T. Moretto and E. Friis-Christensen
Danish Meteorological Institute,
Lyngbyvej 100, DK-2100 Copenhagen Ø, Denmark,
Two strategies for the use of ground magnetometer data in studies of small-scale features in the dayside high-latitude ionosphere are illuminated through a number of examples which, at the same time, demonstrate some of the recent progress within this area. An extensive network of magnetometer arrays are now established providing dense coverage of more than half of the high-latitude northern hemisphere. This fascilitates the adoption of a global perspective as demonstrated in two examples of high-latitude ionospheric travelling convection vortices. Results include detailed descriptions of the evolution of the events during their entire lifetime as they propagate through several local time sectors and/or magnetospheric regions and of their relations to the state and dynamics of the global ionospheric current and convection systems. Another important strategy is to combine different ionospheric data sets. Some results of this are demonstrated through three further examples. One concerns the identification of the ionospheric current system associated with events of poleward moving small-scale auroral structures. The second finds that the convection vortices in an event of a long continual series of vortices are closely related to the position and nature of the convection reversal boundary as identified in incoherent scatter radar data. The final example is a study which determine, in terms of particle precipitation data of low-altitude satellites, the magnetospheric source region of the field-aligned currents that drive impulsive travelling convection vortices and finds this to be inside the plasma sheet.
In addition to the system of global-scale quasi-steady currents involved in the interaction between the Solar Wind and the Earth's magnetosphere, more localised and short term processes occur in the outer magnetosphere. On scale sizes of a few Earth-radii at the dayside magnetopause processes include: flux transfer events (FTE's), impulsive plasma penetration, the Kelvin-Helmholtz instability, and the magnetospheric responses to impulsive changes in the Solar Wind. Presumably, these processes are associated with small-scale current systems, which couple to the ionosphere via electric currents along the magnetic field-lines. The study of the ionospheric response to this variety of different signals at the dayside magnetopause and its boundary layers provide crucial information about the mechanisms of the coupling between the Solar Wind and the magnetosphere. In this way, the study of transient and dynamic phenomena in the high-latitude ionospheric convection patterns and the small-scale field-aligned currents which drive them is highly motivated. Due to their dynamic and localised nature, the puzzle regarding which magnetosphere processes are associated with which ionospheric signatures require the inclusion of ground-based and space-based measurements.
Some of the dayside high-latitude ionospheric signatures that are believed to be associated with the above list of possible magnetospheric processes include ionospheric travelling convection vortices (TCV's), poleward moving auroral forms, and, more generally, extended series of semi-regular ULF pulsations. This list is by no means exhaustive; rather it represents the limited number of features that we are be able to include in this presentation. We shall describe by means of a few examples of studies some of the most recent progress made in the description and understanding of the sample of ionospheric features listed above. The selected examples are all multi instrument studies with ground magnetometer observations being the main experimental data source. For other examples based, for instance, on radar or riometer data we refer to some of the other presentations of this session/meeting (e.g. by Cowley, Lockwood ,Rodger, and Stauning).
The examples are presented below under two headlines, The Global Perspective, and The Combining of Data Sets. This is to emphasize the two underlying principles of the studies, both of which we believe to be important tools in addressing the major unresolved questions in connexion with the above ionospheric phenomena. These include, most prominently, the unambiguous identification of the sources (in the Solar Wind magnetosphere interaction) of each of the ionospheric signatures, a task which is closely related to the question of whether some of them belong to the same processes, maybe under different conditions, and the issue of the relationship between the localised processes and the states and dynamics of the global ionospheric current and convection systems. Viewing localised phenomena in a global (large-scale) frame-work directly gives information on the latter. In addition, it enables the study of the details of the dynamics of those of the features that, even if they are local, move during their lifetime through a large part of the high-latitude ionosphere. Finally it is important for addressing the implications of simultaneous localised signatures in different local time regions. Combining of the data from different ionospheric means of observation, on the other hand, aids the understanding of the physics both of the localised processes themselves and of the processes of the mapping between the magnetosphere and ionosphere.
