CME impact on comet 67P/Churyumov-Gerasimenko
Niklas J. T. Edberg, M. Alho, M. André, D. J. Andrews, E. Behar, J. L. Burch, C. M. Carr, E. Cupido, I. A. D. Engelhardt, A. I. Eriksson, K.-H. Glassmeier, C. Goetz, R. Goldstein, P. Henri, F. L. Johansson, C. Koenders, K. Mandt, H. Nilsson, E. Odelstad, I. Richter, C. Simon Wedlund, G. Stenberg Wieser, K. Szego, E. Vigren, M. Volwerk
CCME impact on comet 67P/Churyumov-Gerasimenko
Niklas J. T. Edberg, (cid:63) M. Alho, M. Andr´e, D. J. Andrews, E. Behar, J. L. Burch, C. M. Carr, E. Cupido, I. A. D. Engelhardt, A. I. Eriksson, K.-H. Glassmeier, C. Goetz, R. Goldstein, P. Henri, F. L. Johansson, C. Koenders, K. Mandt, , H. Nilsson, E. Odelstad, I. Richter, C. Simon Wedlund, G. Stenberg Wieser, K. Szego, E. Vigren, and M. Volwerk Swedish Institute of Space Physics, Uppsala, Box 537, SE-75121, Sweden Aalto University, School of Electrical Engineering, Department of Radio Science and Engineering, PO Box 13000, 00076 Aalto Finland Swedish Institute of Space Physics, Box 812, 981 28 Kiruna, Sweden Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom TU - Braunschweig, Institute for Geophysics and extraterrestrial Physics, Mendelssohnstr. 3 Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, Orl´eans Cedex 2, France Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA Department of Physics, University of Oslo, Box 1048 Blindern, 0316 Oslo, Norway Wigner Research Center for Physics, Budapest, Hungary Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
ABSTRACT
We present Rosetta observations from comet 67P/Churyumov-Gerasimenko duringthe impact of a coronal mass ejection (CME). The CME impacted on 5-6 Oct 2015,when Rosetta was about 800 km from the comet nucleus, and 1.4 AU from the Sun.Upon impact, the plasma environment is compressed to the level that solar wind ions,not seen a few days earlier when at 1500 km, now reach Rosetta. In response to thecompression, the flux of suprathermal electrons increases by a factor of 5-10 and thebackground magnetic field strength increases by a factor of ∼ − , due to increased particle impactionisation, charge exchange and the adiabatic compression of the plasma environment.We also observe unprecedentedly large magnetic field spikes at 800 km, reaching above200 nT, which are interpreted as magnetic flux ropes. We suggest that these couldpossibly be formed by magnetic reconnection processes in the coma as the magneticfield across the CME changes polarity, or as a consequence of strong shears causingKelvin-Helmholtz instabilities in the plasma flow. Due to the limited orbit of Rosetta,we are not able to observe if a tail disconnection occurs during the CME impact,which could be expected based on previous remote observations of other CME-cometinteractions. Key words:
Sun: coronal mass ejections (CMEs) – comets: individual:67P/Churyumov-Gerasimenko – Sun: solar wind
The Rosetta spacecraft arrived at comet 67P/Churyumov-Gerasimenko in Aug 2014, when at 3.6 AU from the Sun.Since then, it followed the comet at a distance of 10-1500 kmfrom the nucleus while the comet passed through perihelion,at 1.2 AU, and outward again until end of mission in Sep (cid:63)
E-mail: [email protected] (NJTE) © The Authors a r X i v : . [ phy s i c s . s p ace - ph ] S e p N. J. T. Edberg et al. mainly through photoionisation of the local neutral gas (Ed-berg et al. 2015; Odelstad et al. 2015; Vigren et al. 2015; Ga-land et al. 2016), but impact ionisation and charge exchangeprocesses also contribute (Burch et al. 2015; Simon Wedlundet al. 2016b). Newly ionised particles immediately feel thepresence of the solar wind convective electric field and arepicked-up by the flow and start to gyrate (Goldstein et al.2015). The first observations of the comet plasma environ-ment were reported by Nilsson et al. (2015), as cometary ionswere measured when at 3.6 AU from the Sun and at a dis-tance of ∼
100 km from the nucleus. The flux of cometary ionsas well as the local plasma density around the nucleus wereobserved to increase gradually as the comet moved closer tothe Sun (Nilsson, H. et al. 2015; Odelstad et al. 2015). Asthe coma grows larger, the interaction with the solar windbecomes more pronounced. To ensure the conservation ofmomentum the solar wind bulk flow is accelerated in theopposite direction from the newly created ions (Broiles, T.W. et al. 2015; Behar et al. 2016). Eventually, plasma regionsand boundaries begin to form, which has also been shownin global 3-D hybrid and MHD simulations (Koenders et al.2013, 2015; Rubin et al. 2014; Huang et al. 2016).Due to the trajectory of Rosetta being in the closevicinity of the nucleus, the full solar wind interaction regionhas not been sampled throughout the mission. The plasmaboundary closest to nucleus is the diamagnetic cavity, whichhas been observed, although intermittently (Goetz et al.2016). The diamagnetic cavity builds up as the neutral-ionfriction force in the