ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus GmbHGöttingen, Germany10.5194/angeo-32-1441-2014A comparison between VEGA 1, 2 and Giotto flybys of comet 1P/Halley: implications for RosettaVolwerkM.martin.volwerk@oeaw.ac.athttps://orcid.org/0000-0002-4455-3403GlassmeierK.-H.DelvaM.SchmidD.https://orcid.org/0000-0001-7818-4338KoendersC.RichterI.SzegöK.https://orcid.org/0000-0002-9740-265XSpace Research Institute, Austrian Academy of Sciences, 8042 Graz,
AustriaInstitute for Geophysics and Extraterrestrial Physics, TU
Braunschweig, GermanyWigner Research Centre for Physics,
Institute for Particle and Nuclear Physics, Hungarian Academy of
Sciences, Budapest, HungaryM. Volwerk (martin.volwerk@oeaw.ac.at)28November20143211144114531October20141November2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/32/1441/2014/angeo-32-1441-2014.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/32/1441/2014/angeo-32-1441-2014.pdf
Three flybys of comet 1P/Halley, by VEGA 1, 2 and Giotto, are investigated
with respect to the occurrence of mirror mode waves in the cometosheath and
field line draping in the magnetic pile-up region around the nucleus. The
time interval covered by these flybys is approximately 8 days, which is also
the approximate length of an orbit or flyby of Rosetta around comet
67P/Churyumov–Gerasimenko. Thus any significant changes observed around
Halley are changes that might occur for Rosetta during one pass of 67P/CG. It
is found that the occurrence of mirror mode waves in the cometosheath is
strongly influenced by the dynamical pressure of the solar wind and the
outgassing rate of the comet. Field line draping happens in the magnetic
pile-up region. Changes in nested draping regions (i.e. regions with
different Bx directions) can occur within a few days, possibly
influenced by changes in the outgassing rate of the comet and thereby the
conductivity of the cometary ionosphere.
Interplanetary physics (interplanetary magnetic fields; plasma waves and turbulence) – space plasma physics (waves and instabilities)Introduction
In this new era of cometary physics, which started with the arrival of the
Rosetta spacecraft at comet 67P/Churyumov–Gerasimenko (67P/CG)
on 6 August 2014, it is worthwhile to take another look at previous satellite
encounters with comets. Rosetta will orbit around the comet and follow it
along its path through perihelion and beyond. Rosetta will thus observe the
temporal variations in the interaction of the outgassing comet with the solar
wind magnetoplasma.
In order to simulate Rosetta orbiting 67P/CG and the changes that can be
expected in the comet–solar wind interaction, three flybys of comet 1P/Halley
by VEGA 1, 2 and Giotto are used. These flybys, within a time span of 8 days
showed significant differences in the interaction in the plasma data; a
similar data set will be generated by the Rosetta Plasma Consortium
RPC, at 67P/CG.
Two different processes are discussed in this paper, first the presence and
generation of mirror mode waves in the cometosheath and then the magnetic
field line draping around the nucleus in the magnetic pile-up region.
Flybys of comet 1P/Halley by VEGA 1, 2 and Giotto
There have been three close flybys of 1P/Halley by Vega 1, 2 and Giotto, all
three within 8 days and along very similar orbits as can be seen in
Fig. .
The orbits of the three close flybys by Giotto (blue), VEGA 1 (red)
and VEGA 2 (green) in cometocentric–solar–ecliptic (CSE) coordinates. The dashed line shows the location of
the bow shock for the Giotto flyby as a guidance. Note that the bow shock
location will differ for the other two flybys. The inset panel shows a
zoom-in to clarify the difference in the orbits near the
comet.
The magnetic field data of these flybys are shown in Fig. .
Note that these data are obtained in shocked solar wind, i.e. within the
cometosheath (the region between the bow shock and the ionopause). It is
clear from this figure that the flybys show very different conditions around
the comet in all components of the magnetic field. The increase of the total
magnetic field Bt during ingress is much more gradual for VEGA1/2
(red/green) as compared to Giotto (blue). Also, note the different sign of
Bx for VEGA1 and VEGA2. For Giotto, multiple sign changes in Bx
around closest approach are found see e.g.discussing “nested”
field line draping regions, whereas VEGA1 only shows one, and VEGA2
shows no sign change at all. Thus within a time span of 8 days, the structure
of the cometosheath has changed significantly. The magnetic field vector
plots of the three flybys in the cometocentric–solar–ecliptic (CSE)
xy plane, containing the dominant magnetic field directions, are shown in
Fig. . In the CSE coordinate system the X axis points
towards the Sun, the Z axis is perpendicular to the orbital plane around
the Sun (positive towards the north) and the Y axis completes the triad.
