Entry of the POLAR Spacecraft into the Polar Cusp under Northward IMF Conditions

 

C. T. Russell1,2, J. A. Fedder3, S. P. Slinker4, X-W. Zhou2, G. Le2, J. G. Luhmann5, F. Fenrich5, M. O. Chandler6, T. E. Moore7 and S. A. Fuselier8

 

  1. Earth and Space Sciences, University of California, Los Angeles

2. Institute of Geophysics and Planetary Physics, University of California, Los Angeles

3. Institute for Computational Science and Informatics, George Mason University, Virginia

4. PPD, Naval Research Laboratory, Washington, D.C. 5. Space Sciences Laboratory, University of California, Berkeley

6. Marshall Space Flight Center, Huntsville, Alabama

7. Goddard Space Flight Center, Greenbelt, Maryland

8. Lockheed Martin, Palo Alto, California

 

Originally published in:
Geophys. Res. Lett., 25 3015-3018, 1998.

 

Abstract

On May 29, 1996 from 0200 to 0800 UT the solar wind dynamic pressure was high ranging from 6 to 8 nPa and the interplanetary field was almost due northward, ranging from 10 to 15 nT in BZ GSM. Even at apogee the POLAR spacecraft should not have entered the magnetosheath according to recent scaling laws. However, the magnetic field was greatly depressed below the value expected indicating the presence of significant plasma energy density throughout the high latitude magnetosphere surrounding the cusp. The presence of this plasma is confirmed by the plasma instrumentation on board the spacecraft. While the entry into this nearly stagnant plasma was gradual, the exit on to polar cap field lines was abrupt. We interpret these observations in terms of the post-cusp reconnection of the strongly northward interplanetary magnetic field.

 

Introduction

Dungey's [1961; 1963] classic studies of the role of the interplanetary magnetic field in the interaction of the solar wind with the Earth's magnetosphere predicts a quite different interaction with strongly northward magnetic fields than with southward magnetic fields. Dungey [1961] predicted that, when the interplanetary field is southward, magnetic flux would be eroded from the dayside magnetosphere and convected over the polar caps into the tail. In the tail reconnection of the magnetic field lines in the two oppositely directed lobes creates closed magnetic field lines that then complete the convection circuit to the dayside replenishing the eroded flux. The response for strongly northward interplanetary fields is effectively just the opposite of that in the southward case. Interplanetary field lines join to tail magnetic field lines above the polar cusps adding a new field line to the dayside magnetosphere. Magnetic flux is removed from the tail but replenished by convection from the dayside. Dungey's models were at best qualitative but have now been quantified by numerical simulation [Fedder and Lyon, 1995] and seem to explain the formation of the boundary layer for northward interplanetary magnetic field (IMF) [Song et al. 1994; Le et al., 1996]. Dungey's northward reconnection scenario should occur only over a limited range of clock angles centered on due northward fields because the same field line could not become connected at both the north and south cusps if its clock angle were too large [Russell, 1972]. The range of clock angles for which Dungey's northward model applies has not yet been determined empirically.

POLAR directly probes the region surrounding the polar cusp. Its orbit is inclined at nearly 90o to the equatorial plane and its initial line of apsides was nearly directly over the poles, with an apogee of 9 RE. On May 29, 1996 on POLAR's 130th orbit when the orbital plane was at a local time of about 1000, the interplanetary magnetic field turned strongly northward with a clock angle of 10o or less (measured from the ZGSM axis) for more than 3 hours. The data obtained on this pass allow us to test and quantify the Dungey northward reconnection model. We examine herein the magnetic measurements of the POLAR spacecraft [Russell et al., 1995], and compare them with both the empirical model of the Earth's magnetospheric magnetic field [Tsyganenko, 1996] and a global MHD model for the northward IMF conditions encountered on this day.

 

Solar Wind and Interplanetary Magnetic Field

Fig. 1. The interplanetary magnetic field [Lepping et al., 1995] and the solar wind dynamic pressure [Ogilvie et al., 1995] as measured at the WIND spacecraft at a solar magnetospheric position of [157, -4, -12] Re.

