AMPTE observations of steady-state reconnection at the dayside magnetopause

Since the introduction of magnetic reconnection into magnetospheric physics by Dungey /1/, the argument over its existence at the dayside magnetopause has raged back - and - forth. Much indirect evidence for dayside reconnection has been compiled. For example, the dependence of magnetospheric processes on the IMF Bz component /2/ and IMF control of energetic solar particle access to the polar cap /3/. However, analysis of IMP6 /4/ and HEOS2 data /5/ failed to find any evidence of high speed ion flows at the magnetopause, an expected signature of magnetic reconnection /6/. It was thus left to the ISEE - 1 and - 2 spacecraft to provide the most powerful evidence for the existence of dayside reconnection. In particular, the observations of high speed plasma flows /7/, which have been shown to be consistent with reconnection models /8/, provide strong evidence. Recently, observations from the AMPTE spacecraft provide further evidence for steady - state reconnection and promise to further our knowledge of the reconnection process.     

Ion signatures of reconnection at the magnetopause

For a rotational MHD discontinuity the bulk flow is Alfvénic in the de Hoffmann-Teller frame. Using AMPTE/IRM data, we present ion distribution functions during three crossings of the dayside low-latitude magnetopause. For these crossings a well defined de Hoffmann-Teller frame can be found, but the field-aligned bulk flow is always slower than the Alfvén speed. Nevertheless, we find signatures in the distribution functions that provide clear evidence for magnetic reconnection: solar wind ions reflected off the magnetopause, “D-shaped” solar wind ion distributions in the boundary layer, and counterstreaming of solar wind ions and ionospheric ions in the boundary layer.     

The magnetopause erosion and the magnetosheath magnetic field penetration into the dayside magnetosphere

The earthward displacement of the magnetopause observed during a southward IMF (or the magnetopause erosion) and its dependence on the solar wind plasma and magnetic field parameters is studied by investigating data of about 30 magnetopause crossings by the ISEE 1 and 2 spacecraft. It is shown that the magnetopause erosion may be explained by a depression of the magnetic field intensity in the dayside magnetosphere caused by the penetration of the magnetosheath magnetic field (component perpendicular to the reconnection line) into the magnetosphere. The penetration coefficient (the ratio of the intensity of the penetrated field to the intensity of the magnetosheath magnetic field) is estimated and found to equal approximately 1.     

Two-spacecraft observations of reconnection at the magnetopause: Model results and data comparison

We revisit an example of “quasi-steady” magnetic reconnection at the dayside magnetopause on February 11, 1998, observed by Equator-S and Geotail at the dawnside magnetopause. Phan et al. [Phan, T.D. et al., 2000. Extended magnetic reconnection at the Earth’s magnetopause from detection of bi-directional jets. Nature 404, 848–850.] reported oppositely directed jets at these spacecrafts and inferred a length of the reconnection line of about 38R_E. Pinnock et al. [Pinnock, M., Chisham, G., Coleman, I.J., Freeman, M.P., Hairston, M., Villain, J.-P., 2003. The location and rate of dayside reconnection during an interval of southward interplanetary magnetic field. Ann. Geophys. 21, 1467–1482.] used measurements from SuperDARN radars to show that the reconnection electric field was variable. Here we complement this work by obtaining snapshots of the reconnection electric field from the in situ observations. To do this, we apply a reconstruction method based on a model of compressible Petschek-type magnetic reconnection. This independent method uses magnetic field observations as input data to calculate the reconnection electric field. We obtain average values of E_rec in the range of 0.4–2.4 mV/m. Further we infer a distance perpendicular to the reconnection line of 0.4–0.6R_E. The model results are compared with the two studies mentioned above. It thus appears that while the transfer of momentum for this event is indeed large-scale, the actual rate depends on the time it is measured.     

Effects of large scale magnetic fields in the dayside ionosphere of Venus

The ion density and magnetic field data from the Pioneer Venus Orbiter for the first three dayside periapsis passes have been analysed to study the effect of the large-scale fields upon the dayside ion density profiles. The peak value of the O⁺ density in a strongly magnetised ionosphere often shows an enhancement as compared to a close non-magnetic orbit. Further, the height of the O⁺ peak shows a positive correlation with the height of the minimum of the magnetic field profile. Contrary to earlier findings, the compressional effects of the magnetic fields are observed even at near-terminator locations.     