outgassing material exceeds the mag-netic pressure force from the outside (Cravens et al. 1987).Also, as the coma grew larger around Rosetta, measurementsindicated a transition from an inner region to an outer re-gion over time, where the boundary in between was inter-preted to be the collisionopause (Mandt et al. 2016). Thecollisionopause is the boundary where the ions become colli-sional and piles up, and its location is dependent on both theneutral outgassing rate and velocity as well as the collisioncross section. The inner region, within the collisionopause,shows significant dynamics in the plasma environment. Or-der of magnitude density variations to the hot/cold plasmamixture occur on timescales of seconds to minutes (Erikssonet al. 2016).Besides the continuous growth and decay of the comaas the heliocentric distance decreases and increases, respec-tively, the plasma environment of the comet also exhibitslarge variations due to the changing solar wind. Edberg et al.(2016) studied four cases of impacting corotating interac-tion regions on the comet from October to December 2014,as the comet activity grew stronger, and McKenna-Lawloret al. (2016) observed two CMEs arriving at 67P in Septem-ber 2014, i.e. soon after Rosetta’s arrival at the comet whenthe outgassing was relatively low. These all impacted whenoutgassing rate was about - particles s − (H¨assiget al. 2015). The CIR impacts caused a compression of theplasma environment present, which led to increased fluxes ofsuprathermal electrons, increased ionisation rate, increasedplasma density as well as an increase in the magnetic fieldstrength.CME impacts on other comets have only been observedremotely. During such observations only large-scale changesin the comets’ comae and tails could be observed due tothe limited resolution of the images. Jones & Brandt (2004) observed how a CME impacted on comet 153P/Ikeya-Zhangand could study how the comet tail appeared scalloped whenthe varying interplanetary magnetic field draped around thecomet. Vourlidas et al. (2007) observed how a CME impactcaused a tail-disconnection event in comet 2P/Encke. Thiswas later modelled by Jia et al. (2009) and it was suggestedthat the sudden magnetic field rotations associated with theCME caused magnetic reconnection to occur in the tail ofthe comet, which was then subsequently disconnected.Here we will present in situ measurement from Rosettaduring a CME impact on comet 67P when close to peri-helion, to study the CME’s effects on the local cometaryplasma environment. In this paper we have used data from all five sensors ofthe RPC (Carr et al. 2007). These are the Langmuir probeinstrument (LAP) (Eriksson et al. 2007), the mutual impe-dence probe (MIP) (Trotignon et al. 2007), the magnetome-ter (MAG) (Glassmeier et al. 2007b), the ion and electronsensor (IES) (Burch et al. 2007), and the ion compositionanalyzer (ICA) (Nilsson et al. 2007). For a detailed descrip-tion of each instrument we refer to the individual instrumentpapers or, for a condensed summary, to the Instrument sec-tion in the multi-instrument study by Edberg et al. (2016).In brief, here we will use electron density and spacecraft po-tential measurements from the LAP1 sweeps, normally atcadency of 96 s or 160 s. The negative of the spacecraftpotential is proportional to the logarithm of the electrondensity (Odelstad et al. 2015) and in the interval coveredhere, if assuming a fixed electron temperature, gives a goodmeasure of the density. The electron density from MIP isderived from the plasma frequency emission line, obtainedin both Short and Long Debye Length modes (SDL andLDL, respectively). The short time scale density variationsin MIP, often large, have been filtered out using a 5-minutesmedian filter and discarding times when the number of MIPmeasurements are considered too small to be representativeof the actual average density. We have also used the vectormagnetic field measurements from MAG at a cadence of upto 20 Hz as well as electron spectrograms from IES and ionspectrograms from ICA, separated in cometary (heavy) andsolar wind (light) ions species.
In September 2015, Rosetta left the near vicinity of thecomet nucleus and began a two-week excursion outward inthe coma to explore the spatial extent and structure of theplasma environment. During this interval the heliocentricdistance spanned 1.34 - 1.41 AU. The trajectory is shownin Figure 1 in the cometocentric solar equatorial coordinatesystem (CSEQ). In this system the x-axis points from thecomet to the Sun, the z-axis is parallel to the componentof the Sun’s north pole orthogonal to the x-axis, and the y-axis completes the right-handed reference frame. From be-ing located in a near-terminator trajectory at around 300km on 22 Sep 2015 Rosetta moved radially outward from
ME impact on comet 67P -1000-8001000-600-400-200 500 10000 Z C SE Q [ k m ] Y CSEQ [km]
600 0 04-Oct800 06-Oct X CSEQ - to the Sun [km]
Figure 1.
