In situ ionospheric current measurements during ROSE rocket flights: evidence for tilted current layers

Journal of Atmospheric and Terrestrial

Physics,

Printedin Gr& Britain.

Vol.54.

No. 6, pp. 725-731,

OU21-9109/92 $5.00+.oO

1992.

Pergamon

Press Ltd

In situ ionospheric current measurements during ROSE rocket flights : evidence for tilted current layers H. Institut

fiir Geophysik

LihR

und Meteorologie der Technischen Universitgt, 3300 Braunschweig, Germany (Received injnalform

Mendelssohnstrasse

3,

6 May 1991)

Abstract-Data recorded by the DC magnetometer on board the ROSE payloads launched into regions of moderate substorm activity have been interpreted in terms of current densities. Adopting generally accepted assumptions on the geometry of the currents, vertical profiles of the Hall current density could be computed. Systematic differences between the results of the upleg and downleg suggest that the assumption of a horizontal current sheet might not be applicable. Reasonable agreement between the observations of both legs is achieved if the Hall current layer is considered to be perpendicular to the ambient geomagnetic field lines.

instabilities in the ionosphere (ROSE et al., 1991, this issue). A triaxial fluxgate magnetometer was flow as part of the scientific payload to peak altitudes of about 120 km. One of the objectives of the instrument was to determine vertical profiles of the Hall current density. There has never before been published current density estimates of this layer derived from vector magnetic measurements. In Section 2 a short description of the relevant instruments is given. In Section 3 the equations needed to calculate the current density are introduced. A rather detailed description of the observed fields and derived current densities is given in Section 4. Subsequently the observations are discussed and a new picture of the current configuration is presented.

1. INTRODUCTION

The ionospheric currents in the aurora1 zone are an important part of the magnetospheric-ionospheric current system. Whenever the currenthensities exceed a certain threshold, they become unstable and plasma instabilities such as the Modified Two-stream Instability are excited. For the understanding of many of the plasma processes the knowledge of the current density and spatial distribution is of interest. Unfortunately, up to this day there is no way to directly measure the current density in the ionosphere. All indirect methods have to make assumptions or use model estimates. Current estimates can, for example, be obtained by the EISCAT facility. One way is to measure height profiles of the electron density and convert them to conductivity profiles with the help of atmospheric models. By multiplying them with Fregion electric fields one can obtain the Hall and Pedersen current densities. Another measurement technique is to fly a magnetometer on board a rocket through the current sheet. Measurements of this kind have previously been reported by PRIMDAHL et al. (1974, 1979, 1984 and references therein). In all cases the magnetometers monitored the change in the field magnitude when traversing the current sheet. The authors had to employ magnetic field models to obtain the nominal field distribution, and in addition they had to make assumptions on the geometry of the currents to obtain estimates of the current densities. In this paper we will present measurements made during the Rocket and Scatter Experiments (ROSE) campaign. The prime objective of the ROSE project was the investigation of the current driven plasma

2. INSTRUMENTATION

The instrument, whose data are reported here, is a triaxial fluxgate magnetometer. A rather detailed description of the magnetometer has been given by L&m and OELSCHL~~GEL(1990). Special features of the instrument are the high resolution combined with fast sampling. The dynamic range of each component covered f 55,000 nT. The field readings were digitized to 16 bit accuracy providing a resolution of 1.7 nT/step. The magnetometer was operated at a rate of 500 vector samples/second, and the bandwidth of the instrument allowed the detection of waves with frequencies beyond 200 Hz. A very crucial quantity for the interpretation of the returned vector magnetic field measurements is the payload attitude. For the reconstruction of the attitude history, data from a star sensor provided 725

H.

126

by INIK, Lulea, Sweden, have been evaluated (SCHMIDTBAUER, 1978). The operational principle of this instrument is that of a slit sensor. Whenever a sufficiently bright star crosses the field of view a light pulse is detected. At the peak of the pulse the payload clock is sampled and the detection time transmitted to the ground. From the series of detection times first the stars observed have to be identified and subsequently the motion of the payload can be reconstructed. This attitude determination system has been proven to be very reliable on payloads where the change rate of the attitude angles is low. In the case of the ROSE project, however, with apogees as low as 120 km, random disturbances caused by winds badly influenced the data evaluation. Only by a combination of magnetometer and star sensor data, as has been described by MUSCHINSKIand L~~HR (1989), could a sufficiently accurate attitude be obtained for all the flights.

