ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-34-1-2016Mass-loading, pile-up, and mirror-mode waves at comet 67P/Churyumov-GerasimenkoVolwerkM.martin.volwerk@oeaw.ac.athttps://orcid.org/0000-0002-4455-3403RichterI.TsurutaniB.GötzC.AltweggK.BroilesT.BurchJ.CarrC.CupidoE.DelvaM.DósaM.EdbergN. J. T.https://orcid.org/0000-0002-1261-7580ErikssonA.HenriP.KoendersC.LebretonJ.-P.MandtK. E.NilssonH.https://orcid.org/0000-0002-7787-2160OpitzA.RubinM.https://orcid.org/0000-0001-6549-3318SchwingenschuhK.Stenberg WieserG.https://orcid.org/0000-0002-4260-2937SzegöK.https://orcid.org/0000-0002-9740-265XVallatC.VallieresX.GlassmeierK.-H.Space Research Institute, Austrian Academy of Sciences, Graz, AustriaInstitute for Geophysics and Extraterrestrial Physics, TU Braunschweig, GermanyCalifornia Institute of Technology, Pasadena, California, USAPhysikalisches Institut, University of Bern, Bern, SwitzerlandSouthwest Research Institute, San Antonio, Texas, USASpace and Atmospheric Physics Group, Imperial College London, London, UKWigner Research Centre for Physics, Institute for Particle and Nuclear Physics, Hungarian Academy of Sciences, Budapest, HungarySwedish Institute of Space Physics, Uppsala, SwedenLaboratoire de Physique et Chimie de l'Environnement et de l'Espace, Orleans, FranceSwedish Institute of Space Physics, Kiruna, SwedenRosetta Science Ground Segment, European Space Astronomy Centre, Madrid, SpainM. Volwerk (martin.volwerk@oeaw.ac.at)15January201634111516October20151December20156December2015This 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/34/1/2016/angeo-34-1-2016.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/34/1/2016/angeo-34-1-2016.pdf
The data from all Rosetta plasma consortium instruments and from the ROSINA
COPS instrument are used to study the interaction of the solar wind with the
outgassing cometary nucleus of 67P/Churyumov-Gerasimenko. During 6 and 7 June
2015, the interaction was first dominated by an increase in the solar wind
dynamic pressure, caused by a higher solar wind ion density. This pressure
compressed the draped magnetic field around the comet, and the increase in
solar wind electrons enhanced the ionization of the outflow gas through
collisional ionization. The new ions are picked up by the solar wind magnetic
field, and create a ring/ring-beam distribution, which, in a high-β
plasma, is unstable for mirror mode wave generation. Two different kinds of
mirror modes are observed: one of small size generated by locally ionized
water and one of large size generated by ionization and pick-up farther away
from the comet.
Space plasma physics (charged particle motion and acceleration; nonlinear phenomena; waves and instabilities)Introduction
The theory of the interaction of an outgassing comet with the solar wind
magnetoplasma started with the explanation of the formation and physics of
the cometary ion tails by and . With the
beginning of the space age and spacecraft-flybys of comets in the last
century, e.g. VEGA 1, 2, Giotto, ICE, Sakigake and Suisei by comet
1P/Halley, Giotto at 26P/Grigg-Skjellerup and ICE at 21P/Giacobini-Zinner,
much has been learned about the various physical processes taking place in
the plasma around the outgassing cometary nucleus.
In the current century, on 20 January 2014 the Rosetta spacecraft
was woken up after 18 months of hibernation, and the
spacecraft cruised towards its rendezvous with comet
67P/Churyumov-Gerasimenko (67P/CG). On 6 August 2014 Rosetta arrived at its
target, and started its escort phase, following the comet along its orbit
from pre- to past-perihelion. 67P/CG's perihelion was on 13 August 2015.
In this paper the data from the Rosetta Plasma Consortium instruments
RPC, are used to study the interaction of the outgassing
nucleus of comet 67P/CG and the solar wind magnetoplasma at a time when the
comet is closing in on its perihelion. Unlike the previous missions mentioned
above, Rosetta does not perform a quick flyby of the comet, but remains at
the comet, moving at a very slow pace of ∼1 m s-1. This means that
Rosetta RPC can follow the development of the interaction of the solar wind
with the increasingly more actively outgassing nucleus as comet 67P/CG heads
towards perihelion, and the decreasing activity after perihelion.
After initial arrival a new phenomenon was found, now called the “singing
comet” ; ∼40 mHz waves generated by a cross-field
current instability created by freshly ionized, not yet magnetized water ions
within the Larmor sphere sphere with radius of 1 Larmor
radius, of the comet. At that time, these newly created ions also
indicated the “birth of a magnetosphere” for which the
spatial distribution of the low-energy plasma was discussed by
. However, “conventional signatures” such as Alfvén waves
or cyclotron waves were not observed.
Later in the mission, with comet 67P/CG approaching its perihelion, the
activity of the nucleus increased significantly. Various strong outbursts
were observed by the Rosetta NAVCAM, see Fig. , which mainly
shows reflected sunlight on dust grains, and these might significantly
influence the plasma interactions. discussed the link between
gas and dust emissions. Indeed, in the second half of July 2015, the
outgassing of the nucleus was so strong that a diamagnetic cavity was created
which extended well past the ∼180 km distance of Rosetta from comet
67P/CG see also
http://blogs.esa.int/rosetta/2015/08/11/comets-firework-display-ahead-of-perihelion/. have predicted distances of ∼25 km for
the diamagnetic cavity distance under quiet conditions. Such strong outburst
conditions have not been modelled yet. In a diamagnetic cavity the outflowing
neutral gas and plasma is strong enough to keep the solar wind and its
embedded magnetic field at bay, pushing it away from the nucleus see
e.g.. This creates a magnetic field-free region around the comet.
