Magnetic Sector Boundaries Research Articles

MHD Modeling of the Background Solar Wind in the Inner Heliosphere From 0.1 to 5.5 AU: Comparison With In Situ Observations

AbstractThe accurate prediction of solar wind conditions in the interplanetary space is crucial in the context of both scientific research and technical applications. In this study, we simulate the solar wind throughout the heliosphere from 0.1 to 5.5 astronomical units (AU) with our improved heliospheric magnetohydrodynamics (MHD) model during the time period from 2007 to 2017. The model uses synoptic magnetogram maps as input to derive the inner boundary conditions based on a series of empirical relations such as the Wang‐Sheeley‐Arge (WSA) relation. To test the performance of this model, we compare the simulation results with in situ measurements from multiple spacecraft including ACE/WIND, Solar TErrestrial Relations Observatory, Ulysses, Juno, and MErcury Surface, Space ENvironment, GEochemistry, and Ranging at different latitudes and heliocentric distances. There is an overall agreement between the model results and solar wind observations at different latitudes and heliocentric distances. Statistical analysis for Year 2007 reveals that our model can predict most of the corotation interaction regions, high‐speed streams, and magnetic sector boundaries at 1 AU. In addition, the bimodal structure of the solar wind for different latitudes is well reproduced by the model which is consistent with Ulysses data. This study demonstrates the capabilities of our heliosphere model in the prediction of the large‐scale structures of the solar wind in the inner heliosphere, and the model can be used to predict the ambient solar wind at locations of planets in the solar system such as Earth and Jupiter.

A link between high-speed solar wind streams and explosive extratropical cyclones

A link between solar wind magnetic sector boundary (heliospheric current sheet) crossings by the Earth and the upper-level tropospheric vorticity was discovered in the 1970s. These results have been later confirmed but the proposed mechanisms remain controversial. Extratropical-cyclone tracks obtained from two meteorological reanalysis datasets are used in superposed epoch analysis of time series of solar wind plasma parameters and green coronal emission line intensity. The time series are keyed to times of maximum growth of explosively developing extratropical cyclones in the winter season. The new statistical evidence corroborates the previously published results (Prikryl et al., 2009). This evidence shows that explosive extratropical cyclones tend to occur after arrivals of solar wind disturbances such as high-speed solar wind streams from coronal holes when large amplitude magneto-hydrodynamic waves couple to the magnetosphere-ionosphere system. These MHD waves modulate Joule heating and/or Lorentz forcing of the high-latitude thermosphere generating medium-scale atmospheric gravity waves that propagate energy upward and downward from auroral zone through the atmosphere. At the tropospheric level, in spite of significantly reduced amplitudes, these gravity waves can provide a lift of unstable air to release the moist symmetric instability thus initiating slantwise convection and forming cloud/precipitation bands. The release of latent heat is known to provide energy for rapid development and intensification of extratropical cyclones.

Validation for solar wind prediction at Earth: Comparison of coronal and heliospheric models installed at the CCMC

Multiple coronal and heliospheric models have been recently upgraded at the Community Coordinated Modeling Center (CCMC), including the Wang-Sheeley-Arge (WSA)-Enlil model, MHD-Around-a-Sphere (MAS)-Enlil model, Space Weather Modeling Framework (SWMF), and heliospheric tomography using interplanetary scintillation data. To investigate the effects of photospheric magnetograms from different sources, different coronal models, and different model versions on the model performance, we run these models in 10 combinations. Choosing seven Carrington rotations in 2007 as the time window, we compare the modeling results with the Operating Mission as Nodes on the Internet data for near-Earth space environment during the late declining phase of solar cycle 23. Visual comparison is proved to be a necessary addition to the quantitative assessment of the models' capabilities in reproducing the time series and statistics of solar wind parameters. The MAS-Enlil model captures the time patterns of solar wind parameters better, while the WSA-Enlil model matches with the time series of normalized solar wind parameters better. Models generally overestimate slow wind temperature and underestimate fast wind temperature and magnetic field. Using improved algorithms, we have identified magnetic field sector boundaries (SBs) and slow-to-fast stream interaction regions (SIRs) as focused structures. The success rate of capturing them and the time offset vary largely with models. For this quiet period, the new version of MAS-Enlil model works best for SBs, while heliospheric tomography works best for SIRs. The new version of SWMF with more physics added needs more development. General strengths and weaknesses for each model are diagnosed to provide an unbiased reference to model developers and users.