3. CURRENTDENSITY

DERIVED

FROM AMPERE’SLAW

It is not possible in general to determine the current distribution from a single site magnetic field measurement. If one wants to derive an estimate of the current density from field observations, assumptions, for example on the geometry of the currents have to be made. According to the given rocket trajectory (up to 120 km) Hall and Pedersen currents will be most important. Both these currents are commonly approximated by horizontal sheet currents. The basic relation for the evaluation of the current density is Ampere’s law curl B = pOj

(1)

where j is the current density, p0 the permeability and B the magnetic field produced by the current. Solving equation (1) for j gives for the two current components in the horizontal plane :

Unfo~unately, none of the spatial gradients of the magnetic field has been measured directly. The quantities available are d@/dt and the velocity u. The temporal derivative of the x component, for example, can be written as follows : dB,

dt -

as,

as,

at -I-~pi-

aB* a3, ayup+ aZvz.

(3)

LOHR

Only the last term on the right hand side is needed for equation (2); all the others are expected to be significantly smaller. Similar assumptions have also to be made for the other field components. In practice this means that, if the current density has no spatial gradients in the horizontal plane, only in the vertical direction and does not show temporai variations, j can be computed as follows :

Problems are expected for cases where the velocity components are small. This is particularly true for v, around apogee. Also vY is rather small due to the northward direction of the trajectory. 4. O~ERVA~ONS Magnetic field measurements presented here have been obtained in the lower ionosphere. The first two rockets of the ROSE project were launched from Andenes, Norway. They reached peak altitudes of 115 km and had a ground range of about 60 km for heights above 100 km. The two other rockets launched from ESRange, Sweden went up to about 125 km, but only had a range of 30 km above 100 km. For all four flights high resolution magnetic field data could be sampled. For the purpose of current determination the deviations of the observed field from the main field are important. To achieve this information the magnetometer raw data have to go through several processing steps : -Remove instrumental effects and compensate for payload disturbances. -Transform data from payload system into geographic system. -Subtract geomagnetic reference field (IGRF) from the data. A result of this procedure is shown in Fig. 1. Displayed are the three components of the magnetic field vs flight time for flight Fl . The directions of the components are as commonly used : x : northward, y : eastward, and z: vertically down. A rather prominent feature on all three components is a sinusoidal signal. It is the result of an imperfect description of the coning motion from the attitude data. The coning is the parameter which is determined least accurately by the star sensor.

In silu ionospheric

current

measurements

during

ROSE rocket flights

-270 Flight

r c

180

90

L x

,Sl

Fig. 2. Observed field differences during the flight Fl for the north, x and vertical, 2 component. The data have been averaged over one coning rotation period. Field variations are believed to be due to ionospheric currents. The steep gradients in x reflect the crossing of the Hall current layer.

Fl ‘; 2 x

time

0

component can be used to decide whether we are north or south of the current centre. The large field changes during flight F3 suggest that the Hall currents were strongest then.

-90

t” 60

120

160

200

240 Flight

time

Is1

Fig. 1. Differences between the measured magnetic field during flight Fl and the reference field (IGRF). The harmonic oscillations are residuals of the coning motion and are due to imperfect attitude data. The first and last 10 s of the flight data are also considered to be not too reliable.

To overcome this problem the field data have been averaged over one coning period. The first and the last ten seconds of the graphs in Fig. 1 are not too reliable, again because of attitude problems at these low altitudes. Figures 2-5 show the averaged and truncated magnetic field data obtained during the four flights. Only the x and z components are shown, since we will discuss solely the Hall current, ,j,, in this paper and therefore the y-component is according to equation (4) of no concern. In all cases the x component shows strong gradients being indicative for the crossing of the Hall current sheet during up and downleg. The z component varies much less, and the sign of this

Fig. 3. The same as Fig. 2, but for flight F2.

H. LUHR

128

Figure 6 shows a stacked plot of the Hall current densities computed according to equation (4). Results in the vicinity of the apogee have been omitted, since they gave unrealistic values. During all flights except

Fig. 4.

for Fl westward electrojets have been observed, which is consistent with the results of the electric field measurements (KOHL and RINNERT, 199 1, this issue). The curves of the current density are rather spiky.

