Local time and interplanetary magnetic field B_y dependence of field-aligned currents at high altitudes

X.-W. Zhou, C. T. Russell, and G. Le

Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California Los Angeles

Originally published in: J. Geophys. Res., 105, 2533-2539, 2000

Abstract. Magnetic field measurements from the Polar spacecraft reveal a field-aligned current system in the high-latitude, high-altitude magnetosphere that is the extension of the traditional region 1 and region 2 field-aligned current system observed at low altitudes. On average both the low- and high-altitude observations show that the region 1 and region 2 currents flow in opposite senses on the morning and afternoon sides. Recent studies reveal that negative (dusk to dawn) interplanetary magnetic field B_y extends the upward flowing region 1 current onto the morningside and positive B_y extends the downward flowing region 1 onto the afternoonside in the northern hemisphere. We use Polar magnetic field data to study both the local time and interplanetary-B_y orderings of the high-latitude, high-altitude, field-aligned currents. We find that the interplanetary-B_y component plays a very important role in controlling the flow sense of the field-aligned current at high altitudes. Except for small clock angles, the effect of the negative (positive) interplanetary B_y is an upward (downward) current both in the afternoon and morning sectors.

Field-aligned currents communicate stress from one part of the magnetosphere to another and play an important role in the solar wind-magnetopsheric-ionospheric interaction. One of the first large-scale statistical studies on the characteristics of field-aligned currents (FAC) was undertaken by Iijima and Potemra [1976a, 1978], using magnetometer observations from the low-altitude, polar-orbiting TRIAD satellite. They identified two regions of large-scale FACs encircling the polar cap. Region 1 current is located poleward of region 2 and flows into the ionosphere on the duskside, while region 2 is in the opposite sense with outward current flow on the dawnside and inward current flow on the duskside. Region 1 current is thought to lie along the poleward boundary of the auroral oval and map to the magnetopause and outer regions of the magnetosphere. Region 2 current lies along the equatorward edge of the oval and maps to within the magnetosphere and possibly to the ring current. A distinct set of FACs has been observed from TRIAD measurements [Iijima and Potemra, 1976b], sitting in the dayside cusp and poleward of the region 1 current system. Near noon, this cusp current has a sense opposite to that of region 1; i.e., it flows upward at prenoon and downward postnoon. This current has been called region 0.

Later study revealed a correlation between the FAC and interplanetary magnetic field (IMF) B_y conditions especially near noon [e.g., McDiarmid, 1979; Erlandson et al., 1988]. Erlandson et al.'s [1988] study shows that the flow direction of the region 1 current near noon depends on the IMF B_y component: When IMF B_y is negative, the duskside region 1 current shifts to prenoon, while when IMF B_y is positive, the dawnside region 1 current shifts to the postnoon. However, McDiarmid et al. [1979] regarded the cusp FACs as part of the prenoon downward region 1 current when interplanetary B_y<0 and as part of the postnoon upward region 1 current for interplanetary B_y>0. In this way, the currents around noon could be described as two oppositely directed region 1 currents which overlapped in a pattern determined by the IMF B_y.

Most of the early field-aligned current studies were based on observations from low-altitude, polar-orbiting satellites, but we expect that these currents extend deep into the distant magnetosphere. Observations from OGO 5 and ISEE 1 and 2 among other satellites [Aubry et al., 1972; Iijima, 1974; Chun and Russell, 1997] confirm these expectations. Polar is now observing FACs at high-altitude, and the initial results [Russell et al., 1997] show that the high-altitude current system is consistent with that at low altitudes. However, statistically, much of our knowledge of FACs and their relationship to the solar-terrestrial interaction is based on low-altitude studies, especially in the noon sector, a region inaccessible to OGO 5, ISEE 1 and 2 and IMP 5. Nevertheless, the behavior near noon in the neighborhood of the cusp remains confusing, at least in part due to temporal variations. Below we use data from the Polar magnetic field experiment (MFE) [Russell et al., 1995] to conduct a statistical study of the high-altitude, high-latitude magnetospheric FACs on the dayside and to examine the local time distribution and the IMF B_y effects on the FAC.

