, D., Kuhrt, E., Richter, I., 2007b. The ROSETTA mission: flying towards the origin of the solar system. Space Sci. Rev. 128, 1–21.
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A 3-D global MHD model for the effect of neutral jets during the Deep Space 1 comet 19P/Borrelly flyby. Icarus 196, 249–257. Lipatov, A., Motschmann, U., Bagdonat, T., 2002. 3D hybrid simulations of the interaction of the solar wind with a weak comet. Planet. Space Sci. 50, 403–411. Midgley, J.E., Davis Jr., L., 1963. Calculation by a moment technique of the perturbation of the geomagnetic field by the solar wind. J. Geophys. Res. 68, 5111. Musmann, G., Neubauer, F.M., Lammers, E., 1977. Radial variation of the interplanetary magnetic field between 0.3 and 1.0 AU: observations by the HELIOS 1 spacecraft. J. Geophys. 42, 591–598. Neubauer, F.M., et al., 1986. First Results from the Giotto magnetometer experiment at the comet Halley. Nature 321, 352–355. Neubauer, F.M., et al., 1993. First results from the Giotto magnetometer experiment during the 26P/Grigg–Skjellerup encounter. Astron. Astrophys. 268, L5–L8. Newbury, J., Russell, C., Phillips, J., Gary, S., 1998. Electron temperature in the ambient solar wind: typical properties and a lower bound at 1 AU. J. Geophys. Res. 103, 9553–9566. Pittichova´, J., Woodward, C.E., Kelley, M.S., Reach, W.T., 2008. Ground-based optical and SPITZER infrared imaging observations of comet 21P/GIACOBINI– ZINNER. Astron. J. 136, 1127–1136. Raeder, J., Neubauer, F.M., Ness, N., Burlaga, L.F., 1987. Macroscopic perturbations of the IMF by 1P/Halley as seen by the Giotto magnetometer. Astron. Astrophys. 187, 61–65. Rayman, M.D., Varghese, P., 2001. The Deep Space 1 extended mission. Acta Astronaut. 48, 663–705. Rayman, M.D., Varghese, P., Lehman, D.H., Livesay, L.L., 2000. Results from the Deep Space 1 technology validation mission. Acta Astronaut. 47, 475. Rayman, M.D., 2002. The successful conclusion of the Deep Space 1 mission: important results without a flashy title. In: 53rd International Astronautical Congress, Houston, IAC-02-Q.5.2.03. Reinhard, R., 1986. The Giotto encounter with comet Halley. Nature 321, 313–318. Richter, I.D.E., Brinza, D., Cassel, C., Glassmeier, K.-H., Kuhnke, F., Musmann, G., Othmer, C., Schwingenschuh, K., Tsurutani, B., 2001. First direct magnetic field measurements of an asteroidal magnetic field: DS1 at Braille. Geophys. Res. Lett. 28, 1913–1916. Schmidt, H.U., Wegmann, R., 1982. Plasma flow and magnetic fields in comets. In: Wilkening, L. (Ed.), Comets. The University of Arizona Press, Tucson, Arizona. Scarf, F.L., Coronti, F.V., Kennel, C.F., Gurnett, D.A., Ip, W.-H., Smith, E.J., 1986. Plasma wave observations at comet Giacobini–Zinner. Science 232, 377–381. Smith, E.J., Tsurutani, B.T., Slavin, J.A., Jones, D.E., Siscoe, G.L., Asoka Mendis, D., 1986. International cometary explorer encounter with Giacobini–Zinner: magnetic field observations. Science 232, 382–385. Soderblom, L.A., et al., 2002. Observations of comet 19P/Borrelly by the miniature integrated camera and spectrometer aboard Deep Space 1. Science 296, 1087–1091. Tsurutani, B.T., Lin, R.P., 1985. Acceleration of 447 keV ions and 4 2 keV electrons by interplanetary shocks at l AU. J. Geophys. Res. 90, 1. Tsurutani, B.T., Smith, E.J., 1986. Strong hydromagnetic turbulence associated with comet Giacobini–Zinner. Geophys. Res. Lett. 13, 259–262. von Rosenvinge, T.T., Brandt, J.C., Farquhar, R.W., 1986. The international cometary explorer mission to comet Giacobini–Zinner. Science 232, 353–356. Weaver, H.A., Stern, S.A., Parker, J.Wm., 2003. Hubble space telescope STIS observations of comet 19P/Borrelly during the Deep Space 1 encounter. Am. Astron. Soc. 126 (July), 444–451. Young, D.T., Crary, F.J., Nordholt, J.E., Bagenal, F., Boice, D., Burch, J., Eviatar, A., Goldstein, R., Hanley, J., Lawrence, D.R., McComas, D., Meier, R., Reisenfeld, D., Sauer, K., Wiens, R.C., 2004. Solar wind interactions with comet 19P/Borrelly. Icarus 167, 80–88. 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Deep Space 1 at comet 19P/Borrelly: Magnetic field and plasma observations I. Richter a,, C. Koenders a, K.H. Glassmeier a, B.T. Tsurutani a,b, R. Goldstein c a b c
