Dayside Reconnection Research Articles

Interplanetary magnetic field control of magnetotail magnetic field geometry: IMP 8 observations

Four years of IMP 8 magnetic field measurements, 1978‐1982, when ISEE 3 took upstream interplanetary magnetic field (IMF) measurements, have been analyzed to produce full cross‐section magnetic maps of the magnetotail at about 33RE downwind from Earth. This paper describes how the field geometry in the cross‐sectional plane responds to different IMF orientations: dominant By, dominant +Bz, and dominant −Bz. The dominant By case exhibits marked departures from bilateral symmetry that have the sense of superimposing a fraction of the IMF on the symmetrical tail field. However, the “superimposed” perturbation field, measured as a fraction of the IMF, is highly nonuniform: It is maximum in the flanks of the plasma sheet and minimum in the lobes. There is also a rotation of the current sheet which varies with IMF strength and with distance. The two dominant Bz cases show no systematic departures from bilateral symmetry. However, the shapes and the relative sizes of the dipolar and flaring field regions of their cross sections are markedly different. The difference field, obtained by subtracting the positive Bz case from the negative Bz case, shows that the strongest perturbations run the north‐south extent of the flanks, instead of residing in the lobes, as might be expected from the dayside reconnection model.

Structure of reconnection layers in the magnetosphere

Magnetic reconnection can lead to the formation of observed boundary layers at the dayside magnetopause and in the nightside plasma sheet of the earth's magnetosphere. In this paper, the structure of these reconnection layers is studied by solving the one-dimensional Riemann problem for the evolution of a current sheet. Analytical method, resistive MHD simulations, and hybrid simulations are used. Based on the ideal MHD formulation, rotational discontinuities, slow shocks, slow expansion waves, and contact discontinuity are present in the dayside reconnection layer. Fast expansion waves are also present in the solution of the Riemann problem, but they quickly propagate out of the reconnection layer. Our study provides a coherent picture for the transition from the reconnection layer with two slow shocks in Petschek's model to the reconnection layer with a rotational discontinuity and a slow expansion wave in Levy et al's model. In the resistive MHD simulations, the rotational discontinuities are replaced by intermediate shocks or time-dependent intermediate shocks. In the hybrid simulations, the time-dependent intermediate shock quickly evolves to a steady rotational discontinuity, and the contact discontinuity does not exist. The magnetotail reconnection layer consists of two slow shocks. Hybrid simulations of slow shocks indicate that there exists a critical number,M c, such that for slow shocks with an intermediate Mach numberM I≥M c, a large-amplitude rotational wavetrain is present in the downstream region. For slow shocks withM I<M c, the downstream wavetrain does not exist. Chaotic ion orbits in the downstream wave provide an efficient mechanism for ion heating and wave damping and explain the existence of the critical numberM c in slow shocks.

A global magnetosphere‐ionosphere coupling model of substorms

A global model of substorms is proposed on the basis of observational synthesis and theoretical modeling. Since the theoretical basis of the present model is the magnetosphere‐ionosphere coupling (MIC) process, it will be called the MIC model of substorms. Substorms can occur in the MIC model without a new X line formed in the near‐Earth plasma sheet, in contrast to the highly popular near‐Earth neutral line (NENL) model of substorms. Following enhanced dayside reconnection, the ionosphere overloads both the solar wind on open field lines and the plasma sheet on closed field lines. The solar wind responds to the overload by providing more driven energy from the dynamo action on open field lines. The plasma sheet responds to the overload by collapsing itself, i.e., dipolarizing its field configuration to form the substorm current wedge. The explosive intensification during the expansion phase is powered by releasing the magnetic energy stored on closed field lines in the plasma sheet. The stored energy is released by the unloading instability driven by a positive feedback in the substorm current wedge.