The same as Fig. 2, but for flight F3.

5 5-12 .” ? ., -18 E ::: :

_

c-

-24 , -301

80

120

160

200

240 Flight

-90

I 60

120

160

200

240 Flight

tilne

Fig. 5. The same as Fig. 2, but for flight F4.

15

0 t,“e

[sl

Fig. 6. Estimates of the Hall current density for all four flights. Around apogee no reliable results can be obtained. During all flights except Fl we have encountered westward electrojets.

129

In situ ionospheric current measurements during ROSE rocket flights This is due to the differentiation of the field readings. A small temporal variation or spatial gradient produces a sizeable spike in the current density. In spite of the roughness of the curve there is a systematic difference between the upleg and the downleg. Computed current densities are lower on the upleg than on the downleg in all cases. Figure 7 shows in a somewhat different presentation the current density versus altitude, for the upleg in the upper part and/or the downleg in the lower part. The height range of the Hall currents varies only by a few kilometers from event to event. The lowest is F 1 with a centre height of 106 km and the highest is F4 with 112 km. The trend already noted in Fig. 6 is even more clear in Fig. 7. On the upleg the peak current densities range around 20 ,uA/m’, while on the downleg they are more around 30 PA/m*. On the other hand the thickness of the sheet is clearly larger on the upleg

120

110 z d > z d

100

i

90

120

F 30 1 2 1 C 2

100

such that similar height integrated current densities evolve for both legs. These differences are rather surprising since the sampled regions are separated by only a few tens of kilometers. 5. DISCUSSIONS

In the previous sections it has been shown that it was possible to obtain Hall current density profiles with the vector magnetometer flown on board the ROSE rockets. A comparison with the observations from the other experiments reveals that the regions of enhanced plasma fluctuations coincide well with the location of the Hall current sheet (ROSE, 1991 ; RINNERT, 1991; SCHLEGEL, 1991; all this issue). It was not possible within this project to provide estimates of the Pedersen current. The reason for this can be read from equation (4). Since the Pedersen current flows at higher altitudes than the Hall current, the rockets should have gone higher up. The apogee of the trajectories was, at best, half way through the Pedersen sheet. No reliable values for j, can be obtained for flight segments where both the velocity components aYand vZare small. One feature that is not too satisfying is the asymmetry of the derived current densities between the upleg and the downleg. A reason could be that one of the assumptions adopted for calculating the currents is not valid. It has been assumed that the current sheet is horizontal. If we go away from this picture and allow for a certain tilt, we no longer can take the velocity component v, in equation (4), but rather have to use the component perpendicular to the sheet. For a tilt in the north-south direction the perpendicular velocity, up, would be-in contrast to v,different on the upleg and the downleg at the same altitudes. A reasonable assumption for the tilt might be that the current sheet is perpendicular to the ambient magnetic field. The inclination of the field lines in this region is about 12’. Such a tilt would favourably explain the observed differences. In Table 1 corrected and uncorrected current densities have been put side by side for

,”

Table

90

1. Comparison

Altitude Flight

(km)

Fl

106

F4

112

Fig. 7. Hall current

density estimates vs altitude, (top) for upleg, (bottom) for downleg. Current densities are systematically lower on the upleg than on the downleg. The estimated thickness of the layer, on the other hand is larger for the former.

between corrected current densities Current _~~

uncorrected

density (PA mm ‘) .~~..~

Uncorrected 19 32 20 26

and

Corrected 23.5 26 22 24

Leg UP Down UP Down

H.

730

,/*-*\ 1M

-

l\_._._./.

/.

