Neutral Sheet Crossings Research Articles

Timescales for the penetration of IMF By into the Earth's magnetotail

AbstractPrevious studies have shown that there is a correlation between the By component of the interplanetary magnetic field (IMF) and the By component observed in the magnetotail lobe and in the plasma sheet. However, studies of the effect of IMF By on several magnetospheric processes have indicated that the By component in the tail should depend more strongly on the recent history of the IMF By rather than on the simultaneous measurements of the IMF. Estimates of this timescale vary from ∼25 min to ∼4 h. We present a statistical study of how promptly the IMF By component is transferred into the neutral sheet, based on Cluster observations of the neutral sheet from 2001 to 2008, and solar wind data from the OMNI database. Five thousand nine hundred eighty‐two neutral sheet crossings during this interval were identified, and starting with the correlation between instantaneous measurements of the IMF and the magnetotail (recently reported by Cao et al. (2014)), we vary the time delay applied to the solar wind data. Our results suggest a bimodal distribution with peaks at ∼1.5 and ∼3 h. The relative strength of each peak appears to be well controlled by the sign of the IMF Bz component with peaks being observed at 1 h of lag time for southward IMF and up to 5 h for northward IMF conditions, and the magnitude of the solar wind velocity with peaks at 2 h of lag time for fast solar wind and 4 h for slow solar wind conditions.

O+ ion conic and plasma sheet dynamics observed by Van Allen Probe satellites during the 1 June 2013 magnetic storm

AbstractThe Van Allen Probe satellites were near apogee in the late evening local time sector during the 1 June 2013 magnetic storm's main phase. About an hour after crossing the ring current's “nose structure” into the plasma sheet, the satellites encountered a quasiperiodic sequence of 0.08–3 keV O+ ions. Pitch angle distributions of this population consistently peaked nearly antiparallel to the local magnetic field. We interpret this population as O+ conics originating in the northern ionosphere. Sequences began as fairly steady state conic fluxes with energies in the ~ 80 to 100 eV range. Over about a half hour buildup phase, O+ energies peaked near 1 keV. During subsequent release phases lasting ~ 20 min, O+ energies returned to low‐energy starting points. We argue these observations reflect repeated formations and dissolutions of downward, magnetically aligned electric fields (ε||) layers trapping O+ conics between mirror points within heating layers below and electrostatic barriers above. Nearly identical variations were observed at the locations of both satellites during 9 of these 13 conic cycles. Phase differences between cycles were observed at both spacecraft during the remaining events. Most “buildup” to “release” phase transitions coincided with AL index minima. However, in situ magnetometer measurements indicate only weak dipolarizations of tail‐like magnetic fields. The lack of field‐aligned reflected O+ and tail‐like magnetic fields suggest that both ionospheres may be active. However, Southern Hemisphere origin conics cannot be observed since they would be isotropized and accelerated during neutral sheet crossings.

An empirical model for the location and occurrence rate of near‐Earth magnetotail reconnection

We introduce an empirical model for the location and rate of occurrence of magnetic reconnection in the near‐Earth magnetotail. The model is constructed using Geotail data, including the set of diffusion region and fast tailward flow events identified in Nagai et al. (2005). Events are arranged by their XGSM location. The event occurrence profile is normalized by the time spent by Geotail within a distance d of the modeled neutral sheet. To locate the neutral sheet, we use the empirical model of Fairfield (1980). We organize our data via a coordinate system that conforms to the Fairfield neutral sheet model. In this new coordinate system, we calculate the deviation of physical neutral sheet crossings from their Fairfield‐model‐predicted locations. We equate d to constant multiples of this standard deviation (0.5 RE, 2 RE, and 5 RE). Using these values, we find that our deduced rate of occurrence of reconnection events has a magnitude consistent with those from similar calculations found in the existing literature. We then use our empirical model to analyze the predicted ephemeris for the Magnetospheric Multiscale (MMS) mission. We predict that during its upcoming near‐tail survey phase, MMS will observe between nine and four diffusion region events (nominal estimate of 7), and between 57 and 31 fast tailward flow events (nominal estimate of 47). Because of the restrictive nature of the criteria used to identify diffusion region events, we emphasize that the number of predicted events that we calculate represents a conservative estimate.

