1 Introduction

The QSAT project began in 2006 as an initiative by graduate students at “Department of Aeronautics and Astronautics of Kyushu University”. They are developing the spacecraft bus. “Space Environment Research Center of Kyushu University” and “Laboratory of Spacecraft Environment Interaction Engineering of Kyushu Institute of Technology” are developing the payload instruments. The primary objectives of the QSAT project are (1) to investigate plasma physics in the Earth’s aurora zone in order to better understand spacecraft charging, and (2) to conduct a comparison of the field-aligned current observed in orbit with ground-based observations (for further details, see Tsuruda et al. 2009). This objective can be achieved with simultaneous measurement of the magnetic field disturbances caused by field-aligned currents (FACs) and plasma densities in geospace, and the electric potential of the satellite. Comparing these observed quantities, we will clarify the relationship of the FAC disturbances with spacecraft charging.

In addition to this objective, we have another scientific interest in the study of FACs. FACs always flow in geospace and play an essential role in the energy transfer from the solar wind into the magnetosphere and ionosphere. FACs are roughly measured in three regions around the Earth in terms of a three-dimensional current system: polar region, middle latitude region and equatorial region. Each region has two types of the flow direction: one type of FACs is the current into the ionosphere from the magnetosphere and the other is the current away from the ionosphere into the magnetosphere. There are various current systems in the magnetosphere. They play the important role in the process that the transverse momentum and energy are transmitted from the source region to the other. The current systems consist of the FAC system around the polar region and at the equatorial region, as well as the Sq current. The polar FACs are caused by the solar wind–magnetosphere dynamo, while the Sq ionospheric current is caused by the atmosphere-ionosphere dynamo. However it is not clear what kind of dynamo causes the equatorial FACs. Thus a better understanding of FACs leads to the establishment of the interpretation of the connecting currents among the polar FACs, the Sq ionospheric current and the equatorial FACs.

Space Environment Research Center is developing the magnetometer. We use a fluxgate sensor with electronics and a data processing unit. In Sects. 2 and 3 we describe the detail science objectives of this mission and the general feature of FACs, respectively. In Sect. 4 we explain the comparison of geospace and ground FAC Data. In Sect. 5 we summarize the science goals. Finally, in the appendix we introduce the concept and function of the magnetometer.

2 Scientific Objectives

FACs at high latitude were first detected by the 1963 38C satellite (Zmuda et al. 1970; Armstrong and Zmuda 1970). After the fist in situ observation, many studies have proposed the observation model of large-scale FACs and characteristics of FACs, i.e. the spatial configuration of the FACs flow direction pattern, seasonal variation (Christiansen et al. 2002), average distributions of FACs over the polar region, and FACs density distributions in MLT (see, for example, Iijima and Potemra 1976). However, characteristics of the equatorial FACs are not yet understood clearly in comparison with polar FACs. Thus the science mission is to study the FACs in both polar and equatorial regions, using a fluxgate magnetometer by observing the magnetic field disturbances caused by FACs in both regions. Specifically, the scientific objectives are (1) to better understand the quiet and active FACs in the polar region, and (2) to investigate FACs in the equatorial region.

A related objective is to compare the FACs observed in orbit and on ground by the MAGDAS (MAGnetic Data Acquisition System; Yumoto et al. 2006). This comparison is important because once we understand the relationship, the ground-based data alone can monitor the FACs in geospace. MAGDAS has been deployed by Space Environment Research Center (SERC), Kyushu University in the world. Using this magnetometer network, we will estimate and monitor FACs in geospace. If we can predict the perilous region where the spacecraft charging occurs, the space hazard and also the social loss of benefit will be reduced.