THE GLOBAL PERSPECTIVE
This aspect we shall illustrate by means of two case studies of events of ionospheric travelling convection vortices which were undertaken as part of the GEM (US Geospace Environment Modelling program) collaboration. The aim was to study this particular type of magnetic impulsive events using an extensive data set resulting from the simultaneous operation of the extended IMAGE and the MACCS networks, which in combination with the magnetometer networks of CANOPUS and GSC in Canada and the network of DMI in Greenland provide a dense coverage of almost half the high-latitude northern ionosphere.
The verification and analysis of impulsive events in geomagnetic recordings as TCV events are usually done from the data of single meridional chains of stations by means of merged equivalent convection vector plots, introduced by Friis-Christensen et al. (1988), an example of which is displayed in Figure 1a with data from the Greenland West Coast stations for the event of Oct 14, 1993. The global set-up enables the use of global snap-shots as proposed by Glassmeier and Heppner (1992) for analysing the shape, position, and dynamics of the convection vortices during the course of the event. An example is displayed in Figure 1b for the same event at the time 13:02:20 UT. In this image, each circle marks the position, in magnetic latitude and magnetic local time (MLT) of one of the observation sites and the vectors represent (as in Figure 1a) the variations of the equivalent convection field. The colour shading depicts the corresponding variations of the field-aligned current (FAC) density distribution, which is estimated from the divergence of the deduced ionospheric Pedersen current vector field. Red represents currents into the ionosphere, which presumably drive counter clock-wise convection vortices, and green represents currents out of the ionosphere driving clock wise convection vortices. Observed in the image at this time of the event is the presence of four convection vortices and the associated FAC injections at their centres (a not quite split set of red currents spreading across noon at roughly 75 ° and two green currents at roughly 70 ° and 7 and 16 MLT, respectively).
Fig. 1a. Time-series of equivalent convection vectors for the West Greenland chain of stations (IAGA codes right, and Inv. Lat. left) for the interval 12:50-13:15 UT on Oct. 14, 1993. Three vortices are identified as sketched.
Fig. 1b. Instantaneous global display of the equivalent convection vectors and the inferred FAC density variations (color shading) at 13:02:20 UT for the event on October 14, 1993.
Fig. 1c. Same as above at different instants one minute apart for the event on December 18, 1993.
Pressure Pulse TCV Event
To study the evolution of the event we examined the entire sequence of snapshots (one for each 20 sec data point), and this has indeed proven even more illuminating when animated on a computer screen. Summarising the event, three vortices, in turn, were seen to form around local magnetic noon and move westward. Concurrent eastward moving vortices matching the first two of these were also observed. The pre-noon vortices, however, were of much larger amplitude and had a more clearly described motion than the post-noon ones. The GOES6 satellite (at approximately 8 MLT) recorded a compressional signature of the magnetic field at a time when the downward (into the ionosphere) FAC filament passed over this position. Consequently, it was concluded that this event very likely belong to the class of TCV events as the ionospheric response to the encounter at the magnetopause of a Solar Wind pressure pulse for which much observational and theoretical documentation exist, e.g. H. Lühr et al., 1996, Lühr and Blawert, 1994, Yahnin et al., 1995, Sibeck, 1993, Glassmeier, 1992, Friis-Christensen et al. (1988), and references therein. Some new verification of existing results and also some new observational features resulted from the study.