We will use the Polar magnetometer data to identify the signatures of the FACs. First, we establish a field-aligned coordinate system by using the Tsyganenko T96_01 model [Tsyganenko, 1996]. The field-aligned component (Z) is along the model field vector. The azimuthal transverse component (Y) is determined from the cross product of the model field vector and radial vector. It is generally eastward except in a small region above the polar cusp to the magnetic pole. The outward normal component (X) completes the right-hand triad. Then we subtract the T96_01 model field from the Polar magnetic field and rotate the detrended data to the field-aligned coordinate system.

The detrending of the observed magnetic field by the model field will only leave variations of the observed field, which we interpret to be magnetic signatures of the localized field-aligned currents. Although the T96_01 model includes a large-scale, field-aligned current which is explicitly built in to the model, this current system does not replicate the often narrow FAC in the region we study here [see Zhou et al., 1997]. We have compared the residual magnetic field in this region with the T96_01 model including the field-aligned currents and then again with the T96_01 model excluding the field-aligned current contribution by forcing these components to be zero. We find that there is very little difference in the magnetic field predicted near the cusp, far too little to affect our field-aligned current identifications.

We identify magnetic perturbations by their appearance in the azimuthal transverse component of the detrended data (D B_y) in the field-aligned coordinates. This is based on the assumption that FACs in the inner magnetosphere are sheet-like and aligned parallel to L shells. However, if in some instances FACs are filamentary, the magnetic perturbations will be observed in both transverse (D B_y) and outward (D B_x) components; or if FACs are aligned more closely with meridional planes, the magnetic perturbations will be observed in the outward normal component (D B_x). We identify only those D B_y that change by more than 10 nT, and we ignore high-frequency fluctuations. In the northern hemisphere an increase in the D B_y toward high latitudes corresponds to a downward flowing current, while a decrease corresponds to an upward one.

In this paper, we concentrate on the IMF B_y effect on the dayside field-aligned current. All observed cases of crossing the field-aligned current systems by Polar can be divided into eight categories, according to eight possible combinations of (1) the IMF B_y polarity (positive or negative), (2) the crossing local time (morning and afternoon), and (3) the observed direction of the region 1 current (upward or downward). All eight possible categories are schematically shown in Figure 1 as plots of typical variations of the detrended D B_y, observed by Polar as the spacecraft crosses the FAC system from low to high latitude. In all cases, |D B_y| increases, as the spacecraft approaches the region 1 current sheet and then decreases, after the spacecraft crosses it and moves away into the polar cap. In many cases the increase of |D B_y| toward the peak of the region 1 current is preceded by a weaker variation in the opposite direction, owing to a contribution from the region 2 current sheet. However, in this study we concentrate entirely on the region 1 current and hence will not consider the effects of the region 2 current.

Figure 1. The categories of DBy signatures with different IMF By and local time depicted schematically. Along the abscissa (time), the spacecraft moves from low to high latitudes. The four left panels show categories 1-4, which are consistent with the traditional Iijima and Potemra [1978] local time distribution. The increase in the residual field in the eastward component corresponds to a downward current, while a decrease corresponds to an upward current. Categories 5-8 are inconsistent with the traditional pattern.

Of the eight categories shown in Figure 1, 4 cases on the left (Categories 1-4) are consistent with the traditional Iijima-Potemra pattern in the northern hemisphere, i.e., upward region 1 current in the afternoon and downward in the morning sector. We will refer to these categories, as local time ordering. The other four categories (5-8), shown on the right, are inconsistent in this sense, and we will call them local time exceptions.

An alternative way of classification can also be suggested, based on IMF B_y. More specifically, according to the observed spreading of the afternoon region current into the morning sector at times with IMF B_y<0, and the oppositely directed spreading for IMF B_y<0 [McDiarmid et al., 1979], we assume that the gross effect of the IMF B_y<0 upon the region 1 current in the northern hemisphere is equivalent to adding an outward current both in the morning and afternoon sectors. Similarly, the oppositely directed IMF B_y>0 is assumed to result in an additional downward current on both sides from the noon meridian. In this sense, the four plots in the upper row of Figure 1 (categories 1, 2, 5, and 6 ) should be considered as "normal" cases, while the ones in the bottom row are "anomalous," and we will refer them as IMF exceptions. From this viewpoint the categories 7 and 8 are anomalous both in the local time and the IMF B_y sense. In fact, we have never observed examples of these two categories. The local time exceptions are all categories 5 and 6, while the IMF B_y exceptions are all categories 3 and 4.

