magnetic coupling in the solar system

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Sarah Matthews, Danielle Bewsher and Chris Davis discuss the complexities of the solar magnetic field, informed by data from Hinode and STEREO.

The Sun’s magnetic field is carried far away from the solar surface by the solar wind, out to the edge of the heliosphere

and the interface with the galactic neighbour-hood. The environment between the Sun and the outer planets is thus dominated by the processes occurring on the Sun that shape the heliospheric magnetic field, and our understanding of the evolution of planetary magnetospheres and atmospheres within our own and other solar

Magnetic coupling in the solar system

Davis et al.: Magnetic coupling

On 10 October 2008, UK solar, solar–terrestrial and planetary scientists hosted a meeting at the RAS on the theme of “Magnetic coupling in the solar system – from the Sun into the heliosphere”. This meeting was proposed in order to stimulate discussion on how the variations of the solar magnetic field influence other solar system bodies such as comets, planets (including our own) and their moons. Energy from within the convection zone is transferred through magnetic coupling into the corona and the heliosphere beyond on a wide range of spatial and temporal scales

that are related to the different types of magnetic field emergence at the solar surface. Thus in order to understand the global heliospheric magnetic field and its influence throughout the solar system, we have to understand the role that flux emergence on all scales plays in structuring in the global field and the associated solar wind. With the successful launch of two major new space-based missions, Hinode and STEREO (Solar Terrestrial Relations Observatory), scientists are well placed to study the emergence of the Sun’s magnetic field and track it into the heliosphere.

abstract

1: An image from the Heliospheric Imager on the STEREO-A spacecraft, taken on 5 November 2007, showing a coronal mass ejection entering the frame from the right, the Milky Way and Jupiter to the left. (NASA)


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systems relies heavily on our ability to under-stand the whole coupled system. At the outer edges of the heliosphere the galactic neighbour-hood exerts the controlling influence. Studies of solar system physics thus have the potential to extend our knowledge of the universe far beyond the sum of its constituent parts, provided that we can make the connections between the phys-ics occurring in different parts of the system.

The ultimate source of all solar activity and its subsequent impacts within the solar system is of course the Sun’s magnetic field. There are still many areas in which our knowledge of the origin and evolution of the solar magnetic field throughout the solar cycle is incomplete, and the meeting began with a summary of obser-vations of solar activity deduced from sunspot activity (figure 2) in cycles 21–23 by Valentina Zharkova (University of Bradford).

The evolution of sunspots through the solar activity cycle is well known, but recent work has focused on the asymmetries between the two hemispheres. Observations show that the back-ground magnetic field changes through the solar cycle and there appears to be a “royal zone” – a restricted latitude region where magnetic flux can emerge as sunspots. The combined area of these sunspots differs between the northern and southern hemispheres and the same is true for the combined area of active regions. By defining an asymmetry index (S – N) / (N + S), this differ-ence can be quantified. Over a given cycle, the asymmetry decreases but (particularly for active regions) lasts throughout the cycle. The hemi-sphere in which the magnetic field dominates oscillates throughout the cycle with a period of around 2.5 years. The significance of this asymmetry manifests itself in a variety of ways such as in the correlation with the occurrence of flares (seen in Hα emissions).

Lidia van Driel-Gesztelyi (Mullard Space Sci-ence Laboratory) then talked about the emer-gence, evolution and eruption of magnetic flux from the Sun. Such emergence is ubiquitous, even when the Sun is at the minimum of its activ-ity cycle, when the emergence is on a relatively small scale. In her presentation, Lidia concen-trated on large-scale events. Observations show loops emerging from the surface of the Sun and quickly reconnecting, followed by flares. For such energy releases, free energy is required in the magnetic field and it has been proposed that the source of this energy results from twists in the emerging flux. Modelling shows that, if there is no twist in the emerging field, the rising flux tubes quickly fragment. Because the flux rises through a complex turbulent convective zone on its way to the surface, twist in the field is vital if it is to escape being torn apart by vortices in its wake. If such flux is twisted sufficiently, it goes through the helical kink instability and produces complex patterns in the magnetic fields of emerging flux tubes. This brings great

complexity to sunspot regions. But most emerg-ing loops are not strongly twisted. It is common to see “nested” flux emergence: one loop after another in the same region. Large active regions can be formed from many dipoles that emerge separately. Flux emerges over many days to form an active region, gradually dispersing over sev-eral months. Many flares are associated with the formation of such a region, with numbers decreasing steeply as the active region decays, eventually vanishing. Associated coronal mass ejections (CMEs) also occur most frequently at the formation of an active region, but the rate declines to a constant level. This is linked to the local complexity of magnetic field structures. The flare index and CME acceleration corre-late with the complexity of an active region. Twisted magnetic fields are clearly important in the emergence and intensity of active regions as well as in the occurrence of CMEs.

