MAGNETOSPHERIC GEOMAGNETIC COORDINATES FOR SPACE PHYSICS
DATA PRESENTATION AND VISUALIZATION
V. O. Papitashvili1, N. E. Papitashvili2, and J. H. King2
1 Space Physics Research Laboratory, University of Michigan, Ann Arbor, MI 48109, USA,
2 National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
Corrected geomagnetic coordinates, which account for the multipolar geomagnetic field, are frequently used to organize the ionospheric-altitude data. However, realistic organization of data measured simultaneously in the magnetosphere and ionosphere in a some sort of magnetic coordinate system requires a combination of the high-altitude external magnetic fields and the multipolar low-altitude field. Such combinations have been non-existent in the past. A new magnetospheric geomagnetic coordinate system is introduced providing such a combination.
MAGNETIC COORDINATE SYSTEMS
Spacecraft data collected in the Earth's magnetosphere are usually presented in geocentric solar ecliptic (GSE) or geocentric solar magnetospheric (GSM) coordinate systems. Different magnetospheric domains can be mapped down to the Earth's surface or ionospheric altitudes along geomagnetic field lines; then observed phenomena (auroral images, particles, electric and magnetic fields, etc.) can be plotted in geographic coordinates. Beyond this ability to trace magnetospheric field lines, what is really needed is a magnetic coordinate system that is natural at high and low altitudes and that is based on some organized physical principal. A few geomagnetic coordinate systems based on the intrinsic magnetic field sources exist to organize ground-based and satellite data. These are spatial systems such as geomagnetic dipole or eccentric dipole coordinates, geomagnetic field apex coordinates (Richmond, 1995), invariant B-L coordinates (McIlwain, 1961), and corrected geomagnetic (CGM) coordinates (Gustafsson et al., 1992; for more examples see NSSDC Web site http://nssdc.gsfc.nasa.gov/space/cgm). A specific (non-spatial) coordinates "Corrected/Invariant Latitude - Magnetic Local Time" are commonly used to better organize geophysical phenomena according to their MLT location over the mid-latitude and polar regions.
MAGNETOSPHERIC GEOMAGNETIC COORDINATES
The closed geomagnetic field lines in the magnetosphere go through B-field minima which lie in a 3-D near-equatorial surface shaped as "a ballerina skirt". It is known that this surface is of a great importance for plasma particle drifts. Because of external magnetospheric currents, a certain fraction of geomagnetic field lines (i.e., the polar magnetic field flux) will be opened to the interplanetary magnetic field (IMF) at very high latitudes. Some of these field lines (usually those which emanate near magnetospheric cusps) go through B-min above or below GSM X-Y plane; thereby warping the 3-D equatorial surface (Gustafsson et al., 1987; Papitashvili et al., 1992). Then any given contour of iso-B-min values at this 3-D surface can be mapped down to the Earth or the ionosphere using the main field and magnetospheric field models (e.g., Tsyganenko, 1989; Toffoletto and Hill, 1993; Schulz and McNab, 1996). These mapped iso-B-min contours will form new magnetospheric geomagnetic (MGM) latitudes and will depend on Universal Time (UT) and geomagnetic activity. The suggested concept resembles in some instance the approach discussed by Antonova and Ganushkina (1994), but we do not take into account the magnetospheric plasma pressure gradients at this stage.
In this paper we utilized the Definitive/International Geomagnetic Reference Field (DGRF/IGRF) models as the multipolar low-altitude field and the T89c magnetospheric field model (Tsyganenko, 1989; Peredo et al., 1993). For specified UT and 3-hr planetary index of geomagnetic activity Kp, we determine an equatorial warped surface of the set of all along-field-line B-minimum points. Then we construct families of constant B-min contours in this surface and map these contours back to a "source sphere" at or near the Earth's surface. Because of north-south asymmetry of the Earth's magnetic field, the south geographic pole lies on 18.8o CGM longitude and its magnetic noon occurs at 15:30 UT - two hours earlier than magnetic midnight occurs at the north geographic pole (17:30 UT). To make the CGM and MGM coordinates comparable, we chose to define the MGM latitudes to be identical to the corresponding CGM latitudes (for example, 30o, 40o, 50o, 60o, 70o, and 80o) counted on the Earth's surface along the northern portion of zero CGM meridian for Kp = 0 at 16:30 UT of Day 81 (quiet global geomagnetic activity, the north CGM pole is near local noon, spring equinox). The corresponding B-min values are used to construct MGM latitudes in both hemispheres for all magnetic activity conditions.
Fig. 1. Comparison of the iso-B-min contours
(MGM latitudes, red lines) and CGM latitudes (blue lines) mapped at the
Earth's surface. Green lines - geographic coordinates and continent boundaries.
Quiet geomagnetic conditions: Kp = 0; spring equinox - March 22 (Day 81).
Left panel: Northern Hemisphere, 16:30 UT; right panel: Southern Hemisphere,
By definition of the Earth's dipole magnetic coordinates, the 180o geomagnetic longitude is fixed at the Earth's surface connecting the north geomagnetic and north geographic poles. The CGM coordinates follow same approach (e.g., Gustafsson et al., 1992). For some interim period at least, we recommend using the CGM longitudes of any given point to complete the proposed magnetospheric coordinate system and, therefore, fix it at the Earth's surface. As we gain experience with the benefits and limitations of the new "magnetospheric" latitudes as defined herein, we will introduce a new system of magnetospheric longitudes.