In a manner analogous to our local time ordering and exceptions discussed above, we can classify the observed currents according to the IMF-dependence of their D B_y signature (or the flow direction), whether it first increases then decreases or it first decreases then increases when the spacecraft moves from low to high latitude. If we suppose only IMF B_y is controlling the D B_y signature, then, when IMF B_y is negative, D B_y first suddenly decreases then gradually increases as Polar crosses the FAC system from low to high latitude in the northern hemisphere. Categories 1, 2, 5, and 6 are consistent with this pattern, which we call IMF B_y ordering. Categories 3, 4, 7, and 8 are inconsistent and we refer to these as IMF B_y exceptions.

Figures 2-7 show examples of category 1 to category 6 FACs. The upper panel shows the IMF B_y, B_z, and the time lag from Wind spacecraft. The lower panel shows the three components in field-aligned coordinates. We use the Tsyganenko 96 model as the background magnetic field. Only the residuals between the Polar MFE data and Tsyganenko's model are shown.

Figure 2 shows an example of category 1. The FAC system is crossed on the afternoon side from ~1250-1430 MLT. From 0723 to 0750 UT, D B_y increases. This increase corresponds to a downward flowing region 2 FAC; from 0750 to 0825 UT, D B_y decreases (although overlaid by some fluctuations), indicating that Polar crosses the upward flowing region 1 current. This is consistent with local time ordering. While in this case, IMF B_y is negative (about -2 nT), so the D B_y signature is also consistent with the IMF B_y ordering expected if the current sense were controlled solely by IMF B_y.

Figure 2. Example of category 1. (top) The IMF By and Bz. (bottom) The three components in field-aligned coordinates. We use the Tsyganenko 96 model as the background magnetic field and only the residuals between the MFE data and Tsyganenko's model are shown. The expected time lag between Wind and the magnetosphere is 20 min.

Figure 3 (category 2) is a case on the morningside. The magnetic local time is at ~0900 MLT. From 2105 to 2112 UT, D B_y decreases indicating an upward region 2 current; from 2115 to 2130 UT, D B_y increases corresponding to a downward region 1 current. The IMF B_y is positive at ~2 nT. So the D B_y signature in category 2 is consistent both with the local time ordering and the IMF B_y ordering.

Figure 3. Example of category 2. See caption of Figure 2. The expected delay between Wind and the magnetosphere is 22 min in this example.

Figure 4 (category 3) is a case for local time ordering but it is not consistent with IMF B_y ordering. Here the magnetic local time when Polar passes the FAC system is ~1530 MLT. D B_y first increases, then decreases which is consistent with the traditional afternoon behavior of the region 2, then region 1 current crossing, but it is inconsistent with the IMF B_y ordering with positive IMF B_y. However, the IMF B_y is only ~3 nT, the IMF B_y/B_z ratio is 3/4.5=0.67, and it is far away from local noon. This IMF B_y is probably not strong enough to control the direction of FAC 3.5 hours away from local noon.

Figure 4. Example of category 3. See caption of Figure 2. The expected delay between Wind and the magnetosphere is 17 min in this example.

Category 4 (Figure 5) is also an IMF B_y exception but in this case the IMF B_y is negative and it is on the morning side. From 1507 to 1515 UT, D B_y decreases (i.e., Polar passes the upward flowing region 2 current), then from 1524 to 1535 UT, D B_y increases (i.e., Polar passes the downward flowing region 1 current). The D B_y signature is consistent with local time ordering. We also notice that during the period of the region 1 FAC crossing, IMF B_y is about -1.5 nT and IMF B_z is about -8 nT. This makes the IMF B_y/B_z ratio to be less than 0.25 which is small. Also, this occurs at early morning ~0830 MLT. The implication from this is similar to that found in the category 3 case above, which is that probably the IMF B_y is not strong enough to control the direction of FAC 3.5 hours away from local noon.

