1. Identification and Visualization of Magnetic Conjunctions for Multi- Altitude Cusp Studies W. Keith 1, M. Goldstein 1, T. Stubbs 1, D. Winningham 2, A. Fazakerley 3, H. Reme 4, and A. Balogh 5 1 NASA Goddard Space Flight Center, Code 692, Greenbelt, MD 20771, USA2 Southwest Research Institute, P. O. Drawer 28510, San Antonio, TX 78228, USA3 Mullard Space Science Lab, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK 4 CESR BP 4346, 9 Ave Colonel Roche, Cedex, Toulouse, 31029, France5 Imperial College Space and Atmospheric Physics group, The Blackett Laboratory, London, UK SM53A-0401 Abstract Multi-satellite studies of the magnetosphere often require that the missions be on similar magnetic field lines and/or in the same magnetospheric region. An automated process has been developed to locate such conjunctions and plot the results for inspection. New visualization tools such as SDDAS/Orbit, ViSBARD, and OVT can then be used to further study the data. These techniques are being utilized to study the cusp at low- and mid-altitudes with the DMSP and Cluster missions. The comparison of particle spectrograms from different altitudes in the cusp is important to our understanding of magnetospheric entry processes. The magnetospheric cusps act as conduits through which shocked solar wind plasma can penetrate to low altitudes. Although this plasma entry persists under all magnetospheric conditions, it is a very dynamic and complex process, strongly affected by external solar wind conditions. Low altitude measurements detect a smaller cusp that is crossed relatively quickly, while at mid altitudes the extent of the cusp is larger and the traversals much slower, blurring the distinction between temporal and spatial features and complicating conjunction studies with low altitude data. Orbit and particle data from both missions have been searched over a thirteen- month period for near-simultaneous cusp region crossings. This technique has found a total of 25 good quality conjunctions, one of which will be shown, making use of the visualization tools mentioned above. These conjunctions show similar complex structures that may lead to a greater understanding of particle entry in the cusp. Figure 1 Identification Due to the precession of the Cluster orbit throughout the year, cusp crossings take place primarily at high-altitudes (dayside apogee) in Feb-March-April and at mid-altitudes (dayside perigee) in Aug-Sept-Oct. To date, January through October 2001 and the fall season of 2002 have been searched in this study. Because of the much longer orbital period of Cluster, it was the initial driver in identifying cusp crossings. An automated process searched for times when the low-altitude mapped AACGM position of Cluster was within a pre-determined “cusp box”. These crossing times were checked and refined by browsing the particle data of the CIS and PEACE experiments. Output generated from these crossing times (see Figure 1) give an overview of the Cluster particle data (spectra), as well as an indication of possible magnetic close-approaches with the various DMSP satellites (lines). These times were then searched by an automated process for times when the low- altitude mapped magnetic position of the Cluster centroid (center of mass position) was within 5 degrees (~550 km) of the magnetic footprint of one of the DMSP spacecraft (F12, F13, F14, or F15). Each of these closest- approaches was then plotted +/- 2 minutes and ranked according to the quality of the cusp in the particle data (Figure 2). The best quality DMSP passes combined with the corresponding Cluster passes could then be compared in detail to determine the quality of the conjunction. This has resulted in 165 total Cluster cusp crossings over which the DMSP data has been searched. This has lead to the identification of 299 DMSP magnetic close-approaches, of which 80 have been rated as “good” by visual inspection of the particle signatures. Forty-six of the Cluster cusp passes over the periods so far covered contained at least one viable DMSP crossing, and many contained several. In some cases, the times when the spacecraft were sampling the cusp and the time of closest approach did not coincide, but there was good coincidence on 25 of the 46 days. A representative example of the 25 is presented here from 14 August, 2001 when the Cluster trajectory was a roughly perpendicular mid-altitude crossing (Figure 3). Figure 2 Visualization Once a conjunction has been identified, the problem of understanding and showing how these data fit together becomes largely a problem of data visualization. Three different software tools have been employed for various tasks; OVT for field-line and footprint mapping, ViSBARD for plotting multiple data sets at the spacecraft location, and SDDAS/Orbit for showing flux along ground tracks. In our example pass, Clusters 1,2, and 4 pass poleward through the cusp at about the same time, with Cluster-3 lagging behind about 30 minutes. In Figure 3 (from OVT, edited to add labels), the field lines passing through the Cluster spacecraft are shown at the time of closest approach with DMSP F13. Note that the DMSP field line remains very close to the three Cluster lines throughout its length, indicating a good magnetic conjunction. Figure 8 (from Orbit, labels added) shows the footprint tracings with ion energy flux magnitude coloration. F14 and F15 see the cusp at about the same place at opposite ends of the time range (until F14 data drops out just poleward of the location of the Cluster centroid). F13 samples the cusp along approximately constant Invariant Latitude at the beginning of the period, passing very close to the mapped Cluster location. Ion and electron density data for Cluster are shown in Figure 7 (from ViSBARD, labels added) and indicate a poleward drift of the cusp during this interval (between Cluster-3 and the others), which is consistent with the northward turning of the IMF during this time. Although a lot of data can be shown together in this way (i.e., Figure 5, where electron and ion temperature (glyphs), electron and ion density (arrow colors), ion velocity (arrow 1) and magnetic field (arrow 2) are shown), it can be difficult to assimilate so much data all at once. The relative altitudes also makes it difficult to view similar data from the two missions (as in Figures 6 and 7) in the same