I. B. Iversen

On the development of a magnetospheric substorm influenced by a storm sudden commencement: Ground, balloon, and satellite observations

Previous statistical investigations have revealed a relationship between storm sudden commencements (ssc) and magnetospheric substorm onsets. Little is known about the physical processes constituting this relationship. We used a comprehensive data set for a detailed case study. The ssc occurred on July 6, 1979, at 1930 UT. The substorm expansion phase started 5 min later. The event was preceded by a loading phase of more than 1.5 hours. The loading phase developed in three steps. During each step the cross-tail current sheet suddenly expanded earthward and intensified. The third step, at 1930 UT, coincided with the ssc. It was very likely caused by the interaction of ions with magnetohydrodynamic waves generated by the ssc. This step was followed at 1935 UT by the onset of the expansion phase accompanied by PiB magnetic pulsations, impulsive electron precipitation, and energetic ion injection at GEOS 2 orbit. The dipolarization of the geomagnetic field started 9 min after the expansion phase onset together with energetic electron injection and a decrease in the energetic ion flux. Signatures of Birkeland currents at dipolarization, and a pressure anisotropy P∥ions > P⊥ions during the 4 min before dipolarization, indicate field-aligned processes consistent with partial diversion of the cross-tail current into the ionosphere. The observations are discussed in the framework of a model in which the ballooning mode instability (BMI) developing in the near-Earth plasma sheet is regarded as the trigger process for the expansion phase. Quantitative estimates of the instability criteria show that the necessary conditions for the BMI are fulfilled during the whole loading phase. An energetic ion pressure gradient, which is needed to drive the BMI, was observed during the interval 1931 UT (ssc) to 1944 UT (dipolarization). Signatures of the instability itself, in terms of regular ion pressure gradient variations, were recorded during the same time interval. We conclude that the magnetosphere was potentially unstable for the BMI when the ssc occurred and that the interaction of the ssc with the magnetospheric particles expanded the cross-tail current sheet further earthward, thereby creating plasma conditions in which the BMI could grow. The instability started at 1931 UT, and the BMI in turn triggered the expansion phase at 1935 UT.

A study of the interaction of VLF waves with equatorial electrons and its relationship to auroral X-rays in the morning sector

Abstract Precipitation of electrons in the morning sector of the auroral zone related to magnetospheric substorm activity has been observed via X-ray measurements from balloons. Coordinated observations within the IMS period both aboard balloons and on the geostationary satellite GEOS-2, provided detailed information for the study of the relation between the distribution function of energetic electrons at GEOS 2 and the precipitation at and near the magnetically conjugate point in the auroral zone. The anisotropy of the electron distribution function is determined by the injection processes and by further modifications, e.g. convection and drift. It is large enough to allow the growth of VLF waves responsible for scattering of electrons into the loss cone.

AURORAL ZONE ELECTRIC FIELD MEASUREMENTS WITH BALLOONS

ABSTRACT Through nearly one solar rotation period (June 20 to July 13, 1975) the horizontal component of the electric field of the polar ionosphere, between 65 and 82 degrees corrected magnetic latitude, has been monitored continuously by balloon borne sensors. 1200 hours of data were obtained by ten balloons uniformly distributed over the period. The measurement reported here refers to relations between the electric field, the ionospheric current systems and the interplanetary magnetic field.

Electric Field Measurements with Balloons

Stratospheric balloons have in the last decade been used for investigating the electric field of the magnetosphere. The idea behind this technique is outlined and problems involved are discussed. A number of examples of measurements related to various subjects are given.

Polar Magnetopause Distance

The distance from Earth to the magnetopause at very high latitudes has been measured on the HEOS-2 satellite. The wave experiment on this satellite measures magnetic and electric signals at five frequencies between 20 and 236 Hz. The transition from the magnetosphere into the sheath region is clearly indicated in these signals. Measurements over one half year show periodic variations in polar magnetopause distance around an average of 14.3 R E. The measurements are compared to calculations of the magnetopause distance that take into account the solar wind pressure and the angle between the solar wind velocity and the Earths magnetic dipole. The periodic variations are predicted reasonably well by these calculations, but an average difference in the distance of approximately 20% exists between calculations and measurements.

On the origin of electron precipitation in the auroral zone related to the onset of magnetospheric substorms

Abstract Observations of electron precipitation via X-ray measurements with balloon-borne instruments and electron measurements onboard the geostationary satellite GEOS-2 are used to analyse the relationship between electron precipitation and magnetospheric processes at the onset of a magnetospheric substorm. The observations are interpreted in terms of electron scattering and different acceleration processes at the outer boundary of the plasma sheet and simultaneous movements of its inner boundary.

Electric fields in the middle atmosphere: A combination of fields mapping from both the troposphere and the magnetospere and fields of internal origin

Abstract Electric fields and currents in the middle atmosphere are important elements of the modern picture of this region. Balloon instruments, reaching the level of the stratosphere, have extensively been used for the experimental work. Our knowledge has in the last two decades been increased concerning, e.g., the upper atmosphere potential, the electric properties of the medium itself and on the coupling with magnetospheric (ionospheric) fields and currents. Also measurements have brought about a discussion of the possible existence of hitherto unknown sources.