Substorm activity in Venus's magnetotail

Abstract. The magnetotail of the induced magnetosphere of Venus is investigated through the magnetic field and plasma data of Venus Express. A comparison is made between two neutral sheet crossings. One crossing shows the magnetic field is rather quiet and the plasma instrument indicates a change from energetic (few 100 eV) to low energy (few 10 eV) ions. The other crossing shows more dynamics in the magnetic field, including signatures that are interpreted as characteristic of a reconnection site, and the plasma instrument indicates ions that are energized to 1500 to 2000 eV, in the same magnetospheric region where in the first crossing only low energy ions showed up.

Orientation and propagation of current sheet oscillations

Using the four‐spacecraft Cluster system, we analyze rapid neutral sheet crossings near the Cluster apogee at about −18RE. In case studies of multiple oscillations of the locally quiet plasma sheet as well as in a statistical study of oscillations in dawn and dusk near‐flank plasma sheet portions we typically obtain that locally these dynamical current sheets are very corrugated and that subsequent crossings basically show portions of large‐scale kink‐like waves propagating from the tail center toward flanks. Propagation velocities are in the range of several tens km/s for the locally quiet sheets, and up to 200 km/s during fast flows. These results suggest that the flapping motions are of internal origin and that kink‐like waves are emitted in the central part of the tail by some impulsive source and propagate toward the tail flanks. The wave properties do not match any local excitation mechanism previously discussed so far in the literature.

A wavy twisted neutral sheet observed by CLUSTER

From July to October 2001, the Cluster satellites were located within the magnetotail and crossings of the neutral sheet were frequently observed by the FGM instrument [Balogh et al., 2001]. The neutral sheet often appears to be moving relative to the spacecraft. Multiple crossings of the neutral sheet on August 3, 2001 are analysed. These neutral sheet crossings were rather unusual. Spacecraft 2, which is in the northwestern part of the Cluster tetrahedron, leads the crossing from the north to the south lobe which implies a strongly twisted magnetotail on a local scale. Examining the solar wind interplanetary magnetic field conditions, we postulate that a wavy motion in the dawn‐dusk direction is superimposed on an extremely twisted and warped neutral sheet.

Hot ions in Jupiter's magnetodisc: A model for Voyager 2 low‐energy charged particle measurements

The Low‐Energy Charged Particle (LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive set of directional and energy‐dependent information on the nature of hot ions in the Jovian magnetodisc. The LECP measurements in the energy range 30 keV to 5 MeV, where the ion pressure dominates the total plasma pressure, have been successfully fit to a two‐species convected k distribution function model for hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where the model could be used ranged from 60 to 30 RJ on the dayside (inbound) and 75 to 125 RJ on the nightside (outbound). With this model, the full angular and spectral information from the lowest‐energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in the outer magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near‐Io regions and ionized during their flight through the outer magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasma is subcorotational, reduced from the rigid value by a factor of ∼2, but increases with increasing distance from 30 to 60 RJ in the dayside region and from 75 to 85 RJ in the nightside region. The trend stops beyond 85 RJ in the nightside region, but there is still a substantial corotational flow that extends from 85 RJ to at least 130 RJ. In all the regions studied, the particle anisotropies in the LECP scan plane below ∼2 MeV are believed to result primarily from the Compton‐Getting effect and not from gradient anisotropies or particles executing nonadiabatic orbits as they encounter the neutral sheet. Gradient anisotropies are not important even in the distant nightside neutral sheet region (>85 RJ) below ∼2 MeV. The large flow velocities and increasing heavy ion temperatures are consistent with a strong corotational electric field and imply that the mass loading due to lower‐energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within ∼60 RJ during the Voyager 2 encounter.

Classification of the tailward drifting magnetic structures in the distant tail

The magnetic field observed at the neutral sheet crossings in the distant tail at |x| = 200‐210 Re can be classified into three types. At the sharp and isolated crossings (Type A), the Bz component is positive on average and the tailward extension of the LLBL seems to be the viable explanation, but the presence of large and IMF‐dependent By component suggests that the field lines are not closed but are connected to IMF at both ends. When the neutral sheet crossings occur multiple times, Type B or C is observed. Type B is characterized by the bipolar magnetic signature and represents a plasmoid or flux rope. Type C is characterized by magnetic oscillations with typical time scales of a few tens of seconds. These oscillations may represent the magnetic loops or spirals produced by the tearing. In all cases, the direction of the drift motion derived from the electric and magnetic field data is directed predominantly tailward.

