Geophysical Research Letters

Field-aligned ionospheric current systems observed from Magsat and Oersted satellites during northward IMF

Authors


Abstract

[1] Combination of Magsat and Oersted high-precision magnetic measurements has provided detailed and consistent patterns of ionospheric field-aligned current (FAC) patterns during intervals of northward interplanetary magnetic field (NBZ) conditions. Our analysis has revealed that the NBZ FAC currents extend from two rounded or bean-shaped ionospheric regions centered at 85° geomagnetic latitude and magnetic local times of approximately 0900 for the upward and 1500 for the downward FAC. The locations of the NBZ FAC regions are almost invariant to seasonal variations and to variations in the magnitude of IMF Bz. The NBZ FAC currents are sizeable for summer conditions only and vanishes during equinox and winter seasons. The NBZ FAC intensity is a strongly non-linear function of IMF Bz with a marked threshold at around +5 nT. The observed characteristics indicate that the NBZ currents are the competing high-latitude counterpart of the region 1 (R1) low-latitude boundary layer currents.

1. Introduction

[2] A comprehensive picture of magnetosphere-ionosphere field-aligned currents resulting from the interaction of the solar wind with the magnetosphere was introduced by satellite observations of the magnetic perturbations in space [e.g., Iijima and Potemra, 1976a, 1976b]. These observations have revealed the existence of the more poleward so-called ‘region 1’ (R1) and the more equatorward ‘region 2’ (R2) oppositely directed field-aligned ionospheric currents systems which appear in the morning and evening sectors of the auroral regions at the border of the polar caps [e.g., Iijima and Potemra, 1976a]. In the noon sector at cusp latitudes an IMF By-related current system may form oppositely directed pairs of east-west oriented FAC regions. The more equatorward region of FAC currents appears to be the continuation across local magnetic noon of R1 currents. The more poleward FAC currents were termed ‘region 0’ (R0) or ‘cusp currents’ by Iijima and Potemra [1976b].

[3] The existence of yet another near-polar current system relating to strongly positive (northward) IMF Bz (NBZ) cases was first inferred from ground-based magnetic observations [e.g., Friis-Christensen and Wilhjelm, 1975; Maezawa, 1976]. With the precision magnetic observations available from the Magsat satellite (1979–80), these NBZ FAC currents were described quantitatively [e.g., Araki et al., 1984; Iijima et al., 1984; Iijima and Shibaji, 1987]. A comprehensive modeling of FAC currents and related ionospheric potentials for various orientations of IMF has been presented by Weimer [2001] using a modeling technique based on constructing maps of the scalar magnetic Euler potentials. Through least-squares fits the coefficients of a spherical harmonics expansion are forced to include dependencies on IMF magnitude and direction, solar wind density and velocity and on geo-dipole tilt angle. Using DE 2 magnetic field data Weimer [2001] reveals many features of the NBZ FAC currents.

[4] We have processed data from around 10,000 orbits of Magsat and Oersted high-precision magnetic measurements. The data provide good coverage in magnetic latitude and local time over the polar caps. The basic assumption, like in the quoted works of Weimer [2001], is that the FAC distributions and thus the associated magnetic perturbations are deterministic. Hence, repeated scanning by differently oriented orbits occurring during the same specific conditions will in due time provide a complete mapping of the related 2-D spatial distribution of magnetic perturbations over the polar regions. However, here we have avoided a global functional representation of the magnetic perturbations and FAC currents. Instead, we use bivariate interpolation [Akima, 1978] to derive for a fine grid over the central polar regions numerical values for the magnetic perturbations associated with selected conditions. The available satellite orbits have been sorted on the basis of strict criteria to ensure proper NBZ conditions with steady IMF and solar wind plasma parameters and dominant IMF Bz component at the time of the polar pass.

2. Observations and Data Analysis

[5] The Magsat satellite was operated 8 months following the launch on 30 October 1979 into a sun-synchronous dawn-dusk near-polar orbit with initial perigee at 352 km and apogee at 578 km altitude. The Oersted satellite was launched 23 February 1999 into a near-polar orbit with perigee at 640 km and apogee at 860 km. Its orbit drifts slowly in local time [e.g., Olsen et al., 2000].