Institut f¨ ur Geophysik und extraterrestrische Physik, Technische Universit¨ at Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, United States Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, United States
a r t i c l e i n f o
abstract
Article history: Received 11 November 2010 Received in revised form 17 January 2011 Accepted 1 February 2011 Available online 19 February 2011
On September 22, 2001 the Deep Space 1 spacecraft performed a flyby at comet 19P/Borrelly at a solar distance of 1.36 AU leading the Earth by 743 in longitude. The spacecraft–comet distance at closest approach was 2171 km. The bow shock had a magnetic compression ratio of 2.5 at a distance of 147 100 km from the nucleus. Deep Space 1 first entered the sheath region essentially from the north polar region. Fluctuations from the cometary ion pickup were present throughout the sheath region and even well upstream of the shock, as expected. The magnetic field pileup region had a peak field strength of 83 nT and was shown to be consistent with a pressure equal to the solar wind ram pressure. The peak field location was offset from the time of closest approach. It is uncertain whether this is a spatial or temporal variation. Draping of magnetic fields around the nucleus was sought, but evidence for this was not apparent in the data. A possible explanation is that the interplanetary solar wind was composed of turbulent short-scale fields, and thus the fields were not symmetric about the point of closest approach. During the flyby phase there were in general few intervals of ACE data where there were large scale Parker spiral fields. With the addition of plasma data, the shock properties are investigated. The characteristics of magnetic draping, pileup and fluctuations are explored. These comet 19P/Borrelly results are contrasted with other cometary flyby results. & 2011 Elsevier Ltd. All rights reserved.
Keywords: 19P/Borrelly Comets Cometary shocks Deep Space 1 Field draping Magnetic field Magnetospheres Pileup Solar wind
1. Introduction Cometary research reveals essential facts about the evolution of our universe. The interaction of comets with the solar wind and the interplanetary plasma (e.g. Coates and Jones, 2009) as well as cometary composition can be studied by investigation of the magnetic field in the vicinity of a comet. A particular opportunity for such an investigation was provided by the flyby of Deep Space 1 (DS1) at comet 19P/Borrelly. Plasma observations made during this flyby (Young et al., 2007) reveal an interesting asymmetry in the bow shock location which is probably due to non-spherical neutral distribution profiles on the comet–solar wind interaction region (e.g. Delamere, 2006; Jia et al., 2008). This makes the DS1 flyby at 19P/Borrelly a very interesting topic for further studies. The DS1 mission (Rayman et al., 2000) was the first mission of NASA’s New Millennium Program. This program was created to test and validate advanced technologies for the future space exploration in the third millennium. DS1 was launched successfully at Cape Canaveral on October 24, 1998. The most remarkable feature of DS1 was the first use of an ion propulsion system in a deep space mission. DS1 was equipped with ion engine diagnostic
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E-mail address: [email protected] (I. Richter). 0032-0633/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2011.02.001
sensors (Brinza et al., 2000) including two ultra small three-axes high-resolution fluxgate magnetometers (FGM) shown in Fig. 1. This instrument, a prototype of the ROSETTA magnetometer (Glassmeier et al., 2007a), was developed at the Institute for Geophysics and Extraterrestrial Physics in Braunschweig. The instrument is able to resolve the magnetic field with a resolution of 0.04 nT within the measurement range of 725 000 nT, and it can operate in a temperature range of 150 3 C to þ 150 3 C. In spite of the high vector sampling rate of 20 Hz, it only consumes 200 mW of power. The primary scientific mission objective was a flyby at the asteroid 9969 Braille on July 29, 1999. Here DS1 revealed the first direct evidence for an asteroidal magnetic field (Richter et al., 2001). In the extended mission, the second scientific objective was the encounter with comet 19P/Borrelly on September 22, 2001 (Rayman and Varghese, 2001). This took place at a distance of 1.36 AU from the Sun and 1.47 AU away from the Earth. DS1 passed 19P/Borrelly with a velocity of 16.58 km/s at a closest approach (C/A) distance of 2171 km at 22:29:33 UTC in the solar wind upstream region. 19P/Borrelly flew through the ecliptic plane from south to north, while DS1 cruised in the ecliptic plane. The angle between the 19P/Borrelly-orbital plane and the DS1 trajectory is 90:83 . The angle between the DS1 trajectory and the comet-Sun line is 90:053 (Figs. 2 and 3). In this paper we provide an analysis of the magnetic field measurements at 19P/Borrelly. The magnetic field data are