ON WHETHER OR NOTVOYAGER 1HAS CROSSED THE HELIOPAUSE

The Voyager 1 spacecraft is currently in the vicinity of the heliopause, which separates the heliosphere from the local interstellar medium. There has been a precipitous decrease in particles accelerated in the heliosphere, and a substantial increase in galactic cosmic rays (GCRs), suggesting easy escape of the former across the heliopause, and entry of the latter. The question is, has Voyager 1 actually crossed the heliopause and is it now in the interstellar medium? We contend that the evidence is inconclusive. The direction of the magnetic field observed by Voyager 1 is unchanged from the direction of the heliospheric magnetic field, and different from the expected direction of the interstellar magnetic field. However, the plasma density, which is measured from observations of plasma waves, is similar to the expected interstellar density and much larger than the solar wind plasma density observed by Voyager 2 (which has a working plasma detector) at smaller heliocentric distances than Voyager 1. In this paper, an analytic model is presented that is based upon and is consistent with all Voyager observations, and in which the higher plasma densities measured by Voyager 1 are due simply to compressed solar wind. Thus both the magnetic field andmore » the plasma density observations are consistent with Voyager 1 still remaining well within the heliosheath. The model has a simple test: Voyager 1 should encounter a magnetic sector boundary crossing, where the behavior of particles accelerated in the heliosphere and the GCRs will be different from what Voyager 1 is now observing.« less

Role of atmospheric circulation in the baric field response to the Earth’s crossing of the interplanetary magnetic field sector boundaries

The Earth’s crossings of the magnetic sector boundaries are accompanied by changes in the magnetosphere, ionosphere, and troposphere. We considered the baric field’s response to the crossing of the inter-planetary magnetic field (IMF) sector boundaries during a geomagnetically quiet period. The IMF sign is shown to affect atmospheric pressure in high-latitude regions. The efficiency and sign of the relationship vary during the year. The baric field response to the Earth’s crossing of the IMF sector boundaries is most distinct during equinoxes. It is shown that, during a geomagnetically quiet period, the circulation processes in the atmosphere drive the changes in the atmospheric pressure when the Earth passes from one IMF sector into another.

Dynamics of solar activity and anomalous weather in summer 2010: 1. Sector boundaries: Anticyclone formation and destruction

A close synoptic relationship has been found, on the one hand, between the sector structure of the solar and interplanetary magnetic fields and, on the other hand, the structure of tropospheric parameters (the near-Earth pressure and temperature) in Central Russia during an anomalously hot anticyclonic weather in June–August 2010. It has been stated that the Earth crossed the magnetic sector boundaries in full agreement with the boundaries of meteorological parameters, structuring the formation, stabilization, and decay dynamics of anticyclones according to observations performed at IZMIRAN (Troitsk, φ = 55°, λ = 37°).

The influence of solar wind on extratropical cyclones – Part 1: Wilcox effect revisited