[6] We use Ampère-Maxwell's law, μoI = ∇ × B, to calculate FAC intensities from a derived 2-D distribution of magnetic perturbations over the polar regions. The magnetic perturbations are found by subtracting model field values from the measured data. For the Magsat measurement the GSFC(12/83) geomagnetic reference field model for epoch 1980 [Langel, 1987] was used. In addition to the internal field contributions, the contributions from the magnetospheric ring current have been included through the Dst-dependent terms given by Langel [1987]. For the Oersted magnetic vector data the Oersted (10c/99) field model for epoch 2000 [Olsen et al., 2000] has been used to calculate the model field. After subtraction of the Dst-corrected model field from Magsat 0.5s or 5s samples and Oersted 1s samples all perturbation data are converted to 5 sec averages. This temporal resolution corresponds to a spatial resolution of around 35 km along the orbit. The positions of the Magsat and Oersted satellites are defined in standard altitude adjusted corrected geomagnetic (aacgm) coordinates and magnetic local time.

[7] The magnetic perturbation vectors are resolved in field-aligned and transverse components using the reference model field to define the local field direction. The transverse components are further resolved in sunward and dawn-duskward components, which are transformed along the field to a 120 km reference level. For the two transverse components a bivariate interpolation procedure [Akima, 1978] is now invoked to derive a fine–grid perturbation vector field at 120 km altitude over the polar caps. The interpolation includes all available samples within each group of passes complying with a specific set of selection criteria. An example of the results from the procedure is shown in Figure 1 for the Z3SS case (se definitions below) derived from 33 south polar satellite passes. The double vortex structure, which depicts regions of downward and upward FAC, is clearly identifiable.

Figure 1.

Magnetic perturbation vector field for the southern polar cap referred to 120 km altitude. Magsat and Oersted data been combined for cases of +5 < IMF Bz < +10 nT during summer season passes.

3. Interplanetary Magnetic Field and Solar Wind Plasma Parameters

[8] For Magsat data the IMF and solar wind plasma parameters used for the selection criteria are based on IMP8 and ISEE3 satellite data. For the Oersted mission IMP8 and ACE (level 2) data have been used. Timing of data samples have been adjusted corresponding to translation with solar wind velocity from the satellite to a reference position at 12 Re in front of the Earth. The delay of FAC intensities in the polar ionosphere relating to the IMF at the reference location is considered to be around 10 min [Stauning, 1994]. The relevant IMF parameters to characterize the conditions are now defined to be the average values of the 12 Re reference data over an interval from To-25 min to To-10 min where To is the time of the central polar Magsat or Oersted pass. We have also calculated the corresponding values for the preceding interval from To-40 to To-25 min and the RMS variations during the entire interval from To-40 to To-10 min in order to specify limits for IMF variability. Selection criteria for groups Z1 to Z4 are shown in Table 1.

Table 1. Selection Criteria for Interplanetary Magnetic Field Parameters for Specific NBZ Cases
CaseIMF BxIMF ByIMF BzdBaBrmsbUnit
  • a

    dB is max extension of range for previous set of IMF param.

  • b

    Brms is max rms value of IMF parameter during total range

Z1−2<Bx<2−1<By<10<Bz<221nT
Z2−5<Bx<5−2<By<22<Bz<532nT
Z3−10<Bx<10−3<By<35<Bz<1053nT
Z4−20<Bx<20−5<By<510<Bz<20105nT

[9] We have, furthermore, grouped the orbits according to season, that is, in summer (S), winter (W) and equinox (E) cases, and we distinguish between north polar (N) and south polar (S) passes. Accordingly, we have formed groups designated by descriptive letters ranging from Z1SW (northward, weak IMF, southern pass, and winter season) to Z4NS. From the 2-D magnetic perturbation vector fields we have derived the FAC currents by calculating the curl from the numerical data using planar geometry for the polar cap. Figure 2 displays the resulting distribution in polar coordinates using a color code to represent FAC intensities and directions. Yellow and red shades represent upward (positive) currents while blue shades represent downward FAC. The illustration in Figure 2 for the Z4NS selection represents northern polar cap summer conditions for the strongest NBZ case (+10 < Bz < +20 nT) for which the number of available passes in the group was adequate for the mapping.