However, the Rosetta RPC magnetometer did still measure a very small magnetic
field, which is an indication for the not-fully corrected offsets of the
magnetometer, which can be either inherent or arise from stray fields from
the spacecraft. In this paper the measured fields have been used to correct
the offset.
NAVCAM image of comet 67P/CG on 5 June 2015, showing the structuredness
of the escaping dust from the nucleus. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0.
In this paper a first overview and discussion is given of the events taking
place on 6 and 7 June 2015. There is a ∼6 h quasi-periodic
variation in the neutral and plasma density . First the
effect of the mass loading on the induced magnetosphere is discussed,
including magnetic field pile-up and draping, relating it to variations in
the solar wind. Second, the behaviour of the freshly created ions and the
resulting mirror-mode wave activity is investigated.
(a) The
time-energy spectrogram of ion monitor of the IES instrument. (b) The
time-energy spectrogram of the ICA instrument. The low cut-off during the
interval on 7 June between 07:00 and 15:00 UT is caused by a mode change of the
instrument. (c) The three components of the magnetic field in CSEQ
coordinates. (d) The magnetic field strength. (e) The MIP-deduced electron
density. (f) The LAP P1 current. (g) The deduced electron and ion density
from the IES instrument. (h) The neutral density from ROSINA COPS. (i) The
location of Rosetta in CSEQ coordinates. (j) The ion velocity components from
the IES instrument. (k) The angle η between the velocity vector and the
radial direction to the comet and the angle ζ between the ion velocity
vector and the magnetic field vector. The vertical lines are at 02:45, 08:30,
15:30, 21:00, 00:30, 04:15, 09:30, 16:45, and 22:00 UT.
Mass loading of the induced magnetosphere
On 6 June 2015 there was a higher-than-usual gas-outflow from the comet,
which loaded the induced magnetosphere with neutral gas and plasma. The
combined data of the six instruments discussed below, for the 2-day
interval of 6–7 June 2015 are shown in Fig. . From top
to bottom the following is shown: the Ion and Electron Spectrometer IES,
time-energy spectrogram, the Ion Composition Analyser ICA,
time-energy spectrogram, the low-pass filtered magnetic field components in
Cometocentric Solar EQuatorial (CSEQ
CSEQ: A cometocentric
coordinate system with the x axis pointing towards the Sun, the z axis is
aligned with the rotational axis of the Sun, and the y axis completes the
triad.
) coordinates from the Magnetometer MAG,; the
magnetic field strength, the Mutual Impedance Probe MIP,
deduced electron densities; the LAngmuir Probe LAP, P1
current, the IES ion and electron density; the Rosetta Orbiter Spectrometer
for Ion and Neutral Analysis ROSINA, Cometary Pressure
Sensor (COPS) neutral density; the location of the spacecraft with respect to
the comet; the IES ion velocity in CSEQ and the angles of the ion velocity
with the radial direction to the comet and with the magnetic field
direction.
In both the IES and the ICA, an increase in ion counts and energies in the
ion channels starting at approximately 18:00 UT is seen. There is an increase
in energy from ∼10 eV to up to ∼500 eV for both instruments,
where IES seems to show a sawtooth-like behaviour with a quasi-period of
around 4 to 6 h as shown in Fig. .
The neutral gas density measured by COPS of ROSINA is shown in Fig. h. A semi-periodic density fluctuation at a
quasi-period of ∼6 h, and a few maxima at ∼ 08:30 and ∼ 15:45 UT and a very strong peak at ∼ 21:00 UT are seen. The second and
third bursts (vertical dashed lines) coincide well with the start of energy
increases in the IES and ICA data in Fig. .
It is clear from comparing panels a, b, d and g in Fig. that a severe change occurs in the environment around
comet 67P/CG; the magnetic field strength starts to increase around 11:00 UT,
when at the same time IES and ICA data show an increase in counts and
energies of the ions.
Left-top: power spectra of the three magnetic field components (in
CSEQ coordinates) on 6 June between 15:00 and 17:00 UT. Left-bottom:
fourth-order polynomial detrended spectra with ±95% confidence levels
indicated by horizontal solid and dash-dotted red lines. Top- and bottom-right: Same as left for 6 June 21:00–23:00 UT.
As the total magnetic field strength increases, the fluctuations in the
magnetic field are also enhanced: the field increases from average B¯≈27 nT with a standard deviation σ≈11 nT during 00:00
to 12:00 UT to B¯≈41 nT with σ≈16 nT during 12:00
to 24:00 UT. In the early hours of 7 June the magnetic field strength has
returned to a lower value B¯≈30 nT with σ≈12 nT,
and the IES densities in Panel e return to the values as at the beginning
of 6 June and the ion densities and the LAP P1 current follow the COPS
neutral densities in Panel f. It should be noted that near 24:00 UT on 6 June
the magnetic field strength decreases to a very low value of Bm≈4 nT.
There is an interesting correlation between the data from ROSINA COPS neutral
density and the densities measured by the RPC instruments. In Fig. the vertical dashed lines are coincident with the
maxima in the COPS data, with the black dashed lines marking the “regular”
6 h maxima. The sharp density peaks at the maroon coloured dashed lines
are artifacts created by reaction wheel offloading on the spacecraft. There
appears to be a delay in the response in the IES time-energy spectrogram to
the increased neutral density. After a neutral density maximum, the count
rate and the ion energy increase and drop just before a new neutral density
maximum is reached again. This may be due to the ionization time, and will
have consequences for when RPC-measurable ions can be observed after neutral
injection. However, this is beyond the scope of this paper.