For the first time, images of the formation of the vortices being born in the noon region are presented. One FAC filament at a time appears. It then intensifies and spreads in longitude, and subsequently moves away while the next filament develops. Also for the first time, a significant apparent equatorward movement of the FAC filaments, of about 4-5 degree of magnetic latitude over roughly 5 hours of MLT, is reported. The equatorward parts of the filament moves faster and intensifies (as a separate filament) giving an elongated and tilted appearance to the vortices. This feature has been reported by many previous studies (see the list of references above). In general much substructure and individual filamentary dynamics of the vortex structures are indicated by the data. The sequence of snapshot displays, we believe, gives the best verification of this, as yet, and in addition provides the first step towards a more specific description of the dynamics. Significant dynamics of the moving current system has been suggested by several of the previous studies. Finally, in agreement with most previous results, the event was observed to maximise in intensity of the main vortex in the 9 MLT region. However, at the same time a strong asymmetry in the development of the event in the pre-noon and post-noon sectors was observed. Earlier statistical results show a similar asymmetry, see e.g. the discussion in Lühr and Blawert (1994), but this is the first evidence to suggest that the amplitude asymmetry applies even to a single event. This could issue a significant challenge to the modelling of these events. So far, only the models of Sibeck et al. (1996) and H. Lühr et al. (1996) have made an attempt to account for the asymmetry of the statistical distribution, but based as they are on the statistical distribution of other observations, the location of the low-latitude boundary layer (LLBL) region and the ionospheric conductivity distribution respectively, neither of these seem able to adequately explain this new result.
TCV Event Merged With the Dynamics of the Large Scale Ionospheric Current System
As the second example we shall comment briefly on the results of applying the same techniques to a very different TCV example. This event occured on December 18, 1993 and many of the same features as for the above event are observed. For example, the elongation and tilting of the vortices, the associated apparent southward movement, and the filamentary substructure and complex dynamics. However inherently this event seems to be of a different class. First, vortices, even of very large amplitudes, are observed only in the post-noon region while almost no activity is observed pre-noon. Secondly, the event seems to be closely related to a sudden intensification of the global ionospheric convection which is observed approximately seven minutes after the onset of the TCV event. These features are verified in the sequence of snapshot displays. The vortices, three in turn, first appear around 11 MLT and move eastward. At the time of the generation and starting movement of the third vortex strong intensifications of the equivalent convection are observed at almost all observation sites as is illustrated by the two images in Figure 1c. The intensifications in the dawn and dusk regions manifest themselves as the establishment of a very large morning (merging) convection cell covering almost all of the polar cap. The FAC intensifications of the vortices at noon and post-noon seem to merge with the global intensification event and during its subsequent slow evolution and fading no consistent vortex signatures can be seen any longer. This is, we believe, the first direct observations to suggest that, at least some, TCV events must be considered in the context of the dynamics of the large-scale ionospheric current system, that is the Region 1, Cusp, and Cleft field aligned currents. To our knowledge the observations of this event will not fit in any of the existing models. The lack of Solar Wind data for this event is very unfortunate.
THE COMBINING OF DATA SETS
This aspect we shall discuss on the basis of three examples. The first two studies concern poleward moving auroral forms and continual ULF pulsations, respectively. Then the results of one more TCV study in which the question of magnetospheric source region mapping is addressed on the basis of particle precipitation data shall be described briefly.
The Ionospheric Current System of Poleward Moving Auroral Forms
Part of the ongoing discussion in connection with the magnetic impulsive events, including the TCV events, is the question whether some of them could be the ionospheric signature of the flux transfer events, thought to result from large, short-lived enhancements in the reconnexion rate. The class of optical events named midday auroral breakup events by Sandholt et al. (1989) and Poleward Moving Auroral Forms (PMAF's) by Fasel et al. (1994) constitutes a widely-recognised candidate ionospheric signature of these phenomena.
We shall take as our next example the results of a very detailed analysis of the relations between such auroral events and the associated ground magnetic signatures presented by Øieroset et al. (1996). The most prominent event as identified in the meridian scanning photometer at Ny Ålesund is illustrated in Figure 2a. An event of equatorward movement of the whole diffuse aurora, marked DD because it is believed to result from a directional discontinuity in the inter-planetary magnetic field (IMF), starts a sequence of poleward moving small-scale intensifications (numbered). A corresponding series of poleward moving deflections are observed in the most poleward sites of the IMAGE chain of ground magnetometers. Both data sets yield an estimate of roughly 2 Km/s for the speed of the poleward movement. The resulting current system to fit these observations, and those of other similar events as well, is displayed in Figure 2b. A narrow elongated current sheet of upward current on newly open field lines associated with the main optical signal and sheets of downward return currents poleward and equatorward are proposed. This is in good agreement with existing models for the case of local post-noon observations and IMF BY negative conditions.
