This section aims to discuss the effect of field aligned currents on pulsation measurements.
However, we shall first briefly give a general description of these current. The circulation of magnetic field in a pattern of closed closed loops within the magnetosphere as a consequence of tangential drag of solar wind is one of the sources that produces the field aligned current sys- tem. They flow on two shells surrounding the Earth (see Figure 3.6). The field aligned currents couple to the currents flowing parallel to the local magnetic field. They were first proposed by Birkeland (1908) to explain polar elementary substorm and other polar magnetic disturbances.
As shown in Figure 3.6 the field aligned currents flow on two shells completely surrounding the Earth. At the polar regions the magnetosphere is dominated by the portions of ionospheric currents flowing in enhanced-conductivity channels at high latitudes. These currents include large horizontal currents that flow in the ionosphere E-region, called auroral electrojets (Erick- son and Winckler, 1973). These currents are linked to the outer parts of the magnetosphere by field-aligned currents which are central to magnetosphere-ionosphere coupling. The electro- jets are generally confined to the high latitudes auroral oval during magnetically quiet periods.
However during disturbed periods the electrojet increases in strength and even expands to both higher and lower latitudes. This expansion results from two factors: enhanced particle precip- itation and enhanced ionospheric electric fields (Erickson et al., 1979).
CHAMP DATA (FGM-LPFF-0): 2002 02 13
-2 -1 0 1 2
Bcom (nT)
-400 -300 -200 -100 0 100 200
Btor (nT)
-600 -400 -200 0 200 400
Bpol (nT)
0700 0800 0900 1000 1100 1200
UNIVERSAL TIME -100
-50 0 50 100
GcLat (deg)
Figure 3.7: The three field-aligned components, i.e. compressional (Bcom), toroidal (Btor) and poloidal (Bpol) together with the latitudinal profile from top to bottom.
The circular orbit of CHAMP makes possible its homogeneous coverage of the Earth’s sphere.
High currents such as FAC are observed by the satellite as it approaches the polar regions. Their effects appear in CHAMP magnetic field data as the disturbances of tens of nT amplitudes as shown in Figure 3.7 when the satellite approaches higher latitudes. In Figure 3.7 shown on the top three panels are field-aligned magnetic field components i.e. compressional (Bcom), toroidal (Btor) and poloidal (Bpol) filtered in the Pc3 frequency range. It can be inferred from the lati- tudinal profile shown in bottom panel of the figure that there are disturbances in magnetic field data that are more pronounced inBtor and Bpol when the satellite is centered around the polar regions. The currents of components perpendicular to the field line are larger than the currents of component along the field line. In Figure 3.7 the transverse components i.e. Btor and Bpol
are largest, indicating that they are due to FAC. For Pc3 studies it is desirable to avoid inter- vals dominated by disturbances caused by the flow of high currents. These disturbances when filtered may leak into the Pc3 frequency band and can be erroneously interpreted as Pc3 pulsa- tions. Also the rapid movement of the satellite across field lines in the polar regions may result in spatial structures that can be falsely interpreted as pulsations. The approach taken in this work to avoid false interpretation of non pulsation oscillations as pulsations is explained below.
Instead of relying on the previous results, we carried out a statistical analysis experiment to determine the region where FAC occurs. The aim of the experiment was to establish the latitudes where CHAMP registers these currents which are consequences of FAC. With the
CHAMP DATA (FGM-LPFF-0): 2001 03 15
-0.10 -0.05 0.00 0.05 0.10
Bcom (nT)
-40 -20 0 20 40 60
Btor (nT)
-10 -5 0 5 10
Bpol (nT)
1412 1423 1434 1445 1456
UNIVERSAL TIME -100
-50 0 50 100
GcLat (deg)
Figure 3.8: Field aligned components magnetic field measurement and latitudinal profile.
knowledge of the Pc3 amplitude cut-off, the latitudes where CHAMP registers these high cur- rents can be estimated. The studies of Junginger and Baumjohann (1988) reports the significant correlations between the spectral power level of magnetospheric pulsation and Kp index. The successful completion of the experiment required knowledge about the magnetic activity in the magnetosphere. Therefore the experiment was carried out for various geomagnetic conditions approximated by Kp index values. The Kp index values used were obtained from INTERMAG- NET station Hermanus Magnetic Observatory (HMO) archive. In selecting events we scanned a number of pole to pole CHAMP trajectories. The events were then grouped in terms of status or conditions of the magnetosphere represented by Kp index values. Statistically for Kp values 1 and 2 significant high currents were noticed at approximately the 780 geocentric latitude.
This was the case for both hemispheres. For Kp = 3 and 4, currents were noticed around the 690 latitude, whereas for 5 and 6 these currents were observed between the 600 and 550 geocentric latitude. Values of Kp higher than 6 were not considered for the experiment because of the possibility of an unstable magnetosphere and a high probability of magnetic substorm occurrence at these Kp index values.
The values established are consistent with other spacecraft observations e.g. Triad (Zmuda et al., 1966). The first in-situ observations (Zmuda et al., 1966) and subsequent studies (Arm- strong and Zmuda, 1973; Iijima and Potemra, 1976a,b; Waters et al., 2001) deduced that the average FAC pattern centers around the 700 magnetic latitude. The classic pattern consists of two rings, region 1 at higher latitude and region 2 at lower latitude with overlap in the pre- midnight region. Figure 3.9 shows the basic pattern of the field-aligned current distribution
Figure 3.9: Distribution of the field-aligned currents in the ionosphere (Iijima and Potemra, 1976b).
(Iijima and Potemra, 1976b) yielded by statistical treatment of magnetic field measurements made on board the Triad spacecraft.
Figure 3.6 summarizes schematically current flows in the magnetosphere responsible for mag- netic activity. The equatorial electrojets over the equator are also responsible for the magnetic activity. The sharp spike in Bpol of Figure 3.8 centered around 14:34 UT when CHAMP was crossing the equator is attributed to the equatorial electrojet current flows.