B. Nikutowski

Force-free magnetic field extrapolation for MHD boundary conditions in simulations of the solar atmosphere

Context. In recent years the accuracy of magnetic field observations in the solar atmosphere has made considerable progress. Similar progress is being made in computer modeling of complex plasma systems and computer capabilities. Aims. To consider observed solar magnetic field structure in numerical simulations a new extrapolation method for solar magnetic fields is used to incorporate such fields into three-dimensional MHD simulations Methods. The simulation employs a new modified linear magnetic field extrapolation which is specifically designed to satisfy symmetry conditions which are generic to MHD models. The model domain includes photosphere, chromosphere, and corona. Results. The new model is applied to several solar field configurations and results are compared to three-dimensional field structure from observations and another extrapolation method. The new model provides a simple and efficient method for the simulation of observed solar magnetic field structures by constructing a three-dimensional initial field that is consistent with symmetry boundary conditions of MHD simulations.

Equator‐S Observations of Boundary Signatures: FTE's or Kelvin‐Helmholtz Waves?

The observation of a bipolar variation of the magnetic field component normal to the magnetopause is usually interpreted in terms of magnetic flux transfer events, FTEs. Our MHD simulations indicate that such signatures can also be explained by the Kelvin-Helmholtz instability (KHI). We have analyzed Equator-S data on March 10, 1998 at the dawnside magnetospheric flank and compared our 2D MHD simulation results with these observations. Equator-S encountered strongly perturbed magnetic signatures and very thin magnetic boundaries with brief intervals satisfying the Walen relation. The de Hoffmann-Teller (HT) velocities for these brief intervals were much larger than that of the overal 36 min period.

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.

EQUATOR-S OBSERVATION OF RECONNECTION COUPLED TO SURFACE WAVES

Abstract In the morning sector of its near equatorial orbit Equator-S observed large magnetic oscillations in the Pc 5 frequency range. The poloidal part of the observed oscillations is usually interpreted as being caused by surface waves of the magnetopause driven by a Kelvin-Helmholtz instability (KHI). We found, however, that these oscillations are sometimes additionally superposed with reconnection signatures. This occurs during periods of northward interplanetary magnetic field. In fact, the Walen relation as a test for reconnection is satisfied for many short periods of time. This indicates transient and thin layers of reconnection, embedded in the large scale motion. We compared our observations with appropriate magnetohydrodynamic simulations allowing reconnection in the case of shear flows. Comparing the simulation results with the magnetic field and plasma observations we found good agreement of simulations and observations. We conclude that Equator-S has observed reconnection embedded in Kelvin-Helmholtz instability generated vortices in its nonlinear phase.

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.

INTERBALL magnetotail boundary case studies

Abstract We present two examples of INTERBALL-1 data near both the high and low-latitude tail magnetopause (MP) under disturbed conditions. For the high-latitude case, MAGION-4 data determine the scales of the MP current sheets which are in the order of 100–500 km for the main ones, 50–200 km for Flux Transfer Events (FTEs) and a few km for the fine structures and ULF turbulence. The MP speed was 15–30 km/s. The energetic protons in the magnetosheath (MSH) provide evidence of reconnection upstream of the spacecraft (S/C). The tailward flows grow for the northward MSH magnetic field when the reconnection site is believed to be shifted tailward of the cusp. The inner boundary layer (BL) after the disturbance consists of tailward and earthward flowing plasma of MSH origin and cold mantle plasma flowing tailward The earthward flow is evidence of reconnection tailward of the S/C, which is regarded as a specific feature of the disturbed conditions. Local production of a plasma-sheet-like plasma at high latitudes is argued based on the inner BL plasma characteristics. The following features are observed in both cases: (a) FTEs for both northward and southward MSH fields; (b) waves in the current sheet vicinities over ten mV/m and 15 nT peak-to-peak; (c) electron fluxes with scales down to a few km with extra heating especially parallel to the magnetic field; (d) outer turbulent boundary layers with a deflected magnetic field; (e) ions with time-energy dispersion-like features and deflected ion fluxes. In the downstream dawn region at the transition between the low-latitude boundary layer and the plasma sheet (LLBL/PS), multiple MP encounters are observed. In the LLBL parallel electron intensifications correlate with ULF magnetic fluctuations.

Accelerated particles from turbulent boundary layer

Abstract We study the high latitude turbulent boundary layer (TBL), where the essential magnetosheath (MSH) ion heating (in 1.5–3 times) is seen in more than 80 % cases within ‘diamagnetic bubbles’ -structures with highly reduced magnetic field. The high- energy tails of the core particle distributions in TBL exceed 20keV for ions and 10keV for electrons on April 21, 1996. We find indications also for further acceleration of a small fraction of protons there up to 300 keV. More energetic particles could have maximum intensity just inside magnetopause that is consistent with their magnetospheric origin. Simultaneous Polar and Interball-1 data in TBL and MSH on May 29, 1996 demonstrate heating of the MSH core ions in cusp, TBL and stagnant MSH. In the TBL the O + ions might be heated as well (up to few keV) and even be backscattered towards the ionosphere together with the protons > 18 keV . The dominant incompressible Alfvenic waves in TBL constitute an intermediate chain for the transformation of the solar wind kinetic energy into the auroral particle acceleration/heating. The most intensive energy transformation occurs over cusps and along the boundary between the mantle and low latitude boundary layer down to X = − 6 R E . It provides primary heating for MSH ions in both cusp proper and boundary cusp.

First ELF wave measurements with the Equator-S magnetometer

Abstract The magnetometer onboard the Equator-S satellite is very sensitive and has a high sampling rate of up to 128 Hz. These specifications allow for the first fluxgate magnetometer measurements of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dayside magnetosheath. The so-called lion roars, typically seen by the Equator-S magnetometer at the bottom of the magnetic troughs of magnetosheath mirror waves, are near-monochromatic packets of electron whistler waves lasting for 0.2–1 sec. They are right-hand circularly polarized shear waves with typical amplitudes of 0.5–1 nT at frequencies of 15–40 Hz, i.e., around one tenth of the electron gyrofrequency.

Interball Observations Of Field Aligned Current Signatures Due To Collisionless Reconnection

We use INTERBALL-1 observations in the Earth magnetotail to search for magnetic field signatures of field aligned currents, which are expected to be generated by collisionless reconnection. The occurrence of such currents has been demonstrated by means of kinetic plasma simulations (Buchner et al., 1998b). We address their magnetic signatures using tail data of INTERBALL-1 obtained in 1996. We have found the predicted B y excursions near reconnection sites in the tail current sheet. The typical B y variations can be seen all over in the tail between X GSM = -12R E and +24R E , between Y GSM = -11R E and +12R E . Here in this paper we present two examples, one near the midnight meridian and one in the dusk tail, the first close to the current sheet midplane and the second at some distance from it.

A high-latitude boundary layer crossing-INTERBALL measurements and MHD model results

Abstract We analyze the highly variable magnetic field behavior after a high-latitude outbound boundary layer crossing of the INTERBALL-1 main satellite on August 26, 1995, using data obtained by the fluxgate magnetometer FGM-1. We suggest that the oscillations, observed immediately after the first magnetopause crossing, are due to a Kelvin-Helmholtz instability. We verify this suggestion by appropriate MHD simulations. For the sake of a direct comparison we present the simulation results in a frame moving through the propagating structure at the relative speed of the satellite along its orbit. As a result we have found that the observed signatures compare well with the simulated crossing of a Kelvin-Helmholtz unstable magnetopause.