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Z
Magnetic Field Data available
143200 km @ 20:00 UTC Bow Shock Borrelly Sun
X
90.05° CA: 2171 km @ 22:29:33 UTC
Fig. 1. The fluxgate magnetometer sensor. Fig. 3. The flyby geometry. The figure shows the xz-projection of the schematic DS1-trajectory in cometo-centered-solar-equatorial (CSEQ)-coordinates, where x is pointing toward the Sun, z is the component of the rotation axis of the Sun which is perpendicular to x, and y completes the right-handed system. The shown bow shock is just a sketch.
Fig. 2. The celestial situation at the flyby time on September 22, 2001. Orbits are displayed in ECLIPJ2000 coordinates. Here x points from the Sun to vernal equinox, y is in the ecliptic plane pointing against the orbital motion of the Earth and z completes the system to be right handed.
contaminated by spacecraft magnetic fields, requiring corrections to remove the interferences. Therefore, we describe the method used to extract the real magnetic background field; namely applying a long-term temperature model of the ion engine fields and a spike detector to get rid of thruster-induced signatures. Furthermore, the unknown DC-level of the background field will be adjusted using the Parker model. With the calibrated data the signature of comet 19P/Borrelly is investigated and compared with the data of different cometary encounters.
2. Data reduction and calibration The two FGMs (inboard: IB, outboard: OB) of DS1 are located outside the spacecraft on a 50-cm-long boom, close to the ion engine beam for diagnostic purpose. The separation distance between both sensors is 46 cm. At this short distance from the spacecraft, the FGM observations are strongly influenced by the ion engine permanent magnets, the field of which is temperature
dependent, the intermittently emitted ion beam, and the magnetically activated hydrazine thrusters used for attitude control of the spacecraft. Due to these interferences, extensive data processing (Richter et al., 2001) is required to extract the scientific signatures from the raw data. First, the instrument raw data is adjusted using the results of the ground calibration (temperaturedependent sensitivity and misalignment of the sensor) and rotated into s/c-coordinates. In the next step, the temperaturedependent influence of the ion engine magnets (permeability is temperature dependent) is eliminated using a linear, long-term model of their magnetic moments. Such a model was developed using quiet phases of the magnetic field over a period of half a year for different temperatures of the ion engine. These temperature dependencies are specified for every sensor location/orientation. The largest gradient ( 2.6 nT/K ) is seen at the IB Z-axis sensor. The ion engine magnetic fields at T ¼ 0 3 C varied from 190 to 6390 nT for the six different sensors. This temperature behavior of the magnets did not change during the mission, therefore a trustable model could be generated. During the flyby interval from 17:00 to 00:00 UTC the engine cooled from 40 to 0 3 C. The temperatures at the FGMs, however, were nearly constant ðT ¼ 2 3 C 7 2 3 CÞ. Spikes originating from the hydrazine thrusters are eliminated by a spike detector. The parameters of this filter are chosen empirically to match the detector with the thruster firings, which occurred rather infrequently at the 19P/Borrelly flyby, and generated spikes of about 15 nT amplitude and 0.3 s duration in the magnetic field data. Due to this large amplitude, the spikes can easily be identified and removed using a short time/long time filter: every data point (short time sample) which deviates