The magnetic field components in CSE coordinates, total field Bt
and magnetic pressure Pb of the three flybys by Giotto (blue), VEGA 1
(red) and VEGA 2 (green).
The orbits of the three close flybys by Giotto (blue), VEGA 1 (red)
and VEGA 2 (green) in CSE coordinates with the magnetic field vectors in the
CSE xy plane plotted along the spacecraft orbits.
Mirror-mode waves
In order to see how the magnetoplasma changes around the comet the
magnetometer data are studied for the presence of mirror-mode (MM) waves
within the cometosheath using the magnetic-field-only method presented by
. A sliding window minimum variance analysis (MVA)
is performed, whereby MMs are identified by the following
criteria:
Angle γ between minimum variance direction and background magnetic field is larger then 80∘;
Angle α between maximum variance direction and background magnetic field is smaller than 20∘;
The amplitude of the waves δB/B>0.3.
This method has been shown to work well see e.g. and in order to improve on the determination of the MMs a
lower limit on the goodness of the MVA can be set using the minimum
λmin and intermediate λint eigenvalues of the
MVA matrix through the following: λint/λmin>5. There
can be short intervals between MMs in which the criteria as stated above are
not fulfilled. However, if that interval is shorter than 20 s the
neighbouring MM intervals are considered to be part of one larger interval.
Examples of mirror-mode waves in the Giotto data. Top: total
magnetic field with low-pass filtered (background) magnetic field as a dashed
line; middle: electron density; bottom: angles α/γ between
minimum/maximum variance direction with the background magnetic
field.
In order to generate MM waves, there needs to be a source, e.g. a ring
distribution of the ions see e.g.,
where there is an asymmetry between the parallel and perpendicular
temperature of the ions. This temperature asymmetry can either be generated
at the quasi-perpendicular bow shock through perpendicular heating of solar
wind ions or be generated by local ionisation and pick up. It has been shown
by that at Venus the source of the MMs is at the bow shock
see also, after which they show turbulent
diffusion as suggested by , whereas at Halley the source is
pick up of local ions as the bow shocks at comets are typically weak
. Using VEGA 1 and 2 plasma data have shown the
presence of slowed-down solar wind plasma and picked up cometary ions at
distances between 1.5 to 0.5 million km from the comet, within the bow
shock.
A current discussion on the presence of MMs in planetary magnetosheaths
generated by ion temperature anisotropies can be found in and
gives an overview of MMs in the magnetosheath and heliosheath,
and their distinguishing features from magnetic decreases (also known as
magnetic holes).
Giotto
The automated MM search is applied to the Giotto magnetometer data
at a data resolution of 1 s, over the interval 13 March 1986
19:00 UT until 14 March 1986 06:00 UT, which spans the interval that the
spacecraft is inside the cometosheath. Two examples of MM wave intervals are
shown in Fig. , but see also for further
examples. The total magnetic field strength is shown as well as the electron
density, indicating a good anti-correlation. Also the two angles α and
γ are displayed; the red/green coloured intervals show where the
search criteria are fulfilled.
An overview of all MM events found is given in Fig. , where
the length, the relative amplitude, the MVA goodness and the average
background magnetic field strength are shown. The red markers show the events
for which the MVA goodness is good (λint/λmin>5). The gray box shows the interval of the magnetic pile-up region (MPR)
in which no MM events are expected see e.g.. It is
clear that there are many more events before the MPR than after, indeed the
last 3 h do not show any activity and are therefore omitted in the figure.
Overview of all the time intervals with MMs for the Giotto flyby.
Shown are the length of the interval, the goodness of the MVA, the strength
of the MMs and the average background field strength. The magenta circles show
for which MMs the MVA goodness is greater than 5. The gray-shaded area is the
magnetic pile-up region in which no MMs are expected.