Figure 5. Example of category 4. See caption of Figure 2. The expected delay between Wind and the magnetosphere is 36 min in this example.

Figure 6 shows an example of category 5. From 0853 to 0858 UT, D B_y decreases. This is an upward flowing region 2 FAC. From 0905 to 0920 UT, D B_y increases. This is a downward flowing region 1 FAC. This FAC system is sitting at 1130-1330 MLT (mainly afternoon). This case is inconsistent with local time ordering. However, IMF B_y is strongly positive (almost 8 nT), so it is consistent with IMF B_y ordering.

Figure 6. Example of category 5. See caption of Figure 2. The expected delay between Wind and the magnetosphere is 37 min in this example.

An example for category 6 is shown in Figure 7. It shows a current system that is consistent with the IMF B_y ordering but not with the local time ordering. D B_y first increases as Polar crosses the downward region 2 current then decreases as it enters the upward region 1 current. The magnetic local time is from ~0950 to 0930 MLT. In this case, IMF B_y is strongly negative at about -5 nT.

Figure 7. Example of category 6. See caption of Figure 2. The expected delay from Wind to the magnetosphere is 25 min in this example.

We have examined all the dayside (0600-1800 MLT) FACs from March 16, 1996, to December 31, 1997. For a total of 427 cases we have 209 and 218 on the afternoonside and morningside, respectively. For IMF B_y positive, there are 220 cases, for IMF B_y negative 207. Thus the number of cases is similar for the different local time and IMF B_y conditions. Figure 8 shows the statistics of all the cases studied. About one third of all the 427 cases we studied are in category 1 and about another third are in category 2. Categories 5 and 6, which we call "MLT exceptions," are inconsistent with the traditional FAC pattern. About 15% of the cases fall into these two categories.

Figure 8. The statistics of all 427 cases from March 16, 1996, to December 31, 1997. About one third are in category 1 and about another third are in category 2. Category 3 and 4 comprise ~21%. Category 5 and 6 comprise 15%.

If we assume that the IMF B_y effect completely controls the observed pattern, then we should see everywhere for positive B_y a downward region 1 current (the traditional morning pattern); and for negative IMF B_y, an upward region 1 current (the traditional afternoon pattern). So categories 1, 2, 5, and 6 are consistent with in this IMF B_y pattern ordering. Categories 3 and 4, the IMF B_y exceptions, are about 21% of the cases.

Figure 9a shows the local time distribution of the 63 MLT exceptions. Most of them are near noon. Figure 9b shows the IMF B_y strength relative to IMF B_z. This can also be thought of as the clock angle of the IMF around the solar wind direction. Figure 9c shows the strength of IMF B_y for these 63 events. Usually IMF B_y is strong (>2nT) when there is an MLT exception.

Figure 9. (a) The local time distribution of the 63 "MLT exceptions." Most of them are near noon. (b) The distribution of cases versus the relative strength of IMF By and Bz for these exceptions. (c) The distribution of cases versus the absolute magnitude of IMF By.

Figure 10a shows the local time distribution of the 89 "B_y exceptions." This distribution is almost uniform from 0900 to 1700 MLT. Figure 10b shows the IMF B_y strength relative to IMF B_z. Figure 10c shows the strength of IMF B_y for these 89 events. Usually, IMF B_y is weak or comparable relative to IMF B_z (IMF |B_y/B_z| <2.5 in most cases).

Figure 10. (a) The local time distribution of the 89 "By exceptions." (b) The distribution of cases versus the relative strength of IMF By and Bz for these exceptions. (c) The distribution of cases versus the absolute magnitude of IMF By.

The above statistical study shows that the traditional local time distribution of region 1 /region 2 currents observed at low-altitudes also holds at high altitudes except for cases of strong IMF B_y near noon, i.e., categories 5 and 6. The y component of the IMF controls the direction of the large-scale FACs in the noon sector.