Abstract. A sun-weather correlation, namely the link between solar magnetic sector boundary passage (SBP) by the Earth and upper-level tropospheric vorticity area index (VAI), that was found by Wilcox et al. (1974) and shown to be statistically significant by Hines and Halevy (1977) is revisited. A minimum in the VAI one day after SBP followed by an increase a few days later was observed. Using the ECMWF ERA-40 re-analysis dataset for the original period from 1963 to 1973 and extending it to 2002, we have verified what has become known as the "Wilcox effect" for the Northern as well as the Southern Hemisphere winters. The effect persists through years of high and low volcanic aerosol loading except for the Northern Hemisphere at 500 mb, when the VAI minimum is weak during the low aerosol years after 1973, particularly for sector boundaries associated with south-to-north reversals of the interplanetary magnetic field (IMF) BZ component. The "disappearance" of the Wilcox effect was found previously by Tinsley et al. (1994) who suggested that enhanced stratospheric volcanic aerosols and changes in air-earth current density are necessary conditions for the effect. The present results indicate that the Wilcox effect does not require high aerosol loading to be detected. The results are corroborated by a correlation with coronal holes where the fast solar wind originates. Ground-based measurements of the green coronal emission line (Fe XIV, 530.3 nm) are used in the superposed epoch analysis keyed by the times of sector boundary passage to show a one-to-one correspondence between the mean VAI variations and coronal holes. The VAI is modulated by high-speed solar wind streams with a delay of 1–2 days. The Fourier spectra of VAI time series show peaks at periods similar to those found in the solar corona and solar wind time series. In the modulation of VAI by solar wind the IMF BZ seems to control the phase of the Wilcox effect and the depth of the VAI minimum. The mean VAI response to SBP associated with the north-to-south reversal of BZ is leading by up to 2 days the mean VAI response to SBP associated with the south-to-north reversal of BZ. For the latter, less geoeffective events, the VAI minimum deepens (with the above exception of the Northern Hemisphere low-aerosol 500-mb VAI) and the VAI maximum is delayed. The phase shift between the mean VAI responses obtained for these two subsets of SBP events may explain the reduced amplitude of the overall Wilcox effect. In a companion paper, Prikryl et al. (2009) propose a new mechanism to explain the Wilcox effect, namely that solar-wind-generated auroral atmospheric gravity waves (AGWs) influence the growth of extratropical cyclones. It is also observed that severe extratropical storms, explosive cyclogenesis and significant sea level pressure deepenings of extratropical storms tend to occur within a few days of the arrival of high-speed solar wind. These observations are discussed in the context of the proposed AGW mechanism as well as the previously suggested atmospheric electrical current (AEC) model (Tinsley et al., 1994), which requires the presence of stratospheric aerosols for a significant (Wilcox) effect.

Heliospheric sector boundaries: Single or multiple?

Solar wind data were examined for a period when the Ulysses spacecraft moved roughly parallel to the boundary between inward and outward interplanetary fields at a solar distance of 1.4 AU. Multiple crossings of the magnetic sector boundary were observed. Three tests were used to evaluate whether or not the crossings were due to multiple crossings of a single surface or single crossings of separate surfaces: (1) a correlation between the latitude and longitude directions of the normals to the surface, (2) the radius of curvature of the surface calculated from consecutive crossings, and (3) the variation of plasma parameters across the surfaces. Both multiple crossings of a single rippled surface and single crossings of multiple boundary surfaces were observed.

Pc1 waves in the system of solar–terrestrial relations: New reflections

This paper is devoted to the beautiful and mysterious Pc1 geoelectromagnetic waves that attract the attention of many researchers all over the world as an essential element of the space physics. It is concluded that in spite of the recent progress in the investigation of Pc1 waves, it is still necessary to make a careful study of some unsettled problems posed in the past. Relevant problems and disputable issues of the physics of Pc1 waves are discussed in this paper. The discussion is started with the paradoxical dependence of the Pc1 wave activity on the solar wind plasma density, which is considered as the key problem. It is argued that the solution of this paradox is of paramount importance to understand the 11-year solar cycle variation of the Pc1 occurrence rate, interplay between Pc1 and oxygen ions in the magnetosphere, impact of interplanetary magnetic field sector boundaries on the Pc1 wave activity, and other similar problems. A schematic picture showing the place of Pc1 waves in the system of solar–terrestrial relations is presented.