To make the distribution of the MM events clearer, histograms for ingress
(Fig. top left) and egress (Fig. top right)
show the number of events binned by the number of sequential sliding windows
which means that the total length of the MM interval in seconds is 30 s
longer. From the histograms it is clear that during ingress there are many
more events than at egress. This is partially caused by the shorter period in
the cometosheath after exiting the MPR, but that cannot explain the total
discrepancy. Therefore, there have to be different reasons for this, which
will be discussed below.
VEGA 1
The VEGA 1 flyby occurred on 6 March 1986, 8 days before the Giotto flyby.
The magnetometer data at a resolution of 1 s, between
∼ 04:30 and ∼ 08:30 UT are searched for MM waves. An example of
an MM interval is shown in Fig. , and an overview of all
events found is shown in Fig. . In this case there are fewer
events before the MPR than after, even though the egress time interval is
shorter.
Figure middle panels show histograms of the duration of the
MM intervals, which shows an almost smooth distribution of MM interval
lengths. Apart from there being less events compared to the Giotto flyby,
there are slightly less events during ingress than egress, although there is
not a great difference in total number of events.
Histograms of the number of time windows in which MMs were observed
per event at ingress (left) and egress (right) for Giotto (blue), VEGA 1
(red) and VEGA 2 (green). To obtain the length of the MM interval the length
of the window, 30 s, should be added to the size of the bin. For VEGA 2 the
requirement on the amplitude ΔB/B>0.3 has been dropped, and thus
the number of events is increased.
VEGA 2
The VEGA 2 flyby occurred on 9 March 1986, 3 days after VEGA 1 and 5 days
before Giotto. Here the magnetometer data were used at 1 s resolution for the
interval ∼ 04:15–∼ 07:20 UT, i.e. up until closest approach,
after which the sensor was saturated and no longer useful for data
evaluation. An example of a MM interval is shown in Fig. ,
where possibly only the last interval at 05:58 UT is a true MM. Note that in
this case the amplitudes ΔB/B of the MMs, as shown in
Fig. is much smaller than in the other two flybys discussed
above. The requirement on the amplitude ΔB/B>0.3 was relaxed for
VEGA 2 in order to find more events. For completeness the distribution of the
MMs is shown in Fig. , however this cannot be compared
directly with the other two histograms shown above.
Example of an MM interval in the VEGA 1 data, in similar format as
Fig. .
Overview of all MM intervals for the VEGA 1 flyby, in similar format
as Fig. .
MM determination: a recap
Comparing the MMs during the three flybys of comet 1P/Halley shows a very
diverse situation within a time span of only 8 days. In chronological order,
VEGA 1 observes only few intervals of MM wave activity, with a slight
number-preference for the outbound leg of the flyby. VEGA 2 shows very little
activity, and only when the amplitude limit for the waves is dropped more
than one event is found. The Giotto flyby, however, shows very strong MM wave
activity, with a strong number-preference for the inbound leg of the flyby.
The overview of the magnetic fields, see Fig. , shows that
apart from the difference in the direction of Bx, the cometosheath total
magnetic field Bt remains constant over a large distance for Giotto,
whereas the magnetic field for the VEGA flybys show more variations and a
slow but gradual increase towards closest approach.
Figure shows the vector plots of the magnetic field along
the orbits of the three spacecraft in the CSE xy plane, low-pass filtered
for periods longer than 20 min. This shows that on time scales longer than
20 min the magnetic field strongly rotates in the xy plane for Giotto,
with mainly Bx variations, whereas it remains rather constant for VEGA 1
with major negative By component and VEGA 2 with major positive
Bx component. This probably means that a constant direction of the
magnetic field is not a major player in the creation of the MMs.
Sources of changes in the cometosheath
The outgassing comet is embedded in the solar wind, and therewith two sources
for changes in the cometosheath are already found. An increased dynamic
pressure of the solar wind will make the cometosheath shrink; on the other
hand an increase in outgassing will extend the cometosheath; both will also
influence the magnetic field structure.
Example of an MM interval in the VEGA 2 data, in similar format as
Fig. .
Overview of all MM intervals for the VEGA 2 flyby, in similar format
as Fig. .
