Magnetic Flux Transfer Research Articles

The effect of solar wind structure on magnetospheric energy supply during solar cycle 20

The energy supplied to the magnetosphere by the solar wind has been estimated by computing the incremental energy build up in a simple magnetotail model as a result of magnetic flux transfer from the dayside via reconnection. A method utilizing the auroral AL index as a quantitative measure of the flux transfer process and, hence, the magnetospheric energy supply is presented. In this way the available AL records are examined for the purpose of determining the variation in energy input from the solar wind and its relationship to interplanetary conditions during 9 years of solar cycle 20 (1966–1974). The principal feature discerned on shorter time scales was a large modulation of the input energy with sector structure, while in the annual averages enhancements were observed in 1968, 1973, and 1974 of 19%, 33%, and 60% over the 1966–1972 mean of 3 × 1025 ergs/yr. Interpreting these results as being due to variations in the time history of the dawn‐dusk component of the interplanetary electric field in magnetospheric coordinates, Bz−V, leads to the conclusion that while on shorter intervals the energy supply is dominated by variations in Bz−, associated with both the IMF and the orientation of the terrestrial dipole, on the time scale of years Bz− changes are small compared with the solar cycle variation in V which clearly produced the increase in 1973–1974 as has been concluded in other studies using techniques different from the one employed here.

The half‐wave rectifier response of the magnetosphere and antiparallel merging

In some ways the magnetosphere behaves as if merging occurs only when the interplanetary magnetic field (IMF) is southward, and in other ways it behaves as if merging occurs for all IMF orientations. An explanation of this duality is offered in terms of a geometrical antiparallel merging model which predicts merging for all IMF orientations but magnetic flux transfer to the tail only for southward IMF. This is in contrast to previous models of component merging, where merging and flux transfer occur together for nearly all IMF orientations. That the problematic duality can be explained by the model is compelling evidence that antiparallel merging should be seriously considered in constructing theories of the merging process.

A correlative study of magnetic flux transfer in the magnetosphere

The applied magnetic flux (Φap) of the southward component of the interplanetary magnetic field in GSM coordinates (Bz−) which impinges upon the sunward magnetopause has been compared with the time integral of the auroral AL index during 56 intervals within a 3‐month period in 1969 when interplanetary records from Heos 1 and Explorer 35 were available. The periods of magnetic activity chosen were those for which Bz > 0 and AL < 20 γ at the beginning and end of the interval. It was found that for these intervals, ∫ AL dt ∝ Φap with a correlation coefficient of 0.94. In addition, the empirical relationships between magnetic flux transfer, the applied southward flux, and ∫ AL dt arrived at by Holzer and Slavin (1978) on the basis of expansions and contractions of the forward magnetosphere observed with Ogo 5 are reexamined and improved. With these expressions describing the bulk rates of magnetic flux transfer within the magnetosphere a lower limit of 0.2±0.1 on the local reconnection efficiency at the magnetopause is calculated.

The effect of erosion on the solar wind stand‐off distance at Mercury

Recent studies have provided quantitative measurements of the effect of dayside magnetic reconnection on the position of the earth's forward magnetopause. By scaling these terrestrial observations to Mercury, it is predicted that the mean solar wind stand‐off distance for average solar wind dynamic pressure conditions will be 0.2–0.7Rm inward from its ‘ground state’ position. Furthermore, it is expected that the magnetopause will be eroded and/or compressed to within 0.2Rm of Mercury's surface a significant portion of the time. Empirical formulae derived for the earth's magnetosphere are used to determine both solar wind stand‐off distances and solar wind dynamic pressures for the two Mariner 10 encounters with Mercury's magnetosphere. It is found that for the first encounter when the interplanetary magnetic field was often southward and substorm signatures are observed inside the magnetosphere, the mean stand‐off distance inferred from the boundary crossings is 1.5Rm (for Psw = 6.0 × 10−8 dynes/cm²). At the time of the final encounter, the Mariner 10 magnetometer observed no significant southward component in the IMF and no substorm activity was evident. For this encounter, the mean inferred stand‐off distance is 1.9Rm consistent with the expected effects of magnetic flux transfer within a terrestrial‐type magnetosphere. A dipole moment of 6±2 × 1022 G‐cm³ is calculated from the observed bow shock and magnetopause positions. Finally, the importance of magnetic flux transfer in the solar wind‐magnetosphere‐atmosphere‐surface interaction at Mercury is briefly discussed.