Effects of the April 1994 Forbush events on the fluxes of the energetic charged particles measured on board CORONAS-I: their connection with conditions in the interplanetary medium

The SONG instrument on board CORONAS-I satellite (fluxes of protons E p >70 MeV and electrons E e >55 MeV ) observed the effects of the Forbush effects caused by interplanetary magnetic field sector boundary crossing and coronal mass ejection in April 1994. The latitudinal dependence of these effects is analyzed and compared with data from ground-based neutron monitors at different latitudes. It was found that while measurements by SONG instrument over the polar caps were in good agreement with the data of the neutron monitor at polar latitudes, at the middle latitudes during the D st decreases, the cut-off rigidity variations were probably so strong that instead of the usual short-time decrease the SONG instrument detected a significant enhancement of particle fluxes. The influence of interplanetary medium conditions on the cosmic ray flux is analyzed and discussed.

Disconnection Events in Comets

Cometary and solar wind data are compared with the purpose of identifying the solar wind conditions which are associated with comet plasma tail disconnection events (DEs), i.e., when the plasma tail appears disconnected from the cometary head. The cometary data are from The International Halley Watch Atlas of Large-Scale Phenomena (Brandt et al., 1992a). A systematic visual analysis of the atlas images (Voelzke and Matsuura, 1998) revealed, among other morphological structures, 47 DEs along the plasma tail of comet P/Halley. This work compares the current competitive theories, based on the triggering mechanisms, in order to explain the cyclic phenomena of DEs, i.e., the ion production effects, the pressure effects and the magnetic reconnection effects are analysed. The distribution of the DEs in time or heliocentric distance presents a bimodal character possibly associated with the cometary passage through the magnetic sector boundaries in the interplanetary medium. The 47 DEs documented in 47 different images allowed the estimation of 19 onsets of DEs, i.e., the time when the comet supposedly crossed a frontier between magnetic sectors of the solar wind. The solar wind data are taken from in situ measurements of IMP-8 (King, 1982), which is used to construct the actual variation of solar wind speed, density and dynamic pressure during the analysed interval. These in situ measurements are referenced to the comet by standard co-rotation methods. The preliminary results of this research reveal that the DEs onsets of comet P/Halley are correlated with pressure effects only in 23% of the analysed cases, therefore these effects should not be the principal triggering mechanism of DEs.

The onsets of disconnection events of comet P/Halley on 1985 December 13-14 and 1986 February 22

This work compares cometary and solar wind data with the purpose of determining the solar wind conditions associated with comet plasma tail discon- nection events (DEs). The cometary data are from The International Halley Watch Atlas of Large-Scale Phenomena (Brandt et al. 1992a). A systematic visual analysis of the atlas images (Voelzke & Matsuura 1998) revealed, among other morphological structures, 47 DEs. The solar wind data are in situ measurements from IMP-8, ICE and PVO, which are used to construct the variation of solar wind speed, density and dynamic pressure during the analysed interval. Data from these same spacecraft plus Vega 1 were used to determine the times of current sheet crossings. These data were tted to heliospheric current sheet curves (Hoeksema 1989) extrapolated from the corona into the heliosphere in order to determine the best-t coronal source-surface radius for each Carrington rotation. This work presents the kinematic analysis for two onsets of DEs (1985 December 13.5 and 1986 February 22.2). The DE onset time, when the comet supposedly crossed a magnetic sector boundary, is determined assuming that the disconnected plasma moves away from the nucleus at constant velocity (Voelzke & Matsuura 1998). This is compared with the analysis of (Yi et al. 1994b) which determined the time of disconnection assuming constantly accelerated linear mo- tion. The velocity varies broadly from one DE to another. In this paper, we report the results of an analysis of four DEs observed from 13 through 14 December 1985 and on 22 February 1986. From a kinematic analysis of the wide- eld photographic images we calculated the disconnection time of the 1985 December 13-14 DEs to be December 13.5 and of the 1986 February 22 to be February 22.2, which are in good agreement with Yi et al. (1994b). The solar wind conditions around comet P/Halley, inferred by corotation of IMP-8 data to the comet, were such