Impulsive magnetic energy releaseWhile the majority of the solar physics com-munity believe that reconnection is responsible for impulsive energy release, there are still issues with understanding how this process works, particularly in terms of how fast the energy is released in solar flares. David Tsiklauri (Univer-sity of Salford) spoke about a new, fast reconnec-tion model in the collisionless regime that can address some of these issues. A growing number of numerical simulations have shown that in the collisionless regime, the off-diagonal electron tensor plays an important role in regions where the magnetic field is weak and the plasma is not completely “frozen in” to the magnetic field. In the Sweet–Parker (collisional) model, where advection is balanced by diffusion, any plasma outflow occurs at the Alfvén speed. By making assumptions about the nature of the electron

motion and magnetic pressure, David balanced advection with the pressure tensor. The resulting collisionless model fits the laboratory data well, with the width of the electron diffusion region comparable with the skin depth. The implica-tions of this model for the production of flares are that it predicts reconnection rates around 100 times larger and a heating flux 10 times that predicted by the Sweet–Parker model.

Critical for understanding energy release in the solar atmosphere on all scales, is a knowl-edge of the coronal field because, in many cases, this is where reconnection is believed to take place. The coronal field is notoriously difficult to measure, so many indirect, but very effective techniques have been developed and these were discussed in several posters. Tom van Door-salaere (University of Warwick) and Youra Tar-oyan (University of Sheffield) both reported the spectroscopic detection of MHD oscillations of solar coronal structure by Hinode’s EUV Imaging Spectrometer (EIS) that allowed them to estimate the coronal magnetic field strength in several cases. The oscillation of the example presented by Tom (figure 3) had an amplitude of 1 km s–1 and a period of 296 s, demonstrating the impressive sensitivity of the EIS instrument for these kind of measurements.

Stephane Regnier (University of St Andrews) described a study of coronal Alfvén speeds in an isothermal solar atmosphere, important for understanding energy release in the corona, and the initiation and propagation of CMEs. The global properties of the magnetic field strength above four active regions associated with differ-ent eruptive events were determined and most of the magnetic flux found to be localized within 50 Mm of the photosphere; most of the energy to be stored below 150 Mm, and the magnetic field strength was seen to decay with height more

2: A typical sunspot region in G-band at 430 nm, from Hinode, shows the solar surface in the photosphere with unprecedented clarity, including the typical granulation. (Hinode)

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slowly for a nonlinear force-free field than for a potential field. The derived Alfvén speeds varied by up to two orders of magnitude and showed sensitivity to the magnetic configuration, with average speeds in flaring regions departing strongly from potential field values.

William Simpson’s (St Andrews) poster dis-cussed an investigation into the magnetic field topology of coronal null points and solar flares. Mathematical models have been proposed for both phenomena that require the presence of a magnetic null point in the solar atmosphere. William used magnetic observations and 3-D modelling to investigate these null points as they are very difficult to observe directly. In this way, he could extrapolate the coronal magnetic field from the field associated with the photosphere of an active region. By understanding the 3-D topology of the magnetic field, it becomes possi-ble to determine the positions of the null points. The example presented was the magnetic field in the vicinity of active region AR0486 during an X-class flare. The analysis confirmed the pres-ence of a stable null point near the flare site.

Sotiris Adamakis (University of Central Lan-cashire) and Maria Madjarska (Max-Planck

Institute for Solar System Research and Armagh Observatory) both examined the response of the atmosphere to impulsive energy release by studying flaring plasma and the dynamics of a jet, showing the enormous range of effects seen in all wavelengths. Observations of this kind are extremely important for constraining models, and the increasing quality of the data in terms of spatial, spectral and temporal resolution con-tinuously presents new challenges.