The trajectory of Rosetta in cometocentric CSEQ co-ordinates, with the projection on the three planes shown in grey.The interval includes the time of the Rosetta dayside excursionout to 1500 km. the nucleus at an angle of about 50 ◦ to the comet-Sunline. Moving at a speed of ∼ ◦ ( http://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/2015_09/univ2015_09.html . Mars was roughlyat the same heliocentric distance at this time, but 30 ◦ offfrom the Sun-Rosetta line. Solar wind monitoring data atMars indicate moderately disturbed solar wind signatures,while at Earth there was no indication of a CME arriving.At Venus and Mercury, no solar wind data was available atthis time, unfortunately. The three largest CMEs had linearvelocities of 429 km/s, 586 km/s and 602 km/s, respectively,as determined from the SOHO images. It is not possible toaccurately determine how the CMEs evolve with time andif they for instance merge with each other or slow downor accelerate (see e.g., Rollett et al. 2014), which providesuncertainty in their evolution. If we assume that the threelargest CMEs, released within 4 hours merge together to one Figure 2. (Top) SOHO LASCO2 image of a CME being re-leased on 30 Sep 2015 in the general direction of Rosetta (Imagefrom ). (Bot-tom) Position of the inner planets and Rosetta on 30 Sep 2015.A and B indicate the positions of the Stereo-A and Stereo-Bspacecrafts, which did not observe the CME. There were un-fortunately no spacecraft in operation at Mercury or Venus. AtMars, both the MAVEN and the MEX spacecraft were measur-ing the solar wind intermittently and saw a weaker signatureof the CME (Figure from http://stereo-ssc.nascom.nasa.gov/cgi-bin/make_where_gif ). single CME on its outward journey and assume a velocity of500 ± km/s it would take roughly 4-6 days for it to reachRosetta and 67P at 1.4 AU. It would then be arriving sometime around 4-6 Oct 2015. This coincides with the intervalof the inbound leg of Rosetta’s dayside excursion. As will beshown next, comprehensive evidence is found that the CMEdoes indeed impact comet 67P, and significantly affects theplasma environment.Figure 3 shows an overview of the RPC data gatheredduring the entire dayside excursion, including the interval N. J. T. Edberg et al. (cid:38) eV) ion and electron fluxes (panelb and c), were relatively low and the solar wind was com-pletely shielded at this time. Any undisturbed solar windions, mainly H + flowing at 400 km/s, would have an energyof about 1000 eV. In this figure only cometary ions, whichare separated out by their heavier masses, are shown. Theions and electrons observed here are all of cometary originand have been created through ionisation of neutral parti-cles from the comet and have energies of about 100 eV. Theparticle instruments are capable of measuring species withenergies down to about 10 eV, below which the Langmuirprobe instrument takes over in measuring the cold plasmaproperties. A redder color at higher energies indicates thatmore particles have been accelerated to higher speeds. Thespread in energy at a particular time corresponds to thetemperature of the plasma, which varies throughout the in-terval. The inner region was also characterised by highlyvariable Langmuir probe sweeps, spacecraft potential andplasma density, indicating that the properties of the colderpopulations of electrons and ions ( (cid:46) eV) change rapidly(panel d, e and f). Order-of-magnitude changes occur ontime scales of seconds to minutes. When Rosetta was movingfurther out the local plasma environment changed. Mandtet al. (2016) studied the structure of the plasma environmentduring this excursion in more detail and identified at leastone type of boundary, interpreted as an collisionopause, firstcrossed at a distance of about 600 km during the outboundleg. This boundary crossing was observed, outbound, as anincrease in energy and flux of the cometary ions and elec-trons, the spacecraft potential increased to positive values,the cold plasma density dropped an order of magnitude toabout 10-100 cm − and there was a moderate magnetic fieldincrease at the same time. While simulations of the plasmaenvironment had indicated that the cometary bow shockwould be crossed before reaching 1500 km (Koenders et al.2013; Rubin et al. 2014). Huang et al. (2016) used an MHDsimulation to show that the bow shock was closer to 10000km under perihelion conditions and illumination-driven neu-tral outgassing. It turned out that the cometary plasma en-vironment was more extended than what Rosetta reachedduring the dayside excursion. In fact, the RPC measure-ments showed no indication of the presence of a bow shock,nor that of solar wind ions, once 1500 km was reached.Instead, Rosetta remained in this outer region, the ion-pile up region, for several days and at least until 00:00 on 4Oct 2015. At this time Rosetta was at a distance of 1000 kmand yet another increase in ion and electron energy and fluxwas observed (as opposed to a decrease as would be expectedif crossing the collisionopause inbound again). This was ac-companied by an increase in the magnetic field strength froman average of about 20 nT to an average of about 40 nT aswell as a sudden increase in electron fluxes for energies < eV by a factor 2-5. The cold plasma density and the space-craft potential remained unchanged at this time. These sig-natures are possibly purely due to the dynamics in the comaitself, or some of the earlier, weaker but faster CMEs releasedfrom the Sun on 30 Sep. The suprathermal electron fluxesas well as the ions and the magnetic field strength show sev-eral larger enhancements and decreases in the following 48hours.At about 20:15 UT on 5 Oct 2015 the main impact of theCME occurs. This agrees well in time with when we expectthe CME to arrive and the impact is clearly identified inall RPC data sets as an increase in magnetic field strength,plasma density, ion and electron flux. Before moving on tothe detailed observations during the main impact we notethat after the CME impact event around midnight on 5-6Oct 2015, Rosetta is briefly located in the undisturbed ionpile-up region (i.e. outside the collisionopause) again for afew hours. Around noon on 6 Oct 2015 the collisionopause isfinally crossed inbound as Rosetta continues to slowly moveback toward the comet nucleus. ME impact on comet 67P Smaller CMEs?