Signatures of the magnetotail neutral sheet

The geometry and the magnetic field of the magnetotail neutral sheet at −4RE <X < −21RE are studied with ISEE‐2 magnetometer data. Neutral sheet crossings are determined using a local coordinate system. We show the position of the neutral sheet after the effect of tilt angle is removed, and construct a smooth model to fit it.

Growth‐phase thinning of the near‐Earth current sheet during the CDAW 6 substorm

The thinning of the near‐Earth current sheet during the growth phase of the CDAW 6 magnetospheric substorm is studied. The expansion onset of the substorm occurred at 1054 UT, March 22, 1979. During the growth phase, two spacecraft, ISEE 1 and ISEE 2, were within the current sheet approximately 13RE from the Earth and obtained simultaneous high‐resolution magnetic data at two points in the current sheet. Plasma data were also provided by the ISEE spacecraft and solar wind data by IMP 8. To facilitate the analysis, the GSM magnetic field data are transformed to a “neutral sheet coordinate system” in which the new x axis is parallel to the average magnetic field above and below the neutral sheet and the new y axis lies in the GSM equatorial plane. A model based on the assumption that the current sheet is a time‐invariant structure fails to predict neutral sheet crossing times. Consequently, the Harris sheet model, which allows one to remove the restriction of time invariancy, is used instead. It is found that during the growth phase, a model parameter corresponding to the thickness of the current sheet decreased exponentially from about 5RE to 1RE with a time constant of about 14 min. In addition, the ISEE 1 and ISEE 2 neutral sheet crossings after expansion onset indicate that the neutral sheet was moving upward at 7 km/s relative to the spacecraft. Since both crossings occurred in approximately 80 s, the current sheet thickness is estimated to be about 500 km. These results demonstrate that the near‐Earth current sheet undergoes dramatic thinning during the substorm growth phase and expansion onset.

On the Average Position of the Near-Earth Neutral Sheet.

Magnetic field data from magnetometers on board the ISEE 1 spacecraft are used to examine the position of the magnetotail neutral sheet, which varies in response to variations in the tilt angle of the earth's dipole axis. In the whole data set of the magnetic field from January 1978 to December 1980, 129 neutral sheet crossings are identified in the tail region X<−10 R E |Y|<18 R E and R<22.6 R E . While it is confirmed that the average neutral sheet does warp when the tilt angle is not equal to zero, in contrast to previous warped-neutral sheet models, the points at which the neutral sheet crosses the XY plane move farther away from the sun-earth line, as the absolute value of the tilt angle increases

Pressure anisotropy and radial stress balance in the Jovian neutral sheet

By examining particle and magnetic field data from the Voyager 1 and 2 spacecraft, we have found signatures indicating that the (≳28 keV) particle pressure parallel to the magnetic field is greater than the pressure perpendicular to the field within the nightside neutral sheet (three nightside neutral sheet crossings, with favorable experimental conditions, were used). By incorporating the pressure anisotropy into the calculation of radial forces within the nightside neutral sheet, we find that (1) force balance is approximately achieved and (2) the anisotropy force term provides the largest contribution of the other particle forces considered (pressure gradients and the corotation centrifugal force). With regard to the problem of understanding the balance of radial forces within the dayside neutral sheet (McNutt, 1984; Mauk and Krimigis, 1987), the nightside pressure anisotropy force is larger than the dayside pressure gradient forces at equivalent radial distances; however, a full accounting of the dayside regions remains to be achieved.

Plasma waves in the magnetotail of Uranus

As the Voyager 2 spacecraft left Uranus during its encounter with the planet on January 24, 1986, it passed through the magnetotail over the distance range of about 11 RU to 79 RU. During this interval the plasma wave receiver on board measured only very weak plasma wave intensities. The most remarkable of these waves are found predominantly inside of about 23 RU with an additional event occurring between about 48 and 56 RU. The frequency of these magnetotail plasma waves is generally below about 100 Hz, extending down to the 10‐Hz lower frequency limit of the receiver. In each case the upper frequency limit of the emissions lies below the local electron cyclotron frequency. The plasma waves are found prior to the first neutral sheet crossing and between the second and third neutral sheet crossings, as defined by the Voyager magnetometer investigation. This timing suggests the waves are found only in the negative solar magnetospheric lobe of the tail, corresponding to field lines which intersect the planet near the southern magnetic pole—that which is continuously in the nightside hemisphere during the encounter epoch. The observations are not conclusive as to the mode of propagation of the waves. However, the spectrum is similar in some respects to broadband electrostatic waves observed in similar regions of Earth's magnetotail.