Solar wind
Changes in the solar wind, such as the interplanetary magnetic field (IMF)
direction or dynamic pressure, will lead to a different interaction with the
comet. It is already clear from Fig. that there seem to be
nested draping regions around 1P/Halley during the Giotto flyby
, whereas the draping pattern for VEGA 1 and 2 seem to be
rather constant if differently directed. In order to see the behaviour of the
solar wind during the flybys, the average values for the IMF are shown in
Table .
30 min averages of the solar wind parameters before and after bow
shock crossing.
For the Giotto flyby the IMF direction changes significantly between before
entering the cometosheath and after exiting. Unfortunately, there are no
plasma data available after closest approach. Similarly, for VEGA 1 the IMF
changes drastically between inbound and outbound as is evident by the sign
changes of both Bx and By. For VEGA 2 there is only inbound
information for the magnetic field.
It is clear that 1P/Halley is in a completely different solar wind during the
flybys of VEGA 1/2 as compared with the Giotto flyby, as tabulated in
Table . The dynamic pressure is 4 to 5 times larger in the
former. This results in a very different magnetic pressure in the
cometosheath, which can be seen in Fig. bottom panel, for
VEGA 1 and VEGA 2 the magnetic pressure Pb is a factor of 5 and 4
greater than for Giotto, during ingress.
Cometary outgassing
The outgassing of comets is not a continuous phenomenon, and can vary
strongly during the perihelion passage; see e.g. for
outbursts of comet 1P/Halley; for outbursts of comet P/Faye
1991 XXI; for outbursts of comet 9P/Tempel 1; and
for outbursts of comet C/2012 S1 ISON.
Observations in the visual and in the infrared
have shown that the brightness of comet 1P/Halley in general followed the
expected light curve, however, there were strong short time scale (hours,
days) variations in the brightness of the comet. These enhancements are
connected to bursts in the outgassing of the comet.
investigated the VEGA 1 and 2 plasma data and found that the
density of the plasma flow for the VEGA 1 encounter was much higher than for
the VEGA 2 encounter.
used the Lyα measurement from the Pioneer Venus UV
spectrometer and the OH measurements from the IUE
of comet 1P/Halley to obtain the water production rate. The
flybys of the three spacecraft discussed in the current paper is covered by
the period over which the activity of the comet was monitored, shown in Fig. 8 of
. It was found that the water production had a strong decrease
from ∼1.5×1030 to ∼5.0×1029 molecules s-1 on
7 March. This means that the VEGA 1 flyby was during high-rate outgassing,
whereas the VEGA 2 and Giotto flybys were during low-rate outgassing, in good
agreement with .
Suppression of MMs
The mirror mode wave is generated by freshly picked up ions near the comet
which will create a ring distribution in velocity space with parallel
temperature T‖ smaller than perpendicular temperature T⟂. The
instability criterion for MM waves is given by the inequality
:
1+β⟂1-T⟂T‖<0,
where β⟂=(nkBT⟂)/(B2/2μ0) is the so-called
perpendicular plasma-β. Equation () can be rearranged into
T⟂T‖-1>β⟂-1=B22μ0nkBT⟂,
which indicates that for strong fields less MM are expected. This leads to an
understanding why there are such strong differences in MM occurrence between
the three flybys.
During the VEGA 1 flyby, there was strong outgassing, i.e. strong pickup
of freshly ionised water. This happened in a very strong magnetic field, with
an average magnetic pressure of ∼0.15 nPa during ingress. The solar
wind is slowed down significantly . Apparently, the energy
density in the picked-up ions, determined by density and pick-up velocity, is
large enough to increase β⟂ and hence fulfill the instability
criterion in Eq. ().
During the VEGA 2 flyby, the outgassing dropped by a factor of 3, hence the
pickup of freshly ionised water decreased as well, the solar wind slowed down
similarly to the VEGA 1 flyby. The magnetic pressure, however, in the
cometosheath remained at a high level, ∼0.12 nPa. In this case the
energy density in the picked-up ions is not enough to increase
β⟂ sufficiently. This leads to a lack of MM structures.
