Abstract

The MIST (magnetosphere, ionosphere and solar-terrestrial physics) community has been reinvigorated with a more formal structure, including a council, and a more thematic approach to the regular community meetings. The MIST group is also becoming more involved with meetings in the wider physics and astronomy community. So far the changes have proved successful. Here we report some of the highlights of the annual November meeting, concentrating on the main three themes: Saturn, mesosphere-thermosphere-ionosphere coupling, and the solar wind.

The MIST (magnetosphere, ionosphere, solar-terrestrial) community has been gathering for scientific meetings since 1970. Up until now the organization of MIST has been somewhat informal, with one or two MIST coordinators keeping things ticking over. This year MIST went to the National Astronomy Meeting in Preston for the first time, aware that it needed to open itself to the wider UK astronomy community. At the same time it was decided that the organizational structure should be formalized and a MIST council was formed. Mike Hapgood (RAL) chairs the new council, which also includes Gary Abel (BAS), Chris Arridge (MSSL), Andrew Kavanagh (Lancaster) and Betty Lanchester (Southampton).

One of the first actions of the new MIST council has been to shake up the annual one-day meeting in London in November. This year it was decided that the meeting, on 30 November 2007 at the Geological Scociety, London, should take a more thematic approach to the oral programme, incorporate a poster session, and have invited keynote speakers. The chosen themes were Saturn, mesosphere-thermosphere-ionosphere coupling, and the solar wind, and the meeting was a resounding success with a very lively poster session and a well-attended series of talks.

Saturn

Before the Cassini-Huygens spacecraft arrived at the Saturn system, scientists speculated about the rapidly rotating magnetosphere. Would it operate like Jupiter's giant rotating Io-plasma-filled magnetosphere, or would it be more Earth-like and react predominantly to the prevailing solar-wind conditions? The Voyager spacecraft had suggested that both internal rotation and external solar wind conditions would play an important role. Cassini has now delivered us a wealth of data from the system, and is currently nearing the end of the nominal mission. The data has boosted our general understanding of the system, while raising many further interesting questions. One thing we probably all agree on, however, is that Saturn's magnetosphere is very much “Saturn-like” - that is, unlike any other magnetosphere we have explored.

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Saturn's dynamic aurora , shown from left to right on 24, 26 and 28 January2004. (STSci)

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Saturn's dynamic aurora , shown from left to right on 24, 26 and 28 January2004. (STSci)

A strong UK involvement in the Cassini mission has resulted in a strong representation of our work at the recent London MIST meeting. David Southwood (ESA/IC) opened the meeting with an invited lecture entitled “Saturn pulsations: thoughts on a different magnetosphere”. The first evidence of pulsations near to the planetary period came from the Voyager spacecraft measuring kilometric radio emissions from within the magnetosphere, and these unexpected oscillations were later uncovered from the charged particle and magnetometer data too. The arrival of Cassini at Saturn in July 2004 immediately confirmed that these pulsations were omnipresent in the magnetosphere. Of course, Jupiter's magnetosphere is also dominated by oscillations in the field and particle observations, along with planetary-modulated radio emissions. So why are the observations at Saturn causing such a stir? The answer lies in the internal magnetic field structure. At Jupiter, the magnetic axis is tilted away from the rotation axis by ~10°, making the whole system “wobble” as the planet rapidly rotates. Thus, the planetary modulated signatures at Jupiter are no great surprise. At Saturn, however, the magnetic axis shows no sign of being tilted away from the rotation axis and thus we are led to question the origin of these approximately planetary-period pulsations, the dominant feature of all the field, particle and radio observations.

Further to these initial observations, the period of the Saturn kilometric radiation (SKR) is also found to have changed significantly since the first measurements in the Voyager-era. In fact, the SKR emissions (whose source is thought to be the high-latitude auroral zones near to dawn) show a variation in the observed period of ~1% per year. Therefore the SKR emissions cannot reasonably provide a measure of the planetary rotation rate as was previously assumed. The field and particle observations suggest a rotating perturbation in the magnetosphere, moving both azimuthally in the direction of planetary rotation and radially outwards from the planet. So far, various theories have been offered to explain the properties of the SKR and field/plasma oscillations in the magnetosphere, but the mystery is yet to be wholly understood. A systematic study of the properties of the field and particle signatures is required to further illuminate the problem. New observations were discussed, showing developments in our understanding of the properties of the pulsations - i.e. the period and phasing of the field components, the properties of the amplitude variations with latitude and distance.