Magnetic field and current structures in the magnetosphere of Uranus

We compare Voyager 2 magnetic field and plasma data with theoretical model calculations for the magnetosphere of Uranus in order to derive a global picture from the quite limited set of measurements. In two dimensions we use a tilt‐dependent linear MHD equilibrium model for the entire magnetosphere to calculate the plasma parameter β = 8πP/B² and the resulting B field in the tail plasma sheet. The three‐dimensional model satisfies the MHD equilibrium equations in an approximate fashion; it is used to calculate the large‐scale magnetic field structure, the tilt‐dependent shape and position of the Uranian magnetotail plasma sheet, and the influence of the interplanetary magnetic field (IMF). The main results of the model calculations are the following: (1) The actual solar wind pressure at Uranus was found to be about a factor of 5 higher than a calculated solar wind pressure derived from an adiabatically expanding quiet solar wind. This means that the Uranian magnetosphere must have experienced a major magnetic storm prior to the Voyager encounter. (2) The previous conjecture is supported by the fact that the asymptotic plasma beta parameter in the neutral sheet, β0 = β(|x| ≫ 1), relaxes from β0 = 37 to β0 = 14 between the first and second neutral sheet crossings. (3) The excellent agreement between measured and modeled tail lobe magnetic field values suggests that during the time period of the Voyager encounter the Uranian tail plasma sheet, measured in units of planetary radii, was thicker and less stretched than the average Earth's tail plasma sheet. (4) Despite sizable temporal variations suggested by the data and the model, the Uranian magnetosphere seems to reach the state of quasi‐static, i.e., slowly time‐dependent, MHD equilibrium. (5) In relation to a given IMF orientation the magnetosphere changes periodically every 8.62 hours (the Uranian day is 17.24 hours) from an “open” to a “closed” configuration and back. Thus the convection process, driven by the direct influence of the IMF, is periodically interrupted by the planet's rotation, such that the plasma sheet might not experience its full stretching. Consequently, the Uranian magnetosphere might not experience convectively driven, Earth‐type substorms. On the other hand, the possibility of substorms cannot be ruled out either.

Magnetotail plasma observations during the 1054 UT substorm on March 22, 1979 (CDAW 6)

The paper reports ISEE 1 and 2 plasma observations in the plasma sheet near 15 RE radial distance for the 1054 substorm on March 22, 1979. Noteworthy features are plasma sheet thinning and the brief appearance of rapid tailward flows after substorm onset, strong earthward flows (superimposed on dawnward convection) at the beginning of plasma sheet expansion and a quieting of all plasma properties at substorm recovery. Much of the time, however, plasma flows are small and switch directions. In particular, no strong flows are observed during neutral sheet crossings. Two‐dimensional velocity distribution functions show a complex behavior, with several plasma components often coexisting and at times streaming in opposite directions. The interpretation of the observations in terms of the formation of a neutral line earthward of and close to ISEE at substorm onset is tempting, but three‐dimensional spatial effects as well as rapid temporal changes have to be invoked to obtain consistency.

Electric fields measured by ISEE‐1 within and near the neutral sheet during quiet and active times

A statistical survey of electric fields observed at 146 neutral sheet crossing events of the ISEE‐1 satellite during 1980 has been made. The observations can be divided into two classes — quiet or turbulent — according to the magnitude of the standard deviation in the 3 second spin period sine wave least‐squares fit to the electric field data. Events classified as turbulent are also turbulent in the magnetic field. During quiet crossings, the dawn‐dusk component of the electric field, Ey, is usually constant to ≲ 1/2 mV/m, with larger variations sometimes occurring over 2‐10 minute time scales. During turbulent crossings, Ey fluctuates rapidly with 3‐second‐average peak‐to‐peak amplitudes of 2‐16 mV/m, and instantaneous field magnitudes up to ∼ 30 mV/m. Turbulent crossings correlate with high levels of ground magnetic activity as indicated by AE and ground magnetograms, and with southward interplanetary magnetic field, while quiet crossings occur primarily during periods of low magnetic activity and northward or mixed interplanetary magnetic field.