CME initiation and evolutionThe question of whether or not a reliable indica-tor or precursor for CME onset can be found is a question that has occupied researchers in the field for some time. Deb Baker (MSSL) addressed this question with observations of up-flows in an active region/coronal hole complex. Were these up-flows an enigma or a CME precursor? Hinode EIS was used to make multiwavelength observations of an active region in an equatorial coronal hole from 11 to 18 October 2008. A highly sheared sigmoidal structure containing a filament erupted to form a CME. The dimming and eruption were observed by STEREO EUVI (EUV Imager) and in the SOHO/LASCO corona-graph. The Hinode EIS instrument observed the active region break up and expand over several days. By overlaying SOHO MDI magnetogram data over EIS velocity maps it is possible to see that closed loops were associated with down-flows while there were upflows at the edges of the active region where the magnetic field was strong. The velocity maps also revealed a strong temperature dependence of such outflows, sug-gesting it is the hotter coronal material that is flowing outwards. Simulations of this event reveal three potential types of plasma up-flow: hot reconnection jets; slower (cooler) plasma along new open field lines resulting from inter-change reconnection between the active region and coronal hole; and plasma upflows driven by the active region expanding along compressed neighbouring open field lines of the coronal hole. The intensification of velocities towards the end of the observation period required greater com-pressive forces. Lots of minor eruptions were observed before the CME. Intensified upflows were seen where the active region was adjacent to open coronal hole field lines, which may be a pre-CME signature.

In terms of CME initiation, Danielle Bewsher (Rutherford Appleton Laboratory/Aberystwyth University) discussed the relationship between EUV dimming and coronal mass ejections. While there have been many studies of EUV dimming in association with CME onsets, there has never been a thorough statistical study of this association, covering appropriate temper-ature ranges. Danielle used a large campaign database from SOHO/CDS and SOHO/LASCO to associate dimming events detected at 1 and 2 MK with CME activity. The results confirm

the CME–EUV dimming association using statistical analysis for the first time and stress that one emission line may not be sufficient for associating dimming regions with CMEs.

Richard Harrison (RAL) picked up this topic from a more global viewpoint with interest-ing observations of outflows in STEREO Heliospheric Imager (HI) data. In a sample of two months’ worth of data, 15 CMEs occurred of which seven were preceded by outflows of material in a stream along a narrow range of solar latitudes. Prior to the CME, these out-flows appeared to intensify with a series of dense blobs travelling away from the Sun’s surface. If these streamers are associated with the CME, this could provide another tantalizing possibil-ity for predicting the occurrence of CMEs. After several examples, Richard homed in on one par-ticular event, from 19 April 2007. By plotting the outflow of material crossing a surface of constant elongation, it is possible to map the latitude of the streamers prior to the CME and measure the speed and intensity of these out-flows. Detailed analysis showed that there were indeed enhanced outflows, forming eight or so transient blobs in the two days before the CME. These all appeared to have similar velocities, around 175 km s–1. This is much less than the expected Alfvén speed, which may indicate that there is a geometric factor involved, for exam-ple, if the flows are not in the plane of the sky. Such activity is consistent with reconnection occurring at one foot-point of a magnetic loop structure, releasing previously contained mat-erial. Further analysis is continuing in order to determine whether these “fuses” are physically associated with the subsequent CMEs, what they represent and how often they occur.

One of the keys to differentiating between competing CME models is determining when the flux rope forms relative to the eruption – is it pre-existing or formed as a consequence of the eruption? Lucie Green (MSSL) presented work that used soft X-ray observations of sig-moids from Hinode to answer specifically this question. Lucie discussed sigmoids (“S” shaped structures in the solar atmosphere), which are seen in both active and quiet regions. In active regions, they are found in the same locations as filaments and the “S” shape is related to the twist in the local magnetic field. Sigmoids occur in both positive and negative forms (mirror images of each other), with the “S”-shaped sig-moids generated by a positive (right-hand) twist in the magnetic field, and mirrored “S” sigmoids are generated by a left-hand twist. They are of interest as their occurrence represents a high probability of an eruption occurring, and Lucie showed several examples related to eruptions where it seemed clear that there was stronger evidence for an arcade to flux rope transition, rather than a pre-existing flux rope.

