Heliospheric Current Sheet Crossings Research Articles

An MHD simulation of the inner heliosphere during Carrington rotations 2060 and 2068: Comparison with MESSENGER and ACE spacecraft observations

We present results from a new magnetohydrodynamic (MHD) model of the inner heliosphere. The model is adapted from the well-established Lyon–Fedder–Mobarry (LFM) MHD simulation code, which until recently mostly applied to studies of the terrestrial magnetosphere. We perform quasi steady-state simulations of two Carrington rotations: 2060 and 2068. During both of these periods, the heliosphere remained quiet and undisturbed by transient phenomena, making them well-suited for simulation studies of Corotating Interaction Regions (CIRs). The MHD model of the solar wind is driven at the inner boundary by the Wang–Sheeley–Arge (WSA) model of the corona augmented with empirical relations to infer the solar wind velocity, density, and temperature. Here we report on a validation exercise whereby LFM-helio simulation results are compared with in situ data from the Advanced Composition Explorer (ACE) and MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We find that the model successfully reproduces the large-scale configuration of the inner heliosphere, namely timing and duration of high-speed streams and heliospheric current sheet crossings, as reflected in ACE and MESSENGER observations. Discrepancies between in situ measurements and simulations, such as 1–2 day errors in the time of arrival of a CIR or the strength of the simulated magnetic field at the spacecraft, are attributed to the uncertainty in the specification of the coronal conditions, rather than a poor performance of the solar wind model. More comparisons between different inner heliosphere models driven with identical coronal conditions are suggested as a way to explore their comparative strengths and weaknesses.

The role of volcanic aerosols and relativistic electrons in modulating winter storm vorticity

Small changes in the vorticity of winter storms, responding to solar wind variations, are found in winters from 1957 to 2011, and are greater for winters with higher levels of stratospheric volcanic aerosols. Using 1993–2011 data, the response of the vorticity area index (VAI) is shown to be of larger amplitude when the days of minima in the relativistic electron flux (REF) precipitating from the radiation belts are used, instead of heliospheric current sheet (HCS) crossings, as key days in superposed epoch analyses. The HCS crossings mostly occur within a few days of the REF minima. The VAI is an objective measure of the area of high cyclonic vorticity, and for the present work is derived from ERA-40 and ERA-Interim reanalyses of global meteorological data. The VAI dependencies on the stratospheric aerosol content (SAC) and the REF are consistent with a model in which the ionosphere-earth current density (Jz) affects cloud microphysics. One of the ways in which Jz is modulated is by changes in stratospheric column resistance (S), which is increased by stratospheric aerosols. Because S is in series with the tropospheric column resistance (T), Jz modulation by REF requires that S be not negligible with respect to T. So the Jz modulation and the VAI response appear when the SAC is very high, or the REF reductions (which also increase S) are very deep, and when the product of the SAC and the reciprocal of the REF exceeds a threshold value dependent on T.

The links between atmospheric vorticity, radiation belt electrons, and the solar wind

The links between winter storm intensity and solar wind variations associated with Heliospheric Current Sheet (HCS) crossings are shown to be present in 1997 through 2002 data without the necessity of high stratospheric aerosol loading.The peak values of winter storm vorticity are measured in terms of the 500hPa vorticity area index (VAI) for the Northern Hemisphere, during extended winter months, November through March. Superposed epoch analyses are used, but instead of the key days being defined by heliospheric current sheet crossings as used formerly, they are defined by minima in the absolute flux of relativistic electrons (REF)>2MeV measured at geostationary orbit. Minima in the VAI were found approximately 3days after the REF minima. Corresponding VAI minima were also found in response to the deepest minima in the solar wind speed (SWS). The SWS minima occurred approximately 2days before the REF minima, and 5days before the VAI minima. These results for the 1997–2002 period of low to moderate stratospheric aerosol loading are compared to periods of high loading that occurred between 1964 and 1994. The weaker VAI response in 1997–2002 is evidently due to the reduced aerosol loading, but there is uncertainty in the amount of weakening, due to the greater sensitivity gained by analyzing with respect to the deepest minima in REF or SWS rather than to times of HCS crossings, and the possibility of secular changes in the solar wind and in the magnetospheric relativistic electron populations.