While the magnetosphere appears in this sense to be strongly driven and modulated by the effects of planetary rotation, we have also been intrigued by the variation of the auroral emissions at Saturn, particularly in response to the upstream solar wind. Theoretical expectations have suggested that the aurora will lie at the boundary between open and closed magnetic field lines in the system (as for Earth). Recently, the orbit of Cassini progressed to high latitudes and provided the first opportunity to study Saturn's polar magnetosphere. UV observations of Saturn's southern auroral emissions using the Hubble Space Telescope in January 2007 were coordinated with this phase of the mission. Cassini magnetometer data indicate that a substantial and consistent field perturbation is present in the high-latitude dayside magnetosphere near to noon, particularly in the azimuthal field component. The azimuthal field reduces from strong positive values to near-zero, indicative of a layer of upward field-aligned current. The January HST observing campaign took place during revolution 37. At this time, the upward current signature lay directly on top of the auroral oval when magnetically mapped to the ionosphere. The plasma characteristics suggest that this physical boundary, which is seen near to noon, represents the open-closed field line boundary. This work confirms that the main oval at Saturn (at least near to noon) maps to a strong upward field-aligned current straddling the open-closed field line boundary. The evidence shows, therefore, that the aurora forms through the solar-wind interaction with Saturn's magnetosphere, suggesting that the solar wind remains important at Saturn. This latter work was presented by Emma Bunce (Leicester).

Mesosphere-thermosphere-ionosphere coupling

The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere, lying between about 50 km to 80-90 km altitude above Earth's surface. Because the mesosphere sits above the altitude at which planes fly and below the altitude of orbiting spacecraft, it is one of the poorest understood regions of the atmosphere. However, recent advances in instrumentation are opening up our understanding and showing the mesosphere to be an important region within which many physical processes and regimes interact. A number of the presentations at the meeting emphasized the extent of coupling between different regions of the atmosphere (including the mesosphere) and also between the ionized and neutral components. By combining ground-based measurements with satellite and global model predictions, these complex interactions have been placed in context and led to a clearer understanding of the underlying physical processes. In the first talk of the afternoon Nick Mitchell (Bath) introduced the important role the mesosphere plays in global atmospheric circulation and the range of waves and tides that contribute to the transfer of energy and momentum between the layers. Many interactions occur between these oscillations and can result in modulation of their amplitudes. EOS AURA Microwave Limb Sounder satellite data was shown to demonstrate that very different mechanisms underly the generation of the two-day planetary wave in the summertime mesosphere compared to wintertime mesosphere at high latitudes. This, combined with theoretical work, indicates that the wintertime wave originates in the stratosphere and its enhancement is a result of wave interactions with the background mean neutral flows.

At ionospheric altitudes the background neutral winds also play an important role in moving the ionization into regions of higher or lower recombination and Dimitry Pokhotelov (Bath) is investigating this in the context of the extreme ionospheric events observed during major geomagnetic storms. The ionospheric plasma content can be deduced from characteristics of microwave GPS signals acquired by a ground network of dual-channel GPS receivers. The tomographic inversion of the GPS data in a 3D time-dependent inversion algorithm can reveal the spatial and temporal distribution of ionospheric plasma density. Figure 2 illustrates the distribution of total electron content (TEC) over the northern hemisphere for 30 October 2003, during the “Halloween” geomagnetic storms. Large amounts of high-density plasma are produced in the dayside ionosphere by solar photo-ionization. During the magnetic storm, the ionospheric plasma is lifted to higher altitudes where the recombination loss is smaller, thus creating a dramatic TEC anomaly over the North American continent. A tongue of ionization is carried anti-sunward by high-latitude ionospheric convection flow from North America over Greenland and into Northern Europe. Neutral winds are thought to be the most likely cause of the ionospheric lifting and their influence was also noted by Nalan Balan (Sheffield), who presented a poster that also investigated the global ionospheric response to large geomagnetic storms by combining satellite and ground-based data over 7-12 November 2004. Modelling showed that equatorward neutral winds were the main driver of positive ionospheric storms at low to mid-latitudes in some longitudes, rather than penetration electric fields.

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A map of the total electron content of the ionosphere over the northen hemisphere during the Halloween geomagnetic storm of 2003, showing the boost to the ionospheric plasma. (Dimitry Pokhotelov, Cathryn N Mitchell and Paul S J Spencer from the Dept of Electronic & Electrical Engineering, University of Bath)

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A map of the total electron content of the ionosphere over the northen hemisphere during the Halloween geomagnetic storm of 2003, showing the boost to the ionospheric plasma. (Dimitry Pokhotelov, Cathryn N Mitchell and Paul S J Spencer from the Dept of Electronic & Electrical Engineering, University of Bath)

Treating the neutral and ionospheric flows in a self-consistent manner is possible within global circulation models such as the Coupled Thermosphere Ionosphere and Plasmasphere (CTIP) model, but sometimes limited by low-resolution or parameterized inputs. Emma Whittick (Aberystwyth) has combined model and measurements to understand the patterns of ionization seen in a latitudinal chain of radio tomography receivers. In its standard form the CTIP model uses statistically averaged high-latitude electric potential patterns to drive ionospheric convection. This leads to a lack of sensitivity to the dynamic and large-scale changes that can characterize the ionosphere when reacting to changes in the Interplanetary Magnetic Field (IMF). By using realistic electric potential patterns from the SuperDARN radar network as the ionospheric convection input to the CTIP model, it has been possible to reproduce the main features observed in the tomography data. The large database of SuperDARN convection patterns has allowed both IMF conditions (By negative and positive) to be tested successfully with this technique. Direct measurements of the neutral winds and temperatures over an extended spatial field were presented by Iris Yiu (UCL) from the SCANDI instrument, which has recently been relocated to the new Kjell Henriksen Observatory at Longyearbyen. This provides co-location with the Eiscat Svalbard Radar and Cutlass SuperDARN radars, allowing investigation of the local ion-neutral coupling in detail and in the context of wider global circulation patterns. Both lower and upper thermosphere winds could be measured, which would provide an extra dimension to further CITP and other model tests.