The solar wind transports magnetic fields from the Sun towards the comet. In
the surroundings of the comet a conducting layer exists, created by
ionization of the outflowing gas from the nucleus. As discussed by
the magnetic field cannot pass unimpeded through this region
near the nucleus and gets hung-up, whereas the part of the field lines
further away are still moving with solar wind velocity. This leads to two
phenomena: near the nucleus the magnetic field will pile-up, i.e. increase
in strength, as the field is delivered faster than it can be transported
away. This creates the so-called induced magnetosphere of the comet.
Furthermore, the field lines wrap around the nucleus, get draped, because of
the difference in velocity along the field line. These phenomena have been
well studied during the flybys of other comets in the last century see
e.g..
Massloading at ∼ 15:45 UT
At ∼ 15:45 UT on 6 June, COPS shows a maximum in the neutral gas density
in the quasi-periodic ∼6 h changes. IES shows an increase in energy
and counts of the ions over the following 4 hours, however, note this
signature looks different from what is happening after midnight on 7 June.
The IES ion (electron) density, Fig. e, is
rather peaked and strongly variable and reaches a maximum density at ∼ 14:36 (∼ 13:36) UT, which is most likely the result of the increased
neutral density at ∼ 08:30 UT. After the ∼ 15:45 UT neutral density
maximum the ion (electron) density starts to increase, with a slight maximum
at ∼ 17:40 UT.
With the increased plasma density a simultaneous increase in magnetic field
strength Bm is observed, see Fig. b.
This could be a result of more magnetic pile-up because of the increased mass
loading generating a layer with higher conductivity and thus a longer
diffusion time. It is, however, unclear if an increase in ion density can
actually lead to such a strong increase in magnetic field strength through
increased hang-up. posited that a decrease in ion density at
comet 1P/Halley could be the reason for the disappearance of the nested
draped magnetic field between the flybys of Vega 1 and Vega 2. However, it is
also quite possible that the increase in magnetic field strength and the
increase in ion density are generated by an external source in the solar
wind. This will be discussed in the next section.
Interestingly though, the situation is different from what was observed at
comet 1P/Halley see e.g., where the magnetic
fluctuations disappeared in the pile-up region. At comet 67P/CG the magnetic
fluctuations increase in the pile-up region.
With B¯≈50 nT the gyro frequency of water ions is fc,H2O≈40 mHz. Spectral analysis of the interval 17:00-19:00 UT on 6 June
is performed and displayed in Fig. top-left panel. The
three components of the magnetic field are spectrally analysed
cf. and displayed. In order to find the confidence level
of the peaks, the spectra are fitted by a fourth-order polynomial, which is
subtracted from the spectrum and from the residual (bottom-left panel) the
±95% confidence level is determined see e.g., shown
as a red solid and dash-dotted line. The spectrum shows that the strongest
(highest PSD) component is By, there is a strong peak at ∼4.7
mHz in Bx and By and a peak at ∼5.5 mHz in Bz, and mutual second and third peaks at ∼7.7 and ∼13 mHz. No
significant signal is found at the water ion gyro frequency.
Massloading at ∼ 21:00 UT
At ∼ 21:00 UT COPS showed another maximum in neutral gas density. The IES
ion density increases with a maximum Ni≈430 cm-3 at
∼ 22:40 UT, after which it quickly returns to pre-event values around
Ni≈50 cm-3. Spectral analysis of the interval 21:00 to
23:00 UT of 6 June shows (see Fig. right panels) that the
strongest component is Bx; there is a first mutual peak at ∼2.8 mHz, a second, stronger, peak in Bx is found at ∼4.7 mHz,
whereas for By a second peak is found at ∼6.0 mHz and for
Bz a second peak is found at ∼5.4 mHz. There seems to be
little common behaviour of the three magnetic field components.
Ion motion
The deduced ion velocities from the IES instrument are shown in Fig. h.
On 6 June the ion (H2O+) velocity is
around v¯≈(-12,-1,2) km s-1, with the magnitude of the
components increasing when the mass loading starts around 16:00 UT (but the
increase in magnetic field strength already starts about 2 hours earlier).
Mainly vx and vz (in CSEQ coordinates) increase in
magnitude with strongest change in vx. After the increase in density
and the increase in magnetic field strength disappear, just before midnight,
vz returns to pre-mass-loading values, but vx and vy strongly increase in magnitude with v¯≈(-23,10,1) km s-1,
lasting for many hours. This means that the ions are mainly moving
anti-sunward as discussed by .
In order to determine the propagation direction of the ions the angle η
with the radial direction to the centre of the comet (red line) is
calculated, as well as the angle ζ of the velocity with the local
magnetic field (blue line) in Fig. i.
Basically, over the whole of 6 June the ions are moving perpendicular to the
radial direction to the comet and nearly perpendicular to the magnetic field,
apart from 09:00–15:00 UT, which is related to the rotation of the magnetic
field discussed further below.
Near midnight, after the enhanced mass-loading, the situation changes: the
ions are accelerated in the X-Y plane and move still mainly perpendicular to
the radial vector with η≈110∘. However, the angle with
respect to the magnetic field increases to ζ≈140∘. The
latter is what one would expect for newly formed ions being accelerated by
the motional electric field see also whilst having an
initial velocity at ionization, starting their gyration around the magnetic
field, creating a ring-beam distribution, which can be unstable for
mirror-mode waves . These are the same
kind of ions that, at arrival at comet 67P/CG, caused the so-called singing
, but in a low-density and low-magnetic field environment.
Propagated solar wind
As there is no upstream solar wind monitor at comet 67P/CG, and changes in
solar wind properties are important with respect to the interactions around
the comet, two solar wind propagation models are used: model
the solar wind as an ideal MHD fluid; whereas based on
a ballistic propagation was carried out. The Opitz-Dósa model tries to find
the Parker-spiral connecting Rosetta to the Sun, based on ACE velocity
measurements, assuming that the state of the Sun and solar wind velocity at a
certain Carrington longitude is constant over half a solar rotation. Through
checking a range of possible Carrington longitudes as the origin of the
plasma measured by ACE, the longitude–velocity pair which results in the
least error is chosen. Both methods ignore the latitudinal extension of
Rosetta (7∘) and propagate solar wind only in the ecliptic plane.
