S. A. Romanov

Comprehensive study of the magnetospheric response to a hot flow anomaly

We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (∼7 min) move outward some 5 RE beyond its nominal position and engulf Interball-1. A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in high-latitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

Two spacecraft observations of a reconnection pulse during an auroral breakup

At 1130 UT on November 28, 1995, two spacecraft, Interball-Tail and Geotail, were in a favorable position to study the plasma sheet activity and an auroral breakup observed on the ground near the spacecraft ionospheric footpoints. Both spacecraft were near the neutral sheet, and they were nearly aligned along the magnetic meridian. During the auroral breakup observed at the equatorward half of the auroral oval (also registered as an AKR burst at Interball) both spacecraft simultaneously detected signatures of a reconnection pulse: The earthward plasma streaming and magnetic field dipolarization were observed at 12 R E at Interball, while the tailward energetic ion beam, then the tailward flow and the passage of a plasmoid were observed at 28 R E at Geotail. This pulse seem to proceed inside of the plasma sheet closed field lines, in the region of small (∼ 1nT) background magnetic field at the neutral sheet. At Interball position the onset of fast earthward ion flow, likely initiated by the reconnection pulse, was followed by other manifestations (dipolarization, enhancements of the magnetic turbulence and the energetic particle flux, the intensification of field-aligned currents). Auroral observations showed initial brightening delayed an approximately 1 min after the commencement of the reconnection pulse. The auroral intensification was not accompanied by a significant magnetic disturbance on the ground, and therefore the event can be classified as the pseudobreakup. We estimate magnetic flux transport characteristics and possible location of the onset region in the plasma sheet. We conclude that observations during this event are consistent with the initiation of an auroral breakup by some disturbance (e.g., Alfven wave) generated by the reconnection pulse that commenced in the neutral sheet at ∼15 R E distance.

Gross deformation of the dayside magnetopause

On July 24, 1996, the entire magnetosheath moved past the Interball-1 spacecraft twice within 7 min. Such rapid transits through the magnetosheath suggest the passage of an antisunward-moving boundary wave with an amplitude in excess of 5 RE. A wave of this amplitude requires an order of magnitude decrease in the pressure confining the magnetosphere. Such decreases have been reported only in conjunction with hot flow anomalies, features accompanying interplanetary magnetic field discontinuities, bounded by narrow shocks, and identified on the basis of heated deflected flows transverse to the Earth-Sun line. We identify all these features just outside the Interball-1 magnetopause crossings on July 24, 1996.

High energy jets in the Earth’s magnetosheath: Implications for plasma dynamics and anomalous transport

High energy density jets in the magnetosheath near the Earth magnetopause were observed by Interball-1 [1]. In this paper, we continue the investigation of this important physical phenomenon. New data provided by Cluster show that the magnetosheath kinetic energy density during more than one hour exhibits an average level and a series of peaks far exceeding the kinetic energy density in the undisturbed solar wind. This is a surprising finding because the kinetic energy of the upstream solar wind in equilibrium should be significantly diminished downstream in the magnetosheath due to plasma braking and thermalization at the bow shock. We suggest resolving the energy conservation problem by the fact that the nonequilibrium jets appear to be locally superimposed on the background equilibrium magnetosheath, and, thus, the energy balance should be settled globally on the spatial scales of the entire dayside magnetosheath. We show that both the Cluster and Interball jets are accompanied by plasma superdiffusion and suggest that they are important for the energy dissipation and plasma transport. The character of the jet-related turbulence strongly differs from that of known standard cascade models. We infer that these jets may represent the phenomenon of the general physical occurrence observed in other natural systems, such as heliosphere, astrophysical, and fusion plasmas [2–10].

Substorm‐associated pressure variations in the magnetotail plasma sheet and lobe

Simultaneous pressure measurements by Interball-Tail in the high-latitude lobe and by Geotail in the equatorial plasma sheet were analyzed for 30 substorms which exhibited significant pressure changes. At the onset of a few substorms we observed equatorial pressure peaks with magnitudes up to 50% higher than those in the lobe. These pileups are probably rather localized, and their properties are consistent with plasma sheet thickening between two active regions in the tail. During expansion and recovery phases of more than half of substorms, we observed equatorial pressure depletions relative to the high-latitude lobe pressure. These depletions can last more than 2 hours and are likely formed during the substorm expansion phase near the equatorial plane behind (tailward of) the strongly dipolar near-Earth magnetotail region. The observed pressure gradient is probably a nonstationary feature and can be compensated partially by magnetic tension on the curved field lines. Magnitude and history of the solar wind dynamic pressure appear to significantly influence substorm scenarios in the magnetotail. Possible existence of the pressure difference should be taken into account in single-spacecraft substorm studies.