Solar wind

The final session of the meeting was made up entirely of contributed talks focused on the solar wind. The solar wind is the continually expanding atmosphere of the Sun. The out-flowing stream of charged particles drags along the frozen-in magnetic field and fills the solar system. As well as its importance as the driver of the Earth's (and indeed other planets') magnetosphere, the solar wind also serves as a natural plasma laboratory ideal for studying fundamental physical processes such as turbulence.

The primary tools for probing the solar wind are in situ spacecraft. While the information collected is from a single point, it is possible to probe the large-scale structure using energetic particle measurements. Olga Malandraki (IC) demonstrated this using solar energetic particle (SEP) events observed by the Ulysses and ACE spacecraft to trace the large-scale magnetic topology of the interplanetary magnetic field. If a magnetic field line observed in interplanetary space has both foot points on the solar surface, then SEPs should be seen streaming in both directions along that field line. Similarly, if only one end is rooted on the solar surface then SEPs will only be seen travelling in one direction. Finally, if neither end is connected to the Sun, i.e. a plasmoid topology, then no SEPs will be observed. Malandraki et al. have used this diagnostic to investigate the topology/connectivity of the magnetic fields within interplanetary coronal mass ejections (ICMEs). The current paradigm suggests that within an ICME both foot points of the field are connected to the solar surface in a flux-rope-like structure. However, Malandraki et al. have used both Ulysses and ACE observations of energetic solar particles to show connectivity at only one end of the field line within an ICME. Indeed, within one event there is a brief period where no SEPs are observed, suggesting a plasmoid-like structure.

Other techniques available to investigate the large-scale structure and evolution of ICMEs include interplanetary scintillation and spacecraft imagers. Gareth Dorrian (Aberystwyth) presented a case study where the two techniques were compared. Interplanetary scintillation (IPS) makes use of observations of a distant radio source made at two locations on Earth separated by a large baseline. The difference in the variation of the signal at the two ground stations is used to infer structure in the solar wind flowing across the line of sight. The IPS observations indicated the probable passage of an ICME on 16 May 2007. Coincidentally, the IPS ray path passed directly through the field of view of the heliospheric imager (HI) on-board the STEREO spacecraft. This highly sensitive camera, able to remotely view ICMEs in interplanetary space, did see an ICME like disturbance at the same time. The combination of these two remote-sensing techniques could well give valuable insights into the large-scale interplanetary structure of ICMEs.

Returning to Saturn, Caitriona Jackman (IC) used Cassini magnetometer data to investigate the ambient solar wind upstream of the planet, both before and after Saturn orbit insertion. To a first order the ambient interplanetary magnetic field can be approximated by the Parker Spiral. This is the spiral-like structure produced by a combination of radial propagation, solar rotation and the interplanetary plasma being frozen to the interplanetary magnetic field. The spiral angle - the angle the magnetic field makes with the radial direction in the ecliptic - can be easily estimated for a given solar-wind velocity and radial distance from the Sun. There is also a latitudinal change but it is unnecessary to consider this for Saturn. Jackman compared the angles from times when Cassini was immersed in the solar wind, to the predicted spiral angles. The data agreed remarkably well with this simple model. The measured spiral angles differed from Parker's predictions by ~7.5° and 1.5° in the inward and outward sectors respectively.

Turbulence is a fundamental physical process and interplanetary space plasmas are an ideal laboratory to study this phenomena. Ruth Nicol (Warwick) has been using Ulysses high-speed solar-wind measurements to quantify the magnetic fluctuations embedded in the solar wind. The high-speed wind is relatively simple in structure, containing no solar transients or interaction regions. It is thus the “simplest” case to work with. Examining how the inertial range and 1/f like region varies with time and thus latitude and Sun spacecraft radius, they have found that the inertial range is quite robust to these changes whereas the 1/f like region does seem to vary. This suggests a coronal origin for the 1/f like region. However, the evolution of this region into the inertial range is remarkably stable, suggesting that no matter what the different source region of the turbulence, it eventually evolves into the same structure depicted in the classic inertial range. Further investigation into turbulence and coherent magnetic structures was presented by Robert Wicks (Warwick). He used simultaneous observations from the ACE and WIND spacecraft to calculate the spatial dependence of the correlation of different solar wind quantities. By using both cross-correlation and mutual information techniques he was able to calculate both linear and nonlinear measures of correlation. He found that spatial structures (density and magnetic field magnitude) vary quite markedly between solar minimum and maximum, suggesting a coronal origin of some of the spatial correlation, whereas magnetic field components showed little variation.

More Information

Full details of the MIST meeting programme can be found at http://www.mist.ac.uk

Author notes

Gary Abel, Emma Bunce, Eoghan Griffin and Adam Rees report from the first new-style discussion meeting of the reorganized MIST community, which took place in London in November.