During the Giotto flyby the outgassing of the comet remained at a low
level, however, also the magnetic pressure in the cometosheath was
significantly reduced to ∼0.03 nPa. The solar wind velocity is still
low, comparable to the VEGA flybys . The reduction of the
magnetic pressure decreased β⟂ again sufficiently to fulfill the
instability criterion of Eq. (). With the lowered magnetic
pressure it is easier to generate MMs, thus the increased number of events
compared with VEGA 1.
This lets us conclude that the internal (outgassing) and external (solar wind
dynamic pressure) influences on the cometosheath both determine the
occurrence rate of MMs around 1P/Halley. However, this does not explain the
strong asymmetry that has been measured between the ingress and egress legs
of Giotto's flyby. A closer look at the magnetic pressure for the Giotto
flyby (Fig. bottom panel) shows that during egress the
pressure varies to values at least double that during the ingress leg of the
flyby. Therefore, the same reasoning as above can be given: during egress the
magnetic pressure is often too strong and thus the instability criterion
Eq. () is less likely to be fulfilled, thereby reducing the
number of MMs.
Magnetic field line draping
Magnetic field line draping, i.e. the hanging up of the IMF in the
conducting ionosphere around the cometary nucleus was first proposed by
for the mechanism by which cometary ion tails are formed. The
“far end” of the field line is transported by the solar wind, whereas the
part near the comet is slowed down by the conducting layer. Thus the field
wraps around the nucleus, bending from the IMF direction into the
XCSE direction.
Using the Giotto data at comet 1P/Halley, used vector plots of
the magnetic field along the trajectory of the spacecraft, similar to
Fig. , and showed that there were many directional changes
in the draping pattern. Looking at the “nesting” of these directional
changes, they were able to connect these changes on the ingress part with
those on the egress leg of the orbit. This showed that there was a pile-up of
old IMF layers around the comet. A comparison of field line draping around
comet 21P/Giacobini–Zinner and Venus, creating a magnetotail, has been
studied by using ICE and PVO data from flybys through the
respective (induced) magnetotails.
Top panel: the magnetic field data from Giotto with the 10 intervals for which field line draping was studies marked in gray. The red
dotted lines show the boundaries between differently draped field as
determined by . Bottom panels: field line draping fits,
according to Eq. () for three intervals: a bad fit with
|rc|≤0.8; a peculiar fit with positive slope A and a
good fit with rc≥0.8.
introduced a method to identify regions where the magnetic
field has a draped-like structure. The magnetic field data are projected on a
pre-encounter solar wind coordinate system, where XIMF is along
the solar wind velocity, YIMF is given by the cross product of
the solar wind velocity and the IMF and ZIMF completes the triad.
The encounter magnetic field is projected onto this coordinate system
producing BX,IMF and a transverse BT,IMF. The
latter is projected onto the radial direction from the comet to the
spacecraft, producing Brad, see Fig. . In the
case of field line draping there is a linear dependence:
Brad=ABX,IMF+C.
Indeed, found that for the Giotto flyby at both ingress and
egress, close to the comet there is a linear dependence. They show that the
draping is a feature of the MPR, whereas in the cometosheath the field is too
turbulent to show any such feature. Similarly, have shown that
for the VEGA 1 encounter at comet 1P/Halley there is field line draping
within the MPR, but no such feature outside.
Magnetic field draping around a comet and field projections at two
different locations showing that the radial projection of the draped magnetic
field changes direction for “over-draped” field
lines.
A more detailed look
used the full MPR, with exclusion of closest approach where
the spacecraft entered the diamagnetic cavity, to determine the relationship
between Brad and BX,IMF. In this section a closer
look at the data is taken; shorter time intervals related to the different
draping regions that were determined by .
Top panel: the magnetic field data from VEGA 1 with the eight
intervals for which field line draping was studies marked in gray. The black
area shows the region where the magnetometer was saturated and which was
excluded from analysis. Bottom panels: field line draping fits, according to
Eq. () for three intervals: a bad fit with |rc|≤0.8; and two good fits with |rc|≥0.8 pre- and
post-closest approach, respectively.
In Fig. the magnetic field data from Giotto are shown
with the intervals that are investigated for draping marked by a gray
background. The intervals were chosen for relatively constant Bx, which
seem to coincide well with the boundaries for the draping that were
determined by , marked by dotted red lines. The results of the
fit given by Eq. () are shown in Table . Clearly,
not all fits are good as is evident from the correlation coefficient,
rc, being small. Three
examples of “bad”, a “peculiar” and a “good” fits are shown in
Fig. .