Factors influencing the intensity of magnetospheric substorms

A magnetospheric substorm is the sequence of processes which occur throughout the magnetosphere following a southward turning of the interplanetary magnetic field and the onset of dayside reconnection. An isolated substorm consists of three phases, growth, expansion and recovery. Two primary processes make up a substorm. The directly driven component visible in the global, two cell current system is powered by the direct projection of the solar wind electric field onto the conducting ionosphere. The unloading component seen as the one cell intensification of the westward electrojet near midnight is powered by processes internal to the plasma sheet. The intensity of substorms can be measured by a variety of activity indices, the most common of which are magnetic indices. The solar wind controls the intensity of substorms through the process of magnetic reconnection. Time series analysis has established that 40 min after a southward turning of the interplanetary magnetic field the strength of the electrojets is correlated with solar wind parameters through a nonlinear coupling function similar to V 2B Sin 4( θ 2 ) ( where θ = IMF clock angle about Sun vector). Linear filter analysis of the magnetospheric system response shows that the average impulse response for the AL index is a Rayleigh function peaking at a lag time of 60 min and diminishing to zero after 3 h. The average response function for a sequence of moderate substorms is bimodal with peaks at 25 and 70 min, while more intense activity is uni-modal with only the 25 min peak. Response functions for individual substorms have peak amplitudes and delays that can be substantially different from the average. The linear analysis predicts less than half the variance in the AL index when high resolution indices are used. These results imply that reconnection on the day and night side are delayed relative to the solar wind by differing amounts which vary with conditions in the solar wind, magnetosphere, and ionosphere. Currents are driven through the ionosphere that are proportional to the reconnection rates which are in turn proportional to the solar wind electric field, but their strength depends on ionospheric conductivity. Conductivity depends on season, prior activity, and the way in which a particular substorm develops. Computer simulation of substorms with a simple ‘leaky tap’ model suggests that one reason for lack of short term predictabilty is likely to be that the magnetosphere is nonlinear with behavior depending both on previous conditions and the strength and wave form of the solar wind input. New techniques of analysis of magnetic indices support this conjecture. As with the Earth's surface weather, accurate space weather predictions will probably require extensive real-time observatory networks both in the solar wind and magnetosphere.

Field-aligned currents at the dayside magnetopause, reconnection and magnetic helicity

The magnetosphere is a complex spatially and temporally dependent plasma physical system which links, in both directions, the solar wind and the ionosphere. A crucial linking mechanism is due to geomagnetic field-aligned currents, driven by electric fields; changes in these two quantities determine the energy flow. Dawn, noon and dusk Region I currents link the northern and southern auroral ovals just inside the magnetopause, and Region II currents close just beyond the plasmapause. Attention is focused on mechanisms for the generation of field-aligned currents under non steady-state conditions, in particular magnetic field twisting during the process of reconnection.

Structure of the dayside reconnection layer in resistive MHD and hybrid models

The structure of reconnection layer at the Earth's dayside magnetopause is studied based on one‐dimensional simulations for the evolution of an initial current sheet. A resistive MHD code and a hybrid code are used in simulations. Initially, the magnetopause current sheet separates two uniform plasma regions: magnetosheath (high plasma density and low magnetic field) and magnetosphere (low plasma density and high magnetic field). In MHD simulations, it is found that steady intermediate shock, time‐dependent intermediate shock (TDIS), Alfvén wave pulse, slow shock, slow expansion wave, or contact discontinuity can be formed as a result of magnetic reconnection. The current density in the magnetopause‐boundary layer region is mainly concentrated at the intermediate shock on the magnetosheath side. High‐speed flow is observed in the boundary layer. The Alfvén wave pulse, which propagates to the magnetosphere, is found to have a large amplitude in plasma flow speed. In hybrid simulations, the reconnection layer has striking differences from that in MHD simulations: (1) Because of the mixing of hot magnetospheric plasma and cold magnetosheath plasma along magnetic field lines, the contact discontinuity does not appear in the reconnection layer. (2) In general cases, the TDIS evolves quickly to a steady rotational discontinuity with a constant width. In the singular case in which the magnetic fields in the magnetosheath and in the magnetosphere are exactly antiparallel, a weak steady intermediate shock is formed. (3) No clear shock front can be identified for the slow shock, and the slow expansion wave is strongly modified as compared with that in the MHD simulations. (4) The boundary layer is a transition region between the magnetosheath and the magnetosphere with a mixing of plasmas from the two sides; the plasma density decreases monotonically from the magnetosheath side to the magnetosphere side, while the total temperature and magnetic field strength increase monotonically toward the magnetosphere side. (5) A decrease in T⊥/T∥ is observed in the boundary layer. Our results indicate that in general cases rotational discontinuities bound the reconnection layer at the dayside magnetopause.