Latitudinal distribution of the solar wind properties in the low- and high-pressure regimes: Wind observations

Abstract. The solar wind properties depend on λ, the heliomagnetic latitude with respect to the heliospheric current sheet (HCS), more than on the heliographic latitude. We analyse the wind properties observed by Wind at 1 AU during about 2.5 solar rotations in 1995, a period close to the last minimum of solar activity. To determine λ, we use a model of the HCS which we fit to the magnetic sector boundary crossings observed by Wind. We find that the solar wind properties mainly depend on the modulus |λ|. But they also depend on a local parameter, the total pressure (magnetic pressure plus electron and proton thermal pressure). Furthermore, whatever the total pressure, we observe that the plasma properties also depend on the time: the latitudinal gradients of the wind speed and of the proton temperature are not the same before and after the closest HCS crossing. This is a consequence of the dynamical stream interactions. In the low pressure wind, at low |λ|, we find a clear maximum of the density, a clear minimum of the wind speed and of the proton temperature, a weak minimum of the average magnetic field strength, a weak maximum of the average thermal pressure, and a weak maximum of the average β factor. This overdense sheet is embedded in a density halo. The latitudinal thickness is about 5° for the overdense sheet, and 20° for the density halo. The HCS is thus wrapped in an overdense sheet surrounded by a halo, even in the non-compressed solar wind. In the high-pressure wind, the plasma properties are less well ordered as functions of the latitude than in the low-pressure wind; the minimum of the average speed is seen before the HCS crossing. The latitudinal thickness of the high-pressure region is about 20°. Our observations are qualitatively consistent with the numerical model of Pizzo for the deformation of the heliospheric current sheet and plasma sheet.Key words: Interplanetary physics (solar wind plasma)

Latitudinal distribution of the solar wind properties in the low- and high-pressure regimes: Wind observations

The solar wind properties depend on λ, the heliomagnetic latitude with respect to the heliospheric current sheet (HCS), more than on the heliographic latitude. We analyse the wind properties observed by Wind at 1 AU during about 2.5 solar rotations in 1995, a period close to the last minimum of solar activity. To determine λ, we use a model of the HCS which we fit to the magnetic sector boundary crossings observed by Wind. We find that the solar wind properties mainly depend on the modulus |λ|. But they also depend on a local parameter, the total pressure (magnetic pressure plus electron and proton thermal pressure). Furthermore, whatever the total pressure, we observe that the plasma properties also depend on the time: the latitudinal gradients of the wind speed and of the proton temperature are not the same before and after the closest HCS crossing. This is a consequence of the dynamical stream interactions. In the low pressure wind, at low |λ|, we find a clear maximum of the density, a clear minimum of the wind speed and of the proton temperature, a weak minimum of the average magnetic field strength, a weak maximum of the average thermal pressure, and a weak maximum of the average β factor. This overdense sheet is embedded in a density halo. The latitudinal thickness is about 5° for the overdense sheet, and 20° for the density halo. The HCS is thus wrapped in an overdense sheet surrounded by a halo, even in the non-compressed solar wind. In the high-pressure wind, the plasma properties are less well ordered as functions of the latitude than in the low-pressure wind; the minimum of the average speed is seen before the HCS crossing. The latitudinal thickness of the high-pressure region is about 20°. Our observations are qualitatively consistent with the numerical model of Pizzo for the deformation of the heliospheric current sheet and plasma sheet.Key words: Interplanetary physics (solar wind plasma)

Properties of interplanetary magnetic sector boundaries based on electron heat‐flux flow directions