Interpretation of observations on July 24, 1996 detected by Interball-1 as pulsed reconnection at low latitude magnetopause

A method for investigating reconnection events is presented. The approach is based on advantages of non-linear spectral analysis named by the Method of Global Minimum and magnetic field measurements. The technique allows to reveal a presence and plasma properties of different particle species, waves, and time intervals of development of non-linear processes in the reconnection layers. We put into practice the approach to study the event on July 24, 1996 detected by Interball-1 and interpreted as gross deformation of the dayside magnetopause of about 5 R_E caused by a process at the bow shock by (Sibeck et al., 1998). Our method of spectral analysis of the data reveals that plasma detected during the event consists of ions both solar wind and ionosphere origin. The spectrum of the magnetic field data is determined by periods caused by gyro-motions of different species of the solar wind plasma (Fe⁺⁶, O⁺⁶, He⁺⁺) and the ionosphere ions (N⁺⁺, He⁺), and power non-stationary (transient) oscillations at period T120 sec. (frequency8 mHz. We reveal the exchange of plasma from the solar wind to the magnetosphere during the event too. Based on results of our analysis we suggest that the simplest explanation of the discussed event is that these are signatures of transient reconnection of interplanetary and terrestrial magnetic fields.     

Dynamics of the dayside aurora

Observations of auroral particles by rockets and satellites show the existence of structured “inverted V” type precipitation events with electron characteristic energies ranging up to several hundred eV in the post-noon dayside auroral region. Two station ground-based measurements from Cape Parry, NWT and Sachs Harbour, NWT, Canada from meridian scanners and auroral all-sky TV cameras are used in conjunction with rocket-borne electron measurements to study the temporal and spatial characteristics of the associated auroral emissions. The frequent occurrence of narrow, transient auroral arcs with lifetimes of 1–2 minutes, resulting from the inverted V events, is discussed.     

Dynamics of the dayside Aurora Australis

Current dayside optical studies of Aurora Australis from the Amundsen-Scott Research Station at the South Pole (74 degrees magnetic latitude) show some striking differences from optical results reported from Svalbard. A 6-channel meridian scanning photometer operating during the past three austral winters shows, in particular, the 630 nm emission is much lower, on average, than the Arctic dayside aurora and very weak on some days. The 558 nm intensity is higher relative to 630 nm suggesting the incoming electrons have a higher average energy. There are notable differences in auroral forms, giving further evidence of asymmetries in the two dayside ovals.     

About the equilibrium and stability of the magnetopause

This paper offers a model of the magnetopause based on the theory of the contact discontinuity; the boundary layer between the two states of space plasma. The structure of the magnetopause is explored for the effects of polarization, and the profiles of the polarizing electrostatic field are obtained.     

Collisionless reconnection in the magnetotail

Recent developments regarding collisionless reconnection in current sheets with a finite normal magnetic field component (B_z) are reviewed. In 2-D x, z configurations the ion tearing mode is stabilized by the electron compressibility. When the y dependence is included, cross-field current instabilities can be excited. Of these, the drift kink mode appears to be particularly important. 3-D electromagnetic particle simulations indicate that this mode can act as the precursor to the growth of tearing modes and subsequent reconnection.     

Lessons from laboratory experiments on reconnection

Magnetic reconnection has been studied in a laboratory experiment designed to model the basic two-dimensional neutral sheet configuration. However, the focus has been put on the inner region of the neutral sheet where the ions are effectively unmagnetized and MHD concepts are violated. In this parameter regime driven reconnection is governed by the fast dynamics of electrons. In true neutral sheets (B_z ⋍ 0) the current is carried by electrons. Thin current sheets (Δz ≳ c/ω_pe) rapidly form multiple X and 0 points due to the onset of the collisionless electron tearing mode. Magnetic energy is transported along the separator at the speed of whistler waves rather than Alfvén waves. Due to space charge separation the reconnection electric field E_y is, in general, not constant along the separator but localized near boundaries, nonuniformities in density and magnetic fields which limit the current I_y. This leads to localized particle acceleration, formation of anisotropic velocity distributions and instabilities. Reconnection and energization can be spatially separated which shows the importance of investigating both the global current system as well as critical local plasma properties. Experiments of current sheet disruptions are performed which demonstrate the processes of magnetic energy storage, transport, conversion and dissipation. Double layers and shock waves can be produced by current disruptions. The laboratory experiments show new dynamic features of reconnection processes not considered in MHD models yet relevant to narrow current sheets or the center of thick sheets.     