Bernhard Kliem (MSSL) then broadened the

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discussion towards what drives the eruption of coronal flux out into the heliosphere. These eruptions are defined by an instantaneous onset and rapid acceleration indicative of the release of stored energy followed by a huge expansion at a rate that exceeds the local Alfvén speed. The CME is thus characterized by three main phases: the final two are acceleration and propa-gation into the solar wind, but they are preceded by a long-term evolution of magnetic topology. Bernhard stressed the point that CME models are not exclusive and all probably play a part in CME onset to some degree. All invoke a flux rope and reconnection and modelling work is able to reproduce the observations well. In those that contain twisted magnetic fields, the degree of twist drives the growth phase of such features and can lead to a CME. Kink instabilities can-not explain everything, however, because some events are seen as simple expanding loops. The torus instability – well known in fusion research – requires an external field to retain the field in a loop structure. If the external field decreases rapidly, the contained material is released. It seems that we might gradually be converging towards the point where we have a CME equiva-lent of the canonical solar flare model.

Towards the Earth and beyondIn terms of determining the impact that a CME might have on Earth or the terrestrial planets, being able to track accurately an ejection and its evolution en route is of key importance. STEREO is designed to do precisely this and Jaz Pearson (UCLan) demonstrated how it is possible to track an event from the solar corona (with STEREO/EUVI data) into the heliosphere at around 1 AU with the STEREO/HI. For an event on 25 March 2008, Jaz used two methods to determine the direction of the mass ejection. First, he used the stereo views of the event in the STEREO EUVI cameras to reconstruct the solar longitude of the expanding magnetic field structures. Secondly, he tracked the event into the HI images on STEREO-A. By making the assumption that a CME will be travelling at a constant speed within the HI field of view, it is possible to use the apparent acceleration across the wide-field optics to estimate the angle of propagation with respect to the space-craft. The two estimates were consistent with each other for this event. Thomson scattering of photospheric light results in events that are most prominent when they are propagating at right-angles to the Sun–spacecraft line. As the STEREO spacecraft diverge, this will favour the observations of CMEs propagating along the Sun–Earth line and such techniques will pro-vide a powerful method for investigating Earth-impacting CMEs in future observations.

Alexis Rouillard (University of Southampton) also discussed a CME observed by STEREO on the east limb of the Sun with the HI instrument

on the ahead spacecraft. It was possible to track this CME all the way from the Sun to Venus. A “V”-shaped structure was seen accelerating out in the STEREO coronagraph COR2. This structure appeared to hit others ahead of it, and two features looked to be the outer loop and inner “V”-shaped structure of the same event. Fitting model parameters to this structure gave a best-fit for a flux rope some 50–60° off the Sun–spacecraft line. Tracking these structures through the HI images using the techniques employed by Jaz Pearson, showed that the direc-tion of propagation was only a few degrees in longitude from Venus and the Messenger space-craft. In situ observations from Venus Express and Messenger indicated the passage of a flux rope. Its structure and orientation correlated well with the HI observations and indicated that the structure had expanded at between 30 and 40 km s–1 in the radial direction.

Of course, our ability to have confidence in the results that we obtain from these analyses relies heavily on the quality of the data and the calibration. Danielle Bewsher showed a new method for deriving the instrument pointing for the HI along with other optical parameters, by comparing the locations of stars identified in each HI image with the known star positions predicted from a star catalogue. The pointing and optical parameters are varied in an auton-omous manner to minimize the discrepancy between the predicted and observed positions of the stars. With this method, the HI images can be considered as self-calibrating.

The Earth is, of course, not alone in being affected by all that the Sun has to throw at us, and the effects of varying solar activity on the evolution of planetary atmospheres may well have a key role to play in determining conditions for life. Focusing on impacts of the solar wind and solar transients on the terrestrial planets within our solar system, Janet Luhmann (Uni-versity of California, Berkeley) presented the the-ories and observations of the ways in which the solar wind affects Earth, Mars and Venus. This is a particularly interesting time for progressing such studies, as not only is there a comprehen-sive suite of solar observatories in operation, but there are also missions operating at each of the terrestrial planets. Janet highlighted the difference in conditions and subsequent effects at Venus and Mars compared to the Earth. Although they both have atmospheres, Venus and Mars do not have active magnetic fields and so solar storms interact with their atmo-spheres directly. The ultraviolet aurora on Venus responds to conditions in the heliosphere, while aurorae have been observed on Mars where the solar wind is concentrated in the martian atmos-phere by pockets of crustal magnetic field. Janet then emphasized that the main drivers of solar activity are those events that enhance the solar wind speed and dynamic pressure and BZ, viz.

co-rotating interaction regions (CIRs) caused by a region of fast wind compressing a region of slower wind ahead of it and CMEs.