Magnetic field reconnection patterns at the dayside magnetopause: An MHD simulation study

Magnetic field configurations associated with dayside reconnection are studied by a global two‐dimensional incompressible MHD simulation. Possible dayside reconnection patterns revealed in the simulation include magnetic field configurations associated with steady state single X line reconnection, impulsive multiple X line reconnection, and bursty single X line reconnection. Three physical parameters, the global magnetic Reynolds number Rm, the critical current density Jc for the resistivity enhancement, and the solar wind Alfvén Mach number MASW are found to be the most important parameters in determining the reconnection pattern. First, it is found that single X line reconnection tends to occur for small Rm (&lt;100) and multiple X line reconnection takes place for large Rm (&gt;200). Second, in the presence of a current‐dependent resistivity and large Rm various patterns of multiple X line reconnection are observed. Third, for a large MASW dayside reconnection tends to occur in the high‐latitude region. It is found in particular that multiple X line reconnection can indeed generate the primary flux transfer event signature, i.e., the bipolar pulse of normal magnetic field component. In addition, at the trailing edge of the bipolar pulses, enhanced plasma speed is observed in the simulation.

The evolution of field‐aligned currents as a function of substorm phase

ISEE 1 and 2 measurements of magnetic changes across nightside field‐aligned currents at mid altitudes in the inner magnetosphere are examined to determine the average ionospheric extrapolated characteristics and properties of region 1 and 2 currents as a function of substorm phase. The properties of these currents studied include current intensity, density, layer width, and velocity and are consistent with earlier studies at lower altitudes. In general, region 1 and 2 currents behave differently during substorm growth and expansion, but are comparable in the recovery phase. During a substorm, region 1 current intensity ranges from 0.4 to 0.6 A/m and peaks during the expansion phase. Region 2 intensity varies from 0.15 to 0.35 A/m and reaches a maximum during recovery. The density of region 2 currents remains essentially steady at 1.4 μA/m², while region 1 decreases from 5 to 1.3 μA/m². The width of these current sheets throughout a substorm remains in the range from 100 to 500 km. The velocity of these sheets is for the most part inwards with speeds from 50 to 200 m/s. Even though these current sheet crossings were not originally selected as substorm associated events, they all occurred during one of the phases of a substorm. This indicates that substorms play a major role in the generation of nightside region 1 and 2 currents detected by the ISEE spacecraft. These results are consistent with a model in which these currents flow to couple the outer magnetosphere and the ionosphere in response to the stresses applied by both dayside and nightside reconnection. Region 1 currents are responsive to variations in the stress caused by both reconnection regions, whereas region 2 currents are responsive to only nightside reconnection.

Comet Halley remote plasma tail observations and in situ solar wind properties: VEGA- [formula omitted] IMF/plasma observations and ground-based optical observations from 1 December 1985 to 1 May 1986

Properties of the interplanetary magnetic field and solar wind plasma are analysed from continuous observations from 1 December 1985 to 1 May 1986 made by the Vega spacecraft (Riedler et al., 1986, Nature 321, 288; Gringauz et al., 1986, Nature 321, 282). Several high speed streams and flare-related shocks are found; the magnetic field sector boundaries are also seen. Many Earth-based observations of disconnection events in the plasma tail of comet Halley are now available and several mechanisms [changes in the solar wind (Jockers, 1985, Astron. Astrophys. Suppl. Ser. 62, 791; 1986, Asteroids, Comets and Meteors II, Reprocentralen HSC, Uppsala), interplanetary magnetic field features, e.g. day-side reconnection (Niedner and Brandt, 1978, Astrophys. J. 223, 665) or night-side reconnection (Russell et al., 1986, J. geophys. Res. 91, 1417)] have been proposed to explain the occurrence of such events. We investigated the correlation between the IMF sector boundaries measured by Vega and the observed disconnection events; such a correlation would support the day-side reconnection explanation. It is found that only in 50% of the considered events does a correlation between these two phenomena exist. For the other cases, a sector boundary of the IMF sweeping over the comet cannot explain the occurrence of disruptions of the main plasma tail. An indication is also seen that density enhancements (e.g. as they occur for high speed streams) in the solar wind are connected with large tail events.