frame. The summaries presented at right of the three visualization packages reflect their particular application in this conjunction study and are not intended to be comprehensive reviews. The opinions expressed are those of the primary author. Conclusions This search method has resulted in the identification of 25 good conjunctions within the magnetospheric cusps between the Cluster and DMSP missions. As the example shown here indicates, there is much that can be learned with such conjunctions about the nature of plasma entry in the cusps that would not be possible with the individual measurements.The visualization tools used in this work each bring unique abilities to the problem, however, none are well enough developed yet to do all that is required (Table 1). The best result comes from knowing when to use each one and exploit its strengths, while avoiding the pitfalls that each also bring. OVT Orbit Visualization Tool is primarily useful for plotting spacecraft orbits in various coordinate systems and for magnetic field line tracing using one of several field models with user-supplied solar wind parameters. The OVT development team is lead by Kristof Stasiewicz at IRFU in Uppsala Sweden (http://ovt.irfu.se). Despite some quirkiness in using the software, it is pretty much the only game in town if you want a 3D visualization of Tsyganenko 2001 field lines displayed along with satellite positions (Figure 3). Figure 3 Of particular interest in this study is the ability to plot magnetic footprints at the Earth’s surface (Figure 4) and the ability to show an intersecting field line over it’s entire length. Figure 4 The line-by-line data entry for the field model parameters is a bit clunky, however the data is actually stored in simple ASCII files. A data dump from a convenient source (in this case time-lagged ACE data from IDFS) into the appropriate format yields an acceptable solution over certain time ranges, although an automated process would be far superior. Parameters for the Magnetopause, Bowshock, and polar cap potential work in the same way and can therefore be as dynamic as the input values they are given. Orbits, calculated from compact orbit element files, may be plotted in GSE, GSM, SMC (SM), GEO or GEI, while magnetic footprints use GEO or SMC. A disadvantage for some is the lack of a version compiled for Macintosh OS X. While it is presumably possible to compile the Unix source for this platform, it has not to my knowledge been done. The Windows version, on the other hand, self-installs easily and is currently the version of choice for usability. ViSBARD The aptly named Visual System for Browsing, Analysis, and Retrieval of Data is under active development at NASA Goddard Space Flight Center, lead by Aaron Roberts. ViSBARD provides 2D and 3D displays of scalar and vector data at the spacecraft location (http://nssdcftp.gsfc.nasa.gov/selected_software/visbard/). While the “retrieval” portion of the system is still in the very early stages and the loading of data can at times be tedious, the software will read CDF and ASCII data (via Resource Description Files) and robustly display them in a variety of useful ways in GSE coordinates (Figure 5-6). Figure 5 Figure 6 ViSBARD can be “tricked” into displaying other coordinates, although the Earth position becomes inaccurate. In Figure 7, some of the data from Figure 5 (density) is shown in SM coordinates. Some of the cusp displacement at Cluster-3 can now be seen to be due to diurnal motion of the magnetic pole. Figure 7 Pre-compiled binaries for Windows, Mac and Linux make it simple to install and update. Future work that will make this software much better include Tsyganenko field lines and the ability to read and display in different coordinate systems. Orbit Orbit is one of many applications within the Southwest Data Display and Analysis System. SDDAS development is lead by David Winningham at Southwest Research Institute (http://www.sddas.org/). Like ViSBARD, Orbit can plot a variety of vector and scalar data along an orbital path. Because it does little bookkeeping with regard to units and coordinates, it is very versatile, however the user must be careful that the resulting display makes sense. Orbit has been used in this study primarily to visualize the magnetic ground tracks of the various missions to give a common context to the various altitudes (Figure 8). Figure 8 This is an example of when the user must be careful. When first plotting ground-tracks, the non-rotating Earth map gave the wrong impression regarding geographic location, so the user-designed grid-map shown was used to accurately give context to the magnetic position data. Figure 9 shows the Cluster PEACE electron density in GSE (top) and SM (bottom), analogous to Figures 5 and 7. Orbit can only show one scalar or vector per satellite (for up to four satellites), which may be a relief to those still trying to sort out Figure 5. Figure 9 The large repository of data in the IDFS format allows for a wide variety of inputs, as well as the ability to plot data after it undergoes mathematical computations via files called SCFs (as in Figure 8 where the ion flux has been integrated on-the-fly). It cannot read data of other formats, however, and there is currently no built-in ability to perform coordinate transforms on the data. Orbit is available as part of a suite of applications within SDDAS, with binaries available for Solaris, HP-UX, Irix, Linux, Mac OS X, and Windows. The Windows version requires UNIX emulation (such as Cygwin) and an X- server, but is still useable. ItemOVTViSBARDSDDAS/Orbit Supported OS WindowsVia Cygwin Linux MacintoshSource UNIXSource Models Tsyganenko Field87,89,96,01planned MagnetopauseIMF, pressureNot time-dependent BowshockPressure, MNot time-dependent Electric potentialIMF, velocity EarthNon-rotating Sun MarsNon-rotating OtherUser-defined Display Time rangeOrbit trackDynamic Time stepSelection or midpointAll steps shown Scalar Data Vector DataMagnitude Multiple Vectors Data Sources ASCIISingle formatVba or Via RDF CDFVia RDF IDFSVia CDF export Long Term Orbit File 2-line Orbit Elements Derived quantitiesVia SCF Online retrievalCDAWeb Beta Features 2D data displayOther SDDAS Save sessionNot combined Coordinates5 geocentricGSE (others planned)Arbitrary XYZ Image Outputpnm, bmp, tifpng, gif, jpg, tifPS, animated gif Support Table 1