Interaction between magnetic clouds and the heliospheric current sheet at 1 AU as it is observed by one single observation point

The effect of Magnetic Clouds (MCs) on the Heliospheric Current Sheet (HCS) local structure is yet an open question. Although it is widely accepted that a magnetic cloud has an important effect on the HCS shape, their structural relation, either the MC is part of the HCS or not, is not completely solved. Moreover, the problem grows up when trying to investigate three dimensional structures using one single observation point. We propose an approach to the MC–HCS study using magnetic models for the MC and local HCS structures, which are able of determining their relative orientation from one single spacecraft data. Three events have been selected in which an MC passage was observed close to HCS crossings. The results confirm the strong effect of MC passage on the HCS local orientation and they seem to be consistent with MCs propagating out of the HCS at 1 AU.

Solar wind effects on Jupiter non-Io DAM emissions during Ulysses distant encounter (2003–2004)

Aims. We analyze solar wind data from the Ulysses spacecraft during the distant Jupiter encounter from 2003 November to 2004 March as well as Nancay decametric (DAM) radio observations non-controlled by Io. Methods. The Ulysses solar wind data are balistically propagated towards Jupiter and are correlated with Nancay non-Io DAM emissions. Results. It is found that the average solar wind dynamic pressure around the time of DAM emissions is 1.7 times higher than its average value during the Ulysses encounter. The occurrence of fast forward (FS) and reverse (RS) interplanetary shocks and heliospheric current sheet crossings (HCS) is correlated with the occurrence of non-Io DAM emissions. We note an enhanced probability of occurrence of non-Io DAM emissions after the expected arrival of FS, RS and HCS at Jupiter. However, about half emissions (54%) did not seem to be associated with these interplanetary structures. We also note that the average duration and power of non-Io DAM emissions are enhanced during periods associated with those interplanetary structures. Conclusions. From the results obtained in this work it seems that non-Io DAM emissions occur during intervals of enhanced solar wind dynamic pressure. Yet, there is no direct correlation between the non-Io DAM emissions duration or power versus the solar wind pressure values and the interplanetary shock Mach number.

North‐south asymmetry of the location of the heliospheric current sheet revisited

The possible latitudinal offset of the location of the heliospheric current sheet (HCS) is an important question, since it has impact on the understanding of various phenomena including the solar dynamo and the modulation of cosmic rays. In the declining phase of the previous, 22nd solar cycle, in 1993, a southward displacement of the HCS by 10° was proposed to explain the north‐south asymmetry of energetic charged‐particle fluxes measured by Ulysses. Other observations supported the north‐south asymmetry as well, and it is now widely accepted by the scientific community that in 1993, the HCS was displaced 10° southward. However, in reality, the Ulysses magnetic field measurements did not give direct evidence for such a large displacement of the HCS. Here we revisit the question and extend our previous study for the declining phase of solar cycle 23 as well. Careful analysis of the HCS crossings observed by Ulysses during the fast latitude scans shows that a southward displacement of the HCS by 2°–3° is possible and consistent with the data for cycles 22 and 23. The impact of the HCS location on the latitudinal gradients of energetic particle fluxes is discussed.

Monitoring the heliospheric current sheet local structure for the years 1995 to 2001

We have monitored the heliospheric current sheet (HCS) local structure through a 6.5 year period starting in January 1995. This interval begins with near solar minimum conditions and finishes in solar maximum conditions. We have used data from Wind mission, mainly the magnetic field instrument (MFI) and the solar wind experiment (SWE). Our work is focused on the HCS local inclination and the solar wind conditions around the HCS crossings, with a particular interest on their evolution along the ascending phase of solar cycle 23 and its relationship with solar wind phenomena such as magnetic cloud and stream interaction regions. We defined a real HCS crossing when a magnetic field minimum, showing a polarity reversal is observed and QeB (where Qe is the electron heat flux in solar wind) reverses its sign through an interval no longer than 60 min. The results suggest that the HCS local structure is more dependent on the solar wind conditions than on the solar cycle stage.