Figure 2.

Field-aligned current intensities referred to 120 km altitude over the northern polar cap for Z4 cases of +10 < Bz < +20 nT and for summer season. Upward currents are in yellow/red shades, downward in blue.

[10] The two bean-shaped intensifications seen at the dayside inside the 80° aacgm latitude circle are the projections to 120 km level of the NBZ FAC currents, upward in the morning sector (red) and downward (blue) in the evening sector. In the morning sector just equatorward of the 80° circle the downward R1 region (blue) and further equatorward the upward R2 region (yellow/red) appear. In the afternoon sector the upward R1 region (yellow/red) and further equatorward the downward R2 region (blue) are seen. The NBZ FAC intensities peak at values just above 1 μA/m2. The total NBZ FAC amounts to +0.46 MA (upward) in the 0600–1200 MLT sector and to −0.49 MA (downward) in the 1200–1800 MLT sector. The total R1 current amounts to −0.35 MA in the morning sector and to +0.30 MA in the evening sector. At the nightside of the polar cap all currents are weak.

[11] Figure 3 displays a complete sequence of FAC current maps for the Z3 cases (+5 < Bz < +10 nT) which include summer, equinox, and winter seasons and north as well as south polar passes. Again, the summer cases show strong signatures of NBZ FAC's while the NBZ signatures are hardly significant in the equinox and winter cases. In all cases the NBZ FAC regions are clearly separated from the R1 FAC regions.

Figure 3.

Composite plot of FAC current intensities over the northern (top fields) and southern (bottom fields) polar caps and for different seasons. Cases with +5 < Bz < +10 nT (Z3) have been included.

[12] Figure 4 presents the NBZ FAC variations with season and IMF Bz strength. The horizontal axis has fields for the seasons (winter-equinox-summer). Each field is subdivided to represent IMF Bz levels from Z1 to Z4 (c.f., Table 1). The solid (open) squares/circles represent upward (downward) northern/southern FAC cases. The symbols are positioned horizontally according to season and IMF level and vertically according to the scales to the left. The top field displays the aacgm latitudes for the center of gravity for the NBZ FAC regions. For all cases these latitudes are close to 85°. The middle field displays for the NBZ FAC centers their magnetic local times. They are consistently close to 0900 and 1500 MLT, respectively, for the upward and the downward current regions. The bottom field displays the integrated NBZ FAC currents for the 0600–1200 and 1200–1800 MLT sectors, respectively, (anticipated negative FAC's have been reversed). During the winter season the total FAC is weak and fluctuates around zero. For the summer cases a strongly non-linear variation is evident. For IMF Bz levels Z1 and Z2 (cf., Table 1) the FAC currents are just identifiable at around 0.05 MA while for level Z3 the total NBZ FAC current jumps to around 0.3 MA and stagnates at around 0.4 MA for level Z4. From the analysis we have also derived (not shown) the latitudes for the transitions between R0, R1 and R2 FAC current regions versus local magnetic time. The transition between R0 and R1 occurs at 80° +/−2° for all dayside MLT's, for all seasons and for all northward IMF Bz cases (Z1–Z4).

Figure 4.

Summary diagram to present NBZ FAC locations (magnetic latitude and local time) and integrated FAC intensities for different seasons and varying IMF Bz levels.

4. Discussions

[13] The mapping of NBZ currents obtained in this study, basically, confirms the results of Weimer [2001] and some of the earlier works like, for instance, Araki et al. [1984]. The extensive data base now available adds to define further characteristic features of the NBZ FAC system.

[14] The seasonal behavior with vanishing NBZ FAC currents during winter conditions, no doubt, relate to the extremely low conductivity present in the central polar cap ionosphere during this season. At the high latitudes in question, that is, above 80° magnetic latitude, solar UV ionization processes are weak or even absent during winter. Furthermore, as the combined result of the high latitudes in question and the infrequent occurrence of substorms during NBZ conditions the energetic particle fluxes are very weak; hence the precipitation-induced ionization is low. The resulting ionospheric conductivity in the central polar caps could well be more than an order of magnitude lower during winter than during summer conditions. This feature impedes the closure of FAC in the winter ionosphere and may cause the magnetospheric field-aligned currents to close in the opposite (summer) ionosphere.