Figure 1 shows the interplanetary magnetic field in solar magnetospheric coordinates and the solar wind dynamic pressure over the time period of interest on May 29, 1996. The measurements have been shifted in time to the expected arrival time at 10 RE in front of the Earth, using the solar wind velocity along the Earth-sun line. We see that the magnetic field was strong throughout the interval from 0000-0800 UT ranging from 12 to 16 nT. Initially the GSM Z component is about 6 nT, but at close to 0300 UT it increases to 16 nT and then maintains a value of between 11 and 16 nT until after 0800 UT. The Y component ranges from about -4 to 4 nT so that the clock angle remains within 20o of being due north with an average angle of about 5o from due north. The bottom panel shows the solar wind dynamic pressure similarly lagged. The solar wind dynamic pressure is high throughout the interval ranging from 6 to 8 nPa, being closer to 8 nPa when POLAR, as we discuss below, was passing through the magnetosheath-like plasma within the magnetosphere.

Fig. 2. The POLAR orbit and the expected magnetopause location shown as the distance from the solar magnetic (SM) X-axis versus the distance along it [Zhou and Russell, 1997]. The SM X direction points along the dipole magnetic equator in the noon meridian. The expected magnetopause was derived for more typical solar wind conditions of 2 nPa. The irregularly scaled orbit is obtained by multiplying the orbital radius by the sixth root of the dynamic pressure.

While the solar wind pressure was about four times that typically experienced in the solar wind, such an increase should not be enough to push the high latitude magnetopause across the spacecraft. Figure 2 demonstrates this by scaling the size of the POLAR orbit by the sixth root of the solar wind dynamic pressure. The smooth orbit is shown as a plot of the distance from the Earth-sun line versus the distance along it. The magnetopause drawn is that expected for a 2 nPa solar wind dynamic pressure and northward IMF in solar magnetic coordinates [Zhou and Russell, 1997]. The irregularly shaped "orbit" is scaled along radii from the origin at each POLAR location. We see that at all times POLAR remains inside the expected magnetopause position. We note that there is evidence for an indentation of the magnetopause in the near cusp region [e.g. Petrinec and Russell, 1995]. Thus the most probable time for exiting the magnetosphere would be around 0700 UT.

 

POLAR Observations

Fig. 3. Magnetic field measurements at 6-second resolution along the POLAR trajectory in the GSM coordinate system from 0230 to 0800 UT on May 29, 1996. The Tsyganenko 96 model for the average solar wind conditions during this period is shown by the dashed line.

The initial operation of the POLAR magnetometer has been discussed by Russell et al. [1997]. Figure 3 shows the POLAR magnetic measurements from 0230 to 0800 UT on May 29, 1996 in the GSM coordinate system together with the expected magnetic field from the T96 model [Tsyganenko, 1996]. This model includes IMF and solar wind effects based on empirical statistical studies, and we use the conditions shown in Figure 1 to control the model. POLAR is proceeding outward and northward in the prenoon sector during this period. Throughout the period shown in Figure 3 beginning deep in the magnetosphere and lasting until 0710 UT, the magnetic field is depressed below the expected values. Although the magnetic field is generally depressed in the immediate neighborhood of the high altitude cusp [Zhou et al., 1997] the depression here is much greater and more extensive than usual. Measurements by the Thermal Ion Detector (TIDE) [Moore et al., 1995] show that the plasma in this region has magnetosheath-like temperatures and composition throughout, but with low flow velocities. Characteristic densities, temperatures and flow speeds during this period were about 100 cm-3, 70 eV and 70 km/s respectively. These flow velocities are antisunward and parallel to the magnetic field except for a brief period in the interval 0635-0655 UT when the flow became sunward but still generally parallel to the magnetic field. The TIMAS ion mass spectrometer [Shelley et al., 1995] indicates that the plasma consists mainly of protons with a significant component (at least several percent) of doubly ionized helium of magnetosheath origin. Energetic (several keV) singly ionized oxygen of ionospheric origin is also observed throughout the interval. When the oxygen flux is high enough to determine its distribution and flow, this plasma is coming from the northern high latitude ionosphere flowing antiparallel to the magnetic field. Toward the end of the period beginning at about 0640 UT, the field strength recovers periodically to and above its undisturbed value until about 0710 UT when it remains at an elevated level. We interpret the region after 0710 to be polar cap field lines with possibly one end open to the solar wind but certainly extending well down the tail and being fairly devoid of plasma.