more than 8 nT from the moving average (taken over 12 points, long time sample) is removed from the data set. After resampling the data to 1 s means and taking into account the s/c-attitude information (provided by SPICE kernels, see Acton, 1996), the magnetic field data are available in comet centered solar equatorial coordinates (CSEQ), where x is pointing toward the Sun, z is the component of the rotation axis of the Sun which is perpendicular to x, and y completes the right-handed system (y is parallel to the Sun’s equatorial plane). This coordinate system is chosen as the x-axis as the comet-Sun line is the major symmetry axis and the Sun equatorial plane is the major symmetry plane for the solar wind.
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In the last step of the raw data processing, the absolute DC level of the external magnetic field has to be estimated. Unfortunately it is not possible to determine the absolute value of the magnetic field at the location of DS1 directly, as the in-flight calibration techniques for spinning satellites (e.g. Anderson et al., 2001) could not be applied, because DS1 is not a spinning threeaxis stabilized spacecraft. Furthermore the encounter did not happen at the Earth where the measurements could have been adjusted with reference data. There was also no possibility to use the IMF data measured by e.g. WIND or ACE, as these spacecrafts were not located at a conjugated field line with respect to the position of DS1 at 19P/Borrelly. Therefore, the only reasonable way to estimate the sensor offsets is to assume a Parker-type solar wind magnetic field in the assigned region and to set the mean residual field components, as measured by the sensors in a quiet period before the encounter, to the theoretically expected values of the Parker field at that location at that time. The Parker field was calculated by means of the spatial Parker field decay law (e.g. Musmann et al., 1977). We checked the results of this adjustment with the Hedgecock-algorithm (Hedgecock, 1975), which is able to determine the unknown offsets under the assumptions that the magnetic field modulus is constant during a certain time interval. As a result the offsets could be evaluated within the order of a few nanotesla. Although the performance of the magnetometer is slightly degraded by the mentioned s/c-generated effects, the overall accuracy of the corrected data is in the order of a few nanotesla and therefore sufficient to reveal the comet’s magnetic signatures discussed below, as the observed magnetic field inside the bow shock will be shown to be much higher. The plasma instrument is described in Young et al. (2007). We use the reduced values of solar wind speed vsw, solar wind mass msw, density n and temperature T to calculate the Alfve´n and magnetosonic speeds. The latter are used in our shock analysis. For this we need to calculate the Alfve´nic and magnetosonic Mach numbers MA ¼ vsw/vA and Mms ¼vsw/(v2A +v2s )1/2 with the Alfve´n speed vA ¼ B=ðm0 nmsw Þ1=2 , the speed of sound vs ¼ ð53 P=ðnmsw ÞÞ1=2 and the total pressure P ¼ nkðTe þTp Þ þ B2 =ð2m0 Þ. In the equations above, the mass and number density are determined over all species. We simplify our calculations by assuming that the plasma consists solely of protons, i.e. msw ¼mp. Due to the lack of measured electron temperatures we looked for an alternative estimate to calculate the plasma pressure. For example, Feldman et al. (1977) assumed Te ¼2Tp; Newbury et al. (1998) found Te ¼141 000 K. We calculated the magnetosonic Mach number with both methods and found out that the results did not differ substantially. Therefore, we used the numerical value by Newbury et al. in this paper for a coarse estimate of the electron temperature. Despite the inconvenient geometry relation between DS1 and ACE, an inspection of the ACE data has been done for a time interval of 10 days before the comet encounter. This coarse overview revealed that the solar wind at that time was highly turbulent, the magnetic field did only for very few time intervals behave as a standard Parker-spiral and changed the polarity (z-component) several times. Thus, neither the ACE plasma nor the ACE magnetic field data were taken into account for in depths comparison with the DS1 data.
