Ado. Space See. Vol.5, No.4, pp.127—130, 1985 0273—1177/85 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © COSPAR INTENSE CURRENT STRUCTURES DURING LOW GEOMAGNETIC ACTIVITY AND THEIR RELATION TO SMALL-SCALE MAGNETIC PERTURBATIONS SEEN BY THE INTERCOSMOS BULGARIA-1300 I. Arshinkov, A. Bochev, P. Nenovsky, P. Marinov and L. Todorieva Central Laboratory for Space Research, Bulgarian Academy of Sciences, 6 Moskovska str., Sofia 1000, Bulgaria ABSTRACT This paper deals with the initial observational data on the field- aligned currents (FAG) and the small-scale transverse magnetic perturbations (SSTMD) in the cusp region obtained from the magnetometer on board the Intercosmos Bulgaria—1300 satellite. The magnetic field has been investigated by a high accuracy flux-gate magnetometer, IMAP, designed in the Central Laboratory for Space Research(CLSR), Sofia. For low geomagnetic activity 10 meridional passes (September-December 1981) have been examined. Intense FAG are observed in the noon sector of the summer auroral region. SSTMD are superimposed on a weak cusp current as a perturbation in the prenoon sector of the winter auroral region. INRODUCTION The most detailed systematic geomagnetic investigations of the regions 1 and 2 field-aligned current (FAG) have been made so far by the Triad satellite/i/. Large-scale perturbations poleward of the region 1 in the cusp sector (10—14 MLT) have been also observed /2,3/. The latter are called “cusp currents’ and come relations between their appearance and the interplanetary magnetic fiela (IIVIF), espLcially Its azlmuthMl component (By), are established /5/. Small-scale transver~magnetic disturbances (SSTMD) appearing noleward from ~he re~±0n I nost Srequdntly in the dawn-noon sector are also reported /4/.They are characterized as irregular perturbations with an average amplitude’-~’100 nT and scale variation of about lOs or 80km persisting even during very quiet conditions. The soviet-bulgarian satellite Intercosmos Bulgaria-1300 provides simultaneous geophysical data to investigate the above phenomenon. The ionospheric- magnetospheric experiments on board the satellite are part of the space project Bulgaria-1300 which is a Bulgarian National Space Program. The present paper deals with the initial observational data on cusp currents and SSTMD obtained from the magnetometer during low geomagnetic activity. EXPERIMENT DESCRIPTION The Intercosmos Bulgaria-1300 was launched into nearly circular polar orbit with inclination 81.9° and perigee (apogee) 826 (904) km on August 7,1981. The satellite is three—ax:ea stabilized within 10. Aboard the spacecraft the magnetic field is measured by a three-component flux-gate magnetometer,IMAP, manufactured by a proprietory technology in the CLSR, Sofia. It is a high accuracy instrument with main technical characteristics as follows/6/: dynamic ranget 64000 nT, resolution 2.5 nT, absolute accuracy 0.025%, frequency range 0 - 2.5 Hz.The magnetometer is calibrated in the CLSR and is metrologicaly certified in the D. ‘.Mendeleev Institute, Leningrad. The sampling rate often used is 12.5 or 3.125 vector measurements per second depending on the mode of the telemetry. The data is stored for 0.5 or i.5h respectively in the onboard ment~ay. The data which has been initially recGd-ved refer to the satellite magnetometer axes (fig.1). The F component is nearly in E-W direction e.i. parallel to the magnetic field of the current sheeto. It is therefore used for an express detection of FAG. The Earth’ s main magnetic field is calculated and substracted from the measured vector values. 127