Figure 3.
Overview of RPC data from the dayside excursion andespecially including the interval of the CME impact. The panelsshow (a) the magnetic field strength, (b) spectrogram of ICA mea-sured heavy (cometary) ions, (c) spectrogram of IES measuredelectrons (d) LAP sweeps (current collected is colour-coded, biaspotential fed to the probe on the vertical axis) (e) spacecraft po-tential measured by LAP (f) electron density measured by LAP(black dots) where data points during spacecraft attitude changesare excluded together with MIP density estimates (red dots), (g)distance to the comet and the Sun from Rosetta (h) longitudeand latitude (projected down on a sphere), SZA, SAA (attitudeangle) and Local time (LT) of Rosetta.
N. J. T. Edberg et al.
Figure 4.
Time series of RPC data covering the interval ofthe CME impact. The panels show (a) magnetic field magnitudeand components in CSEQ coordinates, (b) spectrogram of solarwind ions (c) spectrogram of electron fluxes (d) Langmuir probesweeps, (e) spacecraft potential, and (f) electron density derivedfrom the Langmuir probe sweeps.
ME impact on comet 67P Figure 4 gives a sub-set of the RPC data in order to studythe detailed response to the CME impact on the comet. Inthe spectrogram in panel b we now show solar wind ions,rather than cometary ions as in the previous figure. Theion instrument is capable of separating ions depending ontheir mass and charge and as the solar wind mainly con-sists of H + ions they are easily separated from the heaviercometary H O + and CO + , CO + ions. As stated above theundisturbed solar wind would appear around 1000 eV, withsome spread in energy, but here these H + ions appear at amuch lower energy indicating that the solar wind has beenslowed down by the cometary coma. The magnetic field aswell as the plasma data present several interesting featuresin this interval. Firstly, the CME appears to cause anotherincrease in the magnetic field strength (panel a), from about40 nT to about 60 nT at 20:10 UT and then to a maximumbackground field of about 100 nT around 02:00 on 6 Oct, i.ean increase of a factor of ∼ − , and the spacecraftpotential drops from about +1 V to -10 V, indicating a sig-nificant increase in the flux of electrons. During the time ofthe CME impact, MIP was unfortunately operated in LDLmode, designed for plasma densities lower than about 300cm-3, thus missing the CME itself. The large-scale magneticfield orientation changes, which occur over a time span ofhours, are probably associated with the large-scale magneticflux rope commonly seen across a CME.Furthermore, and of particular significance, the solarwind ions, which had been absent in Rosetta ICA and IESparticle data since April 2015, were briefly observed againduring this event, by the ICA instrument, from 23:00 UT on5 Oct until 04:00 UT on 6 Oct (panel b). This suggests thatthe plasma environment had been compressed significantly,such that the solar wind ions could briefly reach the detector,and provides further evidence that these signatures in thecometary plasma environment are indeed caused by a solarwind event, such as a CME.Next, we will discuss particular effects of the CME im-pact in more detail, focusing on the solar wind ion obser-vations in 3.1, the increased plasma density and fluxes inSection 3.2, and finally the magnetic field spikes in Section3.3. After April 2015, the solar wind ceased to be observed byRosetta, which was located deep inside the coma (Mandtet al. 2016). The solar wind did not reappear again untilseveral months after perihelion. However, during the CMEimpact reported here the ICA instrument did in fact observesolar wind protons penetrating the coma. Panel b of Figure4 shows a spectrogram of these solar wind ions. The solarwind ions (protons) have been slowed down during their pathto Rosetta and end up at roughly the same energy as thecometary heavy ions (compare panels b in Figures 3 and 4).The similar energy unfortunately makes them harder to dis-tinguish from each other by the ICA instrument. But as ICAis a mass-resolving instrument it is possible to separate theheavy cometary ions from solar wind ions. The solar windions are clearly observed here, but the fluxes are relativelylow compared to e.g. solar wind spectra from before April2016. In the interval 02:00-03:00 UT on 6 Oct there appearsto be