3. The magnetic signature of comet 19P/Borrelly Fig. 4 is an overview plot which displays the 1 s magnetic field and 1 min solar wind velocity measurements for the 19P/Borrelly encounter. This interval is about 7.5 h around C/A at 22:29:33 UT (Rayman, 2002) and is denoted by a vertical dashed line. All available magnetic field data are shown.
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The solar wind speed decreases from 360 km/s at 17:00 UTC to 70 km/s at 22 : 30 UTC. The speed then increases as DS1 departs from the comet. The speed is decreased due to the solar wind pickup of cometary ions. As the solar wind becomes more massive, the speed slows down conserving total momentum. The magnetic field magnitude shows similar features. The magnetic field magnitude is 3 nT at 17:00 UTC and increases to 83 nT in the pileup region near closest approach. A closer look at this figure shows a sharp increase of the magnetic field magnitude at 20 : 00 UTC. This occurs at a distance of about 147 000 km to the comet and about 2:30 h before C/A. The field decrease in the Bx- and By-component starts at about 19:45 UTC, the increase in the Bz begins somewhat later at about 20:00 UTC. This structure may be a cometary shock – the exact determination will be done later in this section using the plasma data. Fluctuations, as many as seen during the ICE encounter with comet 21P/Giacobini–Zinner (Tsurutani and Smith, 1986), and the Giotto encounter with comets 1P/Halley and 26P/Grigg–Skjellerup (e.g. Glassmeier et al., 1989; Glassmeier and Neubauer, 1993), are observed during the entire interval shown from 16:30 to 24:00 UTC. There are small magnetic oscillations upstream of the shock, which are due to instabilities associated with the solar wind pickup of cometary ions. The large fluctuations can be easily seen at and after the shock. There are large variations of about 710 nT in all three components and in the field magnitude. The spectral behavior of this variation not shown in detail, but will be the subject of a future work. It is also noted that there is an asymmetry in the magnetic field magnitude about closest approach, both in the peak and average values. The peak magnetic field of 83 nT is observed at 22 : 35 UTC about 5 min after C/A. The average magnetic field is 18 nT from 21:00 to 22:00 UTC, while from 23:00 to 24:00 UTC the field is higher, 23 nT. Similar features have been noted at other comets. This will be discussed below. Fig. 5 shows a high-resolution interval near the shock from 19:00 UTC until 21:00 UTC. The top three panels show the solar wind speed, density, and proton temperature. The bottom panel contains the derived Alfve´n and magnetosonic Mach numbers (e.g. Tsurutani and Lin, 1985). The bow shock is denoted by a vertical line. There are clear parameter jumps at the shock. The solar wind velocity decreases from 290 to 235 km=s, the density increases from 5 to 9 cm3 , the ion temperature increases from 4:2 104 to 5:8 104 K and the magnetic field magnitude from 6 to 15 nT. The magnetosonic Mach number also decreases from 4 to near 1:5. The post shock Mach number is not below unity possibly because of uncertainties in the plasma and magnetic field parameters. Values of 20% changes will lead to post shock Mach numbers less than 1.0. DS1 traversed the comet 19P/Borrelly system from the north to south (cf. Figs. 2 and 3) and encountered the shock region at r CSEQ ¼ ð4600,41 400,147 100Þ km approaching from the ( y+ z)-quadrant. The comet bow shock can be assumed to have a parabolic shape (e.g. Schmidt and Wegmann, 1982), thus the normal to the shock should have a significant z-component. This is tested by doing a normal calculation using the coplanarity method (Colburn and Sonett, 1966). For this the cross product of the magnetic field vectors immediately upstream of the bow shock at 19:55 UTC and immediate inside the sheath at 20:05 UTC has been calculated. To stabilize the results and minimize the effect of waves, 30 s averages have been used. The calculated shock normal determined from the coplanarity calculation is nCESQ ¼ ð0:67,0:21,0:79Þ. The shock normal therefore has an angle of 553 relative to the comet-Sun direction (xCSEQ) and deviates 23 from the normal of a simple parabolic model bow shock with a subsolar distance of 100 000 km, a 150 000 km distance in the z direction over the comet, and a flaring factor of