[15] Another specific observation is the steady location of the center of the NBZ FAC system at 85° aacgm latitude and 0900 and 1500 MLT, respectively. These latitudes are poleward of the cusp latitudes defined, for instance, by Newell et al. [1989] for the positive IMF Bz conditions considered. Hence the NBZ FAC regions must be characterized as the ionospheric projections of mantle/lobe regions extending into the magnetosphere from the top and bottom magnetopause. The NBZ FAC regions are delimited equatorward at the cusp/cleft latitude and their constant positions agrees with the observation that the cusp/cleft location changes little with varying IMF Bz when Bz > 0 [Newell et al., 1989]. An important question is the possible source mechanism for the NBZ FAC currents. We suggest that the complementary characteristics of R1 and NBZ currents with respect to location, direction and intensity variation with IMF Bz indicate that their generation mechanisms are similar. The dayside R1 currents are presumably generated in the low latitude boundary layer (LLBL) just inside the magnetopause at the front and along the flanks of the magnetosphere [e.g., Potemra, 1994]. In the low latitude R1 source regions the magnetospheric field direction is northward. The R1 currents are enhanced during increasingly negative IMF Bz since this orientation favors merging processes. The equivalent FAC generation mechanism in the high-latitude boundary layers (HLBL) have reversed geometry since the magnetospheric field in the possible source region (the frontal mantle and lobe regions) has a southward component. The NBZ FAC, accordingly, should flow opposite of R1, that is, up at the morning side and down at the evening side, they should be located poleward of the R1 FAC, and they should increase with increasing positive IMF Bz. To make the HLBL equivalent to the LLBL with closure of field-lines near the Earth rather than extending far into the tail requires a fairly high (threshold) level of positive IMF Bz. These anticipated features agree well with observations.

5. Conclusions

[16] Using the combined database from the Magsat and Oersted missions with strict criteria to select cases of steady solar wind and magnetospheric parameters and dominant northward interplanetary field (NBZ) conditions has enabled the detection of NBZ-related field-aligned currents (FAC) with an unprecedented detail and consistency. The analysis has specifically illustrated the following features:

  1. The NBZ FAC system comprises a pair of upward and downward currents flowing along field lines extending from rounded or bean-shaped regions of the polar ionosphere. The regions are centered at around 85° aacgm magnetic latitude and at 0900 MLT for the upward currents and at 1500 MLT for the downward current. The locations of the NBZ FAC currents are invariant to season and to IMF Bz level.
  2. The NBZ FAC currents are of substantial magnitude at the dayside polar cap and during the summer season only. They vanish at the nightside and during the winter season.
  3. The total downward and upward NBZ FAC currents are about equal and amounts typically to 0.4 · 106 A at IMF Bz = +10 nT for the summer season.
  4. The total NBZ currents are strongly non-linear functions of the IMF Bz level. They are vanishing small for IMF Bz < +5 nT, rises strongly for +5 < Bz < +10 nT, and level out for IMF Bz > +10 nT.

[17] The characteristics found for the NBZ FAC system suggest that these currents constitute a competing high-latitude boundary layer counterpart to the low-latitude boundary layer R1 FAC system.

Acknowledgments

[18] We gratefully acknowledge the use of ACE data supplied from the ACE Science Center (ACS) and IMP8 and Magsat satellite data made available by the National Space Science Data Center (NSSDC). The Oersted satellite is operated by TERMA Electronics and the Danish Meteorological Institute (DMI). The magnetic data have been processed by the Oersted Science Data Center (OSDC) at DMI. The Oersted satellite project was funded by the Danish Ministry of Transport, the Ministry of Research and Information Technology, the Ministry of Trade and Industry, and the Danish Research Councils. Further support of the mission was provided by NASA, ESA, CNES, and DARA.

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