In his talk, Alexis Rouillard also showed details of how CIRs evolve as they move out into the heliosphere using STEREO HI and in situ data. HI observations show blobs of mat-erial travelling out into the CIR structure that change the nature of the CIR as it rotates from the location of the STEREO-B spacecraft to the STEREO-A spacecraft, and the effect that is seen in the in situ STEREO data indicates how important it is to our understanding of these events to have multiple perspectives (figure 4).

CMEs are, of course, comparatively large and are the most studied of all transients in the inter-planetary medium. They exhibit strong solar-cycle dependence because they are associated with active regions, with differences between each cycle determined by the dominant polar-ity of the interplanetary magnetic field. Con-secutive cycles have an alternating preference that governs the behaviour of the shock. If the interplanetary magnetic field is southward after a CME shock, reconnection occurs with the Earth’s magnetic field initiating a sudden onset of a geomagnetic storm. A major challenge in CME research is to connect the behaviour of a CME with the nature of the associated active region. While the polarity of a CME is of great importance in predicting how it will interact with the Earth’s magnetic field, the relationship between the polarity of the CME and the source region is not simple. A southward component in the interplanetary magnetic field engages the magnetosphere like a clutch in a vehicle, while more subtle changes are seen when the field is northward. On this topic, Laura Bone’s (MSSL) poster discussed the interaction of active region and quiescent filaments associated with a flare and CME. The CME erupted on 19 May 2007, accompanied by a flare, and resulted in a mag-netic cloud near Earth that was registered by the STEREO-B and WIND spacecraft on 22 May. Two independent filaments, one in an active region and the other quiescent, were observed in the days before the flare and eruption and seen to merge. Heating cycles preceded the final filament disappearance, heating and eruption observed by Hinode, STEREO and TRACE.

The effect of the interplanetary medium at Earth is not just confined to CMEs and CIRs, however. The occurrence of solar energetic protons (SEPs) also varies with the solar cycle; the emergence of magnetic flux at times of peak solar activity shields the Earth from cosmic rays and so the cosmic ray flux at Earth is anti-correlated with the solar cycle. SEPs have been associated with atmospheric changes such as a modulation of stratospheric ozone while cosmic rays have been associated with the seeding of high-altitude clouds, modulating the Earth’s albedo, making their study important for long-

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term climate-change research. Janet Luhmann also highlighted the fact that

ionized particles in planetary atmospheres can be lost to space as the solar wind impacts on a planetary atmosphere. At the Earth, the mag-netic field largely contains these within the mag-netosphere, but there is evidence from both the Mars Express and Venus Express missions that this mechanism is occurring at these planets and, in the case of Mars, this may be responsible for the loss of the martian atmosphere. Current loss estimates do not take solar activity into account, but at times of increased solar activity the flux of O+ ions lost in this way can increase by a factor of 100. Do these interactions dictate the fate of atmospheres in our solar system and in other planetary systems?

Global field and the solar cycleHaving touched on the effects on other planets, the discussion focused back on understanding the global evolution of the heliospheric field and its effects. Mat Owens (Imperial College Lon-don) discussed the possible role of CMEs in the evolution of the Sun’s global magnetic field dur-ing the solar cycle. Observations of photospheric flux have long been known to demonstrate a strong solar-cycle variation, but this emission is associated with magnetic field structures that close in the corona and do not extend into the heliosphere. Although we only have single point measurements of the heliospheric field from spacecraft, it is apparent that the variation of total magnetic flux in the heliosphere is also closely associated with the solar cycle. Observa-tions of supra-thermal electrons (STEs) allow us to infer whether the field lines that contain them are open or closed. Open field lines contain a single beam, while counter-streaming electrons are observed on closed field lines. The latter case is nearly always associated with CMEs that add closed flux to the heliosphere. Averages of the intensity of the interplanetary magnetic field