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Vsw[km/s]
|B| [nT]
Bz [nT]
By [nT]
Bx [nT]
September 22, 2001: DS1@BORRELLY, CSEQ-Coordinates, 1s data 60 Closest Approach 22:29:33 40 20 0 -20 -40 -60 60 40 20 0 -20 -40 -60 60 40 20 0 -20 -40 -60 90 80 70 60 50 40 30 20 10 0 400 350 300 250 200 150 100 50 0
17:00
18:00
19:00
20:00 21:00 TIME [UTC]
22:00
23:00
00:00
Fig. 4. The upper panels show the magnetic field data in cometo-centered-solar-equatorial (CSEQ) coordinates (1 s averages). The bottom panel visualizes the solar wind speed measured by the PEPE instrument.
1.5 (Huddleston et al., 1990). The observed bow shock distance is in the same order as simulated by Jia et al. (2008) for different types of neutral distributions. No magnetometer measurements were made on the outbound pass near where a potential shock would have been, because the magnetometer onboard DS1 was classified only as a diagnosis instrument for the ion engine and therefore got low priority for scientific measurements. After crossing the inbound bow shock, the fluctuation level increases dramatically. This behavior has already been recognized during the previous encounters at 1P/Halley, 26P/Grigg–Skjellerup, and 21P/Giacobini–Zinner (Neubauer et al., 1986; Scarf et al., 1986; Smith et al., 1986). The cometary magnetic field directionality has been examined to study the possibility of field draping around the nucleus, as first proposed by Alfve´n (1957). Other cometary encounter data have shown these features (Raeder et al., 1987; Smith et al., 1986; Israelevich et al., 1996). For the data acquisition from the shock to the end of the magnetic data acquisition period (from 20:00 until
24:00 UTC) the direction of the magnetic field in CSEQ coordinates is plotted in (xy)- and (xz)-planes. These are the top and bottom panels of Fig. 6. To the right of the data panel sketches of potential draped magnetic fields are drawn. For the top panel, a red vector indicates Bx 4 0 and By o0 and blue indicates Bx 40 and By 4 0. The other two combination of magnetic field directions are given in the legend. For the bottom panel, blue indicates Bx 4 0 and Bz 40 and red indicates Bx 40 and Bz o 0. In the top panel, the magnetic fields measured just after entering the post shock region are red corresponding to a symbolic field line direction indicated by the red arrow in the upper right figure. For large-scale draped fields, meaning that the field direction changes continuously and smoothly, one would expect the outbound post closest approach field in the direction of the black arrow. Instead it is found that the outbound fields are represented by blue arrows. This direction is not consistent with simple draping as the orientation is opposite to the expected. In the xz-plane shown in the bottom panel the situation is similar. The post shock fields are blue. Therefore, for draped fields,
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PLASMA PARAMETER 350
Mach Number [1]
Temperature Tp[K]
Density n[cm^-3]
Vsw[km/s]
Bow Shock 300 250 200 150 35 30 25 20 15 10 5 0 140000 120000 100000 80000 60000 40000 20000 0 25
Ma Mms
20 15 10 5
0 09-22 09-22 09-22 09-22 09-22 09-22 09-22 09-22 09-22 19:00 19:15 19:30 19:45 20:00 20:15 20:30 20:45 21:00 Date\Time (UTC) Fig. 5. Relevant plasma parameters for the DS1 19P/Borrelly encounter. The velocity, density, and ion temperature have been measured by the PEPE instrument. The Mach numbers are computed using magnetometer and PEPE data.
one would expect that the post closest approach fields would be in the direction of the green arrow. Instead they are represented by red arrows, which means that the measured field direction is more or less opposite to the expected direction. Again, this is inconsistent with large scale draped fields. Some possible explanations will be given in the conclusion section.