The field components in the plasma-laden region before this time are distorted moderately from the expected directions. Until 0315 UT the inclination and declination of the field (not shown), that measure the deviation from the horizontal direction and out of the magnetic meridian respectively, were less than 10o different than expected. However, from 0315 until around 0640 UT, the field direction becomes highly distorted even though the spacecraft is deep in the magnetosphere. This distortion is best exemplified by the BZ GSM component in Figure 3 that has a sign opposite that expected after about 0430 UT. The magnetic field direction that should be downward in this region is upward, roughly in the direction of the external field draped over the dayside magnetosphere. Hence this region has some of the characteristics expected for the magnetosheath i.e. the approximate field direction and the presence of plasma, but, except close to the two field reversal regions at 0415 and 0700 UT, the field is fairly steady and the plasma is flowing slowly. Thus this region is not the undisturbed magnetosheath.

 

Comparison with the MHD Model

Fig. 4. Magnetic field measurements at 6-second resolution along the POLAR trajectory in solar magnetic coordinates with the IGRF 95 removed together with the global MHD model with the dipole field removed for the actual time varying solar wind conditions on May 29, 1996.

Fig. 5. Noon-midnight meridan cut of global MHD simulation at 0530 UT on May 29, 1996. The colored contours show the number density and the white lines show the magnetic field lines in the noon midnight meridian. The peak density in the magnetosheath is 200 cm-3. Black bands denote 20 cm-3 contours. The pink field line passes through the POLAR spacecraft at the position of the cursor. The irregular symbols mark the location of null points in the magnetic field.

A new MHD simulation similar to that described in Fedder et al. [1997] was carried out for this time period using IMP-8 measurements of the solar wind parameters. Figure 4 shows the observed residual magnetic field with the IGRF model removed (solid line) compared with the expected residual magnetic field obtained from this time varying global MHD model with its dipole field removed. The field line configuration in the noon-midnight plane and density contours are shown in Figure 5. The cross marks the position of POLAR in the simulation at 0530 UT, (2.1, 0.4, 7.7) RE in solar magnetic coordinates. There is excellent agreement of the observation and the model throughout the simulation until about 0700 UT when POLAR enters the polar cap field lines and the simulation has not yet reached the polar cap. The reason that the magnetic field is northward rather than the usual southward after 0400 UT is due to the stretching of the geomagnetic field lines toward the northward IMF merging sites near the terminator planes. The white crosses mark these reconnection points. There is a depression of the magnetic field strength over the observed range of position. This is caused by the presence in the simulation of plasma with an energy of about 100 eV and a density of about 80 cm-3 similar to the temperature and density observed by TIDE. The pink line in Figure 5 is the field line through POLAR at 0530 UT. This field line has just recently reconnected in the north. Although at the time of this snapshot the line extends far down the tail in the southern hemisphere, we emphasize that reconnection is occurring at both null points throughout the simulation trapping plasma on closed field lines. IMP-8 was used to provide the solar wind input to the simulation because it was much closer to the Earth than WIND. Thus, the simulation stops at 0730 UT when the IMP-8 measurements cease and we have no information on the location or sharpness of the entry on to polar cap field lines in the simulation. The sharpness of the transition in the polar cap field lines seen by POLAR could be due to a rapid motion of the boundary or a thin boundary.