A global model of cometary tail disconnection events triggered by solar wind magnetic variations

Presented in this paper are the results of a time‐dependent, fully three‐dimensional self‐consistent ideal magneto‐hydrodynamics (MHD) model of disconnection events (DEs) in the cometary plasma tail. Understanding the interaction of cometary plasma with the solar wind is a very important problem in space plasma physics. This study focuses on the detailed MHD processes which occur during DEs based on typical comets with ideal solar wind conditions, as well as different experiences of DEs triggered by different forms of heliospheric current sheet (HCS). The crossing process against a typical HCS, which is interpreted as various tangential discontinuities (TDs), is simulated. Tail rays and/or disconnected tails are formed in such simulations. With our line‐of‐sight integration results, we conclude that observers from different directions relative to the IMF plane will see two different types of DE image series. The magnetic reconnection that occurs in our results confirms previous publications from different models. With additional cases, our study also provides the limitation for such DEs to be triggered: the angle of magnetic field rotation across the TD should be within 90° to 270°. The effects of various intervals of multiple HCS crossings are explored to show that a 2‐h minimum interval is needed for consecutive multiple HCS to trigger a DE. In addition, such DEs caused by multiple HCS crossings have more tail rays and appear closer to observations than those resulted by single HCS crossings. In addition to presenting the first global DE time evolution with a well‐resolved contact‐surface, our results are compared morphologically with observational images. Quasi‐quantitative agreement with observational measurements is found by investigating the recession speed during the DE. Moreover, the importance of having a solar wind condition being able to trigger a frontside magnetic reconnection is found critical for such DEs to originate. The importance of a well‐resolved contact surface to this problem is discussed regarding to the mass loading effect. On the basis of our deductions and simulations, an empirical equation and data table to estimate the minimum upwind boundary distance is derived with respect to the shock distance, which is sensitive to the cometary production rate.

Correlation between energetic ion enhancements and heliospheric current sheet crossings in the outer heliosphere

Voyagers 1 and 2 observed highly‐variable beams of energetic ions in the foreshock region upstream of the termination shock (TS). At Voyager 2 (V2), the ion intensities are generally not related to the plasma properties. At Voyager 1 (V1), the beams are often coincident with crossings of the heliospheric current sheet (HCS). The V1 intensity peaks occur when the HCS is crossed from negative to positive magnetic polarities and V1 is within a few AU of the TS. Two mechanisms are considered: current sheet drift and streaming of ions from the TS along magnetic field lines which are parallel to the HCS. The current sheet drift hypothesis predicts that enhancements observed at V2 will occur when the HCS is crossed in the opposite direction, from positive to negative magnetic polarity, since V2 is at southern heliolatitudes.

Analysis of the heliospheric current sheet fine structure: Single or multiple current sheets

When studying the heliospheric current sheet (HCS) local structure, it is not unusual to find wide HCS crossings. In this paper, we present one crossing that appears to have a complex internal structure composed of three parallel sheets and several possible HCS crossings that are consecutive and are on the order of minutes. Depending on their origin, different scenarios can explain multiple current sheets such as complex structures of helmet streamer at the Corona flowing into the solar wind, local waviness in the HCS structure, local oscillations of the HCS, and inverted magnetic fields or planar magnetic structures (PMS) close to the HCS. Distinguishing among these scenarios using just one observational point is very difficult because all of them are 3D structures. Nevertheless, we think that nearly parallel sheets are more likely in the first and in the last scenarios, i.e. multiple helmet streamer structure and PMS. In order to make the distinction between them, we have studied the possible reversal in the Q e· B sign for every event. Our results suggest that the existence of not-wide HCS composed of multiple parallel sheets cannot be rejected.

On the variability of the stratospheric column resistance in the global electric circuit

There is a discrepancy between the observed amplitudes of vertical current density ( J z ) and electric field ( E z ) in the global circuit responding to inputs modulated by solar activity, as compared to modeled amplitudes. These responses to estimated changes in ionization production are larger than modeled and increase with increasing content of stratospheric aerosol. The discrepancies could be resolved if the column resistance of the stratosphere and upper troposphere is greater than previously calculated due to two effects. The first is the presence of very thin cirrus, subvisible clouds, aerosol and polar stratospheric clouds near the tropopause level and in the lower stratosphere. They raise the resistivity there so that the changes in the lower energy galactic cosmic ray flux, at those altitudes and at middle to high latitudes where the changes are larger than elsewhere, can increase the sensitivity of the global circuit to the solar modulation of the GCR energy spectrum. The second effect is postulated to be most important at times of high stratospheric aerosol loading and is attributed to the presence of a layer of ultrafine H 2SO 4/H 2O aerosol particles that is only present in the descending part of the stratospheric diabatic circulation at middle and high latitudes. With average levels of precipitating relativistic electron flux at those latitudes, the layer has little effect on the ionosphere–Earth column resistance, but for very low flux levels, such as following solar wind heliospheric current sheet crossings, the reduced ionization production allows the column resistance of the layer to rise to become a non-negligible fraction of the total ionosphere–Earth column resistance. This variation in column resistance modulates J z at those latitudes, consistent with the variations that have been inferred from the sparse observations of J z that are available and from observations of other phenomena thought to be causes and effects of the J z variations.

