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

1. Introduction

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.,

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.

2. Field-Aligned Current Identification

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

Categories of Field-Aligned Current Patterns

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

Figure 1. The categories of DB signatures with different IMF _{y}B 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 _{y}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

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

3.1. Example Observations

Figures 2-7 show examples of category 1 to category 6 FACs. The upper panel shows the IMF *B _{y}*,

3.2. Category 1

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

Figure 2. Example of category 1. (top) The IMF B and _{y}B. (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._{z} |

3.3. Category 2

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

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. |

3.4. Category 3

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

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. |

3.5. Category 4

Category 4 (Figure 5) is also an IMF *B _{y}* exception but in this case the IMF

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. |

3.6. Category 5

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

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. |

3.7. Category 6

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

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. |

4. Statistics of the FAC patterns

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

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

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

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 B and _{y}B for these exceptions. (c) The distribution of cases versus the absolute magnitude of IMF _{z}B._{y} |

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

Figure 10. (a) The local time distribution of the 89 "B exceptions." (b) The distribution of cases versus the relative strength of IMF _{y}B and _{y}B for these exceptions. (c) The distribution of cases versus the absolute magnitude of IMF _{z}B._{y} |

5. Discussion and Conclusions

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

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

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

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

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 B 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._{y} |

The IMF *B _{y}* dependence of the field-aligned current system is also consistent with MHD simulations.

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. |

6. Conclusions

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

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

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

**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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