We have used the solar wind electron heat flux flow directions to determine the interplanetary magnetic field polarities for the ISEE 3 period from 1978 to 1982. This technique assumes that the heat flux electrons flow away from the Sun along magnetic field lines. It provides the field polarities independently of the field directions. The resulting distribution of sector durations and the changes in that distribution with solar activity cycle are presented for four 1‐year periods. The large‐scale sectors expected from extrapolation of the Stanford source surface maps are present along with a population of small‐scale sectors with a peak in the time range of 9 hour to 1 day. About half the small‐scale sectors contain bidirectional electron (BDE) flows, suggesting their origins in coronal mass ejections. We also examine cases of false polarities, in which the directions of the fields imply polarities opposite to those determined from the heat‐flux directions. These constitute only 6 to 8% of all the hourly averages of the data. The majority (78%) of these false polarity regions were not associated with BDEs, and 75% were of only 1 or 2 hour durations. False polarity regions tended to lie nearly orthogonal to the spiral field angles and at relatively high latitudinal angles. While multiple (≥ 3 in 24 hours) current sheet crossings are common, we find no cases consistent with a wavy current sheet.

Morphological analysis of the plasma structures of comet P\\Halley

Five hundred and thirty-one images from September 1985 to July 1986 are analysed for the purpose of identifying, measuring and correlating the morphological structures along the plasma tail of P\\Halley. These images are from The International Halley Watch Atlas of Large-Scale Phenomena (Brandt et al., 1992). A systematic visual analysis revealed 124 wavy structures along the main tail and 27 along the secondary tails, 109 solitary waves (solitons) along the main tail and 37 along the secondary tails, 12 Swan-like (Hyder et al., 1974; Niedner and Brandt, 1980; Jockers, 1985) tails, 47 disconnection events (DEs) (Niedner and Brandt, 1979; Jockers, 1985; Celnik et al., 1988; Delva et al., 1991) and 23 knots (Matsuura and Voelzke, 1990; Voelzke, 1996; Voelzke et al., 1997). While the wavy structures denote undulations or a train of waves, the solitons refer to the formations usually denominated kinks (Tomita et al., 1987). The distribution of the DEs in time or heliocentric distance presents a bimodal character possibly associated with the spatial distribution of the magnetic sector boundaries in the interplanetary medium. In general, it is possible to associate the occurrence of a knot and\\or a Swan-Tail with the occurrence of a DE, but the last one may occur independently. The 47 DEs documented in 47 different images allowed the derivation of 19 onsets of DEs (Table 1Table 1Onset time of 19 DEsDE numberOnset time (UT)Possibly related reports11985 Nov. 12.7621985 Dec. 05.6131985 Dec. 13.54Niedner (1986), Yi et al., 1994b41985 Dec. 14.1651986 Jan. 01.9061986 Jan. 07.5271986 Jan. 09.63Niedner (1986), Tomita et al., 1987, Niedner et al., 199181986 Jan. 10.16Niedner (1986), Tomita et al., 1987, Niedner et al., 199191986 Jan. 10.76101986 Feb. 22.17Niedner (1986), Yi et al., 1994b111986 Mar. 08.90Niedner (1986), Wu and Qiu, 1987, Niedner and Schwingenschuh, 1987121986 Mar. 14.05131986 Mar. 15.87Feldman et al., 1986, Celnik et al., 1988, Yi et al., 1994a141986 Mar. 20.01151986 Apr. 03.38161986 Apr. 06.45Niedner (1986), Celnik et al., 1988171986 Apr. 07.70181986 Apr. 11.75191986 Apr. 15.34), i.e., the time when the comet supposedly crossed a frontier between magnetic sectors of the solar wind. The present analysis indicates that in a given DE the disconnected plasma moves away from the comet nucleus at constant velocity (this conclusion differs from results in the literature). Such a velocity, however, varies broadly from one DE to another. Nine onsets of DEs were determined before the perihelion passage with an average of the corrected velocities Vc equal to (260±87) km s−1, and ten after the perihelion with an average equal to (130±37) km s−1. This asymmetric behaviour of Vc in relation to the perihelion may be due to the asymmetric way by which the radial component of the orbital velocity of the comet combines with the velocity of the solar wind in the cometocentric frame of reference. The mean value of the corrected wavelength λc measured in 16 different wavy structures is equal to (2.2 ± 0.2)×106 km. The mean value of the corrected cometocentric phase velocity Vpc is equal to (114±31) km s-1 and the mean amplitude A of the wave is equal to (2.8±0.5)×105 km. There is a tendency for A andλc to increase with increasing cometocentric distance.