4. The magnetic pileup region at 19P/Borrelly and comparison to other comets The top panel of Fig. 7 displays the magnetic field magnitude for the 19P/Borrelly encounter. The peak field is 83 nT and occurs at 22 : 34 UT. This peak value occurs 5 min after closest approach (22:29.33 UTC: indicated by a vertical line). It is interesting to mention that the peak fields at the other three comets encountered to date (Neubauer et al., 1986, 1993; Smith et al., 1986) have similar values like 19P/Borrelly and are sometimes offset from closest approach as well. The second panel
shows the 1P/Halley magnetic field, the third the field of 26P/ Grigg–Skjellerup and the bottom panel is the 21P/Giacobini– Zinner field. Vertical lines indicate the closest approach. The peak fields for 1P/Halley are about 56 and 65 nT on the two sides of the magnetic cavity (there are zero fields close to the comet nucleus). This is taken from Neubauer et al. (1986). The closest approach at 1P/Halley was 00:03 UTC. The first peak was 4 min prior to C/A and the second was 2 min after C/A. Thus, there is a small but obvious asymmetry in the fields. The distribution of these peaks is strongly related to the flyby geometry. The peak field at 26P/ Grigg–Skjellerup was 88:7 nT at 15:18:48. The closest approach time was 15:18:43 746 s. Thus, the peak field was within the uncertainty of the C/A time. For 21P/Giacobini–Zinner, the ICE spacecraft went through the tail of the comet. The two peak tail lobes were 58 and 57 nT occurring at 11:01 and 11:04 UT. The time of closest approach was 11:03 (Smith et al., 1986). Smith et al. have said ‘‘The magnetotail crossings are asymmetric with respect to the current sheet, being 160 s inbound and 270 s outbound. Without further analysis, it is uncertain whether to
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Fig. 6. The flyby geometry of the DS1 encounter with comet 19P/Borrelly and magnetic field directions in the most relevant planes.
attribute such differences to structure or time variation.’’ The same can be said for 19P/Borrelly. There is a definite asymmetry in the peak field/pileup region. It is uncertain at this time whether this is due to actual physical structure or time variations. It is interesting to note that the peak magnetic fields for all comets examined here are similar in magnitude. The 19P/Borrelly, 1P/Halley, 26P/Grigg–Skjellerup and 21P/Giacobini–Zinner fields are 83, 56/65, 88 and 58 nT, respectively. This can be explained by examining the pressure balance between the solar wind ram pressure and the magnetic field pressure. At this time, the solar wind ram pressure was 2.6 nPa. The peak magnetic field at 19P/ Borrelly was 83 nT, giving a magnetic pressure of 2.7 nPa. The two values are reasonably close. Thus, the magnetic field draping effect (Midgley and Davis, 1963; Zwan and Wolf, 1976) has squeezed out the plasma and left primarily a low beta high field region. Another feature obvious from Fig. 7 is the relative strengths of the shocks at the four comets. Earlier in our discussion of the 19P/ Borrelly shock, we noted a magnetic jump of 2:5. All of the other cometary shocks have comparable values. These are the typical values indicated by computer simulation models (Lipatov et al., 2002). An overview of relevant parameters of the mentioned comets is given in Table 1. Parameters of the earlier encounters have been compiled using Glassmeier et al. (1997), Huddleston et al. (1992), Israelevich et al. (1996), Neubauer et al. (1986, 1993), Reinhard (1986), Smith et al. (1986), and von Rosenvinge et al. (1986). The DS1 values come from Boehnhardt et al. (1999), Pittichova´ et al. (2008), Soderblom et al. (2002), Weaver et al. (2003) and Young et al. (2004). The bow shock and the contact surface entities were calculated using the formulae
of Cravens (1986). The bow shock distance of 19P/Borrelly is derived from formulae of Schmidt and Wegmann (1982). The estimates of the perpendicular bow shock distances were calculated with a parabolic flaring factor of 1.5 as used in Huddleston et al. (1992). The modeled and measured shock distances of the comets show a maximum deviation of 20%. Besides the already mentioned feature the table shows that the chance to identify a magnetic cavity was given only at the GIOTTO flyby at 1P/Halley. All the other encounters happened in distances too far away from the contact surface.