It is clear from the fit results in Table that there is an
anti-correlation between Brad and BX,IMF, something
also found by other studies and at e.g. Venus and Mars which
have a cometary-like interaction with the solar wind . However, this current study finds that near closest approach there
is a positive slope of the fit, as seen for intervals 4, 5 and 6 in
Table . This indicates that the magnetic field is
over-draped, bending towards the axis of the tail as was observed e.g. at Venus . A schematic of the change between Brad
positive and negative can be seen in Fig. , where for
B1 it is seen that Brad is projected outward, whereas for B2
it is found that Brad is projected inward. This phenomenon of
positive correlation can only happen on the “night-side” of the comet and
because of the orbit of the spacecraft, seen in Fig. , such a
region is not crossed during egress. Speculatively, there could even be the
possibility that a closed loop around the nucleus is created by a tail
reconnection event.
Magnetic field line draping fit results
Brad=ABX,IMF+CGiotto (13 March 1986) no.timefitrcpoints123:38–23:46-1.38X+6.55-0.70451223:47–2353-0.82X+5.42-0.82479323:56:30–23:590.26X+6.120.36141400:01–00:03:300.58X-4.990.7141500:04:30–00:081.00X-18.210.92197600:13:30–00:15:30-1.00X-2.16-0.8982700:19:30–00:21-1.11X-6.74-0.6474800:22:30–00:27:30-0.86X+2.70-0.82258900:32–00:36:30-0.81X+8.38-0.782471000:41–00:56-1.02X-3.37-0.89765VEGA 1 (6 March 1986) 107:00–07:03-0.44X+25.83-0.53180207:04–07:05:30-0.69X+17.46-0.63368307:06–07:08-0.77X+15.58-0.76120407:08:30–07:10-0.64X+19.63-0.8290507:11:30–07:12:30-0.89X+14.80-0.9460607:14:30–07:16-1.25X+24.33-0.9890707:23:30–07:26-1.72X-5.15-0.99150807:26:30–07:29-1.36X-12.36-0.82150VEGA 2 (9 March 1986) 105:10–05:16:30-0.17X+9.10-0.20390205:23–05:25-0.06X+9.51-0.04681305:37–05:51-0.18X+7.22-0.30829405:52:30–06:05-0.68X+14.86-0.57744506:09–06:21-0.24X+3.54-0.20720606:25:30–06:37-0.54X+8.69-0.58690706:39–07:00-0.37X+4.35-0.371246807:15:45–07:18-1.59X+82.87-0.92133
At VEGA 1 the same study is performed see also and the
chosen intervals can be found in Table and seen in
Fig. . The interval in black is where the magnetometer
saturated, which is not taken into consideration for data analysis. Only near
closest approach inside the MPR, intervals 5, 6 7 and 8, there is a very
clean draping signature, with high values of the correlation coefficient rc.
Further away from the comet in the cometosheath the correlation is bad with
low values of the rc (see Table ). This is in agreement with
the earlier statement that the turbulence in the cometosheath dominates the
draping pattern. The same holds for the draping during the VEGA 2 flyby,
shown in Fig. . Only the short interval 8, before closest
approach shows a clear field line draping signature.
Top panel: the magnetic field data from VEGA 2 with the seven
intervals for which field line draping was studies marked in gray. Bottom
panels: field line draping fits, according to Eq. () for three
intervals: two bad fits with |rc|≤0.8; and the only good fit with |rc|≥0.8.
Interpretation of draping fit
There is clearly a difference in the slope A of the fits to
Eq. () for the good cases. The first difference observed in the
Giotto data is that A can not only be negative, the usual result, but also
positive, where the field line is over-draped into the direction towards
the axis of the cometary ion tail as explained above and in
Fig. .
However, it is also seen that the slope of the draping fit for good fits,
i.e. |rc|≥0.8 (see Table ), varies between
-0.64≤A≤-1.72 as the spacecraft move past the cometary nucleus.