a gap in the solar wind observations. There is a sig-nificant decrease in the solar wind fluxes at this time butas the signal gets weaker the uncertainty in distinguishingthem from cometary ions by the instrument also increases.The drop out of solar protons at this time is therefore to beregarded as a lower limit of the fluxes. The solar wind ionswere observed to be deflected typically some ◦ − ◦ fromthe comet-Sun line. The cometary ions have a preferred di-rection in the anti-sunward direction, but are scattered intheir direction by a few tens of degrees.Rosetta was at this time at about 800 km from the nu-cleus. Earlier, when at 1500 km, i.e. at the furthest distancefrom the nucleus during the excursion, the solar wind wasnot seen at all. The magnetic field strength also increased atthe time of the CME impact, as discussed earlier, indicatinga compression of the plasma in the coma. If interpreting theappearance of the solar wind ions as a pure compression, theCME then compressed the plasma environment to at leasthalf its previous size on the dayside. As can be seen in panel f of Figure 4, the cold plasma den-sity (measured by LAP) increases by as much as one orderof magnitude for two 1-hour long intervals in the morning of6 Oct 2015. The spacecraft potential goes significantly nega-tive, from +1V to -10V, indicating that the electron densitymust be increased, to provide a higher flux of electrons tothe spacecraft. Alternatively, the electron temperature canbe increased to provide the higher fluxes, but as LAP doesnot measure any increased temperature this does not appearto be the case. These signatures coincide with the large mag-netic field spikes appearing and also with large-scale mag-netic field rotations. At the same time, the flux of the moreenergetic electrons increases significantly, albeit more pro-nounced so for the second interval. The solar wind ions, onthe other hand, do not appear to show a maximum of fluxesat the same time as the density peaks.To investigate if this density increase is due to increasedimpact ionisation from suprathermal electrons we calculatethe ionisation frequency from three different suprathermal
N. J. T. Edberg et al.
Figure 5.
IES electron energy spectra during three intervals be-fore and during the CME impact. Up to an order of 10 increasein electron fluxes for energies below 200 eV during the CME im-pact are observed during the CME impact (blue and red line),compared to before (black line). electron spectra in this time interval. Figure 5 shows thethree spectra. The times are indicated in the figure and cor-respond to before CME impact, during elevated flux beforethe main CME impact, and during the time of maximumsuprathermal electron fluxes. Combining these spectra withelectron impact ionization cross sections of H O (Itikawa &Mason 2005), we obtain impact ionisation frequencies f E of . · − s − , . · − s − , and . · − s − , respectively.For these calculations we have assumed isotropic electronfluxes and corrected the measured electron fluxes for thespacecraft potential as derived from LAP measurements.A more thorough treatment of electron impact ionisationfor comet 67P can be found in Galand et al. (2016). Forcomparison, the H O photoionization frequency is approx-imately · − / d = . · − s − , where d =1.41 AU isthe heliocentric distance (Vigren et al. 2015). If assumingproportionality between the total ionization frequency andthe electron number density the enhanced electron impactionization would only bring about a factor of 2.5 increase inthe electron number densities. Hence, the observed increaseddensity by a factor of 10 can clearly not be attributed solelyto increased particle impact ionization.Charge exchange is another process that could cause en-hanced plasma density (Gombosi 1987; Burch et al. 2015).Considering H + + H O → H + H O + to be the dominatingcharge exchange process (Simon Wedlund et al. 2016b), wecan estimate roughly its contribution to the increased den-sity. An average energy spectra of solar wind ion flux (mainlyprotons) is shown in Figure 6. This spectrum is averagedover an interval when the LAP measured electron densityis increased to about 100 cm − . The H + flux is typicallyF H + ∼ · cm − s − eV − at maximum, at an energy E max of 200 eV. The charge exchange cross section σ cx is equal to Figure 6.