Daily changes in global cloud cover and Earth transits of the heliospheric current sheet

Changes in cloud cover are found to occur for periods of a few days following Earth transits of the heliospheric current sheet (HCS), provided also that the transits occur in years of high stratospheric aerosol loading. Using global cloud products from the International Satellite Cloud Climatology Project (ISCCP) D1 data series, epoch superposition analyses were made for various samples of HCS events. For the period August 1991 to June 1994 for the stratospheric aerosol loading due to the Pinatubo eruption, the analysis of the data in 30° geomagnetic latitude intervals revealed that cloud anomalies that were significant and negative were located in the Southern Hemisphere high and middle latitudes, and anomalies that were significant and positive were found in both hemispheres at low latitudes. When the key days in the superposed epoch analysis were determined by minima in the relativistic electron flux, rather than by the HCS crossings, the location of the significant negative anomalies was in the northern high latitudes, and the location of the significant positive anomalies was in middle latitudes in the Northern Hemisphere. The spatial and temporal patterns of these cloud cover variations are in broad agreement with the expected opposite variations at high and low latitudes of the current density Jz in the global electric circuit caused by the relativistic electron flux variations, during periods when the aerosol loading has made a large increase in stratospheric resistivity.

The global heliospheric magnetic field polarity distribution as seen at Ulysses

Abstract. The Ulysses spacecraft is in a near-polar solar orbit with a period of 6.2 years. The heliospheric magnetic field polarity detected by Ulysses from its 1992 Jupiter encounter to the current time is presented, following ballistic mapping of the polarity information to the solar wind source surface, at approximately 2.5 solar radii. The spacecraft’s first foray to polar latitudes and first rapid heliolatitude scan occurred in 1994–1995, near a minimum in solar activity. The heliospheric current sheet during this period was confined to low heliolatitudes. In 2000–2001, Ulysses returned in situ data from the same region of its orbit as in 1994–1995, but near to the maximum in solar activity. Unlike at solar minimum, heliospheric current sheet crossings were detected at the spacecraft over a wide heliolatitude range, which is consistent with the reversal of the solar magnetic dipole occurring during solar maximum. Despite complexity in the solar wind parameters during the latest fast latitude scan (McComas et al., 2002), the underlying magnetic field structure appears consistent with a simple dipole inclined at a large angle to the solar rotational axis. The most recent data show the heliospheric current sheet returning to lower heliolatitudes, indicating that the dipole and rotational axes are realigning, with the Sun’s magnetic polarity having reversed.Key words. Interplanetary physics (interplanetary magnetic fields; sources of the solar wind) – Solar physics, astrophysics and astronomy (magnetic fields)

Scales of heliospheric current sheet coherence between 1 and 5 AU

The structure of the heliospheric current sheet (HCS) at Ulysses and at Wind is compared during the period of near‐radial alignment in 1998. Electron heat flux and magnetic field data are used to determine true magnetic field polarity. During the solar rotation period surrounding alignment there was considerable disagreement between the HCS crossings at Wind and those predicted from the corresponding source surface map, both in number and in location, consistent with the disordered, temporally varying solar wind at this ascending phase of the solar cycle. Despite this complexity the four crossings closest to the time of radial alignment at Wind were successfully identified in Ulysses data with use of a one‐dimensional hydrodynamic code. Further, minimum variance analysis for the first two crossings, which were separated by only 16 hours at Wind, indicated coherent propagation of a large‐scale warp in the HCS. Analysis of the local structure of the HCS on the four crossings, however, revealed a high level of variability both from case to case and from one spacecraft to the other. For example, the third crossing at Wind was a single‐sheet crossing adjacent to structures with fields folded back on themselves and a brief period of counter‐streaming electrons implying a transient structure. At Ulysses multiple sheets were encountered. At the fourth crossing Wind passed through a counterstreaming event with a flux rope signature containing a south pointing axis, while Ulysses passed through a flux tube with little field rotation and northward pointing field. The results are consistent with the view that the heliospheric current sheet is coherent as a global structure but highly variable in local structure over angular distances of a few degrees.