Traditionally, on the dawnside in the northern hemisphere, region 1 current flows downward, but with negative IMF B_y, the traditional downward region 1 on the dawnside near noon is flowing upward. We interpret this dawnside upward flowing current as the extension of the duskside region 1 across noon to the morningside as Erlandson et al. [1988] did using the low-altitude orbiting Viking data.

Similarly, on the duskside, region 1 current usually flows upward, but with positive IMF B_y, the traditional upward flowing region 1 on the duskside is flowing downward. In a similar way to the situation for upward region 1 current on the morningside, the meridian that separates the dawnside and duskside region 1 FAC shifts to the afternoon when IMF B_y is positive.

If we adopt the view that region 1 current lies along the poleward boundary of the auroral oval and maps to the magnetopause and outer regions of the magnetosphere, which means that the field lines carrying the current are open and reconnected to the IMF, then it is easy to understand the role of IMF B_y. Figure 11 sketches the configuration of the magnetic field on the dawn-dusk meridian. Two field lines originating from the polar cap are shown as solid lines. The upper figure shows the normal magnetosphere configuration for IMF due southward. Local noon separates the field lines with the other ends connected to the dawnside magnetosphere from those connected to the duskside. When IMF B_y is negative (and IMF B_z negative), reconnection pulls some open field lines on the dawnside toward dusk in the northern hemisphere as shown in Figure 11b. To support the shear in the magnetic field between these field lines and the dipole field lines the shear layer must carry downward flowing current even though the feet of the field lines are on the dawnside. This explains the D B_y signature of category 6. Also intuitively, the amount of twisted field lines depends on the magnitude of the IMF B_y component. The case is just the opposite for duskward and southward IMF which is shown in Figure 11c.

Figure 11. A schematic view of the magnetic field configuration on the dawn-dusk meridian and its relation to the field-aligned current for different IMF By conditions. The big arrows represent the field-aligned current direction. (a) IMF due southward, (b) IMF dawnward and southward, and (c) IMF duskward and southward.

The IMF B_y dependence of the field-aligned current system is also consistent with MHD simulations. Fedder et al. [1997] compared Polar MFE data on May 19, 1996, with their MHD simulation. In fact, this case is an example of category 6 discussed here. They also calculated the field-aligned current pattern at ionospheric altitudes and presented the FAC density contours for this day. On the results, we may qualitatively sketch these field-aligned current density contours for positive and negative IMF B_y conditions as shown in Figure 12. Although maybe exaggerated, this pattern is very similar to the IMF dependence of the ionospheric convection patterns [Rich and Hairston, 1994]. In our observations we do not see categories 7 and 8 which are inconsistent with either IMF ordering or MLT ordering. This also supports the conclusion of the MLT and IMF B_y roles in controlling the FAC behavior.

Figure 12. Schematic field-aligned current density contours in the northern hemisphere. The solid contours are current flowing downward, dotted contours are current flowing upward. (a) IMF due southward, (b) IMF dawnward and southward, and (c) IMF duskward and southward.

A statistical study of the local time and IMF B_y dependence of the field-aligned currents on the dayside has been performed by using the Polar MFE data from March 16, 1996, to December 31, 1997. The major conclusions are as follows:

1. Polar is probing the field-aligned current in the high-latitude, high-altitude northern hemisphere of the magnetosphere. The region 1/region 2 current pattern originally reported at low altitudes is preserved in the higher-altitude Polar measurements at distances up to 9 R_E except for cases of strong IMF B_y and near noon.

2. When IMF B_y is positive, the downward current on the dawnside extends across noon into the early afternoon side, and vice versa for IMF B_y negative. IMF B_y ordering holds except when IMF |B_y/B_z| is small.

3. The IMF B_y effects on the field-aligned current pattern at high altitudes are consistent with those observed at low altitudes by Viking and with the expectations of the stresses applied to the magnetosphere in MHD models of reconnection. Thus the IMF B_y component plays a significant role in controlling the flow sense of field-aligned currents at high altitudes.

Acknowledgements. This work was supported by the National Aeronautics and Space Administration under research grant NAG5-3171.

Hiroshi Matsumoto thanks T. Araki and another referee for their assistance in evaluating this paper.

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