MHD Wave Propagation in the Magnetosheath: Recent Results.

We discuss recent results from dual spacecraft observations of MHD fluctuations in the afternoon magnetosheath. ISEE 1 and 2 magnetic field and plasma data were used to determine MHD wave modes and directions. Wave directions were inferred from inter-spacecraft phase delays in the magnetic field. Our results draw attention to some unusual aspects of MHD wave propagation in the magnetosheath. Often coherence between the spacecraft is hard to obtain despite separations as small as 1000km. When coherence is significant the spectra appear to be dominated by guided MHD modes: both slow and Alfven (intermediate) mode signals have been identified with good inter-spacecraft phase information. Two patterns of phase propagation have been inferred. Sometimes guided MHD waves appear to propagate from localized sources in the vicinity of the bow shock along the local wave characteristics. (The wave characteristics in the magnetosheath are greatly modified by the plasma flow: the upstream characteristic commonly has a substantial components perpendicular to the field and flow.) At other times waves seem to be propagating along the inward shock normal, possibly as a result of spatially-coherent sources along the bow shock. We suggest that the flow-modified wave characteristics may play an important role in determining the flow of guided wave energy in the magnetosheath. This has implications for the transmission of upstream waves to the magnetosphere and dayside reconnection as we discuss.

Tail‐like reconfiguration of the plasma sheet during the substorm growth phase

Tail‐like reconfiguration observed in the near‐earth plasma sheet during the substorm growth phase is surrounded by enhanced convection except inside and tailward of the reconfiguration region itself. The required distribution of enhanced convection can be driven by an enhanced dayside reconnection. The enhanced dayside convection can be transmitted across open field lines by the tangential discontinuity to reach the last open field line in the tail lobe in about 20 minutes. This can cause Bx to increase in the plasma sheet. On the other hand, the enhanced dayside convection can also be transmitted across closed field lines by the fast rarefaction wave to arrive at the near‐earth plasma sheet (x ≤ −10 RE) in about 15 minutes, causing Bz to decrease in the plasma sheet. Thus, a tail‐like reconfiguration can start within about 15 to 20 minutes after a southward turning of the IMF (interplanetary magnetic field). The duration of the growth phase can be identified with the M‐I coupling time scale of ∼30 to ∼40 minutes. The region‐2 field‐aligned current is shown to increase to 0.2 μA/m2 at the ionospheric level, resulting from the tail‐like reconfiguration.

The excitation of plasma convection in the high‐latitude ionosphere

Recent observations of ionospheric flows by ground‐based radars, in particular by the European Incoherent Scatter (EISCAT) facility using the “Polar” experiment, together with previous analyses of the response of geomagnetic disturbance to variations of the interplanetary magnetic field (IMF), suggest that convection in the high‐latitude ionosphere should be considered to be the sum of two intrinsically time‐dependent patterns, one driven by solar wind‐magnetosphere coupling at the dayside magnetopause, the other by the release of energy in the geomagnetic tail (mainly by dayside and nightside reconnection, respectively). The flows driven by dayside coupling are largest on the dayside, where they usually dominate, are associated with an expanding polar cap area, and are excited and decay on ∼10‐min time scales following southward and northward turnings of the IMF, respectively. The latter finding indicates that the production of new open flux at the dayside magnetopause excites magnetospheric and ionospheric flow only for a short interval, ∼10 min, such that the flow driven by this source subsequently decays on this time scale unless maintained by the production of more open flux tubes. Correspondingly, the flows excited by the release of energy in the tail, mainly during substorms, are largest on the nightside, are associated with a contracting polar cap boundary, and are excited on ∼1‐hour time scales following a southward turn of the IMF. In general, the total ionospheric flow will be the sum of the flows produced by these two sources, such that due to their different response times to changes in the IMF, considerable variations in the flow pattern can occur for a given direction and strength of the IMF. Consequently, the ionospheric electric field cannot generally be regarded as arising from a simple mapping of the solar wind electric field along open flux tubes.

