Nickolay Ivchenko

Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere

The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted, but how they could be maintained is still a matter for debate. The spatial and temporal behaviour of the electric fields—a knowledge of which is crucial to an understanding of their nature—cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200 s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.

Quasiperiodic oscillations observed at the edge of an auroral arc by auroral turbulence 2

The Auroral Turbulence II (AT2) sounding rocket carried three payloads into the auroral ionosphere where they crossed several arc structures. At the border of an auroral arc a quasiperiodic structure was observed by the magnetic and electric field instruments as well as by the particle detectors. The variations were temporal oscillations, but existed only in a narrow (≈ 7 km) region transverse to the arc, with a correlation length along the arc of at least several km. The relation between the electric and magnetic field amplitude indicates the Alfvenic nature of the variations. Field aligned electron precipitation is correlated to the field variations. The narrow band nature of the oscillations and frequency around 0.6 Hz is consistent with waves confined in the ionospheric Alfven resonator.

Miniaturized digital fluxgate magnetometer for small spacecraft applications

A novel design of an Earth field digital fluxgate magnetometer is presented, the small magnetometer in low-mass experiment (SMILE). The combination of a number of new techniques results in significant miniaturization of both sensor and electronics. The design uses a sensor with volume compensation, combining three dual rod cores in a Macor® cube with the side dimension of 20 mm. Use of volume compensation provides high geometrical stability of the axes and improved performance compared to component compensated sensors. The sensor is operated at an excitation frequency of 8 kHz. Most of the instrument functionality is combined in a digital signal processing core, implemented in a field programmable gate array (FPGA). The pick-up signal is digitized after amplification and filtering, and values of compensation currents for each of the axes are determined by a digital correlation algorithm, equivalent to a matched filter, and are fed to a hybrid pulse-width modulation/delta-sigma digital-to-analogue converter driving the currents through the compensation coils. Using digital design makes the instrument very flexible, reduces power consumption and opens possibilities for the customization of the operation modes. The current implementation of the design is based on commercial off-the-shelf components. A calibration of the SMILE instrument was carried out at the Nurmijarvi Geophysical Observatory, showing high linearity (within 6 nT on the whole ±50 µT scale), good orthogonality (22 arcmin) and very good temperature stability of the axes.

Multiple‐point electron measurements in a nightside auroral arc: Auroral turbulence II particle observations

We report here three-point measurements of bursty, velocity-dispersed, field-aligned electron precipitation at the poleward edge of a northward-moving, post-breakup, nightside auroral arc. The three-point measurement allows detection of the proper motion of the inverted-V arc, which is shown to be 550 m/sec northward. The velocity dispersion patterns are fitted to find the source altitude of the precipitation bursts as a function of distance from the poleward edge of the arc. These source points are interpreted to trace out the low-altitude boundary of the inverted-V potential drop, which is seen to rise both in time, and in the northward direction. The precipitation bursts under the inverted-V are seen to have an arc-aligned velocity which varies with time, and which is consistent with the measured E × B local drift speed.

Inhomogeneous transverse electric fields and wave generation in the auroral region: A statistical study

We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region ...

Using multispectral optical observations to identify the acceleration mechanism responsible for flickering aurora

We present an analysis of flickering (2-10 Hz) auroras observed with a state-of-the-art multispectral imaging system, Auroral Structure and Kinetics, located in Tromso, Norway. Short (1-2 s) period ...

A statistical study of the magnetosphere boundary crossings by the Geotail satellite

Geotail magnetometer observations from 1995-97 were used to identify 1369 dayside magnetospheric boundary crossings. Statistical properties of this set of multiple crossings are presented and discussed in terms of models for magnetopause motion. Our results indicate that equatorial boundary crossings are caused by north-south aligned ridges propagating azimuthally around the magnetosphere. Perpendicular orientations of the magnetospheric and magnetosheath magnetic fields inhibit the boundary motion. For the garden hose orientations of the interplanetary magnetic field (IMF), magnetospheric boundary motion is favoured in the dawn sector, while orthospiral orientations favour motion in the dusk sector. These results suggest that pressure pulses generated at the quasi-parallel bow shock drive the boundary motion.

Multispectral observations of auroral rays and curls

Two cases of discrete aurora are presented, in which auroral curls and auroral rays, respectively, were seen. The aurora was imaged by two spatially separated imagers with a long-pass filter ( main ...

Constraints on an exosphere at Ceres from Hubble Space Telescope observations

We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived br ...

Coexisting structures from high‐ and low‐energy precipitation in fine‐scale aurora

High resolution multi-monochromatic measurements of auroral emissions have revealed the first optical evidence of coexisting small-scale auroral features resulting from separate high and low energy populations of precipitating electrons on the same field line. The features exhibit completely separate motion and morphology. From emission ratios and ion chemistry modeling, the average energy and energy flux of the precipitation is estimated. The high energy precipitation is found to form large pulsating patches of 0.1 Hz with a 3 Hz modulation, and non-pulsating co-existing discrete auroral filaments. The low energy precipitation is observed simultaneously on the same field line as discrete filaments with no pulsation. The simultaneous structures do not interact, and they drift with different speeds in different directions. We suggest that the high and low energy electron populations are accelerated by separate mechanisms, at different distances from earth. The small scale structures could be caused by local instabilities above the ionosphere.