The cusp/magnetosheath interface on May 29, 1996: Interball‐1 and Polar observations

On May 29, 1996, under steady strong northward IMF and high solar wind dynamic pressure conditions both Polar and Interball cross field lines that pass through the northern cusp and apparently close to the post-cusp reconnection site. The magnetopause current observed by Interball consists of two quite distinct layers, an inner broad current that is quite turbulent and another current that is quite abrupt and quiet. Polar also crosses current layers, similar to the Interball inner one. These observations support a model in which cusp field lines experience essentially stochastic behavior but on average provide topological connection between the cusp and magnetosheath.

Turbulent Boundary Layer at the Border of Geomagnetic Trap

A new phenomenon was discovered on the basis of analysis of the Interball project data. A hot plasma flow is thermalized through the formation of “long-operating” vortex streets and local discontinuities and solitons in a distributed region over polar cusps. Plasma percolation through the structured boundary and secondary reconnection of fluctuating magnetic fields in a high-latitude turbulent boundary layer account for the main part of solar wind plasma inflow into the magnetospheric trap. Unlike local shocks, the ion thermalization is accompanied by the generation of coherent Alfvén waves on the scales ranging from ion gyroradius to the radius of curvature of the averaged magnetic field, as well as by the generation of diamagnetic bubbles with a demagnetized heated plasma inside. This “boiling” plasma has a frequency region where the spectrum is different from the Kolmogorov law (with slopes 1.2 and 2.4 instead of 5/3 or 3/2). The fluctuation self-organization in the boundary layer (synchronization of three-wave decays) was observed on certain frequency scales.

Anomalous interaction of a plasma flow with the boundary layers of a geomagnetic trap

Using the data from the Interball-1, GEOTAIL, THEMIS and CLUSTER satellites, we propose a mechanism of anomalous magnetosheath dynamics. This mechanism yields that plasma boundaries can be locally deformed over distances comparable to its thickness. In particular, the magnetospheric boundary, the magnetopause, is deformed over distances up to a few Earth radii (RE) under the pressure of supermagnetosonic plasma streams (SPSs), instead of reacting to plasma pressure decreases, as it was previously thought. Supermagnetosonic plasma streams having a kinetic pressure a few times larger than the solar wind pressure and the magnetic pressure behind the magnetopause, can crush the magnetopause and even push it outside the mean bow shock position, as determined through the average pressures balance. Anomalous magnetosheath dynamics is initiated by plasma flow anomalies (FAs), triggered by rotational discontinuities, by jumps in the solar wind pressure and by interplanetary shocks, which all interact with the bow shock. We show that the generation mechanism for SPSs, adjacent to the FA, is connected with the compensation of the FA flow reduction by the SPS enhanced flow, which is produced by polarization electric fields at the FA edges. Statistically, SPSs are extreme events, relayed with intermittency and multifractality inside the boundary layers of the geomagnetic trap. In this way, SPSs provide “long-range” interactions between global and microscales. A similar role may be played by fast concentrated flows in the geomagnetic tail, in fusion devices, in astrophysical plasmas and in hydrodynamics.

Dynamic Interaction of Plasma Flow with the Hot Boundary Layer of a Geomagnetic Trap

The study of the interaction between collisionless plasma flow and stagnant plasma revealed the presence of an outer boundary layer at the border of a geomagnetic trap, where the super-Alfvén subsonic laminar flow changes over to the dynamic regime characterized by the formation of accelerated magnetosonic jets and decelerated Alfvén flows with characteristic relaxation times of 10–20 min. The nonlinear interaction of fluctuations in the initial flow with the waves reflected from an obstacle explains the observed flow chaotization. The Cherenkov resonance of the magnetosonic jet with the fluctuation beats between the boundary layer and the incoming flow is the possible mechanism of its formation. In the flow reference system, the incoming particles are accelerated by the electric fields at the border of boundary layer that arise self-consistently as a result of the preceding wave-particle interactions; the inertial drift of the incoming ions in a transverse electric field increasing toward the border explains quantitatively the observed ion acceleration. The magnetosonic jets may carry away downstream up to a half of the unperturbed flow momentum, and their dynamic pressure is an order of magnitude higher than the magnetic pressure at the obstacle border. The appearance of nonequilibrium jets and the boundary-layer fluctuations are synchronized by the magnetosonic oscillations of the incoming flow at frequencies of 1–2 mHz.

Long periods of the ULF wave activity in the Earth's magnetotail lobes

Abstract A closer look at the temporal evolution of the wave activity in the lobes of the Earths magnetotail seems to suggest a long periodic modulation of the wave intensity. In order to test this suggestion we analyzed time series of the ULF electric field fluctuations in the northern lobes of the Earths magnetotail, measured on board Prognoz-8. As a result we find that the wave activity seems to be modulated with a basic period of about 68 minutes and harmonics near 34, 21 and 16.5 minutes. We speculate that this periodicity is due to a global effect of the three-dimensional Earths magnetosphere.