Magnetic impulses and associated optical signatures in the dayside aurora

We present five magnetic impulse events observed at South Pole and in the magnetically conjugate area at Iqaluit. The magnetic signatures recorded during these events appear to be similar to those discussed by several authors as possible ionospheric signatures of sporadic dayside reconnection via the flux transfer process. Coordinated ground‐based image‐intensified all sky camera data were acquired at South Pole during the events. The optical data show that prior to the onset of each of the events the dayside cusp aurora is several degrees poleward of the station, signifying event initiation on closed field lines. Overhead aurora appears at the onset of the magnetic event and precipitation seems to expand over a large region equatorward of the quiescent cusp aurora. As the event decays the aurora dies away and the quiescent dayside cusp aurora, in four out of five cases, returns poleward of the station. This morphology is inconsistent with the model in which the ground based signature of an FTE is caused by a field aligned current configuration moving over a ground station in the antisunward direction. The observations show temporal events which begin at or inside the boundary of closed field lines. These are consistent with sudden precipitation activity on flux tubes which were previously equatorward of the cusp/cleft boundary and which became temporarily disconnected (non‐dipolar).

Reconnection and solar terrestrial coupling

In recent years much progress has been made in establishing the mechanisms for mass, momentum and energy transfer from the solar wind into the terrestrial magnetosphere; in particular the importance of reconnection, at least at disturbed times, is generally agreed. In the most simple steady open magnetospheric model, the occurrence of reconnection is important because it fundamentally changes the magnetic topology. In the steady model, the reconnection rates on the dayside and at night match and the internal magnetospheric circulation is steadily driven by the solar wind. We discuss the processes that occur in the steady situation with particular attention to the current systems that are set up. Not only do the global Region I field aligned currents fit well with the open model but also recent work provides a simple explanation of the cusp Birkeland current system which directly associates it with the dayside reconnection process. We conclude with brief comments on the inclusion of unsteady circumstances.

A unified view of the influences of the IMF By on geomagnetic activity.

The correlation between the solar wind parameters and geomagnetic activity represented by several indices is utilized to examine the effects of the azimuthal component of the interplanetary magnetic field on solar wind-magnetosphere coupling. It is suggested that the |By| dependence of geomagnetic activity can be expressed by using the theoretical equation of the dayside reconnection. It is emphasized that the influence of By on different indices varies significantly. The seasonal variations of the By dependence of the AL index is discussed in the light of the viscous-like interaction hypothesis.

Substorm processes in the geomagnetic tail and their effect in the nightside auroral zone ionosphere as observed by eiscat

Current understanding of magnetospheric substorms is reviewed with special emphasis on the relation between space-based and ground-based observations. It is pointed out that the traditional means of monitoring substorms from the ground (by using magnetometers, riometers and auroral observations) give only a selective picture of the whole phenomenon, related to the precipitation of electrons with energies above 1 keV. Measurements by incoherent scatter radar, such as the European incoherent scatter facility (eiscat), give a more complete and continuous picture. The ‘neutral line’ model of substorms provides a natural, physical basis on which relevant data can be interpreted. In this picture, two sources of flow are anticipated in the nightside auroral zones, one ‘directly driven ’ (with a delay of 15-20 min) by the interplanetary magnetic field (imf) B z component and associated with dayside reconnection, and the other appearing typically an hour after southward turnings of the imf and associated with rapid tail reconnection during substorms. Evidence for the influence of both sources of flow is found in nightside eiscat data. These data also reveal that, overall, the nightside ionospheric flow and plasma param eters often vary in a quasi-periodic way with a period of ca. 1 h. In two cases in which concurrent interplanetary data are available it appears that the periodicity is inherent in imf B z , but this is not expressed unmodified in the auroral zone because of the presence of the two sources of flow which depend on imf B z in different ways.