Response of magnetotail plasma at lunar distance to changes in the interplanetary magnetic field, the solar wind plasma, and substorm activity

From an analysis of particle and field data obtained on January 19 and 20, 1973, we have determined several important characteristics of the plasma populations in the geomagnetic tail at lunar distance and their relationships to conditions in the solar wind and interplanetary magnetic field (IMF). During an extended period of low geomagnetic activity with the IMF Z component predominantly northward the plasma sheet at lunar distance is observed to cool significantly. Ion temperatures are observed in the range kTi ∼200–400 eV with electron temperatures of ∼100 eV. Number densities are approximately 1 cm−3. During this period the plasma mantle is also present as an extensive but tenuous region of plasma with densities ni = ne ∼10−2 cm−3, bulk velocity VB ∼100 km/s, and ion temperature kTi ∼3 eV. Following a sharp southward turning of the IMF Z component, there is a net transfer of magnetic flux to the tail. At lunar distance the magnetic field in the lobes increases in magnitude, and the density of the plasma mantle increases sharply. The time delay between the southward turning and the observed changes at lunar distance is consistent with the southward IMF being the cause of the increase of the magnetic flux and the plasma mantle density. The electrons in the plasma mantle display a core and halo distribution indicating a solar wind origin. The plasma sheet is observed to become hotter with increasing geomagnetic activity, reaching ion temperatures in excess of 1 keV and electron temperatures up to 900 eV.

Magnetic flux transfer associated with expansions and contractions of the dayside magnetosphere

An empirical magnetopause model is developed by using Ogo 5 magnetometer data, solar wind data from three satellites, and auroral zone magnetic indices. By employing this model it is found that the average displacements of the dayside magnetopause due to changes in solar wind dynamic pressure psw are comparable with, but greater than, the average displacements associated with the transfer of magnetic flux between the nose and the tail of the magnetosphere. After correcting for changes in psw, multiple encounters of the magnetopause occurring over distances of ≥0.5 RE on inbound or outbound legs of the satellite orbit are termed contraction or expansion events, respectively. Events occurring during periods of significant ring current, as judged by the Dst index, are excluded from the study. The remaining 24 expansion and contraction events are attributed to net changes ΔΦ in the total magnetic flux present in the dayside magnetosphere. The model dependence of the results is discussed, and for the events observed it is found that 2 × 1015 < ΔΦ < 2 × 1016 Mx. For the expansion events, ΔΦ shows a positive correlation with the time integral of the auroral zone magnetic index AL over the temporal extent of the observation. A similar correlation between the net flux losses during contraction events with the southward component of the IMF flux impinging on the magnetosphere is complicated by flux returning from the tail during the erosion process. Displacements associated with net flux transfers were observed in all regions of the dayside magnetopause accessible to the satellite, a finding suggesting that the entire dayside magnetosphere is affected by the flux transfer process. The bearing of these results on magnetic reconnection in the earth's magnetosphere is briefly discussed.

Impulsive Flux Transfer Events and Solar Flares

Experimental and theoretical aspects of the fundamental process of magnetic flux transfer (‘field line reconnection’) are reviewed. Explosively rapid events are observed in a laboratory magnetoplasma device designed to test certain aspects of neutral point theories of the solar flare mechanism. In these events, ultra-fast release of magnetic energy through reconnection of field lines is effected by a conduction mode instability in which an abrupt upward transition to anomalous resistivity takes place in the neighbourhood of the X-point. The events are triggered when the X-point current density exceeds the conduction mode instability threshold. The energy that is released is that which is associated with the induced current system that supports field line reconnection under ordinary quiescent conditions. Many characteristics of the laboratory events and their associated phenomena, when appropriately scaled, show remarkable agreement with corresponding flare observational data. Some new directions for further development of flare theory are therefore recommended. These experiments indicate that a number of assumptions commonly made in theoretical analysis of the reconnection process are inappropriate. The flux transfer process can be well understood theoretically in terms of a simple circuit analogue which models the laboratory process.

Small-Scale Solar Magnetic Fields

The observed properties of small-scale solar magnetic fields are reviewed. Most of the magnetic flux in the photosphere is in the form of strong fields of about 100–200mT(1–2 kG), which have remarkably similar properties regardless of whether they occur in active or quiet regions. These fields are associated with strong atmospheric heating. Flux concentrations decay at a rate of about 107Wb s-1, independent of the amount of flux in the decaying structure. The decay occurs by smaller flux fragments breaking loose from the larger ones, i.e. a transfer of magnetic flux from smaller to larger Fourier wave numbers, into the wave-number regime where ohmic diffusion becomes significant. This takes place in a time-scale much shorter than the length of the solar cycle.The field amplification occurs mainly below the solar surface, since very little magnetic flux appears in diffuse form in the photosphere, and the life-time of the smallest flux elements is very short. The observations further suggest that most of the magnetic flux in quiet regions is supplied directly from below the solar surface rather than being the result of turbulent diffusion of active-region magnetic fields.

SQUID operation with ferromagnetic core superconducting dc transformers

We successfully used ferromagnetic core (Cryoperm 10) superconducting transformers to enhance the transfer of external magnetic flux to a symmetric two-holes rf biased quantum interference device (SQUID). The flux transfer ratio has been enhanced up to a factor of ten in comparison with air core superconducting transformers. It has been observed that the presence of ferromagnetic cores in the holes of the SQUID does not introduce additional noise, while it introduces a reduction in the Q of the 19 MHz tank circuit. The experimental results are discussed in the framework of a detailed analysis of the behaviour of the complete SQUID plus tank circuit plus ferromagnetic core superconducting transformer circuit.