/*

1 zcm,

P\*$JZ .-.-.-.

c 100 " ;

0

-

~l~~py~~ 4

I south-

_:

north

The Hall Fig. 8. Sketch of the proposed current geometry current is made up of a series ot- tilted strips being per-

pendicular to the ambient magnetic field. Overlying this is a typical trajectory of a ROSE rocket. The top trace shows the resulting

field variations in the veriical component track at 120 km altitude.

along a

the flights Fl and F4. The chosen altitude corresponds to the centre height of the current sheet. The differences in the current density do not vanish completely but they have become considerably smaller. Also the apparent difference in the thickness of the current layer could be explained by the proposed geometry. A problem arising with the picture of a tilted Hall current layer is that the current sheet in the north would go to higher and higher altitudes and eventually leave the range of Hall conductivity. It would also be expected that the northbound ROSE payloads would encounter the Hall current sheet at a lower altitude on the upleg than on the downleg, which has not been observed. To overcome this problem we might assume that the Hall current is made up of many individual tilted current strips. In order to test this idea a simple model calculation has been made. The basic geometry is a series of plane sheet currents, infinitely long, tilted by 12” and 50 km wide. Figure 8 shows a cross-section of the current configuration and a typical rocket trajectory. A good indicator for the structure of the current is the vertical component of the magnetic field, B,, since it varies only little with altitude. Using a sheet current density J = 0.5 A/m flowing westward, the magnetic field along a north-south profile at an altitude of 120 km has been computed. In the upper part of Fig. 8 the resulting B, component has been plotted. It shows periodic variations with the same wavelength as the width of the current sheet. An inspection of the observations shown in Figs 2-5 reveals that the z components exhibit similar undulations in all cases. This similarity is of course no unambiguous proof for the current configuration considered but it is another piece of evidence. With the help of the trajectory data we can scale the wavelength of the observed undu-

LfJHR

lations. For the flight F 1 we calculate 45 km and for F2, F3, F4 values around 20 km appear. Hall current strips as narrow as this are not in conflict with the concerns mentioned before. The idea of Hall current sheets perpendicular to the magnetic field would explain nicely the pronounced peak in STARE backscatter power at O”-aspect angle (NIELSEN, 1988). Since the plasma instability responsible for the backscatter is confined to the Hall current sheet, maximum power is expected when the beam traverses the layer parallel to the sheet. A question that could be raised is: why has this structuring not been observed before? It is the low trajectory of the ROSE rockets which is in favour with the observations. We do not know of any vector magnetic field measurements from a similar trajectory, which have been interpreted in terms of current densities. Rockets with higher apogees go almost vertically through the E-region and the angular difference between the upleg and the downleg are rather small. Systems like EISCAT are also not able to detect the tilt of the current layers. It would, of course, be highly desirable to have an independent confirmation of the tilting and filamentation of the Hall current. A suitable rocket trajectory should have a large ground range and a peak altitude of about 150 km. An ideal probe, however, would be a tethered satellite at E-region altitudes circling the Earth in a polar orbit.

6. CONCLUSIONS

Vertical current density profiles of the Hall current have been obtained by evaluation of the DC-magnetometer data sampled during the ROSE rocket flights. Somewhat surprising is a systematic difference of the profiles between up and down legs. In all four cases the current densities are lower and the sheets are thicker on the upleg than on the downleg. These apparent differences can be explained by a tilted hall current sheet. An orientation of the current sheet perpendicular the ambient geomagnetic field is consistent with the observations. The data of the vertical magnetic field component provide evidence that the northsouth extension of the individual Hall current strips ranges between 20 and 50 km. Such a filamentation of the Hall current requires an adequately structured Pedersen current and might therefore be an indication for the narrow scale of individual field-aligned current circuits. Acknowledgements-The author thanks W. OelschlSigel and H. Plattner for designing, fabricating and testing the mag-

In situ ionospheric

current

measurements

netic field instruments. The author also thanks P. Hertz for his support and competent advice during the magnetic test at IABG. The author would like to give special credit to C. Willecke for her dedicated efforts in evaluating the data and

during

ROSE rocket flights

731

to H.-M. Mauer and H.-P. Brunke for analysing the attitude data. The ROSE project has been supported by grants from the Bundesministerium ftir Forschung und Technologie under contract # 01 OM 8605.

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1992 1990

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1989 1988 1984

J. atmos. terr. Phys. 54, 733. Report MPAE-W-40-90-18, Max-Planck-Institut Aeronomie, Lindau, pp. 102. ESA SP-291, Europ. Rock. Ball. Progr., 111. J. geophys. Res. 93,4119. Planet. Space Sci. 32, 561.

1974 1979

J. geophys. Res. 79,4262. J. geophys. Res. 84, 6458.

1992 1992 1992

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fiir