In Fig. the slope is plotted as a function of radial distance
from the comet. There seems to be a weak trend for the slope to increase with
decreasing distance to the comet. Also the offset C varies strongly, -18≤C≤82, but that is a direct consequence of the fact that
Brad is the projection of Btrans on the radial direction
and thus Brad and BX,IMF do not add up to B.
The slope |A| of the draping fit as given in Eq. ()
as a function of radial distance to the comet.
There is evidence of only little nesting of different draping regions for
VEGA 1 , compared to what has been seen during the Giotto flyby
. Near closest approach before the saturation, during intervals
4, 5 and 6, there is a good draping fit. However, from
Fig. it is clear that 4 and 6 have oppositely directed
Bx, with interval 5 having basically Bx=0. However, for the
VEGA 2 flybly there is no change in the direction of the draped magnetic
field. This means that either VEGA 2 did not approach the cometary nucleus
close enough to actually penetrate deeply into the pile-up region, or the
changes in the environment between the two flybys caused the nested fields to
dissipate. Zooming in on the vector plots of the two flybys, as shown in
Fig. , shows that VEGA 2 is clearly entering a region where
there was already a directional change for VEGA 1.
The solar wind dynamic pressure remained basically the same for VEGA 1 and 2,
however, the outgassing of 1P/Halley was strongly reduced. This reduction
will lead to less ionisation which will have two effects: (1) less ion
pick up by the solar wind IMF and thus less slow down; and (2) a lesser
conductivity of the ionosphere and thus less hang-up of the field lines. Both
effects will lead to a faster transport of the “old” magnetic field away
from the pile-up region.
With the lesser solar wind dynamic pressure during the Giotto flyby, lesser
pick up can significantly decelerate the solar wind in the cometosheath, as
was observed by e.g. . Checking the solar wind IMF the day
before the encounter of Giotto (12 March 1986, not shown), reveals several
directional changes in all three components of the magnetic field. Convection
of these structures and subsequent hanging-up in the MPR with only slow
diffusion will lead to the observed nested draping.
Vector plots of the low-pass filtered magnetic field for VEGA 1
(red) and 2 (green), zoomed in on the inner 10 000 km around the cometary
nucleus. VEGA 2 enters well into the region where VEGA 1 shows changes in the
sign of Bx, but VEGA 2 shows only one sign of
Bx.
Connection between mirror mode waves and field line draping
Mirror mode waves, as explained above, are generated by a temperature
asymmetry with an instability criterion given by Eq. (). The
temperature asymmetry can be generated by heating at the quasi-parallel bow
shock see e.g. or by magnetic field line draping and
pile-up see e.g..
Ions attached to the magnetic field are transported through the magnetosheath
towards the obstacle in the flow (be it planet or comet). When the magnetic
field lines reach the conducting layer around the obstacle they drape
themselves, thus creating a magnetic pile-up region in which the magnetic
field strength increases. The first adiabatic invariant μ=mv⟂2/2B makes that with increasing field strength the perpendicular
temperature of the ions increases, thereby increasing the temperature
asymmetry and augmenting the MM instability.
It has been shown by that MMs are created by local pick up of
ions near the cometary nucleus. At comet 1P/Halley the cometopause, the
boundary that separates the region of fast flowing solar wind protons
dominated plasma from the more slowly flowing cometary ions dominated plasma,
was located at ∼1.6×105 km from the nucleus .
The first MM encountered by Giotto was measured at ∼(-0.3,-1.1.-0.1)×106 km, well away from the cometopause, which means the picked-up
cometary ions will also experience this energisation, and thus more and
stronger events should be expected near the MPR.
The time development of various parameters around comet 67P/CG
obtained from the model by : the distance comet–Sun; the
magnetic field strength of the IMF and the pile-up region; the ion density;
the distance of the bow shock and the ionopause from the comet; the radius of
the pick-up cycloid at the pile-up boundary. The gray-shaded area shows the
interval during which the cometosheath has a size greater than 2 ion gyro
radii.
Figure shows the Giotto events, where the MPR is
grayed-out. There is a clustering of more events at the MPR, but no clear
increase in strength. Inside the MPR events are found which are comparable to
those outside the MPR. However, have shown that the waves
outside and inside the MPB are rather different, where outside the MPB the
electron density variations and the magnetic field variations are in
anti-phase (as expected for MMs) and inside the MPB these are in phase and
are identified as a fast magnetoacoustic waves see also.