Incidentally, the global magnetometer set-up of the above TCV case-studies also proved valuable for this analysis in that it helped identify the ionospheric convection pattern and its changes, thereby, in the absence of direct measurements, also implying the IMF conditions causing them. This is illustrated schematically by the sequence of cartoons in Figure 2c. At around 10:50 UT the state shifts from a pattern strongly dominated by a negative IMF BY to a pattern which is much less dominated by BY (i.e. BY has gone less negative or BZ has gone more negative). This is interpreted in terms of the projection of the reconnection X-line to indicate that the Ny Ålesund meridian by the shift is moved into the X-line region where then the FTEs, which are presumably caused by pulsed reconnection at this line, are observed.
Fig. 2. All parts of this figure are from Øieroset et al. (1996).
a. Meridian scanning photometer data from Jan. 14, 1994. The MSP measures line-of-sight intensities of auroral emissions as a function of zenith angle. Observations of the auroral red (630.0 nm) and green (557.7 nm) lines of atomic axygen are displayed. The MSP scans along the magnetic north-south meridian. The equatorward boundary of the aurora is marked with a dotted line.
b. Sketch of the ionospheric current system associated with an FTE. Length-scales as estimated from the data are for the width of the central current sheet 300-400 Km and for the longitudinal extent greater (but probably not much) than 1000 Km.
c. A schematic illustration of the evolution of the convection pattern leading to the expansion of the X-line signature into the postnoon sector. The 70 deg. invariant latitude circle is displayed and the assumed X-line signature is indicated by the bar just outside this circle. The position of the station in Ny Ålesund is indicated with a black dot and the background aurora is shown as the hatched area. 06, 12, and 18 MLT are marked, and in addition the 14 MLT meridian through Ny Ålesund is shown.
Continual Series of Ionospheric Convection Vortices
In addition to the impulsive TCV events dealt with above which typically has a duration of 10 minutes, events of similar pulsations are observed, which in contrast go on for hours at a time and result in long series of travelling convection vortices. One such case is recently reported by Clauer and Ridley (1995). Their study, which combines ground magnetic and incoherent scatter radar flow data from Greenland, constitutes our second example of this group. On August 4, 1991 during an interval from about 11:00 - 14:00 UT high latitude coherent ULF waves were observed in the Greenland magnetometers. The pulsations were seen at the east coast, with a phase delay at the MAGIC stations in central Greenland, and further phase delayed at the west coast as well. The recorded delays correspond to a longitudinal phase velocity of approximately 1.3 Km/s and the characteristics of the horizontal magnetic variations match a series of convection vortices travelling westward over a meridional line of stations. This is illustrated by the equivalent convection vector plot for the West Greenland chain of stations, for which local magnetic noon is about 14 UT, in Figure 3a. Such pulsations are reported on previously and they have been explained as the ionospheric manifestation of boundary layer waves coupled to the ionosphere by travelling Alfven-waves. In particular, it has been suggested that these waves originate from instabilities in the LLBL (as for example the Kelvin-Helmholtz instability) which can be driven by strong flow gradients. Significant evidence for this scenario is presented by the other data sets for this case. First, the ionospheric convection reversal boundary (CRB) was determined throughout the event with the line-of-sight ionospheric flow data from the incoherent scatter radar at Søndre Strømfjord. The identification for each scan (approximately 6 minutes apart) of the shape and position of the CRB is displayed in Figure 3b. The boundary separating westward flow to the north and eastward flow to the south, as would be expected for the pre-noon convection cell, is seen to be generally oriented along lines of invariant magnetic latitude but, in addition, shows undulations of the CRB from one scan to the next. The centres of the convection vortices (Figure 3a) all appeared very close to the CRB. Secondly, the position of the CRB was confirmed by the drift meter measurements of two DMSP satellite over passes. In addition, the DMSP particle precipitation data provide an estimate for the mapping of field lines between the satellite and the outer magnetosphere, which for both passes indicated positions inside the LLBL. The results of these observations are combined with the radar estimates of the CRB in the sketch of Figure 3c.