The heliospheric current sheet

The heliospheric current sheet (HCS) is the boundary between open oppositely directed magnetic field lines which commonly originate as the outward extension of the solar magnetic dipole. The dipole tilt, the rotation of the Sun, and the outward propagation of the solar wind cause peaks and valleys in the current sheet which spiral outward. The HCS extends throughout the heliosphere to the greatest distances reached by Pioneer and Voyager. It serves as a magnetic equator, and solar wind parameters including speed, temperature, density, and composition vary with distance from the HCS. Extrapolated back to the Sun, especially near solar minimum, the HCS corresponds to the low‐latitude streamer belt. Both features are closely related to a neutral line obtained by extrapolating photospheric magnetic fields to a source surface at several solar radii. The current sheet and sector structure persist throughout the solar cycle including solar maximum. At 1 AU the width of the HCS is approximately 10,000 km while a surrounding plasma sheet is thicker by a factor of ∼30. The field inside the HCS does not simply decrease to a null and then reappear with the opposite sense. Instead, the field rotates at nearly constant magnitude from one polarity to the other. In spite of theoretical expectations that fields on opposite sides of the HCS will merge or reconnect, there is little evidence that such is occurring. Many scientific questions remain unanswered. What are the global properties of the HCS near solar maximum, and how faithfully are they reproduced by source surface models? Are multiple HCS crossings caused by waves on the current sheet or by multiple current sheets? What is the effect of coronal mass ejections on the HCS and vice versa?

The Bastille day Magnetic Clouds and Upstream Shocks: Near-Earth Interplanetary Observations

The energetic charged particle, interplanetary magnetic field, and plasma characteristics of the 'Bastille Day' shock and ejecta/magnetic cloud events at 1 AU occurring over the days 14 - 16 July 2000 are described. Profiles of MeV (WIND/LEMT) energetic ions help to organize the overall sequence of events from the solar source to 1 AU. Stressed are analyses of an outstanding magnetic cloud (MC2) starting late on 15 July and its upstream shock about 4 hours earlier in WIND magnetic field and plasma data. Also analyzed is a less certain, but likely, magnetic cloud (MC1) occurring early on 15 July; this was separated from MC2 by its upstream shock and many heliospheric current sheet (HCS) crossings. Other HCS crossings occurred throughout the 3-day period. Overall this dramatic series of interplanetary events caused a large multi-phase magnetic storm with min Dst lower than −300 nT. The very fast solar wind speed (≥ 1100 km s −1 ) in and around the front of MC2 (for near average densities) was responsible for a very high solar wind ram pressure driving in the front of the magnetosphere to geocentric distances estimated to be as low as ≈ 5 RE ,m uch lower than the geosynchronous orbit radius. This was consistent with magnetic field observations from two GOES satellites which indicated they were in the magnetosheath for extended times. A static force-free field model is used to fit the two magnetic cloud profiles providing estimates of the clouds' physical and geometrical properties. MC2 was much larger than MC1, but their axes were nearly antiparallel, and their magnetic fields had the same left-handed helicity. MC2's axis and its upstream shock normal were very close to being perpendicular to each other, as might be expected if the cloud were driving the shock at the time of observation. The estimated axial magnetic flux carried by MC2 was 52 × 10 20 Mx, which is about 5 times the typical magnetic flux estimated for other magnetic clouds in the WIND data over its first 4 years and is 17 times the flux of MC1. This large flux is due to both the strong axially-directed field of MC2 (46.8 nT on the axis) and the large radius (R0 = 0.189 AU) of the flux tube. MC2's average speed is consistent with the expected transit time from a halo-CME to which it is apparently related.