Observations of interactions between interplanetary and geomagnetic fields

Magnetospheric effects associated with variations of the north‐south component of the interplanetary magnetic field (IMF) are examined in light of recent experimental and theoretical results. Although the occurrence of magnetospheric substorms is statistically related to periods of southward IMF, the details of the interaction are not understood. In particular, attempts to separate effects resulting directly from the interaction between interplanetary and geomagnetic fields from those associated with substorms have produced conflicting results. One can, however, say with some assurance that the transfer of magnetic flux from the day side to the night side magnetosphere, as evidenced by equatorward motion of the polar cusp and increases of the magnetic energy density in the lobes of the geomagnetic tail, is a direct consequence of the southward IMF. On the other hand, the formation of a macroscopic X‐type neutral line at tail distances less than 35 RE appears to be a substorm phenomenon. Although other plasma and field phenomena in the outer magnetosphere and at low altitudes may be directly associated with the southward IMF, the evidence is less convincing. The quantitative results that have been obtained are compared with current theoretical models of the reconnection process.

Mass motion and heating in a magnetic neutral point system

We present optical spectroscopic and magnetic probe observations of a laboratory discharge device, in which magnetic field line reconnexion occurs at an x−type neutral point. Time-resolved spectral-line profiles of the ionized argon line A II λ4348 are presented. These allow the determination of Doppler shifts and broadenings near the neutral point. Plasma heating occurs at the neutral point and downstream from it. Plasma is ejected from the downstream sides of the neutral point region at the local Alfvén speed at all times. This is equal to about one-tenth the upstream Alfvén speed at the one time it is known. The downstream magnetic Mach number or ‘local reconnexion rate’ is found to be nearly independent of the plasma conductivity and time. The rate of transfer of magnetic flux from the upstream to downstream regions is strongly dependent on conductivity and time.

Plasma instability at an X-type magnetic neutral point

Time-resolved measurements of temperatures, density, and other parameters which describe the plasma at the X-type neutral point of the double inverse pinch laboratory discharge device are reported. Turbulent resistivity develops in this experiment without the usual application of an external electric field. The electric field is generated by field line reconnection or magnetic flux transfer. Various conductivity mechanisms are examined including binary collisions, space charge current limitation (Alfv́en-Carlqvist instability), and other current driven instabilities. The evidence suggests that the ion-acoustic instability is responsible for the observed current cutoff and turbulent conductivity. However, this instability must be driven well above its normal threshold. The competing effects of electron runaway and wave-particle interactions are discussed.

Experimental Study of Magnetic Flux Transfer at the Hyperbolic Neutral Point

A laboratory study is reported herein of a plasma flow process sometimes referred to as the severing and reconnecting of magnetic field lines at a hyperbolic or X-type neutral point. It is found that a hyperbolic pinch develops at the neutral point resulting in plasma compression. The Ohmic electric field of the pinch current enables field line reconnection to take place through resistive diffusion. The reconnection rate is substantially increased by a reduction in the size of the pinch region effected by flow adjustment through pairs of stationary slow mode shocks in a manner similar to that deduced and described by Petschek. Strong evidence is presented for this wave-dominated configuration. However, it is found that the pinch current becomes progressively concentrated at the neutral point and along the shock loci until eventually a critical current density is reached marked by a sudden and large anomalous increase in resistivity as predicted by Friedman and Hamberger. At this point the wave-assisted diffusion mode is terminated and there is observed: (1) a cutoff in pinch current, (2) a large increase in the electric field, (3) corresponding increase in reconnection rate, and (4) generation of a system of large amplitude fast-mode waves. The waves, bearing evidence of the onset of plasma turbulence, effect a rapid and gross redistribution of flux among the cells defined by the separatrix, and blow the slow mode shocks off downstream. The increased electric field raises the possibility of rapid magnetic energy dissipation through acceleration of particles to high energy.

Propagation of the sudden commencement of July 8, 1966, to the magnetotail

A sudden magnetic field increase associated with the July 8, 1966, sudden commencement (sc) was observed by the OGO 3 satellite in the magnetotail. By use of the IMP 3 and Explorer 33 observations made by Ness and Taylor of the interplanetary shock that caused the sc, it is shown that the magnetospheric propagation of the field increase toward the tail is faster than the propagation of the interplanetary shock just outside the bow shock. Conclusions drawn include (1) that the observed magnetic field increase in the tail is unlikely to be due to an increased lateral pressure of the postshock solar wind gas from the side of the tail, and (2) that the transfer of additional polar magnetic flux to the tail due to the increase in the solar wind pressure on the front side of the magnetosphere can account for the observed tail field increase.