ICA solar wind ion energy spectra during the CME im-pact, when the LAP measured electron density was significantlyincreased. The solar wind is significantly reduced both in energyand flux as it reaches Rosetta. . · cm − for H + with an energy of 200 eV (Mada et al.2007). The ion production rate (or charge transfer rate) isthen f cx = σ cx F H + E max = . · − s − , which is compa-rable to the photoionization and electron impact ionisationfrequencies. However, charge exchange is not a net source ofplasma, but rather changes the composition of some of theplasma from solar wind ions to heavier cometary ions. Asthe heavier ions will have lower velocity than the solar windions (in order to conserve momentum) there will be a pileup of plasma and an effective increase of the density. Thisincrease depends on the local fraction of solar wind that ischarge exchanging and it is challenging to calculate the exactdensity increase this would yield. More sophisticated modelsare needed, which is beyond the scope of this paper, and weleave that for any future study. We will settle with simplystating that there will most likely be a significant densityincrease caused by charge exchange at this time. However,we can also mention that preliminary results from hybridmodels (Simon Wedlund et al. 2016a) using variable inputconditions to simulate the effects of a solar wind pressurepulse, such as the one studied here, are in tentative agree-ments with our results. Most importantly, in the simulation,the density may increase several times when the pressurepulse impacts (Alho et al. 2016).The compression due to the increased solar wind dy-namic pressure cause another factor of 2.5 increase (de-termined from the increase in background magnetic fieldstrength, which we assume to be frozen into the plasma),which brings us close to being able to explain the factor totalof 10 increase in density, if taking into account the uncer-tainty of the simplified models in calculating the increasedionisation rates. We also note that the maximum electronimpact ionisation (the blue spectra in Fig. 5) does not occurwhen the measured solar wind flux are at maximum (bluespectra in Fig. 6), such that the maximum effects of chargeexchange and electron impact ionisation might not occur atthe same time. It is also possible that some of the increased ME impact on comet 67P Figure 7.
Time series of MAG (a) magnetic field magnitude, (b)components, (c) clock angle and (d) cone angle as well as (e) LAPdata from an 8 min interval during the CME impact. The greyshaded regions indicate 8 magnetic flux ropes structures. density is due to the changing field direction and that thecold plasma is accelerated by an electric field in the directiontoward Rosetta, as discussed by Vigren et al. (2015).
The third and final feature we observe arising as the CMEimpacts are the large amplitude magnetic field spikes pre-sented in Figure 4. To investigate the nature of these spikesmore carefully, we show in Figure 7 a further zoomed inpart of this interval, focusing on the early morning of 6 Oct2015. In this figure, we now include the high-time resolu-tion (57.8 Hz) ion current measured by LAP1, rather thanthe lower resolution sweep derived parameters. The contri-bution from photo-electrons has been subtracted from thismeasured current, such that the measured current should beproportional to the ion flux. (The subtracted photo-electroncurrent was about 25 nA and determined on a daily basisfrom the characteristics of the combined sweeps that day).The grey shaded regions in Figure 7 indicate eight selectedevents, where the magnetic field strength increases to reachat least 150 nT in all but one case. The intervals are de-termined by eye as when the field is increased and the fieldorientation change occurs. The clock angle = arctan ( B y / B z ) show clear rotations of the magnetic field in these intervals,while the cone angle = arccos ( B x /| B |) shows an increase fol-lowed by a decrease during the events. The rotations are allin the same sense. The ion current typically increases a fac-tor of about 1.5-3 during the spikes. However, the currentincreases are not always in concert with the magnetic fieldsignatures. The current rather increases during a short in-terval within the field rotations periods, or at the edges ofthem.We have performed a minimum variance analysis (MVA) (Sonnerup & Cahill 1967) on these and several moresimilar spikes throughout the two day interval 5-6 Oct 2015.The MVA is a single spacecraft method for obtaining theorientation of a stationary magnetic field structure in space,e.g. the normal of a current sheet or the axis of a magneticflux rope. More specifically, the MVA gives the direction ofminimum, maximum and intermediate variance of the mag-netic field, which forms a right-handed coordinate system.The three orthogonal directions come from the eigenvectorsof the co-variance matrix of the magnetic field components,calculated over a short time (the magnetic field spike, in thiscase). The eigenvector with the smallest associated eigen-value is the minimum variance direction. The original mag-netic field vectors can then be transformed into this newcoordinate system so that, for example, one of the compo-nents is directed normal to the stationary structure assumedto exist in the space where the spacecraft is located.From the results of the MVA, which we will show next,we find that at least 40 spikes appear to be magnetic fluxropes in this two day interval. Figure 8 shows the results ofthe MVA from one of the events presented in Figure 7. Thegrey shaded region again indicates the interval of the mag-netic field spike/flux rope passage. The hodograms of themagnetic field components in MVA-coordinates show char-acteristic circular pattern, if plotting the components in thedirection of maximum and intermediate variance direction,and a near-straight line if plotting the maximum and mini-mum components, which are typical signature of flux ropes(e.g Elphic & Russell 1983). Also, the eigenvalues λ l , λ m , and λ n , associated with the maximum, intermediate and mini-mum eigenvectors (i.e. the direction of minimum, mediumand maximum variance, respectively) are shown next to thehodograms. The ratio λ m / λ n is well above 10, which is alimit for the accuracy of the determination of the eigenvec-tors. Hodograms of the other seven events from Figure 7 areshown in Figure 9, and they all have the similar character-istic shapes as the previous event.We note that not all spikes in the two day interval ap-pear to be flux ropes, which indicates that the MVA anal-ysis might not always work as intended and/or that thereare also other dynamical processes at play, e.g. waves andother instabilities. We have not checked each individual spikethroughout this interval, but rather focused on the largestamplitude spikes. We also note that there are current in-creases which are not associated with an identified flux ropebut still with a field orientation change, e.g. at 02:35:20UT and 02:37:30 UT in Figure 7. The identified flux ropesare not to be confused with the large-scale flux rope acrossthe CME itself, but are rather short duration ( ∼ N. J. T. Edberg et al.