Configuration of Jupiter's magnetic tail and equatorial current sheet

Recent research reports by Behannon et al. (1981) and Connerney et al. (1981) are summarized. It is noted that the analysis made of the detailed neutral sheet crossings by the minimum variance method shows a consistent result with regard to the orientation of the neutral sheet in the magnetic tail as a two-dimensional surface rocking back and forth about the Jupiter sun-line as the rotation of the planet leads to a precession of the tilted dipole magnetic axis. The occurrence of neutral sheet crossings is found not to be consistent with any of the axially symmetric theoretical models proposed earlier on the basis of the 1974 Pioneer 10 observations. It is noted that a simple nonaxially symmetric model has been developed on the basis of the Voyager results which indicates the strong control upon orientation by the interaction of the solar wind with the Jovian magnetosphere. The model is described as simple because it improves the fit of theory to observation but uses fewer parameters. A quantitative model of the magnetodisc equatorial current sheet has been developed for the inner magnetosphere region which matches well the in-situ magnetic field observations.

Wavy nature of the magnetotail neutral sheet

Crossings of the magnetotail neutral sheet are examined with 20 s averaged magnetic field data from Imp 5 satellite. It is shown that for some multiple neutral sheet crossings, the By component reverses its sign between adjacent crossings. This feature can be explained in terms of a wavy profile of the neutral sheet along the dawn‐dusk direction, but not in terms of flapping motion or wavy profile of the neutral sheet along the tailward direction. It is suggested that the magnetotail neutral sheet often shows departure from a plane geometry.

The influence of the direction of the geomagnetic dipole on the position of the neutral sheet

An analysis of neutral sheet crossings detected by the NASA-Goddard Space Flight Center magnetometer on the Explorer 34 satellite has shown that the diurnal wobble of the geomagnetic dipole measurably influences the position of the neutral sheet at 30 RE. Viewed in the solar-ecliptic coordinate system, the dipole wobble originates from the 11° angle between the earth's spin axis and the dipole axis, has a period of 24 hours, and can be resolved into components of motion perpendicular and parallel to the earth-sun line. The perpendicular component exerts a torque on the magnetotail and causes the neutral sheet to pivot about an axis approximately parallel to the solar wind direction. The parallel component encourages the neutral sheet to move above and below the solar-magnetospheric equatorial plane with an amplitude of 1 RE. This latter motion is consistent with the neutral sheet's being hinged at a geocentric distance of 5.25 RE. Although hinging is not as dominant a feature of neutral sheet motion as pivoting at 30 RE, both are in phase with the 24-hour precession of the dipole axis about the earth's spin axis.

Spatial distribution of energetic plasma sheet electrons

The spatial distribution of energetic plasma sheet electrons (E ≥ 50 kev) out to a radial distance of 24 RE using data from the University of California at Los Angeles electron spectrometer and fluxgate magnetometer experiments on Ogo 5 is presented. A comparison of distributions in geocentric solar magnetospheric coordinates (GSM) prepared with and without the use of a neutral sheet model indicates that the use of such a model facilitates organization of plasma sheet data. The distribution in the YGSM- ZN plane, where ZN is the distance above or below the neutral sheet in the ZGSM direction, for limited values of XGSM shows a tendency for the energetic electrons to concentrate near the neutral sheet. However, no characteristic behavior is seen in electron fluxes during individual neutral sheet crossings. The percentage of flux occurrence above a given flux threshold falls off rapidly with distance from the neutral sheet. Contours of constant percentage of occurrence diverge slightly from the neutral sheet at local times away from midnight. This effect decreases with increasing flux threshold. Contours in the XGSM-YGSM plane for a limited range in ZN are extended toward dawn, and thus a limited dawn-dusk asymmetry is indicated. This effect is limited to the extent that percentages of occurrence comparable to those on the dawnside are found in the region about midnight. The degree of asymmetry increases as the flux threshold is increased. The contours in the XGSM-ZN plane converge toward the neutral sheet with increasing distance in the antisolar direction (−XGSM). A comparison of this result with the results from Imp 1 indicates that XGSM is preferable to the radial distance R for organizing tail data.