Magnetopause coupling processes and ionospheric responses: a theoretical perspective

In recent years much progress has been made in establishing the mechanisms for mass, momentum and energy transfer from the solar wind into the terrestrial magnetosphere; in particular, the importance of reconnection, at least at disturbed times, is generally agreed. In the simplest circumstances, where dayside and nightside reconnection rates are balanced and steady, the simple open magnetospheric model would pertain. In general, however, reconnection is unsteady, day-night flux transfer occurs in an irregular way and the full complexity of the solar-windmagnetosphere-ionosphere system becomes apparent. A hierarchy of coupling times, for each of which different processes dominate, needs to be considered.

The universal time variation of geomagnetic activity

Geomagnetic activity exhibits both annual and daily variations. We examine these variations in the Am index which was designed to provide an accurate measure of the Universal Time variation of geomagnetic activity. The dayside reconnection process can quantitatively account for the observed variation in both indices. Control of these variations by the angle of the flow relative to the dipole axis is weaker than control by reconnection.

Questions on the dayside reconnection in connection with magnetospheric convection and open-close boundary.

In examining the magnetic field linkage between the outer magnetospheric structures and the polar ionosphere, some questions are brought forward on 1) the efficiency of the dayside magnetic reconnection in driving the magnetospheric convection, 2) magnetospheric source regions of electric potential across the polar cap and region 1 field-aligned currents, and 3) the open-close boundary of magnetic fields in high latitudes, in connection with the dayside polar cusp phenomena such as the cusp field-aligned currents, DPY currents and electric fields. Examination of these questions lead to the conclusion that1) the dayside magnetic reconnection is not operative, or plays almost no role in driving the magnetospheric convection even if it is operative;2) except for small dayside parts, the region 1 field-aligned currents, as well as the potential peak regions in high latitudes, are connected with the dawn and dusk parts of the tail plasma sheet, probably with substorm source regions;3) the small dayside part of the region 1 field-aligned currents is probably linked with the inner edge of the low latitude boundary layer, and the “cusp” or “NBZ” currents are likely connected with the tail boundary layer (plasma mantle layer);4) magnetospheric magnetic fields tend to be open (merged with the solar wind magnetic fields) at the magnetospheric tail flank, but the “open” field appears to be a rather statistically averaged concept;5) electric charge separation due to temporal and spatial fluctuations in the cross-tail current associated with magnetospheric substorms (whether well-defined or not) in the tail, in collaboration with non-uniform curvature drift currents, probably plays a major role in sustaining the polar cap potential and region 1 field-aligned currents, and therefore6) the electric potential difference across the polar cap is not directly due to the solar wind -vs×Bs, and7) energy and momentum transfer from the solar wind at the dawn and dusk tail flank in the form of a cross-tail current is probably the primary source of the magnetospheric convection.In this connection, it is also shown that a simple consideration of the motional induction electric fields and currents within a boundary layer (mixing or interaction layer) due to relative motion among the solar wind, the mixing layer and the magnetosphere is almost enough to account for the changes in the dayside cusp phenomena associated with the changes in the interplanetary magnetic field.

Evidence for periodic reconnection at Uranus?

The unique orientation of Uranus at the time of the Voyager 2 encounter results in a convection dominated magnetosphere. Plasma and magnetic field data from the tail magnetosheath are presented. Velocity decreases of 5‐10% seem to occur with a 17 hour period. At least four repetitions of this decrease are observed, in all cases when flow passes over the dayside polar cap. One possible interpretation of these features is that they are signatures of dayside reconnection. The cause of the velocity decreases would be drag on the reconnected flux tubes which are coupled via Birkeland currents to the ionosphere. The coupling efficiency for power transfer between the solar wind and Uranian magnetosphere implied by these decreases is consistent with previous determinations of this quantity.