Fig. 3. All parts of this figure are from Clauer and Ridley (1995).
a. Time-series of equivalent convection vectors for the West Greendland chain or stations for the interval 11:30-13:30 UT on Aug. 4, 1991. The axis for northward displacement is centered at the station of STF. The approximate locations of field-aligned currents are indicated.
b. Line-of-sight velocity measurements from a series of azimuth scans made by the radar at Søndre Strømfjord. The velocity vectors are color coded with away velocity being red and toward velocity being blue. Geodetic north is at the top of each plot and lines of constant magnetic invariant latitude are drawn across each frame. Also shown are the locations of ground magnetometer stations within the radar field of view. The convection reversal boundary is indicated by a black line in each scan.
c. Schematic diagram showing the locations of the convection reversal boundary identified by DMSP satellites (filled) and by the incoherent scatter radar (open) and the ionospheric projection of the LLBL inferred from DMSP particle data.
One feature to question the interpretation in terms of a K-H instability driven by the Solar Wind - boundary layer interaction, however, is the fact that the waves are observed across the noon region (East Greenland is at MLT of 11 - 14 during the event). The usual expectation for the K-H generation mechanism would be to see the instability grow along the flanks away from local noon where the magnetosheath velocity is relatively high. This leads the authors to suggest that for this case, where quite unusual solar wind conditions of a very large and dominating IMF BY component along with strong Solar Wind velocity prevail, the instability could originate in the regions close to noon of high latitude magnetospheric flows and shear boundaries which are likely to result from the Solar Wind - magnetosphere electrodynamic interaction under these conditions.
Source regions of TCV events
The last example from Yahnin and Moretto (1996), obtained by combining ionospheric data sets with DMSP particle precipitation data, estimate the likely source region of the FAC filaments which produce TCVs for a few previously reported events. The results for one of the events are summarised in Figure 4. This event is the very first, defining TCV event described by Friis-Christensen et al. (1988) in their pioneering paper in GRL. It consists of a twin vortex system, observed to pass westward over the West Greenland chain on June 28, 1986 at around 10:13 - 10:14 UT where the West coast was at 08 MLT. The path of the leading vortex centre is marked in the figure by two large circles. Three relevant DMSP satellite passes are available for this event and their traces too are displayed in Figure 4 and on each trace the precipitation boundaries detected during that pass are marked. It is seen that the field-aligned currents seem to originate close to the CPS/BPS (Central Plasmasheet / Boundary Plasmasheet) boundary and consequently are several degrees equatorward of the BPS/LLBL boundary and the LLBL region itself, which has so far been considered a likely source region. This result is well confirmed by the other events of the study and also by the results of a similar previous study of Yahnin et al. (1996).
Fig. 4. This figure is from Yahnin and Moretto (1996). It depicts on a geographic map the locations of the magnetic stations (small filled squares) and of the TCV centres (large circles) for the event of Friis-Christensen et al. (1988). Trajectories for three relevant DMSP satellite passes (two of which are in the conjugate region in the Southern hemisphere) are sketched and on each the detected precipitation boundaries are marked according to the legend in the figure. The UT times for the passages of the CPS/BPS boundary are also given. The shaded region is confined by the most equatorward latitudes of the CPS/BPS and BPS/LLBL boundaries, respectively.
Some essential questions regarding the ground-based observations of dayside small-scale dynamic features are still largely unsolved. For example, the unambiguous determination of how the various identified phenomena are related to the potential magnetospheric source processes and, in turn, how these relate to the processes driving the large scale ionospheric convection and current systems. The puzzle of the possible inter-relations amongst the many various ionospheric features constitutes another example. Two strategies for the use of ground magnetic data have proven useful, and in our opinion provide a promising means also for the future, in addressing these problems. This has been demonstrated by a small number of examples of recent results obtained in such studies.
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