In Fig. the propagated solar wind parameters of both models are
shown in panels d–g. There is a difference in some of the parameters: the
Tao-model propagates the tangential magnetic field component, mainly in the
y direction of the CSEQ coordinate system; whereas the Opitz-Dósa-model
propagates the radial magnetic field component, mainly in the x direction
of CSEQ.
The Tao-model shows that the tangential component of the magnetic field
Bt,t slowly increases in strength and after midnight from 6 to 7
June quickly reverses in sign. With the increase in Bt,t the density
Nsw and dynamic pressure Pdyn also increase. The
Opitz-Dósa-model shows that the radial magnetic field, Br,o,
slowly changes from negative to positive, indicating a heliospheric plasma
sheet crossing, which would explain the increase in solar wind density.
However, this could also be a signature of a corotating interaction region
impinging on the comet's plasma surrounding .
As the solar wind velocity does not change during this interval, the increase
in dynamic pressure is only created by an increase in ion density, which is
clear through the same profiles in panels f and g. The solar wind density in
the Tao-model increases by a factor of 4 from Nsw≈2 to
Nsw≈8 cm-3 over ∼18 h. The Opitz-Dósa-model
shows a lesser increase of a factor ∼2.
(a) The magnetic field in CESQ coordinates. (b) The magnitude of the magnetic
field. (c) The electron and ion velocities deduced from IES. (d) The
propagated solar wind magnetic field in cylindrical coordinates. (e) The
propagated solar wind velocity in cylindrical coordinates. (f) The propagated
solar wind density. (g) The propagated solar wind dynamic pressure.
As the modelling of the solar wind propagation cannot be perfect, in Fig. the Tao parameters have also been plotted, shifted by -6 h
as dotted lines. The shift improves the correspondence with the
Opitz-Dósa-model and with what is observed at Rosetta. This shows that with
the increase of the density and the dynamic pressure, the magnetic field
strength measured by Rosetta increases, as should be expected. The
Opitz-Dósa-model has a maximum in between the non-shifted and shifted
maxima of the Tao-model. Indeed, in general the magnetic field strength in
panel b follows the dotted curves in panels f and g rather well.
Interestingly, after shifting the Tao-model by -6 h the change in Bt,t occurs near the change in Br,o.
The factor 8 increase in solar wind density could have a significant
influence in ionization of the outflowing gas if electron/ion-neutral
collisions are important. The creation of a diamagnetic cavity shows that at the location of Rosetta collisions are indeed
important. The increased counts, energy and density in the IES and ICA data occur
during the shifted increase in solar wind density.
Pile-up and draping
With the increase in plasma density and magnetic field strength, generated by
the increased solar wind dynamic pressure and density, the magnetic field is
expected to get more piled-up, as observed, and possibly more draped. For the
whole interval the clock (ξ) and cone (ψ) angle of the magnetic
field is calculated:
ξ=tan-1BzBy,ψ=tan-1BxBy2+Bz2,
the result of which is shown in Fig. a. Before the
increase in density at ∼ 16:00 UT there are variations in both angles,
the clock angle ξ rotates from ∼180 to ∼90∘
then back to ∼-100∘ and back again to ∼140∘. As the
mass loading starts and Bm increases the clock angle ξ remains
constant. The cone angle ψ varies also from ∼0∘ (i.e. in
positive X axis direction, towards the Sun) with slight variations in phase
with the clock angle ξ, moving slightly away from the x direction up to
ψ≈-45∘ and then returns to remain constant at ψ≈20∘ during the interval of increased density and magnetic
field strength.
Panel (a) from top to bottom: the magnetic field components, the total
magnetic field strength and the clock ξ and cone ψ angle of the
magnetic field. Panel (b): the magnetic field plotted along the spacecraft
orbit, with 5 min resolution, in the CSEQ X-Y plane. The blue region depicts
the interval of compressed magnetic field. The dotted lines show where
Bx<0. Panel (c): same as panel (b) but in the CSEQ Y-Z plane. The axes
in panel (b) and panel (c) have different scales for better visibility.
When the field strength starts to decrease at ∼ 21:00 UT, and reaches a
very low value, Bm≈4 nT around midnight, the cone angle
ψ slowly increases to ∼85∘, i.e. far away from the
x direction, whereas the clock angle ξ varies strongly because of the
large oscillations in the magnetic field components, the largest of which are
also visible in the cone angle.
As there is neither undisturbed solar wind data, nor real undisturbed field
around the comet, the draping analysis as proposed by and
applied to comet 1P/Halley see also cannot be
applied. However, the magnetic field direction and behaviour can be looked at
in hedgehog-plots, as in Fig. b and c for the
magnetic field vectors in the X-Y and Y-Z plane along the orbit of the
spacecraft. It is clear from the data in Fig. a, that
on 7 June Bx is the main magnetic component, on 6 June before and
during the increased density period By and Bz dominate. In
the hedgehog-plots the period of the magnetic pile-up is shown in blue,
showing that the direction of the field remains constant. Also, it is clear
from both the cone and clock angles and the hedgehog-plots that the main
rotations of the magnetic field take place before the increase in density,
whereas the blue-coloured region and the later data show a rather stable,
slightly increasing, large-scale magnetic field, only disturbed by the strong
oscillations after 7 June 06:00 UT.
The compression of the induced magnetosphere increases the magnetic field
strength Bm without a significant change in direction shown by ξ
and ψ. There are small rotations of the field, after the pile-up but the
magnetic field pattern does not seem to show any significant change in
draping direction.