Model calculations of the dayside ionosphere of Venus

Model calculations of the dayside ionosphere of Venus are presented. The coupled continuity and momentum equations were solved for O₂⁺, O⁺, CO₂⁺, C⁺, N⁺, He⁺, and H⁺ density distributions, which are compared with measurements from the Pioneer Venus ion mass spectrometer. The agreement between the model results and the measurements is good for some species, such as O⁺, and rather poor for others, such as N⁺, indicating that our understanding of the dayside ion composition of Venus is incomplete. The coupled heat conduction equations for ions and electrons were solved and the calculated temperatures compared with Pioneer Venus measurements. It is shown that fluctuations in the magnetic field have a significant effect on the energy balance of the ionosphere.     

磁层顶通量传输事件的经验重构

在地球磁层顶附近观测到的通量传输事件（Flux Transfer Event,FTE）一般被认为是瞬态局域磁重联的产物,是太阳风质量、动量和能量进入地球内磁层的重要通道.重构FTE的磁场结构可促进对其形成、演化过程及其与周围等离子体环境相互作用的理解.Grad-Shafranov重构法和磁通量绳拟合法等传统磁场重构方法适用于满足特定物理条件的磁场结构.基于平面线性插值原理,设计了一种不受具体物理条件限定的二维FTE磁场结构重构法.模型测试以及对THEMIS和Cluster卫星簇分别观测到的两个FTE的实际应用表明,在合适的多卫星位形条件下,该方法能快速有效重构出FTE的磁场空间分布,有助于推测FTE的磁场线位形,理解卫星测量数据的时间变化,以及分析等离子体物理量相对于FTE的磁场空间分布特征.     

Magnetic reconnection on the Sun: ESPD Senior Prize Lecture

Magnetic reconnection is a fundamental process for changing the magnetic topology and converting magnetic energy into other forms on the Sun, such as heat, flow energy and fast particle energy. In two dimensions it is fairly well understood, although some aspects still need to be developed. In three dimensions, it behaves very differently and a substantial body of theory and numerical experiment has now been built up, including reconnection at null points, separators and quasi-separators.Some aspects of solar flares can be understood with 2D reconnection models, but other aspects such as the shapes of flare ribbons, the acceleration of particles and the creation of twist in erupting flux ropes need a 3D understanding. A paradigm shift in our understanding of coronal heating by reconnection has been stimulated by dramatic new observations of photospheric flux cancellation from SUNRISE and from SST together with the realisation that it may well be driving nanoflare heating events and possibly campfires.     

Driven reconnection in the near-Earth plasma sheet

In some recent MHD simulations of the near-Earth plasma sheet we studied onset and evolution of reconnection due to non-linear resistive instabilities. In our present contribution we show that these non-linear instabilities can be amplified significantly by inflow through the plasma sheet boundary and we discuss the consequences of that driving mechanism on the global dynamics of the instabilities. For high magnetic Reynolds numbers we find thin current sheets developing.     

A review of observed variability in the dayside ionosphere of Mars

Recent measurements by Mars Global Surveyor and Mars Express have greatly increased the number of observations of the martian dayside ionosphere available for study. Together with earlier measurements from the Viking era, these datasets have been used to investigate variations in well-known properties of the martian dayside ionosphere and to discover new ionospheric features. The dayside ionosphere includes the main peak, called the M2 layer, and a lower layer, called the M1 layer. In the topside, above the M2 layer, electron densities exponentially decrease with increasing altitude.     

Ion dynamics of magnetic reconnection in the magnetotail

We discuss the kinetic processes of plasma thermalization, acceleration, and mixing in magnetic reconnection. Non-Maxwellian, gyrotropic ion distribution functions such as anisotropic ion beams in the plasma sheet boundary layer (PSBL) and counter-streaming ions (CSIs) in the plasma sheet are often observed during a plasmoid passage of a satellite in the Earth's magnetotail. Non-gyrotropic ion distribution functions are also sometimes observed just after the passage of the plasmoid. We study the behavior of non-Maxwellian ion distribution functions observed by GEOTAIL. We further study theoretically the ion dynamics by using a particle-in-cell simulation, and discuss the role of non-Maxwellian distribution functions in magnetic reconnection.     

The role of helicity in the reconnection process

The change of helicity for magnetic reconnection is calculated with the help of a kinematic model. The results confirm the approximate conservation of magnetic helicity for reconnection in an almost ideal plasma as e. g. the solar corona. Different examples show the contributions of twist or linkage of flux tubes to the total helicity change in the process. However, they also show that helicity may be produced for reconnection processes in more extended non-ideal regions which is due to new magnetic flux linked to the reconnected flux tubes.