Context and heliographic dependence of heliospheric planar magnetic structures

An analysis is presented of planar magnetic structures (PMSs) detected in the solar wind by the Ulysses spacecraft between its launch in October 1990 and the end of 1998. In all, 667 such structures were found, with durations between 6 hours (the minimum in the search algorithm) and 66 hours. The total amount of magnetometer data found to represent planar periods accounted for 9% of all data returned from the instrument during 1990–1998. PMSs are found to be a common occurrence within corotating interaction regions. Around one fifth of PMSs can probably be accounted for by the passage of shocks through the solar wind, leading to alignment and amplification of preexisting features. Heliospheric current sheet crossings were found to be close to about 50% of PMSs. The results thus favor several previously proposed PMS formation mechanisms. After other causes of PMSs such as draping around the interplanetary counterparts of coronal mass ejecta are taken into account, there still remains a sizeable minority of events that have no obvious association with other solar wind features. It is suggested that some of these unexplained PMSs may have been caused by compressional processes such as draping around transient features and had remained in the solar wind when the signatures of the features responsible were no longer recognizable.

Disconnection Events (DEs) in Halley's Comet 1985–1986:: The Correlation with Crossings of the Heliospheric Current Sheet (HCS)

Disconnection events (DEs) from Halley's comet are analyzed as a group (in order to minimize the impact of uncertainties in any single event) for correlation with solar-wind features. This analysis utilizes new coronal source-surface model results and new fitting techniques to determine the best calculated heliospheric current sheet (HCS) location. Details for several DEs are presented.The results confirm that the 19 DEs are associated with crossings of the HCS. No other property of the solar wind shows a one-to-one association with DEs. Thus, theoretical efforts should concentrate on conditions at HCS crossings (see Y. Yiet al., 1996,J. Geophys. Res.101, 27,586–27,601).

Very low frequency waves in the heliosphere: Ulysses observations

An overall profile of plasma wave activity in a frequency range of 0.2 to 448 Hz in the solar wind is presented using 6 years of Ulysses data which cover a large range of heliographic latitudes (0° to ±80°) at distances from 1 to 5 AU from the Sun. The spacecraft has continuously observed wave activity with peak power below the local electron cyclotron frequency ƒce. Four distinct types of fluctuation phenomena have been observed: (1) enhanced electromagnetic fluctuations associated with the interplanetary shocks and heliospheric current sheet crossings and other solar wind turbulence; (2) enhanced electromagnetic fluctuations associated with compression regions of high‐speed stream interfaces, which were observed in periods of increasing solar wind velocity; (3) electric fluctuations associated with the expanding solar wind, which were observed in periods of decreasing solar wind velocity; and (4) enhanced electric fluctuations in the high‐latitude fast solar wind plasma. The fourth type of wave activity was observed nearly continuously with the relative power observed peaked near the local ƒce. The first three types of waves were observed in the heliomagnetic streamer belt flows, where the spacecraft frequently encountered enhanced solar wind turbulence, interplanetary shocks, and current sheet crossings. The electromagnetic wave bursts (types 1 and 2) are likely to be whistler mode. The occurrences of apparently electrostatic waves during periods of expanding solar wind are coincident with significant reductions in the electron heat flux intensity. The generation mechanism of these electrostatic waves is still under investigation, but the observations may imply that these waves reduce the intensity of the heat flux through enhanced wave particle scattering associated with a heat flux instability.

Galactic cosmic ray modulation and the passage of the heliospheric current sheet at Earth

We have examined the effect of heliospheric current sheet (HCS) crossings on cosmic ray intensity (CRI), solar wind speed, interplanetary magnetic field (IMF) and the cosmic ray solar diurnal anisotropy in free space. Data from eight neutron monitors and three muon telescopes, covering a wide energy and latitude range, were separated into two epochs, 1971–1979 (A > 0) and 1981–1990 (A < 0) according to the solar magnetic polarity. The CRI is more strongly affected during the A > 0 than during the A < 0 epoch. The increase in CRI associated with A-T HSC crossings was larger in magnitude than that of T-A crossings, and the magnitude was dependent on the rigidity of the particles. A symmetric negative cosmic ray gradient was observed with respect to the HCS. HCS crossings have no observable effect on the CRI observed by shallow underground detectors. Changes in solar wind speed and IMF magnitude, near HCS crossings, are related to the distance of the Earth from the sheet. We found no consistent changes in the solar diurnal variation related to HCS crossings.