Solar Wind Modulation of the Global Electric Circuit and Apparent Effects on Cloud Microphysics, Latent Heat Release, and Tropospheric Dynamics.

The solar wind modulation of the energy distribution and flux of galactic cosmic rays causes 11 year cycles in the latitude distribution of air-earth current density in the global electric circuit. Similar changes occur with solar activity on the day to day time scale. At least 3 independent data sets correlate changes in tropospheric winds and vorticity with measured and modelled changes in air-earth current density in the troposphere. These are (1) the decadal oscillations in winter in the North Atlantic region, with period averaging 11 ± 2 years over the last 120 years, and which appear to have been phase locked to the solar cycle except for a period of generally low solar activity prior to 1925; (2) the decrease in 500 mbar Vorticity Area Index (VAI) in winter at times of Forbush decreases of cosmic ray flux; and (3) decreases of 500 mbar VAI in winter at times of solar wind magnetic sector boundary passages, when volcanic aerosols from Agung and El Chicon were in the global stratosphere, together with solar wind sector modulated relativistic electron precipitation. One possible physical mechanism connecting changes in air-earth current density with changes in tropospheric dynamics is via the accumulation of electrostatic charge at cloud tops affecting the microphysics of ice nucleation and precipitation, with consequences for latent heat release and the intensification of winter cyclones.

An examination of directional discontinuities and magnetic polarity changes around interplanetary sector boundaries usingE > 2 keV electrons

Past studies of interplanetary magnetic sector boundaries have been based on the assumption that one can determine the field polarities by comparing the field directions with those of the nominal Parker spiral angles. Previous investigators have found evidence for decreases of ∣B∣, the magnitude of the magnetic fieldB, and increases of Θ, the angle betweenB and the ecliptic plane, at sector boundaries. Others have argued that the characteristic thickness of sector boundaries exceeds that of tangential discontinuities, making sector boundaries a separate class of structures.

Stratospheric volcanic aerosols and changes in air‐earth current density at solar wind magnetic sector boundaries as conditions for the Wilcox tropospheric vorticity effect

A correlation between tropospheric dynamics and solar wind magnetic fields that disappeared in the early 1970s reappeared with a new injection of volcanic aerosols into the stratosphere. A similar pattern of correlation has been found for changes in current density in the global electric circuit and for changes in relativistic electron precipitation. Several other weather and climate variations have been found to correlate with changes in air‐earth current density due to solar wind modulation of the global electric circuit. The accumulation of electrostatic charge on supercooled droplets at cloud tops responds to air‐earth current density changes. A mechanism linking the effects of charge accumulation to changes in ice nucleation, precipitation efficiency, latent heat retention and perturbations in atmospheric dynamics is thus as an explanation for this and other solar wind ‐ atmospheric electricity ‐ weather and climate correlations.

The disconnection events of 1986 April 13-18 and the cessation of plasma tail activity in Comet Halley in 1986 May

The disconnection events (DEs) in the 1986 April 13-18 time period appear to be the last major ones of comet Halley's 1986 apparition. We determined the time of the first disconnection event in this sequence, April 13.9±0.1 days, from kinematic analysis of photographic images. We show here that at the time of this DE, comet Halley had just crossed a magnetic field sector boundary and was in a moderate-density solar wind region with a speed of approximately 350 km s -1 . A sequence of DEs began approximately 1 day after the April 13.9 DE and was easily visible on April 16 and 17. Based on IMP 8 data, we show that this sequence was associated with a complex magnetic structure of the solar wind, showing multiple reversals