Before the massloading, between 06:18 and 13:08 UT, the X component of the
magnetic field has reversed, from positive to negative.
Magnetic field rotations in the pile-up region of comets are of interest, as
they are usually the “memory” of the magnetic field of the solar wind
conditions earlier. At Comet 1P/Halley a large set of nested draping regions
were found . These oppositely directed magnetic fields have to
be separated by current sheets and bring the possibility of magnetic
reconnection in the cometary coma see e.g..
The rotation of the magnetic field, as shown in Fig. does
not show up clearly in the propagated solar wind data. There is a short
change of sign in Bt,t of the propagated solar wind data between
∼ 00:00 and ∼ 02:30 UT on 6 June. The minimum field is only Bt,t,min≈-0.5 nT.
Neither does the propagated radial Br,o
component show a field reversal signature.
The boundaries of this ∼7 h region are studied in more detail, and a
zoom in on the intervals 05:50–06:50 and 12:40–13:40 UT is shown in
Fig. . The magnetic field data and the ion velocity have been
transformed to the minimum variance coordinates system MVA,indicated
by subscripts min, int and max derived from the magnetic field, and
the red dashed line is the low-pass filtered data with a shortest period of
10 min. Clearly, there is a lot of wave activity in the data. There is little
change in the ion velocity direction over the two rotations of the field,
only in rotation 1 the vmax seems to change direction after the
Bmax has changed sign, i.e. moved to the other side of a current
sheet. Although in principle this could be a signature of component
reconnection, the plasma data are too sparse to draw such a conclusion.
Using the low-pass filtered data (periods longer than 10 min), the field
changes by ΔBmax≈21 nT over a time-span of 11 min.
With a spacecraft velocity of vsc∼1 m s-1, assuming the rotations
convect over Rosetta with this velocity, making ΔL≈660 m, and
using Ampère's law,
∇×B=μ0J,
to calculate the current density (neglecting the displacement current):
ΔBΔL≈μ0J⇒J≈25µAm-2.
For the second rotation the field change is ΔBmax≈35 over a time span of 7 min, which leads to a current density of J≈ 66 µA m-2. Because of the assumed slow convection velocity
ΔL remains small, an upper limit for ΔL can be found under
the assumption of frozen in fields and a convection velocity of ∼10 km s-1,
which would significantly decrease the current density by a factor
∼104.
Zoom
in on the two B-field rotations 1 (left) and 2 (right). Each column showing
the magnetic field data in MVA coordinates: Bmax, Bint and
Bmin, the black lines show the data and the red lines show the
low-pass filtered data. The bottom panel shows the IES ion velocities in MVA
coordinates. The vertical dashed line shows the Bmax=0 crossing.
Eight 1 h intervals of the magnetic field strength Bm,
left column before midnight of 6 until 7 June and right column after midnight.
The cyan line is Bm and the blue is the low-pass filtered field. The
magenta line is the IES ion density and the blue dotted line is the IES
electron density, the red line and dots are the MIP electron density in the
left panels and the LAP P1 current in the right panels. Because of the
various different quantities plotted in the panels there are no labels on the
y axis.
Crossing from 6 to 7 June: mirror-mode waves
Pick-up of freshly ionized ions into a streaming magnetoplasma leads to the
creation of a ring/ring-beam distribution in velocity space, which is
unstable see e.g.. Depending on the
plasma-β this can lead to either ion cyclotron waves (low-β) or
mirror-mode (MM) waves (high-β). In the case of comet 67P/CG, the
plasma-β is high and thus MM waves are expected. They were also
observed e.g. at comet 1P/Halley see e.g.. The instability criterion for MM waves is given by:
1+βperp1-TperpT|<1,
where Tperp and T| are the ion-temperatures
perpendicular and parallel to the background magnetic field and
βperp is the perpendicular plasma-β determined only using
Tperp. The MM wave behaves in such a way that the perpendicular
pressure pperp of the plasma is in anti-phase with the magnetic
pressure pB, while the total pressure remains constant.
On 7 June the ion density returned to pre-event values, the magnetic
activity, however, remains. To study the difference in the 4 hours before
and after midnight, the magnetic field and plasma data are plotted in Fig. .
It is clear from the panels in Fig. that during the last 4 h
of 6 June (left panels) the MIP electron density variations (red dots)
seem to be in phase with the low frequency variations of the total magnetic
field. After 6 June ∼ 23:00 UT there is no MIP density available anymore
and after 7 June ∼ 00:10 UT LAP P1 currents are available as a proxy for
the plasma density. Over the first 4 h of 7 June, Fig.
right panels, there often seems to be an anti-correlation between the
total magnetic field Bm and the LAP P1 current.
Starting at 6 June around ∼ 23:00 UT quasi-periodic dips occur in the
magnetic field strength, some of which seem to be anti-correlated with the
LAP P1 current. This could imply that the freshly mass-loaded magnetospheric
magnetic field is mirror-mode unstable see e.g.. As the resolution of the plasma data is too low to
check the pressure balance over the MM structures, the magnetic-field-only
method by is used to investigate the data for MM waves. These
waves are expected to have strong magnetic field variations, ΔB/B,
and they are non-propagating structures, only convected by the streaming
magnetoplasma in which they are embedded. This means that in an MVA the
minimum variance direction should be perpendicular to the background magnetic
field and the maximum variance direction along the background magnetic field.
A study by showed that the angle between maximum variance
direction and background field was smaller than 30∘.