Figure 8.
Results from an MVA analysis of one identified fluxrope. The top panel shows the MAG data in CSEQ coordinateswith the grey shaded region indicating the flux rope. The secondpanel shows the LAP1 ion current, which is proportional to iondensity. The third panel shows the MVA coordinates from thetime indicated by the grey shaded region, and the lower two pan-els show hodograms of the magnetic field components in MVA-coordinates. The eigenvalues as well as the ratio between theeigenvalues associated with the intermediate and the minimumcomponents are also stated. anti-Sun direction. The magnetic field direction in the inter-val around the time of the CME impact is shown in Figure11, projected on the x-y and x-z planes. For most of thetime, before the large event around midnight on 5 Oct, thefield direction of the magnetic field is generally in the -x,-y-direction. Several orientation changes occur toward the endof this interval, when the CME main impact occurs, but be-fore this the global field direction still has a preferred direc-tion. The magnetic field direction is generally perpendicularto the axes of the identified flux ropes.Some of the flux ropes observed here are unusually largefor magnetic field magnitude and peak at over 200 nT inthree events. These are peak field strengths larger than thatof flux rope structures observed anywhere elsewhere in inter-planetary space. However, larger flux ropes do exist in thesolar corona. At Mars, large amplitude flux ropes have beenobserved to form through the interaction between crustalmagnetic fields and the solar wind and reached peak mag-netic field strength of 180 nT (Brain et al. 2010). However,the background field (outside of the flux rope) is at least 50nT here, while for the event at Mars it was about 20 nT,so the relative increase is larger at Mars. In the ionosphereof Venus, flux ropes have been observed to reach about 100nT (Zhang et al. 2012), although most are on the order of
Figure 9. ∼
700 m/s in the radial di-rection (H¨assig et al. 2015), and as the ions form primarily
ME impact on comet 67P Figure 10.
MVA vectors of 40 identified events in the interval4 Oct 2015 - 6 Oct 2015. The purple vectors show the minimumvariance direction, which are aligned with the rope axis. The redand the orange arrows indicate the maximum and intermediatevariance direction, respectively, The 40 ropes all have very sim-ilar orientation and are directed slightly off the direction to thenucleus at (x,y)=(0,0) through ionisation of the neutrals their bulk flow will ini-tially be roughly the same. However, it is not obvious thatthe flux ropes at the comet move with the neutral gas flowspeed, since Rosetta was at this time at a distance of 800 kmfrom the nucleus and in the ion-pile up region. In fact, IESmeasurements during the excursion indicate an ion velocityof ∼
10 km/s, as stated in Section 3.2. If the flux ropes aremoving with speeds in the interval 1-10 km/s they wouldbe on the order of 10-100’s km large, in the direction of theflow (the velocity of Rosetta is insignificant). The flux ropesobserved at Mars reported by Brain et al. (2010) and Behar-rell & Wild (2012), were on the order of ∼
100 km, and thecommonly observed flux ropes in the ionosphere of Venuswere found to have a radii of 6-15 km (Russell & Elphic1979; Elphic & Russell 1983). However, the high-amplitude( ∼ Figure 11.