5. Summary and conclusion Concluding these investigations, it can be stated that in spite of the strong magnetic disturbances of DS1, interesting magnetic field features at comet 19P/Borrelly have been noted. The bow shock had a magnetic compression ratio of 2.5 at a distance of 147 100 km from the nucleus. The sheath region was first entered almost over the north pole at r CSEQ ¼ ð4600,41 400, 147 100Þ km by DS1. Fluctuations from the cometary ion pickup were present throughout the sheath region and even well upstream of the shock, much as expected. The magnetic field pileup region had a peak field strength of 83 nT and was shown to be consistent with a pressure equal to the solar wind ram pressure. The peak field location was offset from the time of closest approach by 5 min. Draping of magnetic fields around the nucleus was sought, but evidence for this was not apparent in the data. A possible explanation is that the interplanetary solar wind was composed of turbulent short-scale fields and thus the fields were not symmetric about the point of
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Fig. 7. The magnetic field magnitudes for 19P/Borrelly, 1P/Halley, 26P/Grigg–Skjellerup and 21P/Giacobini–Zinner. The ordinates for all four panels (field magnitude) are plotted to the same scale and the abscissas (time axes) are different for comparative purposes. The time scale of 26P/Grigg–Skjellerup has been reversed so that the shock is on the left.
Table 1 Relevant cometary parameters. Comet
21P/Giacobini–Zinner
1P/Halley
26P/Grigg–Skjellerup
19P/Borrelly
Mission Flyby date Flyby time Encounter region C/A distance (km) C/A velocity (km/s) Heliocentric distance (AU) Dimensions (radius or axes) (km) Gas production rate (1/s) Solar wind velocity (km/s) Solar wind density (1/cm3) Observed shock distance along trajectory (km) Modeled subsolar bow shock distance (km) Modeled perpendic. bow shock distance (km) Observed peak magnetic field (nT) Time of peak field tC=A 7 Dt (min) Modeled contact surface distance (km) Modeled contact surface field (nT)
ICE 11.09.1985 11:03 Tail 7800 21 1.03 R ¼1.82 2.5 1028 400 6.2 110 000 55 000 83 000 58/57 2/ + 0.5 310 70
GIOTTO 14.03.1986 00:03 Upstream 610 68.4 0.89 15 8 8 8 1029 380 6.0 1 200 000 769 000 1 150 000 57/65 4/ + 2 4440 65
GIOTTO 10.07.1992 15:18:43 Tail 330 14 1.01 R ¼1.3 7 1027 350 8.6 25 000 14 000 21 000 88.7 0 115 71
DS1 22.09.2001 22:29:33 Upstream 2171 16.6 1.36 844 3.5 1028 362 5.2 147 000 95 300 143 000 83 +5 480 57
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closest approach. During the flyby phase there were in general few intervals of large scale Parker spiral fields. Considering all of these pieces, a consistent picture of the magnetic field signature was revealed. Although the FGM was included onboard DS1 as a ’guest instrument’ to serve as ion engine diagnostic tool, it gathered interesting data during the flybys at the asteroid 9969 Braille and the comet 19P/Borrelly. The methods for the science analysis presented here will be enhanced for the ROSETTA project (Glassmeier et al., 2007b). Once ROSETTA will arrive in the comet 67P/Churyumov-Gerasimenko system the instruments of the ROSETTA Plasma Consortium (RPC) will collect unique data and reveal new knowledge about cometary plasma physics.
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