MVA is applied to the RPC-MAG data over a sliding window, and the angles
θ of the minimum variance and ϕ of the maximum variance
directions with respect to the low-pass filtered (longer than 10 min)
background magnetic field are determined, as well as the relative amplitude
ΔB/B defined as twice the variation: ΔB/B=2(B-Bbg)/Bbg. In order for MM identification, the structures have to fulfill
the following criteria: θ≥80∘, ϕ≤20∘ and
ΔB/B≥1. In Fig. an 8-hour interval is
shown, on which the MM determination has been performed. Panel a shows the
IES electron (blue) and ion (cyan) density, in panels b–e the 1 s resolution
MAG data are shown in black with the low-pass filtered data overplotted in
red. Panel f shows ΔB/B and panel g the angles θ (green) and
ϕ (red). There are regions where the above criteria are fulfilled, but
it is difficult to see in this figure. Therefore, short intervals will be
analysed separately below.
(a) The IES electron and ion density. (b–e) The magnetic field components and
field strength in black and the band-pass filtered data in red. (f) The
ΔB/B from the MM identification procedure. (g) Angles θ
(green) and ϕ (red) between the background magnetic field the minimum
variance and the maximum variance direction respectively.
A zoom-in on two 10-min intervals of Fig. , and adding
the density data of either MIP or LAP is shown in Fig. .
In the first interval 22:30-22:40 UT there are short periods where the
criteria are almost fulfilled, the maximum variance angle ϕ is rather
large. Unfortunately, the electron density estimated by MIP is unavailable
when the plasma frequency is out of the frequency range of the instrument, or
when the electron density is small enough and the electron temperature high
enough for the Debye length to be much larger than the instrument
emitter-receiver length scale. This makes it difficult to find a correlation
between Bm and Ne for the whole time series. Before 22:35 UT,
when θ>80∘ it is difficult to interpret the electron
density and thus the inset panel zooms in once more on the interval 22:31–22:32:30 UT.
There it is clear that the MIP electron density is in anti-phase
with the non-filtered magnetic field strength (cyan).
Zoom
in on two intervals with different mirror mode waves. From top to bottom the
magnetic field components; the magnetic field strength with overplotted in
red the MIP electron density (left) or the LAP P′ current. The bottom panels
shows the angles θ and ϕ from the MM determination procedure. The
inset in the left column shows a blow up of a short interval which shows that
the MIP electron density is in anti-phase with the non-filtered Bm
data.
During the second interval of 01:10–01:20 UT, the LAP P1 current acts as a
proxy for the plasma density. In this case it is clear in Fig. right panels that θ and ϕ are close to the MM
criteria. The two strong dips in Bm in the first 5 min show that
as the field strength decreases the current increases.
This means that the mass-loading of the induced magnetosphere of comet 67P/CG
created an unstable ion population through pick-up (a ring/ring-beam
distribution), which relaxes through the generation of mirror-mode waves.
Indeed, such a distribution was posited above when looking at the ion
velocity direction with respect to the background magnetic field. The
question whether such a distribution is able to develop in the
cometosheath
under the above conditions is addressed in the discussion section below.
On 7 June, the MM structures have, on average, a timescale of 100≤Tmm≤150 s, which will be compared to a characteristic length scale of
pick-up ions, being the Larmor radius. Assuming that the newly formed ions
are picked up with the local (decelerated) solar wind velocity vSW, the
gyro frequency ωc,i and radius ρc,i are given by
ωc,i=qiBmi,ρc,i=vperpωc,i,
also assuming that vperp=vSW and that the structures are
transported with vSW over the spacecraft and have a size of αρc,i the timescale is given by
Tmm=αρc,ivSW=αωc,i.
This means that for these assumptions the solar wind velocity drops out of
the equations and the crossing time is given by known and measured
quantities. For water ions at a magnetic field strength of Bm≈20 nT this leads to Tmm≈9α s. With the measured
Tmm mentioned above this leads to 11≤α≤16, which is
similar to what was found by at comet 21P/Giacobini-Zinner,
αGZ≈12, but much larger than what was found by at
comet 1P/Halley, αH≈ 1–2. Taking the ion velocity as
measured by IES, the Larmor radius for water ions becomes ρH2O,i≈280 km.
For the interval 22:30–22:40 UT it is clear that the size of the alleged MM
structure is much smaller than in the later interval discussed above. An
estimate from the inset panel in Fig. shows that the MM
structures have a timespan of ∼10 s. The field strength is slightly
higher at Bm≈25 nT, which means from Eq. () that
Tmm≈ 7 α. This means that α≈1.4, which is
more in line with the results of for freshly picked-up water
ions near comet 1P/Halley.
Similar MM intervals can be found earlier, 19:10–19:12 UT, as shown in Fig. left panel. The timespan is again Tmm∼10 s,
with a field strength Bm≈55 nT this leads to α≈2.7. Again, at the beginning of the interval, the MIP electron
density seems to follow the filtered magnetic field strength (blue). In the
middle, just after 19:11 UT, the electron density is almost in anti-phase with
the non-filtered magnetic field strength (cyan). And MMs can also be found
later, 01:21–01:40 UT, as shown in Fig. right panel.
Here the structures are again larger, on the order of Tmm≈100 s and α≈11 with the LAP P1 current in anti-phase with the
magnetic field strength.
Left: zoom in on interval 19:10–19:12 UT, the MIP electron density
is following Bm,fil (blue) in the first minute. Just after 19:11 UT
the MIP is almost in anti-phase with the non-filtered Bm (cyan) for
about 30 s. Right: zoom in on interval 01:21–01:40 UT showing that the LAP P1
current (red) is in anti-phase with Bm.
Change of MM shape
A closer look at Fig. right panel shows that the structures,
identified as mirror-mode waves, are changing in shape. Indeed, in the top
panel the structures seem to be mainly dips in the magnetic field strength,
Bm, but at later times the structures seem to become asymmetric. A
zoom-in on three intervals of 20 min is shown in Fig. ;
the data are shifted along the y axis in order to make the difference
between them more visible. The LAP P1 current is shown as grey asterisks
overplotted on each interval. The three intervals are different in behaviour:
the first interval 01:20–01:40 UT (blue) shows mainly strong dips in Bm; the second interval 02:20–02:40 UT (green) shows strong asymmetric dips
in Bm and a large variety in structure sizes; the third interval
03:20–03:40 UT (red) shows in the beginning deformation of the waves, strong
periodic peaks with moving peaks super-imposed.