Indeed, it seems that the magnetic pile-up boundary (MPB), where the strong
increase of the magnetic field strength starts, separates two plasmas with
different qualities. showed that he MPB can be described by as
a rotational discontinuity with strongly differing plasma anisotropies on
either side. Studying suprathermal electrons, showed that
there was a clear plasma discontinuity at the MPB.
Implications for Rosetta
Rosetta has arrived at comet 67P/Churyumov–Gerasimenko and will start to
orbit the comet and follow it along its way to perihelion and beyond. This
means that during the lifetime of Rosetta's mission, the comet will encounter
strongly differing solar wind conditions. Rosetta's orbits around or flybys
of 67P/CG take on average 1 week. The above flybys of comet 1P/Halley are
therefore representative of what can be expected at 67P/CG with respect to
the ever changing interaction of the outgassing comet and the dynamic solar
wind.
67P/CG is a weakly outgassing comet, and the interaction region around the
nucleus is only slowly growing, as shown in Fig. 11.
Recently, have used both a multifluid-MHD and a hybrid code to
model the interaction of the solar wind with a weakly outgassing comet. The
magnetic pile-up region is expected to be fully developed in May 2015 and the
bow shock is not expected to appear until July 2015. This means that the
magnetic field line draping can studied from rather early on in the mission
during the escort phase. However, for the mirror mode waves the situation is
different as they will develop outside the MPR but inside the bow shock, in
the cometosheath. The expected global morphology of the interaction region of
the solar wind with the outgassing 67P/CG has recently been discussed by
.
Figure shows the time development of various parameters: the
distance of the 67P/CG to the sun, the solar wind and pile-up magnetic field
strength, the density, the distance of the bow shock and the ionopause and
the pick-up ion cycloid at the pile-up boundary, as taken from the
simulations by . From measurements at comet 1P/Halley it was
found that the size of the water mirror mode waves was on the order of 1 to 2
ion gyro radii , therefore it stands to reason that the size of
the cometosheath should at least be 2 ion gyro radii. In Fig. the
gray bar shows the region in which the gyro radius is less than half the bow
shock distance. This indicates that there will be only a small time window in
which these waves can develop from ion pick up near the comet.
Conclusions
The opportune flybys of comet 1P/Halley by three spacecraft over a time span
of 8 days have shown how strongly the interaction of an outgassing comet with
the solar wind can change over a time interval which is similar to a Rosetta
flyby of/orbit around comet 67P/Churyumov-Gerasimenko. Changes in the dynamic
pressure of the solar wind will have influence on the generation of
mirror-mode waves in the cometosheath, as well as a change in the outgassing
rate of the comet. Increased solar wind dynamic pressure will compress the
magnetosheath and inhibit MM growth, whereas increased outgassing will
enhance ion pick up and assist MM growth.
Magnetic field line draping and pile-up near the cometary nucleus leads to nested draping, i.e. regions of differently directed magnetic field in
the MPR. The change in outgassing rate, and therewith the change in
ionisation and ionospheric conductivity leads to a disappearance of the
nested structures (between VEGA 1 and VEGA 2), whereafter a new nested
structure is build up (between VEGA 2 and Giotto).
Although comet 1P/Halley was up to 2 orders of magnitude stronger in
outgassing than 67P/CG will be, the basic physical processes will remain the
same. The yield of the three 1P/Halley flybys discussed in this paper for the
Rosetta mission is to show that the cometary environment can change quickly
and well within the planned duration of the Rosetta flybys.
Acknowledgements
The authors would like to acknowledge the PDS/PPI for the Giotto and VEGA1/2
magnetometer data. The work of K.-H. Glassmeier, I. Richter and C. Koenders
was financially supported by the German Bundesministerium für Wirtschaft
und Energie and the Deutsches Zentrum für Luft- und Raumfahrt under
contract 50 QP 1001 for Rosetta. D. Schmid acknowledges support by the
Austrian Science Foundation (FWF) under contract P25257-N27. Topical Editor L. Blomberg thanks two anonymous referees
for their help in evaluating this paper.
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