3 The Feature of FACs

3.1 FACs at Auroral Latitude

In the polar region, the distribution of large-scale FACs encircles the magnetic pole as the auroral oval. There are mainly two regions (see Iijima and Potemra 1976); the poleward currents denote as Region 1 (R1) and the equatorial currents denote as Region 2 (R2). The R1 current tends to flow toward and away from the ionosphere in the dawn sector and dusk sector, respectively. The R2 current is the opposite to that of the R1 current. The intensity and density of R1 currents are statistically larger than those of the R2 currents in all magnetic local time (MLT) (Iijima and Potemra 1978). Also the intensity of currents depend on the magnetosphere activity (for example; substorms, seasonal variation). The total intensity of FACs in one hemisphere during active conditions of the magnetic disturbance is higher than that during quiet conditions (Iijima 2000). The distribution of R1/R2 may vary strongly in position for various conditions (interplanetary magnetic field north-south component, seasonal variation, etc. …), however this configuration is very persistent. A large number of studies have reported features of FACs observed in-situ. The average values for the primary features of FAC are as follows; (i) the amplitude range of transverse magnetic disturbance is about from −500 to +500 nT, (ii) FACs density is 0.2 μA/m2, (iii) the apparent period is about 2–5 min.

3.2 FACs in the Equatorial Region

Olsen (1997) confirmed the current direction of the low-latitude meridional current system as suggested by dynamo calculations; upward current at the dip equator and field-aligned downward current at low latitude. In addition, at middle latitudes, there are interhemispheric field-aligned currents (IHFACs). These are believed to be caused by the interhemispheric imbalance of the ionospheric Sq current system (Van Sabben 1966, 1969, 1970). They may flow from the summer to winter hemisphere during morning and in the opposite direction during daytime and evening. We expect the following average values for the primary features of FACs, i.e. (i) the amplitude range of transverse magnetic disturbance is about from −20 to +20 nT, (ii) FACs density is up to 10 nA/m2 (Fig. 1).

Fig. 1
figure 1

A schematic illustration of the field-aligned currents (FACs) in the polar region. The blue lines are currents flowing away from ionosphere, the red lines are currents flowing into ionosphere. The dash line is a path of QSAT satellite (Colour online)

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4 Comparison of Geospace & Ground FAC Data

Haraguchi et al. (2004) found that at the ground foot point of FACs in the ionosphere, magnetometer array on the ground can detect a sharp increase in the northward magnetic component, and latitudinal distribution has one positive peak when FACs flow in and out (R1/R2). They show a simple model for the above observation. Pedersen current JP flows in the ionosphere, which connects R1 and R2 currents. Also Hall current JH generally is produced in the direction perpendicular to both the electric field of JP and the magnetic filed. The magnetic variation by JP does not appear on the ground magnetometer, because of cancellation by the ionospheric divergent current. However, JH produces magnetic variations in the northward component on the ground, according to the Biot–Savart law. Furthermore, the interpolation of the values from six ground CPMN magnetometer stations along the 210 magnetic meridian has a good consistency of ground FACs magnetic field with the estimated DMSP satellite data (see Fig. 2). Therefore, we propose to monitor the FACs intensity using the ground-based magnetometer chain data.

Fig. 2
figure 2

Comparison of geospace & ground FAC data (modified Haraguchi et al. 2004). Blue lines are FAC magnetic fields observed at MAGDAS/CPMN stations. Red line is FAC fields on the ground estimated by DMSP satellite data. The area painted by blue and red are regions where DMSP satellite passed through the polar region (R2 and R1). Each gray box is the ground magnetometer stations in the polar region along 210 MM (magnetic meridian) (Colour online)

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5 Summary

QSAT is a micro-satellite mission being developed jointly by the Kyushu University, the Kyushu Institute of Technology and the Fukuoka Institute of Technology. The science mission is to study the FACs in both the polar and equatorial regions, using a fluxgate magnetometer by observing the magnetic field disturbances caused by FACs in these regions. An important part of the science mission is to compare satellite and ground-based data (MAGDAS). If we can establish the relationship between the space and ground-based FACs data, we can conduct long-term study of the solar wind–magnetosphere–ionosphere coupling in the future by mainly using data from ground-based magnetometer arrays.