COMPARISONS OF THE CGM AND MGM COORDINATES
Figure 1 shows a comparison of the CGM and MGM latitudes over both the Northern and Southern Hemispheres for quiet geomagnetic conditions (Kp = 0) on March 22 where the Northern Hemisphere is plotted for 16:30 UT, and the Southern Hemisphere - for 04:30 UT. The corresponding CGM and MGM latitudes are identical along zero (360o) CGM longitude in the Northern Hemisphere. (See this meridian on Figure 2). These latitudes are of the best coincidence in the sub-auroral/auroral regions (50o - 60o), but the mid-latitude iso-B-min contours are displaced poleward (up to 5o) against corresponding CGM latitudes in the Eastern Hemisphere. The iso-B-min contours vary significantly in the polar caps: they group together near magnetic midnight but often they cross lower CGM latitudes displacing by 5o-10o for different UT.
We select the last closed (at the Earth's surface) iso-B-min contour to be equal 0.01 nT, i.e., the near-pole geomagnetic field lines go beyond of the T89c "modeling box". (The box dimensions are the following: XGSM from -70 RE to 15 RE, YGSM =±30 RE, and ZGSM =±30 RE.) Therefore, the red shaded area in all figures can be considered as the model's "quasi-open" field line region. These "quasi-open" polar cap regions are significantly reconfigured with an increase of Kp-index (Figure 2; note that the MGM latitudes are now plotted for every 5o). Peculiar "contour islands" can be formed at the dayside and sometime near midnight because of significant warping of the 3-D equatorial surface near the cusps - this requires further investigation. There are significant UT and seasonal dependencies in the differences between the high latitude MGM and corresponding CGM latitudes (Figure 3).
|Northern Hemisphere||Southern Hemisphere|
NEW APPROACHES IN THE MAGNETOSPHERIC AND INTERHEMISPHERIC STUDIES
The proposed new magnetospheric coordinates can provide new insight and tests for the globally visualized morphology and dynamics of various magnetospheric domains and their relevance to the known and/or observed phenomena in the geomagnetic field and magnetosphere/ionosphere plasmas. It is expected that a variety of magnetospheric and ionospheric data (easily available from the NSSDC's Web site http://nssdc.gsfc.nasa.gov/space and Space Physics CD-ROMs) can better be organized and interpreted in this new coordinate system. The new MGM coordinates can also be utilized for better comparisons between the ground-based (magnetometers, radars, riometers, Fabry-Perot interferometers, all-sky cameras, etc.) and satellite observations (e.g., auroral images from POLAR spacecraft, magnetic fields and particle measurements from GEOTAIL, INTERBALL, POLAR, FAST, RSTED). A variety of geophysical observations made over both polar regions can better be compared in this new coordinate system because, by definition, the MGM coordinates account for both the internal and external magnetic field sources and map down to the Earth or any given ionospheric altitude the realistic geomagnetic field shells containing a single B-min and two "magnetic mirror" points along the same field line. Thus, the global modeling of the magnetospheric and ionospheric electrodynamics and plasma convection in these new MGM coordinates may help to understand better the interhemispheric conjugacy in a development of global magnetic storms and substorms.
Fig. 3. The CGM longitudes (blue lines) and MGM latitudes (red lines) over the Northern Hemisphere for Kp = 3, 16:30 UT, and different seasons: left panel - September 22 (Day 265); right panel - December 22 (Day 356).
We thank Georg Gustafsson for initiation of this work as well as Nikolai Tsyganenko and Arthur Richmond for valuable discussions. V. O. P. acknowledges support from the National Science Foundation grant ATM-9523329.
Antonova, E. E., and N. Yu. Ganushkina, Selection of the coordinate system for description of magnetospheric domains at magnetostatic equilibrium, Geomagn. Aeron., 34, No. 4, 58 (1994).
Gustafsson, G., N. E. Papitashvili, and V. O. Papitashvili, An improved magnetic coordinate system for auroral oval studies, in Structure and Dynamics of the Polar Current Systems, Proc. Intern. Symp. "Polar Geomagnetic Phenomena" (Souzdal, USSR, 1986), Polar Geophysical Institute, Apatity, 102 (1987).
Gustafsson, G., N. E. Papitashvili, and V. O. Papitashvili, A revised corrected geomagnetic coordinate system for Epochs 1985 and 1990, J. Atmosp. Terr. Phys., 54, 1609 (1992).
McIlwain, C. E., Coordinates for mapping the distribution of magnetically trapped particles, J. Geophys. Res., 11, 3681 (1961).
Papitashvili, V. O., N. E. Papitashvili, G. Gustafsson, K. B. Baker, A. Rodger, and L. I. Gromova, A comparison between two corrected geomagnetic coordinate systems at high-latitudes, J. Geomagn. Geoelectr., 44, 1215 (1992).
Peredo, M., D. P. Stern, and N. A. Tsyganenko, Are existing magnetospheric models excessively stretched? J. Geophys.Res., 98, 15,343 (1993).
Richmond, A. D., Ionospheric electrodynamics using magnetic apex coordinates, J. Geomagn. Geoelectr., 47, 191 (1995).
Schulz, M., and M. McNab, Source-surface modeling of planetary magnetospheres, J. Geophys. Res., 101, 5095 (1996).
Toffoletto, F. R. and T. W. Hill, A nonsingular model of the open magnetosphere, J. Geophys. Res., 98, 1339 (1993).
Tsyganenko, N. A., A magnetospheric magnetic field model with the warped tail current sheet, Planet. Space Sci., 37, 5 (1989).