Magnetic field vectors projected on to the (top) x-yplane and (bottom) x-z plane along the Rosetta trajectory from00:00 UT 4 Oct 2015 until 12:00 UT 6 Oct 2015. Red colours indi-cate an out-of-plane component, blue and in-to-plane componentand green in-plane. The magnetic field is mainly directed in the-x,-y-direction before the large field rotations on 6 Oct 2015. (Goetz et al. 2016). Magnetic field spikes were also observedin close proximity to the magnetic cavity events, togetherwith sudden density enhancements (Goetz et al. 2016; Eriks-son et al. 2016). However, those were smaller in size andappeared to be more regular. It is possible that those arethe same type of structures, but that the ones presentedhere have grown in size compared to the average cases closeto the cavity, and been transported outward with the gasflow. During the transport outward they would likely be-come somewhat deformed. A possible scenario could be that N. J. T. Edberg et al. as the CME impacts on the comet, the plasma environmentis initially compressed and the density increase. As both themagnetic and thermal pressure consequently increases, andthe CME pressure eventually decreases, the plasma regionsand boundaries formed in the near-nucleus plasma couldmove outward. This would then lead to both the diamag-netic cavity growing and the flux ropes seen around thecavity events being transported outward. This is howeversomewhat speculative.Flux ropes, which in principle are the same as mag-netic islands, flux transfer events or plasmoids, typicallyform when there are large shears in a plasma, or, as an effectof magnetic reconnection. Large shears are certainly possi-ble between the outflowing ionospheric plasma (1 km/s) andthe solar wind flow (400 km/s). Goetz et al. (2016) reportedthat the magnetic cavity events where probably associatedwith Kelvin-Helmholtz instabilities, during which flux ropescould also form. Magnetic reconnection is an other possibleformation process. The ions are not magnetized in the comawhile the electrons mostly are, which makes this an envi-ronment where magnetic reconnection processes could pos-sibly occur. The largest amplitude flux ropes are observedin conjunction with the global field direction changes (seeFigure 4). A possible scenario would be that magnetic fieldswith different orientation meet as they convect through thecoma, electrons decouple from the magnetic field, reconnec-tion occurs and plasmoids/flux ropes are formed. Similarideas have been proposed to occur at Venus when interplan-etary magnetic field reversals propagate through the iono-sphere of Venus (Edberg et al. 2011; Vech et al. 2016). Thebursty nature of the magnetic field spikes during the CMEimpact, makes them appear similar to what has been ob-served in the magnetosphere of Mercury, where bursts offlux transfer events have been observed (Slavin et al. 2012).Particle-in-cell simulations have shown that magnetic recon-nection could form such bursts of plasmoids as the tearinginstability disrupts the initial current sheet (Markidis et al.2013). However, further studies would be required to deter-mine if magnetic reconnection could actually occur in thecomet environment.Due to the orbit of Rosetta being on the dayside of thecomet in this interval, we cannot study what is happeningto the ion tail during the CME impact. Previous remote ob-servations of comet-CME interactions have shown that taildisconnection events can occur as a CME impacts (Niedner& Brandt 1978; Vourlidas et al. 2007; Jia et al. 2009). Thisprocess is usually attributed to magnetic reconnection in thecomet tail. Here, we are possibly seeing magnetic flux ropesbeing formed close to the nucleus instead. These are not thesame as tail disconnection events although the processes in-volved could be similar. Some minor amount of plasma willstill be carried away in the flux tubes. How much plasma iscontained in the flux ropes is challenging to estimate sincethe exact scale and structure of them are somewhat unclear.However, if the density is on the order of 600 cm − and theflux rope has a radius of about 100 km and is 600 km long,it would contain ∼ particles. This assumes that eachflux rope extends from 800 km down to 200 km (roughlywhere the magnetic cavity events were observed), and thatthe density is not decreasing with distance within the fluxrope. We have observed how a CME impacts on comet 67P whenthe comet was at 1.41 AU from the Sun (past perihelion).Rosetta was at this time on its inbound leg from the day-side excursion located at about 800 km from the nucleus.The plasma environment is significantly disturbed duringthe impact. The cold plasma density increases by as muchas a factor of 10, to reach a maximum of 600 cm − , thesuprathermal electron flux (10-200 eV) increases by a factorof 5-10, and the background magnetic field increases by afactor of ∼ ACKNOWLEDGEMENTS
Rosetta is a European Space Agency (ESA) mission withcontributions from its member states and the NationalAeronautics and Space Administration (NASA). The workon RPC-LAP data was funded by the Swedish NationalSpace Board under contracts 109/12, 135/13, 166/14 and114/13 and Vetenskapsr˚adet under contracts 621-2013-4191and 621-2014-5526. Support for RPC-MAG is provided bythe German Ministerium f¨ur Wirtschaft und Energie andthe Deutsches Zentrum f¨ur Luft- und Raumfahrt undercontract 50QP 1401. The work on IES was supported in
ME impact on comet 67P part by the U. S. National Aeronautics and Space Admin-istration through contract 1345493 with the Jet Propul-sion Laboratory, California Institute of Technology. Workat LPC2E/CNRS was supported by CNES and by ANR un-der the financial agreement ANR-15-CE31-0009-01. C.S.W.is supported by the Research Council of Norway grant No.240000. This work has made use of the AMDA and RPCQuicklook database, provided through a collaboration be-tween the Centre de Donn´ees de la Physique des Plasmas(CDPP) (supported by CNRS, CNES, Observatoire de Parisand Universit´e Paul Sabatier, Toulouse) and Imperial Col-lege London (supported by the UK Science and TechnologyFacilities Council). REFERENCES
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