Left
panel: three 20 min intervals of the magnetic field strength Bm
data, shifted to enhance visibility – blue 01:20–01:40 UT, green 02:20–20:40 UT,
red 03:20–03:40 UT. The grey stars show the LAP P1 current. Right panel:
the Fourier power spectra for the three intervals. The coloured arrows at the
top mark the peaks discussed in the text.
Spectral analysis is performed on these three intervals. It is clear from
Fig. right panel, that the three intervals have different
spectral content: the first interval (blue) has a peak at f≈6 mHz
and a minor peak at f≈13 mHz, the second interval (green) shows a
plateau-like structure around f≈10 mHz; the third interval (red)
shows a clear double peaked structure at f≈9 and f≈19 mHz
with a minor peak at f≈51 mHz, which explains the beat-mode that
can be seen in the red trace in Fig. left panel. It is not
very clear from the LAP P1 current to deduce that these structures are
mirror-modes, although the Lucek method indicates that they are.
Discussion and conclusions
For the first time in space research history a spacecraft is following a
comet along its orbit from pre- to post-perihelion, entering regions around
the comet that up to now had not been accessed. Also the outgassing of comet
67P/CG at arrival in August 2014 was at a much lower level than for any other
mission. During the period discussed in this paper the outgassing rate is
around 1027 molecules s-1, which is several orders of magnitude smaller than at
comets 27P/Grigg-Skjellerup or 1P/Halley . This
means that the interaction of the solar wind with the outgassing comet is
different, which was clearly illustrated through the discovery of the
“singing comet” by , an unexpected plasma instability created
by the not-yet-magnetized freshly produced ions near the comet. This is the
context in which the results of this paper should be interpreted:
measurements much closer to a cometary nucleus than ever before, with low
outgassing rate and a very slowly moving spacecraft relative to the nucleus.
The data from RPC MAG have been calibrated, however state
that: “The short boom length implies that the spacecraft is heavily
contaminating the magnetic field measurements. At this stage of the
investigation it was not possible to completely remove these quasi-static
spacecraft bias fields from the measured magnetic field values.”. In this
current paper, the observations of the diamagnetic cavity have been used to obtain values for non-corrected bias fields
originating from the spacecraft. Assuming that the diamagnetic cavity should be
field-free see e.g., the measured fields in the cavity have
been subtracted from the data. This leads to a greatly improved determination
of the mirror mode waves using the magnetic-field-only technique
, as the examples shown in Fig. would not
have been selected without bias-field offset correction.
The mass loading of the induced magnetosphere of comet 67P/CG, as indicated
by the Rosetta ROSINA-COPS and RPC plasma instruments showed an interesting
behaviour on the 2 days discussed in this paper (6–7 June 2015). At the
beginning of 6 June COPS shows an increase in neutral density (first dashed
line in Fig. near 02:45 UT), but the RPC plasma
instruments do not show any significant response. With the second strong
increase in neutral density near 08:30 UT, there is some increase in energy in
the ions and the IES electron and ion density slowly increase. However, after
the third maximum in COPS near 15:45 UT, both IES and ICA start to show a
significant increase in both counts and energy of the ions. Indeed, with
every next neutral gas maximum there is a slow increase in counts and
energy.
The observed variations in the solar wind parameters, such as directional
changes and increase in dynamic pressure, in both solar wind propagation
models used in this paper, led to several interesting phenomena:
Before the increased density and the pile-up region there was a rotation
of the magnetic field. This is probably related to changes in the field
direction of the solar wind magnetic field, generating nested draped fields
around the comet.
Depending on the assumption how fast Rosetta crosses this structure
the current densities in the current sheet are tens or µA m-2 or
several nA m-2.
There is increased ionization and energization of gas from the cometary
nucleus in both IES and ICA.
The magnetic field strength increased by a factor of >3 up to ∼60 nT,
increasing the magnetic pressure. With the ion density on the order of 100 cm-3
and the ion temperature a few 105 K, this means that the plasma
beta β∼10.
The newly created ions are accelerated by the motional electric field,
however, the effect only becomes apparent after the pile-up region is exited
by the spacecraft.
In the pile-up region there is evidence for mirror-mode structures,
generated by the newly created ions, with a size between one and three
water-ion gyro radii.
Outside the pile-up region there are clear signatures of mirror-mode
waves, with a much larger size of 10 to 16 water-ion gyro radii.
Outside the pile-up region there is a development of the mirror-mode
structures, where at later times there are three dominant frequencies
present, which leads to strong deformation of the mirror-mode waves signature
in the MAG data.
The above results leave a few points to discuss which will be addressed below.
Nested draping: The change in direction of the magnetic field as observed in the Rosetta data
does not show up clearly in the propagated solar wind magnetic field. The
Tao-tangential field seems to go negative for a short period in the
non-shifted data in Fig. at the beginning of 6 June. The
Opitz-Dósa-radial magnetic field basically shows a heliospheric current
sheet crossing. Because of the draping and hanging-up of the magnetic field
around the comet, it is difficult to find a one-to-one correlation between
the solar wind field signatures and the draped field signatures. The layer of
differently directed field at Rosetta may be the result of an older interval
outside that presented in the figure. The difference in field strength can be
explained through the compression by the solar wind pressure.
Changes in the magnetic pile-up region: Rosetta is located well inside the MPR of comet 67P/CG, which is clear from
the high magnetic field strength measured by MAG, Bm≥20 nT and
the expected solar wind magnetic field strength Bsw≈2 nT.