 

Discussion and Conclusions

Dungey [1961; 1963] examined 2 singular cases of magnetospheric behavior, with the interplanetary magnetic field northward and with the interplanetary field southward. Empirically we now know that the behavior of the magnetosphere does qualitatively follow the southward Dungey prescription over a wide range of angles, when the IMF is even slightly southward [see for example Russell, 1975]. However, the magnetosphere cannot follow the Dungey prescription for northward IMF except for a narrow range of IMF angles centered around the northward GSM direction because the interplanetary magnetic field line must attach itself to the magnetosphere at two quite separate points [Russell, 1972]. On May 29, 1996 it appears that the magnetosphere was able to interact in the way Dungey proposed for northward IMF and thereby trapped magnetosheath plasma in a substantial volume of the dayside magnetosphere. In both the simulation and the TIDE observations significant magnetosheath plasma is seen as early as 0300 UT at a geocentric distance of only 6 Re. There is no sharp boundary between the original magnetospheric plasma and the magnetosheath plasma, apparently added to the magnetosphere by the northward merging, as evident in the color contours in Figure 5 or as registered by the TIDE instrument. The ion composition data clearly show the presence of dense magnetosheath plasma on field lines connected to the ionosphere. The broad rotation of the magnetic field exemplified by the reversal in Bz between 0400 and 0500 is effectively an incipient magnetopause formed within the "trapped" magnetosheath plasma. Upon exiting the region, POLAR did encounter a distinct boundary or wall to the dense plasma. We interpret this as the boundary between open tail flux and newly closed magnetic field lines. Just before crossing the wall the POLAR spacecraft encountered a region of very weak and turbulent magnetic fields with magnitudes sometimes approaching zero. The plasma flow in this region often was sunward as well. Thus we interpret the spacecraft to be in the near vicinity of the actual reconnection site. The simulation as summarized by Figure 5, which is representative of the later period as well, confirms this interpretation. The T96 empirical model is not as successful as the MHD model because it does not predict the extreme "hairpin" bending of the field near the boundary in the neighborhood of the cusp. We expect that the reason for this discrepancy is that little data has been obtained in this region under these conditions and that the necessary averaging over space and time would tend to smooth over sharp features. The T96 model also underestimates the depression in field magnitude associated with this cusp-like plasma.

In short the behavior seen by POLAR is quite similar to that expected from the global MHD simulation. The distortion of the magnetic field seen when the IMF is due northward and the appearance of magnetosheath plasma deep in the magnetosphere are both predicted by the model. The POLAR spacecraft seems to have always been on field lines with at least one foot in the ionosphere although whether both feet were always connected or whether briefly both feet became disconnected awaits further point by point comparisons.

 

Acknowledgments

The research at UCLA and UCB was supported by the National Aeronautics and Space Administration under research grant NAG5-3171. JAF was supported by NASA grant NAG5-4727 and by NSF grant ATM-9613815. SPS was supported by NASA grant W-19-771 and by ONR. Computations were supported by a grant of HPC time from the DoD HPC Shared Resource Center, CEWES.

 

References

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C. T. Russell, Earth and Space Sciences, and Institute of Geophysics, University of California, Los Angeles, CA 90095-1567
(email: ctrussell@igpp.ucla.edu)

X-W. Zhou and G. Le, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567
(email: xwzhou@ipgg.ucla.edu; guan@igpp.ucla.edu)

J. A. Fedder, Institute for Computational Science and Informatics, George Mason University, Virginia

S. P. Slinker, Plasma Physics Div., Naval Research Laboratory, D.C. 20375
(email: slinker@ppdu.nrl.navy.mil)

J. G. Luhmann andF. R. Fenrich, Space Sciences Laboratory, University of California, Berkeley.
(email: jgluhmann@sunspot.ssl.berkeley.edu)

M. O. Chandler, Marshall Space Flight Center, Huntsville, Alabama
(email: michael.chandler@msfc.nasa.gov)

T. E. Moore, Goddard Space Flight Center, Greenbelt, Maryland.

S. A. Fuselier, Lockheed Martin, Palo Alto, California.
(email: fuselier@agena.space.lockheed.com)


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