and the CME rate are well correlated. Matt left the interesting question of whether this demon-strated that CMEs were the cause of the solar cycle variation for another day, but did address how the flux was removed to return the flux lev-els back to their minimum value. It appears that the most likely mechanism is interchange recon-nection, where a CME closed loop is opened. This has been confirmed by observation where counter-streaming electrons are replaced by a single beam. Observations also show that this reconnection tends to favour one polarity foot-point over another, thereby preserving the polar-ity of the open field lines. CME foot-points tend to follow the same rules as sunspots, thereby providing an effective way of transporting flux and therefore generating the polarity cycle. Dur-ing the rising phase of the cycle, open flux fills the polar regions, while CMEs transport closed flux into the heliosphere around the equator. Interchange reconnection takes place at the trailing edge of these foot-points, transporting open flux from the pole to the equator. At solar maximum, the dominant polarities flip and in the declining phase equatorial flux is transported to the pole. There is sufficient flux to explain the field reversals so long as the CME foot-points are separated by more than 5°. It may be that CMEs are not facilitating all flux transport with such a mechanism, but it is certainly an important process. The solar dynamo drives the system, but CMEs are the response to this driver.

The current solar cycle has been generating substantial interest because of the unusually high number of spotless days observed during the current minimum, the reduced value of the photospheric magnetic field strength and the heliospheric magnetic field observed at 1 AU, raising questions about whether we may be seeing a long-term change in solar activity, or whether the current cycle is anomalous. Bob Forsyth (Imperial College London) presented a comparison of the solar heliospheric magnetic

field measured over two solar minima by the Ulysses spacecraft. Ulysses has been making observations over two solar cycles and is now on its third orbit of the Sun. From the first orbit, the solar magnetic field was clearly symmetric, with fast solar wind over the two polar coronal holes, approximating to a tilted dipolar field. In addi-tion to the high-latitude polar coronal holes, these observations revealed the ubiquitous pres-ence of large-amplitude Alfvén waves while the composition of the plasmas from the fast wind regions indicated that they originated from regions of lower coronal temperatures. Three orbits on, the solar magnetic field structure has become more complex. The polar coronal holes extend over far greater latitudes, while the tilt of the current sheet has increased by 10°. There has also been a reduction in flux density by around 34% since the minima of first orbit. According to observations of the Wilcox solar observa-tory, the photospheric field has also reduced by a similar amount between the two cycles and this may be, at least in part, the cause of the decrease observed by Ulysses. This reduction will have led to a reduction in the magnetic field wave power and many other solar wind param-eters, as discussed by Tim Horbury (Imperial). He showed that the amplitude of waves and tur-bulence in the high-speed wind is about 40% lower than the previous minimum. The “global” observations of Ulysses compare well with point measurements made by the Advanced Composi-tion Explorer spacecraft at the first Lagrangian point whenever Ulysses is within 30° in longi-tude, even if Ulysses is at high latitudes. Hence the observed changes affect the entire helio-sphere and may have important consequences at Earth, in areas such as increased cosmic-ray flux. Recent modelling has shown that the mag-netic flux powers the solar wind. Lower coronal temperatures lead to a reduction in the size of the heliosphere and this could explain why the Voyager spacecraft crossed the heliospheric ter-mination shock sooner than expected.

As the meeting concluded, it was clear that while our understanding of complex solar phen-omena and their influence on the heliosphere was being greatly enhanced by both theory and observations, there are still many unresolved questions and many new and exciting challenges in understanding the interactions between the many different components that comprise our solar system. The organizers would like to thanks all the participants. ●

Sarah Matthews is Head of Solar and Stellar Physics at UCL’s Mullard Space Science Laboratory, [email protected]. Danielle Bewsher is a solar physicist working with SOHO, STEREO and Hinode at the Rutherford Appleton Laboratory. Chris Davis is the Project Scientist for the STEREO Heliospheric Imagers at the Rutherford Appleton Laboratory.

4: STEREO’s observations from three distinct vantage points in space help scientists to unravel complex regions in the solar atmosphere. (NASA)

magnetic coupling in the solar system

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