The build-up of a magnetic pile-up region is related to pressure balance from
the draped magnetic field pushing outward and the solar wind dynamic pressure
pushing inward towards the comet. With the increase in solar wind pressure,
this balance is disturbed and the field gets compressed more. This is what is
observed in the MAG data, where the 4-fold increase in dynamic pressure leads
to a magnetic field strength increase by a factor ∼2.5 from ∼20 to ∼55 nT. This agrees well with the expected increase, which would
be Pdyn,max/Pdyn,min.
Ionization increase: Looking at a longer data set of the IES ion energy spectra, it is clear that
this increase in counts and energy of the ions is limited to a period of
≤18 h, which corresponds to the increased solar wind dynamic
pressure, which is caused by an increase of the solar wind density. This
means that an enhanced number of solar wind electrons is also entering the
pile-up region, which increases collisions and ionization as observed by RPC.
After this period the IES densities follow the periodicity in the COPS
neutral density, indicating that the increased ionization was indeed
generated by the higher solar wind density.
Ring/ring-beam distribution: A ring/ring-beam distribution is assumed necessary for the generation of the
mirror mode waves. However, do the pick-up ions have enough time to develop
such a distribution? The IES ion velocity in the increased pile-up region
shows that the ions are basically moving perpendicular to the magnetic field.
With a magnetic field strength between 20 and 55 nT and a velocity of ∼12 km s-1 the gyro frequency is 0.1≤ωc,i≤0.25 s-1
and the gyro radius is 50≤ρc,i≤120 km. In order for a
ring distribution to occur, the collision frequency must be much smaller than
the gyro frequency. The collisional time is given by τcoll=(nσiv)-1, where σi∝10-16 m2 is a typical
ion-neutral collisional cross-section . Using typical values n≥106 cm-3 and v=10 km s-1 this leads to τcoll≈103-104 s. With a gyro period of 25≤τc,i≤60 s this
means there is ample time for the ions to create a ring-beam distribution and
the location of Rosetta with respect to the comet, ∼225 km shows that
the coma is large enough for full gyrations of the ions with the gyro radii
mentioned above.
Different sizes of MMs: Within the pile-up region, in the second half of 6 June, at high density, the
mirror mode waves are between one and three water gyro radii in size. This is
“as expected” from newly created H2O+, as measured, e.g. at comet
1P/Halley . Many hours later, on 7 June, there are much larger
MM structures in the MAG data, with a size between 10 and 16 gyro radii. The
larger structures could possibly be generated by diffusion of smaller size
MMs as described by :
λ(L)=αρc,i[1+(ωc,iL/32u)],
where the source size has been changed to αρc,i. Putting in
the measured values (λ(L)=14,α=2,u=10 km s-1) and solving
for the diffusion distance L∼105 km shows that the large structures
cannot have evolved from diffusion of the small structures in the pile-up
region. Thus these large structures find their origin in MMs created further
upstream in the comet's coma. Where exactly cannot be determined as the
source size α of the MMs further upstream is unknown.
Structure deformation: The main ion species discussed in this paper is H2O+, however, CO+
and CO2+ were almost equal to that of water. showed that
the detector signal of the ROSINA instrument for all three species was on
average ∼2×105 particles (20 s)-1, with only variations depending
on which side of the comet is facing Rosetta. Assuming that the two main
frequencies in the spectrum of the third interval in Fig. ,
with deformed (beating?) MM waves, are related to gyro frequencies of pick-up
ions, then the ratio of the frequencies should possibly be related to the
ratio of the masses of the ions. The low frequency waves are at ∼9 and
∼19 mHz, which have a frequency-ratio of ∼0.47, the mass-ratio of
water with carbon(di)oxide is 0.6 / 0.41. The ratios are close, which might
suggest that there are indeed different kinds of MMs at the same time. This
would ask for an interaction of multiple kinds of MMs in one multi-component
plasma, which has not been discussed in the literature.
The Rosetta mission around comet 67P/Churyumov-Gerasimenko offers excellent
opportunities to investigate processes that have been observed during flybys
of other comets. Due to the slow motion of the spacecraft with respect to the
comet an in-depth view is obtained of the interaction of the solar wind with
the outgassing comet. This paper gives a “short” first discussion of a
2-day interval of the data. With the spacecraft in basically the same
location near comet 67P/CG this gave the possibility to study the reaction of
the induced magnetosphere with respect to the increased solar wind dynamic
pressure. Furthermore, in this way temporal variations in the cometosheath,
e.g. the changes in the characteristics of the mirror mode waves were
studied. Numerical modelling of the events showed in this paper is underway,
as well as theoretical investigations into the various mirror-mode waves in a
multi-ion pick-up plasma.
Acknowledgements
Rosetta is an ESA mission with contributions from its Member States and NASA.
We acknowledge the staff of CDDP and IC for the use of AMDA and the RPC
Quicklook database (provided by a collaboration between the Centre de
Données de la Physique des Plasmas, supported by CNRS, CNES, Observatoire
de Paris and Université Paul Sabatier, Toulouse and Imperial College
London, supported by the UK Science and Technology Facilities Council). 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 1401
for Rosetta. C. Carr and E. Cupido thank the UK space agency for support of
the Imperial College RPC team. T. Broiles, J. Burch and K. Mandt
acknowledge support by NASA for work on IES through contract #1345493 with
the Jet Propulsion Laboratory, California Institute of Technology. N. J. T. Edberg
acknowledges support from the Swedish National Space Board and the
Swedish Research Council. A. Opitz and M. Dósa are grateful to Z. Nḿeth
for the deep discussions and L. Földy for his computational
support. The authors acknowledge the ACE and OMNI databases for solar wind
data.
The topical editor, E. Roussos, thanks two anonymous referees for help in evaluating this paper.
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